Abstract

This study revises the early-magmatic, late-magmatic, and hydrothermal mineral assemblages occurring in rocks of agpaitic affinity, mainly represented by nepheline syenites and phonolites, from the Poços de Caldas alkaline massif. Mostly composed of different types of silicates, these rocks include rare minerals of of distinct groups, such as sulfides, oxides, carbonates, phosphates, sulfates, niobates, and notably halides, with villiaumite (NaF) as an important phase indicative of extreme alkali enrichment. Hydrothermal alteration products of nepheline and alkali feldspar typically consist of feldspathoids and zeolites. The rare minerals are of varied origin, from magmatic crystallization to hydrothermal and weathering processes. In general, they tend to occur interstitially or form euhedral crystals, especially when filling cavities or growing out from their walls. Petrological conditions involving the formation of the agpaitic rocks and their mineral assemblages are discussed.

KEYWORDS:
igneous petrology; alkaline rocks; agpaitic rocks; mineral chemistry

INTRODUCTION

The Late Cretaceous isolated circular structure referred to in literature as the Poços de Caldas massif represents the second largest known alkaline igneous occurrences worldwide, extending over an area of more than 800 km² in southeastern Brazil (Fig. 1). Emplaced into a Neoproterozoic basement mainly composed of granitic and gneissic rocks, probably as a caldera-like structure, the Poços de Caldas massif also includes significant amounts of aeolian sandstones of the Cretaceous Botucatu Formation (Ellert 1959Ellert R. 1959. Contribuição à geologia do maciço alcalino de Poços de Caldas. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, Geologia, (18):5-63. https://doi.org/10.11606/issn.2526-3862.bffcluspgeologia.1959.121851
https://doi.org/10.11606/issn.2526-3862.... ). Tectonically, it is associated with the Cabo Frio magmatic lineament, a curved, 60 km wide, and 1,150 km long WNW-ESE-trending belt encompassing several alkaline intrusions from the east of Poços de Caldas to the oceanic margin near Cabo Frio (Almeida 1991Almeida F.F.M. 1991. O alinhamento magmático de Cabo Frio. In: Simpósio de Geologia do Sudeste, 2., 1991, São Paulo. Atas…, p. 423-428., Riccomini et al. 2005Riccomini C., Velázquez V.F., Gomes C.B. 2005. Tectonic controls of the Mesozoic and Cenozoic alkaline magmatism in the Central-Southeastern Brazilian Platform. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 31-56.). Lying in a singular position, this isolated massif is distant from the alkaline occurrences that form the Alto Paranaíba Igneous Province to the northeast and the Serra do Mar Province (Thompson et al. 1998Thompson R.N., Gibson S.A., Mitchell J.G., Dickin, A.P., Leonardos O.H., Brod J.A., Greenwood J.C. 1998. Migrating Cretaceous-Eocene magmatism in the Serra do Mar alkaline province, SE Brazil: melts from the deflected Trindade mantle plume? Journal of Petrology, 39(8):1493-1526. https://doi.org/10.1093/petroj/39.8.1493
https://doi.org/10.1093/petroj/39.8.1493... ) to the east. Being quite distinctive in terms of petrographic composition, Poços de Caldas can be briefly described as consisting of two main suites: a felsic and a mafic-ultramafic one (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.). The felsic suite comprises nepheline syenites, tinguaites (subvolcanic pyroxene phonolites), and volcanic phonolites. The syenitic rocks mostly form discrete bodies, accounting for 17% of the entire district, whereas the most abundant tinguaites and altered vesicular phonolites represent 80% of the outcropping areas (Ulbrich and Ulbrich 2000Ulbrich H.H., Ulbrich M.N.C. 2000. The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study. Revista Brasileira de Geociências, 30:615-622. https://doi.org/10.25249/0375-7536.2000304615622
https://doi.org/10.25249/0375-7536.20003... ). The mafic-ultramafic lithologies, in turn, are composed of a variety of fragmented volcanoclastic material (agglomerates, lapilli, and tuffs) and poorly exposed lava flows filling the depressed area of Vale do Quartel, a narrow, irregular, N-S-trending structure at the western border of the massif. Additional rock types include phonolites and lamprophyres cropping out as dikes cutting syenitic rocks in areas such as the Osamu Utsumi open pit uranium mine (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.) or in the Minas Pedras quarry, outside the complex, where they are penetrating into basement gneisses (Vlach et al. 1996Vlach S.R.F., Ulbrich H.H., Ulbrich M.N.C., Gualda G.A.R. 1996. Rochas de afinidades alnoíticas e sílico-carbonatíticas periféricas ao maciço alcalino de Poços de Caldas (M-SP). In: Congresso Brasileiro de Geologia, 39., 1996, Salvador. Anais… 2:128-130., 1998Gualda G.A.R., Vlach S.R.F. 1998. Lamprofilita e normandita (låvenita titanífera) em nefelina sieníticos agpaíticos do maciço alcalino de Poços de Caldas (MG-SP): Caracterização mineralógica e petrográfica. In: Congresso Brasileiro de Geologia, 40., 1998, Belo Horizonte. Resumos… p. 284., Ulbrich et al. 1998Ulbrich M.N.C., Vlach S.R.F., Ulbrich H.H. 1998. Geoquímica de diques de rochas máfico-ultranáficas do maciço de Poços de Caldas (MG-SP). In: Congresso Brasileiro de Geologia, 40., Belo Horizonte. Resumos… p. 469.). Dikes of alnoitic and silico-carbonatitic affinity are described as small bodies lying peripherally to the massif and have been investigated geochemically by Vlach et al. (1996Vlach S.R.F., Ulbrich H.H., Ulbrich M.N.C., Gualda G.A.R. 1996. Rochas de afinidades alnoíticas e sílico-carbonatíticas periféricas ao maciço alcalino de Poços de Caldas (M-SP). In: Congresso Brasileiro de Geologia, 39., 1996, Salvador. Anais… 2:128-130., 1998Gualda G.A.R., Vlach S.R.F. 1998. Lamprofilita e normandita (låvenita titanífera) em nefelina sieníticos agpaíticos do maciço alcalino de Poços de Caldas (MG-SP): Caracterização mineralógica e petrográfica. In: Congresso Brasileiro de Geologia, 40., 1998, Belo Horizonte. Resumos… p. 284.).

Abundant geochronological data obtained by different methods are available for the massif and apparently confirm their long magmatic history (K/Ar age between 64 and 83 Ma, with a median value of 77 Ma, Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.). These authors also reported Rb/Sr measurements giving an isochron age of 78.6 ± 6.6 Ma, and more recent work by Vlach et al. (2018)Vlach S.R.F., Ulbrich H.H., Ulbrich M.N.C., Vasconcelos P.M. 2018. Melanite-bearing nepheline syenite fragments and 39Ar/40Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas alkaline massif (MG-SP), and implications. Brazilian Journal of Geology, 48(2):391-402. https://doi.org/10.1590/2317-4889201820170095
https://doi.org/10.1590/2317-48892018201... yielded an Ar/Ar plateau age of 87 (ca.) Ma for phlogopite crystallization in conduit breccia. This value is older than the “best” presumed (ca. 79 Ma) age proposed by Ulbrich et al. (2002)Ulbrich H.H., Vlach S.R.F., Ulbrich M.N.C., Kawashita K. 2002. Penecontemporaneous syenite-phonolite and basic-ultrabasic-carbonatitic rocks at the Poços de Caldas Alkaline Massif, SE Brazil. Revista Brasileira de Geociências, 32(1):15-26. for the main magmatic event of the massif.

Significant areas of the district, especially those associated with felsic rocks, show evidence of being affected in variable degrees by strong subvolcanic magmatic-related hydrothermal alteration and brecciation, as evidenced by the large number of mineralized occurrences (mainly U, Th, Mo, and Zr), some of them already economically exploited in past decades (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Lapido-Loureiro 1994Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.).

The unusual composition of the felsic rocks and the geological processes that influenced the magmatic evolution of the Poços de Caldas massif are responsible for mineral assemblages, mainly associated with rocks of agpaitic affinity, presenting remarkable diversity and generally having complex chemical compositions. Thus, the main purpose of the present study was to review the data available on that mineralogy, describing the different minerals so far identified in the massif that include some well-known and others that still need additional information for a better characterization.

PETROGRAPHY

The dominant felsic suite in Poços de Caldas is represented by highly evolved SiO₂-undersaturated rocks of syenitic (either intrusive or fine-grained) composition, giving rise to the formation of two distinct petrographic lineages based on their alkali content and mineralogy: a transitional agpaitic (a paragenesis proposed by Marks and Markl 2017Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... , and also adopted by Guarino et al. 2019Guarino V., De Gennaro R., Melluso L., Ruberti E., Azzone R.G. 2019. The transition from miaskitic to agpaitic rocks, as highlighted by the accessory phase assemblages in the Passa Quatro alkaline complex (Southeastern Brazil). The Canadian Mineralogist, 57(3):339-361. https://doi.org/10.3749/canmin.1800073
https://doi.org/10.3749/canmin.1800073... , showing the presence of minerals typical of miaskitic rocks) and an agpaitic one. Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. and Vlach et al. (2018)Vlach S.R.F., Ulbrich H.H., Ulbrich M.N.C., Vasconcelos P.M. 2018. Melanite-bearing nepheline syenite fragments and 39Ar/40Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas alkaline massif (MG-SP), and implications. Brazilian Journal of Geology, 48(2):391-402. https://doi.org/10.1590/2317-4889201820170095
https://doi.org/10.1590/2317-48892018201... also defined the former association as “intermediate miaskitic varieties.” The miaskitic lineage is clearly more abundant (118.7 km² of outcrops vs. 24.8 km²) and older than the agpaitic, as reported by Ulbrich (1984)Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich and Ulbrich (2000)Ulbrich H.H., Ulbrich M.N.C. 2000. The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study. Revista Brasileira de Geociências, 30:615-622. https://doi.org/10.25249/0375-7536.2000304615622
https://doi.org/10.25249/0375-7536.20003... , and Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.. Revising the global agpaitic rocks, Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... concluded that in a composite magmatic complex consisting of several miaskitic intrusive units, the agpaitic units are mostly younger compared to the miaskitic ones. In general, according to these authors, miaskitic rocks usually contain zircon, baddeleyite, and titanite as their main accessory phases, whereas the agpaitic ones are characterized by a more complex mineral composition, including aenigmatite, astrophyllite, eudialyte, lamprophyllite, F-disilicates, and wadeite. However, apparently, the Poços de Caldas miaskites mostly contain titanite as a frequent accessory phase, being the mineral of early crystallization in the assemblages (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ). In their review on agpaitic rocks, Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... stated that the quite unusual mineralogy of the agpaitic rocks is due to the presence of some specific elements in their composition, such as large ion lithophile elements (K, Rb), halogens (F, Cl), volatiles (CO₂, H₂O), rare earth elements (lanthanides), and high field strength elements (Zr, Ti, Nb, U-Th). Also, the assemblages result from the enrichment of alkalies (particularly Na) and iron in major mafic constituents and the occurrence of accessory phases of high Na-Ca content. All such chemical evidence is found in Poços de Caldas minerals and has been confirmed by studies performed by Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., Gomes et al. (2021)Gomes C.B., Azzone R.G., Enrich G.E.R., Guarino V., Ruberti E. 2021. Agpaitic alkaline rocks in southern Brazilian Platform: a review. Minerals, 11(9), 934. https://doi.org/10.3390/min11090934
https://doi.org/10.3390/min11090934... , and Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... . Similar characteristics are commonly noted in other well-known agpaitic occurrences around the world (Sørensen 1997Sørensen H. 1997. The agpaitic rocks: an overview. Mineralogical Magazine, 61(407):483-498. https://doi.org/10.1180/minmag.1997.061.407.02
https://doi.org/10.1180/minmag.1997.061.... , Andersen et al. 2010Andersen T., Erambert M., Larsen A.O., Selbekk R.S. 2010. Petrology of nepheline syenites pegmatites in the Oslo Rift, Norway: zirconium silicate mineral assemblages as indicators of alkalinity and volatile fugacity in middly agpaitic magma. Journal of Petrology, 51(11):2303-2325. https://doi.org/10.1093/petrology/egq058
https://doi.org/10.1093/petrology/egq058... , 2018Andersen T., Elburg M., Erambert M. 2018. Contrasting trends of agpaitic crystallization in the Pilanesberg alkaline complex, South Africa. Lithos, 312-313:375-388. https://doi.org/10.1016/j.lithos.2018.05.015
https://doi.org/10.1016/j.lithos.2018.05... , Marks and Markl 2017Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... ). Yet the agpaitic mineral assemblages in Poços de Caldas comprise mainly silicate phases (the eudialyte group being the most typical minerals) and species belonging to other groups, such as oxides, phosphates, carbonates, and fluorocarbonates, more scarcely, sulfates, and niobates.

The massif is also marked by the presence of strongly agpaitic petrographic types described in literature as lujavrites and khibinites (eudialyte-bearing nepheline syenites variable in color index, texture, and grain size; Ulbrich and Ulbrich 2000Ulbrich H.H., Ulbrich M.N.C. 2000. The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study. Revista Brasileira de Geociências, 30:615-622. https://doi.org/10.25249/0375-7536.2000304615622
https://doi.org/10.25249/0375-7536.20003... ) that are particularly abundant in the northern ring area known as Pedra Balão and also occur at the western side of the district (Morro do Serrote) and at its eastern margin (Morro do Taquari), as shown in Fig. 1.

MINERAL COMPOSITION

Poços de Caldas agpaitic minerals are classified based mainly on their chemical composition, which is mostly determined through wavelength dispersive spectrometry (WDS) microprobing and energy dispersive spectrometry (EDS) methods. Additional information for the chrystalochemical characterization of such constituents derives from optical studies, x-ray crystallography, Raman, and infrared spectrographic analysis; all techniques were routinely used by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. in the systematical investigation of Poços de Caldas minerals to confirm their identification and to help distinction between phases of similar characteristics.

Some rare, exotic mineral phases in Poços de Caldas agpaitic rocks are due to crystallization from highly evolved magmas enriched in HFSEs and other incompatible elements and halogens (Gomes et al. 2021Gomes C.B., Azzone R.G., Enrich G.E.R., Guarino V., Ruberti E. 2021. Agpaitic alkaline rocks in southern Brazilian Platform: a review. Minerals, 11(9), 934. https://doi.org/10.3390/min11090934
https://doi.org/10.3390/min11090934... ). In general, as mainly suggested by the studies of Ulbrich and co-workers over the years and by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... , these minerals occur as euhedral phases in the rock groundmass or as irregular, fibrous, or prismatic aggregates. They are usually early- to late-magmatic minerals (mostly represented by different types of silicates), and the assemblages also contain phases originated from hydrothermal and typical weathering processes (carbonates and clay minerals).

The Prefeitura and Bortolan quarries (decommissioned in the 1980s), both including nepheline syenites and fine-grained equivalents (phonolites), respectively, as their main rock types, were the most important sources of samples for the investigation of Poços de Caldas mineralogy. A Th-REE supergene deposit covering a stockwork of magnetite dykes and dykelets at Morro do Ferro has also provided samples for mineralogical studies in the past (Waber 1990Waber N. 1990. Mineralogy, petrology and geochemistry of the Poços de Caldas analogue study sites, Minas Gerais, Brazil: II. Morro do Ferro, Poços de Caldas. Poços de Caldas Report, 3, 121 p., 1992Waber N. 1992. The supergene thorium and rare earth element deposit at Morro do Ferro, Poços de Caldas, Minas Gerais, Brazil. In: Chapman N.A., McKinley I.G., Shea M.E., Smellie J.A.T. (eds.). The Poços de Caldas project: natural analogues of processes in a radioactive waste repository. Journal Geochemical Exploration, 45:113-157., Lapido-Loureiro 1994Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.).

Chemical data on the agpaitic minerals are listed in Tables 1–13, arranged according to their chemical classification into different groups as it is traditionally adopted in literature. Except for polezhaevaite (Table 1) and chlobartonite (Table 2), both minerals analyzed by the EDS method, all the chemical data were determined by the WDS technique and mostly reported in wt% of oxide concentrations.

Halides

This mineral group is represented in the Poços de Caldas massif by villiaumite {NaF}, a low-temperature hydrothermal phase forming commonly euhedral crystals that vary in color from deep red to colorless (Fig. 2). Villiaumite is a crystallization phase of the so-called hyperagpaitic assemblages formed during the final stages of agpaitic systems, indicating extreme alkali enrichment (Marks and Markl 2017Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... ). The mineral contains a high Na content of 51.72%, as reported for the Poços de Caldas phonolitic rocks by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo..

Other hydrothermal constituents of the massif are members of the fluorite group, especially the Sr-bearing strontiofluorite {SrF₂}, with an abundant Sr concentration (59.11%), and the associated polezhaevaite-(Ce) and the undescribed La-dominant equivalent, “polezhaevaite-(La),” which occurs as white aggregates of diminute crystals (Fig. 3). Data on the latter two species, described by EDS in Brazilian rocks for the first time, are shown in Table 1.

Sulfides

Sulfide minerals of unusual composition are rare, with chlorbartonite {K₆Fe₂₄S₂₆(Cl,S)} being the only species identified so far that presents as irregular aggregates and microcrystals (Fig. 4). EDS analyses by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. revealed the high K content of the mineral (Table 2). Common sulfides in the miaskitic and agpaitic rocks are galena, molibdenite, pyrite, pyrrhotite, and sphalerite, as reported by Biondi (2005)Biondi J.C. 2005. Brazilian mineral deposits associated with alkaline and alkaline-carbonatite complexes. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 707-750. and Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. at the Osamu Utsumi mine and the mineralized deposits of Agostinho and Morro do Ferro, and are linked to intense hydrothermal alteration processes responsible for the formation of mineral deposits of U, Zr, Mo, and Th.

Oxides

The oxide minerals found in the Poços de Caldas massif are of varied composition and origin; the phases they constitute bear different cations that occupy structural sites either in isolated form or combined with other elements. Known minerals include baddeleyite {ZrO₂}, found in the Osamu Utsumi mine (Biondi 2005Biondi J.C. 2005. Brazilian mineral deposits associated with alkaline and alkaline-carbonatite complexes. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 707-750.); cerianite-(Ce) {CeO₂}, reported by authors including Frondel and Marwin (1959)Frondel C., Marwin U.B. (1959). Cerianite, CeO2, from Poços de Caldas, Brazil. American Mineralogist, 44(7-8):882-884., Fujimori (1982Fujimori K. (1982). Silicato de T.R. (Ce, La, Nd, Ca, Th)SiO4 e cheralita (Th, Ca, Ce, La)(PSi)O4 minerais responsáveis pela anomalia do Morro de Ferro, Poços de Caldas, MG. In: Congresso Brasileiro de Geologia, 32., 1982, Salvador. Anais… p. 669-671., 1984Fujimori K. (1984). Minerais toríferos do Morro do Ferro, Poços de Caldas, MG. In: Congresso Brasileiro de Geologia, 23., 1984, Rio de Janeiro. Anais… p. 4448-4452.), and Lapido-Loureiro (1994)Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21.; sporadic loparite-(Ce) {(Na,Ce,Ca) (Ce,Th)(Ti,Nb)₂O₆} cited by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... as tiny crystals in the groundmass of nepheline syenites; manganoan ilmenite {Fe²⁺TiO₃} (Atencio et al. 1999Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98.); pyrochlore-group mineral {(Ca,Na)₂Nb₂O₆(OH,F)} (Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ); and pyrophanite {Mn²⁺TiO₃} forming either isolate grains or botryoidal aggregates (Fig. 5). Chemical data for loparite-(Ce) and pyrophanite are listed in Table 3, indicating a complex composition for the former mineral containing high amounts of REE and abundant MnO in the latter. Additional oxide minerals include U-Th-Ti-bearing phases, such as anatase {TiO₂}, brannerite {UTiO₆}, thorianite {ThO₂}, and uraninite {UO₂}, mainly found in rocks of the Osamu Utsumi or Cercado areas. Most minerals are genetically related to hydrothermal processes in tinguaitic breccias and phonolitic lavas and have been mentioned by Lapido-Loureiro (1994)Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21. and Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. in association with the mineralized bodies of the central regions of the complex.

Carbonates

Carbonates are widespread minerals typically present in late phases, originated by hydrothermal or deuteric alteration. In the Poços de Caldas massif, they occupy rock interstices or cavities and are generally associated with different rock types, especially syenitic ones. The carbonate species vary considerably in terms of chemical composition, being represented by two groups, one including Fe- and Sr-bearing varieties and another with important amounts of REE-carbonates. The Fe- and Sr-bearing group of minerals includes ankerite {(CaFe²⁺,Mg)(CO₃)₂} (Atencio et al. 1999Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98.), kutnohorite {(Ca,Mn²⁺)(CO₃)₂} (Schorscher and Shea 1992Schorscher H.D., Shea M.D. 1992. The regional geology of the Poços de Caldas alkaline complex: mineralogy and geochemistry of selected nepheline syenites and phonolites. Journal of Geochemical Exploration, 45(1-3):25-51. https://doi.org/10.1016/0375-6742(92)90121-N
https://doi.org/10.1016/0375-6742(92)901... ), and the very common strontianite {SrCO₃}, which presents in white to green tones and a botryoidal habit. The second, REE-bearing mineral group is represented by light rose, anhedral to euhedral crystals of ancylite-(Ce) {CeSr(CO₃)₂(OH).H₂O} (Fig. 6) and ancylite-(La) {LaSr(CO₃)₂(OH).H₂O}, in addition to fine white-to-colorless aggregates of burbankite {(Na,Ca)₃(Sr,Ba,Ce)₃(CO₃)₅}. A Ba-free variety has been described on some sites by Matioli et al. (1994)Matioli D., Atencio D., Coutinho J.M.V. 1994. Barium-free burbankite from Poços de Caldas, Minas Gerais, Brazil. In: General Meeting of the International Mineralogical Association, 16., 1994, Pisa. Abstracts… p. 268.. Initially identified in Poços de Caldas rocks by Wedow Jr. (1967Wedow Jr. H. 1967. The Morro do Ferro, Thorium and Rare-Earth ore deposits, Poços de Caldas district, Brazil. United States Geological Survey Bulletin, n. 1195-D, 34 p.), a bastnäsite {(Ce,La,REE(CO₃)F}-group mineral was pointed out by Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. as a primary mineral associated with thorite, monazite, pyrochlore, zircon, oxide secondary phases, and several other minerals. Thorbastnäsite {ThCa(CO₃)₂F₂.3H₂O} was described in the Th-REE-(Nb) deposit of Morro do Ferro by Lapido-Loureiro (1994)Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21.. Chemical data on ancylite and Ba-free-burbankite are presented in Table 4, showing variable contents of REE, CaO, and SrO for both these minerals. Burbankite is also distinguished by its abundant Na₂O and low BaO content. An occurrence of a lanthanite {(La,Nd)₂(CO₃)₃.8H₂O}-group mineral (Waber 1990Waber N. 1990. Mineralogy, petrology and geochemistry of the Poços de Caldas analogue study sites, Minas Gerais, Brazil: II. Morro do Ferro, Poços de Caldas. Poços de Caldas Report, 3, 121 p.) has been discovered in the Morro do Ferro deposit, but no quantitative chemical data are available for such a constituent. However, an energy dispersive spectrum obtained by Waber (1990)Waber N. 1990. Mineralogy, petrology and geochemistry of the Poços de Caldas analogue study sites, Minas Gerais, Brazil: II. Morro do Ferro, Poços de Caldas. Poços de Caldas Report, 3, 121 p. suggests a Nd:La ratio near 1:1, indicating an intermediate mineral between lanthanite-(Nd) and lanthanite-(La) (Atencio 1990Atencio D. 1990. “Coutinite”, “coutinhite” and “neodymite” discredited as identical to lanthanite-(La) and lanthanite-(Nd). Mineralogical Magazine, 63(5):761-762.).

Phosphates

Secondary phosphate minerals mainly formed from hydrothermal alteration are found in syenitic rocks of miaskitic to agpaitic affinity, massive to scoriaceous lavas, and lamprophyres of the district and are marked by variable proportions of Sr, reaching a maximum value of 58.66% SrO in stronadelphite {Sr₅(PO₄)₃F} (Azzi 2019Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo.). Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... reported stronadelphite with up to 64.3% SrO and additional phosphatic accessory phases such as strontium-bearing apatite {(Ca,Sr)₅(PO₄)₃F}, fluorcaphite {SrCaCa₃(PO₄)₃F}, and phosphate-bearing britholite-(Ce) {(Ce,Ca)₅(SiO₄, PO₄)₃(OH,F)} in rocks of the massif. Britholite is the only phosphosilicate mineral identified so far, occupying cavities in association with pectolite and götzenite crystals (Fig. 7). Representative analyses for sample P7 297 for both Ce- and La-rich britholite are listed in Table 5, britholite-(Ce) showing a slight enrichment in SrO. Conversely, the rare crystals of britholite analyzed by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... in nepheline syenites present a low SrO content (1.1%). Within the apatite supergroup, Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. recognized belovite-(Ce) {NaCeSr₃(PO₄)₃} described in rocks of the Bortolan quarry as fine colorless to light gray aggregates. Except for the F content of 2.42% in the first analysis, EDS results for two grains of sample P7 123 (Table 5) indicate a relatively homogeneous composition for this mineral. The occurrence of phosphatic minerals such as cheralite {CaTh(PO₄)₂}, previously identified by Fujimori (1984)Fujimori K. (1984). Minerais toríferos do Morro do Ferro, Poços de Caldas, MG. In: Congresso Brasileiro de Geologia, 23., 1984, Rio de Janeiro. Anais… p. 4448-4452., lithiophyllite {LiMn²⁺(PO₄)}, and xenotime-(Y) {Y(PO₄)} in the Osamu Utsumi mine (Cercado and Agostinho deposits) and Morro do Ferro deposits (Morro Alto and Conquista) areas is also mentioned by Neves and Atencio (2019)Neves P.C.P., Atencio D. 2019. Enciclopédia dos minerais do Brasil: elementos nativos e halogenetos. 2ª ed. Canoas: Ulbra, 344 p.. Florencite-(Ce) {Ce,Al₃(PO₄)₂(OH)₆} was noticed by Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. and Azzi et al. (2017)Azzi A., Atencio D., Andrade M.B. 2017. Rare Earth minerals from Poços de Caldas alkaline province, Brazil. In: European Rare Earth Resources Conference, 2017, Santorini. Abstracts…, p. 23..

Sulfates

Mostly hydrated in composition, sulfate minerals are secondary phases resulting from weathering and alteration processes. Waber (1992)Waber N. 1992. The supergene thorium and rare earth element deposit at Morro do Ferro, Poços de Caldas, Minas Gerais, Brazil. In: Chapman N.A., McKinley I.G., Shea M.E., Smellie J.A.T. (eds.). The Poços de Caldas project: natural analogues of processes in a radioactive waste repository. Journal Geochemical Exploration, 45:113-157. mentioned the occurrence of alunite {KAl₃(SO₄)₂(OH)₆} and jarosite {KFe³⁺(SO₄)₂(OH)₆} in the Morro do Ferro deposit, whereas Waber et al. (1992)Waber N., Schorscher H.D., Peters T. 1992. Hydrothermal and supergene uranium mineralization at the Osamu Utsumi mime, Poços de Caldas, Minas Gerais, Brazil. In: Chapman N.A., McKinley I.G., Shea M.E., Smellie J.A.T. (eds.). The Poços de Caldas project: natural analogues of processes in a radioactive waste repository. Journal Geochemical Exploration, 45:53-112. recorded the presence of the same minerals in the oxidized zone of the Osamu Utsumi mine. In this mine, Atencio et al. (1994)Atencio D., Osako L.S., Rondani M. 1994. Aluminocopiapite, alunogen and other supergene sulphates from Poços de Caldas, Minas Gerais, Brazil. Anais da Academia Brasileira de Ciências, 66:501. described different sulfates, such as rozenite {FeSO₄.4H₂O}, aluminocopiapite {Al_2/3Fe³⁺₄(SO₄)₆(OH)₂.20H₂O} (and perhaps other copiapite group-minerals), alunogen {Al₂SO₄)₃.17H₂O}, coquimbite {Al,Fe₃ (SO₄)₆(H₂O)₁₂.6(H₂O)}, a halotrichite {FeAl₂(SO)₄.22H₂O}-group mineral, and gypsum {CaSO₄.2H₂O}. Botryoidal aggregates of blue coquimbite, which turn white with increasing relative humidity, were cited by Atencio and Coutinho (1997)Atencio D., Coutinho J.M.V. 1997. Coquimbita com molibdênio, mina Osamu Utsumi, Polos de Caldas, Minas Gerais, Brasil. Anais da Academia Brasileira de Ciências, 69(3):433.. The blue color returns, however, when the mineral is kept at approximately 100°C. Chemical analysis revealed the presence of 0.49% Mo, which is apparently responsible for the blue color of the mineral. Despite the incorrect name [coquimbite-(Mo)], coquimbite was described by Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418. in rocks of the U-Zr-Mo deposit of Campo do Cercado. Thenardite {Na₂(SO₄)} is listed by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. as a mineral whose complete description in the massif still lacks supporting data. The rare presence of baryte {Ba(SO₄)} in phonolites and lamprophyres and celestine {Sr(SO₄)} in phonolites is pointed out by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... .

Niobates

Fersmite{(Ca,Ce,Na)(Nb,Ta,Ti)₂(O,OH,F)₆} is the only niobate species known, identified by Fujimori (1984)Fujimori K. (1984). Minerais toríferos do Morro do Ferro, Poços de Caldas, MG. In: Congresso Brasileiro de Geologia, 23., 1984, Rio de Janeiro. Anais… p. 4448-4452. in rocks of the Morro do Ferro deposit, where it occurs in association with other rare accessory thoriferous minerals, such as “bastnäsite,” cerianite-(Ce), cheralite, coffinite, and thorbastnäsite.

Silicates

The silicates are early- to late-stage magmatic accessory phases, with F-silicates (sorosilicates) being the most typical group mineral in agpaitic rocks.

Nesosilicates

Only a few nesosilicate minerals occur in the Poços de Caldas massif, namely coffinite {U(SiO₄).nH₂O} and thorogummite (discredited as a valid species in 2014, IMA14-B). This name has been used to characterize heterogeneous mixtures of secondary, non-crystalline minerals originated by the alteration, hydration, and metamictization of thorite {Th(SiO₄)}, both identified in the Morro do Ferro deposit, and a new Fe³⁺-rich cerite group mineral under study, present in the Bortolan quarry. Coffinite is a rare uraniferous mineral initially described by Fujimori (1982)Fujimori K. (1982). Silicato de T.R. (Ce, La, Nd, Ca, Th)SiO4 e cheralita (Th, Ca, Ce, La)(PSi)O4 minerais responsáveis pela anomalia do Morro de Ferro, Poços de Caldas, MG. In: Congresso Brasileiro de Geologia, 32., 1982, Salvador. Anais… p. 669-671. and later confirmed by Lapido-Loureiro (1994)Lapido-Loureiro F.V. 1994. Terras Raras no Brasil: depósitos, recursos identificados, reservas. MCT-CNPq-CETEM, 189 p. Estudos e Documentos, n. 21. as an ore primary mineral set in a gangue bearing thorogummite and other minerals (allanite, bastnäsite, and cerianite) associated with the Morro do Ferro deposit.

F-Disilicates

F-disilicates form a mineral assemblage with a great number of phases, mostly already described in Poços de Caldas agpaitic rocks. They are highly complex in composition, contain Na-Ca-HFSE (Ti, Zr, and Nb)-halogen (F) as important and characteristic elements, and occur in the groundmass of phonolites or in the interstices of nepheline syenites. The F-disilicates are represented in the Brazilian and Paraguayan agpaitic rocks by the rinkite-(Ce) and wöhlerite (or cuspidine) groups (Table 6, modified; Gomes et al. 2021Gomes C.B., Azzone R.G., Enrich G.E.R., Guarino V., Ruberti E. 2021. Agpaitic alkaline rocks in southern Brazilian Platform: a review. Minerals, 11(9), 934. https://doi.org/10.3390/min11090934
https://doi.org/10.3390/min11090934... ). In bold are endmembers known to occur in the Poços de Caldas district. Tables 7 and 8 list chemical composition data available for several of these minerals.

Based on its optical properties, fersmanite was first recognized in Poços de Caldas by Ashry (1962)Ashry M.M. 1962. Studies of three rock samples from Brazil. Boletim da Sociedade Brasileira de Geologia, 11(1):89-107. in the eudialyte syenites (khibinites) of the northern border of the massif. However, despite being more recently listed by Atencio et al. (1999)Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98. as a local secondary accessory phase occupying vugs and fractures within phonolitic rocks, no data are available on the mineral's composition. The minerals described as “giannettite” or “hainite” correspond today to götzenite or bortolanite, according to the current classification of seidozerite-group minerals (Sokolova and Camara 2017Sokolova E., Camara E. 2017. The seidozerite supergroup of TS-Block Minerals: nomenclature and classification, with change of the following names - rinkite to rinkite-(Ce), mosandrite to mosandrite-(Ce), hainite to hainite-(Y) and innelite-1T to innelite 1A. Mineralogical Magazine, 81(6):1457-1484. https://doi.org/10.1180/minmag.2017.081.010
https://doi.org/10.1180/minmag.2017.081.... ). Light yellow götzenite and bortolanite occur as prismatic crystals up to 2 cm long. Bortolanite (Day et al. 2022Day M.C., Sokolova E., Hawthorne F.C., Horváth L., Pfenninger-Horváth E. 2022. Bortolanite, Ca 2 (Ca 1.5 Zr 0.5 )Na(NaCa)Ti(Si 2 O 7)2 (FO)F 2, a new rinkite-group (seidozerite supergroup) TS-Block Mineral from the Bortolan quarry, Poços de Caldas massif, Minas Gerais, Brazil. The Canadian Mineralogist, 60(4):699-712. https://ui.adsabs.harvard.edu/link_gateway/2022CaMin..60..699D/doi:10.3749/canmin.2200001
https://ui.adsabs.harvard.edu/link_gatew... ) is a type of mineral from Brazil. Representative chemical analyses of both minerals are given in Table 7. They do not differ greatly in composition, with the götzenite crystals being slightly enriched in CaO and impoverished in ZrO₂ and Na₂O with respect to the bortolanite. Lamprophyllite is a typical mineral, long known to occur in Poços de Caldas eudialyte syenites (khibinites) either as fine isolate tabular crystals or as fibroradial aggregates (Ashry 1962Ashry M.M. 1962. Studies of three rock samples from Brazil. Boletim da Sociedade Brasileira de Geologia, 11(1):89-107.), being promptly distinguished from other species by its peculiar yellowish-honey (dominant) to brown-reddish shades. The mineral was studied to some extent by Gualda (1998)Gualda G.A.R. 1998. Variações químicas em minerais máficos e a evolução dos magmas agpaíticos do corpo Lujaurítico-Chibinítico do Anel Norte – Maciço alcalino de Poços de Caldas (MG-SP). Monografia de Trabalho de Formatura, Institute of Geosciences, Universidade de São Paulo, São Paulo., and Gualda and Vlach (1998)Gualda G.A.R., Vlach S.R.F. 1998. Lamprofilita e normandita (låvenita titanífera) em nefelina sieníticos agpaíticos do maciço alcalino de Poços de Caldas (MG-SP): Caracterização mineralógica e petrográfica. In: Congresso Brasileiro de Geologia, 40., 1998, Belo Horizonte. Resumos… p. 284., and by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. and Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... years later. Chemical analysis results obtained by all these authors reveal lamprophyllite to be a Ti-silicate with high concentrations of SrO and Na₂O (Table 7). MnO is present in varying amounts. BaO contents are significant as well. Fluorlamprophyllite is a type of mineral from Brazil (Andrade et al. 2018Andrade M.B., Yang H., Downs R.T., Farber G., Contreira Filho R.R., Evans S.H., Loehn C.W., Schumer B.M. 2018. Fluorlamprophyllite, Na 3 (SrNa)Ti 3 (Si 2 O 7)2 O 2 F 2, a new mineral from Poços de Caldas alkaline massif, Morro do Ferro, Minas Gerais, Brazil. Mineralogical Magazine, 82(1):121-131. https://doi.org/10.1180/minmag.2017.081.027
https://doi.org/10.1180/minmag.2017.081.... ). It occurs in nepheline syenites at the Morro do Serrote. Rinkite-(Ce) is found in the syenitic rocks in association with mosandrite-(Ce), which is another mineral of the same group. Both phases consist of Ca-Ti-Na-F-rich silicates of slightly different composition. CaO is more present in rinkite-(Ce), while REE is more abundant in mosandrite (Table 8). Normandite, a mineral also bearing Ca, Ti, Na, and F as major elements, is distinguished from the latter by the higher amounts of TiO₂, MnO, and FeO and the lower quantities of CaO and REE. Comparing the composition of some F-disilicate minerals from the Poços de Caldas massif and the nearby alkaline complexes of Itatiaia and Passa Quatro, investigated by Melluso et al. (2017)Melluso L., Guarino V., Lustrino M., Morra V., De Gennaro R. 2017. The REE- and HFSE-bearing phases in the Itatiaia alkaline complex (Brazil) and geochemical evolution of feldspar-rich felsic melts. Mineralogical Magazine, 81(2):217-250. https://doi.org/10.1180/minmag.2016.080.122
https://doi.org/10.1180/minmag.2016.080.... and Guarino et al. (2019)Guarino V., De Gennaro R., Melluso L., Ruberti E., Azzone R.G. 2019. The transition from miaskitic to agpaitic rocks, as highlighted by the accessory phase assemblages in the Passa Quatro alkaline complex (Southeastern Brazil). The Canadian Mineralogist, 57(3):339-361. https://doi.org/10.3749/canmin.1800073
https://doi.org/10.3749/canmin.1800073... , respectively, Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... drew attention to the presence of mosandrite and normandite and the absence of låvenite in the former. However, normandite, the Ti-equivalent of låvenite, was reported in Poços de Caldas rocks by Gualda and Vlach (1998)Gualda G.A.R., Vlach S.R.F. 1998. Lamprofilita e normandita (låvenita titanífera) em nefelina sieníticos agpaíticos do maciço alcalino de Poços de Caldas (MG-SP): Caracterização mineralógica e petrográfica. In: Congresso Brasileiro de Geologia, 40., 1998, Belo Horizonte. Resumos… p. 284., Atencio et al. (1999)Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98., and Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418..

Cyclosilicates

Cyclosilicates are important constituents of agpaitic rocks in the massif, mainly represented by members of the eudialyte group. Table 9 lists all phases reported in the Poços de Caldas massif. Table 10 presents the chemical compositions determined for some minerals.

The chemical composition of catapleiite, a mineral of bright yellow color and granular habit found in Poços de Caldas rocks, was analyzed by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. and Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... . It is a replacement mineral in agpaitic and transitional agpaitic nepheline syenites (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ). Table 10 shows analyses regarding two samples studied by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo., whose composition varies as indicated by their ZrO₂ and alkali content. Recent data on the mineral by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... are very consistent, showing a high presence of ZrO₂ (29.20–32.22%) and Na₂O (9.05–17.00%). Eudialyte, which occurs in bright reddish pink to pinkish purple shades, is the most important rare metal silicate presenting as a primary or secondary (mainly formed as anhedral crystals after titanite in phonolites, Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ) phase, in high modal amounts in lujavrites and khibinites (9–12%, Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.). In these rocks, it represents a rock-forming mineral that appears concentrated in centimeter- to decimeter-sized patches. Eudialyte forms a complex solid solution with manganoeudialyte, ferrokentbrooksite, and kentbrooksite as endmembers. As noted by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... , the mineral exhibits a pronounced variability in the concentration of MnO, FeO, SrO, ∑REE, and Cl in nepheline syenites in relation to phonolites. Chemical data on the eudialyte group was first made available by Gualda and Vlach (1996)Gualda G.A.R., Vlach S.R.F. 1996. Eudialitas-eucolitas do maciço alcalino de Poços de Caldas, MG-SP: quimismo e correlações com comportamento óptico. In: Congresso Brasileiro de Geologia, 39., 1996, Salvador. Anais… 3:34-36.. Manganoeudialyte, a type-mineral from Brazil, is found in rose to violet shades in lujavrite-khibinite bodies outcropping at the northern edge of the massif, concentrated in centimeter-sized patches interstitial to the main minerals of the rocks (Nomura et al. 2010Nomura S.F., Atencio D., Chukanov N.V., Rastsvetaeva R.K., Coutinho J.M.V., Karipidis T.K. 2010. Manganoeudialyte – A new mineral from Poços de Caldas, Minas Gerais, Brazil. Zapiski RMO (Proceedings of the Russian Mineralogical Society), 139(4):35-47.). Compared to eudialyte, the available manganoeudialyte analyses (Table 10) have a lower SiO₂ and CaO and a higher SrO and MnO content that reaches a maximum value of 9.56%. Small amounts of Nb₂O₅ and REE are also present. Gaidonnayite, which was preliminarily studied by Matioli and Atencio (1994)Matioli D., Atencio D. 1994. Gaidonnayite from Poços de Caldas, Minas Gerais, Brazil. Anais da Academia Brasileira de Ciências, 66(4):500-501., forms isolate bright yellow subhedral crystals. Small polycrystalline spherules of variable color have also been described. Partial chemical analyses demonstrate that the mineral is a Zr-silicate containing significant amounts of alkalies, especially Na₂O (Table 10). Wadeite crystals were described by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... as occupying the interstices of some nepheline syenites. The homogeneous representative chemical composition of sample PC39b grains is presented in the above-mentioned table. The mineral, which corresponds to a Zr-silicate, is enriched in K₂O with a low ∑REE₂O₃ (0.35%) content. Rare crystals of an ill-defined phase having SiO₂ (42.24%), ZrO₂ (26.83%), K₂O (10.44%), and Na₂O (6.94%) as major constituents were tentatively identified by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... as georgechaoite. Briefly mentioned by Atencio et al. (1999)Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98. is hilairite, while ferrobrooksite and kentbrooksite are reported by Atencio et al. (2000)Atencio D., Coutinho J.M.V., Silva J.F. 2000. Kentbrooksite, ferrokentbrooksite and eudialyte from Poços de Caldas, Minas Gerais. In: Conference Mineralogy and Museums, 4., 2000, Melbourne. Abstracts… p. 12.. Little information exists in the literature on the latter three minerals.

Except for niobophyllite, which is only known to be present in phonolitic rocks of the Bom Repouso alkaline occurrence (Rosa 2012Rosa P.A.S. 2012. Geologia e petrologia da suíte alcalina de Bom Repouso, MG. Master Dissertation, Institute of Geosciences, Universidade de São Paulo, São Paulo.), all the other inosilicate members are found in Poços de Caldas rocks (Table 11). Aenigmatite, a Ti-Na silicate with abundant FeO and important amounts of MnO, occurs in the groundmass of phonolites. Astrophyllite was described for the first time in Poços de Caldas by Ashry (1962)Ashry M.M. 1962. Studies of three rock samples from Brazil. Boletim da Sociedade Brasileira de Geologia, 11(1):89-107. in khibinites of the northern border of the massif, presenting as fine lamellar aggregates yellow-brownish in color. Chemically, it is characterized by the high proportions of TiO₂, MnO, and K₂O and the abundance of FeO. Lorenzenite, first described by Gualda and Vlach (1997)Gualda G.A.R., Vlach S.R.F. 1997. Lorenzenita em nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. In: Simpósio de Iniciação Científica, 5., 1997, São Paulo. Resumos. 2:420. in nepheline syenites, is present as colorless euhedral isolate crystals that are mostly prismatic in shape. It is distinguished by its unusual chemical composition, with a marked prevalence of TiO₂ (43.46%) and abundant Na₂O (17.4%). Initially mentioned by Atencio et al. (1997)Atencio D., Coutinho J.M.V., Matioli D., Giardullo P., Ulbrich M.N.C. 1997. Quartzo, narsarsukita e tainiolite em rochas alcalinas de Poços de Caldas, Minas Gerais, Brasil. Anais da Academia Brasileira de Ciências, 69(3):432., narsarsukite was only identified in sample P7 A6 by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo. as having an irregular and variable habit and exhibiting yellow to yellow-brownish shades. As evidenced by the chemical composition reported in Table 12, the mineral corresponds to a Na-Ti-silicate bearing high amounts of FeO. Pectolite crystals are colorless to white and tabular in shape, appearing either in isolated form or as fibroradial aggregates. Subordinate to pectolite, serandite was identified in only two samples by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo.. Both minerals were first identified by Gualda and Vlach (1997)Gualda G.A.R., Vlach S.R.F. 1997. Lorenzenita em nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. In: Simpósio de Iniciação Científica, 5., 1997, São Paulo. Resumos. 2:420.. Pectolite is richer in CaO, while serandite shows a higher MnO content. The amount of Na₂O is almost constant in both species (Table 12). Recent analyses of pectolite by Guarino et al. (2021)Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... present high CaO (21.15–30.36%) and Na₂O (8.94–10.19%) contents and a quite variable amount of MnO (1.84–14.94%). However, as noticed in Poços de Caldas rocks by Atencio et al. (1999)Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98., the information on calciohilairite (hilairite), kupletskite, and elpidite is incomplete and needs to be confirmed by further investigations.

Phyllosilicates

Phyllosilicate minerals are not abundant in the Poços de Caldas massif, being represented by only three members: neptunite {KNa₂LiFe²⁺₂Ti₂(Si₈O₂₄)}, tainiolite {KLiMg₂(Si₄O₁₀)F₂}, and tuperssuatsiaite {Na₂(Fe^3+,Mn²⁺)₃(Si₈O₂₀)(OH)₂.4H₂O)}. A few alteration minerals are also present, namely berthierine 1M {(Fe²⁺,Fe^3+,Al)₃(Si,Al)₂O₅(OH)₄} and chamosite {(Fe²⁺Mg,Al,Fe³⁺)₆ (SiAl)₄O₁₀(OH,O)₈}, both reported by Atencio et al. (1999)Atencio D., Coutinho J.M.V., Ulbrich M.N.C., Vlach S.R.F., Rastsvetaeva R.K., Puscharovsky D.Y. 1999. Hainite from Poços de Caldas, Minas Gerais, Brazil. The Canadian Mineralogist, 37(1):91-98., and fluorapophyllite {(Na,K)Ca₄Si₈O₂₀F.8H₂O)}, referred to by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo..

Neptunite exhibits an intense red color, occurring as subhedral prismatic crystals. EDS chemical data shown in Table 13 indicate that the mineral corresponds to a Ti-Fe-rich silicate with significant alkali content. Present as a colorless to white material in tabular, acicular, or finely aggregated habit, tainiolite is a rare Li-rich phase so far known to occur in two Brazilian localities, Araxá (Traversa et al. 2001Traversa G., Gomes C.B., Brotzu P., Buraglini N., Morbidelli L., Principato M.S., Ronca S., Ruberti E. 2001. Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil). Anais da Academia Brasileira de Ciências, 73(1):71-98. https://doi.org/10.1590/S0001-37652001000100008
https://doi.org/10.1590/S0001-3765200100... ) and Poços de Caldas. Partial compositional values for tainiolite from the Poços de Caldas massif are also included in Table 13. No chemical data are available for Li₂O. The chemical composition of the mineral includes high concentrations of MgO and K₂O, which is quite similar to that determined for the Araxá phase. Tuperssuatsiaite fibers and needles form isolate crystals or aggregates of different shapes, such as rosettes and tuffs. Irregular forms are also present. Although reported by various authors, a full description of the minerals in the massif is only presented by Atencio et al. (2005)Atencio D., Coutinho J.M.V., Vlach S.R. 2005. Tuperssuatsiaite from the Bortolan Quarry, Poços de Caldas, Minas Gerais, Brazil. The Mineralogical Record, 36(3):275-280.. Chemical compositions are reported in Table 13, with the most complete analysis revealing that the hydrated mineral (12.63% of H₂O) is a Fe-rich silicate with important alkali concentrations.

Alteration products

They are mainly referred to as different products that originated from late-magmatic activities developed under hydrothermal and/or weathering conditions. In general, the revised literature does not provide much information on the behavior of these products, which have been mainly identified based on optical features; no detailed investigation is presently available for such minerals.

Hydrothermal minerals

The breakdown of nepheline {(Na,K)Al₂Si₄O₁₆} (and subordinate sodalite {(Na,K)AlSiO₄} as a primary constituent), as a result of alteration processes that took place mainly in central areas of the massif, gives formation to secondary phases mostly represented by distinct feldspathoids, zeolites, and some carbonates.

Feldspathoids usually include analcime {NaAlSi₂O₆.H₂O}, leucite {KAlSi₂O₆}, haüyne {Na₃Ca(Si₃O₈)O₁₂(SO₄)}, nosean {Na₈(Si₆Al₆)O₂₄SO₄.H₂O}, cancrinite {(Na,Ca,□₈)Al₆Si₆)O₂₄(CO₃,SO₄)_2.2H₂O}, and a rare member of the cancrinite group, vishnevite {Na₈(Al₆Si₆)O₂₄(SO₄).2H₂O}. This last variety, which is found as fine brown-reddish aggregates, was analyzed by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo., with the data indicating high contents of SiO₂ (39.47%), Al₂O₃ (33.79%), alkalies (Na₂O 12.41% and K₂O 7.96%), and SO₃ (7.13%).

Zeolites, usually present as prismatic crystals or aggregates filling cavities, have natrolite {Na₂Al₂Si₃O₁₀.3H₂O} and mesolite {Na₂Ca₂Si₈O₁₆O₃₀.8H₂O} as their most frequent phases. A few rare varieties, such as gonnardite {(Na,Ca)₂(Si,Al)₃O₁₀.3H₂O} and both thomsonite, thomsonite-(Ca) {(Na,Ca)(Al₅Si₅)O₂₀.6H₂O} and thomsonite-(Sr) {(Na,Sr₂)(Al₅Si₅)O₂₀.6-7H₂O}, were recognized in rocks of the Bortolan quarry by Azzi (2019)Azzi A. 2019. Caracterização cristaloquímica de minerais do complexo alcalino de Poços de Caldas, MG. Post-Doctoral Scientific Report, Institute of Geosciences, Universidade de São Paulo, São Paulo..

Hydrothermal processes are also responsible for the origin of mineral deposits in the massif, mined in the past for elements such as U-Th-Zr-REE. Strong hydrothermal activities promoted the circulation of fluids, leading to the formation of sulfides and oxides as well as a gangue consisting of minerals of varied composition.

Weathering minerals

Deuteric conditions result in the origin of minerals that are chemically variable in composition and have a wide distribution within the massif. Weathering assemblages contain sulfates such as barite and celestine (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ), Al-rich phases formed by the alteration of feldspars and feldspathoids from nepheline syenites and their fine-grained equivalents, and secondary phosphate minerals. According to Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., the main laterite minerals identified in Poços de Caldas bauxites are gibbsite {Al(OH)₃} and goethite {aFeO(OH)}, whereas kaolinite {Al₂Si₂O₅(OH)₄} and halloysite {Al₂Si₂O₅(OH)₄} are less abundant ones. Other aluminous minerals reported by these authors comprise boehmite {AlO(OH)} and litiophorite {(Al,Li)(Mn⁴⁺,Mn³⁺)₂(OH)₂}.

PETROLOGICAL REMARKS

The Poços de Caldas massif is mainly formed by peralkaline igneous rocks showing (Na + K/Al) ratios > 1. The massif's highly evolved rocks are predominantly SiO₂-undersaturated in composition (mainly nepheline syenites and their fine-grained equivalents), referred to as agpaitic suites based on the geochemistry of its major and trace elements and the presence of typical minerals, such as eudialyte, F-disilicates, aenigmatite, and lamprophyllite (Gerasimovskii 1956Gerasimovskii V.I. 1956. Geochemistry and mineralogy of nepheline syenite intrusions. Geochemistry, 5:494-510., Sørensen 1960Sørensen H. 1960. On the agpaitic rocks. In: International Geological Congress, 21., 1960, Copenhagen. Proceedings… 13:319-327., 1997Sørensen H. 1997. The agpaitic rocks: an overview. Mineralogical Magazine, 61(407):483-498. https://doi.org/10.1180/minmag.1997.061.407.02
https://doi.org/10.1180/minmag.1997.061.... , Marks and Markl 2017Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... ). Intermediate rock types showing characteristics common to both miaskitic and agpaitic rocks are relatively frequent (Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.) and might correspond to the “transitional agpaitic” type proposed by Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... , whose members include HFSE minerals that are typical of both associations (e.g., titanite from miaskitic rocks and eudialyte from agpaitic ones). More recently, the latter authors suggested that “agpaitic rocks” might be referred to as a descriptive term involving the distinction of igneous rocks based on their primary magmatic HFSE mineralogy, regardless of their whole-rock composition. They also suggested that agpaitic mineral assemblages should only be applied to igneous rocks characterized by the presence of early and late magmatic minerals. Thus, miaskitic rocks bearing agpaitic minerals only in hydrothermal veins or as clef fillings should not be called agpaitic.

The field relations between miaskitic and agpaitic rocks in composite complexes are generally variable. Only in a few places do agpaitic rocks constitute the dominant rock units within such complexes. Field relations, as discussed in Poços de Caldas by Ulbrich (1984)Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo. and Ulbrich et al. (2005)Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., indicate that the massif might be placed into group 1 of Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... , which includes plutonic to (sub)volcanic rocks with or without volcanic surface expression. Regarding the relations involving miaskitic and associated agpaitic rocks of those authors, they fall into the type B group, which is more typical, consisting of minor agpaitic units of an otherwise miaskitic complex.

Except for three large areas, namely Pedra Balão, Morro do Serrote, and Morro Taquari (Fig. 1), agpaitic rocks in Poços de Caldas usually occur as small bodies that are subordinate and younger than miaskitic units (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.). They are believed to have been derived from an independent magmatic pulse (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ). Such agpaitic rocks are thought to crystallize from highly evolved magmas that are enriched in HFSE and other incompatible elements and halogens (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ). Rocks seem to originate from low-degree partial melting of slowly, fractionally crystallized alkaline basic-ultrabasic melts deriving from an enriched parental mantle source of basanitic (plagioclase-bearing) and/or nephelinitic (plagioclase-free) composition, which evolved by removal of olivine, clinopyroxene, and oxides toward high Sr- and high Ba-phonolitic magma compositions (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ). Continued fractional crystallization resulted in peralkaline phonolite compositions through the removal of nepheline syenites assemblages in subsequent stages, with late and post-magmatic processes playing an important role that leads to the enrichment of residual magmatic liquids in incompatible elements. Late-stage mineral phases are represented by REE-carbonates and REE-fluorocarbonates, bearing CO₂ and H₂O in their composition. According to Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... , the varying timing of evolution processes in agpaitic magmas is responsible for mineral assemblage sequences that can be distinguished as orthomagmatic, late magmatic/pegmatitic, and hydrothermal (OM, LM, and HY associations, respectively). Specific parameters must be met so that different evolution stages are reached, with magmatic-agpaitic assemblages only formed under early magmatic crystallization conditions involving low enough oxygen fugacity (fO₂) and relatively dry magmas (low aH₂O). Such conditions enable subsequent requirements of Fe enrichment, increase in peralkalinity, retention of halogens, and extreme enrichment in HFSEs to be fulfilled. Textural evidence shown by mineral assemblages of Poços de Caldas rocks (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418., Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ) is indicative that they have been affected by processes giving rise to the formation of variable phases that are typical of the diverse stages of evolution of agpaitic rocks (OM, LM, and HY) described by Marks and Markl (2017)Marks A.W.M., Markl G. 2017. A global review on agpaitic rocks. Earth-Science Reviews, 173:229-258. https://doi.org/10.1016/j.earscirev.2017.06.002
https://doi.org/10.1016/j.earscirev.2017... . Particularly, as observed by Gomes et al. (2021)Gomes C.B., Azzone R.G., Enrich G.E.R., Guarino V., Ruberti E. 2021. Agpaitic alkaline rocks in southern Brazilian Platform: a review. Minerals, 11(9), 934. https://doi.org/10.3390/min11090934
https://doi.org/10.3390/min11090934... , the great number of late-stage magmatic specimens recognized in transitional agpaitic to agpaitic rocks belonging to the eudialyte-group and F-disilicate-group minerals drew attention. Additionally, the presence of other characteristic phases is common to agpaitic assemblages, such as aenigmatite and astrophyllite. Alteration parageneses are mainly represented by analcime and other feldspathoids, whereas hydrothermal activities, especially affecting tinguaites and phonolites of large areas of the massif (Ulbrich 1984Ulbrich H.H. 1984. A petrografia, a estrutura e o quimismo de nefelina sienitos do maciço alcalino de Poços de Caldas, MG-SP. Habilitation Thesis, Institute of Geosciences, Universidade de São Paulo, São Paulo., Ulbrich et al. 2005Ulbrich H.H., Vlach S.R.F., Demaiffe D., Ulbrich M.N.C. 2005. Structure and origin of the Poços de Caldas alkaline massif. In: Comin-Chiaramonti P., Gomes C.B. (eds.). Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. São Paulo: Edusp/Fapesp, p. 367-418.), are believed to be responsible for the origin of a great number of primary and secondary minerals of varied composition (Th-U and Nb oxides, Sr-Ba-REE carbonates-fluorcarbonates-phosphates-silicates). Mostly carbonates and sulfates (baryte and celestine) are related to the deuteric stage of crystallization (Guarino et al. 2021Guarino V., Lustrino M., Zanetti A., Tassinari C.C., Ruberti E., De Gennaro R., Melluso L. 2021. Mineralogy and geochemistry of a giant agpaitic reservoir: the Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil). Lithos, 398-399, 106330. https://doi.org/10.1016/j.lithos.2021.106330
https://doi.org/10.1016/j.lithos.2021.10... ).

ACKNOWLEDGMENTS

The authors wish to thank the Brazilian agency FAPESP(processes #2005/254426-1 and #2019/22084-4) for the financial support. Particularly, A. Azzi would like to thank Prof. H. Ulbrich of the Institute of Geosciences for making available for investigation the scientific collection of Poços de Caldas samples.

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