The chemical composition and age of monazite and kularite from titanium ore of Pizhemskoye and Yarega deposits (Middle and Southern Timan)

A study on the typomorphic characteristics and age of the monazite the two giant titanium deposits of the Timan – Pizhemskoye and Yarega, which revealed differences in morphology in the species composition of the inclusions, the grain size, distribution of chemical types of a mineral associated with conditions of crystallization and different sources of the substance. The isochronous Th-Pb monazite age was calculated using the cHIME method. For Yarega monazite built three isochrone with age 1301, 1105 and 778 Ma; for Pizhemsky monazite-kularite one isochrone with age 782 Ma. Source of hith-Th monazite Yarega oil-titanium deposit could be ancient granite batholith and the origin Yarega less-Th monazite and Nd-ce-monazite-kularite Pizhemskoye deposit with an age of ~ 780 Ma could be related to the hydrothermal conversion of the weathering crusts on lamprophyres close in age with lamprophyre (spessartite and kersantite) of chetlassky Kamen.

Monazite is a phosphate of cerium-group light rare-earth elements. It is a rare but very informative compositionally variable mineral occurring in Timan titanium deposits and occurrences. Its typomorphic characteristics are studied to better understand the genesis, age and sources of the substance in titanium deposits (Makeyev, 2016;Makeyev, Dudar, 2001;Makeyev et al., 2008;. The goal of the present project is to study monazite from two Russia's giant titanium deposits: Pizhemskoye and Yarega. The deposits are similar in geological structure and possibly genesis but are different from other titanium deposits in the non-standard phase-mineral composition (ilmenite-pseudorutile-leucoxene-quartz and siderite-leucoxene-quartz) of titanium ores. The non-standard phase composition of titanium ores (Ignatiev, Burtsev, 1997;Kalyuzhny, 1982;Makeyev, 2016;Makeyev, Dudar, 2001) from these deposits would require a preliminary chemical stage to remove silicon from leucoxene and pseudorutile, to manufacture commercial products (artificial porous rutile, etc.) and to launch the economic mining of these deposits that are highly important for Russia. As monazite is a principal recoverable constituent of titaniferous sandstone (because it occurs in the ore as free grains), the economic profitability and cost price of the economic mining of the above deposits will undoubtedly increase.
The goal of the present study is to better understand the chemical composition pattern of monazite grains from the Yarega and Pizhemskoye deposits and to estimate the time of their formation using the cHIMEchemical Th-U-total Pb isochrone method (chemical isochrone method) proposed by K. Suzuki (Suzuki, Adachi, 1991;Suzuki, Kato, 2008). Special analytical conditions were as follows: ThO 2 , UO 2 , PbO: an accelerating voltage of 20 kV, a probe current of 150 nА and a PET analyzer crystal (Table 1). Age was calculated from the results of electron-probe X-ray spectral determination of ThO 2 , UO 2 and PbO using the cHIME method. The positive results of our studies based on this method are described in (Votyakov et al., 2011;Makeyev, Viryus, 2013;Pilyugin, Mukhanova, 2008;Popova et al., 2010;Williams et al., 2007, etc). calculations of estimated ThO 2 * concentration for constructing an isochrone were made using the formula: where ω(ThO 2 ) and ω(UO 2 ) are the mass fraction of thorium oxide (IV) and uranium oxide (IV) in monazite; M(ThO 2 ) and M(UO 2 ) are the molar masses of thorium oxide (IV) and uranium oxide (IV); λ 232 and λ 238 are decay constants for thorium and uranium isotopes, respectively; it is the time taken by the decay.
Age calculation: where m is the inclination of the isochrone. Analytical results for monazite composition and age calculations are shown in Tables 2 and 3. U and Pb were not estimated in all the grains analyzed because of their low concentrations. Therefore, 52 monazite analyses out of 59 were used for age estimation and isochrones construction.

Materials
Yarega monazite. The grains and crystal fragments of Yarega monazite were extracted from an ultra-heavy fraction of a big (several tons) industrial sample. They are small in size (84×49 -110×80, average size 93×63 µm) and display an isometric to mildly elongated irregular shape with a perfect cleavage; no other phases, except for scarce quartz inclusions, were found inside the grains (Fig. 1). Yarega monazite is concentrated to form a fine (< 0.125 µm) non-magnetic leucoxene fraction together with zircon and rutile. The chemical composition of monazite is shown in Table 2 There are three chemical varieties of Yarega monazite: neodymium-cerium, lanthanum-thoriumcerium and lanthanum-cerium. The latter is the most common variety (over 60 % of cases). A high positive correlation between cerium, lanthanum, thorium, yttrium and calcium, on the one hand, and between neodymium, praseodymium, samarium, europium and gadolinium, on the other hand, is observed (Makeyev, Magazina, 2019). The tetrad rule in the chemical composition of Yarega high-thorium monazite is violated in both the first (Lace-Pr-Nd) and second (Sm-Eu-Gd-Tb) tetrads because of a negative cerium anomaly associated with the isomorphic replacement Th+Y+Ca → Ce and a negative Eu-anomaly in the second tetrad. The typochemical characteristics of Yarega high-thorium monazite with a distinctive Eu-anomaly are typical of monazite, which is genetically related to granitoids Schandl, Gorton, 2004;Williams et al., 2007).
Pizhemsky monazite-kularite. Pizhemsky monazite grains were extracted from the magnetic ilmenitepseudorutile concentrate of the grey-coloured sandstone of an industrial sample weighing 250 kg. They are much bigger (521×446 ÷ 228×130, average size is 375×278 µm) than the grains and crystal fragments of Yarega monazite; they are often amoeba-shaped, rounded or ellipsoid and are grey or brown in color. Individual grains are filled with quartz inclusions (up to 10-15 %, Fig. 2) and are similar in shape, color and internal morphology to leucoxene grains. This morphological variety of monazite is called kularite. In addition to quartz, it contains florencite, muscovite and pseudorutile inclusions. Monazite-kularite grains are magnetized by pseudorutile and are concentrated to form an electromagnetic fraction together with ilmenite, pseudorutile and siderite (Lutoev, Makeyev, 2019). The chemical characteristics of monazitekularite are shown in Table 3. There are three chemical varieties of Pizhemsky monazite: lanthanum-cerium, neodymium-samarium-cerium and neodymiumcerium. The latter is the most common (over 80 % of cases). There is a high positive correlation between cerium and lanthanum, on the one hand, and between praseodymium, neodymium, samarium, gadolinium and europium, on the other (Makeyev, Magazina, 2019). The tetrad rule in Pizhemsky monazite-kularite is violated: there is no bending between the first (La-Ce-Pr-Nd) and second (Sm-Eu-Gd-Tb) tetrads, which seems to be due to elevated Nd and Sm concentrations. This could have been provoked by the hydrothermal transformation of the mineral in the weathering crust of the primary source and the removal of some of the constituents.
The diagrams (Figs. 3 and 4) show the correlations of the constituents used to distinguish between the typochemical characteristics of the above two deposits. Pizhemsky monazite contains more neodymium and samarium, while Yarega monazite carries more lanthanum, thorium, yttrium, calcium, uranium and lead.

Results of monazite dating by the cHiMe method
The results of monazite dating, based on 18 and 35 analyses, are shown in Tables 2 and 3. The average age of Yarega monazite, based on "point" determinations in three combinations, is: 1) 1304±22; 2) 1107±39; (rhombs)
The first two isochrones were constructed for high-thorium monazite grains, while the last one for low-thorium grains. This evidence, together with data on the chemical composition of monazite, suggests that monazite grains from three primary sources, considerably differing in age by ~200 and ~320 Ma, occur together in the Yarega deposit.
Only one isochrone (Fig. 6), showing an age (T) of 782 Ma, can be constructed for the Pizhemsky deposit, based on the results obtained. Statistical analysis was done to calculate two isochrones similar in age: one with a free member PbO = -0.007+0.0345·ThO 2 *, R 2 = 0.908; and the other which has no free member and approaches zero, -PbO = 0.0337·ThO 2 *, R 2 = 0.9084. Let us choose the latter isochrone, as is accepted in this type of study. It should be noted that it is a perfect case, when the isochrones for Pizhemsky monazite closely coincide with the average "point" age of the entire combination. Another feature of the present study is that the age values in isochrone III for Yarega monazite coincide with the only isochrone for Pizhemsky monazite-kularite. This may indicate the close nature of the geological object of one of the indigenous primary sources of monazite of the two Timan deposits.

Discussion
The most similar object for discussion of the results obtained is the well-known and well-studied Ichetyu base mineral gold-diamond-rare earth-rare metal-titanium deposit. It is a 0.2-1.5 m thick breccio-conglomerate horizon resting directly on the Malorucheiskaya rock sequence of the Pizhemsky deposit (Makeyev, 2016;Makeyev, Dudar, 2001). Isotopic (Pb-Th and Pb-U) and chemical age estimations (cHIME method) for La-cemonazite and Nd-ce-monazite-kularite from the horizon are known (Krasotkina, 2018;Makeyev, Viryus, 2013;. Local dating of monazite yielded two peaks of 206 Pb-238 U age values: a major peak with an age of 1000-1060 Ma and a minor peak with a maximum of about 570 Ma. 208 Pb-232 Th age, estimated at the same points as 206 Pb-238 U age, yielded only one peak with an age of 500-700 Ma . In addition to monazite, the isotope-geochemical characteristics of rutile and zircon were analyzed (Krasotkina, 2018). It has been shown that the isotope systems and trace element compositions of all the three mineral-chronometers studied (zircon, rutile and monazite) from the Ichetyu ore occurrence are indicative of a major hydrothermal event dated at ~600 Ma. The event does not manifest itself in the rocks that underlie titanium deposits, and is understood as an age similar to the age of formation of Middle Timan titanium deposits.
The results obtained were interpreted as follows. consistent with isochrones I are mostly ellipsoidal grains of a neodymium-cerium variety (except for two thorium grains and two lanthanum-cerium monazite grains). consistent with isochrone II are only grains of a neodymium-cerium variety (monazite-kularite). consistent with isochrone III are all three monazite varieties, but thorium and lanthanum-cerium varieties prevail over a neodymium-cerium variety. The early isochrones (I) seem to indicate the time of formation of rare metal -rare earth high-temperature hydrothermalmetasomatic mineralization in Neoproterozoic shales, which are similar to the Bobrovskoye and Oktyabskoye chetlassky Kamen deposits, Middle Timan.
The middle isochrones (II) can be correlated in age with chetlassky lamprophyres, whose intrusion time was estimated by the Rb-Sr method at 815 Ma (Makeyev et al., 2009). chetlassky carbonatites (600 Ma), in which monazite is one of the most common accessory minerals, were derived in the same period of time.
The latest isochrones for monazite (III) are consistent with the intrusion age of Devonian basalts and probably with the age of formation of the Ichetyu occurrence proper. During this phase monazite could have recrystallized and lost part of radiogenic lead. This assumption is consistent with the geological knowledge of the structure of a productive breccio-conglomerate horizon (in which basalt fragments occur) and shows that Ichetyu breccio-conglomerates are of volcanicfluidisate origin and were formed simultaneously with the intrusion of basalt sills.
These data should be compared with new age dates for monazite and zircon from the Pizhemskoye and Yarega deposits to more precisely date the deposits themselves. Interestingly, the age values for Nd-cemonazite-kularite from the Ichetyu occurrence and the Pizhemskoye deposit are very similar and seem to indicate a common primary source.
With respect to Timan igneous rocks, only chetlassky lamprophyres are the most similar in isotope age (Makeyev et al., 2008;2009) to the chemical age of Yarega low-thorium monazite and Pizhemsky monazitekularite. Rb-Sr-data show that the largest group of chetlassky lamprophyre samples forms an isochrone with a Neoroterozoic age of 819±19 Ma (Makeyev et al., 2009). Hence, it is Neoroterozoic lamprophyres, most similar in age and the mineral composition of accessories (Makeyev, 2016;Makeyev et al., 2008;  2009; 2016), that could have provided a source of ore matter for the two titanium deposits. The prospecting and exploration of the northern Volsk-Vymskaya Ridge is likely to reveal chetlassky-like lamprophyres beneath titaniferous sequences. The species composition of micron-sized mineral inclusions in Yarega zircon (Makeyev, Zhilicheva, 2018), which is closely associated in titaniferous sandstone with the monazite studied, is: quartz, muscovite, F-REEapatite, xenotime, chlorite, K-feldspar, biotite -PASE (phlogopite-annite-siderophyllite-eastonite) -series mica (according to a new nomenclature, International Mineralogical Association). This clearly shows that the entire association is bimica granite. Well-known criteria (Krasotkina, 2018;Schandl, Gorton, 2004;Williams et al., 2007), such as extremely low Th concentration, U/Th ration and the absence of Eu-anomaly, the chemical compositions of Pizhemsky monazite-kularite and Yarega low-thorium monazite are indicative of their hydrothermal genesis. According to the same criteria, a bedrock source for Yarega high-thorium monazite could have been provided by old Timan granitic batholiths that seem to occur in Timan's crystalline basement and are not exposed on the day surface.
It was thought earlier that oil-saturated leucoxene sandstones from the Yarega deposit are of Middle Devonian age, as shown by spore-and-pollen analysis. However, O.P. Telnova of the Yushkin Institute of Geology of Komi Science center of Ural Branch of Russian Academy of Sciences (personal communication), has recently conducted extensive studies of core samples from prospecting holes, which showed that the samples contain both Middle and Late Devonian microfossils. This ambiguity suggests that microfossils were carried to Yarega titaniferous sandstones and distributed there by migrating oil, which was squeezed out from Devonian parent rock and migrated to porous sandstones as the most favorable collector (an anticlinal trap formed by the Yarega tectonic structure). The mineral compositions of leucoxene sandstones of oil and water grades were identical. It means that oil later filled the upper anticlinal portion of the Yarega deposit and that it was not genetically related to the formation of the titaniferous sequence itself. Here, oil also fills the upper ore-free quartz sandstone bed in the anticlinal trap. It is safe to assume that oil carried a mixture of Middle and Late Devonian plant spores and pollen while migrating. So, the Middle Devonian age of the Yarega deposit is not proven. The question remains open. To correctly approach the problem, further isotopegeochemical studies, similar to those carried out earlier in the Pizhemsky deposit (chernyshov et al., 2010), are needed.
The genesis and ages of the two deposits studied are problems of animated debate. It was assumed earlier that the deposits are old placers, but this point of view has not been generally accepted. According to the existing classification of Russia's titanium deposits, the Yarega is clearly a metamorphogenetic bedrock deposit. The two deposits are identical in geological structure and are similar in mineral composition (and possibly age and genesis). The titaniferous sequence of the Pizhemsky deposit is barren and contains no organic remains. Therefore, its Devonian age was tentatively accepted earlier. The Malorucheiskaya sequence is overlain by the Middle Devonian terrigenous rocks of the Pizhemsky suite (D 2 pz) that contain Middle Devonian pollen. Many scholars estimate the age of the Malorucheiskaya titaniferous sequence of the Pizhemskoye deposit from indirect evidence in the Early Devonian − Neoroterozoic time span. Isotope methods were used to precisely date the titaniferous sequence of the Pizhemsky deposit from a Rb-Sr-isochrone as Neoroterozoic, Т = 685±30 Ma (chernyshov et al., 2010). Material for age determination was provided by ore-hosting rocks, such as siltstone and argillite-like clay, as well as leucoxene ore concentrate proper. Thus, the chemical age obtained for monazite supports our earlier data on the isotope age of zircon, monazite and leucoxene. Hence, the Pizhemsky titanium deposit is much older (most probably 600-700 Ma) and was formed in Neoroterozoic time (PR 3 mr).
The authors of the publications that saw the light in the past few years tried to provide arguments in favor of the bedrock rather than other (not placer) genesis of the Pizhemsky deposit (Makeyev, 2016;Makeyev et al., 2008;2009;2016, etc). One of their arguments is the complete absence of the roundness of quartz (at both localities) a major mineral in terms of ore volume. It is sharply angular and clastic − a feature indicative of the short-distance transport of the entire material. Some of the mineral phases, e.g. leucoxene and monazitekularite, may display the pseudo-rounded shape of grain produced by the hydrothermal reworking of the material and the vertical movement of the material in a vertical flow. Therefore, the rounded shape of grains is a convergent feature, which cannot be accepted as strong evidence for the placer origin of ore occurrences and the movement of the material in subhorizontal water flows. The sizes of quartz and other non-metallic and ore minerals (0.1-2.0 mm) are much greater than those of minerals from Riphean fine-grained (0.01-0.10 mm) shale (bedrock). Furthermore, the shale does not contain sufficient amounts of ore constituents. Therefore, the weathering crust after the shale could not have been a bedrock source of the matter that made up the overlying ore sequence. calculations show that to create current TiO 2 concentration in the deposits, the volume of weathering crusts after shale should have been tens of times the volume and thickness of the titaniferous sequences. Thick weathering crusts after Riphean shales have never formed on Timan, and nobody has ever seen them. The underlying shales of the Lunvozhskaya suite (PR 3 ln) were dated by the Rb-Srmethod at T = 816.3±5.2 Ma . No other ore concentrations, which could be a source of ore matter, have been revealed on the day surface near the Pizhemsky deposit. Hence, the probable source (it could be weathering crusts after lamprophyre) was at some depth beneath the deposits, and the ore masses must have moved vertically. Another convincing evidence for the contribution of hydrothermal processes to the formation of both titanium deposits is the presence (3-15 % by volume) of porous and veined siderite with the endogenic isotope composition of carbon (Makeyev, Nosik, 2009). The decay of ilmenite as a primary ore mineral, which gave rise to leucoxene, took place in a hydrothermal process in the presence of endogenic cO 2 . The reaction was as follows: FeTiO 3 + H 2 O + cO 2 + SiO 2 → (2TiO 2 ·SiO 2 ) + TiO 2 + FecO 3 → Fe(OH) 3 + Fe 2 O 3 +СO 2 .
The aforementioned geological, geochemical and mineralogical features and proofs are convincing enough to argue that the Pizhemsky titanium deposit is of volcanic (phreato-magmatic) origin. Its formation of a mud volcano type was contributed to by water vapour and carbon dioxide. The deposit formed after the substrate of the weathering crusts of lamprophyres followed by the long (hundreds of millions of years) metamorphism of ore sequences (Makeyev, 2016).
Thus, two Timan's giant structurally similar titanium deposits, Yarega and Pizhemskoye, resting directly on Neoroterozoic shales, overlain by Middle and Upper Devonian sedimentary rocks and erroneously interpreted earlier as Middle Devonian, are indeed Neoroterozoic. One of the most probable sources of ore matter could have been provided by chetlassky-like lamprophyres similar in the age, species composition and typomorphic characteristics of accessory indicator minerals.

conclusions
The typomorphic characteristics and age of monazite samples from Timan's giant Pizhemsky and Yarega titanium deposits were studied. Differences in morphology (Yarega monazite occurs as crystal fragments, while Pizhemsky monazite is present as hydrothermally altered ellipsoid grains), the species composition of inclusions (quartz inclusions make up 10-15 % of Pizhemsky kularites), grain size (Pizhemsky monazite is 4 times bigger) and the distribution of the chemical characteristics of the mineral (a large portion of Yarega monazites occurs as high-thorium and lanthanum varieties, while Pizhemsky monazite-kularite occurs as neodymium and samarium varieties) were revealed. The isochrone Th-Pb age of monazite was estimated using the cHIME method. Three isochrones for Yarega monazite with the ages of 1301, 1105 and 778 Ma and one isochrone for Pizhemsky monazite-kularite with an age of 782 Ma were constructed. These differences are due to monazite crystallization conditions and a difference in the sources of the matter. A source for Yarega high-thorium monazite could have been provided by old granitic batholiths, and the origin of Yarega low-thorium monazite and ~780 Ma Pizhemsky Nd-ce-monazite-kularite could have been associated with the hydrothermal alteration of weathering crusts after lamprophyres resting presumably beneath the deposits in the shale-quartzite sequence of the crystalline basement, which is similar in age to chetlassky Kamen lamprophyres (spessartite and kersantite). The free form of monazite grains in both deposits allows us to develop technological methods for extracting the mineral from concentrates, which will certainly increase the profitability of their mining.