Detailed paragenesis and Li-mica compositions as recorders of the magmatic-hydrothermal evolution of the Maoping W-Sn deposit (Jiangxi, China)
Introduction
Li-micas have been widely used as indicators of the magmatic evolution of evolved granites and pegmatites within peraluminous or peralkaline series (Tischendorff et al., 1997 and references therein). However, their use in hydrothermal systems is poorly documented. Moreover, most studies that have used magmatic or hydrothermal micas for this purpose were based on the characterization of only one object and/or on a comparison of chemical compositions with other case studies. Compositional trends are well documented for both magmatic trioctahedral (from iron-biotite to zinnwaldite) and dioctahedral (from Li-muscovite to lepidolite) micas (Alfonso et al., 2003, Roda et al., 2007, Roda Robles et al., 2006, Tischendorff et al., 1997 and references therein; Vieira et al., 2011). A few examples of hydrothermal alteration affecting magmatic Li-micas have been described (Johan and Johan, 2001, Johan et al., 2012, Van Lichtervelde et al., 2008) and a Li-muscovite-bearing episyenite has been identified in Brazil (Costi et al., 2002). A number of publications have described the post-magmatic evolutions of Li-micas (Cuney et al., 1992, Henderson and Martin, 1989, Jin-Ung and Kideok, 2015, Marchal et al., 2014, Neiva et al., 2012, Neiva, 2013). Hydrothermal lepidolite is observed at Yellowstone (Bargar et al., 1973) and F-rich micas have been described from acid leaching alteration in the Henderson Mo deposit (Gunow et al., 1980). Finally, hydrothermal Li-micas in W-Sn deposits have been documented only in a few occurrences in Portugal (Neiva, 1987).
Li-micas are common in W-Sn deposits, which are usually emplaced during multiple magmatic and hydrothermal events that affect granites and their country rocks (Costi et al., 2002, Giuliani, 1985, Johan et al., 2012, Neiva, 2013, Tischendorff et al., 1997). Here, we show that the compositional variations of Li-micas can be correlated with the detailed paragenesis in order to determine both magmatic and hydrothermal influences within a single system and allow the identification of multiple magmas and fluid interactions throughout the history of a rare metal deposit. The Maoping Sn-W deposit in the southern Jiangxi Metallogenic Province (South China) provides an opportunity to (i) characterize magmatic and hydrothermal Li-micas of various compositions present at the different stages of the paragenetic sequence defined in the veins, (ii) relate the micas to the compositions of and interactions between the hydrothermal fluids responsible for the deposition of wolframite and cassiterite, and (iii) use, for the first time, the evolution in hydrothermal Li-mica composition through the paragenetic stages to characterize the fluids involved in terms of timing and interactions as well as to characterize the magmatic influences present within the system.
Section snippets
The southern Jiangxi W-Sn Metallogenic Province
The southern Jiangxi W-Sn Metallogenic Province of southern China is located in the Nanling range in the central part of the Cathaysia Block, which is separated from the Yangtze Block to the northwest by the Jiangshan-Shaoxi fault zone (Fig. 1) (Wang et al., 2013 and references therein). The Cathaysia Block is a former Paleoproterozoic craton which stabilized at around 1.85–1.78 Ga (during the Nuna supercontinent amalgamation) and which includes Archean nuclei (presently identified in the lower
Analytical methods
All preparations and analyses described below were carried out at the GeoRessources laboratory (Nancy, France).
Petrographic data were obtained from observations of polished thin-sections using conventional transmitted and reflected light microscopy and a HITACHI FEG S4800 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), using a Si(Li) semi-conductor detector. Cathodoluminescence (CL) was also performed on thin-sections, using a CITL cold cathode
Paragenetic sequence
The paragenetic sequence was constructed using cross-cutting relationships and the textures of granite, greisen and vein infillings in the mine galleries, as well as petrographic observations made in the laboratory. Four types of veins (I to IV) and seven successive paragenetic stages were identified, separated by plastic deformation, corrosion/dissolution and vein reopening events (Fig. 5). Some minerals from the veins were found to occur repeatedly (e.g. up to 5 generations for quartz)
Magmatic-hydrothermal Li-mica trends
The zinnwaldite to lepidolite (polylithionite-rich) solid solution observed in the peraluminous granite micas (Fig. 12), along with, in particular, increase in the Mn/(Mn + Fe + Mg) ratio (Mn#) from 0.23 to 0.27 (Table 1) with increasing Li content can be interpreted as a magmatic fractionation trend along the liquid line of descent. The observed rare metal (W-Sn-Nb-Ta) depletion that accompanies this trend (Fig. 14) can be interpreted to result from an early fluid-melt separation in the granite
Conclusion
By correlating the geochemistry of Li-micas with the detailed paragenesis of the Maoping deposit, we are able to obtain a number of constraints on the magmatic and hydrothermal processes at the origin of the W-Sn deposits in the Nanling region, the richest W-Sn province worldwide. Textures and compositions of Li-micas along successive paragenetic stages allow to identify: (i) magmatic Fe-Li-micas from the granite and the feldspar veins, which are thought to demonstrate an early fluid-melt
Acknowledgements
Electron microprobe analyses and SEM observations were performed at the SCMEM, GeoRessources Laboratory Nancy, France. We are particularly indebted to Olivier Rouer, Lise Salsi and Sandrine Mathieu for their help during data acquisitions. LA-ICP-MS analyses were performed at the GeoRessources Laboratory, Nancy, France, with the help of Chantal Peiffert. This research was supported by the collaboration between Carnot ICEEL-Nancy and Carnot BRGM-Orléans. Access to the Maoping deposit, sampling
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2022, Ore Geology ReviewsCitation Excerpt :The decreases of LREE contents (Fig. 10m) can be attributed to the fractionations of LREE-rich minerals such as monazite and allanite (Bea, 1996; Stepanov et al., 2012) and the decreases of HREE contents (Fig. 11n) are resulted from the fractionation of HREE-rich minerals such as zircon and xenotime (Bea et al., 1994; López-Moro et al., 2017; Zaraisky et al., 2009). Micas can be used as proxies to discriminate the evolution degree of granitoids (Legros et al., 2016; Li et al., 2015; Van Lichtervelde et al., 2007). Rb, Cs, Nb, and Ta are incompatible with most rock-forming minerals such as quartz, plagioclase, and K-feldspar and can be progressively enriched in the melt with progressive crystallization (Icenhower and London, 1996).