Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China

Abstract

The Baiganhu W–Sn orefield in the southeastern Xinjiang Uygur Autonomous Region is associated with Caledonian S-type syenogranites and metasediments of the Paleoproterozoic Jinshuikou Group. Four types of garnets have been identified in the orefield using petrographic and major and trace element data. Grt-I garnets are generally present as inclusions within magmatic quartz in the syenogranites, with end-member formulas of Sps45–53Alm46–53Adr0–1Grs0–1Prp0–1 and rare earth element (REE) patterns that are enriched in heavy REE (HREE) and contain strong negative Eu anomalies. Grt-II garnets are associated with tourmaline and quartz and occur in interstices between feldspars within the syenogranites. In general, the Grt-II garnets have end-member formulae (Sps64–70Alm29–34Adr0–1Grs0–2Prp0) and REE patterns that are similar to the Grt-I garnets although they are more spessartine-rich and contain higher concentrations of HREE. Grt-III garnets coexist with clinopyroxenes and Mo-rich scheelites within skarns developed along the syenogranite and marble contact. Their compositions are Adr62–88Grs1–18Sps3–12Alm0–8Pyr0 and they have relatively flat REE patterns with no negative Eu anomalies. Grt-IV garnets are present as massive aggregates that are often cross-cut by Mo-poor scheelite-bearing calcite veins. Their end-member formulas are Adr4–22Grs62–73Sps5–16Alm2–10Pyr0 and they have slightly domed REE patterns without negative Eu anomalies. Both Grt-III and Grt-IV garnets contain lower concentrations of the HREE (2–3 and 4–32ppm, respectively) than Grt-I and Grt-II garnets (682–1352ppm with Y=1558–2159ppm, and 6051–12831ppm with Y=9663–13333ppm, respectively).The occurrences, mineral assemblages, major element compositions, and REE patterns of the Grt-I and Grt-II garnets suggest they have magmatic origin and crystallized at relatively low temperatures and pressures, whereas the Grt-III and Grt-IV garnets have hydrothermal origin. The association of magmatic garnet (Grt-II) with tourmaline suggests that boron in S-type magmas has an important role in the formation of W–Sn mineralization during magmatic and subsequent hydrothermal processes. All four types of garnets contain low concentrations of W (⩽1.64ppm) and Mo (<0.77ppm), but relatively high and variable concentrations of Sn (22.96–8364ppm). A positive correlation between SnO2 and andradite molecule contents confirms the substitution of Sn4+ for Fe3+ within these garnets. These data suggest that Sn-rich andradite–grossular garnets in skarns may be used as an indicator mineral for W–Sn exploration when combined with other geological, geophysical, and geochemical signatures.