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 Sps 45–53 Alm 46–53 Adr 0–1 Grs 0–1 Prp 0–1 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 (Sps 64–70 Alm 29–34 Adr 0–1 Grs 0–2 Prp 0 ) 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 Adr 62–88 Grs 1–18 Sps 3–12 Alm 0–8 Pyr 0 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 Adr 4–22 Grs 62–73 Sps 5–16 Alm 2–10 Pyr 0 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 4–32 ppm, respectively) than Grt-I and Grt-II garnets (682–1352 ppm with Y = 1558–2159 ppm, and 6051–12831 ppm with Y = 9663–13333 ppm, 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 (⩽1.64 ppm) and Mo ( 2 and andradite molecule contents confirms the substitution of Sn 4+ for Fe 3+ 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.