Garnet trace element geochemistry of Yangla Cu deposit in NW Yunnan, China: Implications for multistage ore-fluid activities in skarn system

Abstract

Garnet is a common hydrothermal mineral in skarn-type deposits, and is useful for constraining the ore-fluid composition. Based on the mineralogy and geochemical features of different types of garnets from the Yangla Cu deposit, we discuss the factors and mechanism of rare earth elements (REE) incorporation into skarn-type garnet, together with the skarn-forming environment. Garnet from Yangla belongs to the grossular–andradite solid solution series, and can be divided into two types (i.e., zoned and unzoned). Early garnet (Grt I) aggregates (avg. And84.96 Gro9.62 Spe + Pyr + Alm5.42) in the proximal skarn is clearly oscillatory-zoned, whilst most of late garnet (Grt II) (avg. And96.26Gro0.58Spe + Pyr + Alm3.16) in the diopside skarn is unzoned. Grt I is slightly LREE-enriched and HREE-depleted with positive Eu anomalies, whereas Grt II is obviously LREE-enriched and HREE-depleted with positive Eu anomalies. Grt I has higher total REE, Y and HFSE contents than Grt II. We suggest that fluid composition and physicochemical conditions greatly controlled the REE incorporation into garnet. The Grt I oscillatory zoning was likely formed by self-organization under relatively low growth rate, leading to Al/Fe fluctuation in the fluid related to the change of external factors (such as fluid boiling). Based on the differences of microscopic texture and geochemical characteristics between Grt I and Grt II indicate that Grt I was likely formed under a mildly acidic and reducing environment and low W/R ratios, whereas Grt II may have formed in a more acidic and oxidizing environment with higher W/R ratios. The changes in redox, pH and W/R ratios cause the compositional variation from Grt I to II during the ascent of the early hydrothermal fluid. During the process of the crystal formation, the oxygen fugacity increased, and the mildly acidic to acidic ore-forming solution shifted from a low W/R ratios condition to a high W/R ratios condition, causing the precipitation of metal sulfide. The fissures of Grt II are infilled by pyrrhotite, chalcopyrite and pyrite, showing that the garnets form prior to the mineralization of copper and can provide space for the precipitation and enrichment of metals.