Geological implications of apatite within the granite-related Jiepai W–(Cu) deposit, Guangxi, South China

Abstract

The Jiepai W–(Cu) deposit is one of the largest Caledonian granite-related W deposits in the Miaoershan–Yuechengling (MY) pluton of southern China. The deposit consists of skarn-type and altered granite-type mineralization along with an ore shoot hosted by altered arkose. This study presents new petrographic and geochemical data for apatite from fresh biotite granite (ApI), mineralized biotite granite (ApII), and ore shoot (ApIII) samples and uses these data to constrain the magmatic and hydrothermal processes that formed the deposit. ApI is associated with primary biotite and has core–rim textures or oscillatory zoning visible during backscattered electron (BSE) imaging, indicating that this apatite has a magmatic origin. ApII is associated with scheelite and secondary biotite and consists of BSE-bright cores (ApIIa) with oscillatory zoning and rare fluid inclusions surrounded by BSE-dark rims (ApIIb) free of fluid inclusions. These relationships suggest that ApIIa and ApIIb record the effects of slight and significant fluid metasomatism, respectively. ApIII appears homogenous during BSE imaging, is associated with abundant scheelite, sericite and quartz, and contains abundant fluid inclusions, indicative of a hydrothermal origin. ApI has increasing total rare earth elements (REE) (2959–14057 ppm) and Y (1598–9099 ppm) contents from cores to rims, high Na2O (0.05–0.2 wt%) and MnO (0.1–0.32 wt%) and low Sr (71.6–86.1 ppm) contents, and low LaN/YbN values (0.55–1.21). ApIIa contains similar REE and Y contents to ApI, but has higher Sr (194–258 ppm) and lower Na2O (<0.03 wt%) and MnO (0.04–0.11 wt%) contents. ApIIb contains similar Na2O, MnO, and Sr contents to ApIIa, but with much lower REE (1766–2021 ppm) and Y (1128–1512 ppm) contents. ApIII apatite has similar Na2O, MnO, REE, and Y contents to ApIIb, but with slightly lower Sr (103–173 ppm) contents and higher LaN/YbN values (0.92–3.28).The large increase in REE contents from core to rim within ApI is indicative of prolonged crystallization, generating W-enriched residual melts and eventually W mineralization. ApIIa and ApIIb contain similar Na, Mn, and Sr contents to ApIII, but have REE contents that are similar to ApI and ApIII, respectively. This suggests that Na, Mn, and Sr were affected by weak fluid metasomatism, whereas REE can only be mobilized by significant fluid metasomatism. The relatively low REE contents and high LaN/YbN values of ApIII reflect the presence of abundant co-precipitated scheelite. These data indicate that ApI, ApII, and ApIII all have different textural and geochemical (Na, Mn, Sr, Y, REE, and LaN/YbN) characteristics and fluid inclusion populations. All of this suggests that apatite is a potential indicator mineral in mineral exploration for granite-related W–(Cu) mineralization.