Apatite as a proxy for imaging the link between multistage hydrothermal alteration and anomalous gold enrichment in orogenic gold deposits: Evidence from the Jiaodong Peninsula, Eastern China

Orogenic gold deposits account for >30 % of the global gold resources. To understand the abnormal enrichment process of gold and to improve their exploration efficiency, it is important to gain insights into the contribution of hydrothermal alteration to gold enrichment. The giant Jiaodong gold province is a world-class gold-mineralized region and is also the largest gold producer in China (with Au reserves of 5760 t). However, the relationship between multistage hydrothermal alteration and gold mineralization here remains equivocal. In this study, cathodoluminescence (CL) and elemental analysis of apatite from the Wang'ershan gold deposit are applied to different alteration assemblages to trace the multistage hydrothermal alteration and to evaluate its contribution to gold enrichment in orogenic gold deposits.

On the basis of petrographic features, four types of apatite are identified as: unaltered apatite (Ap1), apatite from K-feldspar alteration (Ap2), apatite from hematite-rutile alteration (Ap3), and apatite from pyrite-sericite-quartz alteration, which is directly associated with gold mineralization (Ap4). In general, the geochemical characteristics of apatite in K-feldspar alteration (Ap2) are similar to those of magmatic apatite (Ap1), and the fluids are rich in Ca, P, and REEs, which we correlate with magmatic differentiation in a reducing environment. The K-feldspar alteration is genetically related to Mesozoic magmatism but has no direct contribution to the enrichment of gold. In contrast, apatite from the hematite-rutile alteration (Ap3), which shows high oxygen fugacity, F enrichment, and depletion in REEs, is different from the chemistry of hydrothermal apatite in orogenic gold deposits. Combined with the behavior of Fe and Ti and the wide spatial distribution s of the hematite-rutile alteration process, we infer activation and enrichment of ore-forming materials before mineralization, which may also be a potential reason for the high gold endowment in Jiaodong. In the process of hematite-rutile alteration (Ap3) to pyrite-sericite-quartz alteration (Ap4), the fluid temperature, pressure, and oxygen fugacity decreased. The apatite from pyrite-sericite-quartz alteration (Ap4) have higher S, Cl, and Na contents than those from the former alteration, indicating that volatiles were involved in the mineralization process. The halo of hematite-rutile alteration in Ap3 apatite, formed prior to the mineralization. We propose that hematite-rutile alteration is a key indicator of mineralization in orogenic gold deposits.