A reduced skarn gold deposit at Laozuoshan, NE China: Mineralogy, gold occurrence, and metallurgical implications

Skarn deposits are an important source of gold. In China, most Au-bearing skarns are associated with Cu mineralization, representing an oxidized skarn system. In contrast, Au-only skarns, which are induced by a reduced condition without associated Cu ores, are rare. The skarn mineralogy and Au occurrence in such reduced deposits remain poorly investigated, which restricts the strategy determination in Au recovery. The Laozuoshan deposit, located in the central Jiamusi Massif, NE China, is a typical Au-only skarn deposit (32 t Au at 7.38 g/t). In this study, mineral assemblages were identified using a TESCAN Integrated Mineral Analyzer, while major-element compositions of garnet, pyroxene, and native gold were determined by electron microprobe. Trace elements in arsenopyrite, pyrite, chalcopyrite, and sphalerite were analyzed by laser ablation–inductively coupled plasma–mass spectrometry. Pyroxene (Di1–87Hd12–96) is dominated by the hedenbergite endmember, while garnet (Ad13–53Gr45–77) is dominated by the grossular endmember. They exhibit a low garnet/pyroxene abundance ratio of ~1:3 throughout the deposit. Arsenopyrite is more abundant than other sulfide minerals such as pyrite and chalcopyrite, while oxidized species including hematite and magnetite are rarely present. These observations, combined with the facts that the causative diorite is characterized with whole-rock Fe2O3/(Fe2O3 + FeO) ratios <0.4 and that the wall rocks contain abundant graphite, indicate a reduced intrusion–reduced wall-rock system. The Laozuoshan deposit could be therefore classified as a typical reduced skarn Au deposit. Both visible and invisible Au has been recognized in the skarn. Visible Au particles range from 2 μm to 119 μm in size and occur as intergranular Au (47.8%), inclusion Au (47.3%), and fissure Au (4.9%). Invisible Au occurs within solid solutions of sulfide minerals including arsenopyrite, pyrite, chalcopyrite, and sphalerite, as well as in nano inclusion forms in arsenopyrite as Au-Bi-Te or Au-Ag-Pb assemblages. The Au occurrence at Laozuoshan suggests that integrated processing combining flotation, cyanide leaching, and pyrite-enhanced chlorination roasting can significantly enhance Au recovery from sulfide ores. Given the scarcity of reduced Au skarns in China, this study serves as a valuable example for advancing exploration and recovery models for similar skarn Au deposits.