Huachanggou Au deposit, Lüeyang County, Hanzhong, Shaanxi, Chinai

Regional Level Types
Huachanggou Au deposit	Deposit
Lüeyang County	County
Hanzhong	Prefecture
Shaanxi	Province
China	Country

Altered rock-type gold deposit, hosted in altered spilite and altered carbonatite within a ductile shear zone.

The Huachanggou Au deposit is structurally controlled by a series of NWW-trending N-dipping high-angle reverse faults/shear zones in the ore district. Two types of orebody are present: (1) the dominant altered spilite-hosted Au-bearing lodes. Spilite-hosted Au lodes exceed 2 km in length and contain over 20 orebodies within en-echelon the NWW-trending shear zones (F6 and subsidiaries). Orebodies dip north (50°–90°) and are 135–265 m long and 2–10 m thick (locally ≤ 17 m in the dilation jogs of the reversed faults). The Au grade mainly ranges 3–6 g/t (avg. 4.5 g/t), locally up to 229 g/t. And (2) local developed limestone (minor in phyllite)-hosted Au-bearing quartz veins. The limestone-hosted Au lodes occur in the carbonaceous and crystalline limestone, controlled by the fault related reverse shear zones. These orebodies are 0.7–1.2 m thick and > 2 km long, and extend to > 900 m depth. The Au grade ranges 1.0–13 g/t, with higher average grade at 5.8 g/t than these spilite-hosted Au lodes.
Five hydrothermal stages are distinguished: (1) Stage I: quartz–calcite assemblage, characterized by bedding/foliation-parallel sheeted quartz and calcite veins in schist, bulk quartz filled in thrust faults, and quartz–calcite veins in ductile–brittle fractures within limestone. Most veins are intensely deformed, forming breccia, boudinage and lenses. (2) Stage II: fine-grained pyrite–quartz–calcite assemblage. Veins in this stage consist predominantly of pyrite (>90%), with minor quartz and calcite. They commonly occur as veinlets filling quartz breccia of Stage I, along limestone sutures, or at the interface between bioclastic limestone and phyllite. (3) Stage III: medium-grained pyrite–albite–quartz–carbonate–fuchsite assemblage. Veins in limestone fill in ductile–brittle shear fractures and early fault planes, crosscutting the earlier fine-grained pyrite veins, while veins in spilite are mainly localized in shear zone centers and along early penetrative foliation. (4) Stage IV: coarse-grained pyrite–quartz–calcite–sericite assemblage. The coarse euhedral pyrite (0.3–2 cm in size) commonly occurs in the wallrocks and quartz–calcite veins, containing inclusions of chalcopyrite and minor native gold. Pyrite in this stage displays a single set of simple fractures, indicating brittle deformation. (5) Stage V: quartz–carbonate–albite assemblage, represented by narrow, unoriented quartz–calcite veinlets, coarse comb-textured quartz veins, and albite–dolomite veins containing anhedral, unfractured pyrite. These veins crosscut earlier veins and alteration zones.

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