Morphology and chemistry composition of pyrite in the Laowangzhai gold deposit, Ailaoshan orogenic belt, SW China

Laowangzhai gold deposit is one of the largest gold deposits in the Sanjiang Tethys metallogenic domain, whereby pyrite acts as dominate gold-bearing mineral. Five generations of pyrite have been identified in diagenetic-metallogenic process, based on the crosscutting relationships of different auriferous veins, ore textures, and mineral paragenesis. The sedimentary-diagenetic period is mainly characterized by framboidal pyrite which is enriched in Pb, Zn, Mn, Co, Ni and Bi. Hydrothermal gold mineralization period could be further subdivided in to four stages; Stage I quartz-sericite-pyrite stage, Stage II quartz-polymetallic sulfides stage, Stage-III calcite-quartz-arsenopyrite-pyrite stage, and Stage IV calcite-quartz-stibnite-pyrite stage. During the hydrothermal mineralization period, the crystal form of pyrite evolves as coarse xenomorphic grain, through cube and pyritohedron, to cube. These pyrites are also enriched in Pb, Zn, Mn, Co, Ni and Bi, with different enrichment in Au, As, Sb and Cu, which shows the complex of the oreforming fluid. Stages III is the main mineralization stage which is characterized by pyritohedron pyrite, and the pyrite is enriched in Au, As, Sb, Pb, Zn, Cu, Co, Ni and Bi. The hydrothermal period is characterized by the Au positively correlating with As, which suggests the Au and As formed the [Au, As](2-) and [Au(As, S-3)](2-). These two complex compounds replaced [S-2](2-) and then formed pyrite. The ore-forming fluid is characterized by middle-to-low temperature, with high sulfur fugacity, slow cooling rate, and sufficient mineral source.