Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit, southeastern Tibet: Contrast with carlin gold deposit

Abstract

Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization. The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone, southeastern Tibet, was selected to clarify their difference. The alteration and mineralization from the different lithologies, including meta-quartz sandstone, carbonaceous slate, meta-(ultra)mafic rock, quartz porphyry and lamprophyre were researched. According to the mineral assemblage and replacement relationship in all types of host rocks, two reactions show general control on gold deposition: (1) replacement of earlier magnetite by pyrite and carbonaceous material; (2) alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite. Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks, it contains a large portion of Au reserve, indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure. LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks, with Au content reaching to 258.95 ppm. The diagenetic core of pyrite in meta-quartz sandstone enriched in Co, Ni, Mo, Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu, As, Sb, Au, Tl, Pb and Bi.Different host rock lithology has much impact on the alteration and mineralization features. Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration. The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni. However, Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks. It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate (mainly dolomite and ankerite), sericite, pyrite and arsenopyrite develops in all types of host rocks. This is different from the Nevada Carlin type, in which alteration is mainly dissolution and silicification of carbonate host rock. On the other hand, Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.