RESEARCH AND ANALYSIS ON ORE-CONTROLLING FACTORS FOR MOJIANG-YUANJIANG NICKEL-GOLD DEPOSITS IN YUNNAN, CHINA
Fang Weixuan\ Hu Ruizhong\ Xie Guiqing\ Qi Liang\ Su Wenchao (Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002)
Based on research and analysis on ore\|controlling factors and analysis of tectono\|lithostratigraphic units to shed light on Ni\|Au metallogenic processes, it is suggested that the main ore\|controlling factors for the Mojiang\|Yuanjiang Ni\|Au deposits in Yunnan Province include strata\|controlling and tectonics\|controlling. In the Mojiang\|Yuanjiang Ni\|Au mine, the Upper Devonian Jinchang Formation consists of low\|grade metamorphic volcano\|sedimentary rocks formed in a deep\|water, volcanic\|related hydrothermal depositional basin. This Formation is composed of slate, chert, palimpsest siltstone and sandstone interbeded with tuff mudstone, volcanic lava and tuff. Three tectono\|lithostratigraphic units (the Lanshan, Sishibaliangshan and Mahudong) can be recognized in the Jinchang Formation. The Lanshan unit (D\-3jy\+1) was formed by volcanism and volcanic\|related hydrothermal deposition and is referred to as the volcanic\|exhalative\|genetic unit. Nickel\|gold\|bearing pyritic cherts in the Mojiang deposit occur in the lower part of the Lanshan rock\|unit, ranging in age from 358±8.6 (2σ) Ma (Sm\|Nd isochron dating) to 354.7±0.72 (2σ) Ma (Rb\|Sr isochron dating). The Sishibaliangshan unit (D\-3jy\+2) consisting of slate, chert, palimpsest siltstone and sandstone, volcanic lava rock and tuff, is also referred to as the volcanic\|exhalative\|genetic unit. The Mahudong unit (D\-3jy\+3) is composed mainly of a suite of fine\|grained turbidite rocks with acidic volcanic lava and violet\|red sandstones at its top.\; The ring\|like clayized zone and the tectono\|lithostratigraphic sequence of hydrothermal deposits are located at hydrothermal\|water vents that are one of the main ore\|host structures. The Yanshanian brittle\|ductile shear zones that are another main type of ore\|host structures are typified by the proximately horizontal tension cracks, sinistral shear fractures, close\|off brittle\|ductile shear zones, and Cr\|illite was formed at 178 Ma to 76 Ma. The Himalayan brittle tension cleavages are the best ore\|host structures for high\|grade Au orebodies. The Mojiang Ni\|Au deposits probably underwent four main Ni\|Au metallogenic epochs. Firstly, Ni\|Au\|bearing pyritic cherts may be formed by volcanic\|related hydrothermal deposition in Late Devonian. Secondly, the Ni\|Au\|bearing ductile shear zones were perhaps formed at the end of Permian or at the beginning of Early Triassic. Thirdly, the Au\|bearing brittle\|ductile shear zones were perhaps formed by thrust\|nappe\|type shear tectonics during the Yanshanian orogeny. Finally, deep\|source hydrothermal fluids in the Himalayan orogen may superimpose the high\|grade Au orebodies. Much attention should be paid to the laterite\|type gold deposits as a new prospecting target in the future.