Geological signatures of the Baiyun pyrophyllite deposit in Wutai, Shanxi Province: implication for prospecting of Cu-Au ores in deep

doi:10.13745/j.esf.sf.2020.5.47

Abstract

Abstract:

The Baiyun pyrophyllite deposit is ~15 km away to north of Wutai county, Shanxi Province of China, where the basement rocks belong to the Wutai Complex, north-central of the North China Craton. The main orebody occurs as a lens in fracture of the late Archean meta-volcanic rocks, crosscut by the Mesozoic mafic dikes. The wall rocks exhibit phyllic alteration halos which are weakened outwardly from the pyrophyllite orebody. Auriferous quartz sulfide vein and disseminated gold mineralization occur in the phyllic alteration zone. Mineral assemblages of pyrophyllite ores are dominated by pyrophyllite and quartz, with minor of diaspore, kaolinite, illite, sericite and hematite. Accordingly, the lgf(O₂)-pH diagram is constructed that the pyrophyllic alteration fluids were intensively acidic (pH=2-3) with relatively high oxygen fugacity (>HM buffer). Altered zircon yielded a lower intercepted U-Pb age of (136±7) Ma, similar to those of Cu-Au deposits regionally. Whole-rock H-O isotopic data (δD_SMOW=-34‰--20‰; δ¹⁸O_SMOW=11.2‰-13.9‰) of pyrophyllite ores suggest that the fluids for pyrophyllite alteration are cooling magmatic without pronounced effects of meteoric waters. As to the abundances of metals, the Archean wall rocks are markedly higher than those of the pyrophyllite ores. The concentrations of Au and As in the pyrophyllite ores are below the detected limit, suggesting that the physicochemical conditions of fluids for pyrophyllite alteration are not conductive to gold mineralization, inversely, they can scavenge some metals from wall rocks and transfer them upwardly. Our results suggest that pyrophyllite alteration formed by acid and oxidized fluids might be an early stage of evolving fluids for high sulfidation epithermal deposit. Consequently, a prospective porphyry style or skarn type of Cu-Au deposit in deep is suggested.

Key words: pyrophyllite alteration, gold mineralization, hydrothermal alteration, Wutai complex, ore prospecting

CLC Number:

ZHANG Huafeng, ZHANG Shaoying. Geological signatures of the Baiyun pyrophyllite deposit in Wutai, Shanxi Province: implication for prospecting of Cu-Au ores in deep[J]. Earth Science Frontiers, 2020, 27(5): 126-135.

Figures/Tables 7

Fig.1 Geological skeleton map of the Wutai Complex(b) and distribution of the Mesozoic magmatism and structures(c) (b modified from [11]; c modified from [23])

Fig.2 Geological map of the Baiyun pyrophyllite deposit,Wutai of the Shanxi Province

Fig.3 Microscope photos of altered rocks from the Baiyun pyrophyllite deposit, Wutai of Shanxi Province. Mineral abbreviations adapted from [32].

Fig.4 Thermodynamic phase diagram for pyrophyllite and pyrite-phyllic alteration of the Wutai pyrophyllite deposit, Shanxi Province[10,31]

Fig.5 (a)δ18O versus δD of quartz/whole rocks from pyrophyllite and gold deposits of the Wutai area; (b) SiO2 concentrations vs total metal abundances (a after [31]; data of b from [10])

Fig.6 Terry's pH-T evolution chart for common mineral alteration assemblages in porphyry system[7]

Fig.7 Model for the high sulfidation epithermal deposit of porphyry system[8,35]

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