A Comparison of Carlin-type Gold Deposits:Guizhou Province,Golden Triangle,Southwest China,and Northern Nevada,USA

摘要

Several Au deposits in Guizhou Province,southwest China,described as being similar to the highly productive Carlin-type gold deposits in northern Nevada,USA,were examined to identify similarities and differences between the two districts.Samples were collected along transects from lowto high-grade rock,where possible,and from stockpiles at the Shuiyindong,Zimudang,Taipingdong,Yata and Jinfeng (formerly Lannigou) deposits.Methods used to examine ore and alteration minerals included hand-sample description; reflectance spectroscopy using an ASD Terraspec spectrometer;analyses of hand samples by carbonate staining with Alizaren red and potassium ferricyanide;transmitted and reflected light petrography; chemical analyses,mineral identification,and imaging using a JEOL JSM-5610 scanning electron microscope; and quantitative chemical analyses using a JEOL JXA-8900 electron probe microanalyzer.Geochemical analyses of hand samples for 52 elements were done by ALS Chemex.Results indicate both similarities and differences between the two districts.Both districts have similar geologic histories,and deposits at both locations appear to have formed as a result of similar tectonic events,though the district in southwest China lacks evidence of coeval felsic igneous activity;however,the ore-stage minerals and the fluids that produced the minerals and deposits have some significant differences.The Nevada deposits were dominated by fluid-rock reaction in which host rock Fe was sulfidized to form Au-bearing pyrite.Although ore fluids sulfidized host rock Fe in the Guizhou deposits,the timing of Fe metasomatism is unknown,so whether the deposits formed in response to sulfidation or pyritization is unclear.Fluid-rock reaction between an acidic,aqueous fluid and highly reactive calcareous rocks in Nevada caused extensive decarbonatization of host rocks,jasperoid replacement of carbonate minerals,and alteration of silty rock components to illite and kaolinite.In Guizhou,CO2-bearing ore fluids with temperatures and pressures approaching 100 ℃ and 500 bars greater than temperatures and pressures determined for ore fluids in the Nevada deposits,deposited Aubearing pyrite.In examined Guizhou deposits these fluids carbonatized host rocks and formed both replacement and open-space-filling ore.The fluids,which may have been immiscible,were sufficiently overpressured to fracture wall rocks and to create significant open space filled by vein quartz.While deposit architecture,tectonic setting,and host rocks in Guizhou are quite similar to northern Nevada,ore and alteration minerals suggest that ore-forming processes in examined Guizhou deposits have important similarities to processes associated with formation of orogenic Au systems.The Guizhou deposits display characteristics of both Carlin-type and orogenic systems,perhaps indicating formation at conditions somewhat intermediate to conditions for Carlin-type deposits and orogenic systems.