The Occurrence And Enrichment Mechanism Of Ni And Co In The Maochang Bauxite Deposit,Central Guizhou,Southwest China

The karstic bauxite deposits commonly host numerous associated critical metals such as Li,Ga,V,Cr,REEs,Sc,Co,Ni,Nb,Zr,and U.However,it remains unclear how Ni and Co metals were concentrated into mineral phases,especially sulfides and whether Ni and Co have the same derivation as bauxite ores.The Maochang paleo-karstic bauxite deposit as one of the large-scale concealed deposits in China is distinct from other bauxites in the Qianzhong uplift due to the abundant sulfide veins or veinlets.The detailed mineralogical and sulfur isotopic studies on various Al-bearing rock series using the integrated EBSD,EPMA,LA-ICPMS,and LA-MC-ICPMS analyses revealed five results as follows:(1)Four stages of sulfide mineralization as represented by Py I,Py II,Py Ⅲ,and Py IV,from early to late.Relative to other generations of pyrite,Py Ⅲ exhibits high but variable Ni,Co,and As contents,and a successive evolution from Ni-rich Py Ⅲ-a(3.23 wt.% Ni,0.23 wt.% Co,and 0.21 wt.%As on average)and Py Ⅲ-b(2.13 wt.% Ni,0.57 wt.% Co,and 0.27 wt.%As on average)to Co-rich Py Ⅲ-c(0.12 wt.% Ni,4.57 wt.% Co,and 1.03 wt.% As on average)was recognized.Nickel and cobalt are mainly hosted in Py Ⅲ,millerite and violarite.(2)In combination with the whole-rock geochemistry,bauxites were interpreted to originate from the mixing of underlying Loushanguan Group dolomite and black shale of the Niutitang and Liuchapo Formations,and Ni,Co were mostly derived from the black shale sequences.(3)The complex zoning patterns consisted of multi-generation pyrite reflect that coupled dissolution and reprecipitation process dominates the enrichment of Ni and Co in the sulfides.The origin of sulfides are attributed to diagenesis,where decreasing of oxygen fugacity in the fluid resulted in the decoupling of Ni and Co in Py Ⅲ.(4)The sulfur isotopes display distinct fractionation patterns,varying from-6.9 to-16.6‰ of Py Ⅱ and +17.2 to +22.8‰ of Py Ⅲ-b and sphalerite to +25.3 to +28.8‰ of Py Ⅳ.The sulfur isotopic fractionation patterns are interpretd to be related to sequential reduction of a single batch sulfate in a closed system.(5)A two-stage model of Ni-Co enrichment was proposed,consisting of 1)sedimentation of bauxite-bearing rock series with Ni-Co absorbed or bounded by Fe-oxides/hydroxides,and 2)acidic and oxidized basinal fluids mediated Ni-Co releasing resulted from increasing temperature gradient,and successive precipitation of Ni-and Co-bearing sulfides in response to decreasing oxygen fugacity during late diagenesis.The newly established Ni-and Co-enrichment mechanism in the Maochang paleo-karstic bauxite deposit sheds light on the value of the high-sulfur bauxite ores and highlights the economic potential of critical metals in the bauxite deposits.