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MRD-15 Ni-Cu-PGE sulphide deposits

Late-stage mineral formation of Noril'sk ore field and its genetic relations

Ernst Spiridonov, Moscow State University (Russian Federation)
Yulia Gritsenko, Moscow State University (Russian Federation)
Vyacheslav Golubev, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemestry (Russian Federation)
 

Intersecting veins and impregnation composed of U-Ag-Bi-Co-Ni-Sb-As minerals have been met with throughout the Noril'sk magmatic sulfide Ni-Cu-PGE (-Co-Au-Ag) deposit. The mineralization is considered hydrothermal in origin, and the minerals are Ni-Co arsenides and antimonides, carbonates, Zn-Pb-Cu-Ni-Mn-Ag-Bi-Cd sulfides, Pb selenides, native As, Ag and Bi, Ag amalgam and uraninite. Three stages of U-Ag-Bi-Co-Ni-Sb-As mineralization can be defined on the basis of the type of the host Ni-Cu ore: the first stage consists of Co-Ni-Sb-As minerals associated with the bornite-millerite-godlevskite ore type; the second stage is represented by U-Ag-Bi-Ni-Sb-As minerals hosted by the chalcocite-heazlewoodite ore type, and, finally, the third stage consists of sulfoantimonide and sulfoarsenide mineralization related with the pyrite and Ni-Co-Fe thiospinels ore type. A new mineral species ferroskutterudite (Fe,Co)As₃ was discovered in the first stage of mineralization. Ni/Co ratio in carbonate veins, ca 15:1 (number of analyses 880), is not far from that of the primary magmatic sulfide ore (20:1).
There are certain mineralogical differences between impregnated and vein U-Ag-Bi-Co-Ni-Sb-As ore types: maucherite and breithauptite prevail in the impregnated ore, whereas loellingite, native As and nickeline are common minerals in carbonate veins. Maucherite and breithauptite of impregnated ore type contain up to 1.5-2 wt.% Pd, but Pt and Au are below the detection limits. Pd, Pt and Au are all below the detection limit in carbonate veins. Ni-Co-Fe arsenides are common minerals in carbonate veins intersecting the Ni-Cu sulfide ore bodies, whereas far away from the Ni-Cu sulfide ores loellingite and native As prevail in the carbonate veins.
Rb-Sr age of apophyllite in arsenide vein is 164 Ma, and the model Pb-Pb age of galena in the carbonate vein is 143 Ma indicating that the hydrothermal U-Ag-Bi-Co-Ni-Sb-As mineralization is 80-100 million years younger than the trapp formation P₂-T₁ (251 Ma) and the genetically related magmatic sulfide ores. The hydrothermal mineralization is developed in the areas, where Ni-Cu sulfide ores and country rocks are metamorphosed under zeolite facies conditions. PT conditions of the deposition of carbonate, which are co-genetic with arsenides, are T = 216-127°C, P = 0.9-0.1 kb, and correlate with the conditions of the zeolite facies. The ore mineral associations allude that the ore-forming fluids were metamorphic with elevated f_O2.
Pb isotope composition of galena evidences different substance source for the Noril'sk magmatic sulfide ores and for the U-Ag-Bi-Co-Ni-Sb-As mineralization. Pb isotope composition of galena from magmatic sulfide ores is suggestive of the mantle values: ²⁰⁶Pb/²⁰⁴Pb= 18.169-18.172; ²⁰⁷Pb/²⁰⁴Pb = 15.529-15.532; ²⁰⁸Pb/²⁰⁴Pb = 37.870-37.880, whereas Pb isotope composition of galena from hydrothermal U-Ag-Bi-Co-Ni-Sb-As vein refer to the crustal values ²⁰⁶Pb/²⁰⁴Pb=18.517; ²⁰⁷Pb/²⁰⁴Pb=15.677; ²⁰⁸Pb/²⁰⁴Pb=38.796.