The geology, geochemistry and genesis of gold mineralization associated with Archean iron-formation, Lennox Mine, Zimbabwe

Gilligan, Jonathan Mark (1990) The geology, geochemistry and genesis of gold mineralization associated with Archean iron-formation, Lennox Mine, Zimbabwe. University of Southampton, Doctoral Thesis.

Abstract

Epigenetic gold mineralization at Lennox Mine, hosted by oxide iron-formation and adjacent sheared metabasites of the ca. 3.5Ga Mashava Greenstone Belt, Zimbabwe, developed in the context of a right-lateral brittle-ductile shear zone. Two structural styles of gold mineralization are evident within the iron-formation, characterised by pyrrhotite-replacement of magnetite bands. Vein-Sulphide occurs as centimetre-wide pyrrhotite alteration halos enveloping isolated orthogonal fractures in oxide iron-formation. Fluid-rock interaction was limited by static diffusional flow of the ore fluid into the iron-formation. The Sulphide Ore is a sub-stratiform pyrrhotite-gold orebody which represents an ore fluid conduit formed by shearing along secondary shear orientations which were partly controlled by lithological contacts within the iron-formation. The ductile flow of pyrrhotite was critically important as it promoted strain partitioning into the sulphide ore and enhanced fluid-magnetite interaction by a brittle-ductile equivalent of antitaxial veining, continually exposing fresh magnetite to the ore fluid. Sulphidation of magnetite and the consequent destabilisation of aqueous reduced-sulphur species led to the co-precipitation of pyrrhotite, chalcopyrite and gold. Element associations evident in the Sulphide Ore are: Au+ Cu+ Ni+ Fe (gold-sulphide-mineralisation); Mn+ Zn (silicate minerals); Ti+ Rb (primary iron-formation); and Sr (Carbonate). The Sulphide Ore is characterised by the addition of Au, S & Cu to the precursor oxide iron-formation, with approximate enrichment factors of 3x10³, 3x10² & 3x10¹, respectively. Vein-Sulphide pyrrhotite exhibits a restricted sulphur isotopic composition (δ34S = -0.30 ± 0.22% per mil), indicating a magmatic isotopic signature for the sulphur component in the ore. The Quartz Reef, an auriferous quartz vein, was developed in extensional fractures infilling the ore fluid conduit within the iron-formation and adjacent metabasites. Minimum P/T conditions for the mineralization are 375^oC and 4.5kb. Combined SEM/EDX studies of pyrite grains extracted from the vein confirm the importance of sulphide surfaces and edges as foci for precious metal and telluride mineral precipitation. The average background gold content of the unmineralized oxide iron-formation is in the range 2-3ppb and the bulk rock elemental composition is indistinguishable from that of oxide iron-formations world-wide unrelated to gold mineralization. Oxide iron-formation is altered to silicate iron-formation in the zone of mineralization by redox hydration reactions, principally magnetite-destructive formation of grunerite. A Pb-isotope errorchron for the Lennox iron-formation dates a re-setting event at 3274Ma, corroborating a Sebakwian age for the Mashava Greenstone Belt. Galena from veins and fractures cross-cutting the iron-formation has the most unradiogenic Pb-isotopic composition recorded in Zimbabwe: single stage model age is 3364Ma, model μ of 8.36. A Pb-Pb palaeo-isochron for Quartz Reef galenas indicates gold mineralization may have coincided with tonalitic plutonism at ca. 2.9-3.1Ga, and appears to have been coeval with a number of other gold deposits in Zimbabwe. This conflicts with previous metallogenic models emphasising a ca. 2.6-2.7Ga age as the single gold mineralization event in the Zimbabwe craton. Geochemical studies required the development of new XRF and INAA analytical procedures, optimised for the enhanced matrix effects experienced when analysing high-Fe geological material.

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