Fluid-channelling and gold mineralization within the late proterozoic Mwembeshi Shear Zone, Zambia

Kasolo, Pius Chilufya-Bwalya (1992) Fluid-channelling and gold mineralization within the late proterozoic Mwembeshi Shear Zone, Zambia. University of Southampton, Doctoral Thesis.

Abstract

The Mwembeshi Shear Zone (MSZ) is a major Late-Proterozoic strike-slip zone of poly-phase deformation which extends more than 1000km ENE through central Zambia and separates two Proterozoic orogenic belts - the Lufilian Arc and the Zambezi Belt. It shows considerable heterogeneity of strain and variation in tectonic style along its length and is the locus of a large number of gold showings and small gold mines. Despite the proximity of the MSZ, none of these deposits actually occur within the first-order shear zone. On a local scale, the gold deposits occur in second-order or even third-order dilational sites within brittle-ductile structures, with strike lengths generally less than 10km and widths of a few metres, which are interpreted to be geometrically and genetically linked to the crustal-scale MSZ structure. Four deposits, the Dunrobin (quartz vein), Matala (quartz-tourmaline breccia), Chumbwe (auriferous quartz vein), and the Sasare (ferruginous quartz veins) were studied in detail in order to define the dominant controls of distribution of the gold mineralization associated with the MSZ. Individual gold deposits are located in a wide range of host rocks, all metamorphosed to greenschist facies. Pyrite is the dominant sulphide; other minerals are mostly occluded by pyrite and include arsenopyrite, pyrrhotite, galena, sphalerite ± Cu-sulphides. Gold chiefly occurs as sub-microscopic grains on crystal surfaces of pyrite, less commonly as inclusions in pyrite, and rarely as the native metal within quartz. All the gold deposits exhibit evidence of alteration halos, ranging from a few metres up to 25 metres wide, in which hydrothermal alteration overprints earlier metamorphic mineral assemblages. The alteration was synchronous with deformation and is characterized by minor carbonitization, potassium metasomatism (generally sericite), and sulphidation. Alteration is expressed chemically by the introduction of CO_2, SiO_2 ± boron. There is a general enrichment of As, Cu, Bi, V, Cr, Ba and Sb in the ores and the alteration zones, although the abundances of these elements and the extent of alteration vary considerably in each deposit. The mineralizing fluids are H₂O-CO₂-dominated (1-25 mole% CO₂) and salinities range from 1.6 to 12wt% NaCl equiv. The close association of the H₂O-CO₂ fluids with mono-phase carbonic inclusions and the consistency of their bulk composition with the experimentally determined solvii suggest that the fluids are a result of episodic fluid unmixing, reflecting wide fluctuation in fluid pressures.

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