Fluid-rock interaction and gold deposition within a late archaean shear zone, Dalny Mine, Zimbabwe

Carter, Alan Henry Charles (1989) Fluid-rock interaction and gold deposition within a late archaean shear zone, Dalny Mine, Zimbabwe. University of Southampton, Doctoral Thesis.

Abstract

Gold mineralisation at the Dalny Mine, Zimbabwe, is restricted to a brittle-ductile shear zone which transects a sequence of late Archaean (2.7 Ga) Fe-rich tholeiitic basalts and minor intercalated sediments. The lithological associations, together with some preliminary geochemical analyses suggest that the sequence was probably deposited in a marginal basin above a subduction zone. The Dalny shear zone, which extends NE-SW for approximately 10km, was initiated as a Reidel structure associated with dextral strike-slip displacement along the crustal-scale Lily fault zone interpreted here as a late Archaean terrane boundary. Subsequent regional deformation within the Chegutu-Kadoma greenstone belt resulted in upright folding and reverse compressional reactivation of the Dalny shear zone which dips 70^o towards the north-east. The introduction of gold-bearing fluids during this deformational event resulted in a complex sequence of vein emplacement terminating in the development of widespread carbonate veins. Gold is restricted to three characteristic vein types at Dalny and is associated with abundant pyrite and arsenopyrite, together with accessory chalcopyrite, pyrrhotite, tetrahedrite, sphalerite, galena, bornite, tourmaline and scheelite. Gold primarily occurs as inclusions (up to 60μm diameter) within pyrite but also resides as native grains within fractures, and on the surface of pyrite. Gold was deposited from H_2O-CO_2 fluids (5-10 mole % CO_2) of low salinity (averaging 6.74 wt.% NaCl equiv.) and moderate density 0.97 ± 0.05g cm^-3 under P-T conditions of 3.6 ±0.3 kbars and 390 ±20^oC. Similar fluids have been identified from the adjacent Dawn and Arlandzer deposits implying that the auriferous fluids were of regional extent. The δ18O composition of the fluids estimated from δ18O chlorite analyses was 8.18 to 8.69% _o δD compositions ranged from -12.3 to -17.7% _o. The δ34S composition of H_2S within the gold-bearing fluid probably ranged from 3.85 to 4.15% _o. The peak metamorphic timing of fluid introduction and the absence of associated magmatic intrusions combine with the stable isotope data to suggest a source for the fluids involving regional metamorphic dehydration of the mafic volcanic sequence at depth. Extensive fluid-rock disequilibrium at Dalny resulted in the formation of a zoned alteration assemblage distinguished by the extensive development of pyrite immediately adjacent to quartz vein margins, and an ankerite-muscovite assemblage which extends up to 3m from the mineralised zone. Muscovite-chlorite, and calcite-chlorite mineral assemblages are evident with increasing distance from the mineralised zone, the former being characterised by significant enrichments in K_2O, CaO, Ba and Rb together with a consequent depletion of Na_2O. The pH of the gold bearing fluid is estimated at 5.35 ± 0.55 whilst the log fO₂ of the fluid has been constrained at -26.7 (at 390^oC). The log fH₂S of the fluid was 0.093 which is equivalent to 457ppm H₂S assuming 6.7 mole% CO₂. Gold deposition resulted from destabilisation of the AuHS^o complex due to a decrease in the fS₂ of the fluid accompanying sulphide precipitation. The widespread occurrence of later aqueous inclusions within quartz-carbonate veins at Dalny, indicates the occurrence of a late fluid of moderate salinity (9.33 wt.% NaCl equiv.) which was trapped at 275^oC and 1.9 kbars. This fluid may have resulted from the protracted evolution of the earlier gold-bearing H₂O-CO₂ fluid.

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