Gold mineralization in high-grade metamorphic shear zones of the Renco Mine, southern Zimbabwe

Abstract

The Renco gold mine in southern Zimbabwe is the only known major gold deposit in the granulite facies terrane of the northern marginal zone of the Late Archean to mid-Proterozoic Limpopo belt. Gold mineralization is confined to a system of high-temperature mylonite zones characterized by two distinct geometries. These include a series of southeasterly dipping, anastomosing, north-northeast- to east-northeast- trending tabular lodes, termed shallow reefs, and subvertically inclined, easterly plunging pipelike lodes, termed steep reefs. The kinematics and orientation of the mineralized shear zones are consistent with a lateral and frontal thrust zone geometry that formed during the Late Archean thrusting of the northern marginal zone onto the Zimbabwe craton. Gold is spatially and temporally closely associated with sulfide mineralization, including pyrrhotite as the dominant sulfide with minor amounts of chalcopyrite and pyrite. Associated wall-rock alteration comprises a garnet-biotite-quartz + or - siderite mineral assemblage. Mineral textures within the host mylonites, as well as garnet-biotite thermometry, indicate gold deposition at temperatures of about 600 degrees C under mid-amphibolite conditions, slightly postdating regional peak metamorphic conditions. Fluid flow in the high-temperature shear zones was largely controlled by fracture permeabilities. Transient episodes of brittle fracturing during conditions of close to lithostatic fluid pressures were promoted by a pronounced strain partitioning within the narrow shear zones into ductile mylonite bands and brittle-ductile lithons that contain the bulk of the gold sulfide mineralization. The high-grade metamorphic ore and alteration mineral assemblages are overprinted by lower greenschist facies parageneses along brittle faults and cataclasites that are related to the mid-Proterozoic tectonism of the northern marginal zone. It is concluded that the mineralization at Renco illustrates the rare case of a midcrustal high-grade metamorphic gold mineralization in southern Africa where the vast majority of Late Archean lode gold deposits are related to low-grade metamorphic granite-greenstone terranes.