ARCHAEAN GOLD MINERALISATION DURING POST-OROGENIC EXTENSION IN THE NEW CONSORT GOLD MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

摘要

The area around New Consort Gold Mine (NCGM) was complexly deformed during at least five distinct events named D_(1NC) to D_(5NC). The D_(1NC) to D_(3NC) events involve progressive shearing and folding linked to the 3.25 to 3.23 Ga accretionary history of the Barberton greenstone belt (D_(1NC) and D2NC), and to the subsequent ~3.1 Ga emplacement and doming of surrounding batholiths (D_(3NC)). During D_(3NC) the area experienced intense strain partitioning with the development of a network of shear zones that envelop structural domains characterised by locally unique deformation histories. Around NCGM, D_(3NC) events involved early shearing (D3aNC) along the Consort Bar followed by two episodes of 600 m scale folding (D_(3bNC) and D_(3cNC)), resulting in a complex fold interference pattern. Pegmatites intruded during D_(3NC). The critical structures controlling gold mineralisation are D_(4NC) extensional, 10 to 100 m scale, brittle-ductile shear fractures associated with kink bands and crenulation folds that formed concomitantly with gold mineralisation after all the D_(3NC) structures had fully developed, marking a clearly separate event. This network of fractures is distributed along 100 to 200 m wide westnorthwesterly, north-northwesterly and east-northeasterly trending corridors. The distribution of high-grade ore zones was controlled by the intersection orientation of D_(4NC) fractures and suitable host lithologies, mainly the silicified hinge zones of D_(1NC) folds and the siliceous Consort Bar. Because these host lithologies were complexly folded in D_(1NC)-D_(3bNC)-D_(3cNC) fold interference patterns, the 3-D distribution of ore zones is highly discontinuous and complex. D_(5NC) structures represent late reverse faults of unknown age, well developed in the nearby Albion Mine. Kinematic analyses and stress inversion using Bingham tensor solutions and a reduced moment tensorn method, of mineralised D_(4NC) fractures along old gold workings at NCGM; consistently indicate a vertical σ1, and a horizontal northwesterly to southeasterly directed σ3, within an extensional stress regime. Results for the nearby Clutha, Albion and Woodstock mines are similar. This study suggests that gold mineralisation in the NCGM area can be linked to an extensional event that may have developed separately from the accretionary events shaping the craton, and may have coincided with deposition of the Dominion Group.