Orogenic gold-antimony mineralisation within the structurally-complex Murchison Greenstone Belt, South Africa: New insights into regional- to deposit-scale structural controls of orebody development derived from integrated structural mapping and 3D modelling
Tect was approached in early 2020 by Stibium Mining to conduct a thorough review of the structural geology and gold-antimony mineralisation at Mopani Mine, including Alpha-Gravelotte, Beta, Athens and Monarch shafts, which occur along the ‘Antimony Line’. The Archean-aged Murchison Greenstone Belt, situated in the north-western extents of South Africa, is known for its world-class gold and antimony mineralisation that was mined throughout the 1900s. The belt host over 90 known mineralisation loci, including prospects, deposits and open-pit and underground mines. Despite the historical production of the region, recent discoveries or extensions to known high-grade deposits have been elusive.
The ENE-trending Murchison Greenstone Belt is dominated by greenschist to amphibolite facies volcano-sedimentary tectonostratigraphic sequences, surrounded by several Archean granitoids. Deposit-scale mineralisation is centralized along a prominent shear zone, commonly referred to as the Antimony Line, which is a first- to second-order, deep-seated structure characterized by a steeply north-dipping, discontinuous and laterally variable, brittle-ductile shear zone (SZ) corridor. This corridor is locally oblique to lithological layering and exhibits an overall sinistral transpressive (top-to-the-S) reverse kinematics during NNE-SSW directed compression (D1). During subsequent E-W orientated extension (D2), F1 fold structures/fabrics and the Antimony Line shear zone were transposed and steepened during the development of S2 and S2 axial planar, S-shaped asymmetrical folds (F2). Historical orebodies reflect fluid movement and concentration of economic sulphide lodes and quartz-carbonate veins within discrete structural and lithological trap-sites, namely isoclinal fold hinges, boudins and bends within the tectonostratigraphy along Antimony Line SZ.
To advance the understanding on the structural controls of mineralisation and the geometries of underground orebodies, Tect conducted an initial validation of approximately 80 years of historical datasets and drillhole databases, followed by the structural analysis of mine- to license-scale features. This elucidated the structural controls that facilitated preferential fluid throughput and concentrated economic mineralisation. These findings were integrated, along with mine-focused drilling and historical data, into structurally-accurate 3D implicit models of steeply-plunging ore shoots and interconnected, parallel to sub-parallel arrays of mineralised lenses.
This structural interpretation and mineralisation model has provided a framework for guiding and focusing brownfields and greenfield exploration efforts across the Murchison Greenstone Belt. Tect remains actively involved in collaboration with Stibium Mining’s Geoscientists, processing new exploration data, assisting with forward exploration planning and providing guidance on targeting.