Middle Archaean gold deposits in the North Shaw–Coongan Greenstone Belt, Australia: the relative timing of events responsible for gold mineralisation

Abstract

Gold deposits of the Mid-Archaean East Pilbara Granitoid–Greenstone Terrane, Australia, are unique because they characterise metallogenic evolution of the Early Earth. In the North Shaw–Coongan Greenstone Belt, quartz–sulphide lodes within shear and fault zones are host to gold mineralisation. Detailed field mapping of crosscutting structures at the Sharks Well Mining Centre and Blue Bar Mine in the Coongan Greenstone Belt, and at the North Shaw Mining Centre in the North Shaw Greenstone Belt, have established a relative chronology for gold deposits. The results suggest that the accepted classification of regional gold mineralisation, in terms of two events, may need modification. The Sharks Well Mining Centre is hosted by meta-basalt to andesite and carbonatised quartz–chlorite schist of the Mt. Ada Basalt (Warrawoona Group, ca. 3.49–3.31 Ga). The gold-bearing quartz–sulphide lodes are principally situated in the Shark Gully Shear Zone. The lodes and veins formed early in D1 (local) and prior to development of cleavage and folds. Gold occurs in NW-trending fractures that cut across the quartz lodes and veins, suggesting syn- to post-D1 gold mineralisation. The maximum relative age of gold mineralisation at the mining centre is constrained by the age of the Duffer Formation, which is stratigraphically younger than the Mt. Ada Basalt and dated at 3471–3463 Ma. The Blue Bar Mine is hosted by quartz–fuchsite schist of the Central Coongan Shear Zone, which trends northerly and dips steeply eastward. The mine consists of a single, brecciated, gold-bearing quartz lode that is unconformably overlain by scree breccia, and in turn, by the Mount Roe Basalt of the Fortescue Group (ca. 2.7 Ga). The maximum relative age of gold mineralisation at the mine is constrained by the age of the Euro Basalt at 3395–3325 Ma, while the minimum relative age is constrained by the Mount Roe Basalt at ca. 2.7 Ga. The North Shaw Mining Centre consists of several gold-bearing quartz lodes (including the Auroria and Eldorado lodes), as well as a number of historic gold workings (alluvial and shaft mining) on the plains adjacent to the Shaw River. The mining centre is located on the contact margin of the North Shaw Suite of the Shaw Granitoid Complex. The host rocks consist of contact metamorphosed basalt–andesite and hornblende amphibolites. Five stages of (local) deformation are recognised in the rocks of the mining centre: D1—macroscopic folding of the North Shaw Greenstone Belt about a northerly trending fold axis; D2—NNE-trending foliation and folds; D3—development of a NE-trending disjunctive cleavage, folds and conjugate kinks; D4—development of faults and master joints during emplacement of dykes of the Black Range Suite; D5—formation of the WNW-trending Callina Fault Zone. The fault zone crosscuts mafic dykes of the Black Range Suite, which places constraints on the minimum relative age of gold mineralisation in the fault zone. Based on crosscutting structural relationships, the relative and absolute chronology of the gold deposits in the North Shaw–Coongan Greenstone Belt indicates they are not all of the same age. The crosscutting structural relationships justify a review of the currently accepted ‘Event 1 or 2’ classification of Huston et al. [Econ. Geol. 97 (2002) 691]. Two new events, one at 3325–2774 Ma, and one younger than 2.7 Ga, also affected the mafic and ultramafic rocks of the Warrawoona Group.