Andreas G. Mueller

Structural control of greenstone-hosted Gold mineralization by transcurrent shearing: A new interpretation of the Kalgoorlie mining district, Western Australia

Abstract The Kalgoorlie mining district encloses the Golden Mile and Mt. Charlotte mining areas, and has produced 1200 metric tons of gold. It is classed as one of the largest Archaean gold deposits in the world. The district is located in the Norseman-Wiluna greenstone belt of the Yilgarn craton, Western Australia. This greenstone belt is characterized by NNW-trending transcurrent shear zones, some of which are traceable over more than 500 km strike length. Mineralization in the district is controlled by second-, third- and fourth-order structures located between principal shear zones of regional extent. An early deformation phase of regional folding (D1), associated with prograde regional metamorphism of low grade, preceded two phases of wrenching. The geometry of mineralized shear zones in both mining areas, as well as field and microstructural studies, allow us to distinguish an early phase of transpressional sinistral shearing (D2) from a later phase of transcurrent dextral shearing (D3). The earlier sinistral shear system in the Golden Mile area was in part reactivated, but its gross original geometry was preserved. These observations are consistent with clay model experiments carried out by the authors. Hydrothermal carbonate-sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event. Mineralization within individual shear zones forms tabular orebodies, whereas pipe-shaped ore shoots occur at the intersection of shear zones. The ore shoots contain high-grade telluride mineralization locally. Relative timing relationships show that mineralization postdates the peak of regional metamorphism. Hydrothermal activity in the district was preceded by the intrusion of calc-alkaline porphyry dykes of granodioritic to tonalitic composition, and accompanied by the intrusion of lamprophyre dykes.

The classification of Western Australian greenstone-hosted gold deposits according to wallrock-alteration mineral assemblages

Abstract The epigenetic, greenstone-hosted gold deposits in the Archaean Yilgarn Block of Western Australia can be classified according to their silicate-carbonate gangue, which forms the innermost wallrock-alteration zone of the structurally controlled hydrothermal systems. The proposed classification is descriptive, and emphasizes associations of prominent, macroscopically visible minerals. The gold deposits are divided into two large groups: high-temperature deposits (400°–700°C) formed at pressures of 3–5 kbars in the lower parts of the greenstone belts (10–15 km depth), and medium-temperature deposits (250°–400°C) formed at pressures of 1 to 2 kbars in the upper parts of these belts (3–7 km depth). The high-temperature deposits are usually located in metamorphic terranes of medium grade (amphibolite facies), and are characterized by either microcline-muscovite-andalusite, garnet-pyroxene-biotite or amphibole-biotite-calcite alteration in the ore zones. Deposits showing the latter two alteration styles may also be classified as skarns in the descriptive sense of Einaudi and Burt (1982). The classic medium-temperature (mesothermal) deposits, studies of which form the basis of all current models on Archaean gold metallogenesis, represent only about one half of the total population of deposits in the Yilgarn Block, although they dominate by total gold production, as they include the only giant deposit in Western Australia, i.e., the Golden Mile at Kalgoorlie. The medium-temperature deposits are usually located in metamorphic terranes of low grade (greenschist facies), and are characterized by biotite-ankerite-albite or sericite-ankerite-albite alteration in the ore zones. It is possible that some of these deposits grade into amphibole-biotite-calcite skarns at depth.

Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

In situ U-Pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the Witwatersrand basin, South Africa, reveals two episodes of tectonothermal activity. A major event between 2.06 and 2.03 Ga is recorded in the Wit-watersrand and Transvaal Supergroups in the northwestern and central basin, and broadly coincides with the ca. 2.06 Ga Bushveld event. In the central and southern basin, a previously unrecognized event has been dated between 2.14 and 2.12 Ga. The widespread geographic and stratigraphic occurrence of Bushveld-aged monazite and xenotime, including both auriferous reefs and unmineralized strata, indicates that metamorphism and fluid flow associated with magmatism was pervasive, affecting most of the succession (>10 km thick) in the central and northern parts of the basin. The metamorphic phosphate dates, which are younger away from the complex, indicate a lag of 20–30 m.y. between emplacement and phosphate growth in the central basin (∼100 km south), suggesting that heat related to magmatism was transferred southward at an average rate of 3–5 mm yr −1 . The absence of 2.06–2.03 Ga phosphates in the Welkom goldfield at the southern end of the basin implies that Bushveld-related heating and fluid flow did not affect this part of the basin. The intergrowth of ca. 2.045 Ga monazite with gold in quartz-pebble conglomerate from the West Rand goldfield indicates that fluid flow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.