J.R. Vearncombe

Late Archean convergent margin tectonics and gold mineralization: A new look at the Norseman-Wiluna Belt, Western Australia

On a world scale, the most important hydrothermal gold deposits occur either in Late Archean (ca. 2700 Ma) greenstone belts or at late Paleozoic to Quaternary convergent plate boundaries. In the latter environments, epithermal and porphyry-hosted deposits form during subduction-related magmatism in volcanic arcs, and deep-level meso- thermal deposits form during deformation in continental margin orogenic belts. The mesothermal deposits are similar to Archean gold deposits in greenstone belts such as the Norseman-Wiluna Belt in Western Australia. We here suggest that this similarity exists because the Norseman-Wiluna Belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins, such as the North American Cordillera. It is most likely that the Norseman-Wiluna Belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the Late Archean, the distribution of gold mineralization being controlled by convergent margin tectonics.

Shear zones of the Limpopo Belt and adjacent granitoid-greenstone terranes: implications for late Archaean collision tectonics in southern Africa

Abstract Granulite metamorphism in the > 2.65 Ga Limpopo Belt of southern Africa is related to crustal thickening, due to late Archaean collision tectonics. The Central Zone of the belt is a major thrust sheet of distinctive lithological, structural and isotopic character, separated from adjacent terranes by crustal-scale shear zones, and the Northern and Southern Marginal Zones are up-thrust deep-level granulite facies terranes of the adjacent cratons. Geometrically the marginal zones are mirror images and the Limpopo Belt is essentially symmetrical in N–S section. Conventionally regarded as a N–S collisional orogeny, the gross structural and lithological differences between the Central Zone and the marginal zones, kinematic data from shear zones and geophysical data showing the three-dimensional geometry of the Central Zone are here used to suggest that the direction of emplacement of the Central Zone was also the collision direction. This was from northeast to southwest, with the upthrust, outwardly directed, marginal zones developing in mechanical response to this collision.

3.26 Ga black smoker-type mineralization in the Strelley Belt, Pilbara Craton, Western Australia

Early Archaean (3.26 Ga) volcanogenic massive sulphide (VMS) deposits located within the low strain and very-low metamorphic grade Strelley greenstone belt, East Pilbara, Western Australia, contain exceptionally well-preserved sulphide textures which are directly analogous to those formed in black smoker chimneys at present-day submarine hydrothermal vents. The VMS deposits are associated with seafloor alteration zones in and below silicacarbonate laminites, at the top of a pile of tholeiitic to calc-alkaline intermediate and felsic volcanic rocks with trace element abundances indicative of modem subduction-related processes. VMS mineralization comprises Zn-rich sulphide lenses with significant Cu and barite, and variable Pb and Ag, above Cu-rich stringer zones, all of which are similar to Phanerozoic VMS deposits. Sulphide-sulphate mineralization implicates hydrothermal discharge into sulphate-bearing ocean waters.

Quartz vein morphology and implications for formation depth and classification of Archaean gold-vein deposits

Abstract A simplified scheme to categorise quartz vein morphology from gold deposits is based on the growth direction of quartz or pseudomorphed chalcedony in the veins. The categories are scale-independent and are: (1) face-control, (2) displacement-control, (3) parallel-control, (4) radiating, (5) non-directional control, (6) replacement, and (7) modified. Quartz vein textures do not indicate any particular type of mineralisation. For the depth continuum of Archaean lode-gold deposits of the Yilgarn Craton all seven categories of texture are variously present. However, crustal depth is a major control. Parallel and radiating textures are characteristic of gold deposits formed at the near surface; displacement and non-directional control are typical of deposits at midcrustal (greenschist and amphibolite) conditions; and face-control shows a range of conditions from the near surface to midcrustal. Replacement and modified textures embrace a wide variety of alteration and deformation conditions, and can occur in gold deposits at all crustal levels.

Archaean Au−Ag mineralisation at Racetrack, near Kalgoorlie, Western Australia: a high crustal-level expression of the Archaean composite lode-gold system

The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, fO2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.

Shear zones, fault networks, and Archean gold

Conjugate ductile shear zones developed mostly on granite-greenstone margins are a dominant structural feature of the Yilgarn craton. These greenstone belt–scale shear zones lack both gold deposits and gold-related alteration. Previous interpretations of the shear zones as part of a linked system, as craton-scale terrane boundaries, or as crustal-scale pathways for gold-mineralizing fluids, all appear equivocal. Instead, gold mineralizing fluids in the Yilgarn craton (illustrated here by the gold-rich Yandal Belt) were focused by a network of middle to upper crustal brittle faults in the Late Archean. The faults are 040° to 060° trending dextral, and 090° to 120° trending sinistral; the maximum compression direction is about 075°. Gold mineralization occurs in laminated and other schistosity-parallel quartz veins where brittle faults refract along earlier subsidiary within-greenstone ductile shear zones, and in quartz veins within Late Archean faults.

Fault and stratigraphic controls on volcanogenic massive sulphide deposits in the Strelley Belt, Pilbara Craton, Western Australia

Abstract Early Archaean, FeZnCu volcanogenic massive sulphide deposits of the Strelley Belt, Pilbara Craton, occur at the top of a volcanic dominated sequence, at the interface of felsic volcanic rocks and siliceous laminites, beneath an unconformity overlain by clastic sedimentary rocks. The structure of the Sulphur Springs and Kangaroo Caves VMS deposits is relatively simple, with the present morphology reflecting original deposition rather than significant structural modification. The rocks have been tilted giving an oblique cross-sectional view of discordant high-angle, deep penetrating faults in the footwall, which splay close to the zones of volcanogenic massive sulphide mineralization. Faults do not extend far into the overlying sedimentary cover, indicating their syn-volcanic and syn-mineralization timing. Both the Sulphur Springs and Kangaroo Caves sulphide deposits are located within elevated grabens in a setting similar to massive sulphide mineralization in modern back-are environments. Mineralization at Sulphur Springs and Kangaroo Caves is located at the edge of the grabens, at the site of intersecting syn-volcanic extensional faults.

Heterogeneous brittle deformation in the Devonian carbonate rocks of the Pillara Range, Canning Basin: Implications for the structural evolution of the Lennard Shelf

Deformation documented in the Upper Devonian carbonate rocks of the Lennard Shelf shows a complex history involving discrete synsedimentary structures and faults. The reefal carbonate successions of the Lennard Shelf were deposited along the northeastern margin of the extensional Fitzroy Trough in the Canning Basin. Extensional deformation on the regional scale is concentrated along normal faults trending northwest parallel to the shelf margins, and in structural corridors that are oblique to the strike of the shelf. In the Pillara Range‐Limestone Billy Hills region, a corridor of northeast‐trending deformation is associated with dextral offset of the reef margins, and is interpreted to represent an accommodation zone, comprising individual en echelon sinistral oblique‐slip faults. Synsedimentary deformation controlled reef and platform facies distribution, and caused fracturing with extensive development of neptunian dykes. Later structures, which post‐date marine, radiaxial fibrous cements, are spatiall...

Strain and displacement in the Middle Vale Reef at Telfer, Western Australia

Mineralisation in the Middle Vale Reef at Telfer is coincident with the most deformed rocks in a strata-parallel shear zone in an argillaceous and calcareous horizon. Cleavage to bedding angle and three-dimensional strain calculated by Rƒφ analysis were determined in a profile across the Middle Vale Reef. They show significant volume loss (about 40%), but shear strain was a minor component of the deformation (γ < 1.5). From a balanced section, bedding-plane slip along the Middle Vale Reef appears to have been less than 10 m and the deformation was dominated by material removal resulting in the loss of volume in the host siltstones. The same zone, which was a conduit for fluid during volume loss, reactivated with the addition of quartz-sulphide veins, resulting in a net volume gain. Reactivation of surfaces with a high permeability, such as the Middle Vale Reef, is confirmed as a major control on shear zone-hosted mineralisation.