The Golden Mile, Kalgoorlie; a giant gold deposit localized in ductile shear zones by structurally induced infiltration of an auriferous metamorphic fluid

Abstract

Mesoscopic and microscopic analysis of the structurally controlled gold mineralization of the Golden Mile, Kalgoorlie, has demonstrated that the mineralogically simple gold pyrite mineralization style (70% of production) is restricted to an anastomosing system of narrow high strain zones (ductile shear zones) and to related tension fractures in the intervening unstrained rock. Virtually all ductile shear zone lode structures occur in the Golden Mile Dolerite because it was essentially mechanically isotropic prior to deformation. It is proposed that during the near-upright regional-folding event numerous ductile shear zones were established in an approximately axial planar relationship to the folds, with synchronous conjugate shear zones developed at low angles to this direction. A dynamic analysis shows that the tensional cross lodes can be related geometrically to the three ductile shear zones. Golden Mile-style mineralization is syntectonic and was not deposited posttectonically in structurally prepared sites. Microtextural analysis demonstrates that minerals associated with gold emplacement are strongly modified during strain within the ductile shear zones by ductile deformation dominated by mass transfer processes. In particular there is abundant evidence of crack-seal-type microfracturing within and marginal to grains and in microscopic transgranular sites; this deformation style contributed prominently to the bulk plastic flow of the shear zones. It is proposed that high fluid pressure gradients were established at each crack-seal cycle leading to much enhanced permeability and to considerable focusing of fluid flow through the narrow foliated zones. This structurally induced infiltration mechanism allowed the ore fluid to pervade the deforming zones and also controlled the straining process by moving dissolved mineral components from highly stressed contacts to sites of deposition. Thus the mineralization was introduced during the ductile phase of shear zone evolution, and though mesoscopic veins are present in the ductile shear zones, they are not a necessary part of the gold emplacement event: the majority of veins formed late in the tectonic history. Gold emplacement in the ductile shear zones was controlled by microscopic deformation processes and ductile shear zones are good exploration targets even in the absence of macro-to mesoscopic dilation sites.

Geochimie et mecanisme metallogenique du district aurifere de Hetai sud de la Chine =

“…These deformation zones and their associated alteration played a fundamental role in the formation of gold deposits. This is similar to many other epigenetic gold deposits over the world (Colvine et al, 1988;Phillips et al, 1987;Boulter et al, 1987). The following discussion describes the overall pattern of deformation and alteration related to the emplacement of the gold lodes.…”

“…The former is a set of closely spaced fractures whose orientation parallels the walls of the shear zone, while the E fabric is usually characterized by its occurrence in the intervening unstrained lithons in the deformation system, and by filling of thin (millimeter scale) quartz and carbonate veins and thin selvages of pyrite and muscovite. Some researchers (Boulter et al, 1987) considered them to postdate the S-fabric, but in the Hetai lodes, they seem to have formed at the same time as the S or slightly later but still in the same deformation event. Fabrics S, C, and E were derived from a unified stress field.…”

Minor and Trace Elements in Natural Tetrahedrite-Tennantite: Effects on Element Partitioning among Base Metal Sulphides

“…Two of these samples (V446 and V538) come from the Bleikvassli deposit, Norway, a sedimentary-exhalative deposit which is interpreted to have recrystallized at conditions of granulite facies [82,83]. The other two (G6940 and G11579) come from Kalgoorlie, Western Australia, where Boulter et al (1987) [84] describe recrystallization of pyrite during deformation associated with regional metamorphism. Thus, those assemblages in which chalcopyrite hosts more Ga than sphalerite, both come from environments where sulphides have recrystallized under regional metamorphism.…”

Minéralisations et déformation à proximité de la faille de Davidson, Abitibi, Canada /

“…Enfin la source de l'or n'est pas encore établie, la théorie d'un héritage sédimentaire local s'opposant actuellement à un apport par des fluides hydrothermaux d'une source profonde, voire mantellique (Tomkins, 2010, Pitcairn et al, 2010et Thomas et al, 2010. La profondeur de mise en place est délimitée entre les zones de sub-surfaces et la zone de transition ductile-fragile (Hagemann et al 1992 Yilgarn situé en Australie (Boulter et al, 1987et Mueller et al, 1987 (Kenworthy et Hagemann, 2007), mais également parfois des failles majeures issues d'un milieu fragile (Hagemann et al, 1992). Les structures secondaires sont formées soit en milieux fragiles (Tripp et Vearncombe, 2004), fragiles-ductiles ou ductiles-fragiles (Mueller et al, 1987).…”