Abstract

There has been a long-standing controversy regarding the timing and number of gold mineralization events at Kalgoorlie. Uranium-Pb dating of zircons and hydrothermal monazite and xenotime, as well as 40Ar/39Ar analysis of metasomatic fuchsite and white mica, are used to date pre- to synore dikes, alteration, and orebodies in order to resolve this issue. The majority of gold mineralization at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, are different expressions of a complex mineralizing system that was active at broadly the same time at ca. 2.64 Ga. Gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time, under similar P-T conditions. This giant ore system formed after ca. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at 2680 ± 9 Ma, and intrusion of calc-alkaline feldspar-quartz porphyry dikes at 2670 ± 5 Ma. Gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 ± 6 Ma and overlapped the waning stages of hornblende- and albite-bearing porphyry dike emplacement at 2650 ± 6 Ma and regional metamorphism. Subsequent brittle deformation in the Kalgoorlie gold field was accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the Golden Mile between about 2.61 and 2.60 Ga. This late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks at Kalgoorlie and may be related to uplift and final cooling of the terrane. Despite this late event, the geometry of the Kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization.

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