Geochemical and stable isotopic studies of the Crown and Mararoa reefs, Norseman, Western Australia

Abstract

The quartz reefs of Archean age at Norseman, Western Australia are restricted to the flatter-dipping sections of reverse faults and the gold mineralization is mainly stratabound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events. Bucky' quartz crosscuts laminated quartz and was deposited during an episode of hydraulic fracturing. Gold and sulfides are concentrated on fractures in the laminated quartz reefs. Wallrock assemblages of the Crown and Mararoa reefs require two temporally distinct phases of alteration synchronous with the emplacement of laminated and bucky quartz reef. The first involved the deposition of hydrothermal actinolite and biotite, whereas chloritic overprinting of these assemblages characterizes the second phase of alterarion. Both potassic and chloritic assemblages were modified by the crystallization of metamorphic actinolite and tremolite. In general, single stage alteration is characterized by the addition of volatiles, K 2 0 and CaO, and the loss of Na 2 O. Changes in whole-rock geochemistry during chloritic overprinting of the potassic assemblages include losses of CaO, Al 2 0 3 , Na 2 0 and K 2 0, and a gain of MgO. The δ 18 O values of the Norseman wallrooks range from 5.8 to 9.1 ‰ with a mean value of 7.2 ±0.9 ‰ (16 sampes ). Reef quartzes also exhibit a narrow range of δ 18 O values (11.7 ± 0.4°‰, 26 samples). The wallrock assemblages are cornpatible with a mineralization temperature ≥ 400°C. The calculated δ 18 O value of the water in equilibrium with the quartzes at these temperatures is 7‰. Waters involved in the subsequent metamorphism have identical δ 18 O values which suggest they have a common source, connate fluids and hydrous minerals in the greenstone pile. The observed timing relations and isotope systcmatics suggest that the ore fluids were connate brincs which were mobilized during tectonic dewatering of the volcano-sedimentary pile and channelled up early faults.