40Ar/39Ar constraints on the timing and history of amphibolite facies gold mineralisation in the Southern Cross area, Western Australia*

Abstract

The Southern Cross Greenstone Belt in Western Australia contains structurally controlled, hydrothermal gold deposits which are thought to have formed at or near the peak of amphibolite facies regional metamorphism during the Late Archaean. Although the geological features of deposits in the area are well documented, conflicting genetic models and ore‐fluid sources have been used to explain the observed geological data. This paper presents new 40Ar/39Ar data which suggest that the thermal history of the Southern Cross area after the peak of regional metamorphism was more complex than has previously been suggested. After the main gold mineralisation event prior to ca 2620 Ma, the 40Ar/39Ar ages from amphiboles and biotites sampled from the alteration selvages of gold‐bearing veins indicate that temperatures remained elevated in the region of 500°C for between 20 and 70 million years. These amphiboles and biotites from individual deposits yield ages that are in good agreement with one another to a high precision, implying increased cooling rates after the long period of elevated temperatures. Along the Southern Cross Greenstone Belt, however, amphibole‐biotite pairs from the alteration selvages of gold‐bearing quartz veins, while remaining in good agreement with one another, vary between deposits from ca 2560 Ma to ca 2440 Ma. Amphiboles from metabasalts that are associated with regional metamorphism and not hydrothermal alteration, contain numerous exsolution lamellae that reduce the effective closure temperature of the amphiboles and yield geologically meaningless ages. These age relationships show that the thermal history of the area did not follow a simple cooling path and the area may have been tectonically active for a long period after the main gold mineralisation event before ca 2620 Ma. Such data may provide important constraints on subsequent genetic modelling of gold mineralisation and metamorphism.