AR-40 AR-39 CONSTRAINTS ON THE TIMING AND HISTORY OF AMPHIBOLITE FACIES GOLD MINERALIZATION IN THE SOUTHERN CROSS AREA, WESTERN-AUSTRALIA/

The Southern Cross Greenstone Belt in Western Australia contains struc turally controlled, hydrothermal gold deposits which are thought to ha ve formed at or near the peak of amphibolite facies regional metamorph ism during the Late Archaean. Although the geological features of depo sits in the area are well documented, conflicting genetic models and o re-fluid sources have been used to explain the observed geological dat a. This paper presents new Ar-40/Ar-39 data which suggest that the the rmal history of the Southern Cross area after the peak of regional met amorphism was more complex than has previously been suggested. After t he main gold mineralisation event prior to ca 2620 Ma, the Ar-40/Ar-39 ages from amphiboles and biotites sampled from the alteration selvage s of gold-bearing veins indicate that temperatures remained elevated i n the region of 500 degrees C for between 20 and 70 million years. The se amphiboles and biotites from individual deposits yield ages that ar e in good agreement with one another to a high precision, implying inc reased cooling rates after the long period of elevated temperatures. A long the Southern Cross Greenstone Belt, however, amphibole-biotite pa irs from the alteration selvages of gold-bearing quartz veins, while r emaining in good agreement with one another, vary between deposits fro m ca 2560 Ma to ca 2440 Ma. Amphiboles from metabasalts that are assoc iated with regional metamorphism and not hydrothermal alteration, cont ain numerous exsolution lamellae that reduce the effective closure tem perature of the amphiboles and yield geologically meaningless ages. Th ese age relationships show that the thermal history of the area did no t follow a simple cooling path and the area may have been tectonically active for a long period after the main gold mineralisation event bef ore ca 2620 Ma. Such data may provide important constraints on subsequ ent genetic modelling of gold mineralisation and metamorphism.