The textures and compositions of accessory minerals formed during hydrothermal alteration in porphyry Cu–Au deposits may reflect those processes. Furthermore, if the accessory minerals are stable during weathering processes, they should be able to be used as a guide to mineralization even when the sulphides have been destroyed by weathering. In the Northparkes district, central western NSW, Australia, several Cu–Au ore deposits occur as disseminated or veined sulphides associated with a number of quartz monzonite porphyries intruded into lower Palaeozoic K-rich trachyandesites. This paper reports results from 2000 analyses of rutile grains from 80 samples of rock and soil from around one of these deposits, the E26N porphyry Cu–Au deposit. Within 100 m of ore, rutile grains are larger (commonly with length × breadth >4000 μm2) than the grains further from ore (length × breadth <1500 μm2). The rutile grains closer to ore also are zoned and contain V-rich portions in which V contents are >0.2% (commonly >0.4%) whereas barren rutile generally contains <0.15% V. Niobium contents of >0.15% occur in rutile in the E26N ore zone and in an altered zone within an adjacent earlier intrusion. Thus, Nb in rutile is not as consistently related to mineralization as V in rutile. Vanadium- (and Nb-) rich rutile is present in both the saprolite and soils derived from mineralized rocks at the deposit. Thus, the V contents in rutile can be utilized as a guide to mineralization even in highly weathered material.
- © 2005 AAG/The Geological Society of London