• The geochemistry of the platinum group of elements in felsic rocks as a guide to Cu-Au mineralization.
• The rate of growth and evolution of the continental crust
• Geochronology and geochemistry of detrital zircons and rutile and their application to crustal growth models.

Platinum group element geochemistry in granitoids as a fertility indicator for gold and copper mineralization

The principal research interest of the Group is using the platinum group elements (PGE) to distinguish between ore-bearing and barren suites of granitoid intrusions.   Research activities include ultra high precision analyses of the PGE by isotope dilution in clean laboratories, major and trace element analyses, U-Pb dating of zircons by LA-ICP-MS, and O and Hf analyses of zircon.  The research team comprises Ian Campbell and students Helen Cocker, Jessica Lowczak, Hongda Hao and Nicole Eriks.  External collaborators include Jung-Woo Park  (Soule National University), Charlotte Allen (Queensland University of Technology) and Anthony Harris (Newcrest Mining).

The link between copper, copper-gold and felsic rocks is well known.  What is not known is why some felsic suites are ore-bearing whereas other, apparently similar, suites are barren. What is the fundamental difference between barren and fertile granitic systems?  The hypothesis we are testing is that if a magma becomes saturated with an immiscible sulfide melt and precipitates a significant amount of sulfide prior to becoming volatile-saturated, the chalcophile elements (Cu, Au, Pt, Pd etc) are locked in a sulfide phase in an underlying magma chamber where they are unavailable to dissolve in a hydrothermal fluid and form a Cu or Cu-Au deposit.  Alternatively, if the magma becomes volatile-saturated before significant sulfide precipitation occurs, the chalcophile elements remain in the melt and are available to be collected by the potentially ore forming volatile phase.

PGE geochemistry has been used to identify the timing of sulfide saturation in evolving magma systems, in preference to copper and gold, because they are more sensitive indicators of sulfide saturation and less susceptible to later hydrothermal overprinting. Preliminary results show that barren suites are subject to a high rate of sulfide precipitation early in their evolution when compared with the ore-bearing suites.  Furthermore the timing and rate of sulfide precipitation appears to determine not only whether a suite is ore bearing, but also whether the mineralization is Cu, Cu-Au or Cu-Au-Pd.  If our hypothesis is supported by further testing, PGE geochemistry can be used to distinguish barren felsic suites from potentially ore-bearing suites and to assess the tenor of ore-bearing systems.

Figure 1.  Variation in Pd against MgO (as a measure of fractionation) of basalt to dacite samples from the Niuatahi-Motutahi sub-marine volcanoes of the Lau Basin.  The red arrow indicates the point of sulphide saturation and the blue arrow magnetite saturation.