The ore forming potential of calc alkaline systems : a magmatic perspective
The association of porphyry copper style mineralisation with extensive calc alkaline igneous activity is not in doubt. However, the role of the magmatic system in the formation of such deposits is still not fully understood. Extensive works have investigated the associated alterations, ore petrology, fluid compositions and the theoretical models of ore metal transference from magma to hydrothermal fluid. Despite these numerous studies, little or no information on the chemical composition and physical processes that occur in the magmatic system has been forthcoming. Silicate melt inclusions provide a method of directly sampling the volatile and ore metal content of magmatic systems. A study of a copperrich andesite from Hahajima, Japan, demonstrates that though the majority of calc alkaline systems have the potential to form a porphyry copper deposit, the physiochemical conditions of the magma chamber prevent most from doing so. The melt inclusions contain significant amounts of copper (up to 2000 ppm), which exceed the average andesitic melt copper content by several orders of magnitude. Measured average water content of 3.3 wt%, and a CI/H2O ratio of 0.06 from the melt inclusions are typical of those expected for an arc andesite. The analysis of samples from the Morenci and Chino porphyry copper deposits of the SW USA showed that the melt inclusions contained a maximum of c. 700 ppm Cu. By employing a fractional crystallisation model it is possible to demonstrate that melt responsible for the formation of these deposits was not an exotic copper-rich melt. However, both of these deposits displayed trace element characteristics similar to those of Adakites. A study of copper-rich quartz-included biotites from Morenci demonstrated that the parental magma of this system had a small but significant amount of crustal contamination. The copper enrichment in these biotites is the consequence of the mineral's interaction with a copper-carrying exsolved magmatic volatile phase which was in equilibrium with the melt. The quartz phenocrysts from Morenci, which host the melt inclusions and biotites have a complex history of growth and resorption. These events are faithfully recorded by the quartz's zoning. It is possible by the application of existing crystallisation models to demonstrate that during the later stages of the Morenci magma's crystallisation that a magmatic volatile phase was undergoing cycles of undersaturation, saturation and exsolution. This 'pumping' of volatiles was driven by the phenomenon of undercooling. These studies of calc-alkaline magmatic systems and their contribution to the porphyry copper-forming process, serve to provide some ground truths to the current models used to understand their formation and may provide the basis for the development of future exploration tools.