The ore forming potential of calc alkaline systems: a magmatic perspective

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Description/Abstract

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.