Apatite in porphyry systems and its applications in mineral exploration

Abstract

New techniques in exploration geochemistry are required to reduce the increasing amount of time and resources consumed during exploration of porphyry deposits. This can be achieved through developing our understanding of the processes that result in the formation of mineralised systems, and any associated geochemical signatures which could be recorded in mineral indicators. Apatite chemistry can provide a detailed perspective into porphyry system formation. However, the diversity of apatite occurrence and composition, in addition to its susceptibility to metasomatism, presents added complexity in its application as a mineral indicator. Here the La Granja Cu-Mo porphyry was used as a case study in which the occurrence and composition of apatite from porphyry systems could be investigated. The composition of hydrothermal apatite varies greatly between those from least fractionated, magmatic-derived fluids and distinct sericitic-associated apatite. Compositions of magmatic apatite closely reflect the magma evolution including melt differentiation and the behaviour of magmatic volatiles. Apatite inclusions in zircon provide a means to examine the halogen and sulfur content of a melt. At La Granja halogen ratios, together with additional textural and zircon geochemistry evidence, indicate that the late-stage porphyries crystallised rapidly from fractionated, volatile-rich melts, which alludes to the abrupt termination of the system. The highest concentrations of sulfur were observed within apatite inclusions from the mineralised intrusions. The implied increase in sulfur within the melt occurs after the start of porphyry emplacement and may be an important mechanism in the development of mineralisation. Using the conclusions from this study as a framework it has been possible to develop a series of multivariate discrimination tools effective for exploration. These results demonstrate that studies of apatite chemistry can not only help increase our understanding of porphyry formation, but with careful application can be utilised to aid the exploration of mineralised porphyry systems.