The Application of Apatite for the Study of Magmatic to Hydrothermal Processes in Porphyry Copper Systems

Porphyry copper deposits (PCDs) are magmatic-hydrothermal orebodies that supply 70% of the worlds Cu. Recently, PCD exploration has been pushed towards ‘blind’ targets, not exposed at the surface. Such an approach requires tools capable of determining an intrusive complexes’ proclivity for hosting mineralisation, termed ‘fertility’. Indicator minerals represent one such tool as they may obtain a unique geochemical signature when derived from fertile magmas.

To form a PCD, exsolved magmatic H2O must partition significant quantities of S and Cl, which represent key ore forming constituents and increase the solubility of Cu. Conveniently, apatite can partition Cl, S and H2O, potentially providing a record of their behaviour in PCDs. Therefore, apatite shows great promise as an indicator mineral.

In this study, we carry out e-beam and mass spectrometry techniques on apatite from PCDs and analogous volcanic systems of the West Luzon Arc, Philippines.

We demonstrate that the evolution of apatite F-Cl-OH is unique during H2O-saturated crystallisation. Furthermore, high-S concentrations also suggest S-fluxing in oxidised and fertile crystal mush reservoirs. These processes are identified ubiquitously in zircon hosted apatites from PCDs. Moreover, changes in the volatile composition of apatite from pre to syn-mineralisation magmas in West Luzon may also demonstrate enhanced Cl-extraction from melt crystallising at greater depths. Apatites not included within zircon predominantly undergo hydrothermal dissolution-reprecipitation. The F, LREE and Mn content of such apatites strongly correlates with whole rock alteration assemblage, proving to be a robust predictor of ore-bearing potassic and sericitic alteration.

These findings suggest that apatite is a powerful tool for tracking the magmatic and tectonic processes pertinent to the enhancement of porphyry fertility. By contrast, in systems where apatite has been hydrothermally altered, its composition may provide a useful indicator of ore-associated alteration in detrital sediments.