Accessory minerals as fertility indicators in porphyry copper deposit formation

Overview: Porphyry copper deposits (PCDs) represent relatively small stocks that are commonly intruded at the base of arc volcanoes and above large magma bodies that are the main source of fluids, sulphur and metals. Fertile systems reflect the successful passage of magmas up through the crust where key ore forming processes have occurred. Identifying whether the necessary steps for ore formation have occurred is key to determining prospectivity. However, identifying these processes is often difficult because the host rocks for PCDs are often not representative of the conditions at which ore fluids were generated. The source of such fluids is located deeper and is seldom exposed at the erosion level of the deposit. However, certain robust accessory minerals within PCDs are commonly inherited from deeper, ore-forming regions and retain a time-sequential record of these critical processes. These minerals are also known to host inclusions of less robust minerals such as Cu-rich sulphides. Accessory minerals thus act as powerful proxies for mineralising processes.
This project aims to identify key magmatic-hydrothermal processes in the formation of arc-related PCDs. These processes will be investigated using integrated, high precision, in situ measurements of trace element and isotope suites in multiple accessory minerals (zircon, apatite and titanite) and in mineral inclusions. Quantitative characterisation of accessory mineral assemblages and their inclusions will be carried out using Zeiss’ automated Mineralogic Mining software to determine textural/timing relations.
Taal and Pinatubo represent two active arc volcanoes in the Phillippines and there is strong evidence that the latter is underlain by an active porphyry system. These sites will allow a detailed comparison of magmas associated with active mineralised and barren volcanic systems. By contrast, the Criffell pluton in Southern Scotland is a Devonian granite that has been tectonically juxtaposed with volcanic/shallow intrusive units that are associated with copper deposits. The unusual preservation of both deep and shallow parts of this Cu-bearing magma system makes it an ideal area to investigate mineralisation processes in different parts of a magmatic system.