Approximately two thirds of the world's copper comes from porphyry deposits. These deposits have many distinct attributes, including multiple events in their formation, which affect the reaction series between the hydrothermal fluid and its host rock. This, in turn, defines the alteration and main causes of ore formation. Many porphyry copper deposits are hosted in quartz feldspar porphyries, where the alteration-mineralisation is zoned from barren early sodic-calcic, through potentially ore-grade potassic, chlorite-sericite, and phyllic, to advanced argillic. Chalcopyrite +/- bornite mineralisation in many porphyry copper deposits is largely confined to potassic zones (Sillitoe, 2010). This paper discusses the initial development of a Process Alteration Index (PAI) that focuses on understanding the formation of copper porphyry mineralisation, and supports predictive comminution and metallurgical modelling studies. The PAI can be determined using analytical techniques and focuses on the role of plagioclase and quartz during porphyry deposit formation. Samples from the Highland Valley Copper Mine in British Columbia were selected from a single lithology which displayed a range of degrees of alteration advancing from weak to intense. The samples were analysed using X-ray Diffraction Analysis (XRD) with Rietveld refinement, and the ore hardness determined using the Julius Kruttschnitt Rotary Breakage Tester (JKRBT). A relationship between key alteration parameters which underpin copper ore formation and the ore hardness was observed in the limited dataset available for this research and further work is recommended to determine whether this relationship could be used in predictive mill throughput modelling studies. (C) 2014 Elsevier Ltd. All rights reserved.