Element mobility and alteration types in Iron Oxide Copper and Gold (IOCG) systems

IOCG deposits are economically important providing amongst other resources, around 12% of global copper production and 47% of Australian copper production. A number of different genetic models have been proposed for the formation of IOCG deposits including ore systems for which fluids and metals are sourced from igneous bodies (Hauck, 1990; Groves and Vielreicher, 2001; Pollard, 2001) and others where mineralising fluids are non-magmatic. There are two main non-magmatic models. The first suggests that the key heat source is igneous and contact metamorphism drives thermal convection and development of metal rich brines with possible input of metals from the igneous bodies themselves (Haynes et al., 1995; Barton and Johnson, 1996, 2000; Haynes, 2000). The second non-magmatic model suggests that hypersaline brines are produced by metamorphic reactions at depth and the resulting metamorphic brines become metal rich through wall rock interaction as they migrate and possibly mixing with other aqueous phase to form a deposit (Williams, 1994; de Jong et al., 1997; Hitzman, 2000).

A number of alteration type occurrs in IOCG systems including albitization, scapolitization, “red-rock” alteration (calc-sodic), carbonate alteration, potassic alteration, chlorite alteration as described by Barton (2013). Yet the fundamental relationship between the alteration, the mobility of chemical elements and the formation of the deposits is not well known.
We assess metal mobility during different styles of alteration using a mass balance approach comparing suites of well characterised altered rocks of different types to their least altered parent rocks. We aim to identify which styles of alteration can be shown to mobilise metals and therefore constrain potential sources of metals for IOCG ore deposits in metamorphic terranes, with a focus on Olympic and Mt Isa Provinces in Australia.

Preliminary results of mass balance calculations from the Olympic Province show that potential altered source rocks are significantly depleted in Cu relative to their least altered protoliths. The median Cu and Au mass variation values of rocks albitised at variable degrees (Na alteration) are respectively -87% (range -93% to +258%, n=7) and -27% (range -76% to +69%, n = 7) Similarly rocks with variable potassic alteration (K) have a median Cu mass variation of -52% (range -52% to +186%, n=6) and rocks affected by calc-sodic alteration have Au mass change median of -36% (range -36% to +1656%, n = 10). Mass change in the altered rocks is highly variable with both enrichment and depletion occurring within the same alteration styles. Samples affected by carbonate and potassic alteration are enriched in Au, and calc-sodic and carbonate altered rocks are enriched in Cu. Availability of the particular element in the source rock and lithology play presumably a role in these changes of behaviour in element mobility.