The influence of metal mobility on resource potential in circumneutral pH iron-rich copper mine waste rocks

Abstract

Waste rocks from iron-oxide copper-gold (IOCG) deposits are dominated by Fe-bearing silicates and oxides, with only minor sulphides and potentially no pyrite. Hence, waste rock piles typically have circumneutral pH. However, oxidation of minor sulphides and Fe-bearing minerals can cause localised acidification at the mineral grain scale, especially where Fe oxidation and ferrihydrite precipitation is facilitated by bacteria. This study characterises the general biogeochemical system of waste rock piles at the Salobo IOCG mine, Brazil using a combination of biotic and abiotic leaching experiments on typical, weakly mineralised rocks, and observations of mineralogical changes and associated metal mobility in incipiently weathered, Cu-bearing overburden. Biotic experiments used a strain of the Fe- and S-oxidising Acidithiobacillus ferrooxidans subsp. cultured from the Salobo mine site. Most of the acid generated during oxidation of Salobo rocks is neutralised by alteration of labile Fe-silicates, particularly biotite, chlorite, olivine and amphiboles. Copper in our laboratory experiments was mobilised into solution (up to 1000 mg/l) from disseminated Cu-sulphide minerals during this acidification, and re-precipitated nearby (on the m-scale) within the rock mass during neutralisation. Much of this Cu precipitated with the abundant ferrihydrite, with some Cu precipitating locally with Mn-oxides, suggesting that secondary ferrihydrite and Mn-oxides represent possible exploration targets associated with comparable near-surface, weathered targets. Primary microparticulate gold was found encapsulated in magnetite and Cu-sulphides, with the sulphide-bearing portion of gold liberated during oxidation. Magnetite only demonstrated minor alteration (μm-scale) during short-term acidic oxidation (months). All these alteration processes are expected to occur within the waste piles during storage at the site, and could enhance the metallurgical leachability of these low-grade rocks for future re-processing. In the meantime, the lack of large-scale acidification, and adsorption of mobilised trace metals to the abundant ferrihydrite will ensure that the waste rocks remain environmentally benign.