High-sulphidation (HS) gold, silver and/or copper deposits are generated in both the epithermal and the upper parts of the underlying porphyry environments over vertical intervals of up to 2 km. The HS deposits are generated in advanced argillic lithocaps, which are products of the absorption of acidic magmatic volatiles by voluminous groundwater systems. Mineralisation styles in HS systems reflect depth of formation as well as the interplay between structural, lithological and hydrothermal parameters. The deep parts of HS systems, at depths of >1000 m, are typified by disseminated coppergold mineralisation comprising digenite, chalcocite and covellite in pervasive advanced argillic as well as underlying sericitic alteration. In highly telescoped systems, such
mineralisation may overprint porphyry stocks and associated
quartz-veinlet stockworks. Intermediate levels of HS systems commonly
contain fault-controlled copper-gold mineralisation, typically as enargite in bodies of vuggy residual quartz, silicification and/or massive pyritic sulphide. The shallow parts of HS systems, at depths of <500 m, may host lithologically controlled disseminated mineralisation in which gold and or silver tend to predominate over copper. barren acid-leached zones formed in the steam-heated environment above paleo-water tables may be preserved above or alongside shallow hs deposits.>
The exploration focus is on four principal HS mineralisation styles:
1. copper (eg Chuquicamata, Monywa) or copper-gold (Wafi) in the
deep porphyry-hosted parts of systems preferably, in the case of the
latter, where supergene oxidation is limited and, hence, flotation
may be used for metal recovery,
2. coppergold-bearing replacement mantos and pipes hosted by
carbonate wallrocks in the deep parts of systems (eg Bisbee),
3. high-grade gold in late-stage veins or hydrothermal breccias that
overprint the intermediate to shallow levels of systems (eg El Indio,
Goldfield), and
4. large, bulk-mineable gold deposits in the shallow parts of systems
that were subjected to supergene oxidation, thereby permitting
heap-leach treatment (eg Yanacocha, Pierina).
To these preferred HS styles may be added the low-sulphidation vein
or disseminated goldsilver mineralisation that is commonplace
alongside many HS systems (eg Victoria at Lepanto).
* Originally published in the Proceedings of PACRIM 99,
published by The AusIMM.