| Thomas Bissig, Kirstie Simpson, Richard Tosdal, David Cooke (CODES) |
| Adam Bath (CODES), Ph.D. Candidate |
| Jacqueline Blackwell (CODES), Ph.D. Candidate |
| Kevin Byrne, M.Sc. Candidate |
| Amber Henry, M.Sc. Candidate |
| Meghan Jackson, M.Sc. Candidate |
| Paul Jago, M.Sc. Candidate |
| Janina Micko, Ph.D. Candidate |
| Heidi Pass (CODES), Ph.D. Candidate |
| Wojciech Zukowski (CODES), Ph.D. Candidate |
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Chalcopyrite-pyrite cement in Cowal breccia, E42 deposiit, Cowal, NSW, Australia. |
Introduction
Alkalic gold-(copper) deposits include some of the world's highest-grade porphyry gold resources (e.g., Ridgeway: 53 Mt @ 2.5 g/t Au, 0.77% Cu – 4.26 Moz Au; Cadia Far East: 290 Mt @ 0.98 g/t Au, 0.36% Cu – 9.13 Moz Au), and some of the largest gold accumulations in epithermal settings (e.g., Lihir, 44.7 Moz Au; also Porgera, Cripple Creek, Emperor, Rosia Montana). The alkalic deposits have features atypical of ‘classic' porphyry and epithermal systems that both allow them to be put into these classes as well as distinguish them from the sub-alkalic systems. In contrast to their more common calc-alkalic cousins, there has been little effort made towards developing a coherent model that integrates the characteristics of various alteration styles that can develop in either a shallow- or deep-level alkalic igneous setting. Recent discoveries have raised awareness of the economic importance of the alkalic class of porphyry and epithermal deposits, and have provided opportunities to better define the characteristics of these somewhat anomalous but potentially very metal-rich mineral systems.
The alkalic environment: porphyry versus epithermal
The best-known examples of alkalic Au-Cu deposits are from Mesozoic arcs of British Columbia and the Late Ordovician Lachlan Fold Belt of New South Wales. Other isolated alkalic systems include Dinkidi (Philippines) and Skouries (Greece). These locally high-grade deposits are associated with small volume pipe-like intrusions that may have aerial extents of only a few hundred square metres, which makes them difficult exploration targets (Cooke et al., in press). The alkalic porphyry systems are not associated with advanced argillic alteration assemblages (possibly excluding Dinkidi), and a connection to high sulfidation (HS) epithermal deposits has not been demonstrated despite the local presence of HS alteration systems in New South Wales that are of the same age as the alkalic porphyry deposits. Phyllic alteration in alkalic porphyry systems is typically restricted to fault zones that penetrate the hydrothermal system. Consequently, supergene enrichment zones will be at best poorly developed due to the low sulfide contents of the alteration assemblages. The incipient nature of the peripheral hypogene alteration assemblages (variants on potassic, calc-potassic, sodic, calc-sodic and propylitic alteration) makes identifying the focus for fluid flow difficult when more than several hundred metres away from the mineralized centre. Other manifestations of hydrothermal activity peripheral to alkalic porphyry deposits are poorly documented and their exploration significance has not been assessed rigorously. Nonetheless, initial research in alkalic porphyry deposits has suggested that a systematic vertical and lateral zonation in sulfur isotopic composition surrounds some mineralized porphyry complexes (e.g., Goonumbla and Cadia, NSW, Lickfold, 2001; Wilson, 2003; Didipio, Philippines, Wolfe, 2001; Mt. Polley BC, Deyell and Tosdal, 2004 ) and mineralogical changes have also been noted (e.g. Sketchley et al., 1995; Wilson et al., 2003). These observations lead to the question of what other subtle variations are present outside the main sulfide concentrations.
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Cadia open pit, New South Wales, Australia, May 2002
Some of the world's largest epithermal gold deposits such as Lihir and Cripple Creek are alkalic in nature. Other significant alkalic epithermal deposits include Porgera, Rosia Montana, Emperor and Cowal. The alkalic epithermal systems have features similar to the “low sulfidation” (LS) family of calc-alkalic epithermal deposits. Discriminating features include the presence of roscoelite (e.g., Porgera, Emperor) and anhydrite (e.g., Lihir, Porgera), and negative sulfur isotopic compositions of sulfide minerals (e.g., Cowal, Lihir). These features are indicative of oxidation states higher than expected for calc-alkalic LS systems, and potentially providing evidence of a greater magmatic contribution to the alkalic mineralizing fluids. In addition, the abundance of carbonate minerals provides an opportunity to examine the use of those peripheral (vertical and laterally) to look for subtle geochemical dispersion halos. As with the calc-alkalic systems, alkalic epithermal deposits are best preserved in younger volcanic arcs, although the alkalic systems occur in association with alkalic igneous rocks, implicating anomalous tectonic processes in their formation.
An empirical relationship has been postulated between alkalic epithermal and porphyry deposits, but remains unproven (Jensen and Barton, 2000; Cooke et al., in press; Fig. 1). With the exception of Cowal, no significant alkalic epithermal deposits are known from the major alkalic porphyry belts of western New South Wales and British Columbia, likely relating to the depth of erosion.
Research themes and sites
The integration of detailed structural, geochemical and geochronological information are essential to the development of an improved alkalic model. The study will investigate the following research themes:
- Structural and geological architecture
- Alteration zonation patterns
- Geochemical dispersion and depletion halos
- Geochronology and igneous fertility indices
The aim of the project is to develop a view of the alkalic systems, from the igneous to hydrothermal environment, including some practical guidelines that may prove helpful for exploration. Of the four themes, understanding the 3-D geometry of the alteration and geochemical zonation is crucial. However, such a study requires a firm understanding of the structural geometry and geologic architecture of each site. Study sites include:
- Galore Creek, B.C.
- Mt. Milligan, B.C.
- Mt. Polley, B.C.
- Lorraine, B.C.
- Cowal, N.S.W.
- Porgera, P.N.G.
- Ladolam, Lihir Island, P.N.G.
References cited
Cooke D.R., Wilson A.J., House M.J., Wolfe R.C., Walshe J.L., Lickfold V., Crawford A.J., (in press) Alkalic porphyry Au-Cu and associated mineral deposits of the Ordovician to Early Silurian Macquarie Arc, NSW: Australian Journal of Earth Sciences.
Deyell, C.L., and Tosdal, R.M., 2005, Alkalic Cu-Au deposits of British Columbia: Sulfur Isotope Zonation as a Guide to Mineral Exploration: British Columbia Ministry of Energy and Mines, Paper 2005-1, p. 191-208.
Jensen, E.P., and Barton, M.D., 2000, Gold deposits related to alkaline magmatism: Reviews in Economic Geology, v. 13, p. 279-314.
Lickfold, V., 2001, Volatile evolution and intrusive history at Goonumbla, central western NSW: Unpublished Ph.D. thesis, University of Tasmania, Australia.
Sketchley, D.A., Rebagliati, C.M., and DeLong, C., 1995, Geology, alteration, and zoning patterns of the Mt. Milligan copper-gold deposits: Canadian Institute of Mining and Metallurgy, v.46, p. 650-665.
Wilson, A., 2003, The genesis and exploration context of porphyry copper-gold deposits in the Cadia district, NSW; Unpublished Ph.D. thesis, University of Tasmania, Australia.
Wilson, A., Cooke, D.R., and Harper, B.L., 2003, The Ridgeway gold-copper deposit: a high-grade alkalic porphyry deposit in the Lachlan Fold Belt, NSW, Australia: Economic Geology, 98, p. 1637-1656.
Wolfe, R.C., 2001, Geology of the Didipio region and paragenesis of the Dinkidi Cu-Au porphyry deposit: Unpublished PhD thesis, University of Tasmania, Australia, 200 p.
Recently completed projects
Sulfur Isotope Zonation as a Guide to Mineral Exploration
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