Hannington et al (1996).
What Are VHMS Deposits
VHMS deposits are a globally significant deposit class, exhibiting some of the highest grades of any copper-rich deposit style. These deposits currently account for 22% of zinc, 9.7% of lead, 6% of copper, 8.7% of silver and 2.2% of gold mined globally. They are economically attractive for underground mining, enabling a minimised environmental impact, and commonly occur in camps or districts of numerous deposits.
Aguila is exploring the Sherridon VHMS District in Manitoba, Canada.
Province and District Scale
VHMS deposits form at or near the seafloor in submarine volcanic environments, where metal-enriched hydrothermal fluids associated with seafloor hydrothermal convection discharge at or near the seafloor. Deposits commonly have two parts: a strata-bound tabular body composed of massive sulphide and a second aspect composed of stockwork veins and disseminated sulphides. Host rocks can be either volcanic or sedimentary and they commonly form in clusters.
Gibson et al. (2007).
Because of the mechanism of their formation, VHMS-style occurrences regularly occur as districts or “camps”.These camps are characterised by often more than 50 deposits in a range of geological settings, but all similar enough to enable processing through one centralized mill.
Deposit size often varies widely, with small deposits providing the guide to the largest ones that may lie at depth. Footwall stringer deposits often show higher Cu/Zn ratios than massive sulphide bodies, while structurally remobilized loads may achieve very high grades in a range of geological settings.
There are several ways in which to categorize and classify VHMS deposits. One such way is how gold-rich a deposit is. The Flin Flon mining camp deposits contains appreciable gold, but the deposits are generally considered to be more base metal rich. High gold content may relate to water depth, oxidation sate, fluid temperatures, and magmatic contributions.
Hannington et al. (1999).
Deposits can also be classified based on their associated lithology, with 5 commonly referenced types:
- Mafic-dominated (e.g., Newfoundland Appalachians)
- Siliciclastic-mafic (e.g., Windy Craggy deposit)
- Bimodal-mafic (e.g., Noranda, Flin Flon-Snow Lake and Kidd Creek camps)
- Bimodal-felsic (e.g., Buchans deposits)
- Siliciclastic-felsic (e.g., Bathurst camp)
Barrie and Hannington (1999).
VHMS deposits always form in extensional tectonic environments. The tectonic environment of the Flin Flon deposits including Sherridon, has shown to be juvenile oceanic arc, determined by geochemistry and Nd isotope ratios. Bimodal mafic-dominated volcanism was apparently followed by non-arc (extensional) mafic intrusion. (Zwanzig 2010).
Galley et al. (2007).
The size and morphology of the alteration system is a reflection of VHMS deposit clusters. At the deposit scale, proximal alteration zones may form a halo up to twice the diameter of the massive sulphide lens. A Fe-chlorite-quartz-sulphide±sericite±talc mineral assemblage is commonly associated with the core of stockwork vein mineralization, which becomes increasingly quartz and sulphide-rich towards the lower contact of the massive sulphide lens. In some cases talc and/or magnetite occur at the base of the massive sulphide lens and the top of the alteration pipe.
In the case of the Flin Flon deposits, distal, low-temperature hydrothermal alteration assemblages associated with VHMS may be difficult to distinguish from regional greenschist-facies metamorphic mineral assemblages. When both proximal and regional semi conformable alteration zones are affected by amphibolite grade regional or contact metamorphism, the originally strongly hydrated alteration mineral assemblages change into a coarse-grained quartz-phyllosilicate-aluminosilicate.
Mineral Exploration Strategies
Exploration methods for VHMS deposits are well established, but vary based on depth of overlying cover, ore body geometry and host rock setting. Once within a camp or district, age-equivalent horizons are often sought, as it is recognised the mineralizing systems were active at this paleo-age.
Alteration, chemistry and mineral zonation can provide predictable indictors towards ore. Aguila is utilizing lithogeochemical interpretation workflows to assess these alteration signatures and generate new exploration targets.
Galley et al. (1993).
Geophysical methods commonly include induced polarization (IP), magnetic surveys, and electromagnetic methods (VTEM). Massive sulphide deposits are often directly detectable by geophysics, and may be associated with sulphide-bearing alteration that provides a larger footprint target.
Trans Hudson Orogeny (THO)
The THO is a Paleoproterozoic (2.45 Ga to 1.83 Ga) belt of magmatism and a major volcanic and mountain building event that extends from present-day western Nebraska/eastern Wyoming, northward and eastward through Saskatchewan and Manitoba to Labrador and into Greenland and Scandinavia. The Himalayas can be considered a recent analogue to this belt.
The THO is host to several notable deposits including lode gold and base metal deposits from S. Dakota (Homestake Mine), Saskatchewan and Manitoba (Flin Flon/Snow Lake camp, Lynn Lake), Baffin Island lode gold, west Greenland VHMS and the Bergslagen VHMS district of Sweden.
Corrigan et al. (2007).