Volcanogenic Hosted Massive Sulphide ("VHMS") deposits are base metal-rich mineral deposits, which can also contain lesser amounts of precious metals. Their ores can be major sources of zinc, copper, and lead, with gold and silver byproducts.
VHMS deposits are found worldwide, and often form in clusters, or camps. Several major VHMS camps are known in Canada, these include the Flin Flon - Snow Lake, Bathurst and Noranda camps. These high-grade deposits are often in the range of 5 to 20 Mt, but can be considerably larger. Some of the largest VHMS deposits in Canada include the Flin Flon mine (62 Mt), the Kidd Creek mine (+100 Mt) and the Bathurst No. 12 mine (+100 Mt).
VHMS deposits consist of massive or semi-massive accumulations of sulphide minerals which form in lens-like or tabular bodies parallel to stratigraphy or bedding. VHMS deposits form on, or below, the ocean floor and are typically associated with volcanic and/or sedimentary rocks. Characteristics of well-preserved VHMS deposits include the presence of concordant lenses of massive and semi-massive sulphides which have been exhaled into the ocean as metal-rich brines from black and white smokers, or chimneys. These sulphide zones can overlie discordant (typically copper +/- gold rich) stockworks and/or alteration zones which form below the seafloor.
Over the past 30 years various classification schemes have been proposed for VHMS deposits, based on a variety of criteria (cf. Franklin et al. (1981) and Lydon (1984)). More recently, VHMS deposits have been classified by Barrie and Hannington (1999) and Franklin et al. (2005) based on setting and host rock association into five subclasses, consisting of:
- Bimodal-mafic: hosted by mixed volcanic sequences, typically with greater abundances of mafic than felsic volcanics. Mineralization is often associated with the felsic strata. Examples include the Noranda, Flin Flon-Snow Lake and Kidd Creek camps (Canada). McIlvenna Bay is part of the bimodal-mafic subclass of VHMS deposits.
- Mafic Associated: hosted by mafic volcanic rocks (commonly ophiolites) dominant. Examples include the Cyprus deposits (Oman) and those in the Newfoundland Appalachians (Canada).
- Mafic-siliciclastic: hosted within sequences of mafic volcanic and siliciclastic rocks; felsic rocks can be a minor component; and mafic (and ultramafic) intrusive rocks are common. Examples include the Besshi camp (Japan) and the world-class Windy Craggy deposit (Canada).
- Felsic-siliciclastic: hosted within siliciclastic sediment-dominated settings with abundant felsic volcanics and minor mafics. Examples include the Bathurst camp (Canada) and the Iberian Pyrite Belt (Spain and Portugal).
- Bimodal-felsic: hosted within bimodal volcanic sequences, with greater abundances of felsic than mafic rocks, and minor sediments. Examples include the Kuroko deposits (Japan) and the Buchans deposits (Canada).
VHMS deposits often have a strong metal zonation, this is seen as the segregation of various metal-bearing sulphides throughout a deposit. In general, copper sulphide (chalcopyrite) forms in the central (or higher temperature) parts of the deposit, such as the stockwork and vent-proximal sulphide lenses. Gold concentrations can often be highest in these copper-rich zones. In contrast, zinc and lead sulphides (sphalerite and galena) form in the more distal (or lower temperature) parts of the deposit further away from the vent. Silver is more commonly associated with the zinc- and/or lead-rich parts of the deposit. Generally non-economic iron sulphides (pyrite and pyrrhotite) occur with the base metal sulphides. The iron-sulphides can also be zoned, typically with pyrrhotite associated with zones of more copper-rich mineralization and pyrite associated with zones of more zinc- and lead-rich mineralization.
VHMS mineralization tends to form along a specific favourable horizon within a volcanic, volcaniclastic or sedimentary sequence. Long-lived systems can have several periods of VHMS formation at different favourable horizons. Identification of these favourable horizons can be a significant property- or regional-scale vector for the discovery of new zones of mineralization.
VHMS deposits typically form during periods of rifting, within volcanic arcs, fore arcs, and in extensional back arc basins. Rifting causes extension and thinning of the crust, allowing hot, subvolcanic magmas to rise and form metal-bearing hydrothermal cells that ultimately produce the VHMS mineralization. The localization of lenses of massive and semi-massive sulphides are often controlled by synvolcanic faults and fissures, which permit a focused discharge of hydrothermal fluids on the seafloor. This focused discharge of the metal-bearing hydrothermal fluids, as well as flow and subsequent ponding within paleo-topographic lows, allows for the economic concentration of sulphides in these deposits.