The image below (Galley et al, 2007) shows a schematic cross-section through a siliclastic-felsic VMS deposit. This model applies extremely well to Thor, and the following general observations can be made concerning these deposits:
- They are hosted by submarine volcanic and sedimentary rocks deposited in extensional tectonic settings.
- VMS deposits form from metal-enriched fluids associated with seafloor hydrothermal convection ("black smokers"). Therefore, they are the same age as the host rocks.
- They are a major source of Zn, Cu, Pb, Ag and Au.
- Economic orebodies range from 200 000 tonnes to more than 150 million tonnes in size.
- Typical grades are on the order of 2 to 5% Zn, 1 to 2% Cu, 1 to 2% Pb, 30 to 60 g/t Ag and 1 to 2 g/t Au (Galley et al. 2007).
- Principal ore minerals are sphalerite, galena, chalcopyrite, pyrite (pyrrhotite).
- Stringer-stockwork zones commonly underlie massive sulfides, and may also be a significant source of ore.
- Ore metals can be vertically zoned from iron and copper sulfides at the base of an ore lens through to lead and zinc sulfides on the periphery. Some ore lenses carry significant barite with or above the Pb-Zn sulfides.
- Massive sulfides can grade laterally into distal exhalites characterized by cryptocrystalline quartz, iron oxides, jasper, manganese oxide and elevated (but usually non-economic) metal concentrations.
- VMS deposits occur above extensive footwall alteration zones that form by hydrolysis of feldspar. Primary alteration minerals include sericite, quartz, pyrite, and chlorite. In systems with highly acid fluids, kaolinite, pyrophyllite and even dickite may occur. These minerals are zoned in a systematic fashion from zones of high fluid flux outwards into less-altered host rocks. In metamorphosed VMS deposits, aluminous alteration minerals metamorphose to cordierite, andalusite, or kyanite.
- The geometry of the footwall alteration zone depends on the competency of the host rocks. In sequences dominated by flows and domes, fluid flow is focused by sub-vertical synvolcanic faults, and the alteration zones are pipe-like. In contrast, strata bound alteration (mineralized) zones are more commonly developed in permeable rocks such as tuffs, breccias and sediments, particularly under impermeable cap-rocks such as sills (Gifkins et al., 2005).
Of particular note here is the overage grade of the deposit for Canadian deposits - this is almost exactly the same grade as the Thor deposit, with the exception of silver which is significantly higher than the average grade for Canadian deposits.