Deposit Model
A deposit model is used as an empirical basis for understanding how an ore deposit formed. There are many types of geological models that are used, and exploration and mining of an ore deposit can help to understand exactly how a deposit formed.
The Thor deposit belongs to a unique collection of silver deposits that are extremely high-grade, and have characteristic small tonnages in contrast to what are classically called "massive sulfide deposits". In British Columbia, other examples include the Dolly Varden Mine, Silvertip Mine in British Columbia, and the Keno Hill in the Yukon. in the These types of deposits are particularly difficult to assess in terms of deposit models.
The image below (Galley et al, 2007) shows a schematic cross-section through a siliclastic-felsic VMS deposit. This model appears to apply to Thor given exploration work to date. The following general observations can be made concerning these types of deposits:
The Thor deposit belongs to a unique collection of silver deposits that are extremely high-grade, and have characteristic small tonnages in contrast to what are classically called "massive sulfide deposits". In British Columbia, other examples include the Dolly Varden Mine, Silvertip Mine in British Columbia, and the Keno Hill in the Yukon. in the These types of deposits are particularly difficult to assess in terms of deposit models.
The image below (Galley et al, 2007) shows a schematic cross-section through a siliclastic-felsic VMS deposit. This model appears to apply to Thor given exploration work to date. The following general observations can be made concerning these types of 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 in world markets.
- Economic ore bodies 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, and 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 Canadian deposit grade.