Porphyry copper deposits contain disseminated mineralization, meaning that a large volume of shattered rock contains a ramifying network of tiny quartz veins, spaced only a few centimetres apart, in which grains of the copper ore occur with pyrite. The shattered rock serves as a permeable medium for the circulation of a hydrothermal solution, and the volume of rock that is altered and mineralized by the solution can be huge. Porphyry coppers are among the largest of all hydrothermal deposits, with some giant deposits containing many billions of tons of ore. Although in most deposits the ore averages only between 0.5 and 1.5 percent copper by weight, the tonnages of ore mined are so large that more than 50 percent of all copper produced comes from porphyry coppers.
Porphyry deposits contain most of B.C.’s gold resources and are its main source of gold production. Exploration for these polymetallic deposits has produced a database of over 1,400 porphyry-style mineral occurrences. Roughly 60 of these have gold resource estimates. Significant examples include Prosperity, Kemess Underground, Mt. Milligan, Galore Creek, Schaft Creek, Red Chris, New Afton, Mount Polley, Red Chris and KSM. The top 5 measured indicated porphyry resource estimates alone account for over 100 million contained ounces of gold as of 2015.
Historically, the main sources of gold in the province were mesothermal and epithermal veins. Epithermal gold deposits are a type of lode deposit that contain economic concentrations of gold, silver and in some cases base metals including copper, lead and zinc. Gold is the principal commodity of epithermal deposits, and can be found as native gold, or alloyed with silver. As a lode deposit, epithermal deposits are characterized as having minerals either disseminated through the ore-body or contained in a network of veins. Epithermal deposits are distinctive from low-grade bulk tonnage deposits such as porphyries in that they are typically high-grade, small size deposits. A few characteristics distinguish epithermal deposits. These deposits are found near the surface and mineralization occurs at a maximum depth of 1 km, but rarely deeper than 600 metres. These deposits represent a high-grade, easily mineable source of gold.
Lode deposits are considered primary gold deposits because they are bedrock deposits that have not been moved. They come in a range of shapes and sizes and can form tabular cross-cutting vein deposits but also may be breccia zones, irregular replacement bodies, pipes, stockworks, and other shapes.
In B.C., more than 400 veins yielded 14.6 million ounces, including more than 4 million ounces from the Bridge River Camp, 2.44 million ounces from the Rossland Camp, and 2 million ounces from the Premier Camp (epithermal veins). The Snip intrusion-related, shear-hosted vein ore body produced 1.07 million ounces at an average grade of 24 g/t, representing another attractive high-grade target type. Brucejack, a potential producer in northwestern B.C., is a transitional mesothermal to epithermal stock-work and breccia- hosted deposit. The Blackwater Project, part of an emerging camp on the Nechako Plateau, has characteristics of both high and low sulphidation epithermal mineralization.
Volcanogenic Massive Sulphides (VMS)
Massive sulphide deposits are currently forming in undersea locations characterized by “Black Smokers”. These Black Smokers are plumes of sulphide-rich fluids and represent the venting of hydrothermal fluids, rich in base and precious metals, onto the ocean floor. In contrast to other volcanic-hosted deposits, they form thin, laterally extensive pyrite-rich massive sulfide rock. These deposits are notable for their ore concentrations of copper and cobalt and only minor concentrations of zinc.
Although most VMS deposits mined in B.C. have recovered gold and silver as by-products (e.g. Myra Falls, Britannia), the high grade Eskay Creek mine was exceptional, with average grades of 47 g/t gold and 878 g/t silver. There is potential for the discovery of additional deposits like Eskay Creek between the communities of Stewart and Iskut.
Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that intrude into a carbonate layer such as a dolomite or limestone. Skarns can form by regional, or contact metamorphism and therefore form in relatively high temperature environments. If a skarn has a respectable amount of ore mineralization that can be mined for a profit, it can be classified as a skarn deposit.
MINFILE includes over 900 primary skarn occurrences, nearly 400 of which list gold among the commodities of interest. The largest gold skarn producer was Hedley (Nickel Plate) at 2.5 million ounces. The current producing Yellow Giant Camp includes skarn mineralization.
How Much Gold Is Remaining?
The British Columbia Geological Survey’s MINFILE database lists over 3,380 occurrences for which gold is identified as the primary commodity. Of these, approximately 700 have recorded gold production (400 lode producers and 300 placer).
Between 1858 and 2013, about 32 million ounces were produced from lode deposits and 6 million ounces from placers.
Remaining in-ground gold resources in the province are estimated at 280 million ounces.