GEOLOGY 101 — Epithermal deposits, Part 1

This is the first instalment in a new feature in The Northern Mine, which will consist of 2-part articles. In the first part, we will examine the geologic formation of a particular type of deposit. In the second part, which will run in the following week, the economic viability of such a deposit will be assessed.

An epithermal gold deposit is one in which the gold mineralization occurs within 1 to 2 km of surface and is deposited from hot fluids. The fluids are estimated to range in temperature from less than 100C to about 300C and, during the formation of a deposit, can appear at the surface as hot springs, similar to those found in Yellowstone National Park (in northwestern Wyoming, southern Montana and eastern Idaho). The deposits are most often formed in areas of active volcanism around the margins of continents.

Epithermal gold mineralization can be formed from two types of chemically distinct fluids — “low sulphidation” (LS) fluids, which are reduced and have a near-neutral pH (the measure of the concentration of hydrogen ions) and “high sulphidation” (HS) fluids, which are more oxidized and acidic.

LS fluids are a mixture of rainwater that has percolated into the subsurface and magmatic water (derived from a molten rock source deeper in the earth) that has risen toward the surface. Gold is carried in solution and, for LS waters, is deposited when the water approaches the surface and boils.

HS fluids are mainly derived from a magmatic source and deposit gold near the surface when the solution cools or is diluted by mixing with rainwater. The gold in solution may come either directly from the magma source or it may be leached out of the host volcanic rocks as the fluids travel through them.

In both LS and HS models, fluids travel toward the surface via fractures in the rock, and mineralization often occurs within these conduits. LS fluids usually form large cavity-filling veins, or a series of finer veins, called stockworks, that host the gold. The hotter, more acidic HS fluids penetrate farther into the host rock, creating mineralization that may include veins but which is mostly scattered throughout the rock. LS deposits can also contain economic quantities of silver, and minor amounts of lead, zinc and copper, whereas HS systems often produce economic quantities of copper and some silver. Other minerals associated with LS systems are quartz (including chalcedony), carbonate, pyrite, sphalerite and galena, whereas an HS system contains quartz, alunite, pyrite and copper sulphides such as enargite.

Geochemical exploration for these deposits can result in different chemical anomalies, depending on the type of mineralization involved. LS systems tend to be higher in zinc and lead, and lower in copper, with a high silver-to-gold ratio. HS systems can be higher in arsenic and copper with a lower silver-to-gold ratio.

Many countries have epithermal gold deposits, including Japan, Indonesia, Chile and the western U.S., each of which occupies a portion of the “Rim of Fire,” the area of volcanism that rings the Pacific Ocean from Southeast Asia to western South America. Epithermal gold is also found in British Columbia at the Baker mine, in the Toodoggone district, and near the Taseko River.