GEOLOGY 101 — Carlin-type gold deposits, Part 1

Sediment-hosted disseminated gold deposits consist of fine-grained gold in silty carbonaceous sedimentary rocks. These deposits occur in the Great Basin of the southwestern U.S.

Regions in which this sort of deposit occurs include the Carlin trend, a 60-km-long belt hosting numerous deposits, and the Getchell trend, which extends for 50 km. The Great Basin is a physiographic province on which rests most of Nevada, portions of Utah, Idaho and Oregon, and a small portion of eastern California.

These types of deposits are also known as “Carlin-type” deposits, and occur chiefly in northern Nevada. Production from those formations began in the early 1960s.

The host rocks are predominantly thinly bedded, silty carbonaceous rocks (dolomites and limestones) or shales, though host material at some deposits includes lesser amounts of siliceous rock (silica), intrusive igneous rocks and siliceous breccias.

The gold occurs in arsenic-bearing pyrite and quartz. Gold grains are micron (0.001 mm) to submicron is size. At the Carlin deposit, coarse gold grains measuring up to 0.5 mm in dimension were found in early exploration.

Typically, less than 1% fine-grained sulphides are present in the ore zones.

These sulphides are generally pyrite, though orpiment (As2S3) and realgar (As4S4) may also be present. No other base metal sulphide minerals exist in these sorts of deposits and, other than arsenic (As), elevated concentrations of antimony, mercury, barium and thallium exist. The gold-bearing host rocks are typically strongly altered, with the main types of alteration being decarbonitazation, silicification and argillization.

Decarbonitization is frequently best-developed on a deposit’s furthest boundary, and represents the extraction of carbonate material from the host rock. Silicification is next-closest to the ore, and represents the replacement of the host rock by silica. In places, up to 95% of the rock can be replaced by silica. Such silica-rich rocks are called jasperoid.

Argillization involves development of hydrothermal clay minerals such as montmorillinite and kaolinite, as well as the sericitization of feldspars.

The alteration zonation is not always fully developed, and gold can be present in silicified zones and decarbonitized rocks.

The deposits are closely associated with steep (high-angle) normal faults and permeable horizons (the porous medium through which fluids can flow) in the package of sedimentary host rocks. The deposits formed when hydrothermal fluids flowed along faults until they encountered breccia zones and/or permeable horizons. The fluids then reacted with country rock, producing the alteration and depositing the gold. The process is essentially a selective replacement of carbonaceous rock by silica, pyrite and gold. Jasperoid zones can extend for up to 30 metres from a fault.

These deposits have been described as a type of epithermal gold deposit (see “Geology 101,” T.N.M., April 14/97) that formed through the circulation of hydrothermal fluids near the earth’s surface. Sediment-hosted disseminated gold deposits are now recognized as a distinct group of deposits with greater formational depths (1.5 to 4 km) and higher formational temperatures (greater than 225C).

Current models suggest that these deposits formed from the circulation of meteoric, or atmospheric, water through basement rock. As a result, these deposits exhibit similarities to mesothermal gold deposits (see “Geology 101,” T.N.M., July 28/97). Fluid movement and circulation were apparently related to large-scale tectonic processes during the deposition of gold in the Great Basin region through a combination of mixing, cooling and oxidation of fluids.

— The author is a professor of geology at Memorial University in St.

John’s, Nfld.