by Derek Wilson
Sediment-hosted gold deposits supply a significant amount of the world's gold. Although only the western United States produces gold from such deposits, these structures are also found in China and Peru.
Deposits in Nevada's Carlin trend have produced 750 tonnes of gold; known resources and reserves there are estimated at 2,400 to 3,100 tonnes of gold. Typical deposits contain 1.1 to 24 million tonnes of ore grading between 0.69 and 7.6 grams gold per tonne. Some sediment-hosted gold deposits have grades of up to 20 grams per tonne. At the Carlin trend, more than 93 tonnes of gold have been produced from 8.5 million tonnes of ore.
The deposits have low silver content, with a silver-to-gold ratio of less than 1. Recently discovered deeper deposits, namely the Hardie, contain up to 1.3 million tonnes grading 16 grams gold per tonne.
These gold deposits are exposed near surface, and are mined as open-pit operations. Such mines are generally low-grade and large-tonnage projects. The ore is crushed, piled and treated using heap-leach methods. Heap leaching is a process whereby cyanide-bearing solution is dripped through ore piles, dissolving fine-grained gold out of the rock. The gold-laden cyanide fluid is collected and subjected to further chemical treatment, which precipitates (and concentrates) gold.
Deeper deposits discovered recently along the Carlin trend appear to be hypogene (primary or unaltered concentrations) in nature. If that postulation is correct, then the main sedimentary-hosted gold deposits known today would represent oxidized replacements of hypogene ore. Hypogene deposits, or zones, have been defined at depths greater than 400 metres. Such deposits also contain higher grades (between 6 and 32 grams gold) and contain, in places, more than 10% sulphides. Hypogene deposits are amenable to standard underground mining techniques.
Although these sediment-hosted, disseminated deposits appear to be restricted to the Great Basin of the U.S., there is no reason, geologically speaking, why they couldn't occur elsewhere.
Exploration may have to be directed toward the discovery of these deeper varieties, since the near-surface, oxidized sort may have been subjected to erosion. Exploration should focus on little-deformed carbonaceous sedimentary packages with prominent high-angle faulting and associated alteration (decarbonation, silicification and argillization). The identification of carbonaceous sedimentary rocks is important as these (and faults) provided permeable pathways for ore fluids, as well as chemical traps for gold precipitation.
Regional geochemical surveys for elevated concentrations of arsenic, antimony, barium, mercury and tellurium in carbonaceous rocks are also effective exploration techniques. Geophysical exploration methods would be ineffective, though some fault systems may exhibit a detectable magnetic signature during induced-polarization and resistivity surveys. Exploration for deeper hypogene ore has been conducted on the Carlin trend, particularly near known deposits, through deep drilling.
-- The author is a professor of geology at Memorial University in St. John's, Nfld.