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October 6, 1997
Placer gold deposits, Pt. 1,
by DEREK WILTON

Placer gold deposits form as a result of the breakdown and weathering of existing gold concentrations, erosion of the weathered material and, ultimately, the concentration of that material at a variable distance from its source.

The term "placer" derives from the Spanish word for sand bank or stream eddy. Placer deposits, in the strictest sense, are formed in river systems, but the term is typically used to describe deposits formed in glacial and beach environments.

Placer gold deposits are formed when gold is carried from its source to its site of deposition and concentration by a surface erosional force such as rivers, glaciers, oceans and (rarely) wind.

The formation of gold placers is predicated by two fundamental physical properties of gold. To begin with, gold is dense, with a specific gravity of 19.3 grams per cubic cm -- among the highest for all known minerals or native elements. Also, gold is a native element rather than a mineral (the latter being a naturally occurring inorganic chemical compound), and does not readily react with other elements.

A corollary of this second point is that gold is difficult to dissolve out of rock or minerals. The original source of the gold is unimportant, ranging, as it does, from mesothermal lode deposits to massive sulphide deposits to disseminated sulphides in bedrock to pre-existing placer systems. Placer deposits depend on an original pre-concentration of gold which can be liberated through weathering. Eluvial, or residual, placers are a type of placer deposit in which gold has undergone little transport and actually formed on, or near, the original source through the weathering or erosion of host rock. Owing to its relative chemical inertness, gold remains behind while the surrounding material is removed, essentially concentrating the gold in the weathered remnants.

Gold in placer systems is transported as discreet grains as a result of the metal's inertness. Such grains are said to be detrital, as they are derived from the physical weathering and breakdown of their host rock or mineral, a process known as detrition.Because of the high density of gold grains relative to other rock and mineral material (detritus) carried in the same erosional system, the gold grains must be transported by erosional agents operating with relatively higher energy than that needed to transport normal rock detritus. When the energy exerted by the erosional agent decreases, the gold and other dense detritus will stop moving.

In the case of fluvial (or river) placer systems, detrital gold grains are concentrated in those areas where the current of the stream slows, such as on the slow sides of bends in the river, on the downstream sides of islands or near sand bars. Gold grains move when energy is exerted on them by the transporting medium. The grains will continue to move until the medium loses sufficient energy, whereupon the gold grains will settle out of the transporting medium.

An example of a fluvial placer gold deposit is a mature stream in a valley floor into which numerous subsidiary streams flow. In glacial tills, gold is transported along with other detrital material until the glacier ceases to move, dropping the gold and detritus. The driving mechanism for the formation of placer deposits, therefore, is gravity.

Another innate feature of placer gold deposits is that the material that hosts the gold is unconsolidated sediment (particulate rock that is not cemented together). The host sediment can range from gravels to sand in fluvial systems, as well as to various types of till in glacial deposits or beach sands.

A "pay streak" is the layer of sediment in a placer deposit which is enriched in particulate gold. In fluvial examples, the pay streak frequently occurs in sediments that lie directly on top of bedrock. The pay streak will also contain other dense, hard or inert minerals, such as magnetite, zircon, garnet or chromite.

There is some debate as to whether nuggets in fluvial systems represent purely detrital fragments that were rounded in transport or are the nuclei upon which dissolved gold in the stream precipitated and grew. In some instances, gold grains have greater fineness (ratio of gold to silver) towards the rim, suggesting either preferential removal of silver or precipitation of new gold.

-- The author is a geology professor at Memorial University in St. John's, Nfld.

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October 13, 1997
Placer gold deposits, Pt. 2
by Derek Wilton

Although placer gold deposits have yielded less than 10% of Canada's gold production, their discovery played an important role in attracting settlers to remote areas of the country.

For example, the Cariboo gold rush of the 1860s led many people to the interior of British Columbia, and the Klondike gold rush of 1897-98 did the same for the Yukon. Placer gold deposits were attractive to those early settlers because of their simplicity -- they occur when ore from a bedrock source is milled and concentrated, by natural forces, in the pay streak. Because of the unconsolidated nature of the host sediment, placer gold can be separated easily through simple techniques.

Gold, including nuggets, was first recovered from placer sources thousands of years ago by sifting through sand or gravel horizons. More sophisticated techniques, based on gravity separation and gold's higher density, were developed later.

In Greek mythology, the Golden Fleece sought by Jason and the Argonauts was actually a form of ancient sluice for the separation of detrital gold grains from river gravels; gold-bearing river gravels were flooded over sheep skins, and gold grains were entrapped in the wool.

Placer gold deposits, because of their variable grades and tonnages, are difficult to develop into large commercial operations. Conversely, the ease with which gold can be collected from the sediment makes placer deposits unusual in that individual prospectors with pans can still recover economic concentrations. Gold placers are mined in Siberia, Australia, Colombia and other areas around the world. Placer production in Canada and the U.S., however, has been curtailed somewhat as a result of stricter environmental regulations.

The most typical means of placer production is mining the sediment containing the pay streak and using gravity processing to collect the gold. The sorting usually involves flushing sediment over a separator table (mechanical trap), which collects the gold. These techniques use a considerable amount of water. The mining of placer deposits occurs mainly on surface, but, in the case of deep pay streaks, shafts are sunk through sediment accumulations. With respect to fluvial systems, mining essentially digs up the stream bed. Gold grains can form a plastic mixture, called amalgam, with mercury, which is in liquid form at room temperature. At some deposits, pay streak material can be passed through mercury baths, which removes gold particles. The amalgam is then collected and the mercury driven off, leaving the gold behind. Cyanide solutions can also be percolated through placer gravels to collect gold.

Aside from primary production of gold, placers can also point the way to bedrock gold sources, which is what happened in the California gold rush of 1849.

By following fluvial placers back to their source, prospectors were able to locate richer bedrock mesothermal gold deposits. This practice has become more important in light of environmental concerns with respect to large-scale production from placer systems.

Because gold is a soft metal, the shape and size of gold found in placer deposits can vary. Detrital gold grains become more rounded the farther they travel from the source. For example, eluvial gold grains may take the form of wires reflecting the crystal or intergrowth shapes of the source gold. Abrasion and internal grinding are intrinsic to an erosional system, thus the gold grains will be worked into rounded, nodular shapes as they travel greater distances.

The revelations surrounding the scandal at the Busang deposit of Bre-X Minerals in Indonesia illustrates this principle -- the gold added to core there had the rounded, nugget shape of placer gold, which was reportedly collected from a fluvial placer system. Gold grains from a bedrock source would have had the mineralogical or crystal shapes typical of intercrystalline formation.

In Ontario, Quebec and Newfoundland, geologists have been able to locate bedrock sources of gold by following patterns in the shape of till trains, coupled with determinations of the direction of ice flow in the till. -- The author is a professor of geology at Memorial University in St. John's, Nfld.

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January 12, 1998
Paleoplacer gold deposits, Pt. 1
by Derek Wilton

Paleoplacer deposits consist of placer concentrations of minerals in which the host material is a consolidated rock (sediment comprising weathered detritus that was subsequently cemented together). The prefix "paleo" simply means "ancient."

Gold and uranium are the only commodities mined from paleoplacer deposits. Both commodities can be mined from a single deposit, but often only one or the other is present.

Paleoplacer gold deposits have been mined in the Witwatersrand district of South Africa, the Tarkwaian system of Ghana and in the Jacobina mine of east-central Brazil.

Paleoplacer uranium has been mined at Elliot Lake, Ont., and extracted from the gold deposits of the Witwatersrand district. There is a strong temporal control on paleoplacer uranium occurrences, as these occur only in rocks more than 2.5 billion years old. Gold-bearing paleoplacers are predominantly Archean-aged, but have been mined in rocks as young as 2.1 billion years. Gold in paleoplacer deposits is present as discrete grains. Uranium occurs as uraninite (UO2-U3O8). Like gold, uraninite is a dense mineral with a high specific gravity (6.5-10 grams per cubic centimeter) compared with common detrital minerals. Uraninite is unstable in oxygen-bearing surface waters, and its presence as detrital grains suggests that the earth's early atmosphere was oxygen poor. Some researchers, however, suggest that gold and uranium may be at least partly composed of hydrothermal fluid introduced along faults that bound depositional basins.

The host rock in paleoplacer deposits is quartz pebble conglomorate, a rock containing rounded grains of pure quartz up to 32 mm in diameter. The well-rounded nature and relatively equivalent size of the pebbles defines the host sediment as mature. As such, the particles have been subjected to prolonged agitation in an erosional environment.

This type of sediment forms in a regime of intense weathering and corrosion, wherein quartz is the only common rock fragment to survive, owing to its hardness and resistivity to chemical weathering.

Other minerals are locally concentrated with gold and uraninite. As is the case with a placer deposit, these minerals are dense, hard and/or resistant to chemical alteration. Such minerals include pyrite (in paleoplacer deposits fewer than 2.5 billion years old), platinum group metals, chromite, zircon and arsenopyrite. These minerals are intergranular to the quartz pebbles.

The host rock of a paleoplacer deposit can be composed of up to 3% pyrite. Such rocks are often referred to as pyritic quartz pebble conglomorates. Owing to the differences in their ages, the host rocks of

Witwatersrand-Brazil and Ghana gold ores have subtle compositional differences. The oldest rocks, those found in Witwatersrand and Brazil, are pyritic. The younger rocks of Ghana are hematitic, further reflecting the presence of oxygen in the atmosphere. Uranium does not occur in these younger rocks.

Coal-like layers of organic matter (kerogen) are closely associated with some ore-bearing conglomorate horizons. Gold and uranium are locally concentrated in these organic layers, which are either the remnants of algal mats or the products of later hydrocarbon migration. According to some authors, this organic matter could represent paleo-angal mats. Should that analyses prove accurate, then the mats trapped gold and uranium in either of two ways: physically (from gold and uranium detritus) or chemically (from gold and uranium dissolved in stream waters).

Gold (and uranium) is concentrated in paleoplacer deposits much as it is in placer deposits, in paystreak-like concentrations. The paystreaks are thin sheets of quartz pebble conglomerate interlayered with thicker beds of sedimentary rocks.

In the Witwatersrand deposits, paystreaks can extend for up to 10 km, but are usually less than 3 metres thick. As such, these paystreaks resemble those found in river (fluvial) sediments of modern-day placer deposits. Overall, the host sedimentary rocks were deposited in high-energy fluvial conditions, such as in modern-day braided streams that flow from mountainous regions into alluvial plains. The Witwatersrand rocks are fan delta-like sedimentary horizons deposited at the base of hills from which erosion took place.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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January 19, 1998
Paleoplacer gold deposits, Pt. 2
by Derek Wilton

Paleoplacer gold and uranium deposits are generally mined as underground operations, as the hard-rock host material is usually deep beneath the earth's surface.

The gold mines in South Africa's Witwatersrand region have reached depths of 4 km, and are the deepest operations in the world. The deepening of these mines increases production costs, however, and is part of the reason that gold production in South Africa dropped to 500 tonnes in 1997 from 1,000 tonnes per year in the 1970s.

Ore from paleoplacers is crushed, and sometimes leached, in order to extract gold. These techniques differ from those used at placer operations, where the gold, which is contained in unconsolidated host rock, is won through gravity processing techniques.

Average grades at the Witwatersrand deposits are about 9.2 grams gold per tonne, but have been as high as 19.4 grams. Tonnages are in the order of 4 billion tonnes.

Paleoplacers have been extremely important in terms of the world's gold and uranium resources. Before the explosion of interest in gold deposits in the 1980s, paleoplacer deposits accounted for 75% of the world's gold resources and up to 50% of its uranium resources. More than 42,500 tonnes of gold have been extracted from the Witwatersrand district since mining began there in 1886. The paleoplacers near Elliot Lake, Ont., produced in excess of 140,000 tonnes of uranium, whereas South African paleoplacers have produced more than 130,000 tonnes of uranium.

These deposits form through a subtle interplay between tectonic forces and paleoenvironmental conditions. The sedimentary host rocks form on erosional surfaces that have developed on old rocks. Paleoplacers form in a high-energy fluvial (river) system. Dense detrital gold and uranium grains are deposited when the river flow is no longer fast enough to keep them in motion. Exploration efforts for paleoplacer deposits are usually concentrated on areas that exhibit these geographical properties.

Unlike epithermal or mesothermal lode gold occurrences, paleoplacer deposits are not associated with broad alteration halos (a chemical and mineralogical change in rocks surrounding certain types of gold deposits), which can be used to map potential deposits.

Although geophysical surveys are of little use in the exploration for paleoplacer gold deposits, radiometric surveys can be useful in the search for paleoplacer uranium deposits. These surveys, which employ radioactivity, map the distribution of uraninite, the mineral from which uranium is extracted.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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February 9, 1998
Carlin-type gold deposits, Part 1
by Derek Wilton

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.

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February 16, 1998
Carlin-type gold deposits, Part 2
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.

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