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April 27, 1998 Banded iron formation-hosted gold deposits, Part I by Derek Wilton

Banded iron formation-hosted gold deposits consist of gold intergrown with quartz and/or sulphide minerals in deformed and structurally complicated iron-rich sedimentary rocks. In general, most geologists would define these deposits as a variety of the mesothermal lode gold type.

These deposits mainly occur within Archean-aged (more than 2,600 Ma, or million years old) greenstone belts, though some are Early Proterozoic (ca. 2,100 Ma). Greenstone belts are linear volcanic and sedimentary centres that are engulfed and completely surrounded by granitic-gneissic basement rocks; these belts are typical of the shield areas of northern Ontario and Quebec and the Northwest Territories. The banded iron formation (BIF) host rocks are thinly layered (layers can be measured in centimetres) sedimentary rocks with alternating iron-rich and cherty (silicious) layers; the BIFs can have considerable lateral extents.

There are different types of BIFs, defined on the basis of the mineralogy of the iron-rich layers: if the iron-rich layer is dominantly

magnetite-hematite, then the BIF is termed oxide facies (a sedimentary term meaning a distinctive group of characteristics that distinguish one sedimentary unit from another); if the layer is composed of pyrite and/or pyrrhotite (iron sulphides), then the BIF is called sulphide facies. There are also carbonate- and silicate-facies BIFs.

All BIF's are classified as chemical sediments, which means that they formed through chemical precipitation from seawater on the sea floor. Other sedimentary textures in the BIFs suggest deposition in shallow water on submarine continental shelves.

Gold occurs as native (free) gold intergrown with pyrite and/or pyrrhotite; arsenopyrite and/or magnetite are also present in some deposits. Other accessory and trace minerals are similar to those found in mesothermal lode gold deposits, such as sphalerite, chalcopyrite, tetrahedrite, scheelite, and molybdenite. Mineralogy of the host-rock alteration is predicated upon the fact the rocks are iron-rich. In the case of oxide-facies BIF, primary hematite-magnetite is replaced by pyrite-pyrrhotite with minor siderite (iron carbonate). Quartz, in the form of crosscutting veins, is also a common alteration mineral and, most typically, the gold is intergrown with sulphides in the quartz veins. Chlorite is a common alteration product of silicate minerals here.

Most generally, BIF-hosted gold deposits are thought to form by the reaction of auriferous and sulphur-bearing hydrothermal fluids with the iron oxide (or sulphide) in country rocks, causing precipitation of gold and sulphides. The gold is present in quartz veins or the immediate wallrock, wherein the precipitation reactions occur. As such, the deposits are said to be stratabound (i.e., the gold is contained within a single stratigraphic unit, but the mineralization can cut across the layering in the unit) because the specific chemical horizon responsible for gold precipitation is represented by a single sedimentary horizon. Access to the favorable chemical environments of the BIF for the hydrothermal fluids was provided by large-scale fault and shear systems in a manner similar to that visualized in mesothermal lode gold models.

There is debate as to the origin of a few BIF-hosted gold deposits. Some geologists suggest that gold actually precipitated with the original chemical sedimentary host rocks as sort of a submarine hot-spring that exhaled onto the sea floor. In this model, subsequent deformation of the gold-enriched BIF led to the local remobilization and secondary concentration of gold in highly deformed zones. In other words, the gold was originally precipitated at above-normal concentrations in the BIF but was concentrated up to ore grade with deformation of the BIF. In this case, the gold in the BIF would be classified as stratiform (truly bedded and related to the deposition of the host unit), based on its original pre-deformation form.

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

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May 4, 1998
Banded Iron Formation-hosted gold deposits, Part 2
by Derek Wilton

Banded iron formation-hosted gold deposits are important in terms of Canadian and U.S. gold production, as illustrated by mines such as the Lupin and Musselwhite in Canada and the Homestake in South Dakota.

In general, gold deposits in banded iron formations (BIFs) contain from 0.1 to 100 million tonnes of ore grading between 4 and 30 grams gold per tonne. The Homestake mine, a world-class example of this deposit type, has produced over 1,180 tonnes of gold from 118 million tonnes of ore since operations began in 1876; remaining reserves at the end of 1996 were over 21.5 million tonnes of ore grading 6.72 grams gold.

Lupin has over 9 million tonnes of ore grading 10 to 11 grams gold. The gold is relatively pure, with moderate to low silver content of generally less than 6 grams. The gold ore is mined in a similar manner to that of mesothermal lode gold deposits, with emphasis on veins or sulphide-rich portions of the BIFs.

Since the veins and BIFs are frequently narrow units, mining is typically an underground operation, but there is some production from open pits. The bulk ore is crushed, then fed through a processing and refining plant akin to those in use at archetypal mesothermal lode gold operations. As BIF-hosted gold deposits are restricted to greenstone belt terranes in Archean to Early Proterozoic shield areas, exploration would be directed towards regions such as the Superior and Slave provinces of the Canadian Shield. The main points in both variations to the genetic model for these deposits are that deformation either provided permeable pathways for the gold-bearing ore fluids along faults, or caused remobilization of pre-existing gold accumulations, essentially enriching and upgrading gold concentrations. Exploration would focus on highly deformed, structurally complicated portions of BIFs within greenstone belts, especially where regional fault-shear systems cut through.

The dominant structural style of the deformation manifested at most gold-bearing BIFs is folding; hence contorted fold zones in a BIF would also be a favorable exploration target. Though deformation is strongly developed in these deposits, metamorphic grade usually does not exceed greenschist facies.

Exploration should further zero in on portions of BIFs that are sulphide facies or on areas with sulphide alteration overprinting oxide facies BIF. Since BIFs account for less than 5% of the area of greenstone belts, exploration would first be directed towards locating these sedimentary rocks within the greenstone belt piles. Such exploration would be aided by airborne and ground geophysical surveys over the greenstone belts, since the greatly elevated metal contents of the host rocks make them very electrically conductive and thus discernable by electromagnetic surveys. Also, the rocks' magnetite (plus pyrrhotite) contents make them readily detectable by magnetic surveys. Induced polarization surveys would also be very advantageous in detailed exploration for, and mapping of, these conductive host rocks. Regional geochemical surveys for iron formation and elevated concentrations of gold, iron, arsenic, bismuth

and antimony could also prove effective in exploration.

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

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June 1, 1998
Diamond deposits, Pt. 1 by Derek Wilton

Diamond, which is pure carbon, is the hardest substance known, yet diamonds can be broken relatively easily. Paradoxically, graphite, one of the softest minerals, is a polymorph of diamonds. The essential difference between the relative strengths of these two minerals lies in their crystal structures: in diamonds, carbon atoms are linked in an isometric form, whereas atoms in graphite form linked hexagonal sheets.

Diamond crystal structures are created when carbon is subjected to great pressures (between 45 and 55 kilobars) and high temperatures (1,050 C to 1,200 C). The zone of formation of diamonds, therefore, is below the Earth's crust, in the upper mantle. Rarely, microdiamonds can be found at meteorite impact sites, where shock-derived pressures and temperatures are sufficiently intense to transform carbon into diamonds. Diamonds can also be produced synthetically.

Diamond crystals can revert to graphite if subjected to changes in pressure and temperature over time.

On the Earth's surface, diamonds are found in unusual intrusive ultramafic igneous rocks or in placer and paleoplacer concentrations. In these deposits, diamonds are not hosted by the upper mantle rocks, namely peridotite or eclogite, in which they primarily formed.

The ultramafic igneous rocks that contain diamonds at the Earth's surface are kimberlites or lamproites. Kimberlites are volatile-rich (containing H2O and CO2) potassic, ultrabasic rocks which have an unequigranular grain size, with macrocrysts (magmatic crystals and rock-crystal fragments measuring 0.5 to 15 mm across) and megacrysts (greater than 2 cm and up to 20 cm across) set in a fine-grained matrix. Lamproites are ultrapotassic, magnesium-rich rocks which, unlike kimberlites, contain no CO2.

The feature of kimberlites essential to their containing diamonds is their volatile content, as volatiles cause magmas to intrude explosively from the lower crust or upper mantle to the earth's surface.

Diamonds are carried in kimberlites and lamproites as xenolithic crystals, or xenocrysts. While in transit from their melt sources, magmas pick up diamonds from their host rock and carry them upward. The magma essentially acts as a high-speed elevator, rapidly bringing the diamonds to the Earth's surface. Essentially, diamonds go through the pressure-temperature transition from the depth to surface so quickly that they can't revert to graphite. It is the great hardness of the diamonds that allows them to survive the explosive intrusion.

Kimberlite and lamproite systems have distinctive intrusive architectures. The uppermost part of an intrusive body in a kimberlite is carrot-shaped, and has its roots in dykes and sills (hypabyssal or medium-depth intrusive rocks).

Pipes, which are generally up to 2 to 3 km wide, originate at a root zone, rise through diatreme facies to the crater facies, where the kimberlite actually breaches the Earth's surface. The walls of the diatreme dip at angles of 75 to 85 from the horizontal; the crater walls exhibit shallower dip. The crater is the widest part of the pipe, but it seldom exceeds 2 km in diameter or 250 ha in area.

In contrast, lamproites are not pipe shaped and consist of crater facies fewer than 500 metres in depth. Diamondiferous lamproite craters are up to 1.25 km in diameter with an area of about 125 ha.

Kimberlites and lamproites also contain xenocrysts of garnet and spinel. Diamonds are actually a very minor component of kimberlite and lamproite magmas, whereas other xenocrysts appear in greater abundance. Placer and paleoplacer (Geology 101, T.N.M., Jan 18-25/98) deposits, also known as alluvial deposits, form through the weathering and erosion of diamond-bearing kimberlites or lamproites. Diamonds form detrital placer grains due to their great hardness (they can withstand the erosional processes) and higher density compared with other detrital material. Diamondiferous kimberlites and lamproites are essentially secondary concentrations, whereas placer and paleoplacers deposits are tertiary. -- The author is a professor of geology at Memorial University in St. John's, Nfld.

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June 8, 1998
Diamond deposits, Pt. 2
by Derek Wilton

Diamonds fall into one of four categories. They are, in order of decreasing value: gem, near-gem, industrial and boart. Individual diamonds are measured in carats (1 carat equals 0.2 gram), whereas the grade of diamondiferous rock is expressed in carats per tonne (or carats per 100 tonnes).

World diamond production is in the order of 100 million carats per year. In 1997, the Argyle deposits of Western Australia (the world's largest producer) produced 40.2 million carats from ore grading 3.7 carats per tonne. Highly variable grades can make the value of ore in US dollars per carat quite unpredictable.

The major diamond producing nations are South Africa, Botswana, Australia, Russia and Zaire. In Canada, production from the BHP Diamonds-Dia Met Minerals operation at Lac de Gras, N.W.T., will begin later this year. The vast majority of diamond production is from kimberlites, with only the Argyle deposits providing substantial production from lamproite sources. About 3% of kimberlite pipes, which can occur in clusters of up to 50, contain diamonds, and only 1% of those occurrences are economically exploitable.

Exploration for diamonds, which occur as xenolithic crystals or fragments within kimberlites or lamproites (both of which are intrusive ultramafic igneous rock types), in heavily glaciated areas is difficult because kimberlites and lamproites are soft compared with other rock types, and are likely to be preferentially eroded as a result. The most useful exploration technique, therefore, is geochemical surveying of till and other alluvium. Positive identification of intrusive rocks from a series of samples collected during an exploration program requires detailed petrographic examination and evaluation of the constituent minerals.

Indicator minerals within kimberlite or lamproite need to be geochemically analyzed and classified to determine the intrusion's potential to contain diamonds. The precious stones are a relatively minor mineralogical constituent in those intrusives, though the indicator minerals are sufficiently abundant to be readily evaluated.

Similarly, the composition of indicator minerals in soils, tills and stream sediments can be analyzed to determine if such detrital material was eroded from an area that contained diamondiferous rocks. Indicator minerals include chromite, garnet and ilmenite, each of which has a distinct geochemical signature in diamondiferous rocks.

Critical to the evaluation of diamond potential is the precise analysis of rocks or detrital material, and their indicator minerals, to define their petrological and geochemical compositions. Based on the composition of the sample analyzed, different preparation techniques are required. In order to evaluate a particular kimberlite or lamproite intrusive, bulk samples of more than 30 kg are usually collected. Indicator minerals and diamonds are separated from the sample, producing a heavy mineral concentrate (HMC) for analysis. Heavy minerals will be similarly separated from large bulk samples of detrital material for analysis. The evaluation of diamond prospects is time-consuming owing to the exacting concentration of the minor constituents from such large samples and the precision required to analyze the HMC. Kimberlite and lamproite intrusives often exhibit circular magnetic (mag) or electromagnetic (EM) geophysical anomalies that reflect the elevated mag or EM properties of the intrusives compared with the country rock, usually returning a bull's-eye pattern. The problem with these surveys is that the craters or pipes cover such a small area that it may be difficult to distinguish the anomalies from regional gradients.

Kimberlite and lamproite craters and kimberlite diatremes are initially mined as open-pit operations because the host rocks are usually friable. Underground production is frequently initiated with increases at depth, ely in diatremes. Alluvial sources are mined as open-pit operations. The ore is crushed and diamonds, because of their hardness, are readily separable. -- The author is a professor of geology at Memorial University in St. John's, Nfld.

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