Geology 101

  1. Epithermal deposits, Part 1, April 7, 1997
  2. Epithermal deposits, Part 2, April 14, 1997
  3. Quartz-Carbonate vein gold deposits, Part 1, July 28, 1997
  5. Placer gold deposits, Part. 1, October 6, 1997
  6. Placer gold deposits, Part. 2, October 13, 1997
  7. Paleoplacer gold deposits, Part. 1, January 12, 1998
  8. Paleoplacer gold deposits, Part. 2, January 19, 1998
  9. Carlin-type gold deposits, Part 1, February 9, 1998
10. Carlin-type gold deposits, Part 2, February 16, 1998
11. Banded iron formation-hosted gold deposits, Part I, April 27, 1998
12. Banded Iron Formation-hosted gold deposits, Part 2, May 4, 1998
13. Diamond deposits, Part. 1, June 1, 1998
14. Diamond deposits, Part. 2, June 8, 1998
15. Sedex massive sulphide deposits, Part 1, September 28, 1998
16. Sedex massive sulphide deposits, Part 2, October 5, 1998
17. MVT lead-zinc deposits, Part , October 26, 1998
18. MVT lead-zinc deposits, Part 2, November 9, 1998
19. VMS deposits, December 14, 1998
20. Zinc-copper VMS deposits, Part 1, December 21, 1998
21. Zinc-copper VMS deposits, Part 2, December 28, 1998
22. Zinc-lead-copper VMS deposits, Part 1, January 11-17, 1999
23. Zinc-lead-copper VMS deposits, Part 2, January 18-24, 1999
24. Porphyry deposits, Part. 1, May 31-June 6, 1999
25. Porphyry deposits, Part. 2, June 7-13, 1999
26. Redbed copper deposits, Part 1August 9-15, 1999
27. Redbed copper deposits, Part 2, August 16-22, 1999
28. Skarn deposits, Part 1, November 22-28, 1999
29. Skarn deposits, Part 2, November 29- December 5, 1999
30. Besshi-type VMS deposits (Part I), July 3-9, 2000
31. Besshi-type VMS deposits (Part II), July 10-16, 2000


April 7, 1997
Epithermal deposits, Part 1
by CHRISTINE NORCROSS

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.

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April 14, 1997
Epithermal deposits, Part 2
by CHRISTINE NORCROSS

Epithermal gold deposits, which contribute significantly to the world's gold supply, are an important exploration target which must be evaluated carefully based on the amount of metal they might provide, and at what cost.

The amount of gold in any type of deposit is calculated based on the ore's grade (the amount of gold per tonne of rock) and tonnage (total number of tonnes) available at that grade. The higher the grade of the material, the lower the tonnage required to make recovery economical.

A high-grade deposit could have gold values ranging from 10 to more than 150 grams per tonne, whereas a low-grade deposit grades in the range of 1 to 5 grams. Low-grade deposits may have up to, and possibly more than, 200 million tonnes of rock, whereas a high-grade deposit is frequently smaller. Assay results acquired through drilling are important indicators of a deposit's grade and tonnage. High grades over short distances can be as significant as low grades over longer distances, and both types of deposit can be mined profitably.

Drill results, however, offer only a limited view of a deposit and may be difficult to reproduce. For instance, a single drill hole may intersect a high-grade zone in an otherwise low-grade (high sulphidation-Type epithermal) deposit, giving the appearance of a higher grade than actually exists. Factors other than tonnage and grade come into play in calculating the economic significance of an epithermal deposit. For instance, the presence of other metals in the ore can increase the value of a deposit, and many epithermal deposits contain a significant silver and/or copper content. The price of gold (and other metals) is also an important condition in economic evaluation, as low prices may render small or low-grade deposits uneconomic.

Many epithermal deposits occur in remote regions of under-developed countries, and the construction of infrastructure, such as roads and mills, may be necessary before deposits can be mined. These expenses increase the cost of a mining operation and must be taken into consideration when calculating the economics of a deposit.

Mining and processing methods are also important in determining economics. Since epithermal deposits are often formed at depths of less than 2 km (closer if erosion of overlying material has resulted), many are amenable to relatively less expensive open-pit mining methods. Deeper deposits that can be exploited only through underground methods are more expensive. Finally, recovery methods for epithermal gold deposits can entail either conventional milling or cyanide leaching. The cost of both procedures can increase if gold is contained in minerals that are difficult to process, such as arsenopyrite.

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July 28, 1997
Quartz-Carbonate vein gold deposits, Part 1
by DEREK WILTON

Quartz-Carbonate vein gold deposits (also known as mesothermal lode deposits) form along, and are localized to, major regional fault and fracture systems, but are actually located in secondary or tertiary structures. These vein deposits form from hydrothermal (hot aqueous) fluids, which were derived deep in the earth's crust at a medium geological temperature (250 to 400C).

The fluids use the fault/fracture zones as permeable channels along which to flow from their region of origin until they reach a point wherein any of a number of factors -- chemical reactions with country rock and/or changes in the temperature and/or pressure -- causes the fluids to precipitate. The gold precipitates out of solution along with the quartz vein material. These regional fault systems develop during the waning stages of continental collision and hence can form at significantly later periods than the host rocks; as such, they are termed "epigenetic."

The actual host rocks of the quartz-Carbonate veins are affected by these fault/fracture origins and can range from mylonites to fault gouge. Mylonites indicate deformation under confining pressures sufficiently high that the rock recrystallizes to a fine grain size. This is plastic or ductile behavior, and indicates that the vein formed deep in the earth's crust. Alternatively, if the fault/fracture cuts a rock at a level close to the earth's surface, then it does not have the same confining pressure and hence will break into fault gouge.

Typical quartz-Carbonate vein gold deposits consist of quartz veins with gold, pyrite and/or arsenopyrite. The gold is usually pure gold and can be present in textures ranging from solitary grains to grains intimately intergrown with sulphide minerals. In some deposits, gold is present as "invisible" intergrowths with sulphide minerals such as arsenopyrite (that is, the gold is in the crystal lattice of the sulphide mineral). In other deposits, the gold is not pure but electrum -- a mineral made up of gold, with 20% to 80% silver.

Quartz-Carbonate vein gold systems are characterized by abundant, typically iron-rich, hydrothermal carbonate alteration assemblages which spread into the host rock from the vein. They represent pulses of fluid which flowed along the fracture/fault plane into the surrounding country rock with which they are not in chemical equilibrium, producing chemical reactions and the resultant alteration halo.

Alteration associated with gold mineralization also involves sulphidation (sulphide halos are a characteristic alteration phenomenon of most quartz-Carbonate vein gold deposits) and potassium metasomatism (potassium is usually enriched in the alteration halo around the veins). These halos overprint pre-existing alteration assemblages in the host rock. Any rock type can host these vein systems, but, at best, they are developed in mafic rocks such as basalts, greenstones, gabbros and turbiditic shaley sedimentary rocks; this is attributable to the chemical contrasts between host rock and ore fluids. The ore fluids are silica-rich with carbon dioxide and potassium; hence they react best with mafic rocks, which do not contain free silica but which have calcium-iron-Magnesium silicates that can react with carbon dioxide to form carbonate alteration minerals.

Gold abundances are characteristically low in most geological materials. The average crustal abundance of gold is on the order of 3 parts per billion, and generally no single rock type is preferentially enriched in gold. As a result of the low background contents of gold, a large amount of rock must be affected by the hydrothermal fluids in order for sufficient deposits of dissolved gold to be formed. The general model for these deposits suggests that the associated regional faults have deep roots that extend down to the lower crust. Hydrothermal fluids, which contain gold dissolved from a wide region, are formed, and these are focused up along the faults to higher levels in the crust, where they react with country rock to form lode gold ores.

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

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August 4, 1997
Quartz-Carbonate vein gold deposits, Part 2
by DEREK WILTON

In temporal terms, quartz-Carbonate vein gold deposits apparently have been restricted to specific intervals in the Earth's history, including the Late Archean, Early Proterozoic, early Paleozoic and Early Mesozoic periods. They are best developed in Archean greenstone belts within Archean cratonic areas, such as in northern regions of Ontario and Quebec, Western Australia and southern Africa.

In Canada, the best-known Archean-related mines include the Giant in the Northwest Territories and, in Ontario, the Campbell, Red Lake, Dome, Hollinger and McIntyre, as well as the Kirkland Lake camp. Examples of Proterozoic-related gold-producing regions include Saskatchewan's Star Lake and La Ronge districts. Examples of Paleozoic-Aged formations include the Meguma deposits of Nova Scotia and the Baie Vert occurrences of Newfoundland. Mesozoic-related operations include those in the Bralorne and Caribou districts of British Columbia.

These quartz-Carbonate vein deposits are Canada's primary gold producers and are one of the most important producers worldwide. In general, a minable deposit of this type contains a grade of 6 to 10 grams gold per tonne within 2 to 10 million tonnes of ore.

The drilling and assaying of this sort of deposit can be complicated and fraught with difficulty. The veins themselves usually can be readily mapped through drilling, but determination of the true gold content can be difficult as a result of the so-Called "nugget effect," in which all the gold within an interval can be concentrated in a single point.

The assaying of vein material that is small in quantity but which contains a nugget can yield an erroneously large grade for the system, whereas if a gold-rich nugget within the vein is missed, erroneously low grades can result. To test a deposit properly, sampling must be thorough and completed on a statistically rigorous basis.

Since these veins have rather limited areal extents, the most economically favorable are those with a larger alteration halo. Those halos can also be auriferous, with economically exploitable gold concentrations. Exploration for quartz-Carbonate vein deposits can generally be restricted to orogenic (mountain) or greenstone belts, and the large-scale planar fault-fracture structures therein. Mapping of fault systems and alteration is essential.

Because of its low concentrations in the natural environment, gold is often difficult to detect; hence routine procedures for geochemical exploration (lake sediment surveys, for example) are often too equivocal for tracing the metal in the geological environment. Some elements, particularly antimony and arsenic, are so closely associated with gold that they can be exploration targets in the search for gold since they are much easier to detect. Such elements are known as pathfinder elements.

The best geophysical exploration techniques to use in the search for these types of ore deposits are those that map out fault structures. Techniques employing electromagnetic and magnetic technology would be of little assistance, as the amount of metallic minerals in the veins is usually limited.

These vein systems are planar objects with a much greater length and depth than width, and they are hosted in solid rock. As a result, they are not usually amenable to open-pit mining operations but, rather, are exploited via underground methods.

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

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