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GOLD

General Types of Auriferous Deposits

The mineralization of these particular deposits is characterized essentially by quartz; carbonate minerals, pyrite, arsenopyrite, base-metal sulfide minerals, and a variety of sulfosalt minerals. The principal gold minerals are the native metal and various tellurides; aurostibite occurs in some deposits. Characteristic types of wall rock alteration are generally developed adjacent to and in the vicinity of nearly all deposits in this class. In the old Precambrian rocks, the most common types of alteration are chloritization, carbonatization, sericitization, pyritization, arsenopyritization, and silicification. In the younger rocks, propylitization (chloritization and pyritization) is especially characteristic, and there may also be a development of adularization, silicification, kaolinization, sericitization, and more rarely alunitization.

The elements commonly concentrated in this class of deposits include Cu, Ag, Zn, Cd, Hg, B, TI, Pb, As, Sb, Bi, V, Se, Te, S, Mo, W, Mn, Fe, C02 and SiO2; less commonly Ba, Sr, U, Th, Sn, Cr, Co, Ni, and Hg and Sb are particularly characteristic of the younger deposits. The Au/Ag ratio of the ores is generally greater than 1 in most Precambrian and in some younger deposits; in many Tertiary deposits the ratio is less than 1.

Deposits of this type are widespread in the Precambrian greenstone belts of the world; Examples include Yellowknife, Northwest Territories, Canada; Red Lake and Timmins, Ontario, Canada; Kolar goldfield, India; Kalgoorlie goldfield, western Australia; and the Cam and Motor, Dalny, and other similar mines in.Zimbabwe. Younger representatives are the Mother Lode system of California (Mesozoic); Comstock Lode, Nevada (Tertiary); Goldfield, Nevada (Tertiary); Cripple Creek, Colorado (Tertiary); Coromandel gold belt, New Zealand (Tertiary); Emperor mine, Fiji (Tertiary); Lebong and other auriferous districts, Indonesia (Tertiary); Lepanto mine, Philippines (Tertiary); Kasuga mine, Japan (Tertiary), and the Belaya Gora and other similar deposits in the far eastern Russia (Tertiary).

5. Auriferous veins, lodes, sheeted zones, and saddle reefs in faults, fractures, bedding-plane discontinuities and shears, dragfolds, crushed zones, and openings on anticlines essentially in sedimentary terrains; also replacement tabular and irregular bodies developed near faults and fractures in chemically favourable beds

These deposits are widespread throughout the world and have produced a large amount of gold and silver; they are often referred to as "Bendigo type". The deposits are developed predominantly in sequences of shale, sandstone, and greywacke dominantly of marine origin. Such sequences are invariably folded, generally in a complex manner, metamorphosed, granitized, and invaded by granitic rocks, forming extensive areas of slate, argillite, quartzite, greywacke, and their metamorphic equivalents. Near the granitic bodies, various types (kyanite, andalusite, cordierite) of quartz-mica schists and hornfels are developed and grade imperceptibly into relatively unmetamorphosed slates, argillites, quartzites and greywacke marked by the development of sericite, chlorite and other low-grade metamorphic minerals. Most of the gold deposits are developed in the lower-grade facies. A few economic deposits occur in the granitic batholiths and stocks that invade the greywacke-slate sequences.

The principal gangue mineral in these deposits is quartz; feldspar, mica, chlorite, and minerals such as rutile are subordinate. Among the metallic minerals, pyrite and arsenopyrite are most common, but galena, chalcopyrite, sphalerite, and pyrrhotite also occur. Molybdenite, bismuth minerals, and tungsten minerals are local. Stibnite occurs in abundance in a few deposits, but is relatively rare in most deposits. Acanthite, tetrahedrite-tennantite, and other sulfosalts are not common in these deposits. Carbonate minerals, mainly calcite and ankerite, are common but not abundant. The valuable ore minerals are native gold, generally low in silver, auriferous pyrite, and auriferous arsenopyrite. Telluride minerals are relatively rare, and aurostibite is an uncommon mineral in these deposits.

A few deposits in this category are tabular or irregular replacement (disseminated) bodies developed in carbonate rocks or calcareous argillites and shales. The principal minerals in these deposits are quartz, fluorite, pyrrhotite, pyrite, arsenopyrite, sphalerite, galena, chalcopyrite, and stibnite.

As a general rule, wall rock alteration associated with these deposits is minimal, and the quartz veins, saddle reefs, and irregular masses are frozen against the slate, argillite or greywacke wall rocks. In places, thin zones of mild chloritization, sericitization, and carbonatization are present. Some veins are marked by thick black zones (up to 15 cm wide) of tourmalinized rock. Disseminated pyrite and arsenopyrite are common in the wall rocks of most of these deposits. This pyrite and arsenopyrite is usually auriferous.

The elements exhibiting a high frequency of occurrence in this type of gold deposit include Cu, Ag, Mg, Ca, Zn, Cd, (Hg), B, (In), (Ti), Si, Pb, As, Sb, (Bi), S, (Se), (Te), (Mo), W, (F), Mn, Fe, (Co), and (Ni). Elements in parentheses have a low to very low frequency of occurrence. The Au/Ag ratio in the ores is generally greater than 1.

Deposits in essentially sedimentary terrains are widespread throughout the world. In Canada, Examples occur in the Archean Yellowknife supergroup in the Yellowknife district, Northwest Territories (Ptarmigan, Thompson-Lundmark and Camlaren mines); in the Paleozoic Cariboo group at Wells, British Columbia (Cariboo Gold Quartz mine); and widespread in the Ordovician Meguma group of Nova Scotia. Elsewhere in the world deposits of this type occur in the auriferous Appalachian "Slate Belt" of the United States (Paleozoic); Salsigne mine, Montagne Noire, France (Paleozoic); Sovetskoe deposit, Yenisey region, Russia (Proterozoic); Muruntau deposit, Uzbec Russia (Paleozoic); Bendigo deposits, Victoria, Australia (Paleozoic); and the Pilgrims Rest and Sabie goldfields in the Transvaal System, South Africa (Proterozoic?).

6. Gold-silver and silver-gold veins, lodes, stockworks, and silicified zones in a complex geological environment, comprising sedimentary, volcanic, and various igneous intrusive and granitized rocks

Deposits in this category combine nearly all the epigenetic features described in categories 4 and 5. Quartz is a predominant gangue, and some deposits are marked by moderate developments of carbonates. The orebodies constitute principally quartz veins, lodes, and silicified and carbonated zones. The gold is commonly free but may be present as tellurides and disseminated in pyrite and arsenopyrite. The Au/Ag ratio of the ores is variable depending upon the district and often upon the deposit.


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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011