Guiana Shield offers vast mineral wealth potential

Almost half of Venezuela is underlain by a mineral-rich, unglaciated, Precambrian-aged craton known as the Guiana Shield. This geological entity is analogous to the Canadian Shield and, until recently, was one of the most under-explored regions of the world.

Ever since the discovery by Placer Dome (TSE) of the multi-million-ounce Las Cristinas gold deposit in eastern Venezuela, numerous junior and senior mining companies have been focusing their attention on the Guiana Shield, mostly in search of gold.

One of the more comprehensive reports on the Venezuelan Shield can be found in U.S. Geological Survey Bulletin 2062, published in 1993. Titled Geology and Mineral Resource Assessment of the Venezuelan Guayana Shield, the report was a joint effort of the U.S. Geological Survey (USGS) and Corporacion Venezolana de Guayana, Tecnicia Minera C.A. Much of the following information was culled from this report.

The Venezuelan Shield is a typical Archean-aged greenstone terrain which stretches eastward from Venezuela across Guyana, Suriname, into French Guiana and southward into the Amazon Basin in northern Brazil.

The Shield underlies all of Estado Bolivar, Territorio Federal Amazonas and a portion of Estado Delta Amacuro, south of the Rio Orinoco River. It encompasses more than 415,000 sq. km and is covered primarily by jungle and savannah.

The northern part of the Shield, near the Rio Orinoco River, is relatively flat and makes up the southern margin of the vast plains region known as Los Llanos. The Gran Sabana makes up the southeastern portion of the Shield and includes high, flat-topped mountains called tepuis.

Between these two regions are extensive areas of gently rolling topography covered with jungle.

Territorio Federal Amazonas makes up the southwestern half of the Shield and is a combination of rugged mountainous regions and flat, inundated plains covered by jungle.

The rocks of the Venezuelan Shield can be divided into two older terrains and two younger sequences of sedimentary and igneous rocks.

The oldest terrain comprises the Imataca Complex, which is composed of Archean metamorphic rocks of granulite and amphibolite facies. The second terrain consists of the Early Proterozoic Supamo Complex and the Pastora Supergroup, and is made up of a granite-greenstone belt and associated younger eugeosynclinal rocks.

A transitional suprajacent sedimentary and volcanic sequence is composed of continental clastic and tuffaceous rocks that may be metamorphosed, isoclinally folded or undeformed. The Cuchivero Group, which is the most important unit in this sequence, consists of ash-flow tuff and associated granitic plutonic rocks.

A large portion of the Shield is covered with Early to Middle Proterozoic continental clastic sedimentary rocks known as the Roraima Group. Two ages of diabase dykes (Early Proterozoic and Mesozoic) are present in all terrains and sedimentary sequences.

Cenozoic deposits are mostly restricted to Tertiary deltaic sediments of the Rio Orinoco and floodplains of modern rivers.

More than 450 mines, prospects and mineral occurrences were documented in the USGS report.

Of the compiled occurrences, more than 200 are for gold. Native gold in quartz veins is reported at 125 sites, and placer deposits are reported at more than 80 locations. At some occurrences, both weathered vein and alluvial deposits are present.

At many of the vein-hosted mines and prospects, mineralization has been noted in single quartz veins, quartz stockworks and quartz veins in shear zones. More than 70 of the vein gold mines and occurrences are in the Callao and Lo Increible districts.

The placer operations mostly mine alluvial material, although locally, residual gold is recovered from older terrace deposits or weathered veins. More than 106 diamond mines and occurrences, most of which are alluvial, were documented in the USGS report.

The Venezuelan Shield also contains 14 iron and manganese deposits and prospects, 35 banded iron formation deposits and 40 bauxite occurrences. Other reported mineral occurrences include barite, manganese, titanium, molybdenum, tungsten, alluvial tin, uranium, rare earth element minerals, kaolin, dolomite and sand and gravel.

Known deposit types include Algoma and Superior-type iron, laterite-type bauxite, sedimentary manganese, low-sulphide gold-quartz veins, placer gold, placer diamond, diamondiferous kimberlite pipes, residual and sedimentary kaolin, and carbonatite.

Several other deposit types have not been identified to date, mainly because of deep weathering and forest cover. The USGS believes the terrain is permissive for the occurrence of kuroko-type massive sulphide deposits, thorite/rare earth element veins, tin greisen, uranium, synvolcanic nickel-copper-platinum, rhyolite-hosted tin, porphyry copper and dolomitic marble.

In neighboring Guyana, 75% of the land mass is underlain by rocks of the Shield. The northern portion of the Shield contains greenstone belts and granite-gneiss terrains, while the southern portion consists of granulites, gneisses and metasedimentary belts. Late Proterozoic Roraima Group clastic rocks cap the crystalline Shield units.

The Shield areas contain dozens of lode, eluvial and placer gold prospects. Only one of these deposits, the Omai mine, is currently being exploited. Last year, the mine, operated by Cambior (TSE) in conjunction with partners Golden Star Resources (TSE) and the Guyanese government, produced 206,537 oz. gold at a cash cost of US$259 per oz. At year-end, minable reserves stood at 55.5 million tons grading 0.045 oz. gold per ton.

Other companies exploring in Guyana include Adex Mining (TSE), Altai Resources (TSE), Cathedral Gold (TSE), Exall Resources (TSE), Menora Resources (ME), Minorca Resources (VSE) and Sutton Resources (VSE). Suriname’s portion of the Shield consists of two high-grade metamorphic zones in the western part of the country, and a semi-circular low-grade metamorphic zone stretching from the southeast to the northwest. The high-grade and low-grade metamorphic zones are separated by a vast area of granitoid and metavolcanic rocks. Remnants of the Roraima sandstone formation occur predominantly in the central portion of the country.

Companies active in Suriname include Golden Star,

Canarc Resource (VSE),

KWG Resources (ME), Trev (ASE) and Blue Ribbon

Resources (VSE).