The Guayana Shield in Venezuela is composed of five lithotectonic provinces: 1) an Archean amphibolite-to granulite-facies gneiss terrane; 2) an Early Proterozoic greenstone-granite terrane(s); 3) an Early Proterozoic unmetamorphosed volcanoplutonic complex; 4) Early to Middle Proterozoic continental sedimentary rocks; and 5) Middle Proterozoic anorogenic rapakivi-type granite. Early Proterozoic rocks in the Amazon Federal Territory of western Venezuela are undivided and their relation to other rocks of the Venezuelan Guayana Shield is unclear. Early to Middle Proterozoic continental tholeiitic dikes, sills, and small irregular intrusive bodies and Mesozoic dikes emplaced during the opening of the Atlantic Ocean cut all of the lithotectonic provinces. Major mineral deposits of the Venezuelan Guayana Shield include gold, iron, bauxite, and diamonds. The Archean Imataca Complex, the oldest unit, consists of gneiss and granulite with minor dolomite and banded iron-formation (BIF). Large isoclinal folds, which have been refolded into relatively open folds, are common in the Imataca Complex. Metamorphic grade ranges from granulite facies in the northeast part of the belt to amphibolite facies in the southwest. Deposits of enriched BIF in the Imataca Complex contain more than 2 billion metric tons of iron ore. During the Pre-Trans-Amazonian tectonomagmatic event, between about 2,800 and 2,700 Ma, granitic rocks intruded the Imataca Complex, and injection gneisses and migmatite were developed. The Early Proterozoic greenstone belts, which formed between about 2,250 and 2,100 Ma, consist of a submarine sequence of tholeiitic mafic volcanic rocks, a sequence of tholeiitic to calc-alkaline basalt to rhyolite, and an interval of turbiditic graywacke, volcaniclastic rocks, and chemical sedimentary rocks that characterize the basal, middle, and upper parts, respectively. Layered mafic complexes also occur in the greenstone belts. The greenstone-belt rocks range in metamorphic grade from greenschist to amphibolite facies. Low-sulfide gold-quartz veins are hosted by Early Proterozoic greenstone-belt rocks cut by major shear zones. Deposits of volcanogenic manganese are mined elsewhere in the Guayana Shield; in Venezuela, a moderate potential exists for the discovery of additional deposits of manganese in the greenstone belts. Platinum-group elements (PGE) and chromium are present in anomalous values in the layered mafic complex at Piston de Uroy, Venezuela. Also, PGE are anomalous in mafic metavolcanic and metagabbroic rocks adjacent to an exhalative (Homestake-type) gold prospect in the Cerro La Pinto area. Granitic domes of the Supamo Complex intruded the greenstone-belt rocks between about 2,230 and 2,050 Ma and divided the metasedimentary and meta-igneous greenstonebelt rocks into branching synclinoria between intrusive uplifts. The Trans-Amazonian orogeny was a period of continental collision between about 2,150 and 1,960 Ma, during which the Imataca Complex and the greenstonegranite terranes were deformed and metamorphose...

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Plate 1. Mineral occurrences of the Guiana Shield, Venezuela.... in pocket Mendoza (1985) proposed that as much as 8,000 mt of gold had yet to be discovered in the Venezuelan Guiana Shield, and Newman (1989) reported that Venezuelan gold reserves presently total an estimated 358 mt. The similarity between the geology of the Precambrian Guiana Shield and the Precambrian shields in Canada, Australia, and South Africa, supported by information from regional reconnaissance exploration (Gibbs and Barren, 1983; Sidder and others, 1988;Sidder, 1990) indicates that good potential exists for new discoveries of gold and other mineral deposits in Venezuela. However, the number of undiscovered deposits and amount of contained metal have not yet been determined quantitatively.The richest gold lodes mined to the present are in the El Callao mining district (no. 37 on map).The El Callao mine was the most productive gold mine in the world during the latter part of the Nineteenth Century (Newhouse and Zuloaga, 1929). Cumulative production between 1829 and 1980 was 124 mt of gold (Bellizzia and others, 1981). Peak production was in 1885 when 8,194 kg were produced (Locher, 1974). MINERVEN (a Venezuelan government-owned mining company) produced 2,300 kg in 1986 (Rodriguez, 1987).Locher (1974) estimated that potential gold reserves in the El Callao district are about 84,000 kg from 4.6 x 106 mt with an average grade of 18.33 g/mt gold. Reserves to a depth of 250 m at the underground Colombia Mine (being mined by MINERVEN in 1989) are estimated to be about 14,300 kg from 1.55 x 106 mt with an average grade of 9.2 g/mt gold (Engineering and Mining Journal, 1988).Precambrian greenstone belt rocks are historically the richest source of gold in Venezuela. More than 260 gold-bearing quartz veins are present in the El Callao district. These veins cut Early Proterozoic meta-igneous and metasedimentary rocks that are metamorphosed to the greenschist facies and locally the amphibolite facies (Menendez, 1968). The protoliths consisted of submarine sequences with mafic-ultramafic intrusions, mafic to felsic volcanic rocks, and volcaniclastic, turbiditic, and chemical sedimentary rocks of the Pastora Supergroup and the Botanamo Group. Both tholeiitic and calc-alkaline chemical trends are present in the volcanic rocks.Faults and shear zones localized the mineralization. Mafic metavolcanic and metavolcaniclastic rocks and metasedimentary rocks commonly host the ore. Native gold and minor to trace amounts of pyrite, tetrahedrite, chalcopyrite, bornite, and scheelite are present in the quartztourmaline veins.Carbonate alteration of the wall rocks, as well as propylitization and silicification, extends several meters away from the veins (Banerjee and Moorhead, 1970; Menendez, 1974). The low-sulfide gold-quartz vein model (Berger, 1986;Bliss and Jones, 1988) best characterizes the mineralization in the El Callao district.

A sample of andesitic dike material contains anomalous values of Cu (334 ppm), As (153 ppm), and Mo (7 ppm). A mineral deposit model for epithermal precious metal veins best represents the occurrences in the upper Caura River area.