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.