Tellurides from the Paleoproterozoic Omai gold deposit, Guiana Shield

The Omai deposit, a Paleoproterozoic gold-bearing vein system, is located i n the Barama-Mazaruni Supergroup, a typical greenstone belt in north-centra l Guyana. The deposit is hosted mainly by a shallow-level quartz-monzodiori tic stock (Omai stock), quartz-feldspar porphyry and rhyolite dykes, and, t o a lesser extent, by adjacent mafic volcanic and sedimentary rocks. In the Omai stock, the gold-bearing quartz veins show an uniform pattern, general ly striking 030 degrees and dipping at 20-30 degrees. The veins hosted by t he quartz-feldspar porphyries and rhyolites are stockworks. The metallic mi nerals (<2% of thr vein volume) have a complex mineralogy consisting of var ious sulfides, associated with native elements, tungstates, tellurides, and sulfosalts. Telluride minerals are petzite, calaverite, hessite, tellurobi smuthite, altaite, melonite, coloradoite, and volynskite. The metal invento ry is defined by the Au-Ag-Te-W-Bi-Pb-Zn-Cu-Hg-Mo assemblage, characteristi c of both epi- and mesothermal deposits. The gangue includes mainly quartz, with minor amounts of ankerite, calcite, albite, white mice, chlorite, and epidote. The conditions of ore formation at Omai are estimated from the st ability of telluride - sulfide - oxide silicate assemblages. The bulk of or e minerals and gangue precipitated at low temperatures (220 degrees to 170 degrees C), whereas solutions are characterized by relatively narrow ranges in S-2 (10(-17) to 10(-11)), Te-2 (10(-14) to 10(-10.5)) and O-2 (10(-43) to 10(-36)) fugacities and a weakly acidic pH (between 4 and 5.4). Gold was probably transported as sulfide [Au(HS)(2)(-) and Au(HS)(0) or HAu(HS)(2)( 0)] or thiosulfate complexes, whereas Te was transported as aqueous H2TeO3, Te-2(2-), and HTe- complexes.