Sedimentary rock-hosted disseminated precious metal mineralization at Purisima Concepcion, Yauricocha District, central Peru

Abstract

Sedimentary rock-hosted disseminated gold ores lithologically and chemically similar to those of Carlin-type deposits of the western United States are present in the Yauricocha district, central Peru. The Purlsima Concepci6n deposit is located in the core of a steeply plunging anticline several hundred meters beyond large pipe-shaped Cu-Zn-Pb-Ag-Au replacement orebodies in limestone bordering a late Miocene granodiorite stock. The central part of the stock is potassium-silicate altered and contains high-salinity fluid inclusions. Mineralized impure limestone is generally decalcified and silicified and appears to have been carbonaceous. Original laminated bedding and other sedimentary structures are locally preserved. Characteristic minerals include quartz, rhodochrosite, pyrite, calcite, sericite, and barite. Pyrrhotite, arsenopyrite, plagionite (PbsSbsS17), and altaitc (PbTe) have been recognized locally. Although the chemical composition of the ores is generally similar to that of the Carlin deposit, rocks of Purlsima Concepci6n have higher Fe, Mn, Te, Ag, and T1 and lower Hg contents. Most of the Au-bearing rocks contain between 3 and 6 wt percent Fe, and Te (•4 ppm) and Mn (•4 wt %) are about two orders of magnitude higher than at Carlin. These elevated elemental concentrations are all epigenetic; there is no regional or local geologic or textural evidence for a syngenetic and/or diagenetic origin. The Ag/Au ratio of the ores is about 2.5. Gold correlates positively with Ag, As, Hg, Mo, Sb, Te, and T1 and Ag strongly with Mn. Porous, leached, but not silicified, beds tend to carry higher gold values than silicified rocks. The presence of enargite, tetrahedrite, and high S copper sulfide minerals in the replacement ores and of pyrophyllite, diaspore, alunite, anhydrite, and high-temperature (•400øC), cornpositionally complex, hypersaline fluid inclusions in altered rocks bordering enargite veins in the stock, and the intense dissolution of limestone during deposition of the polymetallic orebodies suggest that the fluids initially had low pH and high f o, andj•, values. The magmatic hydrothermal system may have been similar to that of Julcani and other enargite-tetrahedrite districts in Peru, with the differences in ore types at least in part reflecting the carbonate composition of the host rock at Yauricocha. The elemental chemistry of the Purlsima Concepci6n ores can be qualitatively explained by a process of hydrothermal differentiation in which selective precipitation of base metal and Ag sulfides and sulfosalts depleted the fluids in these components and produced a decrease in Cu/Zn, Cu/Pb, As/Sb, base metal/precious metal, and Ag/Au ratios. The moderate pH and lowfo• andj• values implied by the mineral assemblage of Purisima Concepci6n presumably reflect in part the presence of carbonaceous material. Similar deposits can be expected elsewhere in the Western Cordillera of central and northern Peru where hydrothermal fluids related to Neogene stocks encountered impure carbonaceous limestone beds of the Pari...

Intrusion-related gold deposits

“…However, a genetic connection is supported by the presence of similar, structurally controlled gold-arsenic mineralization as an overprint to the other deposit types in the Bingham district, and by the fact that at least some of this gold mineralization pre-dated end-stage molybdenum introduction in the skarn environment (Cameron and Garmoe, 1987). Porphyry-type mineralization, with or without skarn and/or carbonate-replacement deposits, is also present with intrusions related genetically to the Tai Parit (Sillitoe and Bonham, 1990), Purisima Concepcion (Alvarez and Noble, 1988) and Star Pointer (Smith et al, 1988) gold deposits, and the Cove gold-silver deposit is only 1.5 km from, and apparently related to, the McCoy gold-skarn deposit (Emmons and Coyle, 1988).…”

“…been contributed directly from the subjacent porphyry environment as volatile complexes (Sillitoe, 1983b;Symonds et at., 1987). The acid-sulphate-type enargitegold deposit at Lepanto (see above) forms part of such a shallow advanced argillic environment, whereas the intrusion-hosted enargite-bearing veins at Yauricocha (Alvarez and Noble, 1988;Table 6.4) and the Equity Silver gold deposit (Table 6.5) were emplaced at deeper levels, possibly in the roots of now-eroded advanced argillic zones (Figure 6.8;Sillitoe, 1988). At Equity Silver, the presence of both andalusite and pyrophyllite rather than alunite in the advanced argillic assemblages, and fluid-inclusion homogenization temperatures as high as 400"C (Figure 6.10; Wojdak and Sinclair, 1984) support this interpretation.…”

“…The overall features of the gold-arsenic-(antimony) deposits, including the paucity of base metals and the frequent occurrence of arsenopyrite, realgar, orpiment and stibnite (or their supergene oxidation products) as accompaniments to decalcification and various degrees of silicification and argillization of the carbonate host rocks led Giles (1983), Lindsey (1985), Alvarez and Noble (1988) and Sillitoe and Bonham (1990) to compare one or more of the deposits summarized in Table 6.4 with sediment-hosted (Carlin-type) gold deposits.…”

“…Meyer and Hemley, 1967;Sillitoe, 1983b;Heald et al, 1987). Some of the metals, including Au and Cu, in these fluids, as well as the sulphur, may have Circled numbers and attached boxes denote gold deposit types and the homogenization temperatures CC) and salinities (wt.% NaCl equivalent) of representative ore fluids as determined for the intrusion-related deposits reviewed above and epithermal deposits in general: <Il porphyry type (Alvarez and Noble, 1988;Baker and Andrew, 1988;Bowman et al, 1987;Ewers and Sun, 1988;Morrison et al, 1988;Takenouchi, 1981;Theodore and Blake, 1978;Wojdak and Sinclair, 1984); @prograde skarn (Ewers and Sun, 1988;Theodore et al, 1986;Torrey et al 1986); @ retrograde skarn (Ewers and Sun, 1988;Theodore et al, 1986;Torrey et al, 1986); I!l carbonate-replacement Au-base metals; @ carbonate-replacement Au-As-Sb (Paterson et al, 1987); @ non-carbonate disseminated, stockwork and replacement (Eadington et al, 1974;Handley and Bradshaw, 1986); ® deep acid-sulphate (Wojdak and Sinclair, 1984); @ acid-sulphate epithermal (summary by Sillitoe, 1989a); and ® adularia-sericite epithermal (summary by Sillitoe, 1989a). Based on relationships detailed in Figure 6.8 and Sillitoe (l989a).…”

Le lien genetique entre la mineralisation en or et en mercure et l evolution petrolifere

“…It was first described in the west of the United States in 1962 (Radtke et al, 1980;Berger and Bagby, 1991;Bonham, 1988). However, during the past two decades, sedimentary rock-hosted, disseminated precious metal deposits have become an important source of gold because of their tremendous reserves and huge production (Berger and Bagby, 1991;Alvarez and Noble, 1988).…”