Summary:
Mineralization within the Cerro San Pedro district fits within two general classes of deposit types: 1) intrusion-hosted stockwork and/or disseminated sulphides/oxides within the San Pedro porphyry, and 2) carbonate replacement deposits (CRD) containing oxides and manto-style sulphides within the adjacent limestone.
Typically, the replacement mineralization within the limestone at CSP has one of the following geometries:
• tabular shaped at the porphyry–limestone contact • tabular to elongate fault-controlled veins and breccias
• circular to ovate, shallow to steeply raking breccia pipes/stockwork alteration zones.
The mineralization at Cerro San Pedro has two principal forms: 1) gold and silver within iron oxide minerals, and 2) gold, silver, zinc, and lead (plus minor, locally occurring copper) within sulphides. Both styles of mineralization are hosted within a well-developed system of crosscutting fractures (stockwork) in the San Pedro porphyry and along faults and fractures that extend into the surrounding limestone country rock. In addition, secondary oxide-hosted gold and silver mineralization is present in the nearsurface parts of the Cerro San Pedro district. Here, surficial weathering and oxidation has removed zinc, lead and copper from pre-existing primary sulphide minerals. The majority of the current mineral resources and reserves are contained within the gold and silver bearing oxide portion of the Cerro San Pedro deposit. Although no mineral resources or reserves have been defined for the deeper sulphide-rich portion of the deposit, gold, silver, zinc, lead, and minor amounts of copper were mined from highgrade sulphide deposits along the contact of the San Pedro porphyry and limestone country rocks.
Sulphide-dominated mineralization at CSP occurs in two modes: 1) low-grade porphyry-hosted stockwork and disseminated sulphides, and 2) high-grade polymetallic semi-massive to massive sulphide mantos replacing the adjacent limestones. The dominant sulphide minerals are pyrite, sphalerite, and galena.
Secondary (supergene) gold-silver mineralization in the limestones occurs within an assemblage of vuggy goethite + jarosite + hematite and manganese oxides. These minerals are associated with lead and/or zinc oxides and carbonates, secondary calcite, gypsum and anhydrite as well as other minerals commonly associated with oxidized carbonate replacement sulphide systems. Within the underlying San Pedro porphyry, secondary gold-silver oxide mineralization occurs within an assemblage of supergene goethite + jarosite + hematite and late alunite-kaolinite alteration that has been superimposed on earlier sericitic and propylitic alteration assemblages (Winterbourne, 1999; Petersen et al., 2001). Within the oxidized zone in both the limestones and porphyry, gold and silver typically occur independently as discrete, micron-sized, native grains (Victoria, 1990), a favourable characteristic for cyanide heap-leach extraction
methods.
Primary (hypogene) mineralization in the limestones is typified by semi-massive to massive sulphide replacement bodies consisting of pyrite + pyrrhotite, sphalerite and galena, with minor chalcopyrite. Wallrock alteration is characterized by decarbonatization (“sanding”) of the limestones and local remobilization of carbon proximal to carbonaceous sections of La Peña limestones. Porphyry-hosted sulphide mineralization is characterized by one to seven volume percent pyrite + sphalerite stockwork veinlets and disseminations, with local trace amounts of galena, chalcopyrite, tetrahedrite and arsenopyrite. Gold occurs as micron-sized blebs contained exclusively within pyrite grains; no gold has been observed in association with accessory arsenopyrite or other sulphides. Silver occurs primarily as either acanthite or argentite, but is locally observed as a minor admixture in native gold grains (Victoria, 1990).