History and Geologic Overview of the Yanacocha Mining District, Cajamarca, Peru

Abstract

Discovery and recognition of the Yanacocha district as a belt of northeast-trending, near-surface, high sulfidation-type epithermal gold deposits with strong supergene oxidation was made over a four-year period (1984–1988) through a joint-venture alliance operated by Newmont Mining Corporation with partners Compania de Minas Buenaventura and CEDIMIN (BRGM). Open-pit mining began in the district in late 1993 with initial full-year gold production in 1994 of 300,000 oz and a reported end-of-year reserve of 4 Moz. The production growth curve at Yanacocha over 15 years (1994–2008) has been about 10 times the first full year of production in 1994. In February 2007, Minera Yanacocha (Newmont 51%; Buenaventura 44%; International Finance Corp 5%) poured its 20-millionth gold ounce life-of-mine. Reported 2008 end-of-year oxide reserves within the operating district are >13 Moz gold. Since exploration began in the district, over 20 hard-rock deposits and two gravel fan deposits (La Quinua, La Quinua Sur) have been developed. Cumulative historical discovery-to-reserve conversion costs average <$10/oz Au. The estimated oxide plus sulfide drill-indicated gold endowment within the district exceeds 70 Moz (2,170 tonnes). The high-sulfidation epithermal gold deposits are hosted by volcanic rocks that occur at the southern terminus of the northern Peruvian volcanic belt, a continuous sequence of a north-northwest–trending Miocene-Pliocene suite of bimodal andesite to rhyolite volcanic rocks that extend into southern Ecuador. In the Yanacocha district, the volcanic pile has been subdivided into three groups: (1) the lower andesite sequence, consisting of an intercalated sequence of block and ash flow tuffs, flow sequences with rare, associated flow domes, and an upper zone dominated by ignimbrites and fine-grained, laminated epiclastic sequences that show a transition into the overlying Yanacocha pyroclastic sequence; (2) the Yanacocha pyroclastic sequence, a variable sequence of lithic to lithic crystal tuffs, extensively altered in the central portion of the district and primary host to the majority of gold deposits within the district; (3) the upper andesite-dacite sequence, consisting of intercalated units of andesite to dacite flows, dominated by multiple flow dome complexes in its upper portion. Ar-Ar age dating within the district has yielded ages ranging from 19 Ma (basal lower andesite) to >12 Ma (upper andesite sequence). The entire volcanic pile has been crosscut by multiple phases of phreatic (vapor phase dominant), phreatomagmatic (intrusive component) and hydrothermal breccias, and intruded by multiple late-stage phases of andesite dikes and dacite to quartz dacite plugs, dikes and stocks (10–8 Ma), the latter of which are associated with shallow Au-Cu porphyry-style mineralization that underlies the high-sulfidation epithermal deposits. Alteration in the district consists of multiple stages of advanced argillic assemblages, typical in nature, that are zoned outward from a central core of massive to vuggy quartz to broader envelopes of advanced argillic alteration (alunite-pyrophyllite) to still broader zones of intermediate argillic and propylitic alteration halos. On a district scale, a generally southwest to northeast progression of multiple alteration centers with various timing has been documented through Ar-Ar dating of hydrothermal alunite, with ages ranging from 11.5 to 8.5 Ma. In places the advanced argillic alteration assemblage is overprinted by a later event consisting of creamy colored cryptocrystalline chalcedony plus barite associated with intermediate sulfidation type feeders. Gold mineralization is a late-stage event that was superimposed over all alteration types, most commonly occurring in late-stage fractures with massive, vuggy, and granular quartz, and especially where higher grade gold mineralization (>3.0 g/t Au) is associated with indurated replacement zones of creamy chalcedony. Mineral deposits are commonly localized within pyroclastic host rocks and phreatic breccias that envelop the margins of less permeable phreatomagmatic and within hydrothermal breccia pipes and andesite-dacite dome margins. Geophysical surveys have been employed as part of an integrated approach from the earliest exploration stages in the district. Resistivity and magnetic and potassium radiometric response anomalies are spatially associated with gold on a district-wide and deposit scale. Gravity surveys have added to understanding of basement geometry and have outlined volcanic subbasins. The east-northeast alignment of the Yanacocha gold deposits is interpreted to have been caused by the intersection of a northeast-trending trans-arc crustal break (Chicama-Yanacocha structural corridor). This is superimposed over older, north-northwest to northwest Andean-parallel folded and thrusted Cretaceous-age sedimentary basement terrane (Paleocene-Eocene). Both structural orientations dominate in the Yanacocha district, controlling emplacement of breccias and intrusions as well as formation of gold mineralization. Discontinuous east-west–oriented faults and fracture zones are interpreted to be extensional and control gold mineralization on a deposit scale. The transition from near-surface, gold-dominant mineralization to underlying copper-dominant sulfide mineralization associated with shallow quartz eye-bearing monzonite to tonalite intrusions has been documented by deep (500–1,200 m) diamond drilling beneath the Yanacocha complex (Sur-Norte-Oeste), the largest single gold deposit in the district with an endowment exceeding 18 Moz.