U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

Enargite-Gold Deposits in Marcapunta, Colquijirca Mining District, Central Peru: Mineralogical and Geochemical Zonation of Subvolcanic, Limestone-Replacement

Deposits of High-Sulfidation, Epithermal Character César E. Vidal and Rolando Ligarda

Compañía de Minas Buenaventura S.A.A. Av. Carlos Villarán 790, Urbanización Santa Catalina, Lima 13, Perú

(cesarvid@buenaventura.com.pe)

The Marcapunta enargite-gold deposits are located in the center of the Colquijirca mining district, 310 km north northeast from Lima and 10 km south of the Cerro de Pasco mine. Elevations range from a 4200 m plateau to 4500 m high peaks and ranges. Regional geology preserves folded Permo-Triassic red beds of the Mitu Group overlain by Pucará Group limestones and dolomites of Triassic to Jurassic age, which in turn are followed by carbonate breccias, conglomerates and fresh-water limestones of the Eocene Calera Formation. These units are intruded and overlain by dacitic domes and pyroclastic flows of the Marcapunta volcanic center, with biotites dated at 11.5+0.4 Ma by K/Ar and 12.4+0.1 Ma by Ar/Ar methods. The North-South-trending Falla Longitudinal growth fault determined basin morphology for both the Pucará and the Calera sedimentation; it also controlled the emplacement of the Cerro de Pasco and Marcapunta Miocene volcanic edifices. The Colquijirca mining district has produced Ag-rich ores since pre-colonial Inca times —and particularly in the beginning of XXth century— from primary tennantite-polibasite ores and often supergene-enriched, near-surface, gossanous stromeyerite-native silver occurrences. During the last twenty years the operation has expanded from 1000 tpd to 3500 tpd open pittable Zn (Pb-Ag) ores. Miscellaneous mining campaigns from 1974 to 1982 for Cu (Au) sulfide ores were carried out for two to three months a year from a 500-tpd-underground trackless operation. Albeit composition, silicification and sulfide mineralization are predominantly stratiform in geometry and tend to follow folded beds of the Calera and/or Pucará formations. Historical mining of polymetallic ores in the district adds to an estimate of 13 Mt. Current ore reserves are 8 Mt @ 6.4% Zn, 2.4% Pb, and 3.1 oz/t Ag. The Marcapunta enargite-gold deposits are zoned northwards into the Zn (Pb) Ag ores of Colquijirca and southwestwards into the Zn (Pb) San Gregorio deposits. The Miocene volcanic center at Marcapunta is strongly altered to quartz-alunite ledges with argillic haloes. Alunite separates have been dated at 11.6+0.1 Ma by K/Ar and 10.6+0.1 Ma by Ar/Ar methods. Main silicification and strong quartz-alunite ledges are controlled by several prominent East-West fractures and attain thicknesses from 20 m to 50 m. Mineral assemblages vary outwards from vuggy quartz to quartz-alunite+dickite, illite-kaolinite+montmorillonite and chlorite-calcite envelopes. Subvolcanic copper mineralization surrounds the diatreme and flares outwards along the base of the dacitic domes of the Marcapunta volcano. Semi-massive to massive pyrite bodies, with enargite and traces of covellite with gold, preferentially replace limestone breccias and conglomerates of the Calera Formation and occur trapped between the underlying Mitu Group sandstones and the overlying domes. Ore zones attain thicknesses of 100 m adjacent to the steep diatreme walls and thin out laterally into discrete stratabound manto and breccia horizons. The northern flank of the Marcapunta volcanic center hosts a drill-indicated resource of 50 Mt @ 1.9% Cu, 0.3 g/t Au, 0.6% As, and 23.3 g/t Ag. Calculations are based on 20,964 m distributed in 74 diamond drill holes sited in a 100 m by 100 m grid. The western and southwestern flanks are characterized by a distinct gravimetric anomaly, which was surveyed and has been drill-tested in 2002 by five holes aligned North-South along a 1.8 km belt. Assuming 50% continuity, drill-inferred resources discovered last year are estimated as an additional 50 Mt @ 1.2% Cu, 1.4 g/t Au, 0.3% As, and 17.8 g/t Ag. Gold appears to be enriched in the southwestern portion of the Marcapunta replacement lodes. Cu/As ratios also increase from 3/1 in Marcapunta Norte to 4/1 in Marcapunta Oeste. Metallurgical assessment of these potential Cu-Au ores has —so far— demonstrated ammenability to flotation to produce high-grade, arsenical copper concentrates. Gold recoveries need to improve by roasting or autoclaving combined with cyanidation. Ferric arsenate is the preferred and more stable residue component of the envisioned metallurgical process.

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