UNIVERSIDADDECONCEPCI Ó N Geology of the MM Copper Deposit

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

Geology of the MM Copper Deposit, Chuquicamata District – An Update

Guillermo Müller and Jorge Quiroga Codelco Chile. Isidora Goyenechea 2934. Piso 5. Santiago, Chile ([email protected])

MM is a blind porphyry copper deposit covered by approximately 50 m of barren gravel. The central part of the deposit (MM Central) is located ~5 km north of Calama and 8 km south of Chuquicamata. Both shape and location of the MM deposit are related to a complex, NS-trending, 7-km long by ~300-m wide structural system that corresponds to the southern extension of the West Fault of Chuquicamata. To date, this central block has been explored with 100,000 m of diamond drill holes and 2.7 km of underground workings, with current geological resources amounting to 882 Mt @ 1.02% Cu, 15 ppm Ag, and 566 ppm As, for a cut-off grade of 0.25% Cu. The deposit remains open to the north, south and west.

The discovery of mineralization west of Mina Sur and on the west side of the West Fault in a single hole by Anaconda in the 60’s, and later in four additional holes drilled by Codelco in the 80’s, suggested the possibility for a left-lateral offset of part of the Chuquicamata deposit and stimulated further drilling. The final discovery during the 90’s by Codelco, of at least 2 km of continous, in-situ “Chuqui-like” mineralization (the MM deposit) on the west side of the West Fault has modified the concept of the displacement, which is now under revision.

Geologically, MM is associated with a composite porphyritic intrusion of intermediate composition (granodiorite porphyry, Feldspar Porphyry, dacite dikes) emplaced in Triassic granodiorite (206Pb/238U: 223.6+1.2 Ma) and volcanic rocks of the Agua Dulce Fm. Classic, porphyry-style Cu-Mo mineralization containing bornite, chalcopyrite and digenite accompany K-silicate alteration with stockworks of quartz, Kfeldspar, and anhydrite, at depths of ~600 m below the surface. This core alteration is overprinted and surrounded by Kfeldspar-sericite assemblages with chalcopyrite>pyrite which, in turn, are surrounded by a zone of intense sericite with pyrite>chalcopyrite. An external chlorite halo contains pyrite and copper grades <0.1%. The upper parts of the deposit are dominated by a series of fault controlled hydrothermal breccias and massive sulfide veins, in association with intense advanced argillic alteration with pyrophyllite, alunite, kaolinite, and sericite. Sulfide mineralogy of this zone is dominated by an As-rich, high-sulfidation assemblage with enargite and pyrite associated with important Cu, Ag, and Au grades.

Available 39Ar-40Ar geochronologic data for MM indicate that hydrothermal biotite from the deep-seated porphyry core formed at 34.3 Ma, whereas intermineral sericite formed at 32.5 Ma. Late-stage alunite of the upper, high-sulfidation environment yields ages between 31.8 and 31.4 Ma. This suggests that the hydrothermal system at MM evolved during a life span of approximately 3 m. y., broadly simultaneously with, but independently from, the nearby giant Chuquicamata system. Like Chuquicamata, Escondida and Rosario, hydrothermal telescoping at MM is interpreted to have been responsible for the structurally controlled, partial overprint of high-sulfidation associations over deeper-seated, porphyry-type K-silicate alteration and mineralization. Unlike Chuquicamata and Escondida, however, MM failed to develop a large, supergene chalcocite enrichment blanket, presumably due to early burial and fossilization of the system.

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