Geology of the Concealed MM Porphyry Copper Deposit, Chuquicamata District, Northern Chile
Richard H. Sillitoe, Juan Carlos Marquardt, Fernando RamÍRez, Hugo Becerra, Marcelo GÓMez, 1998. "Geology of the Concealed MM Porphyry Copper Deposit, Chuquicamata District, Northern Chile", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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The MM porphyry copper deposit is located 5 km south of the giant Chuquicamata deposit, and is concealed totally beneath >50 m of Miocene alluvial gravels. Hence geologic understanding is based on extensive core drilling and underground openings.
The MM deposit and related mineralization farther south comprise a semicontinuous tabular zone that extends for 8 km from north to south, is inclined steeply west, and attains a depth of at least 1000 m. The deposit parallels and is delimited along its eastern side by the West fault, a major sinistral strike-slip structure of postmineral age.
The mineralized zone is made up of four steeply inclined panels juxtaposed by postmineral faults subsidiary to the West fault. Panel one, abutting the West fault, comprises weakly chloritized and pyritized andesitic flows. Panels two and three consist of pervasively sericitized granodiorite containing pyrite-poor porphyry copper-type mineralization. The sericitic alteration largely destroyed a pre-existing K-silicate assemblage. Panel four is characterized by structurally controlled sericitic and advanced argillic alteration accompanied by enargite- and bornite-rich, high-sulfidation sulfide assemblages in granodiorite. These alteration-mineralization types grade westward in panel four through weak sericitic to pyrite-poor chloritic alteration, which affects granodiorite, dacite porphyry dikes, and roof-zone andesitic flows.
The high-sulfidation copper zone in panel four comprises irregular stringers, patches, short veins, stockworks, and hydrothermal breccias filled with massive sulfides ± chalcedonic quartz. The hypogene sulfides are dominated by pyrite and enargite, with subordinate amounts of tennantite, bornite, digenite, chalcocite, and covellite, which can give rise to average copper grades of >2 percent. The north-striking structural system that localized the high-sulfidation copper zone is considered as an ancestral West fault.
The MM deposit was assembled structurally as a series of fault slices that are considered to have traveled southward along the West fault. It would seem logical to presume that the slices were detached from the truncated western side of the Chuquicamata deposit. However, although MM and Chuquicamata geology possess similarities, especially with regard to the presence of structurally controlled, high-sulfidation copper zones, there are also several unexplained lithologic and alteration differences between them. These differences may be used to suggest another shallowly eroded porphyry copper system as the source of the MM components. Furthermore, MM lacks the mature supergene profile so important at Chuquicamata, and is essentially a hypogene deposit.