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Andean Porphyries 2018
Porphyry Cu & Au in the Andes
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[Antamina pit]
Looking down into the Antamina pit.    Antamina is a Zn-rich porphyry-skarn Cu-Mo-Ag deposit. It occurs as a shell forming a thick cap above, and downward tapering flanks surrounding a low grade to sub-economic Miocene (~9.8 Ma) calc-alkaline (monzogranitic) porphyry Cu-Mo mineralised intrusion. The mineralised skarn formed in the extensive, shallow, Cretaceous shelf/basinal carbonate sequence of the Peruvian Andes, with ~75% in the massive to thick-bedded, relatively pure limestones of the Jumasha Formation, and ~25% in limestones, marls and shales of the overlying Celendin Formation. The main intrusion has not been unroofed, with only dykes and sills outcropping. In the deposit area the sequence has been subjected to instense thrusting and imbrication and lies within a regional cross-cutting structural discontinuity.
   Metal distribution has a symmetrical lateral zonation outward and upward from: i). a central Mo zone with low Cu in the intrusive core and into the surrounding pink to brown garnet endoskarn; ii). the principal Cu zone, distributed from the broad endoskarn, through the brown, grading outward to green garnet exoskarn to the marble contact; iii). an outer Cu-Zn halo, representing the overlap of the broad Cu zone and the narrower (up to 70 m) Zn periphery; iv). a discontinuous marginal Cu (bornite)-Zn-Bi zone, mainly within the wollastonite skarn locally present between the exoskarn and marble; v). a peripheral Ag-Pb-Zn zone as veins and mantos within wollastonite skarn and marble.
   Antamina is unusual in that the Zn mineralisation, while forming a peripheral zone, is relatively close to the porphyry and is telescoped onto the outer Cu mineralisation in the exoskarn developed in the relatively pure limestone host. Much of the skarn is prograde, with retrograde alteration being structurally controlled and largely restricted to the endoskarn.
   Skarn mineralisation continues undiminished over a vertical interval of at least 2 km, while deep drilling has encountered breccias and increased chalcopyrite, possibly indicating a transition to more significant porphyry style mineralisation at depth (Chang, 2012).
   Antamina illustrated another example of a porphyry-skarn deposit, one that is Zn-rich and is preserved above as well as lateral to a mineralised porphyry intrusion.     Image provided by Song Zhang.

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