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Andean Porphyries 2018
Porphyry Cu & Au in the Andes
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[Yanacocha Pit and core]
On to Cajamarca and the Yanacocha epithermal-porphyry system. The group left Challhuahuacho on the morning of Saturday 10 June for the long drive back to Cusco where they spent the night before departing next morning for Lima with a connecting flight to Cajamarca in northern Peru to arrive late in the afternoon. That evening they underwent yet another set of medical examinations to be judged fit to proceed up to the Yanacocha mine early next morning.
   On arrival at the Yanacocha mine on the morning of Monday 11 June, after a pre-dawn departure from Cajamarca, the group was given a very comprehensive presentation on the setting, geology, structure, mineralisation and alteration of the Yanacocha cluster of deposits. This was followed by a geological overview in the main composite Yanacocha pit (above) followed by the opportunity to study the geology, alteration and mineralisation in sections of the pits. Subsequently the group inspected drill core through the deposits and representative core of the different lithologies and alteration styles, before returning to Cajamarca in the late afternoon to early evening after another long and rewarding day.       Images by Mike Porter.
   Yanacocha, like Toromocho and Antamina, is located within the Peruvian Andes which are characterised by extensive, thick late Palaeozoic to Mesozoic shallow basin to shelf sequences of predominantly carbonate and lesser siliciclastic rocks. These successions were deposited on a broad and stable exotic terrane of Proterozoic rocks accreted to the Amazonian Craton during the Lower Palaeozoic, and are, in turn, intruded and overlain by Cenozoic intrusive and volcanic arcs.
   The deposits at Yanacocha range from the concealed Kupfertel porphyry Cu-Au mineralisation to high sulphidation epithermal Cu-Au deposits superimposed on the lithocap alteration above the porphyry system. Within this system, supergene sulphide Cu blankets and leached oxide Au deposits are developed, as well as the La Quinua transported deposits. As such the mineralised system illustrates the porphyry-epithermal system and was a very valuable, and well presented part of the tour.
   Virtually all of the mineralisation within the Yanacocha cluster of porphyry and epithermal deposits is hosted within intrusive, volcanic and volcaniclastic rocks, with minor skarn alteration on the eastern margin of the corridor where Cretaceous carbonate rocks become the dominant country rocks. In addition, as with almost all of the porphyry-epithermal mineralisation in the Peruvian Andes north of the Abancay Deflection, alteration and mineralisation is of Miocene age.
   The Yanacocha cluster is also a member of a group of Cu-Au, Au-Cu and Au porphyry-epithermal systems found in northern Peru which may be differentiated from a string of major Cu-Mo porphyry deposits that are generally a few to a few tens of kilometres further east.

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