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Andean Porphyries 2018
Porphyry Cu & Au in the Andes
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[Exposed veining Alpala]
Alpala Discovery Outcrop. Left - Sheeted veining in outcrop in the creek bed;   Right - sulphide bearing veining. Channel samples in this vicinity return assays of 1 to 4 g/t Au and 0.3 to 1.3% Cu over horizontal widths of as much as 30 to 50 m.       Images by Mike Porter.
   The Cascabel Project and Alpala deposit are located within the Northern Andes. This segment of the Cordillera is characterised by a series of sliver-like terranes (the Western Tectonic Realm) composed of thick Mesozoic oceanic plateau rocks, too thick to subduct that were accreted to the South American plate during the Cretaceous to Paleocene. These oceanic rocks are bounded to the east by a Late Neoproterozoic to Lower Palaeozoic intra-oceanic arc terrane, which is overlain by a Late Triassic to Jurassic magmatic arc, and had been accreted to the Amazonian/Guyana Craton during the Lower Palaeozoic. The terranes of the Western Tectonic Realm are separated by major dextral reverse faults, and are overlain by Paleocene to Eocene and Oligocene to Quaternary magmatic arcs.
   The Cascabel Project deposits are Cu-Au rich and overlie an oceanic terrane basement, associated with well developed Paleocene to Eocene arc magmatism, located adjacent to one of the major NNE trending faults separating two of the oceanic plateau terranes. NW to NNW cross structures are implied by the structural and intrusive trends in the deposit area.
   The Alpala deposit is a near vertically plunging rod-like body with an ovoid shape in plan which has a vertical extent of ~1 km, the top of which is at a depth of ~800 m with upward and downward pointing fingers. It is associated with a complex of steep, narrow, tapering, Middle to Late Eocene hornblende bearing diorite, microdiorite and quartz-diorite bodies occurring as small stocks and as dykes. These intrude a sequence of andesitic pyroclastic rocks that overlie Eocene shales and greywackes. A late NNW trending hydrothermal breccia cuts the eastern side of the deposit, paralleling the principal trend of the deposit and intrusions.
   Mineralisation evolved from early quartz A type and magnetite veins, followed by quartz-magnetite-chalcopyrite B veins, magnetite-chalcopyrite-anhydrite, chalcopyrite C veins, chalcopyrite-pyrite to pyrite-chalcopyrite CD and D veins, to anhydrite, calcite and then late base metal veining.

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