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Andean Porphyries 2018
Porphyry Cu & Au in the Andes
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[Toromocho pit and core]
Toromocho pit and drill core. Following the presentation, the group was driven to the upper benches of the open pit where the geology and distribution of ore below was explained with the aid of plans. For safety reasons they were not allowed further into the pit. However, the mine geologists had collected a ute (pick-up) load of large representative specimens of the hosts and ore which the group voraciously comminuted into hand specimens to carry away and/or imaged with their phones or cameras.
   After lunch they were taken to the core yard to study some representative holes through the deposit. Then in the mid-afternoon, they returned to the mine gate where the three Prados were waiting to transport them back to Lima by early evening. Yet another long, informative and very rewarding day.       Images by Mike Porter.
   The Toromocho porphyry-skarn Cu-Mo deposit lies in the heart of the Morococha District, surrounded by economically important Cordilleran polymetallic (Zn-Pb-Ag-Cu) vein and replacement bodies distributed over an area of ~50 km
2. Two other smaller porphyry occurrences, Ticlio and Codiaco are found 1 to 2 km NE and ~3 km WNW of the Toromocho porphyry deposit respectively.
   Pre-mineral magmatic activity in the Morococha district, predominantly diorites, commenced at ~14.1 Ma in the mid-Miocene. Granodioritic porphyries were emplaced at 9.3 Ma, followed by an 8.17 Ma feldspar porphyry, the two more significant of at least five porphyry intrusive phases recognised at Toromocho. Prograde skarn and hornfels formation accompanied the granodiorite porphyries, developed within the shelf to shallow basin carbonate rich Triassic to Jurassic Pucará Group country rocks. Breccia pipes, dated from 8.98 Ma, cut all of these porphyries and the skarn. Late mineral 7.75 Ma quartz porphyries cut the skarns and breccias.
   While porphyry-style mineralisation is indicated at ~9.3 and 8.0 Ma at Codiciada and Ticlio respectively, the first mineralisation at Toromocho appears to be quartz-molybdenite veining dated at ~7.9 Ma, followed by Cu persisting to at least 6.8 Ma representing at least two hydrothermal pulses. Cu-Mo mineralisation and the formation of hydrous/retrograde skarn occurred over an area of 6 km
2 from 7.2 to 6.8 Ma during the waning stages of magmatism. These are mainly represented by exoskarns that are either magnesian, composed of serpentine-magnetite±phlogopite and tremolite-diopside-serpentine-chlorite-talc, or calcic andradite-diopside-epidote assemblages.
   Cordilleran polymetallic veining cuts the breccias, porphyries, skarn and porphyry-skarn hosted mineralisation, occurring as sphalerite, galena, tennantite-tetrahedrite, chalcopyrite and enargite, with silver mostly in tennantite-tetrahedrite and as hessite. Polymetallic veins are dated at ~5.75 Ma and polymetallic replacement bodies at ~6.2 Ma, although lesser early ~8.2 Ma polymetallic mineralisation has also been recognised to the NE.
   The porphyry and skarn mineralisation at Toromocho account for 60 and 40% of the resource respectively. Grades are usually higher in the skarn, forming large zones of up to 1.2 to 1.5% Cu. In addition, breccia pipes host higher grades in the porphyry deposit.
   Toromocho illustrates the relationship between porphyry and skarn Cu-Mo and Cordilleran style Zn-Pb-Ag-Cu vein and replacement deposits in the Peruvian Andes.

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