The Trapiche NiCo prospect , Yauli Dome , Central Peru : formed by Miocene magmatic-hydrothermal activity

The Trapiche Ni-Co prospect, province of Yauli, is located in the SE tip of the Yauli Dome, 2 km SE of the Andaychagua Ag-Zn-Pb mine. At Trapiche, discontinuous Ni-Co-Zn-Pb-Cu manto bodies are hosted by weakly metamorphic carbonate and basaltic rocks of the Devonian Excelsior Group. Despite its Ni-Co rich mineral association, Trapiche shares main characteristics with Miocene Cordilleran polymetallic deposits in the area. Resumen El prospecto Trapiche está ubicado en el extremo SE del Domo de Yauli, a 2 km al SE de la mina de Ag-Zn-Pn de Andaychagua. En Trapiche mantos discontinuos con mineralización principal de Co-Ni-Zn-Pb-Cu están hospedados en calizas y basaltos del Grupo Excelsior que muestran un leve metamorfismo. A pesar de su mineralogía rica en Ni y Co, Trapiche comparte las características principales de yacimientos polimetálicos cordilleranos miocenos en el área. Introduction and geological background The Trapiche Ni-Co prospect (11.755532°S, 76.000321°W, 4400 masl), is located 2 km SE of the Andaychagua Mine, Yauli Province, in the western Andean Cordillera of Peru, about 110 km east of Lima. The prospect is located in the SE tip of the Yauli Dome, a NNW-SSE elongated anticlinorium core consisting of Paleozoic rocks that outcrop between Mesozoic and younger rocks. The Yauli Dome is crosscut by Eocene and mainly late Miocene intrusive rocks. Porphyry Cu-Mo and porphyry-related epithermal polymetallic “Cordilleran” (Bendezú and Fontboté, 2009, Fontboté 2020) mineralization including, from south to north, the deposits of Andaychagua, San Cristobal, Toromocho, and Morococha (Beuchat, 2003, Beuchat et al., 2004, Catchpole et al., 2015 a,b) are genetically related to the late Miocene magmatic activity. Main intrusions are aligned in NNW-SSE direction and main faults strike NE-SW and NNW-SSE. The Trapiche area is affected by Pre-Andean tight overturned folds with steep axes dipping to the SW. The host sequence is inverted. The ore is mainly hosted by hydrothermally altered encrinitic limestone (Rosas et al., 2010) located on top (i.e., stratigraphically underlying) basalts that in places contain also minor Ni-Co mineralization (Fig. 1). Both, encrinitic limestones and basalts are intercalated Excelsior organic-rich shales attributed to the Devonian (McLaughlin, 1924, Harrison, 1943, Saintilan et al. 2021) and that may be correlated to lithologically similar Early to Middle Devonian units in southern Peru including the Cabanillas Group (Newell 1949, Boucot et al, 1980). Volcanic rocks at Trapiche are MORB within plate tholeiite alkaline basalts. Excelsior pillow lavas occurring 1 km to the east are within plate sub-alkaline basalts. The sequence is affected by weak metamorphism related to the Carboniferous Gondwanian (formerly, eo-Hercynian) Orogeny (Chew et al. 2016). The presence of nickel and cobalt at Trapiche and other occurrences had been reported by Kobe (1982). In 2007, Volcan Compañía Minera performed an exploration program including 700 m underground workings and 22 short diamond drill holes defining an in average 1.10 m thick and 1050 long discontinuous “manto", with 0.60% Ni and 0.11% Co (Oldham et al., 2008). Zinc contents are in general below 2000 ppm, those of Cu and Pb largely below 1000 ppm, those of Bi, As, Sb lie mostly below 100 ppm, and those of Ag below 10 ppm. Gold contents are low, below 10 ppb whereas those of Pt higher, around 100 ppb in Ni-rich intersections; Pt/Pd ratios around 30. Previous works on Trapiche and similar nearby occurrences suggested, mainly based on the stratiform character of the ore showings and their correlation with basaltic rocks, a syngenetic volcanic-exhalative origin (Kobe 1990; Oldham et al. 2008; Acuña et al., 2008). The present study focused on the Trapiche main manto challenges this interpretation. Mineralization and alteration styles Figure 2 shows a paragenetic sequence of the Trapiche manto. Nickeland cobalt bearing recognized sulfides are millerite (Figs. 3, 4, 5, 6 ) siegenite-violarite (Figs. 4, 5, 6, 7), gersdorffite, cobaltite, and ullmannite. They occur with pyrite, Fe-poor sphalerite (~4-1% mol FeS), galena, chalcopyrite, and tetrahedrite. Sulfides appear as euhedral to subhedral finegrained crystals within small aggregates up to 4 mm in length, mainly filling spaces between the fragments of crinoids (Fig. 3). Fig. 1. Outcrop of the Trapiche manto. It mainly replaces encrinitic limestone and subordinately metabasalt layers intercalated in phyllites of the Excelsior Group with pre-ore quartz veins produced by metamorphic segregation. Fig. 2 Paragenetic sequence at the Trapiche manto. Manganese-rich carbonates are the main alteration minerals in Trapiche. A first generation of Mn>Fe>>Ca-Mg carbonates replaces pervasively the limestone and partly the basalt. Less abundant Mn-Ca-Mg-Fe carbonates (with distinctly lower Mn and Fe contents) occur in late veins and crosscut metamorphic segregation quartz veins (Fig. 8.) Quartz and minor sericite are intergrown with both generations of carbonates. Black alteration crusts of Mn-Fe oxides resulting from the oxidation of the Mnbearing carbonates are observed in the manto outcrops, both in basaltic and carbonate host rocks.