Style of gold mineralisation in the ‘mantos’ of Cori Puno, Eastern Cordillera, Puno, Peru Warren T Pratt1 & Manuel Castro2 1 2 Specialised Geological Mapping Ltd, Station Rd, Urquhart, Moray, IV30 8LQ. UK. +44 7920 886101, info@geologicalmapping.com Coripuno S.A.C., Av. Javier Prado Este 3580 piso 7 Lima 41, +51 988438844, mcastro@coripuno.com.pe 1. Abstract Mantos are generally defined as stratiform mineralised horizons that were more susceptible/reactive to hydrothermal alteration and mineralisation. The term is used in the context of many different styles of mineralisation, including epithermal, porphyry and orogenic. This talk describes ‘mantos’ from Cori Puno, across the watershed from La Rinconada (about 6 km to the southwest), in the Peruvian Cordillera Oriental. The mantos are hosted by andalusite/cordierite schists and black, organicrich slates. We interpret them as flat-lying shear zones and isoclinal fold hinges, almost parallel to the local schistosity/cleavage and bedding. The shear zones are subtle and the ‘mantos’ are marked by major flat quartz veins and swarms of en echelon quartz and sulfide (mostly pyrrhotite) veins. There is a distinct paragenesis of vein types, including ‘Zebra’ veins. 2. Introduction There is very little published information about the mantos at La Rinconada, in the Cordillera Oriental of Peru (Figure 1). Access is difficult, because of social issues and insecurity. Fornari et al (1982) and Fornari & Herail (1991) interpreted the Rinconada mantos as syn-sedimentary, but noted the common presence of bedding-parallel smoky grey quartz veins. We report on the Cori Puno mine. Operated by Consorcio Minero Horizonte (CMH), this produces about 53,000 ounces of gold per year. This gold comes exclusively from flat-lying orebodies, generally described as ‘mantos’. Our article investigates the origin of this gold. 3. Lithostratigraphy At Cori Puno this comprises phyllites/schists of the San Jose Formation, rich in andalusite and cordierite porphyroblasts, overlain by dark slates of the Sandia Formation (Figure 2). The two formations are separated by a distinct regional marker, a clean quartzite bed, approximately 5-30 m thick (Figure 2). The dark slates of the Sandia comprise well bedded, dark grey to black, organicrich mudstones and siltstones. There are also beds of bioturbated meta-sandstone. The slates have a strong tectonic cleavage; metamorphic grade is probably low greenschist, possibly sub-greenschist. (The rocks are best described as slates rather than phyllites, since individual micas cannot be seen.) The slates contain common disseminated pyrite and pyrrhotite, probably of diagenetic origin (anoxic seafloor). The gold-bearing ‘mantos’ occur within both the formations, but the principal examples occur within the Sandia Formation. 4. Intrusions There are small exposures, in the valley bottoms, of a muscovite-bearing granite at Cori Puno. Narrow, folded dikes of granite also cut the San Jose Formation. These intrusions may explain the increased metamorphic grade and widespread cordierite/andalusite (classic contact metamorphic minerals) in the San Jose Formation. 5. Tectonics The rocks in the La Rinconada-Cori Puno tract display unusually flat bedding and cleavage/schistosity. This is largely beddingparallel. There is plenty of surface evidence of overturning and isoclinal folds (Figure 2). There are also lengths of drill core at Cori Puno where the bedding is steeper than the cleavage/schistosity, implying overturning and isoclinal folding. Boudinage of the more competent beds, such as the quartzite marker bed, is widespread. These competent beds also contain abundant quartz tension gash veins. Thrusts have been described from La Rinconada (Fornari et al., 1982). There is a host of post-mineral normal faults that offset the mantos at Cori Puno. There are also widespread crenulation cleavages and kink bands that likewise post-date mineralisation. The Northeast-striking kink bands give rise to a strong Northeast topographic control on creeks and rivers and precipitous cliffs. There is no doubt that true mantos exist in the Cordillera Oriental of Peru. The Vetaspata mantos, close to Sina, on the Bolivian border, are clearly examples of thin reactive beds (more feldspathic sandstone) that interacted with hydrothermal fluids to produce increased sulfides and caused gold mineralisation. These comprise bedding- and cleavage/schistosityparallel zones between 1 and 30 m thick. Despite detailed underground mapping and logging, there is no discernible difference between the mineralised ‘mantos’ and the underlying and overlying strata. For example, they do not appear to contain more organic material. Instead, they are zones of increased veining (both parallel, and oblique, to bedding) and increased disseminated pyrite + pyrrhotite + minor chalcopyrite. The uppermost manto contains abundant stibnite (a typical mineral of high level orogenic gold deposits). The increase in veining seems to be related to a more intense cleavage, isoclinal fold hinges and subtle, easilymissed shear zones and faults. However, the Cori Puno ‘mantos’ are ‘pseudomantos’. They are bedding parallel, but they are not discrete reactive beds. Instead, they comprise tectonic zones, probably within fold hinges, with swarms of grey, and milky, quartz veins. These veins developed whilst the strata were being folded and cleavage was developing. They therefore fall firmly into the category of orogenic gold. They closely resemble the ‘slate belt’ or turbidite-hosted gold deposits of Victoria State in Australia (BendigoBallarat). The Cori Puno (and Rinconada?) veins developed under compressional conditions in the brittle-ductile transition, typical of greenschist facies conditions. A suggested model, showing the two main generations of veins (early grey, late milky), is shown in Figure 4. It shows more intense cleavage within a probable isoclinal fold hinge and a consistent sense of vergence of the veins (Top to Northeast). This creates possible Northwest-striking oreshoots formed by ‘jogs’. 7. Vein paragenesis Acknowledgements Elucidating the sequence (paragenesis) of veins helps us understand the controls on gold mineralisation at Cori Puno. Early veins are a distinct smoky grey colour, as described by Fornari et al (1982) from La Rinconada. These early veins began life as tension gases, but suffered subsequent folding and rotation parallel to bedding/cleavage. They were also boudinaged. They are thus clearly syn-tectonic. Detailed underground sampling and drill core samples show that this generation of veins contains the gold. Many thanks to Gonzalo de Losada (CMH) for permission to describe the mineralisation at Cori Puno. 6. ‘Mantos’ The early grey quartz veins are cut a distinct sequence of flat-lying, bedding parallel major milky quartz veins and common en echelon oblique quartz tension gash veins. The latter give a consistent ‘Top to Northeast’ sense of displacement. ‘Zebra veins’ (Figure 3) comprise early grey, gold-bearing quartz veins cut by regular tension gash veins of milky (non mineralised) quartz. 8. Conclusions References Fornari, M, Herail, G & Laubacher, G. 1982. El oro en la Cordillera Suroriental del Peru: el placer fluvioglacial de San Antonio de Poto (Departamento de Puno) y sus relaciones con la mineralización primaria de La Rinconada. Quinto Congreso Latinoamericano de Geología, Argentina. Actas IV: 369-386. Fornari, M & Herail, G. 1991. Lower Paleozoic gold occurrences in the ‘Eastern Cordillera’ of Southern Peru and Northern Bolivia; A genetic model. Brazil Gold ’91. E A Ladeira (Ed.). Balkema, Rotterdam. ISBN 906191 1958. Sanchez, A & Zapato, A. 2001. Mapa geológico del cuadrangulo de La Rinconada. Escala 1:100,000. Instituto geológico minero y metalurgico, Ministerio de Energia y Minas, Republica del Peru. Illustrations Figure 1 Simplified geological map of the La Rinconada-Cori Puno area. Modified after Sanchez & Zapato (2001). Figure 3 Typical vein textures at Cori Puno, showing early grey (gold-bearing), and later milky, quartz veins. Figure 2 Simplified lithostratigraphy at Cori Puno. Figure 4 Model for the development of the goldbearing veins at Cori Puno.
