Morococha District - Toromocha, Morococha
Cu Mo Ag
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The Toromocho porphyry copper-molybdenum and the Morococha silver-lead-zinc-copper deposits are located in the Morococha mining district, Yauli Province, Junin Department of central Peru, ~140 km east of Lima, 100 km south of Cerro de Pasco, 30 km west of La Oroya and 2 km south of the town of Morococha, at an elevation ranging from 4700 to over 4900 m above sea level (#Location: Toromocho - 11° 36' 32"S, 76° 8' 36"W).
Mining began in the Morococha district in pre-colonial times, before the 16th century, and production has been continuous since the late 19th century. Between 1915 and 1918, much of the district was reorganized and incorporated into the Cerro de Pasco operation. By 1924, Cerro de Pasco was producing from the district at a rate of 1500 tonnes per day from primarily copper ores containing 6% copper. Between 1929 and 1934, Cerro de Pasco excavated the 11.5 km long Kingsmill Tunnel, successfully dewatering all of the Morococha District mine workings above the 4020 m elevation of the tunnel. The Kingsmill Tunnel was still in use in 2014.
The earliest record of the Toromocho deposit was from 1928 when a low grade copper zone was discovered on the margin of the monzonite stock of the San Francisco peak along with several other low grade blocks. Between 1954 and 1955, Cerro de Pasco Corporation undertook an exploration program that confirmed the presence of mineralisation but not the porphyry potential of the district. After 1963, Cerro de Pasco geologists drilled an angle hole from the top of San Francisco peak that intersected oxidised material, but not the main deposit. This was followed in 1966 by a campaign of vertical holes to depths of ~400 m, many of which were terminated in ore grade material. A second Cerro de Pasco campaign was not commenced until May 1970 and involved 39 holes to a maximum depth of ~300 m, followed in 1972 by another 10 holes and a small test pit. In May 18, 1973, the Peruvian Government declared all mining rights in Toromocho as obsolete and transferred the properties to Centromin a Peruvian government entity. From April 1974 to January 1976, Centromin carried a phase of major exploration drilling involving 61 holes. In August 1980, Centromin hired Kaiser Engineers International, Inc. (Kaiser) to prepare a detailed feasibility study of the project. In 2003, Minera Peru Copper Syndicate S.A. acquired an option on the property from Centromin and drilled 5 twinned holes to confirm earlier results. Subsequent engineering and feasibility studies were undertaken (Behre Dolbear, 2012). In 2007, Chinalco acquired Minera Peru Copper and in 2013 commenced mining operations.
Morococha is an underground silver-zinc-lead-copper mine, owned and operated by Argentum, a Peruvian company in which, in 2016, Pan American Silver Corp., through its subsidiary Pan American Silver (Peru) S.A.C., had a 92.01% voting interest. It is located immediately to the north and NE of the Toromocho porphyry copper deposit. Historically, the Gubbins family had begun operating mines in the Morococha District in the 1940s, through Minera Santa Rita S.A. and Minera Yauli S.A., which were subsequently consolidated in the late 1990s into Sociedad Minera Corona S.A. These mines were separate to the Cerro de Pasco operations, which after 1973 were operated by Centromin, until 2003 as detailed above. In 2004, Pan American Silver entered into an agreement with the Gubbins family, to form Argentum to operate the Morococha mines. Few production records are preserved from the early years of mining in the district, although between 1989 and 2003, ~7.9 Mt of ore were mined at a grade of 227 g/t Ag, 0.5% Cu, 1.7% Pb, and 4.6% Zn (Pan American Silver Corp. 2014).
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
District Geology and Mineralisation
The Morococha district vein and porphyry systems cover an east-west elongated area of ~10 x 5 km, and are underlain by a ~2000 m thick sequence of folded Palaeozoic and Mesozoic schists, volcanic rocks and predominantly carbonate, primarily limestone, intruded by multiple, Tertiary age igneous intrusive phases. The limestones have been folded into a generally NNW trending anticline with limbs dipping to the east and west.
There are four main stratigraphic units represented in the Morococha district, from oldest to youngest, namely the:
i). late Permian to middle Triassic Mitu Group continental volcanic rocks and red beds,
ii). late Triassic to early Jurassic Pucará Group sedimentary carbonate and volcanic rocks, and basalts,
iii). the early-Lower to mid-Lower Cretaceous siliciclastric rocks of Goyllarisquisga Group and
iv). Upper Cretaceous Machay Group carbonates of the Chulec, Pariatambo, Jumasha and Celendin Formations (Kouzmanov et al., 2008).
Miocene magmatic activity in the Morococha district started with the emplacement of the mid-Miocene, 14.07±0.04 Ma (Pb/Pb; Kouzmanov et al., 2008) Anticona diorite which dominates the south-western part of the district, but has no indication of associated mineralisation. In the Late Miocene, from 9 to 7 Ma, a series of porphyritic diorites and granodiorites to quartz-monzonites intruded the volcano-sedimentary sequences as well as the Anticona diorite. Most are barren, but some have related porphyry Cu mineralisation. The Toromocho porphyry Cu-Mo deposit dominates the central part of the district and has associated extensive potassic and phyllic alteration zones, affecting feldspar- and quartz-porphyry intrusions. The Potosí, San Francisco and Ticlio porphyritic stocks exhibit K- and/or Na-Ca alteration with associated porphyry-style mineralisation, including quartz-chalcopyrite, quartz-magnetite, quartz-pyrite-molybdenite and quartz-pyrite veinlets (Bendezú et al., 2007). Massive magnetite-serpentine exoskarns and diopside-garnet endoskarns, partly hydrated to epidote, amphibole and chlorite, often bearing polymetallic mineralisation, are found where mainly Jurassic dolomitic carbonates of the Pucará Group are in contact with porphyry intrusions (Catchpole et al., 2008).
The principal post-porphyry mineralisation styles in the Morococha district are (after Catchpole et al., 2008):
• Massive pyrite-quartz bodies - with phyllic alteration halos occur in the fringe areas of certain intrusives and/or as replacement of previously formed breccia zones such as at the base of the Pucará Group, just above the Triassic volcanics.
• Polymetallic replacement bodies - mainly occur as the replacement of tectonic breccias, in part developed along overthrust planes within the Pucará Group, and skarn altered beds of specific limestone bands within the lower units of the Pucará Group (e.g., Manto Italia). Polymetallic bodies fed by polymetallic veins (e.g., Brecha Rosita) are frequently hosted within adjacent pyrite-quartz bodies. Replacement bodies contain assemblages that range from magnetite, chalcopyrite, sphalerite and galena-bearing, pyrrhotite- and pyrite-dominated bodies, to quartz-carbonate-sulphosalts accumulations. Mostly sub-vertical Cu-rich tube-like bodies with rich chalcopyrite, tennantite-tetrahedrite (fahlore) and enargite ores, occur in the central part of the district, located on the rim of the San Francisco intrusive, and were exploited during the early 20th century.
• Epithermal polymetallic veins - Steeply dipping epithermal Zn-Pb-Ag-Cu-bearing veins with phyllic alteration halos are hosted by NNE to ENE trending structures, and at a district scale, cut the intrusive bodies, surrounding sedimentary rocks, skarn alteration, as well as pyrite-quartz and polymetallic mineralisation (Catchpole et al., 2007). Their vertical extent exceeds 1 km, from an altitude of ~5100 m at the highest points of the district, to the lowest mine levels at 4000 m. Three mineralogic zones are distinguished within the vein systems, progressively outward from the centre of the district:
i). a quartz-pyrite rich zone with arsenopyrite, Fe-rich sphalerite and pyrite, and inclusions of pyrrhotite, galena, chalcopyrite and minor Bi-Ag sulphosalts, stannite, scheelite, huebnerite and bismuthinite. This zone is usually followed by
ii). a Cu sulphosalt-sphalerite-galena zone, including economically significant Ag-bearing minerals of the fahlore group, as well as huebnerite, and
iii). a Mn-rich zone with abundant quartz and rhodochrosite, and minor rhodonite, pyrite, fahlore and alabandite, with inclusions of native Te and Ag-tellurides.
This zonation results in the Cu content of these polymetallic Zn-Pb rich veins increasing from the peripheries towards the central parts of the district, and the Toromocho porphyry deposit. This corresponds to a decrease in the amount of sphalerite, galena and Mn-bearing minerals, while the content of chalcopyrite, tennantite, enargite and Cu-Sn-bearing sulphides and sulphosalts increases. This is taken to indicate a higher sulphidation-state of the corresponding mineralising fluids in the central part of the Morococha district.
Epithermal polymetallic veins and replacement ore bodies are economically the most important polymetallic ore body types in the district. In total, there are 20 significant veins that have been mined sporadically for >100 years in the Morococha mining district
The structural setting of the district is dominated by shallowly NW plunging folds, the most important of which is the anticline referred to as the Yauli Dome, which trends NNW and divides the district roughly in half. The volcanic rocks of the Mitu Group outcrop in the core of the dome, with Pucará limestones on the east and limestone and other sedimentary rocks as well as the Anticona diorite on the west limb. Continued compression apparently gave rise to early northwest trending shears, and the updoming intrusion of the quartz monzonite stocks produced an arching of the Yauli Dome and an associated phase of tension faulting generally trending NE-SW, perpendicular to the axis of the anticline.
In the vicinity of the Toromocho deposit the carbonate country rocks belong to the 430 m thick Jurassic Pucará group which comprises dolomites and siliceous limestones, with intercalated basalt and trachyite flows. In the deposit area, the Tertiary intrusives include diorites, granodiorites, quartz, monzonites and quartz porphyries.
The Toromocho magmatic-hydrothermal system is related to multiple porphyritic stocks cropping out in the central part of the Morococha district, including quartz-, feldspar- and granodiorite-porphyries. The porphyry mineralisation related stocks are crosscut by a dacitic porphyry dyke which trends at 100 to 120° and is probably the last magmatic event. It has undergone intense hydrothermal alteration and is therefore interpreted to be pre- to syn-mineralisation. No magmatic body clearly postdating the mineralisation has been encounterd at Toromocho. A large un-mineralised and un-altered porphyritic body, the Yantac porphyry (8.8 Ma; Beuchat, 2003), outcrops to the south of the deposit, and further to the west shows crosscutting relationship with the Anticona diorite.
Contact metamorphic skarns have been formed at the interface between the intrusions and the limestones.
Hydrothermal mineralisation is hosted in both the intrusives and skarns, associated with a large, steeply plunging intrusive breccia pipe composed of clasts of both lithologies, which cuts across the intrusive contact. The copper mineralisation has been dated at late Miocene, 7 Ma, and is contemporaneous with the breccia pipes. These breccias are superimposed on metamorphic skarns which were formed along the contact between the intrusive bodies and the dipping limestone beds. The copper grade is usually higher in the skarn, forming large subhorizontal manto-like higher grade zones.
The deposit occurs as a generally vertical cylindrical mass, which in detail has a complex shape. There is a concentric metal zonation with a cylindrical core of disseminated, stockworks and veinlet copper-molybdenum-silver, surrounded by an almost complete ring or annulus of zinc-copper-silver-gold, mostly as vein deposits, but including bulk disseminated zinc mineralisation. This zone is, in turn, surrounded by the external lead-silver vein systems of the district, as described above.
The Toromocho porphyry deposit exhibits a well developed concentric alteration corresponding to the metal zoning, with a central potassic core represented by secondary biotite, quartz and pyrite which is surrounded by a quartz-sericite phyllic zone and an outer zone of propylitic alteration characterised by epidote, chorite, calcite and sphene.
The primary mineralisation at depth is characterised by chalcopyrite, which has been replaced by chalcocite over a 250 m vertical interval, although remnant chalcopyrite is associated with the chalcocite over much of this thickness. Some enargite has been found in the highest parts of the deposit, usually in generally ENE trending high grade veins, but not in the bulk of the orebody.
Epithermal polymetallic veining overprints the porphyry and skarn mineralisation, and postdates all Miocene porphyritic intrusions (Kouzmanov et al., 2008).
Pre-mining mineral resource figures at Toromocho include:
At a 0.26% Cu cut-off (Peru Copper, 2005):
Measured + indicated mineral resource - 1581 Mt @ 0.49% Cu, 0.015% Mo, 6.8 g/t Ag,
Inferred mineral resource - 257 Mt @ 0.45% Cu, 0.009% Mo, 7.4 g/t Ag.
At a 0.275% Cu cut-off (Peru Copper, 2006):
Proved + probable reserve - 1375 Mt @ 0.51% Cu, 0.018% Mo, 7.06 g/t Ag, plus
Measured + indicated resource - 601 Mt @ 0.37% Cu, 0.016% Mo, 6.82 g/t Ag,
Inferred resource - 151 Mt @ 0.46% Cu, 0.010% Mo, 7.85 g/t Ag.
Current JORC compliant ore reserves are (Chinalco website viewed 2016):
Proved + probable reserve - 1540 Mt @ 0.471% Cu, 0.019% Mo, 6.86 g/t Ag, plus
Measured + indicated resource - 520 Mt @ 0.37% Cu, 0.013% Mo, 6.10 g/t Ag, plus
Inferred resource - 174 Mt @ 0.46% Cu, 0.015% Mo, 11.54 g/t Ag.
The vein mineralisation at the Morococha mine formed along the dominant NE-SW trending tensional fault and fracture systems, that were developed perpendicular to the axis of the Yauli Dome anticline, located to the NE, north and west of the Toromocho porphyry deposit. With the exception of an agglomerate unit in the upper Mitu Group, and the sedimentary breccias in the upper and lower Pucará Group, the Mitu volcanics, Anticona diorite, and much of the sedimentary sequence are good vein hosts. Mineralisation associated with the veining is mostly fracture fill, except in some carbonate hosts where irregular manto replacement can take place in the wall rocks (Wafforn et al., 2014).
Manto replacement mineralisation is generally restricted to receptive stratigraphic horizons where favourable lithologies are intersected by mineralised vein systems or are proximal to pre-mineral intrusives. Mantos can have significant strike lengths where the veins are closely spaced, and can range from <1 to up to 12 m in width. Some of the replacement mineralisation also occurs as irregular, structurally controlled chimneys within generally favourable stratigraphic horizons.
Intrusive contact related skarn bodies are common in the Pucará Group carbonates, generally in areas where pre-mineral intrusives have produced brittle contact related silicification and/or calc-silicate alteration, particularly adjacent to the quartz porphyry-hosted Toromocho disseminated copper system. For the most part these skarns are generally small and irregular, with disseminated rather than massive sulphide mineralisation (Wafforn et al., 2014).
The bulk of the mineralisation at the Morococha mine comprises epithermal Ag-Zn-Pb-Cu veins and associated bedded Ag-base metal replacements/mantos, which together account for the majority of the previously mined and presently known mineralisation. The size and geometry of individual ore shoots are related to lithology and structure. Individual shoots are up to 400 m long, with some having been traced for over 800 m down plunge. Economic vein widths range from 0.5 to >6.0 m. Vein width in the district averages ~1.2 m (Wafforn et al., 2014).
Ore and gangue mineralogy is similar in veins and mantos but varies considerably across the deposit area, as described above. Sphalerite, galena, and chalcopyrite are the principal primary minerals for Zn, Pb and Cu, while Ag is generally present as freibergite (silver-tetrahedrite) or argentiferous galena. Gangue usually comprises quartz, calcite, barite and rhodochrosite (Wafforn et al., 2014).
As described above, the Morococha mineralisation exhibits a distinct lateral and vertical metal zonation, centred on the Toromocho copper deposit. There is also a distinct trend of higher silver grades at higher elevations on the west side of the Morococha mine, with assays of >2200 g/t common above 4800 m a.s.l., and >300 g/t Ag common in the outer silver-lead-zinc zone above the 4400 m elevation. In veins with significant vertical extents, silver grades tend to decrease as zinc grades increase with depth (Wafforn et al., 2014).
Ore reserves and mineral resources at the Morococha mine, as at 30 June, 2014 (Pan American Silver Corp. 2014) were:
Measured + indicated resources - 1.9 Mt @ 180 g/t Ag, 3.45% Zn, 1.39% Pb, 0.49% Cu;
Inferred resources - 8.0 Mt @ 209 g/t Ag, 5.11% Zn, 1.45% Pb, 0.43% Cu;
Proved + probable reserves - 5.1 Mt @ 199 g/t Ag, 4.21% Zn, 1.32% Pb, 0.58% Cu;
Note: reserves are additional to resources.
These summaries are drawn from "Wafforn, M., Steinmann, M. and Delgado, A., 2014 - Technical Report for the Morococha Property, Yauli, Peru; prepared by Pan American Silver Corp., 71p." and "Behre Dolbear, 2014 - Toromocho Project, Resource estimate technical report; prepared for Peru Copper, Inc. by Independent
Mining Consultants, Inc.; 34p."
The most recent source geological information used to prepare this summary was dated: 2014.
Record last updated: 27/9/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Catchpole, H., Bendezu, A., Kouzmanov, K., Fontbote, L. and Escalante, E., 2008 - Porphyry-related base metal mineralisation styles in the Miocene Morococha district, central Peru: in SEG-GSSA 2008 Conference, Student Conference - Johannesburg, July 05-06, 2008, Programs and Abstracts Book, pp. 54-56.|
Kouzmanov, K., Ovtcharova, M., von Quadt, A., Guillong, M., Spikings, R., Schaltegger, U., Fontbote, L. and Rivera, L., 2008 - U-Pb and 40Ar/39Ar age constraints for the timing of magmatism and mineralization in the giant Toromocho porphyry
Cu-Mo deposit, central Peru : in XIII Geological Congress of Peru, Lima, 2008, CD-ROM pp. 1-6.|
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