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Morococha District - Toromocho, Morococha |
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Peru |
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Cu Mo Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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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).
Regional Setting
For details of the regional setting, see the separate Peruvian Segment of the Andean 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 which is underlain by a ~2000 m thick sequence of folded Palaeozoic and Mesozoic rocks that are predominantly carbonates, primarily limestones, with lesser schists and volcanic rocks, all of which are intruded by multiple pulses of Tertiary igneous rocks. The limestones have been folded into a generally NNW trending, doubly plunging anticline with limbs dipping to the east and west forming the elongate Yauli Dome.
The Morococha district occurs on the northwestern margin of the Yauli Dome, the basement of which comprises Palaeozoic metasedimentary rocks of the Excelsior Group, which are discordantly overlain by the Permo-Triassic Mitu Group that dominates the central part of the domal structure (Beuchat 2003; Nagell 1960; Petersen 1965; Rivera and Kobe 1983; Terrones 1949). The Excelsior Group is composed of a heterogeneous suite that includes phyllites, lower Palaeozoic shales, and quartzites that were deformed during late Palaeozoic orogenesis (Lepry 1981).
The main stratigraphic units represented in the immediate Morococha district are, from oldest to youngest, namely the:
• late Permian to middle Triassic Mitu Group continental volcanic rocks, comprising alkaline lavas and pyroclastic flows known locally as the Catalina volcanics, and red bed facies conglomerates, sandstones and shales;
• unconformably overlying late Triassic to early Jurassic Pucará Group, predominantly composed of sedimentary carbonate rocks, interlayered with the thinner Montero Basalt and Sacracancha Trachyte volcanic units;
• the early-Lower to mid-Lower Cretaceous siliciclastric rocks of Goyllarisquisga Group and
• Upper Cretaceous rocks of the Machay Group, which are mainly carbonates of the Chúlec, Pariatambo, Jumasha and Celendin Formations (Kouzmanov et al., 2008).
• Cretaceous Casapalca Formation red beds discordantly overlie the Upper Cretaceous limestones, whilst the rhyolitic Galera lavas and pyroclastic flows lavas and pyroclastic flows unconformably overlie the Upper Cretaceous limestones in the southern part of the district (Bendezú et al., 2012).
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. During the Late Miocene, between 9 and 7 Ma, a series of porphyritic diorites and granodiorites to quartz-monzonites intruded the volcano-sedimentary sequences as well as the Anticona diorite. Although most are barren, the major porphyry Cu mineralisation of the district is related to intrusions of this age.
Mineralisation in the Morococho District occurs in a diverse range of forms, as follows:
• Porphyry mineralisation. The Toromocho porphyry Cu-Mo deposit, which dominates the central part of the district, is the only economic porphyry style mineralisation in the district (in 2015) and is described in more detail under the Toromocho heading below. Mineralisation at this deposit has associated extensive potassic and phyllic alteration zones, affecting granodioritic-, feldspar- and quartz-porphyry intrusions. Elsewhere in the district, the Potosí/Cordiciada (to the NE), San Francisco on the northern margin of the Toromocho deposit, and Ticlio (west of Toromocho) porphyritic stocks exhibit K- and/or Na-Ca alteration with associated economic to sub-economic porphyry-style mineralisation.
The Codiciada composite stock, largely a porphyritic granodiorite, has been affected by strong Na-Ca and moderate potassic alteration, which is overprinted by a sericitic assemblage. Sub-economic porphyry-type quartz-pyrite-chalcopyrite±pyrrhotite and quartz-molybdenite veins are cut by later Cordilleran polymetallic veining. Mineralisation is indicated to have been emplaced at between 9.3 and 8.8 Ma, which is older than at Toromocha.
The small Ticlio porphyry, in the western part of the district, exhibits a zoned pattern of mineralisation and alteration and has been dated at ~8.0 Ma. It comprises a core of high-density stockwork quartz-magnetite±K feldspar veins which grade progressively into an outer zone of intense K feldspar alteration with low-density magnetite and quartz-magnetite veinlets that host small amounts of bornite and native Au inclusions in chalcopyrite. The outermost zone comprises a strong pyrite-quartz-sericitic alteration and skarn mineralisation in adjacent Cretaceous carbonate rocks (Bendezú et al., 2008; Chevalier 2010).
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).
• Skarn hosted mineralisation in the Morococha district is spatially associated with porphyry intrusions, particularly the Toromocho composite (Alvarez 1999; Lowell and Alvarez 2005), Codiciada composite, Gertrudis, San Francisco (Catchpole 2011; McLaughlin and Graton 1935; Petersen 1965), Yantac (Beuchat 2003) and Ticlio stocks (Chevalier 2010). At several locations in the district, phlogopite 40Ar/39Ar ages of skarn adjacent to these intrusive bodies indicate at least two and perhaps three individual skarn forming events over a period of 1.9 Ma. These include 8.81 ±0.18 Ma for the Codiciada skarn (Kouzmanov et al., 2008), 7.2 ±0.2 Ma for the Porvenir skarn adjacent to the Yantac intrusions (Beuchat 2003), and a 6.9 ±0.3 Ma for the Gertrudis skarn (Catchpole 2011). Pucará Group dolomite and dolomitic limestone are replaced by magnesian serpentine-magnetite±phlogopite, tremolite-diopside-serpentine-chlorite-talc and calcic andradite-diopside-epidote exoskarn. Less extensive grossular-diopside and spinel-bearing diopside-grossular endoskarns replaced Miocene porphyries where in contact with Pucará Group carbonate rocks (Catchpole 2011; 2015).
Exoskarn dominated by hydrous skarn mineral assemblages host significant polymetallic (Zn-Pb-Ag-Cu) mineralisation, while more distal skarn-free carbonate-replacement bodies have been found in the Pucará Group. Massive Zn-Pb-Ag-Cu-bearing sulphide bodies up to 20 m thick are found at Manto Italia, which is located close to the western margin of the northern half of the Codiciada composite stock (Catchpole 2011; Paliza and Chavez 2008). Pyrrhotite, pyrite, chalcopyrite, sphalerite and galena are the most abundant sulphides. The peripheries of these bodies are marked by anhydrous diopside-andradite skarn assemblages, whilst the cores are characterised by later hydrous tremolite-serpentine-chlorite-talc skarn and sulphide assemblages (Catchpole 2011).
In addition to the porphyry and skarn-hosted mineralisation, there are significant late to post-porphyry mineralisation styles in the Morococha district, the principal of which are (after Catchpole et al., 2008; 2015):
• Massive pyrite-quartz bodies, which are found in several locations underground and at surface in the central Morococha district, replacing fractured and deformed Pucará Group carbonate host rocks. Most are essentially barren, although they are of economic interest where cut by later Cordilleran polymetallic veins which introduce Zn-Pb-Ag-Cu sulphides e.g. the Brecha Rosita, northern Codiciada; Brecha Riqueza and central Morococha mines. They are mainly developed:
i). along the contact between volcanic rocks of the Mitu Group and brecciated carbonate rocks of the Pucará Group (e.g. Brecha Riqueza), and
ii). along the contact between porphyry intrusions and brecciated carbonate rocks (e.g. Brecha Rosita).
In addition, vertical to sub-vertical cylindrical pyrite bodies are formed along steeply dipping faults (e.g. lower section of the Ombla body). These pyrite bodies may have a banded texture caused by alternating layers of coarse- and fine-grained pyrite, possibly mimicking layering in the carbonate protolith. They are usually composed of 80 to 90 vol.% pyrite, with quartz and accessory rutile. Porphyry intrusive and volcanic rocks in contact with the pyrite bodies have undergone intense sericitic and chlorite alteration. These bodies are similar to the massive pyrite bodies that characterise the Miocene Cerro de Pasco base metal deposit (Baumgartner et al. 2008).
• Cordilleran polymetallic veins - The Cordilleran polymetallic veins occur as steeply dipping epithermal Zn-Pb-Ag-Cu-bearing veins with phyllic alteration halos, occurring within NNE to ENE trending structures. On a district scale, they cut the intrusive bodies, surrounding sedimentary rocks, skarn alteration, as well as pyrite-quartz and other polymetallic mineralisation (Catchpole et al., 2007). They have a vertical extent of >1 km, from ~5100 m above sea level at the highest points of the district, to the lowest levels in the mine at ~4000 m altitude.
The veins are internally well zoned and can be subdivided into three paragenetic stages based on cross-cutting relationships (Catchpole et al., 2011):
i). an early, generally barren 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 enclosed by sericitic alteration halos, and is usually followed by
ii). a composite base metal stage when Zn-Pb-Ag-Cu sulphides are introduced to produce a zone characterised by Cu sulphosalt-sphalerite-galena, including economically significant Ag-bearing minerals of the fahlore or tennantite-tetrahedrite group, as well as huebnerite. The base metal stage can be further subdivided into several sub-stages.
iii). a final Mn-rich carbonate stage characterised by abundant pink rhodochrosite and quartz with minor rhodonite, pyrite, tennantite-tetrahedrite and alabandite, and inclusions of native Te and Ag-tellurides.
The individual stages and sub-stages may be recognised as a discrete phases in a single vein, or be presented as individual veins with clear cross-cutting relationships with other phases. Sphalerite, galena, tennantite-tetrahedrite, chalcopyrite and enargite are the most abundant base metal sulphides. Silver mostly occurs as a trace or minor element in tennantite-tetrahedrite and as the Ag-telluride hessite, whilst galena is generally Ag poor.
There is a lateral zonation within the vein system, where the Cu content of the polymetallic Zn-Pb rich veins increases from the peripheries towards the central parts of the district, and the Toromocho porphyry Cu-Mo 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.
• Carbonate replacement bodies occur as vertical chimneys at the contact between intrusive bodies and carbonate beds, and as mantos and irregular bodies developed along fault zones. They are generally massive sulphide bodies that form where Cordilleran base metal veins intersect faults or lithological contacts that act as hydrothermal fluid conduits. Many of these zones were mined out prior to 1970 and include Cu-rich bodies in the central area, near the San Francisco and Gertrudis intrusions. These deposits frequently 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 same group (e.g., Manto Italia). Polymetallic bodies fed by polymetallic veins are often 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-sulphosalt 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.
Apart from the Toromocho porphyry-skarn deposit, 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 NNW 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.
Toromocho
In the immediate Toromocho deposit area, Tertiary magmatic rocks intrude carbonate country rocks which belong to the 430 m thick Jurassic Pucará Group. This sequence comprises dolomites and siliceous limestones, with intercalated basalt and trachyite flows as described above.
The Toromocho porphyry Cu-Mo deposit is part of a complex magmatic-hydrothermal system related to multiple intrusive events in the central part of the Morococha district. Subsequent to the ~14.1 Ma pre-mineral Anticona diorite, magmatic activity re-commenced at ~9.1 Ma with the emplacement of granodioritic porphyries, followed by at least five porphyry intrusive phases that are recognised at Toromocho. This suite includes two main pulses of feldspar porphyry that have been recognised, including the largely un-mineralised and un-altered Yantac feldspar porphyry to the SW of Toromocha, dated at ~8.81 Ma, and 8.17 Ma feldspar porphyries that occur peripheral to and within the main Toromocho deposit. The youngest intrusion is a 7.75 Ma late-mineral quartz porphyry.
Magmatic activity related to mineralisation persisted for >2 m.y., and includes the granodiorite-, feldspar- and quartz-porphyries. The porphyry mineralisation-related stocks are crosscut by an ~8 Ma dacitic porphyry dyke which trends at 100 to 120° over a strike length of ~10 km. 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 encountered at Toromocho. The large un-mineralised and un-altered 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.
Prograde skarn and hornfels have been formed at the interface between the intrusions and the dipping limestone beds, developed within the shelf to shallow basin carbonate rich Triassic to Jurassic Pucará Group country rocks. Hydrothermal mineralisation is hosted in both the intrusives (~60%) and skarns (~40%), and is associated with a number of large, steeply plunging multistage intrusive breccia pipes, dated from 8.98 to 7 Ma. These breccias cut all of the porphyries and the skarn, crossing the intrusive contact and are composed of clasts of both skarn and porphyry. They are superimposed on the contact metamorphic skarns. Copper mineralisation in these breccias has been dated at late Miocene, 7 Ma, and is contemporaneous with host breccia pipes. Copper grades are usually higher in the skarn, forming large sub-horizontal manto-like higher grade zones.
Cu-Mo mineralisation accompanied the formation of hydrous/retrograde skarn which occurred over an area of 6 km2 from 7.2 to 6.8 Ma during the waning stages of magmatism. These skarns 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.
Extensive potassic and phyllic alteration zones within the porphyries are associated with stockwork quartz-chalcopyrite ±chalcocite, quartz-pyrite and chalcocite mineralisation and quartz-pyrite-molybdenite veins. Chalcocite is a product of supergene activity and is not hypogene. Disseminated Cu mineralisation accompanied by potassic alteration also occurs within breccia pipes in the central part of the deposit.
At least two hydrothermal pulses have been identified. The first produced economic mineralisation at 8.0 to 7.8 Ma (7.97 ±0.11 and 7.77 ±0.11 Ma, Re-Os on Mo; Beuchat 2003), while the most recent event is reflected by biotite cooling ages of 7.2 ±0.3 (K-Ar; Eyzaguirre et al., 1975) and 6.81 ±0.14 Ma (40Ar/39Ar; Kouzmanov et al., 2008).
The San Francisco porphyry stock in the north of the main deposit area contains porphyry-style quartz-molybdenite veins associated with pervasive potassic alteration and an overprinting phyllic assemblage. In the same area, milky quartz veins cut the San Francisco stock and postdate the quartz-molybdenite vein mineralisation. These veins are up to 15 cm thick with sericitic alteration envelopes, and occasionally, vugs containing small muscovite crystals. 40Ar/39Ar dating of muscovite separated from vugs in a milky quartz vein revealed that the muscovite cooling age postdates the last porphyry-type hydrothermal event at Toromocho and San Francisco by at least 1 m.y. and overlaps the 5.7 Ma (adularia and muscovite cooling ages; Catchpole, 2011) of the Cordilleran polymetallic mineralisation in the district. These quartz veins underwent repeated reopening, as indicated by abundant fluid inclusion trails aligned parallel to the strike of the vein (Catchpole et al., 2015). Based on these ages and crosscutting relationships, the milky quartz veins are interpreted to postdate the porphyry-type molybdenite-bearing quartz veining, but predate Cordilleran polymetallic veins in the central Morococha district.
The Toromocho 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 also 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 - 1.581 Gt @ 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.
TOTAL resource - 1.838 Gt @ 0.484% Cu, 0.014% Mo, 6.88 g/t Ag.
At a 0.275% Cu cut-off (Peru Copper, 2006):
Proved + probable reserve - 1.375 Gt @ 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.
TOTAL reserve + resource - 2.127 Gt @ 0.467% Cu, 0.017% Mo, 7.85 g/t Ag.
JORC compliant Ore Reserves and Mineral Resources in 2016 (Chinalco website viewed 2016) were:
Proved + probable reserve - 1.540 Gt @ 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,
TOTAL reserve + resource - 2.234 Gt @ 0.447% Cu, 0.017% Mo, 7.05 g/t Ag.
Morococha
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 both 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. Sphalerite, galena and chalcopyrite are the principal primary minerals for Zn, Pb and Cu, while Ag is generally present as freibergite (silver-tetrahedrite - (Ag,Cu,Fe)12(Sb,As)4S13) 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:
Mineral resources
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;
Ore reserves
Proved + probable reserves - 5.1 Mt @ 199 g/t Ag, 4.21% Zn, 1.32% Pb, 0.58% Cu;
TOTAL Ore Reserves + Mineral Resources - 15 Mt @ 202 g/t Ag, 4.59% Zn, 1.40% Pb, 0.49% Cu.
Note: reserves are additional to resources.
These summaries are drawn from both the published sources listed below and 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 decription was dated: 2018.
Record last updated: 14/9/2018
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.
Toromocho
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Bendezu, A., Catchpole, H., Kouzmanov, K., Fontbote, L. and Astorga, C., 2008 - Miocene magmatism and related porphyry and polymetallic mineralization in the Morococha district, central Peru: in XIII Geological Congress of Peru, Lima, 2008 [CD-ROM] Proceedings 6p.
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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.
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Catchpole, H., Kouzmanov, K., Bendezu, A., Ovtcharova, M., Spikings, R., Stein, H. and Fontbote, L., 2015 - Timing of porphyry (Cu-Mo) and base metal (Zn-Pb-Ag-Cu) mineralisation in a magmatic-hydrothermal system - Morococha district, Peru: in Mineralium Deposita v.50, pp. 895-922.
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Catchpole, H., Kouzmanov, K., Putlitz, B., Seo, J.H. and Fontbote, L., 2015 - Zoned Base Metal Mineralization in a Porphyry System: Origin and Evolution of Mineralizing Fluids in the Morococha District, Peru: in Econ. Geol. v.110, pp. 39-71.
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Kouzmanov, K., Bendezu, A., Catchpole, H., Ageneau, M., Perez, J. and Fontbote, L., 2008 - The Miocene Morococha District, Central Peru - Large-Scale Epithermal Polymetallic Overprint on Multiple Intrusion-Centred Porphyry Systems: in Pacific Rim : Mineral Endowment, Discoveries & Exploration Frontiers, PACRIM Congress, 24-26 November 2008, Gold Coast, Queensland, Extended Abstracts, AusIMM, Melbourne, pp. 117-121
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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, Conference Proceedings CD-ROM pp. 1-6.
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