Colquijirca District - Colquijirca, Smelter, Marcapunta Oeste, San Gregorio, Oro Marcapunta
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The Colquijirca Cordilleran polymetallic epithermal Zn-Pb-Cu-Ag±Au district is located at ~4300 m a.s.l., 310 km NE of Lima and 8 km south of Cerro de Pasco, near Tinyahuarco, in central Peru. Individual deposits within the district include Colquijirca, Smelter (Marcapunta Norte), Marcapunta Oeste, San Gregorio and Oro Marcapunta, which are distributed over a 7 km, north-south to NNE aligned trend.
(#Location: Colquijirca - 10° 45' 13"S, 76° 16' 15"W).
Two main types of epithermal mineralisation occur in the district, both with close temporal and spatial relationships to a Miocene diatreme-dome volcanic complex: i). ~11.9 to 11.1 Ma high-sulphidation Au-(Ag) ores emplaced within the Marcapunta diatreme-dome complex (Vidal et al., 1997; Bendezú et al., 2003), and ii). the economically more important 10.8 to 10.6 Ma Cordilleran Cu-Zn-Pb-Ag(-Au) deposits that are zoned from Cu rich proximal to the dome, to distal Zn-Pb rich, hosted largely in carbonate sequences (Lindgren, 1935; McKinstry, 1936; Vidal et al., 1984; Bendezú et al., 2003, 2008; Bendezú, 2007).
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record. The Colquijirca and nearby Cerro de Pasco districts are related to one of the easternmost Miocene magmatic belts of central and northern Perú, ~360 km east of the current Peruvian trench.
The main feature in the centre of the Colquijirca district is the 12.7 to 12.4 Ma Miocene Marcapunta diatreme-dome complex that intrudes a thick sequence of sedimentary rocks. These are, from the oldest to youngest, Devonian Excelsior Group slates and phyllites, that occur to the east of the Longitudinal Fault (see Structure below), unconformably overlain by red beds of the Upper Permian to Lower Triassic Mitu Group, and limestones and dolostones of the Late Triassic-Early Jurassic Pucará Group. These are unconformably and transgressively followed by continental, mainly detrital and carbonatic rocks of the Early Cenozoic Pocobamba Formation and fresh-water limestone of the Eocene Calera Formation which transgressively overlies the Pocobamba Formation.
In the deposit area, the Pocobamba Formation rests unconformably onto Mitu Group rocks, but is only exposed over a small area as it is overstepped westward by the Calera Formation which to the north of the Marcapunta volcanic complex, mostly rests directly on the Mitu Group. The Calera Formation dips shallowly to the east at 20 to 35°. The Mitu Group occurs in the core of a NNW trending anticlinal feature to the west of the deposits over a width of ~2 km. In the western limb of this structure, it is overlain by the Pucará Group, which dips 25 to 50°W. To the south, the nose of the anticline is intruded by the Marcapunta volcanic centre, and the gently east dipping Calera Formation lies unconformably on the west dipping Pucará Group.
The Marcapunta volcanic centre comprises multiple porphyritic lava dome intrusions of dacitic high-K sub-alkaline composition, which pre- and post-date several episodes of phreatomagmatic brecciation (D. Noble, reported in Bendezú and Fontboté, 2009; Bendezú et al., 2003; Sarmiento, 2004; Bendezú, 2007). The combined complex is an upwardly flaring multiple intrusion and diatreme, which at the surface covers ~3 x 2 km area, elongated north-south. Mineralised Calera and Pocobamba formation rocks are overlain by the flaring volcanic centre/diatreme complex on its northern and western wings.
The principal hosts to mineralisation in the Colquijirca district are the carbonate rocks of the Upper Triassic to Lower Jurassic Pucará Group (San Gregorio) to the south of the volcanic complex, and the Eocene Pocobamba and Calera Formations (Marcapunta Oeste, Smelter and Colquijirca) to the west and north, as follows:
Pucará Group - Predominantly a carbonate sequence that is principally composed of thick-bedded, dark coloured limestone and dolomite, with local shale interbeds and siliceous concretions (Jenks, 1951). This sequence is ~3000 m thick to the east of the north-south Longitudinal Fault which passes <1 km to the east of the main deposits and forms part of the eastern margin of the Marcapunta complex. To the west of this fault, it is only 300 m thick and consists of thin-bedded, light-coloured limestone (Jenks, 1951). In the deposit area, the carbonate rocks are extensively altered, and in the ore deposit, the dolostones and limestones have undergone complete decarbonatisation to an advanced argillic assemblage, basically composed of quartz, alunite and kaolinite, with more than 30% total sulphides (Fontboté and Bendezú, 1999).
Pocobamba Formation - which has been divided into two members,
• Cacuán Member, comprises up to 300 m of shales, sandstones, conglomerates and limestones, characterised as a red-bed sequence. It commences with a lower section, mainly of cross-bedded reddish sandstones, siltstones and mudstones. Conglomerate clasts are <5 cm long, mostly subrounded and cemented by calcareous material, mainly limestone derived from the Pucará Group (Angeles, 1999), with minor sandstones, siltstones and locally, milky quartz. The upper intervals of the member dominantly comprises metre thick beds of whitish limestones that are lacustrine in origin (Angeles, 1999).
• Shuco Member, that is 100 to 200 m thick and comprises poorly sorted monomictic, clast-supported breccias and conglomerates. Near Colquijirca it rests on a variety of pre-Cainozoic rocks, including the Mitu (west of Colquijirca) and Excelsior Groups (at Condorcayán on the northern tip of the Colquijirca deposit). It has been shown to extend over a large area beneath the Calera Formation in the northern part of the Colquijirca district. Pucará Group pebbles, cobbles and boulders are the dominant clasts, with interstitial angular to subrounded granule- to cobble-sized material, including sandstone, cemented by calcite. At Colquijirca, immediately west of the operating open pit, the Shuco member exhibits incipient bedding and is dominated by subrounded clasts, most less than 20 cm in diameter. This member is considered to be a fanglomerate or piedmont deposit (Jenks, 1951; Angeles, 1999), possibly derived by erosion of mainly Pucará Group blocks uplifted during the late Mesozoic to early Cainozoic along the north-south Longitudinal fault (Bendezú and Fontboté, 2009).
The Calera Formation is composed of ~70% argillites, siltstones and sandstones, and about 30% limestones and marls (Jenks, 1951). Angeles (1999) divided the formation into three members.
• Lower member, ~60 m thick, dominated by detrital sedimentary rocks, including tuffs, sandy siltstones and, to a lesser extent, conglomerate beds that are tens of centimetres in thickness, consisting of pebbles of black limestone in an argillic matrix.
• Middle member, ~60 m thick, characterised, by abundant nearly pure carbonate rocks and subordinate metre thick beds of massive, fine-grained argillites and marls, usually rich in organic matter.
• Upper member, ~150 m thick, consisting of intercalated beds of argillites, siltstones, marly dolostones, marls rich in organic matter, varved carbonaceous dolostones and limestones, and some volcanosedimentary beds, including tuffs.
The total thickness of the Calera Formation is a minimum of 250 m (Angeles, 1999), although drilling immediately NE of Marcapunta Hill indicates a 500 m thick sequence. The Calera Formation is indicated to be of late Eocene age, based on ~36 to 37 Ma K-Ar dating of biotite from a tuff layer located near the base of the lower member (Noble and McKee, 1999).
The most prominent lineaments in the district are two major regional north-south reverse faults, north-south fold trends, and a strike-slip fault system. These include the major north-trending Longitudinal Fault, a reverse fault, which passes through, or close to, both the Cerro de Pasco and Marcapunta volcanic centres, and controlled basin morphology during both the Pucará and Calera Formation sedimentation. A second NNW-SSE to north-south trending, pre-Marcapunta complex reverse fault passes to the west of the Colquijirca-Smelter deposits, and emerges to the south of the volcanic complex, where it is east of the San Gregorio deposit. Most of these structural elements are related to Neogene compressive events that affected extensive areas of the central and northern Peruvian Andes (Angeles, 1999). Both the folding and the F1 and F2 fault systems occurred prior to hydrothermal mineralisation and provided important channels for mineralising fluids, particularly in the northern part of the district (Bendezú and Fontboté, 2009).
Alteration and Mineralisation
The Marcapunta volcanic complex has been strongly altered to form cores of residual, locally vuggy quartz, with halos of advanced argillic alteration mainly composed of quartz-alunite and kaolinite-bearing assemblages. Gold and silver, mainly present in veinlets and coatings of oxides, are largely contained within these cores of vuggy quartz, which extends into the adjacent country rock. Locally, close to the diatreme-dome complex, particularly to the west, these quartz-alunite zones and associated veinlets are cut by centimetre wide pyrite-(enargite) rich veinlets and veins generated during the sulphide-rich polymetallic event. These crosscutting relationships, in conjunction with 40Ar/39Ar dating indicate that the polymetallic event (~10.8 to 10.6 Ma) postdated the high-sulphidation Au-(Ag) mineralisation (~11.9 to 11.1 Ma; Bendezú et al., 2008).
Oro Marcapunta prospect
High-sulphidation epithermal Au-(Ag) mineralisation is dominantly hosted within the Marcapunta volcanic complex. It comprises oxidised veinlets and disseminations hosted in vuggy silica veins, that are zoned outward to quartz-alunite and to argillic alteration assemblages that affect most of the Marcapunta volcanic complex. Typical grades in the vuggy silica veins are of the order of 0.2 to 3 g/t Au and 10 to 70 g/t Ag (Vidal et al., 1997), with Ag:Au ratios ranging from 10 to 30:1. The Au-(Ag) bearing veins have vertical dimensions of up to 100 m and are mainly found in the central section of the complex, associated with diatreme breccia and pyroclastic infill. Their morphology is controlled by both lithological (phreatomagmatic breccias) and structural permeability, mainly related to east-west fractures. The deeper sections of vuggy silica veins contain unoxidised Au-(Ag) ores, which contain <5 vol.% of finely disseminated sulphides, and sulphide veinlets that are mainly composed of pyrite-enargite with very minor chalcocite, covellite and sphalerite, accompanied by clays, largely kaolinite, but also smectite and/or illite. Vuggy silica veins and surrounding quartz-alunite zones, which are devoid of veinlets, contain minor amounts of Au-(Ag), interpreted to suggest most of the precious metals were precipitated during veinlet formation.
In several areas, quartz-alunite alteration is observed to post-date Au-(Ag) bearing veinlets, implying several repeated episodes of vuggy silica-quartz-alunite alteration and Au-(Ag) deposition took place at Marcapunta, e.g., Bendezú et al. (2008) showed that alunite samples associated with the high sulphidation epithermal Au-(Ag) mineralisation gave 40Ar/39Ar plateau ages of between ~11.9 and ~11.1 Ma. This range appears to have encapsulated two discrete pulses of Au-(Ag) deposition which can be distinguished, where alunite cemented clasts of Au-(Ag) bearing vuggy silica are cut by alunite-free Au-(Ag) bearing oxide veinlets. This suggests an early Au-(Ag) deposition pulse must have predated the alunite cement precipitation at 11.51±0.06 Ma, and the younger period of Au-(Ag) postdated this age.
Smelter and Colquijirca deposits
The Smelter deposit is located adjacent to, and partially beneath, the northern margin of the Marcapunta volcanic complex, whilst the historic Colquijirca deposit is the northward continuation of the same mineralised zone, connected by a narrower 'neck', and separated by an arbitrary line. Together, the combined sulphide system covers a north-south elongated interval of ~3500 x 400 to 900 m, and has a flat to shallow dip, except to the south where it dips southward at up to 45°, following the contact with the Marcapunta volcanic complex.
Extensive early quartz-pyrite replacement surrounds the main diatreme and the central volcanic and subvolcanic domes of the Marcapunta complex, altering a >200 m thickness of the Calera Formation and underlying Shuco member of the Pocobamba Formation in the Smelter deposit area. Increasingly toward the north, the base of replacement migrates upward through the sequence so that only the Calera Formation and upper stratigraphic positions of the Pocobamba Formation are mineralised, until the Shuco member and the lower portion of the Calera Formation are also unmineralised. Further to the north again, the ores at Colquijirca are restricted to carbonate rocks of the Calera Formation, specifically replacing a 50 to 90 m thick interval of the Middle member (Bendezú and Fontboté, 2009).
At the Smelter deposit, mineralisation is also found within the volcanic rocks of the Marcapunta complex, where lava domes and block and ash deposits up to tens of metres thick are intensely replaced by quartz and pyrite. A small block of economic mineralisation also occurs in the uppermost beds of the Mitu Group red beds (Bendezú and Fontboté, 2009).
Three stages of mineralisation are recognised, i). an early quartz-pyrite stage that produced large, basically barren, quartz-pyrite replacement bodies from Smelter to Colquijirca; ii). the main ore stage composed of zoned arsenical Cu-(Au), located around the volcanic complex at Smelter, grading to the distal Zn-Pb-(Ag) ores at Colquijirca; and iii). a late Au-free Cu stage which generated economically less important mineralisation at Smelter (Bendezú and Fontboté, 2009). In more detail, these stages comprise (after Bendezú and Fontboté, 2009):
i). Early Quartz-Pyrite Stage - which although economically unimportant, contains an estimated 800 to 1000 Mt @ 40 to 50 vol.% pyrite in the Smelter-Colquijirca corridor, and probably a similar volume in the Marcapunta Oeste area. On a district scale, quartz-pyrite replacement roughly has a funnel-shape surrounding at least 75%, and possibly all the main volcanic conduit. Close to the Marcapunta volcanic complex, the geometry of the quartz-pyrite replacement followed the contact between the main intrusive conduit and the carbonate host sequence. In more distal areas, it was basically controlled by the sedimentary host rock bedding and, subordinately, by fractures.
In the Calera Formation, this stage mainly occurs as a simple replacement of limestones and/or dolostones, marls, calcareous argillites and conglomerates, with the original fabric of the host rock commonly preserved. Finely laminated bands as thin as a few tens of microns composed of alternating pyrite- and quartz-rich bands also occur, particularly toward the top of the mineralised sequence in intervals corresponding to varved carbonaceous dolostones and limestones. Strongly silicified and pyritized conglomeratic beds characterise the sequence that consist of alternating clast- and matrix-supported breccias probably from the Shuco Member of the Pocobamba Formation. Relatively rare hydraulic breccias, more common crackle breccias, as well as breccias with monomictic to heterolithic clasts are present in the Calera Formation, with up to 20 cm sulphide-poor clasts derived from carbonate and/or detrital rocks of the hanging wall, set in a quartz-pyrite matrix that accounts for 40 to 60% of the breccia.
Quartz comprises at least 60 vol.% of the quartz-pyrite replacement bodies, occurring mainly as subhedral to subordinate euhedral grains, generally ranging in size between 30 µm to 2 mm, with 10 to 20 vol.% dark cryptocrystalline chert, jasperoid and rare opal. At Colquijirca, numerous <2 m thick beds of chert characterise the early quartz-pyrite stage and occur persistently along the most distal parts of the deposit. These beds most commonly comprise up to 30 vol.% dark to black cryptocrystalline quartz and finely crystallized quartz that contain <5 vol.% <100 µm pyrite inclusions.
Pyrite from this stage represents >90 vol.% of the total sulphides of the entire Smelter-Colquijirca corridor. This pyrite (pyrite I) has up to 1.2 wt.% As (typically 0.2 to 0.6 wt.%), and typically <0.2, but locally >1 wt.% Cu. It also includes minor to trace amounts of extremely fine grained rutile, zircon and scheelite disseminated in the quartz-pyrite matrix. Chalcopyrite, sphalerite, marcasite, and pyrrhotite occur as minute inclusions in pyrite.
The early quartz-pyrite stage displays a gradual increase in pyrite with time, with late vein generations consist mainly of pyrite, overprinting matrix and clasts replaced in the early stages which show only minor pyrite disseminations.
ii). Main Ore Stage - No evidence is apparent for a time gap between the early quartz-pyrite and the main ore stage, with late quartz-pyrite veinlets accompanied by minor to trace amounts of enargite, suggesting the transition was gradational. However, abundant crosscutting relationships indicate the main ore-stage fluids extensively overprinted the early stage. In the internal parts of the system, the quartz-pyrite stage was mainly overprinted by minerals of the copper-bearing ore zones. Beyond the quartz-pyrite front, overprinting of the country rock was mainly by minerals of the zinc-lead bearing zones. From proximal to the internal intrusion, to the external/distal portions of the mineralised system, the main ore-stage zones comprise the:
• Enargite-gold zone - which typically contains 1 to 2% Cu and 0.2 to 1.0 g/t Au. Enargite comprises up to ~90 vol.% of the total sulphide content. Enargite is commonly intergrown with luzonite (~5 vol.%), with variable amounts of mainly pyrite, colusite, tennantite, goldfieldite, ferberite, gold-silver tellurides, bismuthinite, gold, quartz, alunite, zunyite, kaolinite, dickite, smectite, illite and muscovite. The enargite-gold zone is present to 800 m below the surface at Marcapunta in narrow subvertical veins and veinlets that are interpreted to represent the roots of the epithermal polymetallic system. Horizontally, the enargite-gold zone is largely confined to the Smelter deposit. Enargite ranges from a few up to 50 mm in size, and in most cases is coarse grained, commonly anhedral to locally subhedral. From 5 to 10% pyrite (II) was also precipitated during formation of the enargite-gold zone and is the next most abundant mineral after enargite. Gold occurs in two paragenetic phases, an early deposition as irregular inclusions of elongate, <10 µm long electrum (>90 wt.% Au and <10 wt.% Ag) within enargite, which were either precipitated with or after enargite. The second gold phase is associated with up to 50 µm Te-bearing blebs, including locally abundant goldfieldite and tennantite. These blebs replace late generations of enargite along grain boundaries. Gold occurs chiefly as tellurides, with minor amounts of extremely fine grained electrum as inclusions in goldfieldite, including kostovite and nagyagite.
• Enargite zone - which is best developed between the northern end of the Smelter deposit to the central part of the former Principal pit at Colquijirca. This zone has the same mineral associations and assemblages as those observed in the enargite-gold zone but lacks discernible colusite, ferberite, or Au-bearing minerals. As such it is characterised by an assemblage of enargite±pyrite, quartz, bismuthinite, alunite, dickite, kaolinite, smectite, illite and muscovite. At Colquijirca, enargite comprises >95 vol.% of the copper phases, with luzonite only locally as significant as it is in the Smelter deposit. The bulk of the enargite occurs cementing hydraulic breccias and incipient crackle breccias and in veinlets, typically forming massive aggregates of anhedral grains. Pyrite is volumetrically minor, and only in the Colquijirca deposit is it up to 10 vol.%. Alunite is distributed throughout the entire enargite zone at Colquijirca, in an assemblage with fine grained euhedral pyrite II. The enargite zone also contains numerous lenses up to tens of centimetres thick of sphalerite-galena-pyrite-hematite-siderite-kaolinite, which crosscutting relationships suggest, predates the enargite zone.
• Bornite zone - which occurs in lateral gradational contact with the enargite zone and is largely contained within the quartz-pyrite replacement zone. At Colquijirca, the transition from enargite to almost enargite free bornite occurs over an interval of <60 cm. The bornite zone has only been identified at Colquijirca, but presumably also occurs lateral to the enargite zone in the Smelter area. It accounts for <2 vol.% of the global copper resources of the Smelter-Colquijirca mineralised corridor, and forms small discontinuous podlike bodies, mostly near the enargite zone front. These podlike bodies are up to 10 m long, 4 to 6 m wide, and 2 to 3 m thick. Copper grades in places exceed 5 wt.%, but because of its small tonnage is not mined for Cu. The main mineral association in this zone is bornite±tennantite, pyrite, quartz, barite, dickite and kaolinite, in addition to minor to trace sphalerite, covellite, chalcopyrite, chalcocite, digenite, galena and siderite. In this association, part of the pyrite, quartz and sphalerite predate bornite deposition, whereas the rest of the minerals were precipitated after the bornite. Bornite typically occurs as irregular masses of anhedral grains.
• Tennantite zone - is dominated by tennantite, accompanied by major amounts of barite, pyrite and quartz. It surrounds the bornite zone, and where the latter is absent, directly rims the enargite zone. This zone has been identified northward from the line separating the Smelter and Colquijirca deposits, and is best developed in the central part of the Colquijirca deposit. Minor phases of the tennantite zone include kaolinite, dickite, illite, smectite, enargite, chalcopyrite, bornite, stromeyerite, Bi-bearing sulphosalts, and numerous other sulphides and sulphosalts, and is the most mineralogically complex zone of the whole Colquijirca district. Typical grades in the the zone are of the order of 1 to 2 wt.% Cu, 300 to 600 g/t Ag (to >3000 g/t), with variable, generally subeconomic contents of Zn and Pb. It contains some of the highest reported hypogene silver grades in the Colquijirca district. Although usually no more than 4 m thick and 6 m wide, it has good continuity, over intervals of up to hundreds of metres surrounding the enargite and/or the bornite zones, including perpendicular to bedding. Late chalcopyrite replaces tennantite.
• Chalcopyrite zone - which extends for up to several hundred metres northward from the line that separates the Smelter and Colquijirca deposits. It occurs along bedding as a thin rim to the tennantite zone. This zone extends beyond the main early quartz-pyrite replacement front, and is best developed between the centre and the northern end of the Colquijirca deposit. In addition to chalcopyrite, this zone consists largely of pyrite, tennantite, sphalerite, galena, dickite, kaolinite, barite, quartz, siderite and minor amounts of Bi- and Ag-bearing sulphosalts. Typical grades are 0.2 to 0.5 wt.% Cu, 4.0 to 6.0 wt.% Zn, 2.0 to 3.0 wt.% Pb and 100 to 150 g/t Ag. Pyrite is the main sulphide, and can be as much as >30 vol.%. An early pyrite generation is virtually always partially replaced by chalcopyrite and is commonly associated with quartz in strongly silicified thin beds (which are <2 m thick). This early pyrite generation represents up to 90 vol.% of pyrite of the chalcopyrite zone, and are interpreted as probable distal fingers of the early quartz-pyrite replacement. A second pyrite generation is either intimately intergrown with chalcopyrite or occurs as nearly euhedral grains disseminated in chalcopyrite matrix, and has no observed reaction with the chalcopyrite. Tennantite is the second most abundant copper mineral, typically occurring as tiny patches in chalcopyrite. Minor to trace Ag and Bi-sulphosalts are distributed throughout the chalcopyrite zone, but are abundant in numerous small, elongated tabular bodies that are 5 to 20 m long, 1 to 3 m wide and 1 to 2 m thick, mostly located in the central parts of the chalcopyrite zone. In addition to chalcopyrite and Ag and Bi sulphosalts, these tabular bodies contain subordinate amounts of sphalerite and galena.
• Sphalerite-galena zone - which mostly surrounds the chalcopyrite zone but in places it is observed to surround inner copper zones, including the enargite zone. It also contains pyrite, quartz, alunite, kaolinite, dickite, Zn-bearing siderite and hematite, and is best developed in the Colquijirca deposit, where it constitutes the largest Zn-Pb-(Ag) resource of the deposit. Prior to mining, the zone may have exceeded 30 Mt @ 6% Zn, 3% Pb, and 120 g/t Ag. It extends to the northernmost tip of the Colquijirca deposit, and also to the eastern margin of the same deposit, where at La Llave and La Pampa, a resource of nearly 47 Mt @ 3.2% Zn, 1.1% Pb, 44 g/t Ag has been delineated. A similar zone also occurs to the east and west of the Smelter deposit. There is an outward gradation in the mineralogical composition across the sphalerite-galena zone, through three main sub-zones:
a). Alunite-quartz bearing sub-zone with sphalerite, galena, pyrite and barite, that occurs as small podlike bodies typically overlying the enargite zone, where alunite is intergrown with sphalerite, galena, barite and quartz in druselike cavities cementing breccias or as coatings on small geodes in massive sphalerite-galena-pyrite bodies (Bendezú et al., 2008).
b). Kaolinite-dickite bearing sub-zone with sphalerite, galena, pyrite, ±alunite, siderite, which is the most important volumetrically, and mainly occurs surrounding the chalcopyrite zone in the northern half of the Colquijirca deposit. This sub-zone typically contains at least 5 vol.% kaolinite + dickite, and locally up to >50 vol.%. Both kaolinite and dickite fill open spaces within sphalerite, galena and pyrite clusters, whilst fine grained sphalerite, galena and pyrite occur as euhedral grains in a matrix of kaolinite.
c). Siderite-hematite bearing sub-zone with sphalerite, galena and quartz. This sub-zone includes considerable amounts of siderite, quartz and hematite. The siderite may be significantly enriched in Zn. Magnetite commonly partially replaces specular hematite and is most abundant in the external parts of the sub-zone. Other characteristic but less abundant minerals in the sub-zone include marcasite, quartz, fluorite and fine-grained muscovite.
The abundance of sphalerite (Sp) relative to galena (Gn) decreases with increasing distance from the Cu zones, from 8 to 15 vol.% Sp and 3 to 5 vol.% Gn in the alunite-bearing sub-zone (Sp:Gn = 3 to 4:1); to 5 to 8 vol.% Sp and 2 to 5 vol.% Gn in the kaolinite-dickite sub-zone (Sp:Gn = 1 to 3:1); to 2 to 5 vol.% Sp and 2 to 3 vol.% Gn in the siderite-hematite sub-zone (Sp:Gn = 1 to 1.5:1). Galena dominates over sphalerite in the external parts of the siderite-hematite sub-zone. In some small localised sections of the sphalerite-galena zone rich patches of as much as 900 to 1500 g/t Ag are encountered.
• Zn-bearing carbonate zone, which forms an outermost halo, almost completely surrounding the sphalerite-galena zone, and is virtually devoid of sulphides. It basically comprises Zn-bearing siderite and rhodochrosite, with minor quartz in mantos that are <10 m long and wide, and 3 m thick, with grades commonly in the range of 1 to 5 wt.% Zn.
• Barren outer zone, that consists of calcite and subordinate dolomite patches and veinlets above the entire Smelter-Colquijirca mineralised corridor beyond the Zn-bearing carbonate zone. Calcite and dolomite veinlets have been recognised up to several hundred metres north from the Zn-bearing carbonate zone and up to tens of metres above it, accompanied by recrystallisation of the carbonate hosts.
iii). Late Ore Stage - which overprints the enargite-gold, enargite and bornite zones of the main ore stage, and comprises an assemblage dominated by chalcocite with subordinated digenite and covellite and trace amounts of chalcopyrite. This stage is structurally controlled underground in the Smelter deposit, where thin chalcocite veinlets tend to occur along NW trending subvertical faults and fractures, and has been recognised as deep as almost 700 m below the oxidation zone. It is most commonly seen overprinting the enargite-rich zones. Chalcocite typically forms irregular, poorly crystallized aggregates a few centimetres wide, replacing enargite along fractures and veinlets that continue into the main ore-stage minerals, and may also replace pyrite II and late gold-bearing tennantite. Chalcocite is also observed coating pyrite II, and covellite and less commonly digenite may locally be as abundant as chalcocite. The last phases occur as fuzzy, fibrous aggregates with chalcocite or as rims to enargite grains.
Late ore stage chalcocite-(covellite-digenite) are also common in the bornite zone as thin veinlets cutting bornite or as coatings on dissolution vugs. In both cases, chalcocite is >50 vol.% of the assemblage and covellite between 10 and 40 vol.%, with the balance being digenite and, subordinately, chalcopyrite. Locally, pockets a few tens of centimetres across comprise massive chalcocite-(digenite-covellite-chalcopyrite).
Important late ore-stage chalcocite bodies also occur in the Marcapunta Oeste deposit (Vidal and Ligarda, 2004), where they appear, as at Smelter, to be structurally controlled by subvertical faults.
Production reserves and resources - Various reserve and resource estimates have been published, a follows:
Historic production to 2003 - 13 Mt or ore (Vidal and Ligardo, 2003);
Remaining Zn-Pb-Ag reserves, 2003 - 8 Mt @ 6.4% Zn, 2.4% Pb, 96 g/t Ag (Vidal and Ligardo, 2003);
Drill indicated Cu resource, 2003 - 50 Mt @ 1.9% Cu, 0.3 g/t Au, 23.3 g/t Ag, 0.6% As (Vidal and Ligardo, 2003);
Resource at Marcapunta, Smelter North and South, and Colquijirca open pit in 1978
- 310.53 Mt @ 4.1% Zn, 3.1% Pb, 54 g/t Ag, (USGS MRDS database, viewed 2016)
Marcapunta Oeste deposit
This deposit is located below the the western wing of the Marcapunta volcanic complex diatreme, and appears to be the southern extension of the Smelter deposits. It comprises horizontal to sub-horizontal mantos and irregular bodies of breccia, confined within a prospective horizon, sandwiched between the sedimentary rocks of the Mitu Group at the base, and dacitic volcanic rocks of the volcanic complex at the top, with a thickness that varies from 20 to 100 m. The mineralised zone contains enargite-pyrite, grading to covellite-chalcopyrite-digenita-chalcocite, within a gangue of alunita-quartz (El Brocal website).
JORC compliant mineral resources at a 0.5% Cu cut-off, (after AMEC, 2009) are quoted on the El Brocal website (viewed 2016) as:
Chalcocite ore - 17.671 Mt @ 1.45% Cu, 0.34 g/t Au, 0.059% As;
Enargite ore - 31.304 Mt @ 1.24% Cu, 0.74 g/t Au, 0.379% As;
Mixed ore - 26.705 Mt @ 1.12% Cu, 0.59 g/t Au, 0.152% As;
TOTAL - 75.680 Mt @ 1.25% Cu, 0.60 g/t Au, 0.224% As.
San Gregorio deposits
An unmined resource that is located ~3 km south of the Smelter deposit, and just to the south of Marcapunta Oeste. It is the southernmost deposit of the north-south mineralised trend that starts with the Colquijirca open pit ~7 km to the north, and is located <1 km south of the southern margin of the Marcapunta volcanic complex diatreme.
The deposit is composed of stacked orebodies, each of several tens of metres in thickness, dipping SW at moderate to shallow angle, and containing up to 15% Zn and 5% Pb. Mineralisation replaces a 300 m thick carbonate sequence and has lateral north-south elongated dimensions of at least 1000 x 600 m. On the basis of lithostratigraphic correlations, supported by palaeontological observations (A. Pardo in Bendezú, 1997) and trace element analyses, the host carbonate rocks are interpreted to belong entirely to the Upper Triassic to Lower Jurassic Pucará Group (Fontboté and Bendezú, 1999).
The carbonate host rocks are extensively altered, with the richest ores contained in what is locally termed 'sulphide rock'. This rock is the result of complete decarbonatisation of Pucará Group dolostones and limestones, and the development of an advanced argillic assemblage of quartz, alunite and kaolinite, with >30% total sulphides. The ore mineralogy is very fine-grained, commonly <40 µm, generally only comprising sphalerite, galena and very minor marcasite, giving the macroscopic appearance of an unconsolidated detrital rock. Locally, the quartz, together with less abundant alunite and kaolinite, amount to >75 wt.% of the rock, known as 'high silica sulphide rock' (Fontboté and Bendezú, 1999).
A few intercalations of relict host protoliths with a dolomitic composition are preserved within the main mineralised area. Minor bodies of massive Fe-Zn rhodocrosite occur at the transition between relict dolostone and sulphide rocks. There is a zoning progressing from 'sulphide rock' → massive Fe-Zn rhodocrosite → relict dolostones, controlled by rapid pH increase as very acid fluids are neutralised by carbonate country rocks. Fontboté and Bendezú (1999) suggest the extreme acidity of the fluid responsible for the formation of the advanced argillic alteration of the 'sulphide rock' is indicated by the fact that it was able to transport and precipitate up to 15% Al2O3 in originally carbonate rocks that were aluminium deficient.
The upper sections of San Gregorio is capped by a silicified layer up to several tens of metres in thickness, and a tabular body consisting of massive alunite, kaolinite and quartz that is up to 50 m thick. Its morphology and the presence of relict 'sulphide rock' cut by alunite-kaolinite veins suggest that this cap was formed by steam-heated acid waters and that was superimposed on the main mineralisation phase. This cap carries high Ag and, in places, Bi values (Fontboté and Bendezú, 1999).
TOTAL zinc resource at San Gregorio - 79.934 Mt @ 5.223% Zn, 1.528% Pb, 9.6 g/t Ag (El Brocal website (viewed 2016).
The most recent source geological information used to prepare this summary was dated: 2009.
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.
Bendezu R, Page L, Spikings R, Pecskay Z and Fontbote L, 2008 - New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au–Ag and Cordilleran polymetallic ores: in Mineralium Deposita v.43 pp. 777-789|
Bendezu, R. and Fontbote, L., 2002 - Late timing for high sulfidation cordilleran base metal lode and replacement deposits in porphyry-related districts: the case of Colquijirca, central Peru: in SGA News No. 13 p. 1, 9-13.|
Bendezu, R. and Fontbote, L., 2009 - Cordilleran Epithermal Cu-Zn-Pb-(Au-Ag) Mineralization in the Colquijirca District, Central Peru: Deposit-Scale Mineralogical Patterns : in Econ. Geol. v.104, pp. 905-944.|
Bendezu, R., Fontbote, L. and Cosca, M., 2003 - Relative age of Cordilleran base metal lode and replacement deposits, and high sulfidation Au-(Ag) epithermal mineralization in the Colquijirca mining district, central Peru: in Mineralium Deposita v.38, pp. 683-694.|
Fontbote, L. and Bendezu, R., 1999 - The carbonate-hosted Zn-Pb San Gregorio deposit (Colquijirca District, Central Peru) as part of a high sulfidation epithermal system: in Stanley, C.J. et al., (Eds. 1999), Mineral deposits: processes to processing, Balkema, Balkema, Amsterdam, pp. 495-498.|
Vidal, C. and Ligarda, R., 2003 - Enargite-gold deposits in Marcapunta, Colquijirca Mining District, Central Peru: Mineralogical and geochemical zonation of subvolcanic, limestone-replacement deposits of high sulfidation,epithermal character: in 10th Congreso Geologico Chileno, 2003, Universidad de Concepcion, Departamento de Ciencias de la Terra, 1p.|
Vidal, C. and Ligarda, R., 2004 - Enargite-gold deposits at Marcapunta, Colquijirca Mining Dustrict, central Peru: Mineralogic and geochemical zoning in subvolcanic, limestone replacement deposits of high-sulphidation epithermal type: in Sillitoe, R.H., Perello, J. and Vidal, C.E., (Eds.), 2004 Andean Metallogeny: New Discoveries, Concepts and Updates, Society of Economic Geologists, Special Publication 11, pp. 231-241.|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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