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Magistral |
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Peru |
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Cu Mo Ag Sb
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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 Magistral porphyry-skarn copper-molybdenum deposit is located at an elevation of 3900 to 4700 m in the Peruvian Andes, ~140 km east of the Pacific coast port of Trujillo, and 450 km NNW of Lima, in Peru (#Location: 8° 13' 8"S, 77° 46' 31"W).
Magistral is part of the Pasto Bueno-Conchucos district, which was known early in the colonial era as a gold-silver producing area. Early records report the production of ~0.685 t of gold and 1.37 t of silver between 1644 and 1647 (Salazar Suero, 1997). The prominent outcrops of copper oxides at Magistral were probably known at this time, but the first modern records of exploitation date to 1915 when the Garagorri Mining Company built a small smelting furnace to exploit high-grade surface ores from shallow workings, lasting until 1919. The deposit was explored intermittently between 1924 and 1953 by the Cerro de Pasco Corporation. Cerro de Pasco purchased the Magistral concessions in 1950, but no significant work was done until 1969. From 1969 to 1973, Minera Magistral conducted a surface and underground exploration program that focused on copper-bearing skarn mineralisation on the south side of Magistral valley, at and above the valley floor level. Buenaventura Ingenieros S.A. conducted a thorough evaluation of the Magistral deposit in 1980-1981. In 1997, Minero Peru began the process to privatize Magistral by inviting open bidding. An option to purchase the titles to the five Magistral mining concessions was awarded to Inca Pacific in 1999. In 2000, Inca Pacific entered into a joint venture with Minera Anaconda Peru S.A. as Ancash Cobre, to carry out exploration and development at Magistral. In 2004, Inca Pacific Resources Inc. acquired the Anaconda Peru interest in Ancash Cobre. From 2000, Ancash Cobre had conducted drilling and other testing programs and in 2007, prepared a new mineral resource estimate, and completed a final feasibility study. In 2011, Inca Pacific was purchased by Compañía Minera Milpo S.A.A., a division of Brazil's Votorantim Metais Ltda.
Regional Setting
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
Magistral lies near the northeastern end of the Cordillera Blanca, which is predominantly composed of Cretaceous carbonate and clastic sequences, that strike north to NW and are folded into a series of anticlines and synclines with NW-trending axes. The Cretaceous sedimentary pile is bounded to the east by an unconformably underlying early Palaeozoic metamorphic terrane composed mainly of micaceous schist, gneissic granitoid and slate.
The Cretaceous rocks are structurally overlain by black shale and sandstone of the upper Jurassic Chicama Formation that was thrust eastward over a prominent regional structure. The Chicama formation was intruded by granodiorite and quartz diorite related to the extensive 8.2±0.2 Ma Cordillera Blanca batholith, the main bulk of which is ~20 km to the SW of Magistral, with smaller inliers ~10 km to the NW (Dick, 2004).
The Cretaceous sequence comprises:
• Goyllarisquizga Group - a lower dominantly clastic sedimentary succession composed of sandstone, quartzite, shale and minor carbonate, that comprises, from the base, the following formations:
- Chimu, ~140 m of quartzite, with shale interbeds,
- Santa, ~120 m of dark grey dolomitic limestone, marly limestones and shale,
- Carhuaz, ~275 m of sandstones and
- Farrat, ~600 m of white, medium to coarse grained, crossbedded, quartzite and sandstone with intercalated shale and marl.
• Machay Group - a predominantly calcareous, succession, made up of limestone, marlstone, sandstone and calcareous shale, comprising the following formations, from the base:
- Pariahuanca, ~125 m of calcareous sandstones and limestone,
- Chulec, ~275 m of shale, calcareous siltstone, marl and sandy limestones,
- Pariatambo, ~90 m of limestone, grey to black mudstone, shale and tuff,
- Jumasha, ~950 m of limestones with thin marl, calcareous shale and siltstone interbeds, and
- Celendin, >300 m of limestone, marl and thin shale interbeds.
Several major structural elements are evident in the Cretaceous sedimentary sequence in the Magistral district, including anticlines, synclines and thrust faults, the trend of which all swing from NW to north-south near Magistral (Dick, 2004). The imbricate tectonics in the district do not seem to have affected the basement, and are considered to be thin-skinned, consisting of low-angle, east vergent thrust faults and horizontal shortening in the order of 10 km east-west. The leading thrust edge is the Conchucos fault. Numerous NE-trending lineaments cut the low-angle features, resulting in disruption to fold axes, termination of folds, the alignment of intrusive bodies along them, and appear to have had an affect on the location of Quaternary-age glacial valleys.
District Geology
The district scale structural setting is complex, characterised by low-angle reverse faults and upright to overturned north-striking folds. The dominant structural feature in the area is the Huacchara fault, ~1 km to the west of Magistral, which forms a major break in the stratigraphy, with vertical displacements of at least 1000 m, and can be traced for >25 km to the north from Magistral. It strikes north-south and dips about 60°W, juxtaposing siliciclastic sedimentary rocks of the Goyllarisquizga group over the carbonate-dominant Jumasha formation.
East of the Huacchara fault, the stratigraphy is predominantly deformed by a series of thrusts and tight anticlines and synclines with axial planes dipping to the NW, and limbs dipping at between 10 and 50°. Between Laguna Pelagatos and Magistral, a large overturned fold, which is related to the Huacchara fault, is cored by carbonate-rich sedimentary rocks of the Pariahuanca, Chulec and Pariatambo formations, suggesting the stratigraphy at Magistral, particularly the skarn-hosting Jumasha formation, may be overturned. The reverse faults in the Magistral area vary from high- to low-angle, the latter constituting bedding plane thrusts striking NW and primarily affecting the Jumasha and Celendin formations.
West of the Huacchara fault, there is a similar structural setting, comprising a large synclinal fold with an arcuate axis, striking ~NE.
Several stages of deformation, ranging in age from upper Cretaceous to Miocene have been documented (Wilson and Reyes, 1967; Noble et. al., 1990; Wilson et. al., 1995; and Benavides-Caceres, 1999). Red beds of the Upper Cretaceous Chota Formation to the south of Magistral, have been dated at 50 to 44 Ma (Noble et. al., 1990) and discordantly overlie Cretaceous calcareous rocks, evidence of a pre-Eocene event. As the red beds are folded and thrust faulted, a younger tectonic event is indicated. The roughly east-west trending alignment of intrusive stocks in the region of Magistral indicates that preintrusive structures had an affect on the location of intrusions.
The main lithologies in the immediate deposit area can be divided into two domains separated by the regional north-south striking Huacchara thrust fault. The stratigraphy to the east of the fault is dominated by the Jumasha formation, but also contains some of the Celendin formation, but further to the east is underlain by the Pariahuanca, Chulec and Pariatambo formations, which comprise sandstones, marls and black shales.
The Jumasha Formation, which is the principal host to skarn mineralisation at Magistral, is mainly composed of medium to thick bedded limestone (Noblet, 2000), and includes four principal stratigraphic members with a total thickness of ~900 m. The Celendin formation outcrops mainly in the walls of the hanging valleys to the NE of the deposit, and comprises units of grey marlstone, calcareous shale and thinly bedded limestone, and has a stratigraphic thickness of at least 300 m (Noblet, 2000).
The complete section of the regional stratigraphy is exposed to the west side of the Huacchara fault. The lowermost unit is a quartzite member of the Chimu formation, overlain by the Aptian clastic sequences of the Santa, Carhuaz and Farrat formations (Dick, 2004 after Noblet, 2000). These units are overlain by the Pariahuanca, Chulec and Pariatambo formations, which are, in turn, overlain by the Jumasha and Celendin formations.
Intrusive rocks occur as small stocks and dykes of Miocene diorite to quartz monzonite composition. The intrusions, including the Magistral stock, were emplaced along a NE-trending zone extending along the Magistral valley (Dick, 2004).
Thick accumulations of unconsolidated gravel, lacustrine deposits, and talus are found at lower elevations, and are related to fluvioglacial and lacustrine environments associated with alpine glaciation and earthquake activity. The massive blocky talus on the southeast side of the Magistral valley (Arizona and El Indio areas) is the result of landslides caused by the 1946 earthquake (Sassarini, 1973).
Deposit Geology
Jumasha formation - is a well-bedded, dark-grey recrystallised micrite limestone, with thin beds of calcareous shale, siliceous carbonate sediment and recrystallised sandstone. These rocks dip 45°W. The limestone becomes progressively more bleached, with its carbon content decreasing as the Magistral alteration zone is approached. Toward the intrusive, there is generally a sharp contact separating unaltered limestone and metasomatically altered rock or skarn, although distal skarn alteration can occur in limestone up to 150 m outboard of the main contact. Remnants of limestone or marble within the alteration aureole of the Magistral deposit are usually bleached white, and are generally coarser grained than those outside the aureole.
Intrusive Rocks - In plan view, the intrusive mid-Miocene Magistral stock, dated at 15.3 to 14.6 Ma, has an irregular, east-west elongated, 600 x 400 m elliptical shape at the 3950 m elevation, and occupies an area of ~0.24 km2. The upper surface of the stock appears to plunge at ~45 to 60°WSW and is up to 350 m in diameter orthogonal to the plunge axis.
The stock appears to represent a single intrusive body, with an equigranular to porphyritic textures and diorite to quartz monzonite composition. It will be described in more detail in the 'Alteration' section below.
Dykes and Sills - late, little altered and weakly mineralised, coarse-grained dykes are common in the peripheral sections of the Magistral deposit, including the mixed zone. They contain disseminated pyrite with minor chalcopyrite and scarce quartz veinlets. Alteration comprises propylitisation and silicification, and they were probably emplaced in the late stages of the mineralising process. Another later scarce set of subvolcanic unmineralised, andesitic to basaltic-andesite dykes intrude all lithologies including Jumasha limestone. Their textures are aphanitic to porphyritic, with some having phenocrysts of plagioclase, hornblende or biotite.
Structure
Six principal low-angle or thrust faults that strike northerly and dip at 25 to 45°W have been mapped in the deposit area. Two of these, the Keith Glover thrust and the San Ernesto fault have been named (Ramos, 2005). Where the San Ernesto fault intersects the San Ernesto mine, it strikes north and dips 40 to 55°W, and has a strike-slip displacement on the order of 60 m (Ramos, 2005). Three major normal faults have also been identified, the Chavin, B normal and C normal. The Chavin Fault, also strikes north and dips subvertically, with locally measured vertical displacements of 5 to 50 m along the plane of the fault (Ramos, 2005). Numerous smaller unmapped faults have been encountered, marked by fracture and rubble zones in drill core.
Alteration
Several distinct styles of alteration are associated with the Magistral deposit. Metasomatic contact metamorphic alteration affected the Jumasha carbonate rocks during the emplacement of the Magistral intrusion, generating skarn within its thermal aureole. Endoskarn formed within the marginal carapace of the intrusive complex and within peripheral dykes and sills. Synemplacement prograde and post-emplacement retrograde hydrothermal events resulted in widespread porphyry-style alteration in the intrusion and the retrograde alteration of skarn. Fracture controlled silica-pyrite-arsenic alteration in the main deposit, and bands of orpiment-realgar in unaltered limestone, are interpreted to postdate retrograde skarn alteration (Dick, 2004; Ramos, 2005).
Endoskarn - developed in the carapace of the Magistral stock, and in the minor intrusions of the mixed zone outboard of the main intrusive contact. Where less intense, it often exhibits relict granitic or porphyritic texture, with primary plagioclase, hornblende and biotite, and lesser red to brown garnet and pyroxene. Where the endoskarn is well-developed, primary textures are virtually obliterated by an assemblage of red to brown garnet, pyroxene and green garnet.
Mixed Zone - a transitional zone of intercalated skarn and intrusive bodies located between the main endoskarn altered intrusion and the exoskarn. It contains bodies or lenses of skarn, alternating with porphyritic and coarse-grained granitic dykes and sills that are characterised by porphyry style mineralisation and potassic and/or phyllic alteration. The intrusive and skarn intervals each have widths varying from <1 to tens of metres. The skarn is mainly composed of red-brown garnet and clinopyroxene, and has a fine to medium grained granoblastic texture. Endoskarn is commonly also developed in the intrusions. Retrograde alteration, typically composed of calcite-quartz-smectite-chlorite-clay, with K feldspar-biotite-calcite in intensely altered zones, is widespread and locally intense. The intrusions in the retrograde skarn zones often have propylitic alteration overprinting earlier potassic and phyllic phases. Both retrograde-altered skarn and intrusive rocks often have well-developed quartzpyrite-chalcopyrite-molybdenite vein stockworks.
Exoskarn and Distal Skarn - Skarn alteration is developed at the contact between the Magistral intrusion and the Jumasha Formation carbonate rocks. Mixed zone dykes are absent in many areas in the footwall of the Magistral stock, although the exoskarn in these locations is no different to that lying outboard of the mixed zone in other parts of the deposit. Skarn near the intrusive contact is composed of varying proportions of clinopyroxene and red to brown garnet. The
arnet composition is intermediate between andradite and grossularite, whilst the pyroxene is inferred to be intermediate between hedenbergite and diopside (Allen, 2000). Toward the skarn-limestone contact, the proportion of green garnet and clinopyroxene often increases (Ramos, 2005). Lenticular/irregular bodies of distal skarn are found in limestone well beyond the limits of the main skarn contact, typically green-coloured skarns that contain green garnet, lesser brown garnet, abundant clinopyroxene, and minor quartz and wollastonite. Retrograde alteration is well developed in many distal skarns, generally in haloes bordering quartz-chlorite-epidote-calcite veins. The distal skarns lying 50 m or more beyond the main skarn front often have large garnet crystals that form a coarse mosaic with quartz-calcite-chlorite-magnetite or chalcopyrite-pyrite-magnetite filling the large interstices. In some cases, the copper content in these coarse-grained skarns can reach 5% or more. Such coarse grained skarns are suggested to have formed from limestone with high porosity, possibly including fossiliferous (bioclastic) beds.
Hornfels and Skarnoids - assemblages that were developed in thin beds of calcareous shale and siltstone, interbedded within the limestone country rocks.
Porphyry-Style Alteration - The Magistral stock has been subjected to locally strong hydrothermal alteration. The earliest phase is characterised by calcium-silicate (clinopyroxene, tremolite/actinolite), followed by a potassic assemblage of secondary biotite-K feldspar-quartz, and late, overprinting phyllic (sericite-pyrite) phases. In sections of the deposit, known as the 'H facies' (see below), very strong sericite alteration, with wall-rock silicification in strongly quartz-veined zones, overprints and destroys the textures and mineral assemblages of the earlier phases. Both potassic and quartz-sericite alteration facies are important hosts to Cu-Mo mineralisation, although the potassic zones are significantly more important volumetrically than the superimposed quartz-sericite facies. However, where the sericite-quartz alteration and associated quartz veining is most intensely developed, the intrusion is barren. Argillic alteration is characterised by the development of quartz, green clay or sericite, and chlorite associated with weakly developed quartz veining. It has replaced the original rock constituents and only hosts weak Cu and Mo mineralisation. Sections of the main prograde potassic altered porphyry system (i.e., parts of the Sara facies and significant parts of the San Ernesto facies described below) have been subjected by argillic alteration.
Late-Stage Alteration - characterised by a very fine-grained mixture of dark-grey to black silica and pyrite that forms selvages on late-stage fractures in both altered intrusives and skarn. These fractures may contain realgar [AsS], orpiment [As2S3] and other arsenic minerals where they cut garnet-rich skarn and quartz-sericite-altered intrusive rocks. No retrograde skarn minerals are recognised, where these late-stage silica veins cut garnet-rich skarn, suggesting the silica-arsenic event was much later than the formation of retrograde skarn alteration. Orpiment and realgar are also found in unaltered limestone beyond the outer limit of skarn, occurring as distinct bands within the limestone, sometimes containing 30 to 50% of these arsenic sulphides. Mo, without significant Cu, is also found in weakly altered distal skarns, well outboard of the main skarn contact. As, Sb and Mo tend to occur in altered limestone country rock that is not mineralised with Cu. Sb minerals have not been observed to occur in unaltered limestone country rock.
Mineralisation
Historical Surface and Underground Workings - The largest outcropping mineralised zones in the Magistral skarn are named San Ernesto, Arizona, Sara (El Indio) and Chavin. The San Ernesto, Arizona and Sara zones (from west to east) are segments of a continuous, ~1000 m long, by up to 150 m wide skarn deposit, which has been imbricated by postmineral faults, and occurs along the southern and southwestern margin of the Magistral intrusion (Sassarini, 1973). The outcrops of these zones now form a series of resistant knobs and steep bluffs extending 900 m easterly along the south side of the Magistral cirque, at elevations of 4100 to 4360 m. The broad Magistral cirque trends NE-SW across the western half of the intrusion, opening out to the SW and closing to the NE.
In historical underground workings at San Ernesto, the mineralised zone comprises massive skarn, composed of varying amounts of red-brown garnet and pyroxene with minor relict blocks of recrystallised limestone. The skarn contains veinlets, irregular pods, lenses and disseminated aggregates of pyrite, chalcopyrite, pyrrhotite, marcasite and molybdenite, with minor amounts of sphalerite, galena, arsenopyrite and magnetite.
The Chavin (formerly Rio Tinto) zone is located along the northern margin of the same intrusion over a length of >1000 m, interpreted to connect around the western margin of the intrusion with the San Ernesto zone. It outcrops as small scarps and bluffs at elevations of 4100 to 4190 m on the northwest side of the cirque. Small outcrops of mineralised distal skarn, including the La Gringa and Asturias zones, lie 500 to 800 m east of Chavin, peripheral to the inferred trace of the eastern contact of the intrusive.
Exposed skarn in the Chavin zone is largely composed of coarse-grained brown garnet with lesser pyroxene, quartz, minor chlorite and calcite (Sivertz 1999), varying from banded to massive. Narrow dikes and sills of quartz-rich hornblende plagioclase porphyry cut the Chavin outcrops. Coarse-grained aggregates of pyrite with varying amounts of pyrrhotite, magnetite and chalcopyrite occur in lenses, pods and discontinuous bands in massive garnet skarn. Streaks of limonite and manganese oxides with malachite and azurite coat the outcrops in places, marking narrow, north-trending fracture zones. Narrow dykes and sills of quartz-rich hornblende plagioclase porphyry cut the Chavin outcrops. Selected samples of skarn and mineralised dykes vary from <0.15 to >3% Cu, suggesting the mineralisation is patchy
Mineralisation in Prograde and Distal Skarn - Skarn-related mineralisation is preferentially developed close to steeply dipping contacts in most of the zones on the southern sides of the Magistral Intrusion (i.e., the San Ernesto and Arizona zones). In the north and northwest (i.e., Chavin zone), erratically mineralised prograde skarn is found outboard of the mixed zone.
Skarn mineralisation is characteristically disseminated, veined and locally semi-massive to massive sulphides, comprising chalcopyrite, pyrrhotite and pyrite, sometimes with minor molybdenite. Near the San Ernesto zone underground workings, these sulphides occur in a body of hydrothermal breccia with associated semimassive to massive magnetite. Although Cu grades are generally more erratic in skarns compared to the porphyry-style mineralisation, where best developed, it can be >10% Cu, as in the San Ernesto underground workings, where the main ore zone is up to 80 m wide. However, Mo grades are generally lower in skarn than in porphyry style mineralisation, although locally, drill intersections of up to 1.0% Mo have been recorded in skarn, particularly in the San Ernesto zone, where molybdenite occurs as coarse blebs interstitial to coarse garnet crystals.
The distal skarns at Asturias and San Blas zones, to the SW and north respectively, are characterised by a green garnet-pyroxene-wollastonite assemblage. No significant grades of Cu had been intersected at either in 2008, although potential remained.
Mixed Zone and Intrusive Rocks where porphyry style sulphide and iron-oxide mineralisation which is hosted by both intrusive rocks and retrograde altered skarn in the mixed zone, occurs predominantly as stockwork or sheeted complexes of quartz-adularia/orthoclase-epidote veins. Pyrite, chalcopyrite and molybdenite are the most common sulphides in the veins, with variable but usually minor amounts of pyrrhotite, tetrahedrite, tennantite, stibnite, sphalerite, magnetite and hematite. Pyrite, chalcopyrite, tetrahedrite and molybdenite also occur disseminated in the wall rocks, although molybdenite is generally restricted to the close proximity to quartz-sulphide veins, whilst chalcopyrite and pyrite are more widely disseminated. Where mineralised quartz veins are absent, Cu and Mo grades are generally low (<0.4% Cu, <150 ppm Mo). Porphyry-style mineralisation is best developed as a broad shell that straddles the border zone of the Magistral stock, and the adjacent zone of locally endoskarn altered porphyry dykes and retrograde altered garnet-pyroxene exoskarn that constitutes the 'mixed zone'.
Several styles of mineralised quartz veins had been recognised by 2008. The earliest comprise chalcopyrite-dominant quartz-pyrite veins, which are generally more common than molybdenite-dominant veins (Glover, 2000), although, petrographic evidence suggests molybdenite is an early-vein mineral, found on vein margins and disseminated in the adjacent wall rocks. Pyrite is intergrown with vein quartz, and chalcopyrite forms rims on pyrite. The sequence of sulphide deposition is interpreted to have been molybdenite → pyrite → chalcopyrite → stibnite → tetrahedrite (Allen, 2000, 2001). Arsenopyrite occurs within both vein types locally, but is more common within late-stage, fine-grained, grey, sulphide-bearing, quartz-calcite veins (see below). Quartz in the early copper-rich veins is dull grey and translucent. These veins have epidote-chlorite selvages where they crosscut prograde garnet-pyroxene skarn and endoskarn, and chlorite-only selvages where they crosscut the porphyry away from the intrusive contact. Late-stage quartz veins that carry molybdenite are white and opaque with sericitic selvages, and, together with more pervasive silica-sericite alteration, are particularly well developed close to late brittle faults (Glover, 2000).
As the core of the intrusion (in the Sara facies zone) is approached, the mineralised quartz veins gradually diminish in abundance and give way to low-grade disseminated mineralisation. Due to the westerly plunge of the porphyry-skarn system, Cu-Mo mineralisation is better developed on the hanging wall (western) side than on the footwall (eastern). Drill holes collared in the central and eastern sections of the well-mineralised central to western section of the Magistral intrusion (the San Ernesto Facies), terminate in the low-grade core or Sara facies that is exposed at surface further to the east. Very few drill holes have passed through the entire intrusion into the footwall skarn, so little is known of the alteration and quartz veining in the footwall shell to the intrusion, which is represented on the eastern margin of the intrusion at surface.
Late Stage Quartz-Calcite-Sulphide Veins - which contain locally massive, very fine grained pyrite, tetrahedrite-tennantite and arsenopyrite, with occasional traces of chalcopyrite and/or sphalerite. This veining comprise a relatively minor part of the mineralisation, but is widespread, crosscutting both skarn and porphyry style mineralisation. Locally, they are very high grade, and are associated with zones of retrograde alteration, defined by pervasive silica and sericite, which has a characteristic tan colour where it overprints garnet skarn. As with the quartz-molybdenite veins with which they are sometimes associated, these grey, sulphide-quartz-calcite veins commonly occur close to late-stage brittle faults.
Distribution of Mineralisation and Alteration
On the basis of post-alteration textures, compositions, the degree and style of alteration, the density of veins, and the tenor of copper and molybdenum mineralisation, the Magistral deposit has been divided into three separate 'facies', as follows:
i). San Ernesto Facies - has the best-developed porphyry-style alteration and mineralisation, and is characterised by moderate to locally strong potassic and quartz-sericite alteration. It hosts the greater part of the Cu and Mo mineralisation, mainly occurring as stockwork and sheeted quartz-sulphide veins. Disseminated Cu and Mo mineralisation is also found within the wall rock. Although dominantly a quartz monzonite, the quartz content and the ratio of orthoclase to total feldspar vary, with compositions that range from diorite to quartz monzonite as orthoclase and quartz contents increase. Some minor porphyritic phases, interpreted as intrusions into the main porphyry, have quartz phenocrysts and a distinctly different porphyritic textures that range from medium-grained equigranular to porphyritic, with grains (in equigranular rocks) and phenocrysts ranging from 0.5 to 5 mm. The porphyritic varieties have microcrystalline to granular groundmasses composed of fine aggregates of quartz, K feldspar and minor ferromagnesian minerals. The rock typically contains 25 to 35% plagioclase grains or phenocrysts, with up to 10% amphibole and biotite. Phenocrysts of plagioclase and hornblende are subhedral to euhedral, whilst biotite is anhedral. This facies has characteristically been subjected to moderate to locally strong hydrothermal alteration, the earliest of which is a calcium-silicate phase (clinopyroxene, tremolite/actinolite). This was succeeded by potassic (secondary biotite-orthoclase-quartz) and late, overprinting phyllic (sericite-pyrite) phases. It locally contains well-developed sets of multidirectional quartz-sulphide veinlets, which can reach densities of 30 to 40 per metre. The similar veins in the Sara facies are much less frequent.
ii). Sara Facies - occupies an oval shaped area in the eastern central and southern part of the Magistral intrusion, where it is in direct contact, via a variable width of the mixed zone, with skarn altered Jumasha carbonates on its NE, eastern and southern margins in the Asturias, La Gringa and El Indio areas. To the north, west and SW, it is bounded by the San Ernesto Facies which forms the outer crescent shaped margin of the Magistral intrusion.
Endoskarn zones occur near the contacts with exoskarn. This facies is interpreted to represent the core of the Magistral intrusion. Alteration is weak to moderate potassic, with secondary biotite replacing both amphibole and primary biotite. Like the San Ernesto facies, the composition of the intrusions ranges from diorite to quartz monzonite. Its central section has a coarse-grained equigranular texture, although border phases near skarn contacts are sometimes porphyritic. Phenocrysts comprise 40% plagioclase, 8 to 10% hornblende and 6 to 8% biotite in an interstitial matrix (or groundmass, in porphyritic phases) of plagioclase, K feldspar, quartz, amphibole and biotite. Cu-Mo mineralisation occurs in sparse quartz-pyrite-pyrrhotite veinlets, and less commonly in quartz-calcite veins. Weakly mineralised zones of disseminated pyrite-pyrrhotite-chalcopyrite are also present.
iii). H Facies - a late, strongly sericite-altered, intensely fractured alteration zone characterised by dense, overprinting, stockwork and sheeted quartz and quartz-calcite veins. These veins are weakly mineralised to barren, sometimes at a density of >40 per metre, each of which is 5 to 20 mm thick. Vuggy textures are common, with cavities on the margins and cores of the veins, which have strongly silicified alteration envelopes. They are locally so closely spaced that they almost completely replace the original intrusive rock and earlier mineralised veins. They are not restricted to the stock, but in some areas adjacent to the intrusive contact, also cut skarn, forming a rock composed of relict angular silicified fragments and blocks of skarn in a dense stockwork of translucent to white quartz and quartz-calcite veining. The strong overprinting sericite alteration in the core of the facies gradually weakens outwards and has very irregular gradational contacts with San Ernesto facies alteration. This alteration is present in the north-central (Chavin) and southwest (San Ernesto) sectors of the deposit, where it take the forms of irregular lenses near contacts between skarn and the main body of San Ernesto porphyry. This facies characteristically has much lower Cu and Mo grades than the other two facies, although the volume of Cu and Mo mineralisation is inversely proportional to the intensity of the alteration and, particularly, to the volume of barren veins. The intense sericite alteration is strongly texture destructive, most commonly overprinting potassic and propylitic alteration of the San Ernesto facies.
Age dating reported by Kerr (2004) includes:
• Secondary biotite of San Ernest alteration - 15.3±0.7 Ma, and Sara alteration - 15.0±0.5 Ma (K/Ar - Chilean Geological Survey);
• Primary biotite at San Ernest porphyry - 15.1±0.2 Ma, (Ar/Ar - Queens University);
• Hydrothermal molybdenite at San Ernest porphyry - 14.73±0.02 Ma and 14.63±0.02 Ma (Re/Os - Colorado State University).
Reserves and Resources
NI 43-101 compliant mineral resources at a 0.4% Cu cutoff as at December, 2007 were (Kunter et al., 2008):
Measured + indicated resources - 195.555 Mt @ 0.51% Cu, 0.05% Mo, 2.6 g/t Ag;
Inferred resources - 55.399 Mt @ 0.55% Cu, 0.02% Mo, 1.5 g/t Ag;
TOTAL resources - 250.954 Mt @ 0.52% Cu, 0.04% Mo, 2.36 g/t Ag.
NI 43-101 compliant ore reserves at a 0.4% Cu cutoff as at December, 2007 were (Kunter et al., 2008):
Porphyry - Proved + probable reserves - 52.340 Mt @ 0.39% Cu, 0.048% Mo, 2.07 g/t Ag, 0.021% As, 25.0 g/t Sb;
Mixed - Proved + probable reserves - 31.512 Mt @ 0.56% Cu 0.054% Mo, 2.39 g/t Ag, 0.051% As, 65.7 g/t Sb;
Skarn - Proved + probable reserves - 32.915 Mt @ 0.57% Cu 0.048% Mo, 3.49 g/t Ag, 0.061% As, 33.3 g/t Sb;
TOTAL - Proved + probable reserves - 116.767 Mt @ 0.49% Cu 0.049% Mo, 2.56 g/t Ag, 0.040% As, 38.3 g/t Sb;
High As - Proved + probable reserves - 3.186 Mt @ 0.51% Cu 0.033% Mo, 2.19 g/t Ag, 0.241% As, 84.7 g/t Sb;
TOTAL less High As - Proved + probable reserves - 113.581 Mt @ 0.49% Cu 0.050% Mo, 2.57 g/t Ag, 0.035% As, 37.0 g/t Sb.
Published mineral resources at a 0.3% Cu cutoff as at May, 2014 were (Compañía Minera Milpo [a Votorantim subsidiary] website, 2016):
Measured + indicated resources - 128.786 Mt @ 0.55% Cu, 0.06% Mo, 2.8 g/t Ag;
Inferred resources - 103.396 Mt @ 0.56% Cu, 0.04% Mo, 3.7 g/t Ag;
TOTAL resources - 232.183 Mt @ 0.55% Cu, 0.05% Mo, 3.2 g/t Ag.
This summary is drawn, and paraphrased from "Kunter, R., Prenn, N. and Elfin, S., 2008 - Technical Report, Magistral Property, Feasibility Study; A technical report prepared by Samuel Engineering, Inc. for Inca Pacific Resources Inc. 152p."
The most recent source geological information used to prepare this decription was dated: 2008.
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.
Magistral
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