Minas Conga - Perol, Chailhuagon, Amaro
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The Minas Conga cluster of porphyry gold-copper deposits, including Chailhuagón, Perol and Amaro, are located ~15 km NE of the Yanacocha gold deposit cluster, in the Cajamarca Mineral Belt, ~35 km NE of the city of Cajamarcay, and 625 km north of Lima, in northern Peru. Chailhuagón and Amaro are 4 km SSW and 5 km NNW of Perol respectively (#Location: Chailhuagon - 6° 56' 32"S, 78° 22' 1"W;
Perol - 6° 54' 43"S, 78° 21' 38"W, Amaro - 6° 51' 43"S; 78° 22' 53"W ).
The Chailhuagón and Perol deposits were discovered in 1991 by CEDIMIN, a joint-venture between the French government-owned BRGM and Compañia de Minera Buenaventura (CMB), during an exploration project targeting gold deposits as an extension of the Yanacocha complex to the NE. Between 1994 and 2000, CEDIMIN continued exploration on both deposits. Amaro was first recognised as a sub-economic occurrence in 1998-99, but was not shown to be viable until an infill drilling program in 2005-06. In 2001, CMB acquired BGRM's share of CEDIMIN, and the Conga Project was amalgamated into the Minera Yanacocha joint venture between Newmont Mining Corporation and CMB. Further evaluation of the Chailhuagón and Perol deposits in 2004 led to a decision to develop the Conga Project. Environmental baseline studies were initiated from 2005 and project engineering development commenced on 2008, while drilling continued into 2009. Social and political opposition to the project has delayed its development since then.
The Minas Conga Project deposits lie within the central Andean orogenic belt, which consists of folded and thrusted Ordovician to Cretaceous sedimentary basement rocks, overlain by early Tertiary to Holocene volcanic sequences and early to late Tertiary intermediate to felsic intrusive rocks. These Tertiary rocks include the northern Peruvian volcanic belt, a continuous, NW to NNW trending Oligocene to Eocene and Miocene to Pliocene suites of andesite to rhyolite volcanic rocks that extend into southern Ecuador. This volcanic belt is parallel to and follows a marked NNW to NW trending structural belt of elongate fold axes, faults and mainly east vergent thrusts.
The Minas Conga deposits are located towards the SE end of the Cajamarca mineral belt in the Western Cordillera of the northern Peruvian Andes, a generally north-south to NW-SE trending belt of Oligocene to Miocene porphyry copper deposits that extends for 350 km from Cajamarca in the south to the Ecuadorian border and includes two geochemically distinct groups of deposits, namely: i). porphyry Cu-Mo deposits which include La Granja, Michiquillay, El Galeno, Cañariaco and Rio Blanco; and ii). porphyry Cu-Au deposits which include Cerro Corona, Minas Conga and La Carpa.
A number of structural trends are evident in the Minas Conga district. One of the most important is the generally NNW-SSE trend at ~330°, strongly reflected by faults, east vergent thrusts and fold axes. These structures are intersected by a WSW-ENE trend, which corresponds to the projected extension of the NE to ENE trend of the mineralised corridor defined by the the Yanacocha epithermal gold deposit cluster, ~15 km to the SW. The Minas Conga deposits lie within the intersection of these two broad structural zones. In addition, there is a near north-south trend at ~10°, evident as faults and the elongation of some mineralised zones, and the youngest, an east-west trend reflected in key faults.
For detail of the regional setting, see the separate Peruvian Andes record.
The stratigraphic sequence in the Minas Conga project area is as follows, from the base of the Cretaceous (after Knight Piésold Consultores, 2010; Mendoza, 2010):
Goyllarisquisga Group - a basal sedimentary quartzose wedge of Aptian to Lower Albian age, composed of the following units:
Farrat sandstone, ~400 m of white, medium to coarse grained, quartzite and sandstone with crossbedding;
Santa-Carhuaz Formation, consists of grey shales with interbedded dark grey marly limestones and sandstones;
Inca Formation, ~150 m of Lower Albian ferruginous mudstone, sandstone and impure limestone;
Chulec Formation, ~270 m of thinly bedded shale, calcareous siltstone, marl, and nodular and sandy limestones, weathering to a distinctive cream or yellow-brown colour;
Pariatambo Formation, 100-300 m of wavy limestone, variable dark-grey to black mudstone, shale and tuff, restricted to the Perol area at Minas Conga;
Pulluicana Group - which is of Upper Albian to Santonian age, and composed of the following,
Yumagual Formation, an ~880 m thick, extensively developed sequence, which has a para-conformable lower contact with the Pariatambo Formation, and is composed of,
- a basal unit of thin to medium bedded limestones;
- clear to brownish grey, massive to medium bedded limestones and lesser black mudstones to shales, with fossiliferous marl towards the middle of the sequence;
Mujarrún Formation, an ~480 m thick sequence composed of,
- dark grey medium-bedded and nodular limestone and marl;
- fossiliferous, nodular, yellowish marl and grey shales;
- marls, nodular limestones, shales and medium calcareous siltstones bands.
Quillquiñán Group - comprising:
Cajamarca Formation, 100 to 400 m of grey to clear, fossiliferous limestone, nodular limestones and calcareous shales. This formation have been encountered over restricted areas to the north and south of the Minas Conga district.
Celendin Formation, thin shale interbedded with limestone;
Chota Formation, conglomerades, tuffs and sandstones.
The principal younger volcanic unit at Minas Conga is the Fraylones Formation, which overlies an older volcanic suite, that (e.g., Mendoza, 2010). Previously (e.g., Bell et al., 2004), these two volcanic sequences were correlated with the Eocene Porculla and overlying Huambos Volcanics found elsewhere in the district. These two units at Minas Conga comprise:
A lower suite, which may be either the Eocene Porculla Volcanics of the Lower Calipuy Group, the early Miocene 19.5 to 15.15 Ma Upper Calipuy Group or the Lower Sequence of the 14.5 and 8.4 Ma Yanacocha Volcanic Complex. This lower volcanic suite is described as mostly volcanic andesitic tuffs, including lahars, but also contains whitish grey, massive rhyolitic volcanics, with interbedded andesitic pyroclastic and lavas of the same composition. Mendoza (2010) notes similar syn-eruptive andesitic 'lahar' at the Condorcana volcanic centre located to the ESE has been dated at ~12.9 Ma. However, this is younger than the 15.7 Ma 'main productive granodiorite' that intrudes the 'lahar' at Amaro (see below). Hence, is likely a member of the Calipuy Group. See the Regional Setting section of the separate Yanacocha record for detail of these units.
Frailones Formation, (also known as the Peña de Leon pyroclastics?) which averages 100 to 400 m in thickness and is principally composed of acid volcanic tuffs and breccias, with quartz crystals up to 3 mm in diameter and biotite crystals in a feldspathic matrix approaching a dacitic tuff. They also occur as dacitic breccias composed of large blocks of tufa, wrapped in a tuffaceous matrix. Both tuffs and breccias contain fragments with a white fibrous pumice texture. Although most of the group consists of light coloured acid pyroclastics, there are also layers of andesitic tuffs which are generally distinguished by their reddish or purple colour. The unit is mostly well stratified with medium to thick layers, partially compacted, with low levels of ignimbritic welded tuff. Dacitic tuffs which may correlate to this unit have been dated at 11.58 Ma (Thompson, 2002, reported in Mendoza, 2010). This formation most likely correlates with the Coriwachay Dacite-Rhyolite Magmatism, the final phase of the 'Upper Andesite Sequence' of the Yanacocha Volcanic Complex, that is associated with the bulk of the gold mineralisation at Yanacocha.
Multiple intrusions have been recognised at the three porphyry systems (after Knight Piésold Consultores, 2010; Mendoza, 2010):
Picota Diorite - an Eocene pre-mineral intrusion that is exposed as a 1200 x 250 m, crescent shaped body, immediately to the east of the Perol deposit, dated at 43.6 Ma (Laughlin et al., 1968) and 42.03±0.46 Ma (Thompson, 2002, reported in Longo et al., 2010).
Main quartz-feldspar porphyry - which shows a close relationship to the alteration and mineralisation and is the first pulse of a multiphase intrusive with at least three stages, each of which has a similar compositions and structure. The other two stages are the 'intramineral' and 'late' phases listed below. These porphyries contain 20 to 30% plagioclase (from 2 to 8 mm), 2 to 8% quartz (from 2 to 6 mm) and 10% biotite and hornblende (from 2 to 10 mm) in an aphanitic matrix of quartz and plagioclase (Knight Piésold Consultores, 2010). This stage has been dated at 15.80±0.09 Ma (Fluor, 2005).
Intramineral quartz-feldspar porphyry - similar to the 'main porphyry', but less altered and mineralised.
Young quartz-feldspar porphyry - similar to the 'main porphyry', but less- to unaltered and unmineralised.
Main microgranodiorite - which accompanies the main mineralisation and alteration pulse, and has a generally porphyritic texture, containing disseminated pyrite. The main intrusion has a length of ~2 km from north to south and is ~0.4 km wide. It consists of plagioclase, hornblende, biotite and quartz phenocrysts set in a microcrystalline matrix of plagioclase and quartz. As at Perol, there are three phases each of which has a similar appearance (Knight Piésold Consultores, 2010). The early 'main microgranodiorite' has been dated at 15.58±0.12 Ma (Fluor, 2005).
Intramineral microgranodiorite - similar to the 'main microgranodiorite', but less altered and mineralised.
Young microgranodiorite - similar to the 'main microgranodiorite', but less- to unaltered and unmineralised.
Early porphyritic hornblende-diorite - characterised by a variable texture, ranging from a uniform, fine grained (with 'sugar sized' crystals) intrusive to seriate textured porphyry, with partially flow banded or aligned hornblende crystals. It contains phenocrysts of zoned plagioclase, hornblende, and biotite, as well as >1% quartz, set in a groundmass of microcrystalline feldspar, quartz and biotite. Crystals typically range from 1 to 5 mm in length, with plagioclase occasionally >5 mm. It is elongated east-west, and is interpreted to be pre-mineral, with alteration and mineralisation increasing toward the 'main productive granodiorite' intrusion (Mendoza, 2010). A 40Ar/39Ar age on hornblende of 17.30 Ma is reported for this unit (Thompson, 2002, reported in Mendoza, 2010).
Amaro Mirador intrusion - this dioritic stock contains plagioclase, hornblende and biotite with trace to 1% quartz eyes, and is characterised by a seriate porphyritic texture. It is situated NNE of the Amaro prospect and has an east-west, fault-controlled contact with the Yumagual Limestone country rock. It is mostly fresh to weakly chloritised but contains silica-alunite 'ribs' (Mendoza, 2010).
Main productive granodiorite, composed of phenocrysts of plagioclase (3 to 5 mm), biotite (up to 8 mm), hornblende and 3 to 5% quartz eyes (3 to 5 mm) set in a microcrystalline groundmass. Specimens of this intrusion are difficult to distinguish from the intramineral variety on the basis of composition and texture, although at contact, the relative intensity of potassic alteration and abundance of quartz veins is readily recognisable. This phase does not apparently outcrop, but persists at depth, as an overall columnar stock. A U-Pb dating yielded an age of 15.7±0.5 Ma (Mendoza, 2010).
Intramineral granodiorite porphyry, has phenocrysts of plagioclase (3 to 5 mm), biotite (up to 8 mm), hornblende, and 3 to 5% rounded quartz eye phenocrysts (3 to 5mm), set in a microcrystalline groundmass of quartz and K feldspar. It truncates quartz veins at contacts with
the 'main productive porphyry' unit, generally with chilled contacts, and sharp decreases in grade and intensity of alteration. A U-Pb date yielded an age of 15.5±0.3 Ma (Mendoza, 2010).
Hornblende needle dykes - a distinctive set of NNW striking porphyritic dykes with needle-like crystals of hornblende (up to 5 mm) that are sometimes twinned (C. Schnell, 2005, reported in Mendoza, 2010), and contain euhedral plagioclase phenocrysts (1 to 3 mm).
Toward the centre of the deposit, this unit has undergone moderate potassic alteration comprising secondary biotite and magnetite, ~1 vol.% magnetite-quartz veinlets, and trace sulphides, suggesting the dykes, are pre-mineral or intramineral. They cut a distinctive stratabound fragmental andesite interpreted to be a brecciated lava flow or a volcanic debris flow generated by lahar processes (Fisher and Schmincke, 1984; McPhie et al., 1993; Vallance, 2000) dated 12.9±0.7 Ma.
Heterolithic breccia - a heterolithic fragmental rock that commonly contains clasts of the 'main productive granodiorite' cut by quartz-magnetite veins, pink K feldspar fragments, and large unmineralized clasts of granodiorite porphyry, all set in a rock flour matrix. It truncates veins in the 'main productive granodiorite', but in turn, may be cut locally by the latest intrusions. It appears to have an upward-flaring geometry, and contains from 0.1 to 0.2 ppm Au. However, the density of veining and gold grade increase to as much as 0.4 to 0.6 ppm near the margins of the 'main productive granodiorite', partially or entirely due to the physical incorporation of numerous mineralised clasts. Its characteristics suggest a phreatic breccia origin (e.g., Sillitoe, 1985), possibly triggered by emplacement of one of the late intrusions (Mendoza, 2010).
Details of the individual deposits are as follows:
The Perol deposit occurs as two separate mineralised zones, Perol West (~700 x 300 m, elongated NE-SW) and Perol East (300 m in diameter) that is ~400 m to the SE, each centred on a separate steep stock of the 'main quartz-feldspar porphyry'. These two stocks intrude the Eocene Picota Diorite in the east, dacitic volcanics of the Yanacocha Volcanic Complex to the north, and marl, limestone and shale of the middle Cretaceous Mujarrún and Cajamarca Formations, east of the contact with the underlying Yumagual Formation. Separate, mostly concealed stocks of 'intramineral quartz-feldspar porphyry' occur immediately to the west, with only a small outcrop area, while larger Young quartz-feldspar porphyry stocks are found 300 to 500 m to the south and west of the main ore related intrusions. A series of lens shaped dykes of the latter also cut the Yumagual Formation to the SW.
Alteration - zoning is centred on the 'main quartz-feldspar porphyry' stocks, as follows (details of the mineral assemblage of each alteration type will be given in the Amaro descriptions below):
• Endoskarn - developed within the Eocene Picota Diorite, prior to the mineralised intrusions.
• Exoskarn, marble and hornfels - developed within the Picota Diorite, and the carbonate and siliciclastic members of the Mujarrún and Cajamarca Formations. Multiple skarn alteration may have accompanied both intrusion of the Picota Diorite, the 'main quartz-feldspar porphyry', and possibly the 'intramineral quartz-feldspar porphyry'.
• Potassic - prograde alteration that is mostly not exposed at surface, being overprinted by retrograde phyllic, intermediate and advanced argillic assemblages in the upper 200 to 250 m of the preserved system. Potassic alteration is centred on the both the 'main' and 'intramineral quartz-feldspar porphyry' stocks, but extends into the wall rocks, particularly the Picota Diorite.
• Propylitic - which is outboard of the potassic zone, again overprinted by the retrograde assemblages closer to the surface, but being exposed distally to mineralisation, on the outer margins of the retrograde zones at ~400 to 600 m from the 'main quartz-feldspar porphyry'. The exceptions are for intrusions of the Young quartz-feldspar porphyry which are everywhere have only suffered propylitic alteration, even where intruding potassic and phyllic altered 'main quartz-feldspar porphyry'.
• Phyllic - the first of the retrograde alteration types, centred on the 'main quartz-feldspar porphyry' stocks to depths of ~250 m below the current surface, forming vertical cylindrical volumes.
• Intermediate argyllic - largely occurring marginal to the 'main quartz-feldspar porphyry' plugs and the phyllic zones, but better developed to the NE within the Picota Diorite over widths of >500 m and vertical intervals of up to 200 m, thinning away from the stock.
• Argyllic - occurs as a more or less horizontal cap to the intermediate argyllic alteration zone, principally to the NE.
• Advanced argyllic - the uppermost zone, developed over the argillic alteration, principally to the north and NE, including within the dacites of the Yanacocha Volcanic Complex.
Mineralisation is concentrated on the two 'main quartz-feldspar porphyry' stocks, with coherent cylindrical shells of >0.3 g/t Au, enclosing significant cores of >1 g/t Au. Unlike at Chailhuagón, where higher grade ore is associated with the potassic alteration, mineralisation at Perol is largely concentrated on the phyllic zone, locally extending down into the potassic zone at depth, which predominantly only hosts low grades of >0.05 to <0.3 g/t Au. Significant, steep blocks of similar grade mineralisation are found within the endoskarn on the northwestern margin of the Perol West deposit, immediately outboard of the 'main quartz-feldspar porphyry' contact.
Gold occurs as microscopic inclusions in chalcopyrite, bornite and pyrite in a dense stockwork of quartz-sulphide veins, with Au (ppm): Cu% ratios of ~3:1. Pyrite:chalcopyrite ratios vary from 5:1 to 10:1, and 2:1 to 1:1 in the in the phyllic and potassic zone s respectively. An outer annulus of Mo is evident, with grades of >30 ppm, enclosing small core of >150 ppm Mo.
At surface, the Chailhuagón deposit occurs over a north-south elongated area of ~1500 x 750 m, centred on an irregular ~1500 x 500 m outcrop of the 'intramineral microgranodiorite' with numerous apophyses around its margins, and islands of country rock within, enclosing a number of cores of the 'main microgranodiorite'. The latter appear contract downward into dyke like roots, while the 'intramineral microgranodiorite' continues to down without significantly diminishing in width. Wall rock to this intrusive complex is the limestone sequence of the Yumagual Formation, which has been converted to marble in the contact zone, but with only minor skarn alteration. An ~700 x 300 m mass of Young microgranodiorite intrudes the Yumagual Formation to the east, locally in contact with the 'intramineral microgranodiorite', but mostly separated by 100 to 200 m. The young microgranodiorite also intrudes the core of the 'main-Intermineral microgranodiorite' complex.
The deposit is cut by three main directions of faulting, i). an orogen parallel NW-SE set, ii). another group that trends NNE-SSW to north-south and parallels the longation of intrusion, and iii). a NE-SW set that parallels the regional cross orogen trend that controls the Yanacocha mineralised corridor.
Alteration - In contrast to Perol, Chailhuagón lacks significant retrograde phyllic or argillic alteration, with the Au and Cu mineralisation being directly related to potassic alteration. As such, it appears to represent a deeper level within the porphyry system. Th principal alteration styles and their distribution are as follows:
• Endoskarn - is extensively developed within the Intermineral microgranodiorite.
• Exoskarn, marble and hornfels - developed within the enclosing Yumagual Formation limestones. Exoskarn is rare, being restricted to thin lenses along the Intermineral microgranodiorite intrusive contact, while much of the surrounding limestone has been converted to marble, and siliciclastic interbeds are hornfelsed. As at Perol, this alteration may have taken place as multiple events, related to more than one of the intrusions.
• Potassic alteration affects most of both the 'main' and 'intermineral microgranodiorites', with the assemblage in former characterised by K feldspar and the latter by biotite-magnetite.
• Propylitic alteration forms an outer rim to the potassic core, up to 250 m thick within the Intermineral microgranodiorite, apparently overprinting the endoskarn, but not significantly penetrating the marble outside of the intrusion.
• Chloritic alteration is largely found throughout the Young microgranodiorite, including the intrusions within the core of the main, strongly potassic altered intrusive complex, but not in the other lithologies.
Mineralisation - The higher grade >0.75 g/t Au mineralisation is concentrated in the K feldspar altered 'main microgranodiorite', with >0.3 g/t grades persisting to the margin of the Intermineral microgranodiorite. No mineralisation is evident in the Young microgranodiorite. The higher grade core is restricted to the northern half of the intrusive complex, where the 'main microgranodiorite' is best developed. A central ~500 x 200 m zone has a low grade to barren core, surrounded by an annulus that contains 0.75 to >1.5 g/t Au and ~0.2 to >1% Cu.
Mineralisation occurs as sheeted to stockwork quartz veins and disseminated sulphides, where gold is found as microns-sized inclusions in chalcopyrite and bornite. Within the potassic altered zones, the pyrite:chalcopyrite ratio varies from 2:1 to 1:1, and the Au (ppm): Cu% ratio is 3:1.
Mineralisation at Amaro is closely associated with the 'main productive granodiorite', which is largely concealed by the enclosing country rock. That intrusion occurs as a steeply plunging, broadly cylindrical stock with a diameter of ~200 to 250 m. It is flanked by 'intramineral granodiorite porphyry' that increases in width with depth, but is also largely concealed. Dykes and sills of the latter intrude the core of the 'main productive granodiorite' and surrounding country rock, while an upwardly flaring cone of breccia cuts the 'main productive granodiorite', but appears to accompany 'intramineral granodiorite' dykes.
These intrusions are located within the SW margin of an irregular ~600 m diameter intrusion of the Miocene, ~17.30 Ma, 'early porphyritic hornblende-diorite', which has arms that extend for ~500 to 1000 m in east-west, NNW-SSE and ENE-WSW (to NE-SW) directions, following the three key structural trends in the district. Immediately to its NE extremity, this 'early porphyritic hornblende-diorite' is apparently intruded by the 750 x 800 m, wedge shaped Amaro Mirador intrusion. Small intrusions of the 'hornblende needle dykes' are distributed outboard of the 'main productive granodiorite' and 'intramineral granodiorite porphyry'.
All of these intrusions cut NW-SE striking limestones of the middle Cretaceous Yumagual Formation, and the overlying flat to shallowly dipping Tertiary andesite lahars.
Alteration - As at the Chailhuagón deposit, mineralisation is closely associated with potassic alteration. No substantial phyllic assemblage is recognised, whilst intermediate argyllic alteration accompanies the late breccia, and a thin 2 to 5 m cap of supergene argillic alteration is found at the surface (Mendoza, 2010). The key alterations styles are as follows (after Mendoza, 2010):
• Exoskarn, calc-silicate hornfels and endoskarn - skarns are only exposed in one area at surface, where an endoskarn occurs in the eastern section of the 'early porphyritic hornblende-diorite'. Subsurface endoskarn has been intersected along the margins of the 'early porphyritic hornblende-diorite', the Amaro Mirador intrusion (at depth), and the 'intramineral granodiorite porphyry'. Exoskarn is limited and occurs within remnant fingers of limestone enclosed within intrusives.
The principal skarn assemblages in exoskarn are diopside-epidote, chlorite, and/or garnet. Pyrite, traces of chalcopyrite, and occasionally bornite are observed in this alteration, with some high-grade intersections of >1 ppm Au in altered in carbonate wall rocks. Ferromagnesian minerals in the endoskarn are replaced by diopside and grossular garnet. Limestone has been recrystallised to marble where calcareous sedimentary rocks are present near intrusive rocks, often enclosing small local veins of calc-silicate minerals. This alteration may be related to more than one of the intrusions, including an initial phase related to the 'early porphyritic hornblende-diorite', and later generations accompanying introduction of the granodiorites.
• Potassic alteration is exposed over an area with a diameter of ~600 m in the centre of the deposit, and occurs in the most eroded and topographically lowest part of the deposit area. It is typified by early disseminated and later veinlet-controlled magnetite and sulphides, in addition to biotite as disseminations of fine to coarsely crystalline biotite associated with quartz veinlets.
Three intensities of potassic alteration have been distinguished:
i). Biotitised potassic, where secondary biotite completely replaces all primary mafic phenocrysts, centred on and dominating the 'main productive granodiorite' and extending for 200 m outwards into the surrounding 'early porphyritic hornblende-diorite' and andesitic lahars. The density of quartz veinlets ranges up to 70 vol.% within the intense potassic alteration in the core of the system, with the intensity of alteration and Au-Cu grades diminishing outward.
ii). K feldspar potassic, where K feldspar occurs in vein selvages and patchy areas of the matrix, only exposed in small and isolated areas within the the 'early porphyritic hornblende-diorite' at surface, but increases with depth;
iii). Partial potassic, where hornblende±biotite phenocrysts are partially replaced by secondary biotite, predominantly on the peripheries of the system in the 'early porphyritic hornblende-diorite' and andesitic lahars, whilst alteration in the 'intra-mineral granodiorite porphyry' is mainly of magnetite with partial secondary biotite.
Magnetite contents are closely related to the degree of potassic alteration. Rocks containing >3 vol.% magnetite are mostly within the 'main productive granodiorite'. The heterolithic breccia has an estimated ~2 vol.% magnetite, occurring as mineralised clasts of previously altered rock. Sulphides such as chalcopyrite-pyrite are associated with veinlet-controlled magnetite at the surface.
• Propylitic alteration is peripheral to the potassic zone, largely affecting the andesitic lahars, 'early porphyritic hornblende-diorite' and Amaro Mirador intrusion. It is characterised by an assemblage of chlorite, epidote and illite(?), with weak associated pyrite, chalcopyrite and magnetite. It forms topographically resistant ridges, and is poorly mineralised to barren, surrounding the main ore zone.
• Intermediate argillic alteration overlies, or occurs within potassic domains. It comprises a patchy to pervasive replacement of plagioclase and biotite by medium yellow-green to darkish brown sheet silicate minerals (illite or smectite), ±silica, clay, chlorite, pyrite and magnetite, and sometimes calcite, hematite and anhydrite. When overprinting endoskarns, anhydrite-calcite veins are surrounded by wide complex envelopes of brown sericite with occasional chalcopyrite veinlets. This alteration also accompanies the heterolithic breccia as pervasive dark brown sericite.
Mineralisation - An exposed area of ~400 x 500 m above the concealed 'main productive granodiorite', contains ubiquitous quartz veinlets. This veining is hosted within potassic altered rocks, primarily the 'early hornblende diorite' and restricted areas of fragmental andesite and the heterolithic breccia body. Areas with more abundant outcropping quartz veins (1 to 15%) are associated with more intense and coarsely crystalline secondary biotite. This zone corresponds to a slightly more extensive zone of magnetite veining. Even relatively young units, e.g., the 'needle hornblende unit', contain occasional thin magnetite veinlets with sparse sulphides (Mendoza, 2010).
The outcropping veining is the upper part of a <50 to 200 m wide halo of 1 to 10% veining that surrounds the 'main productive granodiorite', which at depth, contains a 50 to 100 m wide, steeply plunging core of >30% quartz veins, locally as high as 70%. Quartz veining persist for >500 m below the surface, with vein densities progressively decreasing outward. There is a positive correlation between abundant magnetite (≥3% as disseminations and veinlets) with the zones of >10% quartz veinlets (Mendoza, 2010).
Most of the quartz veins are 'A' type (e.g., Gustafson and Hunt, 1975; Proffett, 2003), although there is wide textural variety, including coarse quartz-magnetite, multiply-banded quartz-magnetite (some as much as 1 m or more thick) and magnetite-walled veins. In most, chalcopyrite occurs as filling in later microfractures, rather than as primary disseminations, suggesting the sulphide stage follows the quartz-magnetite stage (Moore, 2004, reported in Mendoza, 2010).
Secondary biotite and magnetite alteration of the matrix of the 'main productive granodiorite' is progressively cut by i). 'A' veinlets (quartz-magnetite-chalcopyrite ±bornite) with trace K feldspar selvages; ii). magnetite-quartz veins ±K feldspar selvages; iii). 'B' veinlets (quartz-biotite-magnetite-chalcopyrite ±bornite), which truncate all of the vein types above; iv). chalcopyrite-pyrite veinlets, which are the youngest type (Mendoza, 2010).
Veining within the 'early hornblende diorite' and 'intramineral granodiorite porphyry', are predominantly i). 'B' veinlets (quartz-magnetite ±chalcopyrite), occasionally with K feldspar selvages: ii). magnetite-quartz ±actinolite; iii). quartz-chalcopyrite ±pyrite centre-lines with smectite selvages (Mendoza, 2010).
Flakes of molybdenite are locally associated with 'B' veinlets in the upper part of the system, whereas in the core of system, some molybdenite is locally associated with 'A' veinlets (quartz-chalcopyrite ±molybdenite) with K feldspar selvages.
The Minas Conga deposits are overall low sulphide systems, with four main species in decreasing order of abundance: chalcopyrite > pyrite > minor bornite > sparse molybdenite. These occur in four main associations: i). predominantly pyrite, linked to the heterolithic breccia, although pyrite also increases peripheral to Au-Cu deposit and at higher levels of the system. An isolated volume with >1.5% sulphide, dominantly pyrite, occurs within the fragmental andesite; ii). chalcopyrite≥bornite (±pyrite), mainly within the 'main productive microgranodiorite' and partially within the immediately surrounding wall rocks and 'intra-mineral microgranodiorite'. There are restricted and isolated areas within this association that contain significant bornite. The highest sulphide content of ≥1.5 to 5%, with local zones of up to 20%, forms the core of the mineralised system, accounting for much of the 'main productive granodiorite', decreasing outward; iii). chalcopyrite > pyrite (±bornite), which surrounds the previous assemblage; iv). pyrite>chalcopyrite (±bornite), which in turn, encloses the volume of rock containing the previous association.
Chalcopyrite and bornite account for the bulk of the copper and gold in the deposit, occurring as disseminations in the matrix of the host volcanic rocks, as well as in veinlets. Gold grades increase with the abundance of bornite, although it only accounts for a small percentage of the sulphide present, with the more abundant chalcopyrite controlling the grade, as well as the native gold content.
The distribution of metals is as follows (after Mendoza, 2010):
i). Gold - The >0.3 g/t Au isopleth defines an ~250 m diameter vertical cylinder centred on the 'main productive microgranodiorite', persisting marginally into the surrounding country rocks. Above ~200 m depth, grades are mostly between 0.3 and 1.0 g/t, although below that level, gold levels in this cylinder are predominantly >1 g/t Au, which is almost entirely within the 'main productive microgranodiorite', enclosing significant cores of >1.5 g/t Au. Within the heterolithic breccia, anomalous gold grades of up to 0.5 ppm are evident.
ii). Copper - The >0.1% Cu isopleth defines an irregular ~500 x 350 m vertical ovoid cylinder, also centred on the 'main productive microgranodiorite', containing a core of >0.2% Cu below 200 m depth that closely correlates with the margin of the same intrusion and in turn encloses smaller cores of >0.4% Cu. In sections of the deposit, ≥0.2% Cu defines the same pattern as the than ≥1 g/t Au.
iii). Molybdenum mineralisation forms a shell like cap immediately over the 'main productive microgranodiorite'. Within the well mineralised Au and Cu zones, Mo levels are dominantly <5 ppm, grading up to 5 to 10 ppm Mo in the upper extremities of the 'main productive microgranodiorite'. The main Mo rich cap has been partially eroded immediately above that intrusion where it has a preserved thickness of only ~75 m, thickening to >375 m and ~125 m to the north and south respectively as it dips gently in those directions into the 'early porphyritic hornblende-diorite' and andesitic lahars. This cap contains levels of generally 10 to 25 ppm Mo, with cores of >25 to 50 ppm, and lower grade margins both above and below.
iv). Silver broadly correlates with Au and Cu distribution, with the ≥0.55 g/t Ag isopleth coinciding with higher grades of Au and Cu, although the Ag block expands with depth to a greater degree, beyond the 'main productive granodiorite unit'. In contrast to Cu and Au, the upper 200 m of the Ag zone is not substantially weaker than the deeper mineralisation.
v). Zinc, Lead and Barium - Anomalous Zn values of ≥100 ppm lie outside of the 'main productive granodiorite' beyond the outer fringes of the significant Au and Cu mineralisation. Occasional strongly anomalous Zn (≥0.25%) assays are localised at depth on the eastern side of the deposit, apparently related to exoskarn and/or endoskarn. Skarns and Zn-Pb mineralisation appear to have been created at different times, including an early phase related to the intrusion of the 'early hornblende diorite', and a leter phase related to the 'main productive microgranodiorite'.
Pb is practically absent in the system. Anomalous Ba (≥250 ppm) is most prevalent in the heterolithic breccia bodies and is distal to the upper part of the porphyry system.
Reserves and Resources
Resource estimates at December 31, 2015 are as follows for [Newmont's 51.35% share] and total tonnages (Newmont Reserves and Resources Report, February 17, 2016):
No tonnage included in reserves;
Indicated resource - [356.20] 693.67 Mt @ 0.65 g/t Au, 0.26% Cu, for 450 t Au;
Inferred resource - [118.40] 230.57 Mt @ 0.39 g/t Au, 0.19% Cu, for 90 t Au;
TOTAL indicated + inferred resource - [474.60] 924.25 Mt @ 0.58 g/t Au, 0.24% Cu, for 540 t Au.
This summary is largely drawn from: "Mendoza, N., 2010 - Geology, grade distribution, and metal ratios at the Amaro gold-copper porphyry deposit, Minas Conga district, Cajamarca province, Peru; A thesis submitted as partial fulfillment for the Degree of Professional Science Masters in Economic Geology, the University of Arizona, 149p"; and
"Knight Piésold Consultores, 2010 - Minera Yanacocha S.R.L. Proyecto Conga Estudio de Impacto Ambiental, Informe Final; a report prepared by Knight Piésold Consultores S.A., for Minera Yanacocha S.R.L.,1731p."
The most recent source geological information used to prepare this summary was dated: 2011.
This description is a summary from published sources, the chief of which are listed below.
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Mendoza, N., 2010 - Geology, grade distribution, and metal ratios at the Amaro gold-copper porphyry deposit, Minas Conga district, Cajamarca province, Peru: in A thesis submitted as partial fulfillment for the Degree of Professional Science Masters in Economic Geology, the University of Arizona, 149p.|
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