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The Maricunga (previously Refugio) porphyry gold deposits include Verde and Pancho, which are located 120 km due east of the city of Copiapó at elevations of between 4200 and 4500 masl, within the Maricunga Belt in the Andes of Northern Chile (#Location: 27° 33'S, 69° 18'W).
Gold mineralisation was discovered at Maricunga in 1984, and shortly after acquired by Compañia Minera Maricunga (CMR). CMR undertook geological and geochemical investigations, identifying gold mineralisation in three areas: Cerro Verde, Cerro Pancho and Guanaco. Following joint venture exploration programs with Anglo American, Bema Gold and Amax Gold Inc., commercial production commenced in 1996, operated by Compañia Minera Maricunga (CMM), a 50/50 JV between Bema and Amax. Kinross Gold Corporation (Kinross) acquired Amax's 50% interest In 1998. Between 1996 and 2001, the mine produced more than 28.5 t of gold from 46.0 Mt of ore. Operations were suspended in 2001, and after further exploration to increase the reserve base, resumed full production in October 2005 (Belanger, 2007).
The Maricunga Belt comprises a NNE trending chain of andesitic to dacitic volcanoes that are part of a late Oligocene-Miocene continental margin plutonic volcanic arc. This arc contains a series of predominantly epithermal acid-sulphate gold-silver deposits (including La Coipa) and porphyry gold orebodies (including Aldeberan, Marte & Lobo).
Basement rocks in the Maricunga Belt range in age from Palaeozoic to early Tertiary. The oldest are the Late Carboniferous (Upper Pennsylvanian) to Triassic rhyolite ignimbrites and breccias of the Pantanosa Formation. This formation has been uplifted over a northerly trending, west dipping, reverse fault, and faulted over an ~900 m thick, ~55° east dipping package of interlayered redbeds and greenstones of the Late Jurassic to Early Cretaceous Monardes and Agua Helada Formations. All of these rocks are, in turn, overlain by ~200 m of andesitic volcaniclastic sedimentary rocks correlated with the Late Cretaceous to Early Tertiary Quebrada Seca, Quebrada Paipote and Las Pircas Formations.
Gold mineralisation at Maricunga is hosted by the Early Miocene (22 to 24 Ma; Muntean 1998) Maricunga volcanic-intrusive complex, which is composed of rocks that are largely of intermediate composition. This complex is exposed over an area of 12 km2 and comprises andesitic to dacitic domes, flows and breccias that are intruded by subvolcanic porphyries and breccias (Muntean 1998). There are only minor differences between the various units of volcanic and intrusive rocks in the complex making differentiation of individual units difficult.
The structural trends affecting the Verde and Pancho deposits are predominantly related to fracture systems rather than fault zones. The principal trends at Verde are NNE and NNW fracture systems, whilst at Pancho, the dominant structural trend is NW, both in mineralised and in late, post mineral structures. One of the main structural elements influencing the Pancho deposit is Falla Guatita fault zone, which may have resulted in significant vertical displacement, juxtaposing different alteration zones.
At Verde, gold mineralisation is related to two Early Miocene, annular shaped, quartz-diorite to micro-diorite porphyry plugs, forming an east-west elongated 1.6 x 1.2 km dumbbell shaped complex. The ore is hosted by the outer porphyry, intrusion breccias and dacite porphyry country rocks that ring altered, but unmineralised quartz-diorite porphyry. At Verde West, mineralisation is centred about an elliptical, 175 x 100 m porphyry plug which is elongated at ~330°. The porphyry plug at Verde East is oriented at 35°, with dimensions of ~130 x 80 m. Both porphyries occur within a sequence of intermediate tuffs, porphyries and breccias that host the gold mineralisation. Six major lithologic units that are andesitic to dacitic in composition, have been identified at Verde, namely:
i). Barren post mineral intrusives;
ii). 'Mineralised' post mineral intrusives;
iii). Verde breccia - an intrusive breccia and/or volcanic tuff breccia, with a geometry that suggests an intrusive origin. It is a generally green to greenish-grey, mostly matrix supported breccia, with angular to rounded clasts that range from 2 mm to >2 m in size. The color of the rock is largely due to chlorite, and occasionally epidote. Locally it is mostly matrix-sized material with only sparse clasts of generally monomictic volcanic fragments that are often porphyritic with white plagioclase laths up to 5 mm in length. The unoxidized rock typically contains 0.5 to 1% pyrite. Quartz-magnetite veinlets are common in the mineralised portions of the unit. Locally, the breccia is cut by fine-grained matrix sized material that forms clastic dykes that are generally >2 cm in width, and may have been formed as “fluidised” material injected into fractures at the time of the formation of the unit.
iv). Dacite porphyry - a volcanic to hypabyssal intrusive rock with 20 to 40% plagioclase phenocrysts set in a fine-grained matrix. It contains phenocrysts of biotite, hornblende and sparse quartz. This unit contains the best developed stockwork veining, with some portions containing up to 20% quartz-magnetite±pyrite veinlets.
v). Dacite tuffs;
vi). Barren Laguna tuff.
The most favourable ore hosts are the Verde Breccia and Dacite Porphyry units. The main central porphyry bodies mostly comprise the 'mineralised' and barren post mineral intrusives.
Weak porphyry style alteration assemblages are observed at Verde. Rare potassic alteration has been been encountered, while silicification is local and patchy. Propylitic alteration, although locally variable, appears to be ubiquitous at a mine scale and does not vary laterally or vertically. Supergene/argillic alteration, which directly affects gold recovery, involves sericite and chlorite being replaced by clay minerals, magnetite by hematite and pyrite by jarosite, accompanied by the deposition of limonite, manganese oxides, clay, sericite, jarosite along with gypsum in the more argillic altered zones. This alteration is the result of oxidation and the leaching by meteoric waters that have penetrated to variable depths within the deposits, depending on the fracture intensity and faulting.
The Verde gold mineralisation is interpreted to be the result of the fracturing and concentration of fluids in the carapace of an intrusive plug or stock (Belanger, 2007), with the gold being closely associated with quartz, magnetite, calcite and garnet stockworks. The stockwork veins (~80%) are generally finely banded and dark grey in color, and contain magnetite. The remainder (~20%) are predominantly white quartz veins. Gold is interpreted to have been introduced during at least 2 phases of mineralisation, i). an initial lower grade phase, associated with copper, probably coeval with the porphyry emplacement event, and ii). a higher-grade gold only event, possibly associated with the structurally emplaced veinlet swarms and northwest trending sheeted veinlet zones more evident in Verde East. The Verde East and Verde West mineralised zones cover an area of 1400 x 700 m, with gold mineralisation identified to a depth of ~600 m.
The Pancho deposit is ~2 km NW of Verde, hosted within a sequence of intermediate tuffs, porphyries and breccias. Six main lithologic units have been identified. These are as follows, from older to younger:
i). Hornfels - which comprise <5% of the rocks within the deposit, and have only been identified at depth by drilling in the Pancho porphyry system. They occur as a series of elongate, sub-horizontal masses, intercalated with intrusive breccias and dioritic porphyry, and are characterised by intense widespread silicification.
ii). Diorite Porphyry - a hypabyssal intrusive rock that occupies ~60% of the Pancho intrusive complex, and is the main host of mineralised A, B and T veinlets. It contains 20 to 40% phenocrysts. Biotite or hornblende phenocrysts have been completely altered and replaced by assemblages of chlorite-quartz-sericite-magnetite-hematite. At depth the porphyry clearly shows potassic alteration. A second, associated but volumetrically smaller intrusive phase is also indicated.
iii). Intrusive Breccias - which occupy ~15% by volume of material in the Pancho complex, and are generally elongated and subhorizontal. They are characterised by fragments of dioritic porphyry which vary in size for mm to several cm within a normally fine-grained matrix. These breccias are a significant host to gold mineralisation, within A, B, T veinlets.
iv). Diorite Porphyry-2 - which occurs as a small body restricted to an interval between two drill sections at depths of from 300 to 400 m. It appears to cut the main Diorite Porphyry, but is in turn, is intruded by the Dioritic Post Mineral Intrusive. It has a very similar mineralogy, alteration assemblage and mineralisation to the main Diorite Porphyry body, other than its primary biotite is only partially altered into chlorite, and it has less veinlets than the main Diorite Porphyry.
v). Dioritic Post Mineral Intrusive - which is not strictly 'post mineral' in that it does host some very low-grade associated mineralisation.
vi). Volcanic Breccias - restricted to the upper portion of the deposit, representing ~15 % of the volume of the Pancho deposit. The breccias are normally sub-horizontal and discordant with respect to the other five lithological units.
The gold mineralisation at Pancho is associated with a central core of potassic alteration, manifested by the replacement of mafic minerals by fine grained, secondary biotite and magnetite. Partial replacement of plagioclase by K feldspar has also been observed. Muntean (1998) described a more restricted core to this potassic zone, comprising magnetite-K feldspar-plagioclase replacement, grading outward into the more widespread secondary biotite zone. As at Verde, much of the potassic zone is obliterated or obscured by a later chlorite overprint.
The intermediate to upper sections of the mineralised system are dominated by pyrite-albite-clay-sericite alteration, primarily in the volcanic rocks, but also overprinting the upper parts of the potassic zone. This alteration is late and most likely includes a significant supergene component. The uppermost parts of the system contain hypogene alunite, dickite and pyrophyllite, characteristic of epithermal, high sulfidation alteration. This hypogene assemblage overlaps strong supergene alunite, kaolinite and other clays.
The Diorite Porphyry intrusives and intrusive breccias are the main hosts to mineralised veins, although mineralisation is also found within the volcanic rocks. The main mineralisation associations are: i). a direct association with A, B and T veinlets; ii). zones of silicification, which is normally micro-granular and dark grey in colour, with or without magnetite; iii). an association with intrusive breccias; iv). occurrence in contact zones, such as that between the intrusive and volcanic breccias, or the contact between the intrusive breccia and the Diorite Porphyry.
In the porphyry, the veins are stockwork or sheeted, and are generally subvertical with a strong, preferred NW strike. This NW structural control is evident, not only at outcrop scale, but is also reflected in the NW alignment of the smaller Guanaco deposit another 1 km to the NW. The mineralisation outlined at Pancho (to 2007) covers an area of 800 x 700 m, and has been identified in drilling to depths of >600 m.
Verde, Pancho and Guanaco define a NNW trending alignment nearly 4 km in length. The host andesites surrounding the deposit cluster are altered over an area of some 3 x 2 km. Alteration is principally chloritic and sericitic, with pyrite and magnetite, although evidence of the former existence of biotitic K-silicate is obvious in places, as at Pancho.
Mineralisation is present within quartz veinlets, accompanied by magnetite and pyrite. Veinlet types at Verde include early thin, wavy sericite/chlorite-quartz-magnetite (A-veins), cut by thicker quartz-magnetite-pyrite (B-veins) and later still by quartz-pyrite T veinlets. The overall sulphide content is generally <2%, while magnetite is usually 2 to 5%. Pancho, the central of the three deposits is more typical of a porphyry copper deposit, with >0.1% Cu, although gold grades are similar to those at Verde. Zoning outwards comprises magnetite reduction from a central core, an increase in pyrite, and a decrease in gold. Mafic minerals are altered to secondary biotite and magnetite.
The 'geological reserve' at Verde in 1989, prior to mining, was 216 Mt @ 0.88 g/t Au at a 0.5 g/t Au cut-off. Of this, the mineable oxide reserve at the Verde East and West deposits totalled 101 Mt @ 1.02 g/t Au at a cutoff of 0.5 g/t (Brown and Rayment, 1991) and hypogene copper averages 0.03% Cu (Flores, 1993). Pancho was estimated to contain ~81 Mt @ 0.85 g/t Au in 1993 with hypogene copper grades between 0.05 and 0.2% (Flores, 1993).
Progressive ore reserves and mineral resource estimates for the operation are as follows (after Belanger, 2007):
Verde - Dec 31 2007 - proven+probable reserves - 103.48 Mt @ 0.82 g/t Au = 84.6 t Au, plus
measured+indicated resources - 46.322 Mt @ 0.72 g/t Au = 33.4 t Au,
Pancho - Dec 31 2007 - proven+probable reserves - 176.022 Mt @ 0.66 g/t Au = 115.9 t Au, plus
measured+indicated resources - 65.134 Mt @ 0.57 g/t Au = 37.26 t Au.
TOTAL - Dec 31 2007 - proven+probable reserves - 279.502 Mt @ 0.72 g/t Au = 200.46 t Au, plus
measured+indicated resources - 111.456 Mt @ 0.63 g/t Au = 70.73 t Au, plus
inferred resources - 134.71 Mt @ 0.56 g/t Au = 75.44 t Au.
The following are from Kinross annual reserve and resource reports, published in the following years, with annual production in 2015 of 12.261 Mt of ore @ 0.75 g/t Au mined, yielding 6.6 t of gold:
Dec 31 2010 - proven+probable reserves - 269.8 Mt @ 0.70 g/t Au = 189.39 t Au, plus
measured+indicated resources - 187.601 Mt @ 0.57 g/t Au = 106.622 t Au.
Dec 31 2013 - proven+probable reserves - 90.595 Mt @ 0.75 g/t Au = 67.84 t Au, plus
measured+indicated resources - 126.96 Mt @ 0.66 g/t Au = 84.01 t Au.
Dec 31 2015 - proven+probable reserves - 40.641 Mt @ 0.8 g/t Au = 32.4 t Au, plus
measured+indicated resources - 198.084 Mt @ 0.7 g/t Au = 132.9 t Au,
inferred resource - 53.942 Mt @ 0.6 g/t Au = 32.7 t Au.
This summary is largely based on sections of the report: Belanger, M., 2007 - Technical Report for the Maricunga Gold Mine (Located in the Maricunga District of Region III, Chile), Prepared for Compañia Minera Maricunga and Kinross Gold Corporation.
The most recent source geological information used to prepare this summary was dated: 2007.
Record last updated: 3/11/2014
This description is a summary from published sources, the chief of which are listed below.
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Davidson J, Mpodozis C 1991 - Regional geological setting of epithermal gold deposits, Chile: in Econ. Geol. v86 pp 1174-1186|
Muntean, J.L. and Einaudi, M., 2001 - Porphyry-epithermal transition: Maricunga Belt, Northern, Chile: in Econ. Geol. v.96, pp. 743-772.|
Muntean, J.L., Einaudi, M., 2000 - Porphyry gold deposits of the Refugio District, Maricunga Belt, Northern, Chile: in Econ. Geol. v.95, pp. 1445-1472.|
Sillitoe R H 1995 - Refugio, Chile: in Sillitoe R H, Exploration and Discovery of Base- and Precious-Metal Deposits in the Circum-Pacific Region During the Last 25 Years Metal Mining Agency of Japan pp 26-27|
Sillitoe, R.H., 1991 - Gold metallogeny of Chile - an introduction: in Econ. Geol. v.86, pp. 1187-1205.|
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