CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Image: The Candelaria open pit, Chile.
Porter GeoConsultancy continued its International Study Tour series of professional development courses by visiting a representative selection of the most important iron oxide copper-gold deposits in South America, in the Andes of Chile and the Carajas region of Brazil.
in Para, Brazil,
Sossego in Para, Brazil,
Igarapé Bahia in Para, Brazil,
Alemão in Para, Brazil,
Salobo in Para, Brazil,
Andean Workshop in Copiapo, Chile,
Punta del Cobre Field Workshop Copiapo district, Chile,
Mantoverde northern Chile,
Candelaria northern Chile,
Mantos Blancos northern Chile,
The tour commenced in Marabá, Para state, Brazil on the evening of Sunday 3 June and ended in Santiago, Chile on the evening of Wednesday 13 June, 2007.
Participants were able to take any 4 or more days, up to the full tour, as suited their interests or availability.
The main components of the itinerary were:
Cristalino ...................... Monday 4 June, 2007.
The Cristalino IOCG deposit is located some 40 km to the east of Sossego in a bifurcation of the major regional Carajás Fault in the Carajás district of Para State, Brazil.
Basement in the area is represented by the Xingu Complex which is >2.86 Ga in age and is composed of a variety of rocks, including the ~3.0 Ga Pium Complex and 2.9 Ga Greenstones. These are overlain by the 2.76 Ga Grão Para Group of volcanics and sediments, cut by the 2.5 Ga Estrella Granite and subsequently by 1.9 Ga granites but is overlain by the un-metamorphosed 2.7 to 2.6 Ga Águas Claras marine sandstones.
Cristalino is hosted by volcanics of the Grão Para Group composed of orange dacite and green andesite with minor basalt and in association with hydrothermally altered and disrupted banded iron formations within this same sequence. These iron formations have been upgraded nearby where they constitute part of the Carajás Iron Resources.
Mineralisation is concentrated in a NW-SE trending, sinsitral transpressive zone of shearing over a drilled length of 2200 m and thickness ranging from 10's of metres to 500 m. The shear zone is several hundreds of metres in width and is a splay of the Carajás Fault. The ore zone is generally brecciated and is found in the volcanics below the iron formation and in the lower sections of the iron formation itself. In general the iron formation forms the upper limit to ore and may have acted as a capping. The hydrothermally altered breccia is composed of 5 to 50% sub-angular to sub-rounded fragments.
Mineralisation is associated with the emplacement of 2.7 Ga diorite to quartz-diorite intrusions into the volcano-sedimentary sequence and iron formation.
There are two styles of mineralisation: (i). 60% of which is crosscutting stockwork veins and veinlets, and (ii). 40% breccia ore where the breccia fragments are surrounded by sulphide veins and a sulphide matrix. Mineralisation is also accompanied by magnetite and associated amphibole alteration. The principal sulphides are chalcopyrite and pyrite in a 2:1 to 3:1 ratio. The Copper was introduced after the magnetite and amphibolite alteration, although the highest grades are associated with the amphibole zones. The iron alteration where it affects the iron formation represents addition, not remobilisation of iron.
Hydrothermal alteration progressed from: (i). early widespread actinolite-albite; to (ii). biotite with scapolite and magnetite; to (iii). amphibole with magnetite as hastingsite, grunerite, actinolite and cummingtonite; to (iv). chlorite with albite, magnetite and hematite; to (v). chlorite and carbonate; to (vi). muscovite and carbonate.
The average 3-5% sulphide mineralisation is associated with the last three overlapping phases of alteration and comprise chalcopyrite, pyrite and lesser arsenopyrite with trace Ni-Co sulphides. The gold is in the pyrite.
Indications of Cu mineralisation were first noted in the area in the late 60's to early 70's. Grid geochemistry and geophysics from 1984-87 led to 2 anomalies being drilled in 1988 with some 13 holes in two prospects. The second phase of work was commenced in 1997-98 with more grid mapping, geochemistry and geophysics, culminating in a drill intersection of 38 m @ 1.4% Cu, 0.25 g/t Au between 76 and 114 m depth.
The resultant approximate resource from the subsequent drilling to 2001 amounted to 500 Mt @ 1.0% Cu, 0.2-0.3 g/t Au. According to CVRD, the reserves amount to 261 Mt @ 0.73% Cu.
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Sossego ...................... Tuesday 5 June, 2007.
The Sossego IOCG deposit is located some 40 km to the south of the Carajás townsite in the state of Para, Brazil. It is approximately 80 km SE of Igarapé Bahia and Alemao, and 40 km west of Cristalino.
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Mineralisation is hosted along a regional WNWESE-striking shear zone that defines the contact between the metavolcanosedimentary rocks of the ~2.76 Ga Itacaiúnas Supergroup and tonalitic to trondhjemitic gneisses and migmatites of the ~2.8 Ga Xingu Complex.
The deposit is hosted by granite, granophyric granite, gabbro and felsic metavolcanic rocks representing both suites.
The ore is located in two adjacent centres, Sossego Hill (the SossegoCurral zones) and the larger Sequeirinho (the PistaSequeirinhoBaiano zones) which has a length of 1.6 km and thickness of 150 to 200 m in its central section. These two centres are separated by a major high angle fault.
The Sequeirinho orebodies have been subjected to regional sodic (albitehematite) alteration, overprinted by sodiccalcic (actinolite-rich) alteration accompanying with the formation of massive magnetite(apatite) bodies. Both alteration assemblages exhibit ductile to brittle-ductile fabrics and are cut by spatially restricted zones of potassic (biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages (Monteiro, et al., 2007).
The Sossego Hill orebodies display only weakly developed early albitic and very poor subsequent calcicsodic alteration, although they have well-developed potassic alteration assemblages that were formed during brittle deformation that produced breccia bodies. The matrix of the breccias commonly displays coarse mineral infill suggestive of growth into open space (Monteiro, et al., 2007).
The sulphides of both groups of orebodies were initially accompanied by potassic alteration and a subsequent more important assemblage of calcitequartzepidotechlorite. In the Sequeirinho orebodies, sulphides range from undeformed to deformed, while at the Sossego Hill orebodies they are undeformed. Very late stage, weakly mineralised hydrolytic alteration is present in the Sossego Hill orebodies (Monteiro, et al., 2007).
The dominant sulphides are chalcopyrite with subsidiary siegenite and millerite, and minor pyrrhotite and pyrite in the Sequerinho orebodies, although pyrite is relatively abundant in the Sossego Hill bodies.
In early 2001 the total resource was quoted as 355 Mt @ 1.1% Cu, 0.28 g/t Au, encompassing a mineable reserve of 219 Mt @ 1.24% Cu, 0.33 g/t Au at a 0.4% Cu cut-off and stripping ratio of 3.3:1 wate:ore.
At the commencement of mining in 2004, reserves were quoted by CVRD as 250 Mt @ 1.0% Cu. Montiero, et al., (2007) published a reserve of 245 Mt @ 1.1% Cu, 0.28 g/t Au.
Mineralisation (gold) was initially discovered by garimperos (prospectors) in 1984 within CVRD concessions. The area was tendered to Phelps Dodge in 1996 and the first major intersections were in early 1997.
In 2001 the project was controlled by Mineracao Serra do Sossego, a 50:50 joint venture between Phelps Dodge do Brasil and CVRD. In 2002 CVRD bought Phelps Dodge's share and commenced mining in 2004.
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Igarapé Bahia ...................... Wednesday 6 June, 2007.
The Igarapé Bahia Au-Cu-(REE-U) deposit is located in the Carajás Mineral Province of Para State Brazil.
Igarapé Bahia is hosted by the Igarapé Bahia Group, considered to be a lower greenschist facies metamorphosed unit of the Archaean (ca. 2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit and volcanoclastic rocks. The Igarapé Bahia orebodies represents a 100 to 150 m thick gossan-laterite zone from which significant amounts of gold (>60 t) were mined until 2003. Where not outcropping, the primary mineralisation is obscured by a 250 m thick unconformable siliciclastic unit referred as the Aguas Claras Formation.
The copper-gold mineralisation at the Igarapé Bahia deposit is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. The breccia unit is exposed at or near the surface as a semicircular annulus, with a form similar to a ring complex with a diameter of approximately 1.5 km. The mineralised breccia unit occurs as a 2 km long by 30 to 250 m thick series of fault dislocated bodies on the southern, northeastern and northwestern sections of this structure, dipping steeply outwards at ~75°, and is nearly concordant with the metavolcanic-sedimentary wallrocks.
The economically extracted ore at Igarapé Bahia is largely developed as a supergene gossan-laterite enrichment within the 150 to 200 m thick oxide profile. Three orebodies have been mined at this contact, forming a semi-circular trace at the surface namely, Acampamento - dipping at around 75° to the north-east, Furo Trinta to the south-east, and Acampamento Norte to the north-west, forming an outward dipping domal structure in three dimensions.
The oxide zone is characterised by supergene enrichment and hematite, goethite, gibbsite and quartz. This is underlain by a transition zone that may be up to 50 m thick with enriched supergene malachite, cuprite, native copper and goethite and minor amounts of digenite and chalcocite responsible for high grade Cu and Au. This zone is in turn underlain by primary Cu-Au mineralisation, represented by hydrothermal breccias containing chalcopyrite, bornite, carbonate, magnetite and minor molybdenite and pyrite.
Strong hydrothermal alteration of the host sequence produced intense chloritisation, Fe-metasomatism, Cu-sulphidation (chalcopyrite and bornite), carbonatisation, silicification, tourmalinisation and biotitisation in the primary zone.
Gold-copper mineralisation is localised at the commonly brecciated contact between the metavolcanics and the meta-volcaniclastics-metasediments and comprises, magnetite/siderite heterolithic breccias and hydrothermally altered metavolcanics. These rocks are enriched in REE (monazite, allanite, xenotime, bastnsite and parisite), Mo (molybdenite), U (uraninite), F (fluorite), Cl (ferropyrosmalite) and P (apatite).
Production has been at a rate of around 10 t Au per annum, with the remaining reserve in 1998 being 29 Mt @ 2 g/t Au. The deposit was and is controlled and operated by CVRD/Vale.
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Alemão ...................... Wednesday 6 June, 2007.
The Alemão IOCG Au-Cu-(REE-U) deposit is part of the Igarape Bahia mineralised system in the Carajás Mineral Province of Para State Brazil (see the Igarapé Bahia record).
Alemão is hosted by the Igarapé Bahia Group, considered to be a lower greenschist facies metamorphosed unit of the Archaean (ca. 2.75 Ga) metavolcano-sedimentary Itacaiúnas Supergroup which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit and volcanoclastic rocks. The Alemão ore body underlies the far northwestern margin of the Igarapé Bahia deposit, which represents a 100 to 150 m thick gossan-laterite zone from which significant amounts of gold (>60 t) were mined until 2003. Elsewhere it is obscured by a 250 m thick unconformable siliciclastic unit referred as the Aguas Claras Formation.
The copper-gold mineralisation at the Igarapé Bahia/Alemão deposit is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. The breccia unit is exposed at or near the surface as a semicircular annulus, with a form similar to a ring complex with a diameter of approximately 1.5 km. The mineralised breccia unit occurs as a 2 km long by 30 to 250 m thick series of fault dislocated bodies on the southern, eastern and northern sections of this structure, dipping steeply outwards at ~75°, and is nearly concordant with the metavolcanic-sedimentary wallrocks. The Igarapé Bahia deposit is the thick gossan-laterite zone developed within the top 100 to 150 m of the exposed breccia unit.
The Alemão deposit is located immediately to the northwest of this annular zone, occurring as a particularly magnetite-Cu-Au-enriched down-faulted segment of the Acampamento Norte orebody, the northern most orebody of the Igarapé Bahia deposit. It has dimensions of around 500 m in length, 50 to 200 m thick and has been traced down plunge for at least 800 m below the surface, although the top of the deposit is at a depth of approximately 250 m below Aguas Claras Formation sandstone cover.
The Alemão orebody is hosted by a hydrothermally altered breccia at the contact between the footwall mafic volcanics, with associated BIF and hyaloclastite, and a dominantly coarse to fine-grained metasedimentary sequence in the hanging wall. A set of unmetamorphosed 2.75 to 2.65 Ga quartz diorite and 2579±7 Ma dolerite dykes cut the orebodies, the host metavolcano-sedimentary sequence and the overlying clastic metasedimentary sequence of the Áoguas Claras Formation/Rio Fresco Group.
The breccia has gradational contacts with its wallrocks and is made up of polymitic, usually matrix-supported clasts, composed mainly of coarse, angular to rounded basalt, BIF and chert clasts derived from the footwall unit.
The hydrothermal paragenesis is marked by ferric minerals (magnetite and hematite), sulphides (chalcopyrite-pyrite), chlorite, carbonate (siderite, calcite, ankerite) and biotite with minor quartz, tourmaline, fluorite, apatite, uraninite, gold and silver. Sericite and albite are rare. The mineralisation is represented by hydrothermal breccias and hydrothermally altered rocks. These fall within two groups, namely:
i). massive magnetite-chalcopyrite bands and polymict breccias with a matrix of magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; and
ii). brecciated hydrothermally altered volcanics with chalcopyrite, bornite, pyrite, chlorite, siderite and ankerite both in the matrix and disseminated in the altered country rock.
Several generations of late mineralised veins crosscut the ore breccia and are composed of variable concentrations of chalcopyrite, pyrite, quartz, calcite, chlorite, and fluorite. The veins commonly display open space - filling textures (e.g., comb)
The total estimated resource in 2001 was 170 Mt @ 1.5% Cu, 0.8 g/t Au. More recently CVRD has quoted a reserve of 161 Mt @ 1.3% Cu, 0.86 g/t Au.
The deposit was discovered by Docegeo, exploration arm of CVRD in 1996 and is not in production.
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Salobo ...................... Thursday 7 June, 2007.
The Salobo 3 Alpha IOCG Deposit is located in the Carajás district of Para State, Brazil, and is some 30 km to the north of Igarapé Bahia and Alemao.
The deposit is contained in supracrustal rocks of Igarapé Salobo Group of Archaean age (interpreted to be older than the Igarapé Bahia Group that hosts Igarapé Bahia and Alemao), which is composed of iron-rich schists, metagreywackes, amphibolites and quartzites.
This sequence overlies the basement gneisses of the Xingu Complex composed of partially migmatised gneisses. The original stratigraphic relationships with the basement and within host sequence are masked by intense ductile-brittle shear zones and over-thrusting.
The Salobo 3A deposit extends over an area represented by a NW trending strike length of 4 km, 100 to 600 metres in width. Mineralisation has been recognised to depths of 750 metres below the surface.
Copper mineralisation occurs as chalcocite and bornite, with subordinate quantities of chalcopyrite, together with variable proportions of molybdenite, cobaltite, covellite, gold and silver, lodged in schists with variable proportions of magnetite, amphibole, olivine, garnet, biotite, quartz and plagioclase.
The Cu grade is generally proportional to the magnetite content, with the highest grades in +50% magnetite which have >1% Cu, while the bulk of the ore in a schist with 10 to 50% magnetite and containing 0.6 to 1.1% Cu. Other lithologies within the orebody and shear zone have variable iron contents, with Fe silicates (fayalite, grunerite, biotite) predominating and <10% magnetite, as well as lower Cu and Au.
The ore lies within a major brittle-ductile shear zone that has produced irregularly distributed lenticular shaped ore shoots. The host rocks were progressively metamorphosed to pyroxene hornfels facies, at equilibrium temperatures of 750° C, resulting from sinistral transcurrent transpressive shearing accompanied by oblique thrusting.
A first hydrothermal event developed at temperatures between 650 to 550° C causing partial substitution of chalcopyrite by bornite and chalcocite, accompanied by intense K-metasomatism. This was followed by sinistral transcurrent transtensive shear zone formation, causing green schist facies metasomatism, characterised by intense chloritisation and partial substitution of bornite by chalcocite.
The estimated mineral resources prior to 2000 were of the order of 789 Mt with 0.96% Cu and 0.52 g/t Au.
Following the 2004 feasibility study, CVRD quoted reserves of: 986 Mt @ 0.82% Cu, 0.49 g/t Au at a 0.5% Cu cutoff.
The project was initially a 50:50 joint venture between CVRD and the Anglo American Group, although CVRD now owns it 100%.
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Travelling - Carajas, Brazil to Copiapo, Chile (via Marabá, Brasilia, Sao Paulo and Santiago) ...................... Friday 8 & Saturday 9 June, 2007.
Andean Overview Workshop ...................... Saturday 9 June, 2007.
An overview workshop was run late on Saturday 9 June, 2007 in Copiapo, Chile at the beginning of the Andean segments of the tour to provide a context to the tectonic, geological and metallogenic setting of the IOCG deposits of the Andes and descriptions of other deposits in the region not on the itinerary. The workshop was led by Dr Carlos Arevalo of the Chilean Servicio Nacional de Geología y Minería (SERNAGEOMIN).
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Punta de Cobre district - Field Workshop ...................... Sunday 10 June, 2007.
A field workshop was run in the Punta de Cobre district. It provided a context setting to the major Candelaria deposit and an impression of the anatomy of an important IOCG district, the geologic, structural and metallogenic setting, the distribution of alteration and different styles of mineralisation. The workshop was led by Dr Carlos Arevalo of the Chilean Servicio Nacional de Geología y Minería (SERNAGEOMIN), an acknowledged expert on the district, its geology and mineralisation.
A series of iron oxide Cu-Au(-Zn-Ag) deposits occur within the Punta del Cobre belt which hosts the Punta del Cobre district, sensu strictu and La Candelaria the largest of these deposits.
The Punta del Cobre belt lies within an Early Cretaceous continental volcanic arc and marine carbonate back-arc basin terrane whose sequences are intruded by Early Cretaceous granitoid plutons that form part of the Chilean Coastal Batholith. The Punta del Cobre belt deposits are found fringing the eastern margin of the batholith within (eg., La Candelaria) or just outside the contact metamorphic aureole (eg., the Punta del Cobre district). Andesitic volcanic and volcaniclastic host rocks are intensely altered by biotite-quartz-magnetite. This style of alteration extends much further to the east of the intrusive contact than the metamorphic mineral associations in the overlying rocks that are clearly zoned outboard. Local areas of intense calcic amphibole veining that overprints all rock types occur within the contact metamorphic aureole.
Chalcopyrite mineralization which crosscuts and thus post-dates all of the major metamorphic and metasomatic assemblages is paragenetically late. Deposits found close to the contact of the batholith and the deeper parts of the sequence in the Punta del Cobre district are characterised by abundant magnetite with associated biotite-quartz alteration, which is overprinted by fracture-controlled calcic amphibole, and chalcopyrite-pyrite mineralization. Potassium feldspar-chlorite and/or biotite ± quartz plus magnetite ± hematite occur in the intermediate parts of the hydrothermal system. Up-section and, in places, laterally, these assemblages grade into pervasive albite-chlorite-calcite-hematite that are spatially associated with Cu-Au mineralization in the more distal portions of the system.
Mineralization is controlled by tectonic structures and their intersection with massive volcanic rocks and overlying volcaniclastic rocks. Isotopic ages of alteration minerals associated with the metallic mineralisation indicates that the bulk of the iron oxide mineralization formed between 116 and 114 Ma, and the main copper-gold mineralization between 112 and 110 Ma, and that hydrothermal activity was coeval with both the emplacement of the Copiapó Batholith and regional uplift. They also imply burial during the mineralisation was no greater than 2-3 Km.
Deposits other than La Candelaria account for another 120 Mt @ 1.5% Cu, 0.2 to 0.6 g/t Au, 2 to 8 g/t Ag, with the larger deposits producing up to 1.5 Mt of ore per annum.
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Mantoverde .......... Monday 11 June, 2007.
The Mantoverde mine is located in the Los Pozos district of the Chilean Coastal Cordillera, some 400 km south of Antofagasta and 100 km north of the coastal town of Copiapo, northern Chile. Individual deposits include Manto Verde, Manto Ruso, Manto Monstruo and Monte Cristo.
The Los Pozos district falls within a structural segment bound to the east and west by two branches of the N-S trending Atacama Fault Zone (AFZ), a subduction-related, arc-parallel wrench fault system that extends for more than 1000 km along the Chilean coast. The prominent 340 to 345° trending and 40 to 50°E-dipping brittle Manto Verde fault cuts this segment of the AFZ and controls the main Cu-Au ore zone.
The district is predominantly composed of Jurassic and/or possibly Early Cretaceous andesitic flows and breccias which are interpreted to correlate with either the Jurassic La Negra Formation or the Early Cretaceous Bandurrias Group. These cataclased andesitic volcanics host rocks are intruded by coeval (?) early Cretaceous granitoids of the Chilean Coastal Batholith. Granodiorites and monzonites of the ~130 Ma Las Tazas Plutonic Complex are found in the western part of the Manto Verde deposit, while diorites, monzodiorites, granodiorites and tonalites of the ~127 Ma Remolino Plutonic Complex lie to the east.
The Cu-Au ores of the Los Pozos district are hosted mainly in specularite-dominated tabular breccia bodies (Mantoverde), breccia pipes (Manto Ruso and Manto Monstruo) and stockwork bodies (Monte Cristo). All are associated brittle faults belonging to the AFZ, which were emplaced during an extensional phase of sinistral strike-slip and dip-slip tectonism of that complex Jurassic to early Cretaceous major regional fault system. The Mantoverde deposit consists of three breccia units paralleling the 12 km long Mantoverde Fault for at least 1500 m. The Mantoverde Fault which strikes NNW and dips at 40 to 50° E, connects two major branches of the AFZ.
The hangingwall Manto-Atacama breccia is a ±100 m wide, matrix supported, specularite rich, hydrothermal breccia that follows the Mantoverde Fault along strike and down dip although it thins with depth. The degree of brecciation and mineralisation decreases gradually outwards from the fault into the andesite wall rocks.
In contrast the footwall Manto Verde Breccia is a ±20 m wide cataclasite with relatively sharp mineralisation boundaries, with sulphides being hosted by andesitic volcanic and deformed volcaniclastic rocks where hypogene grades locally exceed 1% Cu with around 0.25 g/t Au and Elevated REE concentrations.
Other Cu-Au deposits also occur on more northwest-trending bends of this fault or the eastern branch of the Atacama fault zone, or at the intersection of the Manto Verde fault and related second order structures.
The predominant ore minerals are chrysocolla, brochantite, antlerite and atacamite with minor malachite and cuprite. Copper minerals are closely associated with iron oxides, with specularite mainly in the footwall of the Manto Verde fault in the northern part of the district, while magnetite accompanies Cu oxides in the southern part in both the hangingwall and footwall blocks of Manto Verde fault. The main specularite rich orebodies are hydrothermal breccias with clasts of volcanic rocks in a specularite rich matrix, with copper mineralisation occurring in association with specularite stockworks in the periphery of these bodies representing a transitional into barren wall rocks. Magnetite rich Cu-Au ore, with variable specularite, occurs as breccias, stockworks and disseminations in tectonized zones at the intersection of the Manto Verde fault and secondary faults.
Primary sulphides occur below the base of supergene oxidation at depths of 40 to 100 m at Manto Ruso and as much as 200 m below surface at Manto Verde. Oxidation has produced copper carbonates, sulphates and silicates with minor Cu chlorides. Supergene enrichment is only thin and poorly developed. At Manto Ruso, the transition between the oxidised zone and primary ore is relatively sharp, with chalcopyrite and pyrite being disseminated in the specularite breccia, in veinlets, and in aggregates in the hypogene ore where copper grades vary between 0.6 and 0.8%. In the primary ore at Manto Verde, chalcopyrite is found as disseminations in the matrix of the hydrothermal specularite breccia and as coarse grains in veinlets and veins intergrown with calcite. In the central part of the district, south of Manto Verde, chalcopyrite and pyrite are associated with magnetite.
Overall there is a vertical zonation in the distribution of the iron oxide species, with magnetite at depth and specularite at shallow palaeodepths. The N-S zonation on a district scale where magnetite is largely absent to the north and increases to the south, is interpreted to reflect differences in the erosion level).
District wide chloritisation and K-silicate alteration is characterised by microcline replacement and veining in the andesite country rock. The K-silicate alteration is interpreted to be related to the intrusion of granite dykes. Hydrothermal biotite is commonly chloritised and is only preserved locally. Sericitisation with associated quartz, is superimposed on the K-silicate phase and is localised in the Mantoverde Fault and neighbouring fracture zones. The latter, with calcite, accompanied the emplacement of the specularite/magnetite and hypogene Cu-Au mineralisation. K-Ar ages of 117 ±3 Ma of sericite from an andesite of the transition zone and of 121 ±3 Ma of sericite from a granite dyke are interpreted as minimum ages for the mineralization.
Fluid inclusion studies suggest boiling and homogenisation temperatures of 215 to 340° C, with a maximum value of 500° C. The geologic features of the deposit are interpreted to indicate a common magmatic-hydrothermal origin to the apatite bearing magnetite deposits of the Chilean Iron Belt, but may represent a younger phase to the copper deficient systems.
The copper resource at Mantoverde to the 800 m level was estimated prior to mining at 120 Mt @ 0.72% Cu (0.2% Cu cutoff) including a mineable reserve of 85 Mt @ 0.82% Cu.
In the mid to late 1990s, exploration identified:
- a Cu oxide resources of the order of 180 Mt @ 0.5% Cu overlying
- a hypogene sulphide resource of 440 Mt @ 0.56 % Cu, 0.12 g/t Au at a 0.20 % Cu cut-off.
The mine is operated and controlled by Minera Mantos Blancos S.A., an Anglo American Group company.
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Candelaria .......... Tuesday 12 June, 2007.
The La Candelaria deposit is located some 20 km south of Copiapo. Like Mantoverde and Mantos Blancos it is localised near the Atacama Fault Zone within the Coastal Cordillera of northern Chile, and the Chilean Iron Belt.
It is hosted by early Cretaceous andesitic rocks, predominantly flows and volcanic breccias that are masked by a thick sequence of limestones with minor evaporites. This sequence has been intruded and strongly metamorphosed and metasomatised within the broad 2.5 km wide contact aureole of a multiphase lower Cretaceous (119 to 97 Ma) batholith which is exposed <1 km to the west of Candelaria. The accepted mine stratigraphy of the sub-horizontal host rocks is based on alteration assemblages. This comprises an intensely biotitised Lower Andesite, overlain by the banded and brecciated Tuff, which in turn is succeeded by an albitised Upper Andesite and the fine grained and biotitised Metasediments.
The ore deposit has a grossly stratabound form, with a thickness of >350 m in its central section. It lies within the Lower Andesite and the Tuff units. The ore is present as veins, veinlets, breccia fillings and coarse to fine-grained disseminations of chalcopyrite in association with 10 to 15% magnetite, lesser pyrite and subordinate pyrrhotite.
The alteration is not obviously zoned, comprising dominant pervasive biotite in the lower 70% of the orebody, accompanied by calc-silicates, mainly actinolite and scapolite. The overlying limestones have been skarn altered, accompanied by restricted gold and copper, although small gold rich copper skarns in this lithology cropped out at two localities above the main Candelaria orebody. While the altered limestones are stratabound garnet bearing skarns, the mineralisation within the andesite has the characteristics of a wall rock porphyry deposit, although no porphyry has been identified. The main alteration assemblage within the andesites are albite-quartz-epidote with pyroxene and scapolite, while the underlying tuffs and conglomerates have albite, quartz, epidote, magnetite, amphibole and scapolite.
This alteration is interpreted to have resulted from a six stage process, namely: i). An early texture preserving albitisation which decreases in intensity and pervasiveness downwards; ii). Biotite-quartz-magnetite to produce widespread brown Fe rich biotite, disseminated quartz-magnetite and large magnetite-quartz bodies, but has no accompanying significant Cu or Au; iii). Main Ore Stage - the bulk of the Cu and Au accompanied this stage, superimposed on the previous barren alteration, and is characterised by iron rich calcic amphibole, chalcopyrite, magnetite and hematite after magnetite with pyrite albite and quartz, plus possibly cogenetic barren scapolite-pyroxene-amphibole and calcic garnet skarns in the overlying Upper Andesites; iv). Epidote-chlorite with minor associated chalcopyrite; v). Hematite-calcite-chalcopyrite stage represented by hydrothermal breccia cements, and locally important mineralisation; vi). Anhydrite and calcite-chlorite stage.
The mineable reserve comprises 470 Mt @ 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag within a geological resource of 600 Mt at a similar grade.
The mine is operated by the Phelps Dodge subsidiary Compania Contractual Minera Candelaria.
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Mantos Blancos .......... Wednesday 13 June, 2007.
The Mantos Blancos copper-silver deposit is located in the Coastal Range of northern Chile, some 45 km NE of the Pacific coastal city of Antofagasta in northern Chile.
It lies within the Atacama Fault Zone and is hosted by a Triassic sequence of acid volcanics, mainly rhyolites and dacite which dip at 10 to 45° SE and, cut upper Palaeozoic sediments and metasediments. These are in turn overlain by Jurassic clastics and limestones, Jurassic andesites and Cretaceous andesites and dacites. In the mine area the host volcanics are intruded by a sill like sub-volcanic andesite body, by sheets of dacite and abundant andesite dykes.
The Mantos Blanco mineralisation displays two superimposed hydrothermal events, namely:
i). an older phyllic alteration probably related to felsic magmatic-hydrothermal brecciation at ~155 Ma, and
ii). a younger (141-142 Ma) potassic, propylitic and sodic alteration phase, coeval with dioritic and granodioritic stocks and sills, and dioritic dykes.
The principal ore formation is genetically related to the second hydrothermal event, and comprises hydrothermal breccias, disseminations and stockwork-style mineralisation, associated with sodic alteration.
The ore stage alteration is dominated by albitisation and silicification that is distributed homogeneously through the volcanic sequence. Specular hematite is found in the barren upper levels and red hematite in the mineralised zones. These represent four principal alteration types, namely:
i). Na metasomatism manifested by albitisation of feldspars as well as albite veining and pore filling;
ii). incipient to intense addition (or locally removal) of Mg and Fe, reflected by chloritisation or bleaching;
iii). intense hematisation in the form of disseminated and stringer specularite and by intense pervasive red hematisation to many of the rocks within the deposit; and
iv). silicification, represented by quartz phenocrysts, microcrystalline aggregates in the groundmass and as occasional veinlets and amygdule fillings.
The mineralisation occurs as irregular bodies of oxide and sulphide copper with economically significant associated silver. The oxide minerals atacamite and chrysocolla are common in the upper levels of the sulphide body associated with faulting and intense brecciation.
The hypogene sulphide assemblages have distinctive vertical and lateral zoning, centred on magmatic and hydrothermal breccia bodies, which are interpreted to constitute feeders to the main mineralisation which is largely distributed in irregular, lenticular bodies roughly parallel to stratification. A barren pyrite root zone is overlain by pyrite-chalcopyrite, and followed upwards and laterally by chalcopyrite-digenite or chalcopyrite-bornite. A digenite-supergene chalcocite assemblage characterises the central portions of high-grade mineralisation in the breccia bodies. Silver is found in the lattices of both the oxide and sulphide minerals and correlated with the Cu grade.
Economic grade ore is found over an interval of 3x1.5 km and to a depth of 450 m. In 1995 the pre-mining resource was calculated at 170 Mt, of which 91 Mt were oxide ore @ 1.4% Cu and 89 Mt of sulphide ore @ 1.6% Cu and 17 g/t Ag.
More recent testing has indicated that the deposit contains around 500 Mt @ 1.0% Cu.
The mine is operated by Minera Mantos Blancos S.A., an Anglo American Group company.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available from the IOCG 07 Tour options page.
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T M (Mike) Porter, of Porter GeoConsultancy
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T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
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