Remote Sensing

Inside the Blog Everything about Remote Sensing and GIS for Geologists

Remote Sensing and GIS for Geologists

Read More

Radomiro Tomic Copper Mine

Radomiro Tomic Copper Mine

Location: 3,000m above sea level in the Atacama Desert of northern Chile

Products: Copper

Owner: CODELCO Production future.

Ore Type: Porphyry copper deposit.

Overview: Radomiro Tomic is the first mine to have been entirely developed by the Chilean state copper-mining company, Codelco. Located at 3,000m above sea level in the Atacama Desert of northern Chile, this new mining and hydrometallurgical operation is 4km from the Chuquicamata mine and smelter. Development was approved in 1995, started in 1996 and was essentially completed in 1997. The initial target capacity was 150,000t/y of copper cathodes but optimisation during construction raised the rating to 180,000t/y by the commissioning date. Prime contractor LX Ltda (Bechtel and ARA in joint venture) completed work early, below the budgeted $641 million. Recovering copper by heap leaching and electrowinning, Radimiro Tomic was inaugurated in 1998. In 1999 Codelco contracted Kvaerner Metals to increase the plant capacity to 250,000t/y of cathode at a cost of $220 million. This expansion was completed in 2001. Codelco initially created a new Radomiro Tomic Division with a streamlined organisation to manage the facility. During 2002, the corporation decided to amalgamate Chuquicamata and RT as one division - Codelco Norte - and developed a consolidated resource exploitation plan for the deposits in this area. About 460 people work at Radimiro Tomic.

Geology of the Deposit: The Radomiro Tomic porphyry copper deposit is located along the West Fissure structural domain, one of the main strands of the Domeyko fault zone (Cuadra and Camus, 1998). The orebody is 1 km wide, elongated north-south, and is completely buried by 30 to 150 m of Tertiary to Quaternary alluvial gravels, below which a thick oxidation zone was developed on granitic bedrock during a semiarid regime.

The host rock of copper mineralization at RadomiroTomic is the Chuqui Porphyry, the youngest intrusion emplaced on the eastern side of a Tertiary intrusive sequence. To the east the Chuqui Porphyry is in contact with a coarse-grained granodioritic intrusion, the Elena Granodiorite, and to the west with the Fortuna Granodiorite Complex (Fig. 2). The latter is a north-northeast–elongated intrusion 22 km long by 5 km wide that comprises five intrusive bodies of fine- to mediumgrained hornblende-biotite granodiorite. This complex, dated 39 to 37 Ma (Dilles et al., 1997) intruded Triassic to early Tertiary andesitic volcanics. Contact relationships between the Chuqui Porphyry and its host rocks are not well defined because information is based only on a few existing drill holes along the margins of the deposit.

The hypogene alteration and mineralization processes are restricted almost entirely to the Chuqui Porphyry, a granodioritic to monzonitic intrusion with medium- to coarse-grained phenocrysts set in a fine groundmass, and they were originated within the West Fissure fault zone. Recent 40Ar/39Ar dating on the Chuqui Porphyry at Radomiro Tomic gives an average age of 32.7 Ma for the K silicate late-magmatic phase and an age of 31.8 ± 0.3 Ma for the sericitic alteration (Cuadra et al., 1997). These ages define a slightly different hydrothermal timing compared to Chuquicamata (33.4 and 31.1 Ma, respectively; Zentilli et al., 1995). At Radomiro Tomic, potassic alteration is younger and the quartz-sericitic event is older than at Chuquicamata, so the time duration of alteration is shorter at Radomiro Tomic than at Chuquicamata.

The mineralized intrusion is buried beneath Tertiary to Quaternary alluvial gravels, with a thickness ranging from 30 m on the east side to 150 m on the west side. As a result the bedrock surface has a relatively gentle slope to the west, with some abrupt changes indicating possible faults, but this is not well-defined yet. The gravels are composed of angular andesitic fragments, 1 to 10 cm in diameter, in a sandy and poorly to moderately cemented matrix. This unit is in direct contact with the underlying leached or oxide zones of the deposit. Locally, the gravel is mineralized with exotic copper reaching thicknesses of tens of meters, mainly along northwest-trending paleochannels according to the paleotopography. A tuff intercalated in the gravels, 2 m below the current surface, was dated in biotite by the K-Ar method at 9.7 ± 0.7 Ma (Cuadra et al., 1997).

Alteration

K silicate alteration is developed pervasively throughout the entire Radomiro Tomic deposit, with the highest intensity found in the area between coordinates 10,000 N and 11,000 N (Fig. 2). It is represented typically by quartz-K feldspar veinlets and biotitization of hornblende phenocrysts. Quartz-sericitic alteration is less abundant and is clearly controlled by northeast- and north-south–striking subvertical structures. These structures are marked by quartz-pyritechalcopyrite D-type veins, with quartz-sericite halos. In the upper part of the oxidation zone supergene argillic alteration is defined by montmorillonite and kaolinite in fractures and as replacement of feldspars.

Hypogene mineralization

Hypogene mineralization follows a concentric distribution of inner bornite-chalcopyrite, intermediate chalcopyrite > pyrite, and outer pyrite > chalcopyrite zones centered around the coordinates 9,700 N and 11,000 N (Fig. 2) and averaging 0.5 wt percent total copper. Minor molybdenite mineralization is present alone or associated with chalcopyrite and pyrite in quartz veins and veinlets striking north and dipping subvertically. Arsenic minerals such as enargite are absent in significant contrast to the Chuquicamata orebody. This difference may indicate a greater degree of erosion for the Radomiro Tomic orebody. Hypogene mineralization continues to at least 400 m below the top of sulfides.

Supergene mineralization

Supergene oxidation and leaching processes affected the hypogene mineralization to an average depth of about 200 m beneath the gravel-bedrock contact (Fig. 3). Supergene mineralization is present immediately below the gravels with a typical vertical distribution of leached oxide zones, a mixed (oxide sulfide) zone, and a secondary sulfide zone. Locally, along quartz-sericite-faulted veins enrichment has reached depths of up to 800 m. The description of the different zones follows.

Fig. 2. Surface geology of the Radomiro Tomic area. The shaded area represents the subsurface geologic projection of the orebody at level 2,825 m.

Copper Mining

The conventional open pit strips at a 1.5:1 waste-to-ore ratio using rotary drills, P&H 4100 shovels, a LeTourneau loader, Caterpillar 793B and Komatsu 330st-capacity trucks. An FFE Minerals gyratory primary crusher near the pit rim supplies coarse ore, which travels to the main processing area via a 9,615t/h Krupp conveyor. The expansion added tertiary cone crushing (SRP Hydrocones) to the secondary Symons cone. Conveyors take stockpiled ore to pre-treatment and stacking on the racetrack-style heap-leach pads. Leached material is reclaimed by a bucket wheel and is conveyed to the dump area. Rahco and MAN-Takraf supplied the crawler-mounted materials-handling equipment. To handle the expanded production, the conveyor drives were fitted with programmable soft braking systems.

Copper Processing

Following acid leaching, the copper is separated from the heap-leach solution by four-stage solvent extraction with Acorga reagents and is fed in solution to the electrowinning tankhouse for recovery as cathodes using sophisticated technology. Four solvent-extraction trains designed and supplied by Outokumpu Engineering (including VSF mixer-settlers, Proscon 2100 NT process control system and OTI 99 titrators) remove the copper. The electrowinning feed and reagent streams are cleaned using molecular sieve coagulators and electrolyte filters designed by Codelco, and by CPT flotation columns. The large and highly automated electrowinning tankhouse was fitted with unique cathode cranes and stripping machines as well as a system of blowers and aerosol chimneys to ventilate the cells. The expansion added 272 cells, two travelling cranes and a cathode washing-stripping machine. Secondary leaching of waste will add 24,000t/y to 28,000t/y of copper to the cathode output.

Copper Production

After start-up, Radomiro Tomic recorded total operating costs of $0.44/lb, producing 162,000t copper in 1998 and 190,100t in 1999. The expansion boosted output to 256,000t/y in 2001 and Codelco hoped to maintain production at around 300,000t/y thereafter. Actual output in 2002 was 297,119t at a cash cost of $0.33/lb.

References

- Economic Geology 96, 401-420 (2001).
- Cameron, E.M., Leybourne, M.I., and Palacios, C. (2007): Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters? Mineralium Deposita 42, 205-218.

Read More

Pascua Lama mine

Pascua Lama mine

Location: Andes Mountains, on the Chilean-Argentine border.

Products: Gold, Silver, Copper.

Owner: Barrick Gold.

The Mine life: The mine life is expected to be 25 years.

Deposite Type:

The gold, silver and copper mineralisation and alteration assemblages at Pascua-Lama are associated with a structurally controlled acid sulphate hydrothermal system hosted by intrusive and volcanic rock sequences of the Upper Palaeozoic and Middle Tertiary age. Alteration and mineralisation are of the high-sulphidation, epithermal type. Throughout the Pascua-Lama district, the alteration and mineralisation appear to have been strongly controlled by structure. This control is most evident along the Esperanza, Pedro and Quebrada de Pascua fault systems. As is typical with high-sulphidation epithermal deposits, the principal metal commodities at Pascua-Lama are gold and silver, the copper content is sub-economic.

The presence of hypabyssal intrusive host rocks that are not related to mineralisation is unusual for high sulphidation deposits, making Pascua-Lama (along with Barrick’s Alto Chicama deposit in Peru, which is hosted by meta-sedimentary rocks) somewhat unique among deposits of this type.

Overview

Pascua-Lama is located at an altitude of 3,800m to 5,200m. The Chilean part of the mine constitutes 75%, while 25% is located in Argentina. The development activities of Pascua-Lama were stopped in April 2013, following a Chilean court's orders on issues of sanitation and violation of the Glacier monitoring plan. The Diaguita indigenous community filed a petition for the closure of the project. Barrick Gold's plea to reopen the project was rejected by a local appellate court in Copiapó, Chile, on 24 April 2013.

The Chile's Supreme Court, however, issued a ruling in September 2013 overturning the Copiapó court order. Following the ruling, Barrick Gold will construct a water management system at the Chilean section of the mine in order to receive environmental approval for the project. The water management system is expected to be completed by the end of 2014.

Barrick Gold announced its decision to temporarily suspend the Pascua-Lama project, in October 2013, in order to reduce its debt burden. Construction of facilities required for obtaining the environmental approval will, however, be completed. The company plans to resume the mine's development in future.

The Argentinean segment was to include critical infrastructure such as the processing plant and tailings storage facility.

Barrick Gold had estimated the development capital cost of Pascua-Lama to be $3bn at 2009 prices, but the construction delay increased the estimated capital costs of the project to approximately $8bn to $8.5bn at 2012 prices. The development of the mine would have created more than 5,500 jobs during construction and more than 1,600 jobs during production phase.

Geological Setting

The Pascua-Lama deposit is situated at the crest of the high cordillera of Region III, along the international border between Chile and Argentina and on the northern edge of a major mineralised trend known as the El Indio belt. This trend, along which a number of major precious metal deposits are located (including the nearby Veladero mine), stretches 47km south of Pascua-Lama to the world-renowned El Indio deposit and adjacent Tambo deposit (both closed).

The geology in the region is dominated by extrusive volcanic rocks that are locally intruded by hypabyssal stocks of varying size and numerous dikes and sills (Figure 6-1). Volcanic activity began with deposition of the Permian Guanaco/Zonso felsic ash flows from a caldera 15km east of Pascua-Lama and subsequent intrusion of the Permian-Triassic Chollay crystalline felsic rocks along the extent of the El Indio belt. These events were followed by intrusion of the Triassic Pascua-Lama granite complex in the immediate vicinity of the Project. Deposition of extrusive volcanic rocks and continued intrusive activity resumed in the Oligocene with the Bocatoma diorite stocks (33-36Ma), the Tilito dacite ash flows (27.2-17.5Ma) the Escabroso mafic andesite and andesitic flows (21.0-17.5Ma), and the Cerro de Las Tortolas I andesites (16.0 ±0.2 -14.9 ±0.7Ma), after which volcanic activity decreased markedly in the vicinity of the El Indio belt. Subsequent activity was confined to the Vacas Heladas intermediate dacitic domes, lava flows and felsic tuffs (12.8-11.0Ma), and the Late Miocene rhyodacite dikes at Pascua. The most recent activity in the region included deposition of the post mineralisation silicic Vallecito rhyolites south of Pascua-Lama in the vicinity of Cerro de Las Tortolas, and the Upper Pliocene Cerro de Vidrio rhyolite. All ages are from Bissig et al., (2000a & 2001) and Martin et al.,(1995).

Regional structure in and around the gold deposits and prospects in the El Indio belt is dominated by northerly-trending high angle reverse faults, normal faults and fold belts oriented parallel to the major structural grain of this portion of the Andean Cordillera. Pascua-Lama is positioned near the center of a northerly trending graben that contains nearly the entire Tertiary volcanic sequence that is distributed along the spine of the cordillera in Chile and Argentina. This graben is bounded by two high angle reverse fault zones, the Baños del Toro/Chollay located 10km west of the deposit and the El Indio zone situated 16km to the east. The graben is cut at Pascua and El Indio by strong, west-northwest fracture zones, which form loci for mineralisation. Large elliptical fracture zones are also present immediately to the east and/or northeast of both El Indio/Tambo and the Pascua-Lama/Veladero deposit areas, and these zones may have contributed to host rock permeability.

Metallurgy and Mineral Processing

The Pascua-Lama (and Esperanza) ore is extremely complex and highly variable, ranging from relatively straight forward oxide zones which are amenable to cyanide leaching, to highly altered sulphide zones containing soluble sulphate minerals with some cyanide-amenable gold/silver and some refractory gold/silver hosted in sulphides. The majority of silver occurs in an enriched blanket of secondary mineralisation in the upper zones of the deposit with silver grades typically four to five times those of the underlying primary zones.

The deposit is hosted in a high-sulphidation hydrothermal system consisting of acidic material that requires a washing stage to remove soluble iron and copper sulphate salts that are detrimental to subsequent processing. Ore material in the deposit is classified as two main types:

Non-Refractory and Refractory, both ore types are crushed, wet ground and washed in similar circuits. The washed Non-Refractory ore is subject to direct cyanide leaching only with pregnant solution which is recovered from the counter current decantation (“CCD”) circuit, treated in a conventional Merrill Crowe (zinc precipitation) circuit to produce gold/silver doré. The washed

Refractory ore is subject to flotation with cyanide leaching of the flotation tails. Solution recovery and precious metal production from the leached tails is via the CCD and Merrill Crowe circuits to produce gold/silver doré. The flotation circuit produces a final gold/silver rich concentrate of nominally 12% copper for export to smelters.

The proposed nominal plant capacity is 45,000t/d of ore, 30,000t/d for Non-Refractory ore and 15,000t/d for Refractory ore, according the following schedule:

• Year 1, Q1: Two lines, 30,000t/d Non-Refractory ore;

• Year 2, Q4: Three lines, 45,000t/d Non-Refractory ore; and

• Year 3, Q3: Two lines, 30,000t/d Non-Refractory ore and one line, 15,000t/d Refractory ore.

Mine Production and Mineral Reserve Estimate

SRK audited the Mineral Reserve estimate that was prepared by Barrick (Table 2). SRK is of the opinion that the estimation strategy and methods employed meet or exceed current industry standards and the reserves have been classified according to CIM guidelines. The LoM plan was based on calculations prepared in mid-2008 for the Feasibility Study and not the end of year Mineral Reserve estimate disclosed in this report.

The difference between the LoM plan and the Mineral Reserve estimate is not considered material to Silver Wheaton.

Mining commences in 2011 with pre-stripping. The amount of pre-stripping required is 66.4Mt and this is scheduled to be mined in an 18-month period using the owner’s equipment. The first ore is produced in late 2012. The production phase commences in 2013. The LoM production schedule is shown in Table 3.

The average ore plus waste mining rate is 66.0 Mt/y, comprising 18.3Mt/y of ore and 48.8Mt/y of waste. The average overall strip ratio is 2.71:1, exclusive of the pre-production period. The average overall strip ratio inclusive of the pre-production period is 2.88:1.

Read More

Genetic Model of Ore Deposits

Magmatic

Or “Mafic Igneous Intrusion Associated Deposits”

Syngenetic : The Ores are formed after their host rocks.

Primary : Nickle.

Secondary : Chromite, Copper, Molybdenum, (PEG).

Host: The most significant magmatic deposits are related to mafic (gabbro, norite) and ultramafic (peridotite, dunite) rocks originated from the crystallization of basaltic and ultramafic magma.

Process : Magmatic intrusions result in partitioning of elements and contamination of the melt by assimilation of the host rock. Gravitation segregation of Sulphur result in sulfide ore to form at the bottom of the melt. It is also possible to produce this type of ore through the meteorite impacts, rift/continental flood basalt-associated sills and dykes, volcanic flows and troctolite intrusions.

Examples : There are several largest magmatic deposits: they are Cr-PGE deposits at Bushveld Igneous Complex, South Africa, Ni-Cu-PGE deposits at The Great Dykes, Zimbabwe, Ni-PGE-Cr deposits at Sudbury “(meteorite impact-unusual), Canada, Ni-Cu-PGE deposits at Stillwater Igneous Complex, Montana, US.

Magmatic Ore Deposits

Sedimentay

The sedimentary deposits are concordant and may be integral part of stratigraphic sequence. It is formed due to seasonal concentration of heavy minerals like hematite on the seafloor. The structures consist of repeated thin layers of iron oxides, hematite or magnetite, alternating with bands of iron-poor shale and chert. The examples are Pilbara BIF, Northwestern Australia, Bailadila and Goa iron ore, India.

Similarly economic limestone deposits are formed by chemical sedimentation of calcium magnesium carbonate on the seafloor. Coal and lignite are formed under sedimentary depositional condition. Evaporite deposits form through the evaporation of saline water in lakes and sea, in regions of low rainfall and high temperature. The common evaporite deposits are salts (halite and sylvite), gypsum, borax and nitrates. The original character of most evaporite deposits is destroyed by replacement through circulating fluids.

For More >>> Here

Sedimentary Ore Deposits

Metamorphic

Metamorphic deposits are formed in different types of metamorphic conditions, ranging from low to high temperature and low to high pressure, due to regional prograde or retrograde metamorphic process and hosted by metamorphic rocks. Minerals like garnet, kyanite, sillimanite, wollastonite, graphite and andalusite are end product of metamorphic process. Three general types of metamorphic deposits are known:  1) copper-rich, 2) gold-rich, and 3) lead-zinc-silver-rich.

1- Copper-Rich Types: These metamorphic deposits are characteristically associated with very low grade to low grade metamorphism.  Most often they form in terrains where mafic or ultramafic basement rocks are overlain upsection by organic-rich sedimentary rocks. Examples Kennicott, Alaska, Ore = Chalcocite + Bornite. White Pine, Michigan, Ore = chalcocite + bornite + chalcopyrite + minor sphalerite.

2- Gold-Rich Types: are of two general types:  1) Archean iron formation types, and 2) quartz-carbonate veins. Archean Iron Formation Types: Occur mostly in Precambrian shield areas.  Vein morphology but most veins apparently concentrated in peculiar iron-rich shales and sandstones which are upgraded by at least one and usually several metamorphic events.  Gold occurs in quartz veins in the silicate or sulfide facies host rocks. Examples Precambrian of Wyoming Jardine, Montana.

Quartz-Carbonate Types: 

Often associated with greenstone belts in shield areas. Ore formed  in structural zones/shear zones which are  regional in scale.  The districts usually contain  large scale folding as well. Examples Valdez  Creek District, Alaska Conn Mine, Eastern Canada  AJ Mine, southeast Alaska.

3- Lead-Silver-Rich: deposits typically contain

galena, sphalerite, and locally tetrahedrite and

chalcopyrite as ore minerals.

Examples Coer de Lane district, Idaho.

For More >>> Here

Metamorphic Ore Deposits

Volcanogenic Massive Sulfide

VMS and VHMS type of ore deposits contribute significant source of Cu-Zn-Pb sulfide +- Au, Ag, formed as a result of volcanic associated hydrothermal events under submarine environments at or near the seafloor. It forms in close time and space association between submarine volcanism, hydrothermal circulation and exhalation of sulfides, independent of sedimentary process. The deposits are predominantly stratabound (volcanic derived or volcano-sedimentary rocks) and often stratiform in nature. The ore formation system is synonymous to black smoker type of deposit.

Examples:

Kidd Creek, Timmins, Canada, is the largest VMS deposit (35 Mt @ 2.2% Cu, 5.3% Zn, 0.22% Pb and 60 g/t At at 2005) in the world. Kidd is also the deepest (þ1000 m) base metal mine. The other notable VMS/VHMS deposits are IPB of Spain and Portugal, Wolverine Zn-Cu-Pb-Ag-Au deposit, Canada and Khnaiguiyah Zn-Pb-Cu, Saudi Arabia.

Deposits of this class have been classified by numerous workers in different ways (e.g., metal sources, type examples, geodynamic setting - see Franklin et al. (1981) and Lydon (1984)). Recently, VMS deposits have been classified according to their setting and rock associations into five subclasses, including (after Barrie and Hannington (1999) and Franklin et al. (2005)).

Mafic associated

VMS deposits associated with geological environments dominated by mafic rocks, commonly ophiolite sequences. The Cyprus and Oman ophiolites host examples and ophiolite-hosted deposits are found in the Newfoundland Appalachians represent classic districts of this subclass.

Bimodal-mafic

VMS deposits associated with environments dominated by mafic volcanic rocks, but with up to 25% felsic volcanic rocks, the latter often hosting the deposits. The Noranda, Flin Flon-Snow Lake and Kidd Creek camps would be classic districts of this group.

Mafic-siliciclastic

VMS deposits associated with sub-equal proportions of mafic volcanic and siliciclastic rocks; felsic rocks can be a minor component; and mafic (and ultramafic) intrusive rocks are common. In metamorphic terranes may be known as or pelitic-mafic associated VMS deposits. The Besshi deposits in Japan and Windy Craggy, BC represent classic districts of this group.

Felsic-siliciclastic

VMS deposits associated with siliciclastic sedimentary rock dominated settings with abundant felsic rocks and less than 10% mafic material. These settings are often shale-rich siliciclastic-felsic or bimodal siliciclastic. The Bathurst camp, New Brunswick, Canada; Iberian Pyrite Belt, Spain and Portugal; and Finlayson Lake areas, Yukon, Canada are classic districts of this group.

Bimodal-felsic

VMS deposits associated with bimodal sequences where felsic rocks are in greater abundance than mafic rocks with only minor sedimentary rocks. The Kuroko deposits, Japan; Buchans deposits, Canada; and Skellefte deposits, Sweden are classic districts of this group.

Volcanogenic Massive Sulfide

Mississippian Valley Type

Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which are epigenetic, stratabound, rhythmically banded ore with replacement of primary sedimentary features. MVT and Irish type deposits are commonly associated with a 'dolomite front' alteration.

Process  The formation of ore minerals occurs later when tectonic processes and mountain building events channel metal-bearing fluids through the carbonate host rock.  MVT deposits are closely associated with orogenic forelands – large valleys running parallel to mountain belts. The deposits formed by diagenetic recrystallization of carbonates creating low-temperature hydrothermal solution that migrates to suitable stratigraphic traps like fold hinge and faults at the continental margin and intra-cratonic basin setting.

The OFM are predominantly sphalerite, galena and barite. Calcite is the most common gangue mineral. Low pyrite content supports clean concentrate with high metal recovery of +95%. Some deposits are surrounded by pyrite/ marcasite halo. Exploring for MVT deposits  involves targeting the locations where MVT mineralization is most likely to occur: carbonate platforms in foreland basins with the necessary structural channels. Aside from outright mineral occurrences, geologists will use existing maps to identify the target host rocks and important geologic structures. Field methods such as stream sediment or soil geochemistry may be used to target lead or zinc mineralisation. Geophysical methods including gravity and magnetic surveys may be employed. Gravity may identify mineralized areas (high density rock) or buried open structures which may have been fluid channels (low density rock). Magnetic surveys may be useful since the magnetic iron sulfide mineral pyrrhotite is known to occur in MVT deposits.Examples There are numerous Zn-Pb-Ag sulfide deposits along the Mississippi river in US, Pine Point, Canada, San Vicente, Central Peru, Silesia, Southern Poland, Polaris, British Columbia, Lennard Shelf and Admiral Bay, Western Australia.

Mississippian Valley Type

Skarn Deposits

Skarns are calcium-bearing silicate rocks of any age, can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skarn, is the mineralogy. This mineralogy includes a wide variety of calc-silicate and associated minerals but usually is dominated by garnet and pyroxene. Skarns can be subdivided according to several criteria. Exoskarn and endoskarn are common terms used to indicate a sedimentary or igneous protolith, respectively. Magnesian and calcic skarn can be used to describe the dominant composition of the protolith and resulting skarn minerals. Not all skarns have economic mineralization; skarns which contain ore are called skarn deposits. In most large skarn deposits, skarn and ore minerals result from the same hydrothermal system even though there may be significant differences in the time/space distribution of these minerals on a local scale.

Major skarn types:

1-Iron (Fe ) Skarns. Is The largest skarn deposits, are mined for their magnetite content and although minor amounts of Cu, Co, Ni, and Au may be present, iron is typically the only commodity recovered (Grigoryev et al., 1990). Many deposits are very large (>500 million tons, >300 million tons contained Fe) and consist dominantly of magnetite with only minor silicate gangue. Ex. Um Nar area, central Eastern Desert, Egypt. Fe Skarn, Iron Oxide Cu-Au, and Manto Cu-(Ag) Deposits in the Andes Cordillera of Southwest Mendoza Province, Argentina.

2-Gold Skarns. Most Gold skarn deposits are associated with relatively mafic diorite, granodiorite plutons and dyke/sill complexes. Most gold produced from skarn deposits came as a byproduct of the mining of other metals, particularly Cu. The term "gold skarn" is used here in the economic sense suggested by Einaudi et al. (1981) and refers to ore deposits that are mined solely or predominantly for gold and which exhibit calc-silicate alteration, usually dominated by garnet and pyroxene, that is related to mineralization. Ex. The Nickel Plate mine in the Hedley district, British Columbia is the largest and highest grade gold skarn in Canada. Fortitude Deposit, Battle Mountain District, Nevada. Gold skarn mineralization at the Crown Jewel.

3-Tungsten Skarns. Tungsten skarns are found on most continents in association with calc-alkaline plutons in major orogenic belts. are associated with coarse-grained, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles. These features are collectively indicative of a deep environment. Plutons are typically fresh with only minor myrmekite and plagioclase-pyroxene endoskarn zones near contacts.

4-Copper Skarns. are perhaps the worlds most abundant skarn type. They are particularly common in orogenic zones related to subduction, both in oceanic and continental settings. Most copper skarns are associated with I-type, magnetite series, calc-alkaline, porphyritic plutons, many of which have co-genetic volcanic rocks, stockwork veining, brittle fracturing and brecciation, and intense hydrothermal alteration. The largest copper skarns are associated with mineralized porphyry copper plutons. These deposits can exceed 1 billion tons of combined porphyry and skarn ore with more than 5 million tons of copper recoverable from skarn.

5-Zinc Skarns. Most zinc skarns occur in continental settings associated with either subduction or rifting. They are mined for ores of zinc, lead, and silver although zinc is usually dominant. They are also high grade (10-20% Zn+ Pb, 30-300 g/t Ag). Related igneous rocks span a wide range of compositions from diorite through high-silica granite. They also span diverse geological environments from deep-seated batholiths to shallow dike-sill complexes to surface volcanic extrusions. The common thread linking most zinc skarn ores is their occurrence distal to associated igneous rocks. Major reviews of zinc skarn deposits include Einaudi et al. (1981), Megaw et al. (1988), and Megaw (1998).

6-Molybdenum Skarns.

Most molybdenum skarns are associated with leucocratic granites and range from high grade, relatively small deposits (Azegour, Morocco, Permingeat, 1957) to low grade, bulk tonnage deposits (Little Boulder Creek, Idaho, Cavanaugh, 1978). Numerous small occurrences are also found in Precambrian stable cratons associated with pegmatite, aplite, and other leucocratic rocks (Vokes, 1963). Most molybdenum skarns contain a variety of metals including W, Cu, Zn, Pb, Bi, Sn, and U and some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be mined economically. Mo-W-Cu is the most common association and some tungsten skarns and copper skarns contain zones of recoverable molybdenum. Most molybdenum skarns occur in silty carbonate or calcareous clastic rocks; Cannivan Gulch, Montana (Darling, 1990) is a notable exception in that it occurs in dolomite. Hedenbergitic pyroxene is the most common calc-silicate mineral reported from molybdenum skarns with lesser grandite garnet (with minor pyralspite component), wollastonite, amphibole, and fluorite. This skarn mineralogy indicates a reducing environment with high fluorine activities. These deposits have not received significant study outside of the Soviet Union and there has not been a modern review since the brief summary by Einaudi et al. (1981).

7-Tin skarns.

Tin skarns are almost exclusively associated with high-silica granites generated by partial melting of continental crust, usually caused by rifting events. The skarn destructive stages of alteration are particularly important in tin skarn deposits. As noted by Kwak (1987), the most attractive ore bodies occur in the distal portions of large skarn districts where massive sulfide or oxide replacements occur without significant loss of tin in calc-silicate minerals like garnet.

Skarn Deposits Type

Black smokers pipe Type

“Black smokers” pipe-type deposits are formed on the tectonically and volcanically active modern ocean floor by superheated hydrothermal water ejected from below the crust. The water with high concentrations of dissolved metal sulfides (Cu, Zn, Pb) from the crust precipitates to form black chimney-like massive sulfide ore deposits around each vent and fissure when it comes in contact with cold ocean water over time. The formation of black smokers by sulfurous plumes is synonymous with VMS or VHMS deposits of Kidd Creak, Canada, formed 2400 million years ago on ancient seafloor.

Black Smokers Deposit Type

Sedimentary Exhalative Deposit

SEDEX ore deposits are formed due to concurrent release of ore-bearing hydrothermal fluids into aqueous reservoir mainly ocean, resulting in the precipitation of stratiform zinc-lead sulfide ore in a marine basin environment. The stratification may be obscured due to post depositional deformation and remobilization. The source of metals and mineralizing solutions are deep-seated superheated formational brines migrated through intra-cratonic rift basin faults which come in contact with sedimentation process. The formation occurred mainly during Mid-Proterozoic period. SEDEX deposits are the most important source of zinc, lead, barite and copper with associated by-products of silver, gold, bismuth and tungsten. Depending on the deposit sub-type they also host variable amounts of valuable by-products including copper, gold and silver.  SedEx deposits are high grade, with an average size of approximately 70 Mt, and can host about 12 percent lead and zinc. SedEx deposits are easily distinguished from many other deposit types by the fact that their formation is the result of minerals being deposited through the discharge of metal-bearing fluids into seawater. This is a strong contrast from other deposit types that are formed as a result of some type of intrusive or metamorphic process. The examples are zinc-lead-silver deposits of Red Dog, Northwest Alaska, McArthur River, Mt Isa, HYC, Australia, Sullivan, British Columbia, Rampura-Agucha, Rajpura-Dariba, India, and Zambian copper belt.

Sedimentary Exhalative Deposit

Residual Ore Deposits

“Residual” deposits are formed by chemical weathering process like leaching which removes gangue minerals from protore and enrich valuable metals in situ or nearby location. The most important example is formation of bauxite under tropical climate where abundance of high temperature and high rainfall during chemical weathering of granitic rocks produces highly leached cover rich in aluminum. Examples are bauxite deposit of Weipa, Gove Peninsula, Darling Range and Mitchel Plateau in Australia, Awaso and Kibi, Ghana, East Coast, India, Eyre Peninsula Kaolin deposit, Australia. Basic and ultrabasic rocks tend to form laterites rich in iron and nickel respectively. Nickel-bearing laterites, may or may not be associated with platinum group of elements, are mined at New Caledonia, Norseman-Wiluna greenstone belt of Western Australia and Central Africa, Ni-bearing limonite overburden at Sukinda, India. The other residual-type deposits are auriferous laterites in greenstone belts (Western Australia), Ni-Co and Cr in laterites on top of peridotites (New Caledonia and Western Australia respectively), and Ti in soils on top of alkali igneous rocks (Parana Basin, Brazil).

Residual Mineral Deposits

Placer Deposits

“Placer” deposits Syngenetic are formed by surface weathering and ocean, river or wind action resulting in concentration of some valuable heavy resistant minerals of economic quantities. The placer can be an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The type of placer deposits are namely, alluvial (transported by a river), colluvial (transported by gravity action), eluvial (material still at or near its point of formation), beach placers (coarse sand deposited along the edge of large water bodies) and paleo-placers (ancient buried and converted rock from an original loose mass of sediment). The most common placer deposits are those of gold, platinum group minerals, gemstones, pyrite, magnetite, cassiterite, wolframite, rutile, monazite and zircon. The California gold rush in 1849 began when someone discovered rich placer deposits of gold in streams draining the Sierra Nevada Mountains. Recently formed marine placer deposits of rutile, monazite, ilmenite and zircon are currently being exploited along the coast of eastern Australia, India and Indonesia.

Placer Deposits

Read More