The iron skarns of the Turgai Belt, northwestern Kazakhstan
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Abstract:
The world-class Sarbai, Kachar and Sokolovsk iron ore deposits of the Turgai belt, in the Carboniferous Valerianovskoe arc of northwest Kazakhstan, contain an aggregate of more than 3 billion tonnes of mineable massive magnetite. The Valerianovskoe arc is the possible westward extension to the South Tien Shan arc that is host to the giant Almalyk Cu-Au porphyry system in Uzbekistan. The magnetite bodies of the Turgai belt replace limestone and tuffs, and are distal to locally proximal to the contacts of gabbro-diorite-granodiorite intrusive complexes. Three main stages of alteration and mineralisation can be recognised at these deposits, namely: (1) pre-ore; (2) the main magnetite forming; and (3) post ore phases. The pre-ore stage is characterised by high temperature, metamorphic/metasomatic calc- and alumino-silicates. The main magnetite ore phase formed when hot, sulphur poor, acidic, iron-, silica- and aluminium rich fluids were structurally focused to dissolve and replace the dominantly limestone hosts. This was accompanied by a skarn assemblage gangue of epidote, calcic-pyroxenes, calcic-garnet and calcic-amphiboles, minor sulphide minerals and high field strength element (HFSE)-bearing accessory minerals such as titanite and apatite. This magnetite-skarn
mineralisation was followed by a late sulphide phase, when comparatively cooler fluids, which produced distinctive and extensive alteration assemblages of sodium-rich scapolite, albite, chlorite and K feldspar, accompanied by chalcopyrite, pyrite and minor sphelarite and galena. The post-ore phase, is characterised by cross cutting barren veins composed of calcite, lesser albite and K feldspar, and minor quartz, and by widespread alteration comprising scapolite, albite and silica, which surrounds the deposit, and extends for several kilometers into the host rock. Many of the geological and mineralogical features of these deposits closely resemble those of IOCG deposits and provinces around the world.
However, as the copper sulphide mineralisation is sub-economic, they may only be classified as either IOCG-style or IOCG-related deposits. Stable isotope (C, O, S) studies have been carried out on a range of sulphides, carbonates and silicates related to the mineralisation. Preliminary results from sulphides intergrown with magnetite support a magmatic source for the sulphur. Oxygen isotope data from associated silicates and iron oxides also support an igneous, or igneous rock equilibrated source for the mineralising fl uids. Carbon and oxygen isotope data from gangue carbonates suggest
carbonate is derived from the interaction of igneous-derived or igneous-equilibrated fl uids with host limestones.Keywords:
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The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca- or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have δ 18 O values of 9.1 to 9.5‰, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower δ 18 O values of 2.9 to 7.3‰. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of δ 34 S values from −3.4 to +12.4‰, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.
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Bornite
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The Marcona Magnetite Deposit, Ica, South-Central Peru: A Product of Hydrous, Iron Oxide-Rich Melts?
Marcona, the preeminent Andean magnetite deposit (1.9 Gt @ 55.4% Fe and 0.12% Cu), is located in the iron oxide copper-gold (IOCG) subprovince of littoral south-central Peru. Fe oxide and Cu (-Zn-Pb) sulfide mineralization was controlled by northeast-striking faults transecting a Middle Jurassic (Aalenian-to-Oxfordian) andesitic, shallow-marine arc and a succession of contiguous, plate boundary-parallel, Late Jurassic to mid-Cretaceous volcanosedimentary basins.
At Marcona, hydrothermal activity was initiated in the earliest Middle Jurassic (161–177 Ma) by high-temperature Mg-Fe metasomatism represented by cummingtonite and phlogopite-magnetite assemblages. Subsequently, during the terminal eruptions (156–162 Ma) of the arc, widespread albite-marialite alteration (Na-Cl metasomatism) was followed by the emplacement of an en echelon swarm of massive magnetite ore-bodies with subordinate, overprinted magnetite-sulfide assemblages, hosted largely by Paleozoic metasilici-clastics. The magnetite orebodies exhibit abrupt, smoothly curving contacts, dike-like to tubular apophyses, and intricate, amoeboid interfingering with dacite porphyry intrusions. There is no convincing megascopic or microscopic evidence for large-scale Fe metasomatism associated with the main, sulfide-poor mineralization. The largest, 400 Mt Minas 2-3-4 orebody is interpreted as a bimodal magnetite-dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma. Oxygen and sulfur stable-isotope geothermometry indicates that the evolution at ca. 159 Ma from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphi-bole-pyrrhotite-pyrite assemblages coincided with quenching from above 800° C to below 450°C and the concomitant exsolution of dilute aqueous brines. Subsequently, chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, in part metasomatic, were deposited from lower temperature (≤360°C) brines.
The Cu-poor Marcona (“Kiruna-type”) magnetite and Cu-rich IOCG deposits in the district, therefore, although spatially contiguous, represent contrasting ore deposit types. The former are interpreted as the product of Fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back-arc basin. The weak associated magmatic-hydrothermal Cu sulfide mineralization at Marcona was generated through melt vesiculation and contrasts with the considerably higher grade Cu- and Ag-rich orebodies of the major Cu-rich IOCG deposits in the Central Andes, e.g., La Candelaria-Punta del Cobre, Mantoverde, Raul-Condestable, and Mina Justa, which were the products of cool, oxidized, hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion.
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Ore genesis
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Breccia
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Felsic
Metasomatism
Ore genesis
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Hypogene
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Stockwork
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Banded iron formation
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The Fe-Cu-Zn sulphide mineralisation of the Polusagi and Canakci areas are two of many cupriferous deposits which occur along the S.E. Anatolian ophiolite belt. The Polusagi stratiform mineralisation is within the lavas of an Upper Cretaceous-Eocene submarine volcanogenic sequence which is thrust over the crystalline Palaeozoic basement rocks. The Canakci mineral isation occurs within the volcanogenic sediments of probable Upper Cretaceous- Eocene age. The host and associated rocks of the mineralisation at Polusagi are studied in detail. Extensive submarine alteration has changed the igneous mineralogy and rock chemistry. Element mobility during the alteration is determined and a chemical classification scheme is proposed on the basis of the immobile elements Zr, Y, Nb, La, Ce and Ti. The igneous rocks belong to a tholeiitic suite and were formed during the early stages of an island- arc development on a Tethyan marginal basin floor. The Polusagi magma originated from a high degree partial melting of an upper mantle source rock and the various rock types were produced by a process of fractional crystallisation involving olivine, clinopyroxene, magnetite, plagioclase and apatite. Mineralogy and mineral chemistry of the Polusagi rocks are studied in detail. The rocks were metamorphosed to prehnite-pvunpellyite facies at higher stratigraphic levels and to greenschist facies at lower, under pressures < 3kb and probably around 2kb, Metamorphism was basically a result of sea-water-rock interaction i.e. ocean-floor hydrothermal metamorphism. Ore mineral chemistry and mineralisation at both areas are examined in detail. The Polusagi massive sulphides are fine grained and composed mainly of pyrite, chalcopyrite and sphalerite with minor fahlore, galena, bornite and covellite and traces of magnetite, hematite and idaite. They show strong mineralogical banding and exhibit features ascribable to precipitation from solutions in an unrestricted environment. The massive sulphides are underlain by pyritic stockwork mineralisation and are overlain by and associated with Fe-Mn-oxide rich siliceous sedimentary horizons. The mineralisation at Canakci is in two forms : massive and disseminated. In many aspects, the former is similar to the massive banded ore and the latter to the stockwork mineralisation of the Polusagi deposit. The Canakci massive sulphide is composed mainly of pyrite, bornite, fahlore, sphalerite and galena with minor chalcopyrite, digenite, idaite, covellite and traces of mawsonite, hexastannite, colusite and native gold, A model is presented to account for the genesis of both deposits. In the submarine geothermal system set up following the volcanism, the sea water percolated through the volcanic pile, became heated and reacted with and leached metals from the rocks. The hydrothermal metal bearing solutions were then discharged onto the sea-floor through channels now represented by the stockwork mineralisation. Upon reaching the sea-water rock interface with temperatures up to 400oC, the solutions mixed with the sea-water and precipitated massive sulphides as a result of a sudden drop in temperature and increases in pH and oxygen fugacity. With further dilution, Fe-Mn oxide rich siliceous sediments precipitated from cool and now oxidised solutions.
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The Rakkurijarvi prospect consists of a group of mineralized magnetite and lithic breccias within the ca. 2.05- to 1.90-Ga Proterozoic supracrustal sequence of the Kiruna district, northern Sweden. Potentially economic grades of Cu and Au, largely in the form of chalcopyrite and other sulfide assemblages, are hosted in brecciated magnetite and metavolcanic rocks. The extent of the mineralization is currently open, both downdip and along strike. The deposit was discovered through an integrated geophysical and geochemical program focused on iron oxide-copper-gold (IOCG)–style mineralization. It is hosted by brecciated greenschist facies metavolcanic rocks within and adjacent to an east-northeast–trending shear zone. The dominant characteristics of the deposit are consistent with the IOCG class and include magnetite and lithic breccias hosted in a metavolcanic sequence, with matrices of albite, actinolite, and calcite surrounded by halos of sodic (albitescapolite) and potassic (scapolite-K-feldspar-biotite) alteration. A distinctive accessory mineral assemblage includes apatite, titanite, and allanite. The paragenesis and textural evolution of the deposit includes early Narich alteration accompanying massive magnetite alteration. The Na-rich alteration is overprinted by potassic alteration (also associated with magnetite), although the paragenesis is complex and multiple generations of both sodic and potassic alteration are recognized. Alteration of lithic clasts to magnetite confirms a metasomatic origin, as opposed to an orthomagmatic origin, for the magnetite mineralization. Re-Os analyses of two separates of molybdenite intergrown with magnetite, interpreted as cogenetic with the sulfide assemblage, yield mineralization ages of 1853 ± 6 and 1862 ± 6 Ma. Reconnaissance bulk-rock chemistry of the host volcanic rocks is consistent with an intermediate volcanic
protolith, but much of the original character of the rocks is masked by albitization and incipient iron, sodic, and potassic alteration. The data also indicate significant element mobility during metasomatism and, in particular, the addition of Ti to the rock mass in biotite and as titanite. The compositions of secondary minerals are consistent with alteration and mineralization caused by highly saline fluids of relatively low F activity. The stable isotope characteristics of calcite, with δ18OSMOW ranging from 9.43 to 19.89 per mil and δ13CPDB ranging from –11.69 to +4.88 per mil, suggest that the fluids of the calcite and sulfide stage were derived from a magmatic
source but had interacted extensively with local sedimentary and volcanic rocks.
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Geology and Alteration Geochemistry of the Porphyry Cu-Au Deposit at Bajo de la Alumbrera, Argentina
This paper describes the igneous geology, alteration mineralogy, and geochemistry, as well as the ore mineralization history of Bajo de la Alumbrera, a world-class porphyry copper-gold deposit in northwestern Argentina. The deposit occurs in the K-rich calc-alkaline Farallon Negro Volcanic Complex, located 200 km east of the main Andean porphyry copper belt of Chile. The deposit consists of a composite stock of dacitic porphyry intrusions and extends into the surrounding andesitic volcanic rocks. Two of the earliest intrusions, now occupying the center of the intrusive complex, show intense hydrothermal alteration and mineralization. These two porphyries host the highest ore grades and were subsequently intruded by several weakly mineralized or barren porphyries.
Intense quartz-magnetite (±K feldspar) alteration pervades the first mineralized porphyry, whereas similar, but lower intensity alteration affected the second. Laterally, this alteration grades into potassic alteration (secondary K feldspar and biotite), which affects all porphyries except the latest dikes. Andesitic wall rocks within a hundred meters of the intrusions are dominated by dark hydrothermal biotite. An outer halo of propylitic alteration (epidote-chlorite-albite-calcite) extends up to 1 km into the andesites. Feldspar-destructive alteration (sericite + pyrite ± clay minerals ± gypsum) overprints the potassic and propylitic alteration in the volcanic rocks and all porphyries. The alteration is locally controlled by faults and late fractures and developed most pervasively upon the outer part of the potassic zone and toward the transition to propylitic alteration in the upper part of the deposit. Mass-balance calculations show small positive changes in net volume for the potassic alteration and minor volume decreases for the chlorite-epidote and feldspar-destructive alteration. The intensely quartz-magnetite veined sample of quartz-magnetite alteration indicates a total volume increase of 350 percent.
Copper and gold are intimately associated at all scales. They occur as composite gold sulfide grains (chalcopyrite, rarely bornite), show a close correlation of ore grades in sample assays, and have a closely overlapping distribution on the mine scale. A well-developed ore shell coincides with potassic alteration and partly with the distribution of intense quartz ± magnetite veins, even though the bulk of the ore minerals now occupy texturally late positions in vugs and fractures. A barren zone with intense quartz-magnetite ± potassic alteration and veins, but little or no sulfides, is interpreted as the focus of upflow of high-temperature ore fluids, prior to ore mineral saturation and coprecipitation of copper sulfides and gold in the ore shell.
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