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 Faleme iron district in the Kedougou-Kenieba inlier of the Paleoproterozoic Birimian Supergroup of West Africa consists of nine major and 19 minor orebodies, distributed in a belt 65 km long and 15 km wide. Two major exoskarn orebodies have total reserves of 320 million metric tons (Mt) magnetite ore with 42 percent Fe, and seven major supergene-enriched orebodies, which overlie endoskarn, have reserves of 310 Mt with 59 percent Fe. Previous workers advocated an (exhalative)-sedimentary origin for the primary ore. In this paper, however, we present strong evidence for contact-metasomatic skarn mineralization associated with microdiorite intrusions, with an emphasis on the Karakaene-Ndi and Goto deposits. The ore deposits of the Faleme district are the only major Precambrian magnetite skarn deposits worldwide that have not been subjected to postore metamorphism. The deposits of the Faleme district have many similarities to Phanerozoic magnetite skarn deposits that are associated with dioritic intrusions and also lack postore metamorphism. The endoskarn of the Karakaene-Ndi deposit is hosted by microdiorite with pervasive albitization. Garnet (And 56 –100 Grs 0 –43 Alm 0 –2 Sps 0 –1 Pyr 0 –1 ) and clinopyroxene (Di 58 –100 Hed 2 –42 ) are early phases. Individual grains of garnet and clinopyroxene commonly are inhomogeneous, with Fe being concentrated at the margin of the grains rather than in the core. Fe-rich zones in clinopyroxene are also enriched in Na. Biotite and magnetite precipitated relatively late, followed by pyrite (2 vol %) ± chalcopyrite ± pyrrhotite ± iss. Aqueous-carbonic fluid inclusions in quartz from one sample have CO 2 concentrations of 5 to 14 mol percent and indicate pressures of 300 to 900 bars and temperatures of 281° to 373°C at total homogenization. The magnesian exoskarn of the Goto deposit is hosted by dolomitic and calcitic marble, which locally contains graphite. Magnetite is associated with prograde clinopyroxene (Di 89 –100 Hed 0 –11 ) and phlogopite-rich biotite (Mg/(Mg + Fe)~0.89) as well as retrograde serpentine. Garnet is scarce. Sulfur concentrations average 1 wt percent. Pyrrhotite and subordinate pyrite ± chalcopyrite ± arsenopyrite ± pentlandite ± cobaltite postdate most of the magnetite. The exoskarn formed under more reducing conditions and at lower temperatures than the endoskarn. Magnetite in the exoskarn has trace element concentrations similar to those of magnetite in the endoskarn but is distinguished from igneous magnetite in granodiorite and magnetite in andesite (possibly of igneous origin) by low concentrations of Cr 2 O 3 and V 2 O 5 . Microdiorite is the most likely source of iron although no igneous magnetite in microdiorite could be identified. Microdiorite with only moderate hydrothermal alteration has lower iron concentrations (1.6–4.5% Fe 2 O 3(total) ) than unaltered granodiorite and weakly altered andesite in the area (avg ~7% Fe 2 O 3(total) ). Iron was probably leached from the moderately altered microdiorite and transferred into zones of intense alteration in microdiorite and host-rock marble. The low degree of hydrothermal alteration of granodiorite and small areal extent of andesite argue against a granodioritic or andesitic source of iron. The supergene ore is derived from endoskarn with about 30 percent Fe and consists of hematite and hydrous iron oxides. The largest orebodies have a thickness of 100 m and are located in the central part of hills that rise 250 m above the surrounding peneplain; the bottom of the enrichment zone usually is centered deep below the top of the hills. The absence of supergene enrichment in exoskarn orebodies probably is due to unfavorable structural conditions. Supergene enrichment of endoskarn was probably not exclusively by residual enrichment, in which Ca, Mg, Na, and Si were removed from the orebody, but also involved addition of Fe from descending solutions to the site of enrichment. Exploration for new supergene iron ore should be focused outside the exoskarn belt between Goto and Safa, as the deposits in this area are unlikely to have experienced major supergene enrichment. The Faleme district has some similarities to the geologic setting of iron oxide-copper-gold districts in which the deposits are hosted by dioritic rocks and may be representative of an iron oxide-rich end member of the iron oxide-copper-gold class. However, only about 3 t of gold have been mined from alluvial deposits in the area and the district is relatively poor in copper.
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The polymetallic Madem Lakkos sulfide deposit in northern Greece is hosted within marble of the Mesozoic (?) Kerdylia Formation, a high-grade metamorphic complex composed of migmatitic biotite gneiss interlayered with marble, hornblende gneiss, and amphibolite. The Kerdylia Formation is invaded by a variety of foliated and nonfoliated intermediate to felsic intrusions of Tertiary age. The Madem Lakkos deposit is long-believed to have formed from a single epigenetic hydrothermal replacement event related to Tertiary magmatism, but this research has recognized the presence of three different and distinct ore types in the deposit that resulted from a much longer and more complex genetic history.Based on ore mineralogy, textures, and geochemistry, the Madem Lakkos ores can be characterized as (1) massive sulfide ore, (2) disseminated sulfide ore, and (3) skarn ore. The massive pyrite-sphalerite-galena ore exhibits abundant and well-developed metamorphic structures and textures that indicate the ore has been metamorphosed to upper amphibolite grade, at temperatures of at least 600 degrees C, together with its marble and gneissic host rocks. These textures include foliated-lineated galena and sphalerite, slip planes and deformation twinning in galena and sphalerite, and granoblastic annealing-recrystallization features with the development of 120 degrees triple-point junctions in galena, sphalerite, and pyrite. Despite its metamorphism, this ore preserves a generally stratiform nature with sharp, unaltered host-rock contacts, a regional and stratigraphic association with chemical and possible evaporitic metasedimentary rocks, compositional layering, and metal zoning that are consistent with formation as a sedimentary massive sulfide deposit.Disseminated sulfide ore, the most abundant type in the deposit, consists of complex veins and irregular manto-type impregnations in altered marble that are composed of pyrite, sphalerite, tennantite, chalcopyrite, arsenopyrite, galena, seligmannite, boulangerite, and minor amounts of a wide variety of additional sulfominerals in a quartz-sericite-manganiferous carbonate gangue. Disseminated sulfide ore transects and has reacted with the earlier massive sulfide ore and does not exhibit evidence of metamorphism. Euhedral zoned crystals with mineral and fluid inclusions, open-space fillings, and complex textural relationships are characteristic of this ore type and indicate that it formed through the replacement of marble by reaction with hydrothermal solutions. Disseminated sulfide ore is enriched in Cu, As, Mn, Sb, and Bi in comparison with the massive sulfide ore.Skarn ore contains pyrite, chalcopyrite, scheelite, and minor amounts sphalerite, galena, and Pb-Bi sulfominerals in a calc-silicate assemblage of gangue minerals that includes andradite-grossularite garnet, diopside, calcite, quartz, epidote, and minor chlorite, actinolite, and magnetite. Textures similar to those found in the disseminated sulfide ore and an absence of metamorphic features are characteristic of the skarn ore. Highly saline fluid inclusions in quartz from the skarn ore suggest that high-temperature, low-pressure porphyry copper-type magmatic fluids were involved in generation of this ore. Skarn ore does not exhibit a spatial relationship to igneous rocks in the mine but may be related to porphyritic quartz diorite stocks a few kilometers to the south that have halos of propylitic and phyllic alteration and porphyry copper-type mineralization.The different ore types are characterized by a very similar lead isotope composition ( 206 pb/ 204 pb = 18.78-18.82, 207 pb/ 204 pb = 15.67, 208 Pb/ 204 pb = 38.88-38.92), which lies within the restricted field of igneous rocks from northern Greece. Although this resemblance between ore and igneous rock lead has been used to support a magmatic origin for the Madem Lakkos and related sulfide deposits, the uniform isotopic composition of all lead in this tectonically active region weakens this argument. If, as is proposed, the massive sulfide ore was initially deposited as a synsedimentary body within the Kerdylia Formation, the modern model age of the lead strongly suggests that mineralization took place only a short time before the rocks were metamorphosed.The superposition of multiple ore types having different mineralogic and chemical compositions, textures, metamorphic grades, and apparent ages indicates a complex, multistage, polygenetic origin for the Madem Lakkos deposit. An interpretation consistent with this evidence is that synsedimentary massive sulfide ore was deposited as a stratiform body within a sequence of probably Mesozoic shallow-water platform carbonate and clastic-volcaniclastic sediments, possible evaporitic sediments, and lesser amounts of volcanic rocks. This ore and its host rocks were metamorphosed to upper amphibolite grade during Cretaceous-Tertiary regional metamorphism.Coregional, post-tectonic intrusions generated heat and magmatic fluids that produced skarn and skarn ore by replacement of marble at temperatures above 360 degrees C. A continuing but cooling convective hydrothermal system mixed magmatic fluids with meteoric water. These hydrothermal fluids permeated marble and massive sulfide ore peripheral to the skarn ore, reacting with them and extensively altering the marble to form disseminated sulfide ore. Massive sulfide ore and related chemical sedimentary rocks were partly dissolved by and incorporated into the hydrothermal solutions, thereby contributing Pb, Zn, Fe, Mn, Ag, Au, and minor amounts of other constituents to the hydrothermal system. Fe, Cu, W, As, Sb, and Bi were probably magmatic contributions.
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