Ore deposits

Ore deposits

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  ORE DEPOSITS ORE DEPOSITSRELATED TO MAFIC, ULTRAMAFIC, INTERMEDIATE TO FELSIC IGNEOUS ROCKS: Kunwar Shruten Chauhan Department of Geology University of Rajasthan Jaipur
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  PAGE 1 ORE DEPOSITSRELATED TO MAFIC AND ULTRAMAFIC ROCKS . Ore deposits formed during fractional crystallization of magmas are designated as magmatic segregation deposits (also known as orthomagmatic deposits). These deposits are the direct crystallization products of magma, usually form in the magma chamber and thus constitute deep- seated intrusive bodies. However differentiated or immiscible melts and crystal mushes can be driven into the walls or roofs of the magma chamber to form ore bodies in the form of dikes, sills, and even extrusive flows. A magmatic segregation deposit may constitute an entire intrusive body, form a single compositional layer within the igneous rock body, or occur as disseminated minerals. Mode of occurrence:A magmatic segregation deposit may occur in any of the following forms: (i) constitute an entire intrusive body (ii) form a single compositional layer within the igneous rock body (iii) may be defined by the presence of disseminated minerals. Orthomagmatic deposits of Cr-Pt-Ti-Fe and Cu-Ni-Fe (-Pt) associated with basic and ultrabasic rocks emplaced in cratonic and active orogenic settings. The worlds major Orthomagmatic deposits are associated with (1) Layered igneous intrusive (Bushveld, Great Dyke and Sudbury) a) Bushveld layered complex contains about 75% of the world’s chromium reserves. The Bushveld complex is large reserves of platinoid ores (composed of Pt+Pd+Rh+Ru+Ir+Os) and the average ore grade is 8.27ppm. Another significant resource of the Bushveld complex is the layered magnetite – rich units associated with titanium and vanadium. b) The Great Dyke of Zimbabwe is a layered igneous intrusion, 532 km long, 5 to 9.5 km wide and consists of chromite layers that occur along the entire length and the individual layers extend across the entire width. c) The Sudbury basin is Shock metamorphic features favours a meteorite impact hypothesis for the eruption of magma. Sudbury Igneous complex (1849 Ma) consists of a lower zone of augite - norite, a thin middle zone of quartz–gabbro and an upper zone of granophyres and these three rock units are reported to be comagamatic. In steep to vertical, radial and concentric dykes that appear to penetrate downward into the foot wall from the base of the complex and referred to as offsets,the inclusion– rich sulphide bearing rock is quartz diorite. Most of the ore bodies occur in the sublayer (inclusion- and sulphide- rich norite and gabbro) whose magma was rich in sulfides with inclusions and peridotite, pyroxenite and gabbro. The sublayer and offsets are at present the world’s richest source of nickel as well as an important source of copper, cobalt, iron, platinum and 11 other elements. (2) Intrusions related to flood basalts of cratonic areas (Norilsk–Talnakh)
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  PAGE 2 Noril’sk deposit, Russia, occurs in the differentiated layered, dominantly gabbroic intrusion. The copper-nickel sulfides form breccia and disseminated and massive ores at the base of the intrusion and vein ore bodies developed in the footwall rocks and the basal portion of the intrusion. (3) Bodies emplaced in active orogenic areas (Eastern Gold Field Province) Eastern Goldfields province, Yilgran craton, Western Australia, is bestowed with several occurrences of orthomagmatic copper-nickel sulfide deposits associated with komatiitic suite of lava flows and related shallow dyke-like or sill-like dunitic intrusions. Assosiated rocks; 1) KIMBERLITES AND LAMPROITES About 20% of the world production of diamonds is derived from kimberlites and lamproites and the rest is extracted from beach and alluvial placers deposits containing diamonds.  Kimberlites and lamproites are carrot shaped and are generally regarded as having been intruded upwards into cratonic areas through a series of deep–seated tension fractures,often in terrains of regional doming and rifting. 2) CARBONATITES  Carbonatites are generally found in: (1) stable cratonic regions along major faults and fault intersections and (2) rift valleys.  Carbonatites are exploited for phosphorus (from apatite), magnetite, niobium (from pyrochlore), zirconia, Rare earth elements (from monazite and bastanasite), barite, strontianite and vermiculite. Only one carbonatite body (Palabora complex, South Africa) is known for large reserves of copper ore. 3) ANORTHOSITES  Anorthosite massifs are essentially Proterozoic in age and confined to terrains of anorogenic intercontinental rifting or incipient rifting tectonic setting in a broad belt across North America, Britain and Scandinavia.  Anorthosite massifs are divisible into two groups (Herz 1976) based on their plagioclase and oxide compositions; they are: (1) Labradorite anorthosite massifs (e.g., Michigamau anorthosite, Labrador; Duluth Gabbro complex, Minnesota), and (2) Andesine anorthosite massifs (e.g., anorthosite massifs of Adirondack Mountains in New York; Allard Lake region in Quebec,Canada and Rosland in Virginia, USA).  Anorthosite massifs are the major source of titanium minerals (ilmenite and rutile). ORE DEPOSITS ASSOCIATED WITH INTERMEDIATE TO FELSIC IGNEOUS ROCKS:
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  PAGE 3 Orthomagmatic depositsassociated with felsic volcanism Felsic magmatic melts are rarely in a position to generate significant Orthomagmatic deposits. But iron ore deposits, consisting magnetite ± apatite encountered at severalplaces are considered as of magmatic segregation origin. These deposits occur both as lava flows and dyke-like bodies Pegmatites and their mineral deposits Pegmatites are coarse-grained igneous or metamorphic rocks, generally of granitic composition. Pegmatite developed at higher structural levels and spatially related to intrusive late tectonic granitic plutons, are marked by minerals with volatile components (OH,F, B) and a whole range of accessory minerals containing rare lithophile elements, including Be, Li, Sn, W, Rb, Cs, Nb, Ta, REE and U. Pegmatites are mined for: (a) Sn, Nb-Ta,REE, Y, Zr, Be and U ores, (b) Gem quality beryl, Chrysoberyl, topaz and tourmaline; optical grade fluorite crystals; piezoelectric quartz crystals, (c) feldspar required for ceramic industry (d) Li-minerals (spodumene, petalite, lepidolite) for the glass industry and manufacture of Licompounds, (e) book-mica (muscovite) for the electrical industry and electronic tubes. Greisen deposits Greisens are granoblastic aggregates of quartz and muscovite (or lepidolite) with accessory amounts of topaz, tourmaline and fluorite formed by post-magmatic metasomatic alteration of granite. Greisens are usually developed at the upper contacts of granitic intrusions and mineralization occurs as large irregular or sheet-like bodies immediately beneath the upper contact of late stage Greisen deposits are mined for tin, tungsten and beryllium. In future, greisens may constitute important sources of Be, Nb-Ta, REE, Y and other elements. Skarn deposits Skarn deposits have been referred to variously in literature as (a) hydrothermal metamorphic, (b) igneous metamorphic, (c) contact metamorphic and (d) pyrometasomatic deposits. They are developed at the contact of intrusive plutons and carbonate country rocks. The latter are converted to marble, calc-silicate hornfelses and/or skarns by contact metamorphic effects. The majority of the world’s economic skarn deposits occur in calcic exoskarns. Skarn deposits are mined for Cu, Fe, W, Pb-Zn, Mo, tin etc; these deposits are generally smaller. Most copper skarn deposits are associated with calc-alkaline granodiorite to monzogranite stocks emplaced in continental margin arcs. A small number of copper skarns occur in oceanic island arcs associated with quartz diorite to monzonite plutons. Giant skarn type iron deposits at Sarbai in Turgai iron ore province, Kazakhstan are associated with diorite and contain 715 Mt of iron ore with 46% Fe. Iron skarn deposit at Iron Springs, Utah (USA) is an ideal example of Skarn development at the boundary between intrusive and limestone.
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  PAGE 4 Bulk ofthe skarn tungsten (scheelite) comes form a few relatively large deposits, Dolomite hosted skarn deposits of talc are encountered in France (Trimouns Mine) and Austria. Minor amount of graphite production comes from skarns from Skaland Mine, Norway. ‡ Sn- and Mo- skarns are typically associated with high-silica (~75% SiO2) well differentiated plutons, while Au- and Fe- skarns relate with relatively less-evolved, low silica (62% SiO2), high – iron plutons. Cu-Zn-and W- skarns are associated with plutons having silica content ranging from 63-69%. Skarn deposits are associated with magmatic rocks emplaced under diversified tectonic settings; Oceanic back arc basin, subduction zone beneath continents and post-subduction or continental rifting environment. Porphyry deposit The term “Porphyry system” relates to a composite (occasionally single), mediumto silicic igneous intrusive complex wherein at least one member displays porphyritic texture, with ~25vol% of phenocrysts of alkali feldspar and/or quartz. Three main types of “Porphyry” ores are recognised: 1) Porphyry copper with two sub-type Cu-Mo and Cu-Au (these two types of deposits represent the Lowell-Guilbert model and Diorite model respectively) 2) Porphyry molybdenum which is subdivided into the “Climax-type” with Sn and W and “Quartz-Monzonite type” (Endoco-type) which grades into Cu(Mo) porphyry with decreasing Mo/Cu ratio. Porphyry-tin, which contains minor/trace amounts of W, Mo, and Bi. Major concentrations of porphyry Cu and Mo deposits are confined to Phanerozoic island arc settings and Andean type continental margins,Porphyry Cu and Moores are also known from Palaeozoic fold belts. Porphyry Mo deposits are mostly of Mesozoic and Tertiary ages. Porphyry Sn deposits are mostly of Tertiary age ; Sn – granites of Asian countries are of Mesozoic age.