Chromite

Abstract: Magnetite and hematite are common minerals in a range of mineral deposit types. These minerals form partial to complete solid solutions with magnetite, chromite, and spinel series, and ulvospinel as a result of divalent, trivalent, and tetravalent cation substitutions. Electron microprobe analyses of minor and trace elements in magnetite and hematite from a range of mineral deposit types (iron oxide-copper-gold (IOCG), Kiruna apatite–magnetite, banded iron formation (BIF), porphyry Cu, Fe-Cu skarn, Fe-Ti, V, Cr, Ni-Cu-PGE, Cu-Zn-Pb volcanogenic massive sulfide (VMS) and Archean Au-Cu porphyry and Opemiska Cu veins) show compositional differences that can be related to deposit types, and are used to construct discriminant diagrams that separate different styles of mineralization. The Ni + Cr vs. Si + Mg diagram can be used to isolate Ni-Cu-PGE, and Cr deposits from other deposit types. Similarly, the Al/(Zn + Ca) vs. Cu/(Si + Ca) diagram can be used to separate Cu-Zn-Pb VMS deposits from other deposit types. Samples plotting outside the Ni-Cu-PGE and Cu-Zn-Pb VMS fields are discriminated using the Ni/(Cr + Mn) vs. Ti + V or Ca + Al + Mn vs. Ti + V diagrams that discriminate for IOCG, Kiruna, porphyry Cu, BIF, skarn, Fe-Ti, and V deposits.

Abstract: The layered mafic rocks of the Bushveld Complex are derived from three magmatic lineages - a lower part comprising the Lower and Critical Zones which crystallized from high-Mg and-Si parent liquids, an isotopically distinct Lower Main Zone derived from more evolved aluminous tholeiitic liquids, and a succession above the Pyroxenite Marker, which includes the entire Upper Zone, stemming from the mixing of residua of these earlier liquids with a final major injection of tholeiitic liquid. The varied mineralogy and thickness of the Marginal Zone indicate that it is not a quenched magma, but represents variable cumulus enrichment into several chemically different magmas, only some of which may be representative of magma producing the layered rocks. The complex was emplaced by repeated injections of magma ranging in volume from small to large. Evidence supporting the concept of multiple injections lies in the presence of distinct breaks in initial Sr-isotopic ratio, textures indicating partial resorption of earlier crystallizing phases, abrupt reversals of fractionation trends defining saw-tooth profiles through successive units, and protracted reversals in mineral compositions through hundreds of metres of section that culminate in primitive olivine-rich cumulates in the Lower and Critical Zones. The small influxes yielded localized partial cyclic units Studies of the Critical Zone in the Western limb, for which more comprehensive data are available than elsewhere, reveal a gradation along ca. 200 km of strike from a more primitive proximal facies in the northwest to a more evolved distal facies in the southeast. This concept helps to explain major problems in the Cr budget of the entire limb, the greater proportion of chromite- and olivine-rich cumulates in the proximal facies, and the more feldspathic character of the distal sequence. In the Eastern limb changes in relative spacing between, and thicknesses of, chromitite layers from north to south suggest a similar process, but the transition is abrupt rather than gradational. No single process can account for all forms of layering. Despite certain limitations, largescale magma mixing is the most plausible mechanism to bring the liquid composition into the primary phase volume of chromite to produce chromitite layers. By contrast, the trace-element chemistry of magnetitite layers suggests that they were derived from relatively thin liquid layers, and that magma addition and mixing did not occur. The origin of the incomplete cycles which may range from pyroxene- and/or olivine-rich to feldspar-rich layers of the Lower, Critical, and Main Zones appears to be intimately bound up with the emplacement of fresh magma batches. These mixed with partially crystallized residual liquids to produce cumulates bearing incompletely resorbed plagioclase inclusions, highly variable Sr-isotopic ratios, and non-cotectic proportions of phases. Chemical fingerprinting of pyroxenes in Critical Zone rocks shows that gravitational sorting must have played some part in the generation of pyroxenites and in yielding non-cotectic norites. Prominent layering near the top of the Main Zone in the Eastern limb cannot be explained by injection of fresh magmas, oscillatory nucleation or crystal settling, leaving the action of density currents as the most probable mechanism. Whereas layering is well developed in parts of the Upper Zone, the cyclicity of the Critical Zone is absent. The existence of an extremely thick liquid column in the chamber during the later stages is indicated by the 2500 m-thick, isotopically homogeneous sequence above the Pyroxenite Marker. Layering and reversals in mineral composition within this interval may have resulted from the breakdown of stratified liquid layers or convective overturn, rather than addition of magma.

Abstract: A mechanism of origin for chromitite layers in stratiform intrusions is described, based on the same physicochemical principles as an earlier explanation but formulated in a geologically very different way. By this new mechanism, the concentrated chromite is precipitated from chromite-saturated picritic tholeiite liquid when this liquid is blended with earlier liquid of the same type that has differentiated to relatively siliceous compositions.

Abstract: hydrocarbons, carbon dioxide and nitrogen have been determined in Chromites forming giant orebodies in the southern part of the Early inclusion-rich samples. (3) In the northern and western part of the Palaeozoic ophiolite sequence of the Kempirsai Massif (Kazakhstan, Kempirsai massif, complex silicate–oxide assemblages formed in Urals) contain a large number of inclusions, i.e. silicates, sulphides, small orebodies of orbicular Al-rich chrome spinel. Chlorite, amalloys, arsenides, and fluids. The chromite orebodies are surrounded phibole, hydrogarnet, sphene, manganoan ilmenite and Ca–Ti oxide by dunite envelopes of variable thickness, which show transitional are documented in addition to Ni sulphides and rare PGM. The boundaries to harzburgite host rocks. The composition of ore-forming formation of chromitite in the Kempirsai Massif is explained in chromites in depleted mantle rocks of the southern part of the massif terms of a multi-stage process involving mantle fluids. Low-Cr, (Main Ore Field) is rather uniform, showing high cr-number high-Al spinel present in small orebodies in the northern and western [100Cr/(Cr+Al), 78–84] and mg-number [100Mg/ part of the massif formed from mid-ocean ridge basalt (MORB)(Mg+Fe), 51–85] values. Smaller bodies of Al-rich spinel in type melts extracted from fertile mantle in an extensional tectonic the northern and western part of the massif (Batamshinsk) have setting. The large orebodies and the amphibole–chromite veins in variable cr-number (38–60) and mg-number (50–88) values. the southern part formed later from interaction of hydrous, secondThree textural types of inclusions in chromite are distinguished: (1) stage high-Mg melts and fluids with depleted mantle in a convergent In Main Ore Field chromites, primary silicate inclusions generally tectonic setting. Metasomatic alteration of the mantle wedge above have high mg-number (>95), Cr and Ni, and are dominated by subducted crust by fluids played an important role in generating pargasitic amphibole, forsterite, diopside, enstatite and Na-phlosecond-stage melts and in releasing metals. gopite. Chromite formed over a temperature range from ~1200° to <1000°C at oxygen fugacities 1–2 log units above the fayalite– magnetite–quartz (FMQ) buffer. A diversity of primary and secondary platinum-group minerals (PGM) is described from the chromitites, including alloys, sulphides, sulpharsenides and arsenides