The formation of late magmatic oxide ores

Bateman, Alan Mara

doi:10.2113/gsecongeo.46.4.404

Cited by 50 publications

(22 citation statements)

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“…Neutral buoyancy (Ryan, 1993) does not necessarily have anything to do with it. Certainly, the very iron-rich melts that are present are too dense (p = 2.7-2.9) to do much else but sink through any loose body of crystals or network of fractures that might open up and remain opened for any period of time, as they do, for example, in the formation of discordant iron-rich pegmatites in layered intrusions (Bateman, 1951;Scoon and Mitchell, 1994). Such liquids need a nearly impermeable mat on which to sit.…”

Section: Discussionmentioning

confidence: 99%

Melt Migration through High-Level Gabbroic Cumulates of the East Pacific Rise at Hess Deep: The Origin of Magma Lenses and the Deep Crustal Structure of Fast-Spreading Ridges

Natland¹,

Dick²

1996

Proceedings of the Ocean Drilling Program, 147 Scientific Results

108

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Gabbroic rocks recovered by drilling and submersible at Hess Deep, eastern equatorial Pacific, record processes of melt migration and channelized magma flow that bear on the origin and persistence of magma lenses and the development of lowercrustal structure at the East Pacific Rise. Rocks from ODP Hole 894G near the summit of the intrarift ridge at Hess Deep, and sampled by submersible from an uplifted marginal horst on the nearby northern slope of the rift, are mainly gabbronorites and olivine gabbronorites that crystallized very near the base of sheeted dikes. North Slope gabbros were sampled without interruption right to the base of sheeted dikes, and differ from Site 894 principally in that the sequence contains oxide-rich ferrodiorites and is cut locally by narrow tonalite dikelets. The highly fractionated rocks plausibly represent the frozen residues of a narrow and thin magma lens which geophysical evidence suggests once lay beneath the axial neovolcanic zone of the East Pacific Rise and its feeder dikes. Site 894 rocks represent the now-frozen immediate substrate, upon which a magma lens once rested, and through which it was supplied with melt.Bulk compositions and modal mineralogy (principally low abundances of oxide minerals) demonstrate that almost all the gabbroic rocks of Site 894, and North Slope gabbronorites deeper than about 200 m below the base of the dikes, are adcumulates with <7% (average 4.4%) of material crystallized from trapped intercumulus melt. Yet, in contrast to adcumulates in layered intrusions, none of these rocks crystallized in close proximity to a major magma body. Lithostratigraphic relationships at Site 894, and the presence of grain-size variations across sharp contacts in several of the North Slope dive samples, show that most crystallization and adcumulus crystal growth occurred in fracture networks or in narrow dike-like bodies averaging about 3 m, but ranging down to as little as 1 cm, in thickness. Expulsion of intercumulus melt was extremely efficient at all of these scales, and-based on evidence from drilling nearby at Site 895 at the crust-mantle transition-evidently was pervasive throughout the entire mass of gabbros down to the mantle. Melt expulsion was not only thorough, but occurred almost immediately, providing a nearly impermeable base into which dense, iron-rich magmas in the thin melt lens could not sink.No gabbroic rock from either Site 894 or the North Slope is layered, although some of the rocks show preferred orientation of plagioclases. There are no monomineralic adcumulates; all are olivine-plagioclase-clinopyroxene or plagioclase-clinopyroxene adcumulates and mesocumulates which crystallized on cotectics. Strongly zoned plagioclases, some enclosed as broken crystals in clinopyroxene or orthopyroxene oikocrysts, and unusually high Cr-contents of clinopyroxenes, attest to initial stages of crystallization from primitive to moderately fractionated basaltic magmas, followed by post-cumulus migration of highly fractionated (ferrobasaltic to ferroan...

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Section: Discussionmentioning

confidence: 99%

Melt Migration through High-Level Gabbroic Cumulates of the East Pacific Rise at Hess Deep: The Origin of Magma Lenses and the Deep Crustal Structure of Fast-Spreading Ridges

Natland¹,

Dick²

1996

Proceedings of the Ocean Drilling Program, 147 Scientific Results

108

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“…Processes responsible for the formation of Ti-V magnetite ore bodies in gabbroic rocks include liquid immiscibility (Lister [30]; Reynolds [31]; Zhou [32]), fractional crystallization of slowly cooled magma with dominant plagioclase buoyancy effect (Charlier [33]), residual liquid injection (Das and Mukherjee [34]), change in oxygen content (Klemm [35]) and change in pressure (Cawthorn and Ashwal [36]). Bateman [37] suggested the concept of late gravitative liquid enrichment for the formation of magnetite ore bodies associated with gabbroic rocks. Crystallization of Ti-V magnetite ores from basaltic magma depends on ferrous/ferric ratio of the liquid, which is a function of fO2, temperature and water content of the magma (Reynolds [38]).…”

Section: Genesis Of Ore Bodiesmentioning

confidence: 99%

Titaniferous Magnetite Deposits Associated with Archean Greenstone Belt in the East Indian Sheild

Mondal¹,

Baidya²

2015

EARTH

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Abstract:In the East Indian Shield, occurrence of titaniferous magnetite deposits associated with the Archean Greenstone belt occur in Kumhardubi, Betjharan and Nuasahi areas of Odisha and Dublabera area of Jharkhand. The ore bodies comprise lenses, veins, bands and patches within gabbroic rocks. Petrogenetic studies have revealed the primary and secondary mineral constituents of the ores such as titanomagnetite, ilmenite, hematite, spinel, cobaltite, goethite, martite, rutile and silicate gangue minerals. Various crystallographic intergrowths are resulted from exsolution & oxidation at different temperatures during cooling of the sub-solidus magma. Chemical analyses show that the ore contains 10.35 -17.68 wt.% TiO2, 0.148 -0.227 wt.% V2O3 and 32.75 -67.39 wt.% Fe2O3. Different geochemical composition diagrams confirm their tholeiitic origin. The formation of the massive ore bodies is referred to late magmatic crystallization from tholeiitic magma followed by Fe-Ti enriched residual liquid injection within the host rocks. Syn to late formation of the magnetite ores along with gabbroanorthositic intrusive with respect to the Archean Greenstone Belt of East Indian Shield is suggested.

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“…The older models acknowledge the existence of genetic connection with the host complexes. They either invoke separation and accumulation of Fe-Ti crystals to form layers or postulate existence of Fe-Ti oxide liquids from which the ores crystallized [45,46,49,[62][63][64][65]. The newer models invoke mechanisms whereby episodic increase in oxygen fugacity triggers the crystallization of enough quantities of Ti-Fe oxides for the development of ore rich layers [47,54,55,58,59,[66][67][68].…”

Section: An Overview Of Petrogenetic Evolution Of Cmuc and The Associmentioning

confidence: 99%

Mineralogy, geochemistry and petrogenesis of the V-Ti-bearing and chromiferous magnetite deposits hosted by Neoarchaean Channagiri Mafic-Ultramafic Complex, Western Dharwar Craton, India: Implications for emplacement in differentiated pulses

et al. 2014

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The Channagiri Mafic-Ultramafic Complex occupies lowermost section of the Neoarchaean Shimoga supracrustal group in the Western Dharwar Craton. It is a segmented body occupying the interdomal troughs of granitoids. The magnetite deposits occur in the northeastern portion; typically occupying the interface zone between gabbro and anorthositic. Mineralogically, the deposits are simple with abundant magnetite and ilmenite. Hogbomite is a consistent minor mineral. Magnetites are typically vanadiferous (0.7–1.25% V2O5). Ilmenite consistently analyses more MgO and MnO than coexisting magnetite. Chlorite, almost the only silicate present; lies in the range of ripidolite, corundophilite and sheridanite. The chromiferous suit occupying eastern side of Hanumalapur block (HPB) contains Fe-Cr-oxide analysing 37.8–11.9% Cr2O3 and 40.5–80% FeOt. In these too, chlorite, typically chromiferous (0.6–1.2% Cr2O3), is the most dominant silicate mineral. Geochemistry of V-Ti-magnetite is dominated by Fe, Ti and V with Al, Si, Mg and Mn contributing most of the remaining. Cr, Ni, Zn, Co, Cu, Ga and Sc dominate trace element geochemistry. The Cr-magnetite is high in Cr2O3 and PGE. Two separate cycles of mafic magmatism are distinguished in the CMUC. The first phase of first cycle, viz., melagabbro-gabbro, emplaced in the southeastern portion, is devoid of magnetite deposits. The second phase, an evolved ferrogabbroic magma emplaced in differentiated pulses, occupying northeastern portion of the complex, consists of melagabbro→gabbro-anorthosite→V-Ti magnetite→ferrogabbro sequence. Increase in oxygen fugacity facilitated deposition of V-Ti magnetite from ferrogabbroic magma pulse emplaced in late stages. The second cycle of chromiferous PGE mineralized suite comprises fine-grained ultramafite→alternation of pyroxinite-picrite→Crmagnetite sequence formed from fractionation of ferropicritic magma. HPB also includes >65m thick sill-like dioritic phase at the base of the ferriferous suit and a sinuous band of coarse-grained ultramafite enclosed within the chromiferous suit; both unrelated to the two mafic magmatic cycles.

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The formation of late magmatic oxide ores

Cited by 50 publications

References 0 publications

Melt Migration through High-Level Gabbroic Cumulates of the East Pacific Rise at Hess Deep: The Origin of Magma Lenses and the Deep Crustal Structure of Fast-Spreading Ridges

Melt Migration through High-Level Gabbroic Cumulates of the East Pacific Rise at Hess Deep: The Origin of Magma Lenses and the Deep Crustal Structure of Fast-Spreading Ridges

Titaniferous Magnetite Deposits Associated with Archean Greenstone Belt in the East Indian Sheild

Mineralogy, geochemistry and petrogenesis of the V-Ti-bearing and chromiferous magnetite deposits hosted by Neoarchaean Channagiri Mafic-Ultramafic Complex, Western Dharwar Craton, India: Implications for emplacement in differentiated pulses

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