Alteration mineralogy,mineral chemistry and genesis of Zhibo iron deposit in western Tianshan Mountains,Xinjiang
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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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The Hongyuntan iron deposit is hosted in pyroclastic rocks of the Lower Carboniferous Yamansu Formation.The ore bodies occur as layers,stratoid bodies or lenses.The principal ore mineral is magnetite,together with minor maghemite,specularite,pyrite and trace chalcopyrite.The gangue minerals include garnet,diopside,actinolite,chlorite,tremolite,epidote,biotite,albite and quartz.The ore structures are mainly of massive and disseminated forms,with occasional banded or veined forms.The ore textures are of subhedral-anhedral granular and metasomatic types.The wall rock alteration shows symmetrical zoning,and the alteration colors change from dark to light from ore bodies outwards.On the basis of observed mineral assemblages and ore fabrics,two periods of ore deposition were recognized,i.e.,skarn period and hydrothermal ore-forming period,which could be further subdivided into four metallogenic stages,namely skarn stage,retrograde alteration stage(main ore-forming stage),early hydrothermal stage and quartz-sulfide stage.Electron microprobe analyses show that the end member of garnet is mainly andradite-grossularite.The composition of pyroxene is mainly diopside-asteroite.The amphiboles is composed mainly of actinolite and tremolite with minor magnesiohornblende.The composition of these skarn minerals suggests that skarn in the Hongyuntan iron deposit is calcic skarn,belonging to metasomatic skarn.The characteristics of main and trace elements suggest that the formation of magnetite was closely related to the skarn.In combination with geological characteristics,the authors suggest that the skarn might have resulted from interaction between Ca-rich pyroclastic and Fe-rich magmatic hydrothermal fluid which was transported along the fault system.The formation of magnetite was hence related to the regressive metamorphism of the skarn.
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The Zankan iron deposit in the West Kunlun metallogenic belt is a newly discovered super large deposit. The Paleoproterozoic Bulunkuole group metamorphic rocks are widely exposed in the ore district,where orebodies mainly occur in hornblende plagioclase schist and biotite quartz schist,locally formed in the contact zone of felsite and biotite quartz schist. The iron deposit consists of seven ore bodies,wherein the No. 1 and No. 3 orebodies are major ones. According to the ore textures and mineral paragenesis characteristics,the formation of the Zankan iron deposit can be divided into three metallogenic epochs: the early sedimentary period,middle metamorphic period and last magmatic hydrothermal period. The magmatic hydrothermal period can be further divided into the skarn stage,hydrothermal replacementstage and sulfide stage. The magnetite with a fine anhedral crystal in the early sedimentary period is mainly distributed in quartz grains in banded ores. This type of magnetite has low TFeO,MgO,MnO,and high TiO2,Al2O3. Compared to the early period magnetite,the middle metamorphic period magnetite has an allotriomorphic granular blastic texture,in banded ores,with higher TFeO,MgO,MnO and lower TiO2,Al2O3. The magnetite of skarn stage of the late magmatic hydrothermal period has an euhedral granular structure and is enriched in TFeO,MgO,MnO,while poor in TiO2,Al2O3. The magnetite with a hypidiomorphic-euhedral granular texture and replacement remnant texture in the hydrothermal alteration stage are mainly distributed in disseminated ores,where the content of TFeO,Al2O3,TiO2,and MnO in this type of magnetite varies in a large range. A comparative analysis suggests that the Zankan iron deposit belongs to a sedimentary metamorphic iron deposit,and has experienced metasomatic alteration by late magmatic hydrothermal processes.
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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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