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1.
The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO2, MgO, Al2O3, FeO and Cr2O3 due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.  相似文献   

2.
The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO2, MgO, Al2O3, FeO and Cr2O3 due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.  相似文献   

3.
Platinum-group element (PGE) and gold inclusions are usually present in peridotites and chromitite deposits associated with ophiolites. Here, we present the first detailed study of the mineralogy of precious metals in ultramafic rocks hosted in the Paleozoic Coastal Accretionary Complex of Central Chile. In these ultramafic rocks the mineralization of precious metals is associated with small meter-size pods and veins of massive chromitite hosted in serpentinite-filled shear zones. Crystallographic orientation maps of single chromite grains, obtained using the Electron-Backscattered Secondary Diffraction technique, allow us to identify two types of chromite in the precious-metal bearing chromitites: (1) Type A chromite, characterized by an average misorientation per grain of ≤ 2° and chemically homogeneous cores surrounded by a porous rim with abundant inclusions of chlorite, and (2) Type B chromite, which exhibits higher degrees of misorientation (2–8°) and porosity, and abundant silicate inclusions, but a relatively homogeneous chemical composition. In situ analyses using EMPA and LA-ICP-MS for major, minor and trace elements indicate that composition of the magmatic chromite is only preserved in the cores of Type A chromite grains. Core to rim chemical trends in these Type A chromites are characterized by a progressive increase of the Cr# with a decrease of the Mg#, loss of Al and addition of Fe2 + in the porous rim. The observed changes in the microstructure and chemistry of chromite are associated with the infiltration of external fluids through shear zones filled with antigorite (± talc) developed in partly serpentinized peridotites (i.e., olivine–lizardite dunites). Thermodynamic calculations using the phase equilibria relations in the system Cr2O3–MgO–FeO–Al2O3–SiO2–H2O (CrMFASH) indicate that Fe2 +-rich porous chromite + chlorite replaced the original assemblage chromite + olivine in the chromitite while prograde antigorite was formed. According to our results this transformation occurred at ~ 510–560 °C when external fluids penetrated the ultramafic/chromitite bodies through shear zones. These temperatures are slightly higher than estimated for the metamorphic peak in the host metapelitic rocks (i.e., ~ 420 °C at 9.3 kbar), suggesting that a hotter ultramafic body was captured by the metasediments of the accretionary prism during their exhumation through subduction channel. Chlorite geothermometry yielded a wide range of lower temperature from 430 to 188 °C, for chlorite present in the porous chromite rims. These results are in agreement with the retrograde overprint under greenchist-facies metamorphism conditions recorded by metapelitic host rocks and minor volcanogenic massive sulphide deposits in the area (300–400 °C, ~ 3–4 kbar). We suggest that although initially decoupled, the chromitite-bearing ultramafic rocks and their metasedimentary host undergone a common metamorphic PT pathway of exhumation during the formation and evolution of the subduction-related accretionary complex.The chromitites contain appreciable amounts of the platinum-group elements (up to 347 ppb total) and gold (up to 24 ppb), present as inclusions of platinum-group minerals (PGM) and alloys as well as native gold. The PGM identified include native osmium, laurite (RuS2), irarsite (IrAsS), osarsite (OsAsS), omeiite (OsAs2), Pt–Fe alloy (possibly isoferroplatinum) and a suite of inadequately identified phases such as PtSb (possibly stumpflite), PdHg (possibly potarite), RhS, Ir–Ni and Ir–Ni–Ru compounds. Only a few grains of osmium and laurite were identified in unaltered cores of chromite and therefore considered as magmatic in origin formed during the high-T event of chomite crystallisation in the upper mantle. The other PGM were located in the porous chromite associated with chlorite or base-metal minerals (BMM) that often fill the pores of this altered chromite or are intergrowth with antigorite in the host serpentinized ultramafic rock. The assemblage of BMM identified in the studied rocks include sulphides [millerite (NiS), polydymite (Ni3S4), violarite (FeNi2S4), galena (PbS), sphalerite (ZnS), chalcocite (CuS)], arsenides [(orcelite (Ni5 − xAs2) and maucherite (Ni11As8)], the sulpharsenide gersdorfitte (NiAsS), and native bismuth. The irregular shape of several PGM grains observed in porous chromite suggest disequilibrium, whereas others exhibit perfectly developed crystal faces with the associated secondary silicate or base-metal mineral suggesting neoformation of PGMs in situ from metamorphic fluids. We suggest that the origin of these PGM inclusions is magmatic, but some grains were reworked in situ when metalloid (i.e., As, Sb, Pb, Zn and Hg)-rich fluids released from metasediments penetrated the ultramafic rocks through active shear zones, once the ultramafic bodies became tectonically mixed with the host metasedimentary host rocks. During this event, gold sourced from the (meta)sediments was also precipitated within chromitites and serpentinites.  相似文献   

4.
Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO2-H2O andMgO-Al2O3-SiO2-H2O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal Pfluid PH2O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.  相似文献   

5.
The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO2-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO2, with minor H2S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO2-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.  相似文献   

6.
《Resource Geology》2018,68(4):352-372
Talc deposits in Nangarhar Province, are hosted by Paleoproterozoic carbonate rocks, metamorphosed to amphibolite facies in the east–west (E–W) trending Spinghar Fault Block. Many deposits in this province have potential economic importance. However, detailed geologic and petrological studies on ore genesis are still lacking. In this study, eight talc deposits and two prospects of the Spinghar Fault Block were investigated. Talc is mainly formed by alteration of the host dolomite marble, magnesite rocks, and tremolitite. Talc ore bodies occur parallel to subparallel to the beddings of the host carbonate rocks. Dolerite occur as dikes and sills and are mostly metamorphosed to amphibolite. Although the amphibolite occurs mostly parallel to subparallel to the beddings of the host carbonate rocks, and talc orebodies, it partly crosscuts the host rocks. Massive layers of tremolitite were observed with most of the talc ore bodies. Quartz veins occur along the gneissosity of gneiss all over the study area. SiO2 and MgO content in talc rocks from all deposits ranged from 49.1 to 65.1 wt% and from 26.1 to 32.9 wt%, respectively. CaO content in talc rocks and magnesite rocks are less than 1 wt%. ƩREE content in talc rocks ranged from 0.1 to 8.9 ppm. Chemical compositions of host carbonate rocks are close to the ideal composition. Concentrations of Al, Ta, Hf, Zr, Th, Cr, Ni, Co, and ƩREE in talc ores and host carbonate rocks were very low and inconsistent with mafic and ultramafic rocks protolith. Therefore, the metamorphosed sedimentary carbonate rocks were likely to be the protolith of the talc ores. The occurrence of parallel to subparallel quartz veins to the gneissosity of gneiss, as well as the presence of hydrous minerals in host carbonate rocks, suggested that hydrothermal fluids were most probably supplied through the gneiss.  相似文献   

7.
《Ore Geology Reviews》2008,33(3-4):629-650
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

8.
The Indian Plate and Kohistan Island Arc are juxtaposed to the north of Peshawar along the Indus Suture, which is characterized by a tectonic mélange consisting of ultramafic, mafic, and a variety of sedimentary rocks. Within metavolcanic greenstones, there are small, lensoid bodies comprising quartz, white mica (up to 4.5 wt% FeO), chlorite (pseudothuringite to ripidolite), pyrite, and abundant chloritoid (up to 60%) and ilmenite. These lenses consist essentially of SiO2, TiO2, Al2O3 and iron oxide. There is a clear petrographic and geochemical (major, trace and RE element) transition from these rocks to their host greenstones, suggesting a common parentage. The greenstones contain high TiO2, Fe2O3 and P2O5. Their chondrite-normalized patterns show enrichment in light REE, depletion in heavy REE, negative Eu and positive Yb anomalies. Because the greenstones have probably been altered, the present set of data is not capable of deciphering the tectonic setting of these rocks, but previous geochemical studies have suggested an island arc setting for volcanic rocks in the mélange. By chemical analogy with lateritized basalts, the chloritoid-rich rocks are considered to be the product of weathering of basalts in tropical, probably equatorial, conditions before the northward drift of Kohistan. Mélange formation and greenschist facies metamorphism occurred during the collision of the Indian Plate and the Kohistan Magmatic Arc, to its north, in the Late Cretaceous-Early Tertiary.  相似文献   

9.
The Güneyocak chromite mineralization is hosted by the Upper Cretaceous Divrigi ophiolitic melange, which consists of serpentinite, serpentinized harzburgite and dunite, gabbro, diabase dikes, pyroxenite, blocks of limestone, and radiolarite. Serpentinites were intensely listwaenitized near the mineralization and in other locations in the study area. The Guneyocak chromite mineralization is of interest because of its internal structure and abundant, repeated chromitite bands, as well as for its chemistry. These features are unusual for ophiolite-hosted chromite. Major-element chemistry shows that the chromites have very high Fe2O3 and MgO and very low FeO. The Guneyocak chromites are classified as of Alpine type on the basis of host-rock lithology and Cr2O3, Al2O3, FeO(T), and Cr/Fe values. However, the very high Fe2O3 and MgO and very low FeO compositions of the chromites do not correspond to those of an Alpine-type chromite deposit. Repeated chromite banding and high Fe2O3 content of the chromite strongly suggest repeated oxygen fugacityf(O2) fluctuations and that the Guneyocak mineralization formed at relatively shallow depths. The Güneyocak chromite is characterized by a slightly boninitic character, which represents high partial melting under conditions of high oxygen fugacity. We conclude that the Guneyocak chromite mineralization formed in the uppermost part of the ultramafic rock series of the Divrigi ophiolitic melange.  相似文献   

10.
Detailed geochemical and mineralogical investigation of four talc deposits in the Eastern Desert of Egypt (Atshan, Abu Gurdi, Darhib and Kashira) suggests that the deposits form a distinct lithological unit within the Shadli metavolcanic rocks. The talc crystallized from the replacement of siliceous carbonate beds locally intercalated with clastic sediments. Th/Yb vs. Ta/Yb ratios of the rocks suggest that the sediments and the host volcanic rocks formed in an active continental margin (ACM) environment. Thus, the talc deposits may represent relict fragments of an ancient, regionally extensive carbonate horizon within the arc-related metavolcanics. The talc-rich rocks, which contain relict carbonate, serpentinized olivine and tremolite, have low (<3 wt%) Al2O3, Cr, Ni (<20 ppm), Co and Sc (<15 ppm) concentrations, precluding mafic or felsic igneous protoliths. The deposits were locally affected by contact metamorphism, giving rise to pyroxene-hornfels and granulite facies assemblages, and by regional metamorphism which produced greenschist-amphibolite grade assemblages. Disseminated sulfides commonly occur in the talc-tremolite-rich rocks (having low Al2O3 concentrations), suggesting that the metals were probably present in the original carbonate beds, but were remobilized and reconcentrated during the various metamorphic events.  相似文献   

11.
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

12.
The mineralogy of the transported Fe-Ni-Cr-laterite ore bodies from the Edessa area in Northern Greece was studied. The special emphasis was on the textural features and chemistry of chromite. The chromite was residually inherited in laterites from weathered ultramafic rocks and it displays zonation. Three main zones were optically distinguished: an inner chromite zone, an intermediate ferritechromite zone and a magnetite rim. These three zones have distinct compositions. The major oxides MgO and Al2O3 decrease from the chromite core to the ferritechromite zone, while FeOt increases and Cr2O3 either increases or decreases. A characteristic chemical feature of the chromite is the very high Mn-content in the ferritechromite zone, up to 20%wt MnO. Chemical zonation has, however, been detected in optically unzoned chromite cores rimmed by magnetite.The zoning and the high Mn-content of the chromite is a result of serpentinization in the presence of Mn-rich fluids, following lateritic weathering and finally Alpine low-grade metamorphism.  相似文献   

13.
The mineralogy of a metasomatic sequence formed between ultramafic and quartzofeldspathic protoliths from the Southern Alps of New Zealand consists of a forsterite-antigorite core surrounded concentrically by zones of antigorite-magnesite, magnesite-talc, talc, tremolite, chlorite and muscovite with discontinuous pods of albite associated with the muscovite zone. On the basis of trace element data the original ultramafite-schist contact is positioned between the present tremolite and chlorite zones.An experimental study of a metasomatic system was undertaken in an attempt to clarify diffusion relationships during the metasomatic event. To simulate the natural event, ultramafic and quartzofeldspathic natural starting materials were tightly packed in a gold tube with a graphite layer between to allow later identification of the original lithological interface. Run conditions were 450° C at 2 kb for 40 days. Phase dissolution and formation were analysed petrographically and component migration was examined with the electron-microprobe. The following hierarchical scheme of component migration, phase dissolution and phase formation is delineated: CO2 migrates from the schist into the ultramafite forming first the antigorite-magnesite zone and then the magnesite-talc zone at higher values of CO2. These zones are then partially overprinted by the formation of talc due to SiO2 metasomatism. The SiO2 is supplied from the schist by the dissolution of quartz and albite in the region adjacent to the ultramafite. The tremolite zone forms at the expense of the metasomatic talc zone upon the introduction of CaO from the schist into the ultramafite. Concurrent with tremolite formation, MgO migrates from the ultramafite into the schist to form the chlorite metasomatic zone. The growth of the chlorite zone causes dissolution of the pre-metasomatic micas and displaces K2O from the chlorite zone further into the schist. Displaced K2O and Na2O are responsible for the formation of the muscovite zone and the albitite pods.  相似文献   

14.
Potential chromite ore deposits of India are situated in Sukinda, Odisha, which may also be considered as a potential resource for platinum group elements (PGEs). This paper reports on PGE geochemistry in twenty six samples covering chromite ores, chromitites and associated ultramafic rocks of the Sukinda ultramafic complex. Platinum group element contents range from 213 to 487 ppb in the chromite ore body, from 63 to 538 ppb in rocks that have chromite dendrites or dissemination and from 38 to 389 ppb in associated olivine–peridotite, serpentinite, pyroxenite and brecciated rocks. The PGEs are divided into two sub‐groups: IPGE (Ir, Os, and Ru) and PPGE (Pd, Pt, and Rh) based on their chemical behaviour. The IPGE and PPGE in these three litho‐members show a contrasting relationship e.g. average IPGE content decreases from chromite to chromitite and associated rocks while PPGE increases in the same order. Appreciable Ag in chromitite (270–842 ppb) is recorded. Positive correlation between IPGE with Cr2O3 and with Al2O3 is observed while these are negatively correlated with MgO. Covariant relationships between Au and Mg in rocks devoid of chromite and between Ag and Fe in chromitite sample are observed. Chromite in all seams and some chromitite samples exhibit an IPGE‐enriched chondrite normalized pattern while PPGE are highly fractionated and show a steep negative slope, thereby indicating that PGE in the parental melt fractionates and IPGE‐compatible elements prefer to settle with chromite. The rocks devoid of chromite and rocks containing accessory chromite exhibit a nearly flat pattern in chondrite‐normalized PGE plots and this suggests a limited fractionation of PGE in these rocks. Variation in the distribution pattern of PGE and Ag in three typical litho‐members of the Sukinda Valley may be related to multiple intrusion of ultramafic magma, containing variable volume percentage of chromite.  相似文献   

15.
《International Geology Review》2012,54(12):1466-1483
Chromitites (>80% volume chromite) hosted in two ultramafic bodies (Lavanderos and Centinela Bajo) from the Palaeozoic metamorphic basement of the Chilean Coastal Cordillera were studied in terms of their chromite composition, platinum-group element (PGE) abundances, and Re-Os isotopic systematics. Primary chromite (Cr# = 0.64–0.66; Mg# = 48.71–51.81) is only preserved in some massive chromitites from the Centinela Bajo ultramafic body. This chemical fingerprint is similar to other high-Cr chromitites from ophiolite complexes, suggesting that they crystallized from arc-type melt similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction mantle. The chromitites display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 180 and 347 ppb, as is typical of chromitites hosted in the mantle of supra-subduction zone (SSZ) ophiolites. Laurite (RuS2)-erlichmanite (OsS2) phases are the most abundant inclusions of platinum-group minerals (PGM) in chromite, indicating crystallization from S-undersaturated melts in the sub-arc mantle. The metamorphism associated with the emplacement of the ultramafic bodies in the La Cabaña has been determined to be ca. 300 Ma, based on K-Ar dating of fuchsite. Initial 187Os/188Os ratios for four chromitite samples, calculated for this age, range from 0.1248 to 0.1271. These isotopic compositions are well within the range of chromitites hosted in the mantle section of other Phanaerozoic ophiolites. Collectively, these mineralogical and geochemical features are interpreted in terms of chromite crystallization in dunite channels beneath a spreading centre that opened a marginal basin above a supra-subduction zone. This implies that chromitite-bearing serpentinites in the metamorphic basement of the Coastal Cordillera are of oceanic-mantle origin and not oceanic crust as previously suggested. We suggest that old subcontinental mantle underlying the hypothetical Chilenia micro-continent was unroofed and later altered during the opening of the marginal basin. This defined the compositional and structural framework in which the protoliths of the meta-igneous and meta-sedimentary rocks of the Eastern and Western Series of the Chilean Coastal Cordillera basement were formed.  相似文献   

16.
The Laowangzhai gold deposit, located in the Ailaoshan gold belt (SW China), is hosted in various types of rocks, including in quartz porphyry, carbonaceous slate, meta‐sandstone, lamprophyre, and altered ultramafic rocks. In contrast to other wall rocks, the orebodies in altered ultramafic rocks are characterized by the occurrence of a large amount of Ni‐bearing minerals. The ore‐forming process of the orebodies hosted by altered ultramafic rocks can be divided into two stages: pyrite‐vaesite‐native gold and gersdorffite‐violarite stages. The contents of As and Sb increased during the evolution of ore‐forming fluid based on the mineral assemblages. Thermodynamic modeling of the Ni‐Cu‐As‐Fe‐S system using the SUPCRT92 software package with the updated database of slop16.dat indicates the fS2 in ore‐forming fluid decreases significantly from stage I to stage II. The decreases of fS2 due to crystallization of sulfides and fO2 due to fluid–rock reaction were responsible for ore formation in altered ultramafic rocks of the Laowangzhai gold deposit. Geological evidence, the in situ sulfur isotope values of pyrite, and the other published isotopic data suggest that the ore‐forming fluid for ultramafic rock ores was dominantly composed of evolved magmatic fluid with the important input of sediments.  相似文献   

17.
Chromite mineralization in metamorphosed dunites from the southern Klyuchevskoy dunite-harzburgite ultramafic complex (Central Urals) has been investigated using geomagnetic surveys along with laboratory studies of ore-forming and accessory spinels of the same genetic type. Magnetization in the study area is carried mainly by accessory Fe-Cr-spinel of a variable Fe2 +(Cr2 -xFex3 +)O4 composition. Metamorphism caused changes in element contents and in both crystal and magnetic structure of the primary nonmagnetic accessory spinel, unlike the almost fresh ore-forming spinel. Thus, ore bodies stand against their host rocks, which is a prerequisite for the use of geomagnetic surveys for exploration of podiform chromite deposits in dunite-harzburgite complexes. Ground magnetic surveys at a test site composed of faulted rocks bearing disseminated chromite mineralization in metamorphosed dunites resolved a chromite ore zone and a fault block boundary showing up as geomagnetic anomalies. Laboratory studies using high technologies (thermomagnetic analysis at 4 to 1000 K, as well as magnetic resonance and magnetic force spectroscopy) revealed, for the first time, magnetic clusters (superparamagnetic phases) in primary nonmagnetic accessory spinel, which are responsible for the magnetic properties of the host rocks. Microscale variations in Cr-spinel correlate with the geomagnetic anomalies recorded by field surveys at the test site.© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.  相似文献   

18.
The results of melt inclusion study are reported for chromites of the Klyuchevsky ultramafic massif, which is the most representative of all Ural ultramafic massifs localized beyond the Main Ural Fault Zone. The massif is composed of a dunite-harzburgite complex (tectonized mantle peridotite) and a dunite-wehrlite-clinopyroxenite-gabbro complex (layered portion of the ophiolitic section). The studied Kozlovsky chromite deposit is located in the southeastern part of the Klyuchevsky massif and hosted in serpentinized dunite as a series of lenticular bodies and layers up to 7–8 m thick largely composed of disseminated and locally developed massive ore. Melt inclusions have been detected in chromites of both ore types. The heated and then quenched into glass melt inclusions and host minerals were analyzed on a Camebax-Micro microprobe. The glasses of melt inclusions contain up to 1.06 wt % Na2O + K2O and correspond to melts of normal alkalinity. In SiO2 content (49–56 wt %), they fit basalt and basaltic andesite. The melt inclusions are compared with those from chromites of the Nurali massif in the southern Urals and the Karashat massif in southern Tuva. The physicochemical parameters of magmatic systems related to the formation of disseminated and massive chromite ores of the Klyuchevsky massif are different. The former are characterized by a wider temperature interval (1185–1120°C) in comparison with massive chromite ore (1160–1140°C).  相似文献   

19.
The Pouébo and Diahot terranes of NE New Caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c. 40 Ma metamorphism. This Eocene high‐P event has been linked with the SW‐directed obduction of the New Caledonian Ophiolite, an extensive ultramafic nappe that dominates outcrop in the south of the island. In the north, ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with, but separated from, the eclogites and blueschists by foliated talc–chlorite–serpentine–carbonate‐bearing rocks. The base of the largest and best‐preserved ultramafic body at Yambé is marked by a distinctive (2 m thick) layer of high‐P mylonite that preserves evidence for early blueschist facies conditions (S1) as inclusions in eclogite facies minerals. Textural evidence preserved in olivine‐bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high‐P terrane as serpentinite tectonites sourced from hydrated mantle, formerly in the hangingwall of the Eocene subduction zone. Serpentinite emplacement accompanied burial of the NE New Caledonian margin at T<500 °C and P<16 kbar. The ultramafic fragments were buried to depths of 50–60 km in the subduction zone, where olivine was stable and coarse‐grained garnet–omphacite‐rich assemblages developed in low strain domains within enclosing mylonites. Host metabasic and metasedimentary rocks from the structurally highest portions of the high‐P belt have a prograde record identical to that of the ultramafic tectonites. The early emplacement and similar P–T history of host rocks and ultramafic masses suggest that NE New Caledonia preserves a fossil slab/mantle–wedge boundary reactivated during exhumation.  相似文献   

20.
The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30–44 wt% Cr2O3; 20–30 wt% Al2O3) are linked with the peridotite-troctolite-olivine gabbro lineage. Two main types were found: (1) Al-rich refractory ores associated with harzburgites and feldspathic periodotites and (2) more Cr-rich varieties associated with lherzolites. — Metallurgical chromite ores (45–53 wt% Cr2O3; 12–18 wt% Al2O3) are linked with the peridotite-pyroxenite-norite lineage. Two main types were also found: (1) Cr-rich metallurgical ores associated with orthopyroxenites and (2) more Al-rich varieties related to clinopyroxenites. — The chemical composition of chromite within the deposits varies depending on the chromite/silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall-rock peridotites. — The geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks.New address: BEB Erdgas und Erdöl GmbH, Riethorst 12, D-3000 Hannover 51The terms chrome-spinels and chromite are used as follows: 1. Chrome-spinel is only used for those occuring as accessory minerals in various ultramafic and mafic rocks (= accessory chrome-spinels). Their chemical composition has been determined only by microprobe analysis. — 2. Chromite is used for ore and ore deposits (=chromitites); the chemical composition has been determined by wet chemistry (AAS) or by microprobe analysis  相似文献   

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