Structural and magnetic microinhomogeneities in accessory spinel of the system Fe2+(Cr2–xFex3+)O4 from the Kytlym massif (Urals platinum-bearing belt)

The microstructure and magnetic properties of accessory Fe–Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe–Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe–Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites. It has been established that the microareas are separations in solid solution Fe²⁺(Cr_2–xFe_x³⁺)O₄, which are enriched in Fe^3 + and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature. The formation of this earlier unknown type of magnetic Fe–Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif. The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum–chromite deposits.     

Magnetic microphases in chrome-spinels from alpine-type ultramafic rocks,Central Urals

The ore and accessory chrome-spinels from metamorphosed dunites of the Cr-bearing Klyuchevskoi alpine-type ultramafic massif are studied. As a result of use of thermomagnetic analysis in the range of 4–900 K, magnetic resonance spectroscopy, and magnetic-force microscopy, secondary magnetic Fe³⁺-enriched microphases chaotically distributed in the primary nonmagnetic mineral were revealed for the first time in accessory chrome-spinels. It was established that the metamorphosed accessory chrome-spinels produce the magnetic properties of the host rocks and the primary nonmagnetic chrome-spinels forming ore bodies remains almost unaltered. This originates the contrast of magnetic properties between the ore body and host rocks and provides the geomagnetic anomaly in the ore-hosting zone.     

Dielectric loss in ore-forming chromium spinel at temperatures from 20 to 800 °C

Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe₂O₃)^? : (FeO/Fe₂O₃)^??, where (FeO/Fe₂O₃)^? and (FeO/Fe₂O₃)^?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe₂O₃ ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr₂O₃) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr₂O₃), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N², with an error of σ = 0.19 (N = T · 10^?2, T is temperature in °C).     

Chromitites from the Luanga Complex,Carajás,Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75 Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150 meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr + Al + Fe³⁺)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20 cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al₂O₃ activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe³⁺ and Fe²⁺ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe²⁺) cations and minor to none exchange between trivalent cations (Cr³⁺, Al³⁺ and Fe³⁺). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe²⁺ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100 Mg/(Mg + Fe²⁺)) and Al# (100Al/(Cr + Al + Fe³⁺)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.     

A secondary precious and base metal mineralization in chromitites linked to the development of a Paleozoic accretionary complex in Central Chile

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 Fe^2 + 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 Fe^2 +-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 (Ni_5 − 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.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

The Neoproterozoic (∼ 820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ∼ 35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ∼ 150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite–biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age.     

The role of silica in the hydrous metamorphism of chromite

Retrograde hydrous metamorphism has produced three types of microstructures in chromite grains from chromitites and enclosing rocks of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive chromitites (60–80 vol% chromite), (i) partly altered chromite with homogeneous cores surrounded by lower Al₂O₃ and MgO but higher Cr₂O₃ and FeO porous chromite with chlorite filling the pores. In serpentinites (ii) zoned chromite with homogeneous cores surrounded by extremely higher Fe₂O₃ non-porous chromite and magnetite rims, and (iii) non-porous chromite grains. The different patterns of zoning in chromite grains are the consequences of the infiltration of reducing and SiO₂-rich fluids and the subsequent heterogeneous interaction with more oxidizing and Fe-bearing fluids. During the first stage of alteration under reduced conditions magmatic chromite is dissolved meanwhile new metamorphogenic porous chromite crystallizes in equilibrium with chlorite. This reaction that involves dissolution and precipitation of minerals is here modeled thermodynamically for the first time. µSiO₂-µMgO pseudosection calculated for unaltered semi-massive chromitites at 2 kbar and 300 °C, the lowest P-T conditions inferred from the Tapo Ultramafic Massif and Marañón Complex, predicts that chromite + chlorite (i.e., partly altered chromite) is stable instead of chromite + chlorite + brucite at progressive higher µSiO₂ but lower µMgO. Our observation is twofold as it reveals that the important role of SiO₂ and MgO and the open-nature of this process. P-T-X diagrams computed using the different P-T pathways estimated for the enclosing Tapo Ultramafic Massif reproduce well the partial equilibrium sequence of mineral assemblages preserved in the chromitites. Nevertheless, it is restricted only to the P-T conditions of the metamorphic peak and that of the latest overprint. Our estimations reveal that a high fluid/rock ratio (1:40 ratio) is required to produce the microstructures and compositional changes observed in the chromitites from the Tapo Ultramafic Massif. The circulation of SiO₂-rich fluids and the mobilization of MgO from the chromitite bodies are linked with the formation of garnet amphibolites and carbonate-silica hydrothermalites (i.e., listwaenites and birbirites) in the ultramafic massif. The origin of these fluids is interpreted as a result of the dissolution of orthopyroxene and/or olivine from the metaharzburgites and metagabbros enclosed in the Tapo Ultramafic Massif.     

Petrology and geochemistry of the Karaj Dam basement sill: Implications for geodynamic evolution of the Alborz magmatic belt

The northeastward subduction of the Neo-Tethyan oceanic lithosphere beneath the Iranian block produced vast volcanic and plutonic rocks that now outcrop in central (Urumieh–Dokhtar magmatic assemblage) and north–northeastern Iran (Alborz Magmatic Belt), with peak magmatism occurring during the Eocene. The Karaj Dam basement sill (KDBS), situated in the Alborz Magmatic Belt, comprises gabbro, monzogabbro, monzodiorite, and monzonite with a shoshonitic affinity. These plutonic rocks are intruded into the Karaj Formation, which comprise pyroclastic rocks dating to the lower–upper Eocene. The geochemical and isotopic signatures of the KDBS rocks indicate that they are cogenetic and evolved through fractional crystallization. They are characterized by an enrichment in LREEs relative to HREEs, with negative Nb–Ta anomalies. Geochemical modeling using Sm/Yb versus La/Yb and La/Sm ratios suggests a low-degree of partial melting of a phlogopite–spinel peridotite source to generate the KDBS rocks. Their low I_Sr = 0.70453–0.70535, ɛNd (37.2 Ma) = 1.54–1.9, and T_DM ages ranging from 0.65 to 0.86 Ga are consistent with the melting of a Cadomian enriched lithospheric mantle source, metasomatized by fluids derived from the subducted slab or sediments during magma generation. These interpretations are consistent with high ratios of ²⁰⁶Pb/²⁰⁴Pb = 18.43–18.67, ²⁰⁷Pb/²⁰⁴Pb = 15.59, and ²⁰⁸Pb/²⁰⁴Pb = 38.42–38.71, indicating the involvement of subducted sediments or continental crust. The sill is considered to have been emplaced in an environment of lithospheric extension due to the slab rollback in the lower Eocene. This extension led to localized upwelling of the asthenosphere, providing the heat required for partial melting of the subduction-contaminated subcontinental lithospheric mantle beneath the Alborz magmatic belt. Then, the shoshonitic melt generates the entire spectrum of KDBS rocks through assimilation and fractional crystallization during the ascent of the magma.     

Petrogenesis of ore-forming and pre/post-ore granitoids from the Kounrad,Borly and Sayak porphyry/skarn Cu deposits,Central Kazakhstan

Ore-forming porphyries and barren granitoids from porphyry Cu deposits differ in many ways, particularly with respect to their adakitic affinity and calc-alkaline characteristics. In this study, zircon U–Pb and molybdenite Re–Os dating, whole rock geochemistry, whole rock Sr–Nd–Pb and zircon O–Hf isotopic analyses were carried out on the ore-forming granitoids from the Kounrad, Borly and Sayak deposits, and also on pre-ore and post-ore granitoids in adjacent regions of Central Kazakhstan. Geochronology results indicate that pre-ore magmatism occurred in the Late Devonian to Early Carboniferous (361.3–339.4 Ma), followed by large scale Cu mineralization (325.0–327.3 Ma at Kounrad, 311.4–315.2 Ma at Borly and 309.5–311.4 Ma at Sayak), and finally, emplacement of the Late Carboniferous post-ore barren granitoids (305.0 Ma). The geochemistry of these rocks is consistent with calc-alkaline arc magmatism characterized by strong depletions in Nb, Ta and Ti and enrichments in light rare earth elements and large ion lithophile elements, suggesting a supra-subduction zone setting. However, the ore-forming rocks at Kounrad and Sayak show adakitic characteristics with high Sr (517.5–785.3 ppm), Sr/Y (50.60–79.26), (La/Yb)_N (9.37–19.62) but low Y (6.94–11.54 ppm) and Yb (0.57–1.07 ppm), whereas ore-forming rocks at Borly and barren rocks from northwest of Borly and Sayak have normal arc magma geochemical features. The Sr–Nd–Hf–O isotopic compositions show three different signatures: (1) Sayak granitoids have very young juvenile lower crust-derived compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.70384 to 0.70451, ɛ_Nd (t) = + 4.9 to + 6.0; T_DM2 (Nd) = 580 to 670 Ma, ɛ_Hf (t) = + 11.3 to + 15.5; T_DM^C (Hf) = 330 to 600 Ma, δ¹⁸O = 6.0 to 8.1‰), and were probably generated from depleted mantle-derived magma with 5–15% sediment melt addition in the magma source; (2) the Kt-1 granite from northwest of Sayak shows extremely enriched Sr–Nd isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.71050, ɛ_Nd (t) = − 7.8, T_DM2 (Nd) = 1700 Ma), likely derived from partial melting of ancient continental crust; (3) other granitoids have transitional Sr–Nd compositions between the Sayak and Kt-1 samples, indicating a juvenile lower crust source with the addition of 10–30% of ancient crustal material. The pre-ore magmatism was probably related to partial melting of juvenile lower crust due to northward subduction of the Junggar–Balkhash Ocean, whereas the ore-forming adakitic rocks at Aktogai, Kounrad and Sayak formed by partial melting of thickened lower crust which subsequently delaminated. The ore-forming rocks at Borly, and the later post-ore barren granites, formed by partial melting of juvenile lower crust with normal thickness. This tectonic setting supports the existence of an Andean-type magmatic arc in the Devonian to the Late Carboniferous, resulting from the subduction of the Junggar–Balkhash oceanic plate. The link between whole rock geochemistry and scale of mineralization suggests a higher metallogenic potential for adakitic rocks than for normal arc magmatism.     

Chromite typology and composition characterized through multispectral specular reflectance

Determination of multispectral specular reflectance is an important tool for ore identification in reflected light microscopy, and may be used for automated characterization of ores. However, reflectance values can be affected by compositional variations in a way that is seldom understood. The aim of the present work is to investigate this problem in chromite, an ore whose composition may show typically large natural variations as member of the spinel family, and whose relatively high reflectance variations are apparently unpredictable. For this research, eighteen samples of chromian spinel covering a large range of compositions in the base of the Hagerthy prism were selected for microprobe analysis and reflectance measure on polished sections. The samples belong to a variety of deposits and types (ultramafic massif, ophiolitic, and metamorphosed types: Ojén and Ronda Massifs, Spain; Mayarí, Moa-Baracoa, Camagüey and Sagua de Tánamo, Cuba; Golyamo Kemenyane, Avren and Yakovitsa, Bulgaria). The specular reflectances are characterized as multispectral values, measured at thirteen intervals (50 nm each) in the VNIR region (Visible and Near-Infrared: 400–1000 nm), using the automated CAMEVA System.The relationship between compositional and reflectance values is studied by multivariate analysis and subsequently tested on independent samples. For this purpose, the samples have been grouped in two sets: a larger population of fifteen samples constitutes the initial data set for mathematical processing, while a selection of three samples of widely differing compositions is used to test the resulting functions, so as to gain a critical appraisal of their validity.The results obtained show that the specular reflectance of chromite depends on composition and can be used to estimate compositional parameters, as #Cr = Cr/(Cr + Al) or #Mg = Mg/(Mg + Fe²⁺), but this relationship is complex and does not allow simple direct determinations, due to the multiplicity of possible changes and coupled substitutions (e.g. Al–Cr–Fe³⁺, or Mg-Fe²⁺, Ni, Zn, Ti …) in the chromian spinel family. On the other hand, the relationship of chromite composition with deposit type is also detectable through the reflectance values, but with a higher uncertainty. Reflectance increases with increasing Cr and Fe³⁺ contents in all cases, but the relationship of reflectance with Mg seems to be specific of the deposit type: while reflectance increases with increasing #Mg in the Ojén chromites, it shows the opposite behaviour in podiform chromites. Although these results should be regarded as preliminary until further studies on larger sample populations can be achieved, they are suggestive of possible practical applications for exploration, e. g. to approach Cr-spinel compositions or chromite deposit typology, early in an exploration campaign, by inexpensive reflected light determination of specular reflectance in a few samples.     

The origin of the Dapingzhang volcanogenic Cu–Pb–Zn ore deposit,Yunnan province,SW China: Constraints from host rock geochemistry and ore Os–Pb–S–C–O–H isotopes

The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive ε_Nd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ³⁴S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.310–18.656, ²⁰⁷Pb/²⁰⁴Pb = 15.489–15.643 and ²⁰⁸Pb/²⁰⁴Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ¹³C_PDB and δ¹⁸O_SMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ¹³C_PDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ¹⁸O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.     

Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble,with special reference to Vietnamese deposits

Oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble from Vietnam and other worldwide deposits have been determined in order to characterize the O-isotope fractionation between calcite and spinel. In Vietnam, the Δ¹⁸O_cc–sp (= 3.7 ± 0.1‰ for six samples from the An Phu and Cong Troi deposits) is remarkably constant. The combination of these data with those obtained on calcite–spinel pairs of Paigutan (Nepal, n = 2), Ipanko (Tanzania, n = 1), and Mogok (Myanmar, = 2) are also consistent with an overall Δ¹⁸O_cc–sp of 3.6 ± 0.3‰ for all the spinel samples (n = 11). The straight line correlation δ¹⁸O_cc = 0.96 δ¹⁸O_sp + 4.4 is excellent despite their worldwide geographic spread. The increment method of calculating oxygen isotope fractionation gave a geologically unreasonable temperature of formation for both minerals at 1374 °C when compared to temperatures obtained by mineral assemblage equilibrium of these marble type deposits, between 610 and 750 °C. The constant Δ¹⁸O_cc–sp reflects a constant temperature for this amphibolite facies assemblage, whose current best estimate is calculated at 620 ± 40 °C, but unquantified uncertainties remain.     

Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt,South Africa: Constraints from fluid inclusion and stable isotope studies

Orogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H₂O-CO₂ composition (~3 wt% NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300 ± 30 °C and ∼2 kbar. Thermodynamic calculations at 300 °C suggest an increase in fO₂ (10⁻³²–10⁻³⁰ bars) and corresponding decrease in total sulfur concentration (0.002–0.001 m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site, the calculated low fO₂ values are consistent with previously determined high Fe/Fe + Mg ratios (>0.7) in associated chlorite, absence of sulfates and restricted positive δ³⁴S values in associated pyrite. Based on the fluid composition, metal association and physico-chemical conditions reported in the current study, it is confirmed that gold in the Amalia fluid was transported as reduced bisulfide complexes (e.g., Au(HS)₂⁻). At Amalia, gold deposition was most likely a combined effect of increase in fO₂ corresponding to the magnetite-hematite buffer, and reduction in total sulfur contents due to sulfide precipitation during progressive fluid-rock interaction.The epigenetic features coupled with the isotopic compositions of the ore-forming fluid (δ³⁴S_ΣS = +1.8 to +2.3‰, δ¹⁸O_H2O = +6.6 to +7.9‰, and δ¹³C_ΣC = −6.0 to −7.7‰ at 300–330 °C) are consistent with an externally deep-sourced fluid of igneous signature or/and prograde metamorphism of mantle-derived rocks.     

Re–Os and Sr–Nd–Pb isotope constraints on source of fluids in the Zhifang Mo deposit,Qinling Orogen,China

The Zhifang Mo deposit is located in the northeastern Qinling Orogen along the southern margin of the North China Craton. The deposit represents a quartz-vein system hosted in the Mesoproterozoic Xiong'er Group volcanic rocks. We identify three hydrothermal stages (early, middle and late), characterized by veinlets of quartz–pyrite, quartz–molybdenite–pyrite–chalcopyrite–galena–sphalerite, and quartz–carbonate assemblages, respectively. Five molybdenite samples from the Zhifang deposit yield Re–Os ages ranging from 241.2 ± 1.6 Ma to 247.4 ± 2.5 Ma, with an isochron age of 246.0 ± 5.2 Ma (2σ, MSWD = 7.4), and a weighted mean age of 243.8 ± 2.8 Ma (2σ, MSWD = 5.5). The Re–Os age shows that the Mo mineralization occurred during the Indosinian Orogeny, and suggests that the mineralization is unrelated to the Yanshanian magmatism or the Paleo-Mesoproterozoic volcanic–hydrothermal event.This study also reports a new Sr–Nd–Pb isotope dataset from ore sulfides in an attempt to constrain the source of the ore-forming fluids. Ten sulfide samples from middle stage of the Zhifang Mo deposit yield I_Sr(t) ratios of 0.710286–0.711943, with an average of 0.711004; ε_Nd(t) values between − 19.5 and − 14.8, with an average of − 16.7; and (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁸Pb/²⁰⁴Pb)_i ratios of 17.126–17.535, 15.374–15.466 and 37.485–37.848, with averages of 17.380, 15.410 and 37.631, respectively. One pyrite from the early stage yield I_Sr(t) of 0.722711–0.722855, with an average of 0.722783, which is higher than those of the middle stage sulfides and suggests equilibration with wallrocks. The ε_Nd(t) values are in the range of − 17.3 to − 16.6 with a mean at − 17.0; and (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁸Pb/²⁰⁴Pb)_i ratios are 17.386, 15.405 and 37.622, respectively. The ore sulfides show higher Pb-isotope ratios, higher ε_Nd(t) and lower I_Sr(t) values than the host rocks. The results suggest that the ore-forming fluids had lower I_Sr(t), and higher ε_Nd(t) values than the ore sulfides, and were possibly sourced from the Dengfeng Complex. The southward subduction of the North China Craton beneath the Huaxiong Block during the Triassic was possibly responsible for the formation of the Waifangshan orogenic Mo system.     

The Blagodatnoe gold-sulfide deposit (Yenisei Ridge,Russia): the nature of geophysical anomalies and the succession and causes of formation of petrophysical zoning

Data on geophysical fields and petrophysical heterogeneity, parageneses and the thermodynamic conditions and age of their formation, and fluid inclusions were used for the genetic reconstruction of petrophysical zoning at the Blagodatnoe gold-sulfide deposit. Petrophysical associations of the preore and ore stages of the deposit formation are clearly reflected in anomalies of the magnetic and natural electric fields and the aureoles of radioactive elements. At the early preore stage (752 Ma), reduced solutions with high activity of K, enriched in U, Th, and, probably, Au, were supplied to intensely foliated tectonic zones. Their interaction with initial metasedimentary rocks gave rise to contiguous zones of quartz–muscovite and chlorite metasomatic rocks. Accompanying graphitization led to a high electrochemical activity of the metasomatic rocks, which generated anomalies of up to –300 mV in the natural electric field; the most intensely carbonized zones became enriched with U (up to 6.5 × 10^–4%) and, probably, Au. The quartz–muscovite metasomatic rocks accumulated Th and K (up to 29 × 10^–4% and 4%, respectively), whereas the chlorite metasomatic rocks accumulated rock-forming elements (particularly Fe), which led to the compaction of these rocks and the acceleration due to gravity in local positive anomalies. The nonmagnetic character of the fresh pre-ore metasomatic rocks suggests the predominantly pyritic composition of early sulfides. At the ore stage (698 Ma), the minerals were deposited from H₂O–CO₂–As–S solutions at 560 to 315 °C. The activity of these solutions caused a redistribution of radioactive elements and a high petrophysical differentiation of the ore-bearing structure. The amplitudes of the anomalies above this structure vary from 500 to 80 nT in the magnetic field and from –130 to + 10 mV in the natural electric field. It has been found that the hydrothermal fluid hardly affected the polarization properties of graphitized rocks at the maximum temperatures but caused an intense removal of U and the development of magnetic pyrrhotite after pyrite. The temperature decrease in the mineral-forming system was favorable for the formation of siderite. Carbonaceous schists which experienced carbonatization lost their electrochemical activity. The binding of carbon dioxide in the solid phase influenced the migration capability of trace elements and their zonal distribution. With this evolution of the solution, Th accumulated at the lower levels of the mineralized zone, whereas the upper levels of the deposit became enriched with U. Productive gold–arsenopyrite–pyrite–pyrrhotite paragenesis with anomalous magnetic susceptibility evolved at the ore stage. The late galena–sphalerite–chalcopyrite paragenesis (365 Ma) was of strictly local occurrence and reduced the magnetic susceptibility of ores.     

Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton,India: Mineralogy and P–T regime

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimberlite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3–6 wt% Na₂O) in eclogites to Ca pyroxene (<3 wt% Na₂O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, enstatite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an X_Mg value of 0.70, which is unusually low compared to the X_Mg range of 0.91–0.97 for all other samples. Garnet in the rutile eclogite is also highly iron-rich with an end member composition of Prp_26.5Alm_52.5Grs_14.7Adr_5.1TiAdr_0.3Sps_1.0Uv_0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component. Geothermobarometric calculations in Cr-rich xenoliths yield different P–T ranges for eclogites and garnet pyroxenites with average P–T conditions of 36 kbar and 1080 °C, and 27 kbar and 830 °C, respectively. The calculated P–T ranges approximate to a 45 mW m^?2 model geotherm, which is on the higher side of the typical range of xenolith/xenocryst geotherms (35–45 mW m^?2) for several Archaean cratons in the world. This indicates that the EDC was hotter than many other shield regions of the world in the mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted and metamorphosed oceanic crust.     

Using platinum-group elements and Au geochemistry to constrain the genesis of podiform chromitites and associated peridotites from the Soghan mafic–ultramafic complex,Kerman, Southeastern Iran

The podiform chromite deposit of the Soghan mafic–ultramafic complex is one of the largest chromite deposits in south-east Iran (Esfandagheh area). The Soghan complex is composed mainly of dunite, harzburgite, lherzolite, pyroxenite, chromitite, wehrlite and gabbro. Olivine, orthopyroxene, and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo₉₄) than those in harzburgites (Fo₉₂) and lherzolites (Fo₈₉). Chromian spinel mainly occurs as massive chromitite pods and as thin massive chromitite bands together with minor disseminations in dunites and harzburgites. Chromian spinels in massive chromitites show very high Cr-numbers (80–83.6), Mg-numbers (62–69) and very low TiO₂ content (averaging 0.17 wt.%) for which may reflect the crystallization of chromite from a boninitic magma. The Fe^3 +-number is very low, down to < 0.04 wt.%, in the chromian spinel of chromitites and associated peridotites of the Soghan complex.PGE contents are variable and range from 80 to 153 pbb. Chromitites have strongly fractionated chondrite-normalized PGE patterns, which are characterized by enrichments in Os, Ir and Rh relative to Pt and Pd. Moreover, the Pd/Ir value which is an indicator of PGE fractionation ranges from < 0.08 to 0.24 in chromitite of the Soghan complex. These patterns and the low PGE abundances are typical of ophiolitic chromitites and indicating a high degree of partial melting (about 20–24%) of the mantle source. Moreover, the Pd_N/Ir_N ratios in dunites are unfractionated, averaging 1.2, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru and Pd anomaly, and their Pd_N/Ir_N ratio averages 1.98 and 2.15 respectively.The mineral chemistry data and PGE geochemistry, along with the calculated parental melts in equilibrium with chromian spinel of the Soghan chromitites indicate that the Soghan complex was generated from an arc-related magma with boninitic affinity above a supra-subduction zone setting.     

Ore-forming process of the Huijiabao gold district,southwestern Guizhou Province,China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO₂ inclusions, CO₂–H₂O inclusions, hydrocarbon inclusions, and hydrocarbon–H₂O inclusions. The ore-forming fluids are characterized by a H₂O + CO₂ + CH₄ ± N₂ system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H₂O + NaCl → H₂O + NaCl + CO₂ + CH₄ ± N₂ → H₂O + NaCl ± CH₄ ± CO₂ with a slight decrease in homogenization temperature and salinity. The δ¹⁸O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_H2O and calculated δ¹⁸O_H2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H₂O–NaCl–CO₂ fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.     

Base metal mineral segregation and Fe-Mg exchange inducing extreme compositions of olivine and chromite from the Xiadong Alaskan-type complex in the southern part of the Central Asian Orogenic Belt

The Xiadong Alaskan-type complex shares much in common with typical Alaskan-type complexes worldwide, while showing some unique features in terms of mineral compositions. Olivine from the Xiadong dunites is characterized by extremely high Fo component of 91.7–96.7 and anomalously negative correlation of Fo with NiO, while chromite is featured by high 100 × Fe³⁺/(Fe³⁺ + Cr + Al) (>70), high 100 × Fe²⁺/(Fe²⁺ + Mg) (>70), high 100 × Cr/(Cr + Al) (>90), low MnO (<0.6 wt%) and TiO₂ contents (<0.5 wt%). To investigate these particular features, we conducted petrographic observation and mineral composition analyses for the Xiadong dunite. A number of Fe and/or Ni sulfides and alloys occurring as inclusions in olivine and chromite indicate that base metal mineral segregation took place prior to crystallization of olivine and chromite and probably induced Fe and Ni depletions in olivine. The FeO and MgO variations in profile analyses from chromite to adjacent olivine are compatible with Fe-Mg exchange. The diffusion mechanism of Fe from olivine to chromite and Mg from chromite to olivine may have elevated both Fo of olivine and 100 × Fe²⁺/(Mg + Fe²⁺) ratio of chromite and further enhanced the decoupling of Fo and NiO in olivine. We thus suggest that base metal mineral segregation and Fe-Mg exchange play important roles in the extreme compositions of the Xiadong dunite. The Ni depletion of olivine and degree of Fe-Mg exchange between olivine and chromite may be used as indicators of mineralization in mafic-ultramafic intrusions.     

Metallogeny of the northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt in the Lhasa terrane,southern Tibet

The northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt (NGPB), characterized by skarn and porphyry deposits, is one of the most important metallogenic belts in the Himalaya–Tibetan continental orogenic system. This belt extends for nearly four hundred kilometers along the Luobadui–Milashan Fault in the central Lhasa subterrane, and contains more than 10 large ore deposits with high potential for development. Three major types of mineralization system have been identified: skarn Fe systems, skarn/breccia Pb–Zn–Ag systems, and porphyry/skarn Mo–Cu–W systems. In this study, we conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf isotopic study of ore-forming rocks in the NGPB, specifically the Jiangga, Jiaduopule, and Rema skarn Fe deposits, and the Yaguila Pb–Zn–Ag deposit. Although some of these deposits (porphyry Mo systems) formed during the post-collisional stage (21–14 Ma), the majority (these three systems) developed during the main (‘soft collision’) stage of the India–Asia continental collision (65–50 Ma). The skarn Fe deposits are commonly associated with granodiorites, monzogranites, and granites, and formed between 65 and 50 Ma. The ore-forming intrusions of the Pb–Zn–Ag deposits are characterized by granite, quartz porphyry, and granite porphyry, which developed in the interval of 65–55 Ma. The ore-forming porphyries in the Sharang Mo deposit, formed at 53 Ma. The rocks from Fe deposits are metaluminous, and have relatively lower SiO₂, and higher CaO, MgO, FeO contents than the intrusions associated with Mo and Pb–Zn–Ag mineralization, while the Pb–Zn–Ag deposits are peraluminous, and have high SiO₂ and high total alkali concentrations. They all exhibit moderately fractionated REE patterns characterized by lower contents of heavy REE relative to light REE, and they are enriched in large-ion lithophile elements and relatively depleted in high-field-strength elements. Ore-forming granites from Fe deposits display ⁸⁷Sr/⁸⁶Sr_(i) = 0.7054–0.7074 and ε_Nd(t) = − 4.7 to + 1.3, whereas rocks from the Yaguila Pb–Zn–Ag deposit have ⁸⁷Sr/⁸⁶Sr_(i) = 0.7266–0.7281 and ε_Nd(t) = − 13.5 to − 13.3. In situ Lu–Hf isotopic analyses of zircons from Fe deposits show that ε_Hf(t) values range from − 7.3 to + 6.6, with T_DM(Hf)^C model ages of 712 to 1589 Ma, and Yaguila Pb–Zn–Ag deposit has ε_Hf(t) values from − 13.9 to − 1.3 with T_DM(Hf)^C model ages of 1216 to 2016 Ma. Combined with existing data from the Sharang Mo deposit, we conclude that the ore-forming intrusions associated with the skarn Fe and porphyry Mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central Lhasa subterrane, respectively. Melting of the ancient continental material was critical for the development of the Pb–Zn–Ag system. Therefore, it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the India–Asia continental collision.