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1.
Luobusaite： A New Mineral   总被引：6，自引：0，他引：6  
A group of mantle minerals including about 70-80 subtypes of minerals are discovered from a podiform chromitite in Tibet, China. Recovered minerals include diamond, coesite, moissanite, wustite, Fe-silides and a new mineral, luobusaite. All of these minerals were hand-picked from heavymineral separates of the podiform chromitite in the mantle peridotite of an ophiolite. The grains of luobusaite are as host mineral with inclusions of native silicon or as an intergrowth with native silicon and Fe-Si phase. Luobusaite occurs as irregular grains, with 0.1-0.2 mm in size, consisting of very finegrained aggregates. The mineral is steel-grey in color, metallic luster, and opaque. The empirical formula （based on 2 for Si） is Fe0.83Si2, according to the chemical compositions of luobusaite. X-ray powder-diffraction data： orthorhombic system, space group Cmca, a = 9.874 （14） A, b = 7.784 （5） A, c= 7.829（7） A, Z=16.  相似文献   

2.
The study on the competitive adsorption shows that the magnitude order of metal ions adsorbed onto oxide and silicate minerals in near-neutral solution with low ionic strength is in mole/nm2 as follows: CaCO3 > quarte > hydromuscovite > kaolinite > Ca-montmorillonite > goethite > gibbsite. These minerals can be divided into three groups according to their surface equilibrium constantsK M of the adsorption reactions, which are the function of the dielectric constants ε of the absorbent minerals. The relationships between constantsK M and mineral dielectric constants ε are described as follows: lgK M 1 = 7.813-26.15/ε lgK M 2 = 9.030-26.15/ε lgK M 3 =11.63-26.15/ε for the adsorption reaction: >SO- + Mn+≥SOMn-1)+ (n = 1, 2, 3) The first group of minerals include quartz, goethite, 1:1 phyllosilicates and other oxide minerals; the second: gibbsite, brucite and 2:1 phyllosilicates; the third: carbonate, sulphate and phosphorate minerals. The appearance reaction constants have a variation of magnitude ±0.5 for different metal ions with the same mineral. This project was financially supported by the National Natural Science Foundation of China (No. 49572091).  相似文献   

3.
Paleoproterozoic carbonaceous shales in the Tim-Yastrebovskii ancient rift, which underwent zonal metamorphism at 350–550°C, contain REE mineralization of silicates (allanite, thorite, and Ce-P huttonite) fluorcarbonates (bastnaesite and synchysite), phosphates (monazite and xenotime), and REE-bearing apatite. The reason for the wide occurrence of bastnaesite and other REE minerals is relatively high REE concentrations in the sulfide-bearing carbonaceous shales, with these elements accumulated in the organic matter in the course of diagenesis. Reaction textures with REE-bearing chlorite, bastnaesite, and allanite suggest that REE-bearing chlorite and bastnaesite provided REE for the forming of higher temperature allanite and monazite. This is corroborated by the REE patterns of the monazite, allanite, and bastnaesite, which are almost identical and are characterized by the strong predominance of LREE. The replacements of REE minerals during metamorphism at 350–550°C took place via a number successive transitions: (1) Mnz → Aln, Chl REE → Bst, Chl REE → Aln, Bst → Aln and (2) Bst → Mnz and Ap LREE → Mnz. These replacements can be accounted for by prograde metamorphic reactions.  相似文献   

4.
Zinccopperite (tentatively named) is a rare native alloy mineral discovered in quartz monzonite-porphyry in the Xifanping area, Yanyuan County, Sichuan Province. It is a new variety of zinc-copper alloy mineral found for the first time in the porphyry-copper deposit in China. Its intergrown minerals are K-feldspar (mainly perthite), albite-oligoclase, quartz and biotite; and the associated minerals include pyrite and chalcopyrite. It is characterized by a golden reflection colour, being isotropic (isometric), with the grain size ranging from 10 to 50 μm, microhardness VHN10= 190 kg/mm2, and reflectance RVis= 67.97%. Electron microprobe (Model JXA-733) analysis shows Cu = 59.15%-62.55% and Zn= 36.32%-39.85%. The crystallochemical formula is Cu6.27-7.0Zn4.0, simplified as Cu7Zn4.  相似文献   

5.
Yarlongite: A New Metallic Carbide Mineral   总被引：1，自引：0，他引：1  
Yarlongite occurs in ophiolitic chromitite at the Luobusha mine (29°5′N 92°5′E, about 200 km ESE of Lhasa), Qusum County, Shannan Prefecture, Tibet Autonomous Region, People’s Republic of China. Associated minerals are: diamond, moissanite, wüstite, iridium (“osmiridium”), osmium (“iridosmine”), periclase, chromite, native iron, native nickel, native chromium, forsterite, Cr-rich diopside, intermetallic compounds Ni-Fe-Cr, Ni-Cr, Cr-C, etc. Yarlongite and its associated minerals were handpicked from a large heavy mineral sample of chromitite. The metallic carbides associated with yarlongite are cohenite, tongbaite, khamrabaevite and qusongite (IMA2007-034). Yarlongite occurs as irregular grains, with a size between 0.02 and 0.06 mm, steel-grey colour, H Mohs: 5?-6. Tenacity: brittle. Cleavage: {0 0 1} perfect. Fracture: conchoidal. Chemical formula: (Cr4Fe4Ni)Σ9C4, or (Cr,Fe,Ni)Σ9C4, Crystal system: Hexagonal, Space Group: P63/mc, a = 18.839(2) ?, c = 4.4960 (9) ?, V = 745.7(2) ?3, Z = 6, Density (calc.) = 7.19 g/cm3 (with simplified formula). Yarlongite has been approved as a new mineral by the CNMNC (IMA2007-035). Holotype material is deposited at the Geological Museum of China (No. M11650).  相似文献   

6.
Situated in the Henan-Shaanxi fault-uplift area on the southern margin of the Sino-Korean Paraplatform, the Jinduicheng porphyry molybdenum deposit is the most important molybdenum producer in China. During Yenshanian the Jinduicheng granite porphyry was emplaced in metaspilite of the Proterozoic Xionger Group, controlled by a NW-trending fault. Mineral compositions are mainly quartz (25–40%), microcline and microcline-perthite (27–40%) and plagioclase (An 8–14, 14–32%), associated with minor biotite and muscovite, and phenocrysts are made up of K-feldspar, quartz and plagioclase. Accessory minerals include magmatite, apatite and zircon. The porphyry contains SiO2 73.83% and K2O + Na2O 8.06% (with K2O/Na2O ratio being 1.82), beloning to the calc-alkaline series. Mineralization occurs in the porphyry body and biotitized and hornfelsized spilite within the exocontact zone. The maximum depth of mineralization reaches 1000 meters below the surface. According to mineral assemblage, ore veins are classified into five types: (1) pyrite-quartz; (2) pyrite-K-feldspar-quartz; (3) pyrite-molybdenite-quartz; (4) pyrite-molybdenite-K-feldspar-quartz; and (5) muscovitefluorite-pyrite-molybdenite-quartz. As the most important economic molybdenum mineral, molybdenite occurs in various forms. Wall-rock alterations show a gross zonation of K-feldspathization-greisenization → silicification → propylitization from the porphyry outwards, of which silicification is most intense and has close genetic relationship with Mo mineralization. Fluid inclusion studies yield homogeneous temperatures ranging from 250 to 240°C, with the main stage between 400 and 300°C. Gas inclusions frequently coexist with multiphase inclusions containing such daughter minerals as halite, sylvite, molybdenite and K-feldspar. Under moderate-high temperatures, lowerfo2, highfs2 and weak acidity conditions, boiling of ore-forming fluids is a prerequisite for the precipitation of molybdenite. Sulfur, oxygen and carbon isotopic compositions suggest that at the high temperature stage (450°C) magmatic water is dominant and at the main ore-forming stage a mixture of magmatic water and meteoric water is expected. At the late stage, the mixture is predominated by meteoric water. Sulfur and molybdenum are mostly of magmatic origin.  相似文献   

7.
On the basis of mineral paragenesis and the chemistry and homogenization temperatures of fluid inclusions, the physicochemical parameters were calculated for the formation of the Dalingkou Ag-Pb-Zn deposit in Zhejiang. From the early to the late stage of mineralization the ore-forming temperature venation was found to be 298.5 °C → 267.0 °C → 217.6 °C → 167.3 °C, with a corresponding pH change of 3.0 ∼ 5.8 → 6.1 → 6.7→ 5.0 ∼ 7.3. The pressure changed from 403.8 to 128.5 atm, andlogf S2-9.9 → -11.2 → < -15; logf o2< -44→ -45.6 ∼ -42.6 → > -44.2; and logf co2 around -1.55. In conjunction with geological observations, the deposit is considered to be of meso-epithermal origin, i.e., it was formed after continental volcanic-subvolcanic activity. The major factors affecting ore precipitation are the decreasing temperature and the increasing pH of ore-forming solutions.  相似文献   

8.
Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X A as well as P and T. The predicted mineral associations for the low- and high-X A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased  相似文献   

9.
A continuous flow method, by a combination of thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), is presented for determination of both H isotope composition and H2O concentration of garnet from eclogite. Together with biotite NBS-30, the garnet was tested by preheating mineral grains at different temperatures. Preheating at 90°C for 12 h was found to be capable of eliminating adsorption water on sample surface. This results in constant δD values and total H2O contents for the garnet, with weighted means of −93 ± 2‰ and 522 ± 11 ppm (wt), respectively. The garnet that was preheated at 350°C for 4 h also gave constant δD values of −86 ± 6‰ and H2O contents of 281 ± 13 ppm (wt). The latter result for the H2O contents agrees with the H2O contents 271 ± 58 ppm (wt) measured by Fourier transform infrared spectroscopy for quantitative analysis of structural hydroxyl in the same garnet. Stepwise-heating TC/EA-MS analyses for the garnet show that the molecular H2O are depleted in D relative to the structural OH and has higher mobility than the structural OH. Therefore, the TC/EA-MS method can be used not only for quantitative determination of both H isotope composition and H2O concentration of hydrous and anhydrous minerals, but also for the concentration of structural hydroxyl after high-T dehydration.  相似文献   

10.
Fluid inclusions that bear halite daughter minerals were discovered in volcanic rocks at Pingnan area in the Dongying sag. The samples of the fluid inclusions collected from the BGX-15 well drill cores are hosted in quartz of diorite-porphyrite. The daughter minerals are identified as NaCl crystals after being observed under a microscope and analyzed by in situ Raman spectroscopy at −185°C. The results of micro-thermal analysis show that the homogenization temperatures of primary fluid inclusions are between 359 and 496°C, and the salinities of fluid inclusions are from 43.26 to 54.51 wt-%. All fluid inclusions in the studied samples can be divided into five types including primary fluid inclusions and secondary fluid inclusions. The fact that five types of fluid inclusions were symbiotic in the same quartz grain implies that immiscibility happened in magma. Due to the decrease in temperature and pressure during the ascent of magma, the fluids became intensively immiscible. This process accelerates the degassing of CO2 from magma, but the remnant fluids with high salinity are preserved in fluid inclusions. Thus, the primary fluid inclusions are mainly in NaCl-H2O fluids and poor in CO2. The results of our study indicate that the degassing of magma and accumulation of CO2 gas at the Pingnan area are relative to the immiscibility of high salinity fluids. This discovery is important because it can help us have a further understanding of the mechanism of magma degassing and accumulation of the inorganic CO2 in eastern China. Translated from Acta Geologica Sinica, 2006, 80(11): 1699–1705 [译自: 地质学报]  相似文献   

11.
The Taikeu ore cluster comprises four tantalum-bearing ore deposits: Taikeu, Ust-Mramorny, Longotyugan, and Neudachny. Their ores are related to microcline-albite and less frequent quartz-albite-mica (phengite) metasomatic rocks replacing granites. The major ore minerals belong to the fergusonite and pyrochlore groups (mainly plumbopyrochlore). The compositional evolution of these minerals is considered in detail. High-Yb fergusonite containing up to 22–27 wt % Yb2O3 was established in the ore cluster for the first time. Its formula is ({Yb0.30Er0.15Dy0.05Lu0.04Tm0.04…}0.61Y0.39)[Nb0.99Ta0.01]O3.96. The density of this mineral determined by microvolumetric method is 6.47 ± 0.06 g/cm3. Fergusonite evolves from high-Yb to low-Yb varieties, with relatively small variations in Er and Dy. It is suggested that this fractionation trend is caused by high solubility of Yb2+ complexes (primarily, fluoride complexes) in aqueous fluid during albitization. The chemical composition of the pyrochlore-group minerals evolves as follows: Na+Ca → U+Th → Pb. Plumbopyrochlore was formed at the late metasomatic stage of tantalum-niobium mineralization. The characteristic features of this stage are a high oxidation potential, on the one hand, and a low activity of sulfide sulfur, on the other hand.  相似文献   

12.
Silicon shows no variation in its chemical valence in nature and exists mainly in the form of silicon-oxygen tetrahedra, so very small silicon isotope thermodynamic fractionation occurs and the resultant silicon isotope variation is limited. Dynamic fractionation of Si isotopes during precipitation of SiO2 from a solution is a main factor leading to substantial variations in silicon isotopes in nature. In this experimental study, we determined the dynamic fractionation factorα for silicon isotopes during precipitation of SiO2 from the solution. And in combination of α, a theoretical explanation is presented of the considerably low δ30Si values of black smokers on modern seafloor, Archean banded magnetite-quartzite and clay minerals of weathering origin, and of clearly high δ30Si values of siliceous rocks in shallow-sea carbonate platforms. This paper won the Paper of Excellence in the Second National Young Scientist Symposium on Geochemistry of Minerals and Rocks.  相似文献   

13.
A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks. Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite (2–18% grossular component) and diopside (X Fe = 0.01–0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside (X Fe = 0.14–0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07–13.67 wt % Cu, 0.48–1.56 wt % Zn, and 0.76–1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2–6.4 wt % Ag). They contain veinlet-shaped AuCu zones with 13.3–14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close to auricupride (AuCu3). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation was estimated at 350–600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem ophiolitic belt.  相似文献   

14.
In this study, magnetic techniques were used to characterize the surface soil from different geomorphologies (i.e., sand desert, oasis, Gobi, and dry lake) in Central Asia. Results demonstrate that the main magnetic minerals in the surface soil are magnetite, maghaemite and haematite with some paramagnetic materials. Cross plots of M rs/M s versus B cr/B c and χfd% versus χarm/saturation isothermal remanent magnetization (SIRM) indicate that the main magnetic grain sizes in surface soil are pseudo single domain (PSD) and multidomain (MD). The samples from West China (i.e., Tarim basin and Junggar basin) are dominated by magnetic minerals with larger grain size, while those from North China (i.e., Alxa plateau, Erdos plateau, and Mongolia plateau) are dominated by fine magnetic minerals. The similarity in magnetic mineral constitutions between the Chinese loess and the surface soils from Central Asia implies that the loess originated from a vast area of arid, semi-arid regions of Central Asia. The low value of concentration-dependent magnetic parameters indicates that the low concentration of magnetic minerals in the surface soils from Central Asia and the magnetic enhancement from the pedogenic take place in both the loess and the paleosols, although the progress is stronger in the latter. Translated from Quaternary Sciences, 2006, 26(6): 937–946 [译自: 第四纪研究]  相似文献   

15.
A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R1, R2-sharp line luminescence and spectra of excitation of λ3- and λ4-components of R1-line of Cr3+-emission had the following results: (1) The Fe2+–Ti4+ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm−1 if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm−1 in crystals with  ≥ . The energy difference is explained by an expansion of the Of–Ok, and Ob–Om edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr3+ substitutes for Al compared to the chromium-free case. (2) The Cr3+ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr3+ 4A2g→4T2g(F)(I), →4T1g(F)(II), and →4T1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm−1. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm−1, Dq(b)=1600 cm−1, B(a)=790 cm−1, B(b)=620 cm−1 (errors ca. ±10 cm−1), again in agreement with values extracted from the λ3, λ4 excitation spectra. The CF-values of set a are close to those typical of Cr3+ substituting for Al in octahedra of other silicate minerals without constitutional OH− as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr3+ in the kyanite structural matrix. The CF-data of Cr3+ type b, expecially B, resemble those of Cr3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997  相似文献   

16.
Summary We have conducted electron microprobe (EMP) analysis of 158 grains of platinum-group minerals (PGM; 0.1–1 mm in size) from 11 placer samples collected from Holocene fluvial placers and buried paleochannel placers at various localities in British Columbia. These grains principally comprise Pt-Fe-(Cu) alloy minerals: Fe-rich platinum [ΣPGE:(Fe + Cu + Ni) = 3.6–7.6], Pt3Fe-type alloy (isoferroplatinum or Fe-rich platinum), subordinate “Pt2Fe”-type alloy (probably, a compositional variant of Fe-rich platinum) and the tulameenite-tetraferroplatinum series. Less-abundant are iridium [Ir-dominant Ir-Os-(Pt) alloy] and osmium [Os-dominant Os-Ir-(Pt) alloy]. Ruthenium [Ru-dominant Ru-Ir-Os alloy] occurs as a single grain. One of these Pt-Fe alloy grains is unusually zoned; its core zone is: Pt74.0Fe20.4Cu1.9Ir1.5Rh1.1Pd1.0Os0.08Ru0.01Ni0.01 (in at%) [ΣPGE:(Fe + Cu + Ni) = 3.5], and its rim zone is: Pt78.5Fe15.5Cu1.7Ir1.5Rh1.4 Pd1.2Ni0.15Os0.06Ru<0.01 [ΣPGE:(Fe + Cu + Ni) = 4.8]. This zoning indicates late-stage removal of Fe and corresponding addition of Pt, probably as a result of interaction with a late fluid phase. Various combinations of minor elements: Ir-Rh, Rh-Pd, and Ir-Rh-Pd are observed in the analysed Pt-Fe-Cu alloys. However, the Ir-Pd pair appears to be prohibited because of crystallochemical factors. Minute PGM inclusions in Pt-Fe alloy grains, likely derived from the Tulameen complex, comprise: hongshiite (Pt1.04Pd0.02 Cu0.93), sperrylite (Pt0.93Ir0.03)Σ0.96(As2.02Sb0.01)Σ2.03, hollingworthite-platarsite (Rh0.74 Pt0.21Fe0.02Pd0.02Ir0.01)Σ1.00S0.91As1.10, cuprorhodsite-malanite (Cu0.91Fe0.03Ni<0.01)Σ0.95 (Rh1.06Pt0.89Ir<0.01)Σ1.95S4.10, a rare Te-rich isomertieite (Pd10.96Fe0.03)Σ10.99(Sb1.13 Te0.94)Σ2.07As1.93, and an unusual Pt-Pd-Rh antimonide [(Pt + Pd + Rh):(Sb + As) = 1.2–1.25], related to genkinite. This antimonide may exhibit a minor solid solution extending from genkinite toward stumpflite. In addition, 20 grains of diopside [Ca46.4–49.1Mg42.8–48.2Fe3.1–8.1; ≤0.59 wt% Cr2O3] and 20 grains of olivine [Fo86.8–91.5 Fa7.9–12.5], from a PGM-bearing placer located in the vicinity of the Tulameen complex, were analysed. The compositional ranges of these placer silicates are comparable to those of clinopyroxene and olivine in the olivine clinopyroxenite and dunite units of the Tulameen complex. The majority of the analysed placer PGM grains were probably derived from Alaskan-type source rocks, whereas an ophiolitic source, associated with the Atlin ophiolite complex, is suggested for the placer PGM deposits in the Atlin area, northern British Columbia. Authors’ addresses: Andrei Y. Barkov, Robert F. Martin, Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada; Michael E. Fleet, Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada; Graham T. Nixon and Victor M. Levson, B.C. Geological Survey, Ministry of Energy, Mines and Petroleum Resources, PO Box 9320 Stn. Prov. Govt., Victoria, British Columbia V8W 9N3, Canada  相似文献   

17.
The distribution of gold in high-temperature fumarole gases of the Kudryavy volcano (Kurile Islands) was measured for gas, gas condensate, natural fumarolic sublimates, and precipitates in silica tubes from vents with outlet temperatures ranging from 380 to 870°C. Gold abundance in condensates ranges from 0.3 to 2.4 ppb, which is significantly lower than the abundances of transition metals. Gold contents in zoned precipitates from silica tubes increase gradually with a decrease in temperature to a maximum of 8 ppm in the oxychloride zone at a temperature of approximately 300°C. Total Au content in moderate-temperature sulfide and oxychloride zones is mainly a result of Au inclusions in the abundant Fe–Cu and Zn sulfide minerals as determined by instrumental neutron activation analysis. Most Au occurs as a Cu–Au–Ag triple alloy. Single grains of native gold and binary Au–Ag alloys were also identified among sublimates, but aggregates and crystals of Cu–Au–Ag alloy were found in all fumarolic fields, both in silica tube precipitates and in natural fumarolic crusts. Although the Au triple alloy is homogeneous on the scale of microns and has a composition close to (Cu,Ni,Zn)3(Au,Ag)2, transmission electron microscopy (TEM) shows that these alloy solid solutions consist of monocrystal domains of Au–Ag, Au–Cu, and possibly Cu2O. Gold occurs in oxide assemblages due to the decomposition of its halogenide complexes under high-temperature conditions (650–870°C). In lower temperature zones (<650°C), Au behavior is related to sulfur compounds whose evolution is strongly controlled by redox state. Other minerals that formed from gas transport and precipitation at Kudryavy volcano include garnet, aegirine, diopside, magnetite, anhydrite, molybdenite, multivalent molybdenum oxides (molybdite, tugarinovite, and ilsemannite), powellite, scheelite, wolframite, Na–K chlorides, pyrrhotite, wurtzite, greenockite, pyrite, galena, cubanite, rare native metals (including Fe, Cr, Mo, Sn, Ag, and Al), Cu–Zn–Fe–In sulfides, In-bearing Pb–Bi sulfosalts, cannizzarite, rheniite, cadmoindite, and kudriavite. Although most of these minerals are fine-grained, they are strongly idiomorphic with textures such as gas channels and lamellar, banded, skeletal, and dendrite-like crystals, characteristic of precipitation from a gas phase. The identified textures and mineral assemblages at Kudryavy volcano can be used to interpret geochemical origins of both ancient and modern ore deposits, particularly gold-rich porphyry and related epithermal systems.  相似文献   

18.
Summary The stoichiometry of pyroxenes {viiiX2+ viY2+ ivZ4+ 2 O6} and garnets {xiiX2+ 3 viY3+ 2 ivZ4+ 3 O12} is re-evaluated by a theoretical crystal-chemical approach that takes into account natural phenomena that do not fit with conventional anhydrous stoichiometric mineralogy: the existence in eclogites of microinclusions of other minerals that may have been exsolved from previous supersilicic or subsilicic UHPM pyroxene or garnet. Different definitions of supersilicic and subsilicic are discussed and the one based on the ability to exsolve SiO2 and leave behind a stoichiometric pyroxene or garnet is recommended for general adoption. The SHAND system (S = Si et al.; H = H; A = Al et al.; N = Na et al.; D = divalents) for projecting multivariate chemical space involving 23 cations and 104 selected natural or potential mineral species on to two essential diagrams (SAND and SHND) is described in full for the first time. Numerous possible chemical exchanges are considered and justified with respect to known mineral phenomena such as cation vacancies, octahedral silicon or protonation. Several new potential end-members are presented, in particular “supersilipyx”, “supersiligar” and “subsiligar”. It is suggested that small quantities of these end-members can be incorporated into UHPM solid-solutions and lead to various exsolution phenomena during eclogite exhumation.  相似文献   

19.
Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM21O38, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2−x(OH)x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb2O5), Cr (up to ∼6 wt% Cr2O3), and OH (up to ∼0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998  相似文献   

20.
Summary The Tuvatu gold–telluride prospect is one of several epithermal gold systems along the >250 km northeast trending Viti Levu lineament, Fiji, which are genetically associated with alkalic magmatism. Vein structures contain a variety of sulfides, native elements, sulfosalts, and tellurides. Calaverite is intimately associated with various vanadium-bearing minerals: roscoelite, karelianite, vanadian muscovite, Ti-free nolanite, vanadian rutile, schreyerite, and an unnamed vanadium silicate. Thermodynamic calculations for the systems V–Al–K–Si–O–H (Cameron, 1998) and Au–Te–Cl–S–O–H at estimated conditions of formation of the telluride-native gold stage at Tuvatu (∼250 °C, ΣAu = 1 ppb, ΣTe = 1 ppb, ΣS = 0.001 m, ΣV = 0.0001 m, and aK = 0.01), show that the stability fields of calaverite, roscoelite, and karelianite converge in pH-fO2 space near the hematite–magnetite buffer and at neutral to slightly acid conditions. Thermodynamic and textural data suggest that these minerals were deposited together at Tuvatu and likely explain the common coexistence of roscoelite and calaverite in epithermal gold systems elsewhere. The presence of magnetite with up to 0.7 wt.% V2O3 in the Navilawa Monzonite is consistent with the derivation of V from the alkalic intrusive rocks, which are also considered to be the source of Au and Te in the Tuvatu deposit.  相似文献   

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