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1.
The two drill holes, which penetrated sub‐horizontal rare earth element (REE) ore units at the Nechalacho REE in the Proterozoic Thor Lake syenite, Canada, were studied in order to clarify the enrichment mechanism of the high‐field‐strength elements (HFSE: Zr, Nb and REE). The REE ore units occur in the albitized and potassic altered miaskitic syenite. Zircon is the most common REE mineral in the REE ore units, and is divided into five types as follows: Type‐1 zircon occurs as discrete grains in phlogopite, and has a chemical character similar to igneous zircon. Type‐2 zircon consists of a porous HREE‐rich core and LREE–Nb–F‐rich rim. Enrichment of F in the rim of type‐2 zircon suggests that F was related to the enrichment of HFSE. The core of type‐2 zircon is regarded to be magmatic and the rim to be hydrothermal in origin. Type‐3 zircon is characterized by euhedral to anhedral crystals, which occur in a complex intergrowth with REE fluorocarbonates. Type‐3 zircon has high REE, Nb and F contents. Type‐4 zircon consists of porous‐core and ‐rim, but their chemical compositions are similar to each other. This zircon is a subhedral crystal rimmed by fergusonite. Type‐5 zircon is characterized by smaller, porous and subhedral to anhedral crystals. The interstices between small zircon grains are filled by fergusonite. Type‐4 and type‐5 zircon grains have low REE, Nb and F contents. Type‐1 zircon is only included in one unit, which is less hydrothermally altered and mineralized. Type‐2 and type‐3 zircon grains mainly occur in the shallow units, while those of type‐4 and type‐5 are found in the deep units. The deep units have high HFSE contents and strongly altered mineral textures (type‐4 and type‐5) compared to the shallow units. Occurrences of these five types of zircon are different according to the depth and degree of the hydrothermal alteration by solutions rich in F and CO3, which permit a model for the evolution of the zircon crystallization in the Nechalacho REE deposit as follows: (i) type‐1 (discrete magmatic zircon) is formed in miaskitic syenite. (ii) LREE–Nb–F‐rich hydrothermal zircon formed around HREE‐rich magmatic zircon (type‐2). (iii) type‐3 zircon crystallized through the F and CO3‐rich hydrothermal alteration of type‐2 zircon which formed the complex intergrowth with REE fluorocarbonates; (iv) the CO3‐rich hydrothermal fluid corroded type‐3, forming REE–Nb‐poor zircon (type‐4). Niobium and REE were no longer stable in the zircon structure and crystallized as fergusonite around the REE–Nb‐leached zircon (type‐4); (v) type‐5 zircon is formed by the more CO3‐rich hydrothermal alteration of type‐4 zircon, suggested by the fact that type‐4 and type‐5 zircon grains are often included in ankerite. Type‐3 to type‐5 zircon grains at the Nechalacho REE deposit were continuously formed by leaching and/or dissolution of type‐2 zircon in the presence of F‐ and/or CO3‐rich hydrothermal fluid. These mineral associations indicate that three representative hydrothermal stages were present and related to HFSE enrichment in the Nechalacho REE deposit: (i) F‐rich hydrothermal stage caused the crystallization of REE–Nb‐rich zircon (type‐2 rim and type‐3), with abundant formation of phlogopite and fluorite; (ii) F‐ and CO3‐rich hydrothermal stage led to the replacement of a part of REE–Nb–F‐rich zircon by REE fluorocarbonate; and (iii) CO3‐rich hydrothermal stage resulted in crystallization of the REE–Nb–F‐poor zircon and fergusonite, with ankerite. REE and Nb in hydrothermal fluid at the Nechalacho REE deposit were finally concentrated into fergusonite by way of REE–Nb–F‐rich zircon in the hydrothermally altered units.  相似文献   

2.
The variant rock types of an Alkaline-Carbonatite Complex (ACC) comprising alkali pyroxenite, nepheline syenite, phoscorite, carbonatite, syenitic fenite and glimmerite along with REE and Nb-mineralization are found at different centres along WNW-ESE trending South Purulia Shear Zone (SPSZ) in parts of Singhbhum Crustal Province. The ACC occurs as intrusions within the Mesoproterozoic Singhbhum Group of rocks. Alkali pyroxenite comprises of aegirine augite, magnesiotaramite, magnesiokatophorite as major constituents. Pyrochlore and eucolite are ubiquitous in nepheline syenite. Phoscorite contains fluorapatite, dahllite, collophane, magnetite, hematite, goethite, phlogopite, calcite, sphene, monazite, pyrochlore, chlorite and quartz. Coarse fluorapatite shows overgrowth of secondary apatite (dahllite). Secondary apatite is derived from primary fluorapatite by solution and reprecipitation. The primary fluorapatite released REE to crystallize monazite grains girdling around primary apatite. Carbonatite is composed dominantly of Srcalcite along with dolomite, tetraferriphlogopite, phlogopitic biotite, aegirine augite, richterite, fluorapatite, altered magnetite, sphene and monazite. The minerals comprising of the carbonatite indicate middle stage of carbonatite development. Fenite is mineralogically syenite. Glimmerite contains 50–60% tetraferriphlogopite. An alkali trend in the evolution of amphiboles (magnesiotaramite-magnesiokatophorite-richterite) and chinopyroxenes (aegirine augite, aegirine) during the crystallization of the suite of rocks is noted. Monazite is the source of REE in phoscorite and carbonatite. Fluorapatite has low contents of REE, PbO, ThO2 and UO2. Pyrochlore reflects Nb-mineralization in nepheline syenite and it is enriched in Na2O, CaO, TiO2, PbO and UO2. Pyrochlore containing UO2 (6.605%) and PbO (0.914%) in nepheline syenite has been chemically dated at 948 ± 24 Ma by EPMA.  相似文献   

3.
Zoned scheelite crystals from the Yubileinoe porphyry gold deposit were studied by EMPA and LA-ICP-MS. The MoO3 content decreases from 10–13 wt % in the crystal center to less than 2 wt % in the rim. Scheelite is enriched in LREE with respect to HREE and has negative Eu and Ce anomalies. Early scheelite has a flat REE distribution pattern with a negative slope, while the latter has asymmetrical convex REE spectra due to the lower La content and higher Nd, Sm, and Gd concentrations. The REE distribution in early scheelite has been established as inherited from ore-bearing granitic rocks, while this distribution in later generations of the mineral was determined by hydrothermal fluid.  相似文献   

4.
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   

5.
贵州省松桃县道坨超大型锰矿床的发现及其成因探讨   总被引：5，自引：0，他引：5  
贵州省松桃县道坨锰矿床是新发现的一个超大型全隐伏锰碳酸盐矿床。文章阐述了该矿床的发现概况及基本的地质和地球化学特征,并应用锰矿石和含锰黑色页岩的元素和碳同位素地球化学特征对菱锰矿的形成机制提出了制约。道坨超大型锰矿床的发现是填图及对区域地质资料综合分析的结果。该矿床具有品位高、厚度大、呈层性好及展布广等特点。其锰矿石的主量元素特征为Al2O3、TiO2、Fe2O3含量较低,P2O5中等程度富集,MnO、MgO含量相对较高,Fe/Mn比值低。在微量元素方面,锰矿石显示出较为明显的Co富集,含锰黑色页岩则显示出较为明显的Co、Mo富集;在稀土元素方面,锰矿石具有较高的稀土元素总量,轻微的"帽式"稀土元素PAAS标准化配分模式及明显的Ce正异常,含锰黑色页岩的稀土元素总量与PAAS接近,其稀土元素PAAS标准化配分模式较为平坦,无Ce异常。碳同位素测试结果显示出菱锰矿富集碳的轻同位素,表明在菱锰矿形成过程中存在有机碳的参与。文章表明,道坨锰矿床内的锰是以氧化物或氢氧化物的形式沉淀,菱锰矿是在缺氧且富含有机物质的成岩环境中转化而成。  相似文献   

6.
The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF2-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 ± 0.1 to 25,000 ± 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 °C and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from ∼2 to ∼0.5 with increasing Gd in fluorapatite, hence a marked non-Henry’s Law behavior, but becomes independent of composition at and above ∼5000 and ∼1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry’s Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry’s Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry’s Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca2+ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry’s Law in their respective compositional ranges investigated in this study.Nonstoichiometry, such as Ca2+ and F− vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry’s Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REEs by natural apatites. Particularly, the non-Henry’s Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites.  相似文献   

7.
In the bottom part of the tongue-shaped, layered granitoid intrusion, exposed in the Western Tatra Mts., apatite-rich granitic rocks occur as pseudo-layers and pockets between I-type hybrid mafic precursors and homogeneous S-type felsic granitoids. The apatite-rich rocks are peraluminous (ASI?=?1.12–1.61), with P2O5 contents ranging from 0.05 to 3.41 wt.% (<7.5 vol.% apatite), shoshonitic to high-K calc-alkaline. Apatite is present as long-prismatic zoned crystals (Ap1) and as large xenomorphic unzoned crystals (Ap2). Ap1 apatite and biotite represent an early cumulate. Feldspar and Ap2 textural relations may reflect the interaction of the crystal faces of both minerals and support a model based on local saturation of (P, Ca, F) versus (K, Na, Al, Si, Ba) in the border zones. Chondrite-normalized REE patterns for the apatite rocks and for pure apatite suggest apatite was a main REE carrier in these rocks. Minerals characteristics and the whole rock chemistry suggest both reduced S-type and I-type magma influenced the apatite-rich rocks. The field observations, mineral and rock chemistry as well as mass-balance calculations point out that the presence of apatite-rich rocks may be linked to the continuous mixing of felsic and mafic magmas, creating unique phosphorus- and aluminium-rich magma portions. Formation of these rocks was initially dominated by the complex flowage-controlled and to some extent also gravity-driven separation of early-formed zoned minerals and, subsequently, by local saturation in the border zones of growing feldspar and apatite crystals. Slow diffusion in the phosphorus-rich magma pockets favoured the local saturation and simultaneous crystallization of apatite and feldspars in a crystal-ladden melt.  相似文献   

8.
Grothite—a rare Al- and F-rich variety of titanite—was identified in two different gold-bearing mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region, Russian Far East. Grothite is associated with quartz, orthoclase, chlorite, muscovite, tourmaline, almandine-spessartine garnet, ilmenite, pyrophanite, magnetite, fluorapatite, and sulfides. Grothite forms numerous scattered lamellar aggregates 20–100 μm in size with a relatively homogeneous structure. The lamellae grow in chlorite or between chlorite and orthoclase. According to microprobe analyses, variations in major elements of grothite are as follows (wt %): 30.56–34.07 SiO2, 7.91–12.71 Al2O3, 22.83–28.29 TiO2, 23.55–29.21 CaO, 0.52–4.25 FeO, and 2.19–6.16 F. It is suggested that grothite appeared in the gold-bearing mineral assemblages of the Berezitovy deposit due to the specifics of the primary composition of the host rocks and physicochemical conditions of the fluid regime at the final stage of deposit formation.  相似文献   

9.
Abstract: The Onsen acid‐sulphate type of mineralization is located in the Desmos caldera, Manus back‐arc basin. Hydrothermal precipitates, fresh and altered basaltic andesite collected from the Desmos caldera were studied to determine mineralization and mobility of elements under seawater dominated condition of hydrothermal alteration. The mineralization is characterized by three stages of advanced argillic alteration. Alteration stage I is characterized by coarse subhedral pyrophyllite with disseminated anhedral pyrite and enargite which were formed in the temperature range of 260–340°C. Alteration stage II which overprinted alteration stage I was formed in the temperature range of 270–310°C and is characterized by euhedral pyrite, quartz, natroalunite, cristobalite and mixed layer minerals of smectite and mica with 14–15 Å XRD peak. Alteration stage III is characterized by amorphous silica, native sulphur, covellite, marcasite and euhedral pyrite, which has overprinted alteration stages I and II. Relative to the fresh basaltic andesite samples, the rims and cores of the partly altered basaltic andesite samples have very low major, minor and rare earth elements content except for SiO2 which is much higher (58–78 wt%) than SiO2 content of the fresh basaltic andesite (55 wt%). REE patterns of the partly altered basaltic andesite specimens are variably depleted in LREE and have pronounced negative Eu anomalies. Normalization of major, minor and REE content of the partly altered basaltic andesites to the fresh basaltic andesite indicates that all the elements except for SiO2 in the partly altered basaltic andesite are strongly lost (e.g. Al2O3 = ‐8.3 to ‐10.9 g/100cm3, Ba = ‐2.2 to ‐5.6 mg/100cm3, La = ‐130 to ‐200 μg/100cm3) during the alteration process. Abnormal depletion of MgO, total Fe as Fe2O3, LREE especially Eu and enrichment of SiO2 in the altered basaltic andesites from the Desmos caldera seafloor is caused by interaction of hot acidic hydrothermal fluid, which originates from a mixing of magmatic fluid and seawater.  相似文献   

10.
南秦岭镇安地区发现了以白钨矿-绿柱石为矿石矿物组合的矿床新类型。为查明该矿床中首次发现的翠绿色祖母绿级绿柱石的致色机理并进一步揭示其成矿机制,本文在野外地质调查的基础上,对绿柱石开展了电子探针与微区X射线衍射等矿物学研究。结果表明:矿区中首次发现的祖母绿级绿柱石,呈自形晶产于石英(方解石)脉中并与白钨矿共(伴)生。祖母绿级绿柱石从核部至边缘V2O3含量较高,分别为0.64%~0.98%和1.04%~1.42%,且有增高趋势。X射线衍射数据表明区内祖母绿为"正常"绿柱石,同时存在Al■Me2+和Be■Li两种类质同象替换机制。V为本区祖母绿主要致色元素,来自于区内碳质板岩、金云母片岩及白云质大理岩等地层,Be、Si、Al等主要元素则来自于深部酸性岩浆岩。本次发现为该矿床中钨、铍矿产资源的综合开发利用提供了基础地质资料,并为南秦岭在区域及深部继续寻找稀有金属矿指出新的找矿方向。  相似文献   

11.
A unique zircon was studied in the gneiss samples collected from the Wadi Abu Rusheid psammitic gneiss using electron scanning microscope and electron probe microanalyses. This zircon can be categorized into two types according to the texture and trace element content: (l) magmatic zircon slightly enriched in HfO2 with ordinary zone. (2) Overgrowths of zircon occur as two species, the first species being highly enriched in HfO2 with irregular zoning. The second species is highly enriched in HfO2 forming a rim around the second species with a very sharp thinner boundary. The first type shows a distinct oscillatory internal zoning pattern without change in shape of this zone and has conspicuous inclusion-free zircon overgrowths with distinct poor concentrations in Y, Hf, Th, U, Nb, and Ta in both rim and core. The second type shows two species, the first one displays distinct irregular interval zoning and irregular overgrowth with abrupt change in composition of these zones with distinct enrichment in Y, Hf, Th, U, Nb, and Ta in the rim relative to the core. The second species is forming a rim around the first species also with distinct enrichment in Y, Hf, Th, U, Nb, and Ta content. These indicate that two events (crystallization environment) have played an important role in the formation of this zircon and largely reflect differences in whole-rock trace element contents between the successive generations of this zircon. The first event is believed to be of magmatic origin giving rise to normal composition of magmatic zircon. The second event shows an intense successive process of metasomatic activity during the formation of the Abu Rusheid radioactive gneiss. Electron microprobe analysis indicates that oscillatory zoned zircon shows poor content of Y, Hf, Th, U, Nb, Ta, and rare earth elements (REE) in the rim and core, while overgrowths of zircon are slightly enriched by these elements. Also, these analyses indicate that the Abu Rusheid psammitic gneiss has been significantly enriched by the thorite mineral (Th content up to 54.72% ThO2) and columbite-bearing minerals (Nb content up to 64.74% Nb2O5, Ta content up to 9.32% Ta2O5). The poor content of REE in overgrowths of zircon indicates mobilization of REE during the metamorphism processes of gneiss.  相似文献   

12.
Xenotime and monazite inclusions in fluorapatite megacrysts from a granitic pegmatite, Gloserheia, Froland, Bamble Sector, southern Norway are described utilizing high contrast backscattered electron imaging of cross sections of a selection of fluorapatite crystals. Electron microprobe analysis is then used to further characterize the xenotime and monazite, as well as (Y+REE) normal and depleted regions in the fluorapatite. In the (Y+REE) normal regions Y2O3 ranges from 0.4 to 1.3 whereas it ranges from below the electron microprobe detection limit to around 0.4 in the depleted regions. Low Y values in monazite (XY?=?0.01?0.05) co-existing with xenotime indicates that inclusion formation in the originally (Y+REE)-enriched fluorapatite must have occurred below 300°C. Formation of the xenotime and monazite inclusions is attributed to fluid-aided coupled dissolution-reprecipitation processes during the later stages of subsolidus cooling of the pegmatite. The fluorapatite megacrysts are hypothesized to have under gone two major fluid-induced alteration events. The first occurred sometime after crystallization was complete at temperatures below 300°C and resulted in the initial formation of the xenotime and monazite inclusions. The second occurred at some later time as the product of a relatively limited fluid infiltration, also under T?<?300°C. This resulted in the formation of (Y+REE)-depleted regions along lattice and cleavage planes while at the same time promoting Ostwald ripening of the xenotime inclusions resulting in larger grains in the (Y+REE)-depleted areas.  相似文献   

13.
This paper reports the results of a geochemical investigation of zircon from a migmatized aluminous gneiss (gn), melanosome (M), and sequential leucosome generations (Lc2, Lc3, Lc4, and Lc5) from an outcrop in the northwestern Ladoga region. The contents of REE, Y, Ti, Hf, Th, U, and P were determined using a Cameca IMS-4f ion microprobe in 12 zircon grains from the aforementioned rocks, in two-three spots in each grain. All of the specimens show rather uniform REE distribution patterns. More significant variations were observed in the light and medium REE (at smaller variations in the heavy REE), as well as in Ti, Y, Th, and U contents between zircons from the host rocks and from the leucosomes. It was supposed that REE-rich zircons from the gneiss and melanosome (without oscillatory zoning) are relics, whereas rhythmically zoned zircons with lower REE contents crystallized in the gneiss in the presence of dispersed anatectic melt. The contents of most REE and Y increase from core to rim in zircons from the gneiss, melanosome, Lc2, Lc4, and Lc5, which is opposite to the compositional trend of zircons from Lc3. It was shown that the decrease of HREE and Y content in zircon in the sequence Lc5 → gn → Lc2, Lc3, Lc4 is related to a decrease in the abundance of these elements in the rocks. The leucosomes do not correspond to a differentiation series of a single melt (there is no variation trends of Rb/Sr, K/Rb, and Rb/Ba in the rock series). The lower Lu/Hf and Sm/Nd values in the leucosomes relative to the host rocks allowed us to suppose that their protolith was gneisses (for Lc2) and migmatites (for Lc4 and Lc3). The similarity of the early migmatites and gneisses to Lc3 with respect to major and some trace elements and almost identical Lu/Hf and Sm/Nd values support the possibility of the formation of this leucosome generation during the beginning of the diatexis of migmatites, which was promoted by a temperature increase. This resulted in a specific trend in the content of some elements during zircon growth in Lc3, which is different from the trend of zircons from other leucosomes.  相似文献   

14.
Extremely U-depleted (<1 ppm) zircons from H8 banded ores in the East Orebody of the Bayan Obo REE–Nb–Fe deposit are presented, with mineral compositions, textures, 232Th–208Pb SHRIMP ages and petrological context. Cores of East Orebody zircon contain up to 7 wt% HfO2 and are zoned, depicting bipyramidal crystal forms. A distinct generation of patchy, epitaxial rim zircon, similarly depleted in U, is intergrown with rare earth ore minerals (bastnäsite, parisite, monazite). Overprinting aegirine textures indicate paragenetically late, reactive Na-rich fluids. Chondrite-normalized REE patterns without Eu anomalies match closely with those from the Mud Tank and Kovdor carbonatitic zircons. Increased HREE in rims ((Lu/Gd)N 43–112) relative to cores ((Lu/Gd)N 6–7.5) and the localized presence of xenotime are attributable to reactive, mineralizing fluid compositions enriched in Y, REE and P. Cathodoluminescence further reveals HREE fractionation in rims, evidenced by a narrow-band Er3+ emission at 405 nm. The extreme depletion of U in core and rim zircon is characteristic for this mineral deposit and is indicative of a persistent common source. U depletion is also a characteristic for zircons from carbonatitic or kimberlitic systems. 232Th–208Pb (SHRIMP II) geochronological data reveal the age of zircon cores as 1,325 ± 60 Ma and a rim-alteration event as 455.6 ± 28.27 Ma. The combined findings are consistent with a protolithic igneous origin for zircon cores, from a period of intrusive, alkaline–carbonatitic magmatism. Fluid processes responsible for the REE–Nb mineralizations affected zircon rim growth and degradation during the widely reported Caledonian events, providing a new example in a localized context of HREE enrichment processes.  相似文献   

15.
http://www.sciencedirect.com/science/article/pii/S1674987113000893   总被引：1，自引：0，他引：1  
Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.  相似文献   

16.
A.M.R. Neiva   《Ore Geology Reviews》2008,33(3-4):221-238
Cassiterite and wolframite compositions from Sn > W- and W > Sn-bearing quartz veins in Northern and Central Portugal are compared to provide evidence on fluid compositions. In Sn > W-bearing quartz veins, euhedral cassiterite shows sequences of alternating parallel darker and lighter zones. The darker zones are pleochroic, oscillatory zoned, exhibit exsolutions of columbite and ixiolite and are richer in Nb, Ta and Fe than the lighter zones which consist of nearly pure SnO2. Cassiterite from W > Sn-bearing quartz veins is usually zoned, with homogeneous and slightly pleochroic darker zones, which are chemically similar to lighter zones. Both zones have inclusions of rutile and rare ilmenite. The darker zones of cassiterite from the former veins are richer in Nb, Ta and Fe contents and poorer in Ti than the darker and lighter zones of cassiterite from the latter veins. This is attributed to differences in the composition of magmatic hydrothermal fluids.Wolframite compositions from Sn > W- and W > Sn-bearing quartz veins do not show any significant distinction, because they precipitate from relatively similar magmatic hydrothermal fluids. In some deposits, most wolframite crystals are homogeneous, but others are heterogeneous. Inner patches, rich in a hübnerite component, rarely occur in crystals from the Filharoso and Panasqueira deposits. Zoned crystals, showing an increase in Fe and a decrease in Mn from core to rim, were found in the Vale das Gatas deposit. Complex oscillatory zoned crystals occur. In the Carris deposit, later wolframite contains inclusions of scheelite, partially replaces it and is richer in Fe and poorer in Mn than earlier wolframite. Wolframite from Sn > W-bearing quartz veins in the Argozelo deposit and W > Sn-bearing quartz veins from Vale das Gatas and Panasqueira deposits has significant Nb content. This does not depend on the Fe and Mn content of the wolframite, but W content is negatively correlated with Nb content. Only very rare single crystals of wolframite show an increase in W and a decrease in Nb from core to rim. Sn > W-bearing quartz veins contain wolframite poorer in Nb than the darker zones of cassiterite, which exsolved columbite and ixiolite. In W > Sn-bearing quartz veins from Panasqueira and Vale das Gatas, the wolframite has a higher Nb content than the cassiterite, which contains rutile inclusions enriched in Nb, because cassiterite and wolframite are derived from two distinct magmatic hydrothermal fluids of different age. The fluid responsible for wolframite precipitation will have a similar composition to that resulting from the evolution of the fluid responsible for cassiterite precipitation in the Sn > W-bearing quartz veins.  相似文献   

17.
The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ18O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO43−/CO32− and F−/CO32− ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.  相似文献   

18.
Acid mine drainage is a major source of water pollution in the Sarcheshmeh porphyry copper mine area. The concentrations of heavy metals and rare earth elements (REEs) in the host rocks, natural waters and acid mine drainage (AMD) associated with mining and tailing impoundments are determined. Contrary to the solid samples, AMDs and impacted stream waters are enriched in middle rare earth elements (MREEs) and heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs). This behavior suggests that REE probably fractionate during sulfide oxidation and acid generation and subsequent transport, so that MREE and HREE are preferentially enriched. Speciation modeling predict that the dominant dissolved REE inorganic species are Ln3+, Ln(SO4)2−, LnSO4+, LnHCO32+, Ln(CO3)2− and LnCO3+. Compared to natural waters, Sarcheshmeh AMD is enriched in REEs and SO42−. High concentrations of SO42− lead to the formation of stable LnSO4+, thereby resulting in higher concentrations of REEs in AMD samples. The model indicates that LnSO4+ is the dissolved form of REE in acid waters, while carbonate and dicarbonate complexes are the most abundant dissolved REE species in alkaline waters. The speciation calculations indicate that other factors besides complexation of the REE's, such as release of MREE from dissolution and/or desorption processes in soluble salts and poorly crystalline iron oxyhydroxy sulfates as well as dissolution of host rock MREE-bearing minerals control the dissolved REE concentrations and, hence, the MREE-enriched patterns of acid mine waters.  相似文献   

19.
Abstract: The North granitic body of the Miyako pluton is located in the Northern Kitakami belt, Northeast Japan. The formation of the scheelite–chalcopyrite–magnetite–bearing aplitic veins and scheelite–chalcopyrite–magnetite–bearing Yamaguchi skarn deposit was closely associated with the formation of the Miyako plutons. Petrographic facies of the North granitic body vary from quartz diorite in marginal zone (zone A), to tonalite and granodiorite (zone B), and to granite (zone C) in the central. The large numbers of aplitic veins distributed around the Yamaguchi mining area are divided into two groups: barren and scheelite–mag–netite–chalcopyrite–bearing aplitic veins. The latter cut massive clinopyroxene skarns of the Yamaguchi deposit, and are composed of plagioclase, K‐feldspar and titanite. Some plagioclase crystals have dusty cores with irregularly shaped K‐feldspar flakes, and clear rims of albite. Textures of plagioclase in the mineralized aplitic veins are different from the idiomorphic textures with sharp plagioclase crystal boundaries that occur in the North granitic body and barren aplitic veins. These textural data suggest that the mineralized aplitic veins were formed from hydrothermal fluid. Changes in the contents of major and minor (Rb, Sr, Sc, Co, Th, U) elements in the North Miyako granitic body are similar to those of zoned plutons formed by typical magmatic differentiation processes. On the other hand, concentrations of REE, especially middle to heavy REE, of granitic rocks in zone C and barren aplitic veins are significantly lower than those of granitic rocks in zones A and B. The hypothetical chondrite‐normalized REE patterns, calculated assuming fractional crystallization from zone B granitic melt, suggest that REE concentrations of the residual melt increased with the degree of fractional crystallization, and changed into a pattern with enriched LREE and strongly negative Eu anomaly. However, the REE patterns of granitic rocks in zone C are different from the hypothetical patterns. Moreover, the REE patterns of magnetite–scheelite–chalcopyrite aplitic veins are quite different from those of granitic rocks. The Cu contents of granitic rocks in the North Miyako body increase from zone A (5–26 ppm) to zone B (10–26 ppm), and then clearly decrease to zone C (5–7 ppm) and drastically increase to the barren aplitic veins (39–235 ppm). Concentrations of Cu in the mineralized aplitic veins are also higher than those of the granitic rocks in zone C. The decrease in REE and Cu contents of granitic rocks from zone B to zone C is not a result of simple magmatic fractional differentiation. Fluid inclusions in quartz from mineralized aplitic veins contain 3.3 wt% NaCl equivalent and 5.8 wt% CO2. It was also demonstrated experimentally that the removal of MREE and HREE by fluid from melt enabled the formation of complexes of REE and ligands of OH‐ and CO32‐. Based on the possibility that the melt of the granitic rocks of zone C and the mineralized aplitic veins coexisted with CO2‐bearing fluid, it is thought that REE were extracted from the melt to the CO2‐bearing fluid, and that the REE in the mineralized aplitic veins were transported by the CO2‐bearing fluid. It is likely that the low HREE and Cu contents of the granitic rocks in zone C could have been caused by the removal of those elements from the granitic melt by the fluid coexisting with the melt. The expelled materials could have been the sources of scheelite–magnetite–chalcopyrite–bearing aplitic veins and copper mineralization of the Yamaguchi Cu‐W skarn deposit.  相似文献   

20.
The Zimudang gold deposit is a large Carlin‐type gold deposit in the Southwest Guizhou Province, China, with an average Au content of 6.2 g/t. Gold is mainly hosted in the fault zone and surrounding strata of the F1 fault and Permian Longtan Formation, and the ore bodies are strictly controlled by both the faults and strata. Detailed mineralogy and geochemistry studies are conducted to help judge the nature of ore‐forming fluids. The results indicate that the Au is generally rich in the sulfides of both ores and wall rocks in the deposit, and the arsenian pyrite and arsenopyrite are the main gold‐bearing sulfides. Four subtypes of arsenian pyrite are found in the deposit, including the euhedral and subhedral pyrite, framboidal pyrite, pyrite aggregates and pyrite veins. The euhedral and subhedral pyrite, which can take up about 80% of total pyrite grains, is the dominant type. Au distributed unevenly in the euhedral and subhedral pyrite, and the content of the Au in the rim is relatively higher than in the core. Au in the pyrite veins and pyrite aggregates is lower than the euhedral and subhedral pyrite. No Au has been detected in the points of framboidal pyrites in this study. An obvious highly enriched As rim exists in the X‐ray images of euhedral pyrites, implying the ore‐forming fluids may be rich in As. The relationship between Au and As reveals that the Au may host as a solid solution (Au+) and nanoparticles of native gold (Au0) in the sulfides. The high Co/Ni ratio (>1) of sulfides and the enrichment of W in the ores all reflect that the gold‐bearing minerals and ore‐forming process were mainly related to the hydrothermal fluids, but the magmatic and volcanic activities cannot be neglected. The general existence of Au and As in the sulfides of both ores and wall rocks and the REE results suggest that the ore‐forming fluids may mainly be derived from the basin itself. The enrichment of Tl suggests that the ore‐forming fluids may be enriched in Cl. The Ce and Eu show slightly or apparently negative anomalies, which means the ore fluids were probably formed under reducing environment. The Y/Ho ratios of ore samples fluctuate around 28, implying the bicarbonate complexation and fluorine were both involved in the ore‐forming process. Combined with the previous studies and our results, we infer that the ore‐forming fluids enriched Au, As, HS? and halogen (F, Cl) were derived from the mixture of reducing basinal fluids and magmatic or volcanic hydrothermal fluids.  相似文献   

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