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1.
Alteration of rocks and localization of uranium mineralization in the northwestern Strel’tsovka caldera are exemplified in the Dal’nee deposit. In the main parameters of hydrothermal mineralization (temperature, pH, pressure, and composition of solution), the Dal’nee deposit differs from the deposits of the Strel’tsovka ore field located in the central part of the caldera. The localization of high-grade stratiform orebodies are interpreted in light of kinematic relations between steeply and gently dipping faults that formed in the tectonic setting of the NE-SW-trending, long-living, right-lateral, strike-slip faulting. The wide halos of argillic alteration and the structural control of uranium mineralization are caused by the fact that the deposit is located at the margin of the geological block, which has developed since the Late Triassic in a regime of extension (pull-apart) to form a depression, which is arranged en echelon relative to the main caldera and comparable to it in area. Currently, this depression is overlapped by sediments of the Sukhoi Urulyungui Basin. Such a structure markedly increases the probability of finding hidden uranium ores associated with low-temperature argillic alteration in the volcanosedimantary rocks and granitoid basement of the northwestern Strel’tsovka caldera.  相似文献   

2.
The Freda-Rebecca Mine is currently the largest gold producer in Zimbabwe. The ore deposit is hosted by two main shear systems crosscutting the Rebecca diorite and Bindura granodiorite (2.65 Ga) as well as Shamvaian metasediments, which are affected by contact metamorphism. Following the intrusion of the Bindura granodiorite, intensive hydrothermal alteration developed preferentially in the dioritic part of the igneous complex (Rebecca diorite). The hydrothermal alteration started with an extensive K-dominated hydrothermal metasomatism in the whole Rebecca diorite. It was followed by less penetrative hydrothermal alteration developed preferentially near shear zones and veinlets. Hydrothermal metasomatism caused microcline and biotite formation, prevailing in the Rebecca diorite. Two main stages of post-metasomatic hydrothermal alteration and mineralization were distinguished, based on spatial relationships between different minerals and some geochemical aspects. In the first stage, an actinolite-tourmaline-arsenopyrite mineralization formed, which is characterized by Ni-Co arsenopyrite associated with actinolite, tourmaline and quartz. The second, lower temperature stage gave a polyphase gold-rich mineralization that developed in four phases which are distinguished by different parageneses including: Phase 1. Actinolite, chlorite, clinozoisite/epidote, quartz, calcite, arsenopyrite, pyrite, pyrrhotite, chalcopyrite. Phase 2. Chlorite, epidote, calcite, gold, native Bi, Bi-Pb sulfides, galena, chalcopyrite, fahlore, pyrite, Fe-gersdorffite. Phase 3. Epidote, calcite, (Ni, Co, Fe) As S phases, Co-Fe-gersdorffite. Phase 4. Chlorite, calcite, quartz. The gold mineralization is exclusively associated with phase 2 and developed in three sulfide parageneses: – Gold + Bi-Pb sulfides + bismuth + chalcopyrite + galena, associated with chlorite (Au 1). – Gold + galena + pyrite, associated with calcite and chlorite (Au 2). – Gold + pyrite + Fe-gersdorffite, associated with epidote, chlorite and calcite (Au 3). The hydrothermal alteration and mineralization formed after the consolidation of the Rebecca diorite and Bindura granodiorite, most probably in the postmagmatic cooling stage. The mineralization was emplaced either synchronously or subsequently to the shear zones which crosscut the consolidated pluton. Not all shear zones are mineralized and different shear zones show different amounts of mineralization and hydrothermal alteration. Thus, it is suggested that during the cooling stage of the pluton and subsequent to the formation of the K-metasomatic zone (microcline + biotite), hydrothermal fluids preferentially followed just forming or pre-existing shear zones. It cannot be excluded that this process developed in a plutonic porphyry copper-like environment, in which the classically hydrothermal zonation did not form due to synchronous tectonic disturbance, which preferentially drives the hydrothermal flow along shear zones. Mineral parageneses and data from chlorite geothermometry indicate that the different stages and phases of hydrothermal alteration reflect cooling stages of the hydrothermal system, from >300 °C in the first stage to about 150 °C in the last phases. Received: 4 January 1999 / Accepted: 13 August 1999  相似文献   

3.
The late-Hercynian granites of Königshain underwent multistage hydrothermal processes. Extensive high-temperature late-magmatic alteration is, for example, indicated by low Zr/Hf and an REE pattern displaying the tetrad effect. Intensive post-magmatic alteration of the granite occurred along brittle structures. At least two main stages of post-magmatic hydrothermal alteration are involved. The first high-temperature stage, which is characterized by albitization and/or quartz leaching (episyenitization), resulted from fluid–rock interaction with late-magmatic fluids that very probably mixed with external low-salinity fluids. Quartz dissolution was triggered by vapour condensation and/or the cooling of these fluids (below 450??°C) along brittle structures. The high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low-temperature fluids at stage 2; the latter is characterized by the chloritization and illitization of episyenites. Almost all major and trace elements were enriched or depleted during one of the main alteration stages. However, Zr, Hf, Th, and Ti were immobile during post-magmatic alteration. The significant depletion of LREE and the enrichment of HREE in albitized samples is controlled by the dissolution of monazite and the new formation of HREE-rich polycrase-(Y) or aeschynite-(Y) during post-magmatic stage 1. Negative Ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2.  相似文献   

4.
Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the 40Ar–39Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial 87Sr/86Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13C-enriched subcontinental lithospheric mantle by the plume.  相似文献   

5.
Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO2, Al2O3, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.  相似文献   

6.
The middle and eastern parts of the Northern Calcareous Alps (NCA) can be subdivided into two distinct units with a lateral boundary marked by abrupt changes in the conodont colour alteration index (CAI-values). The first of these is a northern unit (Tirolikum) with a relatively homogeneous distribution of no or low grade conodont alteration (CAI 1.0–2.0). The thermal overprint is thought to be relatively young and related to a heat flow from the Tauern crystallization. The second unit consists of the Juvavic nappe system (Juvavikum), which is distributed along the southern rim of the NCA but also covers some of the northern parts of the Tirolikum. With respect to its CAI-distribution the Juvavikum is more heterogeneous on a regional and local scale, with some local CAI-inversions. The Juvavikum additionally shows distinctly different sets of CAI-values one with weak (CAI 1.0–1.5) and another with strong alteration (CAI 5.5–7.0) — at present the highest known thermal overprint measured in the NCA. The metamorphism is relatively old as it predates the Late Jurassic—Early Cretaceous gravity tectonic emplacement of the Juvavikum onto the Tirolikum. The high CAI-values of parts of the Juvavic nappe system are though to be related to tectonic burial in an accretionary wedge formed parallel to the closure of the Vardar Ocean. The low CAI values of the Tirolikum apparently exclude a direct juxtaposition of the two units at the time of metamorphism.  相似文献   

7.
The Madjarovo ore district is centred on the exposed section of a Lower Oligocene volcano and consists of radially disposed Pb–Zn-precious metal veins and attendant intermediate sulfidation wallrock alteration. Earlier high sulfidation and potassic porphyry style alterations are found in the centre of the district spatially associated with monzonitic intrusions. The total duration of all mineralization and alteration was ca. 300 ka. Stable isotope analyses (S, O, H) have been carried out on a suite of sulfides, sulfates and silicates from the mineralization, high and intermediate sulfidation alterations and a suite of basement rocks. These data range between the following limits: . We also analysed δD of fluid inclusions in quartz and barite for which we obtained, respectively, the ranges of −43.6 to −78.6 and −58.4 to −67.1‰. The data show that high sulfidation alteration was dominated by magmatic fluids with minor meteoric water, whereas the fluids responsible for the intermediate sulfidation alteration were essentially magmatic. The fluids responsible for the intermediate sulfidation Pb–Zn mineralization were mixed magmatic–meteoric and certainly contained a significant meteoric component. Sulphur is likely derived from basement and/or igneous sources. The evolution of alteration and mineralization styles from potassic, porphyry copper style to high sulfidation to intermediate sulfidation can be understood in terms of changing ore fluid composition resulting from an increasing permeability of the system and an increasingly remote source of magmatic fluid with time. These changes link directly to the geological evolution of this volcanic centre.  相似文献   

8.
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9.
Human has always modified its surroundings for better adaptation which include the building of urban landscapes, dams, and engineering constructions like bridges and other infrastructures. It has been unveiled from the study of River Chel that when the rail bridge was constructed before 1913, the natural width of the river was nevertheless maintained but the problem was initiated and after the road bridge construction in 1970s when the river width was minimized,the morphological changes started in a large scale. Such modifications have affected nature as well as human communities both positively and negatively. This paper will try to portray the evolution of channel shifting and changes of sediment size regime that the River Chel is experiencing due to the construction of bridges across the middle part of its course. So, this study is concerned about the changes in channel pathway since last 100 years and field-based hydrological parameters and sediment size analysis reveals some distinct changes in the channel planform with alteration of sediment size regime. Simulation of the hydraulic modeling in HEC-RAS specifies the probable affected area with bridges and without bridges in both upstream and downstream of the river course. Hence, the study unveils the alteration of the river hydrology and sediment size caused due to anthropogenic effects and impact of such flow analysis has been evaluated through the bridge scour calculation by CUS method. The discussion concludes that the Odlabari road bridge construction (after 1970) has modified the normal hydrological behavior of the river with a higher probability of bank erosion in downstream and floods in upstream resulting river bottleneck condition.  相似文献   

10.
The Yandong porphyry copper deposit, located in the Eastern Tianshan Mountains, Xinjiang, China, is part of the Central Asian Orogenic Belt. The Yandong deposit is hosted by a volcanic complex in the Early Carboniferous Qi’eshan Group and a felsic intrusion. The complex consists of andesite, basalt, diorite porphyry, and porphyritic quartz diorite. The felsic intrusion is a plagiogranite porphyry emplaced within the complex. The diorite porphyry and plagiogranite porphyry yield SIMS zircon U–Pb ages of 340.0 ± 3 and 332.2 ± 2.3 Ma, respectively. Element geochemistry shows that both the complex and plagiogranite porphyry formed in the Dananhu–Tousuquan island arc, a Carboniferous magmatic arc.The diorite porphyry and plagiogranite porphyry are host porphyries, but the plagiogranite porphyry is a productive porphyry. It caused the porphyry-style Cu mineralization and associated alteration. The alteration assemblages include early potassic and propylitic assemblages. These were overprinted by a chlorite–sericite assemblage, which in turn was overprinted by a late phyllic assemblage. The phyllic alteration is associated with the highest Cu grades. The mineralization is recognized to include three stages, from early to late: stage 1, a potassic alteration associated with a chalcopyrite + pyrite assemblage; stage 2, represented by chlorite–sericite alteration with a chalcopyrite + pyrite assemblage; and stage 3, the main-ore stage that is marked by phyllic alteration with chalcopyrite + pyrite ± molybdenite and producing more than 70% of the total copper production at Yandong. Yandong may represent a common scenario for Paleozoic porphyry Cu systems in the Central Asian Orogenic Belt.  相似文献   

11.
The Hillgrove gold–antimony deposit is hosted in late Palaeozoic, biotite-grade metasedimentary rocks and Permo-Carboniferous granitoid intrusions of the New England Orogen. Mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255–245 Ma. Hydrothermal fluids were controlled by extensional faults in a regional-scale sinistral strike-slip fault system. Principal faults in this system were developed in, and possibly evolved from, mylonite zones which were active during Late Permian tectonics. Earliest mineralisation formed scheelite-bearing quartz veins, and these were followed by auriferous arsenopyrite–pyrite–quartz–carbonate veins with minor base metal sulphides. This latter type was accompanied by sericitisation and carbonation of the host rock, with addition of sulphur, arsenic and gold, in zones up to 20 m from veins. Quartz–stibnite veins with electrum, gold, aurostibite, and arsenopyrite form a prominent and economically important hydrothermal type, with little wall-rock alteration but extensive hydrothermal breccia formation and local open-space filling textures. Below a mining depth of 300–500 m, this type passes over a short distance downwards into stibnite-poor gold-bearing veins. Late-stage carbonate–stibnite veins with gold and silver sulphosalts cut all earlier veins, and have open-space filling textures. Aspects of the Hillgrove deposit have similarities to many other orogenic gold deposits in the SW Pacific which have been formed at different structural levels. Hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit, formed progressively during syn-orogenic uplift. Editorial handling: N. White  相似文献   

12.
Monazite [(Ce,LREE,Th,U,Ca)(P,Si)O4], with complex zoning in Th and other elements, is commonly observed in metamorphic and igneous rocks. The hypothesis that this alteration is a product of fluid-mediated element mass transfer has been tested in the piston-cylinder press (CaF2 assembly, cylindrical graphite oven) at 1,000 MPa and 900°C and in cold seal autoclaves on a hydrothermal line at 500 MPa and 600°C. Experiments included a relatively homogeneous monazite-(Ce) (7–8 wt% ThO2) from a heavy mineral sand plus a series of alkali-bearing fluids including 2N NaOH, 2N KOH, and Na2Si2O5 + H2O. Experiments were conducted using BSE imaging, EMP analysis, and both TEM and HRTEM. A subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in Th + Si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior. These ThSiO4-enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks. In the Na2Si2O5 + H2O experiments, scarce inclusions of britholite formed in the altered monazite. The altered monazite is also characterised by strong depletion in Pb, Ca, and Y. Thorium and Si mobility, coupled with the formation of britholite inclusions, during partial alteration in the monazite grain is considered to be the product of fluid-aided coupled dissolution–reprecipitation as opposed to solid-state diffusion. Since other fluids, including NaCl and KCl brines, do not result in the formation of these textures, the experimental replication of ThSiO4-enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced, specifically by alkali-bearing fluids. If true, complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event, presuming that all the original radiogenic Pb has been removed.  相似文献   

13.
The disseminated gold deposit of Agdarreh (24.5 t at 3.7 g/t Au) is hosted in hydrothermally leached Miocene reefal limestone in the Takab geothermal field, which is part of the Cenozoic Urumieh–Dokhtar volcanic arc of NW Iran. Alteration and mineralisation are largely bedding controlled blanket-like and include: (1) pre-ore decalcification; (2) first-stage silicification associated with pyrite (early pyrite with 3–4 wt% As, late pyrite with <1–3 wt% As) and sphalerite; (3) second-stage silicification with precipitation of galena, Pb–Sb–As sulphides, sulphosalts, tellurides and native bismuth; (4) late-stage cinnabar and barite in vugs; (5) oxide ore stage and carbonate alteration (complex Mn–Fe-rich oxyhydroxides, arsenates, sulphates, APS minerals and rutile in residual leached rock and infill of karstic cavities). Gold occurs invisibly in the jasperoids and is enriched in the Mn–Fe oxyhydroxide surface cap of the jasperoids. Gold mineralisation is associated with the hydrothermal metal suite of As, Sb, Hg, Te, Se, Tl, Ba, Zn, Ag, Cd, Bi and Pb, and is characterised by very low Cu contents. Arsenian pyrite probably carried most of the primary (invisible) gold. Native gold occurs in association with the late-stage cinnabar and the oxide ore. The Agdarreh deposit shows many similarities with Carlin-type ore and is interpreted to have resulted from near-surface hydrothermal activity related to the Cenozoic arc volcanism that developed within the extensional Takab graben. The extensive oxidation at Agdarreh may be partly due to the waning stages of hydrothermal activity. Active H2S-bearing thermal springs are locally depositing extremely high contents of Au and Ag, and travertine is present over large areas, suggesting that ore-forming hydrothermal activity occurred periodically from the Miocene to Recent in the Takab geothermal field. The present paper deals with the geological framework, host rocks, characteristic features of hydrothermal alteration and mineralisation, and genesis of the Agdarreh deposit. The results of fluid inclusion and stable isotope studies are in progress and will be given in a forthcoming paper.  相似文献   

14.
《Ore Geology Reviews》2008,33(3-4):629-650
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

15.
Carbonate xenoliths containing olivine and rimmed by kaersutitic amphibole were collected in basaltic rocks of the Basal Complex of La Palma. The mineralogical composition and microscopic appearance may suggest a relationship with carbonatites in general, thus a major element, trace element and stable isotope study was conducted to investigate the origin of the carbonate formation. Based on electron microprobe analyses, the carbonate is calcite with up to 6.3 wt% MgO and 7.2 wt% SiO2. The elevated SiO2 content may suggest a melt origin for the carbonate. However, the C and O isotope compositions of the carbonate xenoliths (δ13C and δ18O around −1‰ and 13‰, respectively) are similar to those of calcite veins and amygdales in basaltic rocks of the Basal Complexes of La Palma and Fuerteventura and are interpreted as produced by fluid degassing and metasomatism by CO2-H2O fluid derived from mobilization of sedimentary material. Trace element contents determined by laser-ablation ICP-MS analyses support the assumed origin, thus, the relationship with carbonatitic melts can be excluded. Based on trace element compositions, the amphibole surrounding the xenoliths is not related genetically to the carbonate. The elevated SiO2 content of the calcite can be attributed to submicron relics of pyroxene, thus, the use of this feature as an evidence for melt origin is questionable.  相似文献   

16.
The Jinbaoshan Pt–Pd deposit in Yunnan, SW China, is hosted in a wehrlite body, which is a member of the Permian (∼260 Ma) Emeishan Large Igneous Province (ELIP). The deposit is reported to contain one million tonnes of Pt–Pd ore grading 0.21% Ni and 0.16% Cu with 3.0 g/t (Pd + Pt). Platinum-group minerals (PGM) mostly are ∼10 μm in diameter, and are commonly Te-, Sn- and As-bearing, including moncheite (PtTe2), atokite (Pd3Sn), kotulskite (PdTe), sperrylite (PtAs2), irarsite (IrAsS), cooperite (PtS), sudburyite (PdSb), and Pt–Fe alloy. Primary rock-forming minerals are olivine and clinopyroxene, with clinopyroxene forming anhedral poikilitic crystals surrounding olivine. Primary chromite occurs either as euhedral grains enclosed within olivine or as an interstitial phase to the olivine. However, the intrusion has undergone extensive hydrothermal alteration. Most olivine grains have been altered to serpentine, and interstitial clinopyroxene is often altered to actinolite/tremolite and locally biotite. Interstitial chromite grains are either partially or totally replaced by secondary magnetite. Base-metal sulfides (BMS), such as pentlandite and chalcopyrite, are usually interstitial to the altered olivine. PGM are located with the BMS and are therefore also interstitial to the serpentinized olivine grains, occurring within altered interstitial clinopyroxene and chromite, or along the edges of these minerals, which predominantly altered to actinolite/tremolite, serpentine and magnetite. Hydrothermal fluids were responsible for the release of the platinum-group elements (PGE) from the BMS to precipitate the PGM at low temperature during pervasive alteration. A sequence of alteration of the PGM has been recognized. Initially moncheite and atokite have been corroded and recrystallized during the formation of actinolite/tremolite, and then, cooperite and moncheite were altered to Pt–Fe alloy where they are in contact with serpentine. Sudburyite occurs in veins indicating late Pd mobility. However, textural evidence shows that the PGM are still in close proximity to the BMS. They occur in PGE-rich layers located at specific igneous horizons in the intrusion, suggesting that PGE were originally magmatic concentrations that, within a PGE-rich horizon, crystallized with BMS late in the olivine/clinopyroxene crystallization sequence and have not been significantly transported during serpentinization and alteration.  相似文献   

17.
《International Geology Review》2012,54(10):1145-1160
Lanjiagou is a porphyry Mo deposit in terms of its alteration zonation and mineralization associated with granitic intrusions and predominance of quartz vein-hosted molybdenum mineralization. It is the largest Mo deposit in North China Craton (404,000 t). There is an intimate spatial/temporal association between all stages of mineralization and Early Jurassic granitic intrusions at Lanjiagou. Most of the molybdenum was emplaced during the principal hydrothermal (PH) stage (184.6 ± 1.3 – 185.6 ± 1.4 Ma), contemporaneously with intrusion of fine-grained porphyritic granite (188.9 ± 1.2 Ma) into a granite batholith (193 ± 3 Ma). The PH mineralization stage is mainly hosted by a quartz-dominated stockwork associated with phyllic alteration in the fine-grained porphyritic granite. This stage was followed by the late hydrothermal (LH) activity. Thick Mo-rich quartz veins were emplaced during the LH stage and cut the porphyry ore bodies. A ring breccia zone formed during the last hydrothermal stage and apparently cuts both the porphyry and the quartz vein ore bodies. The main hydrothermal vein stages have predominantly concentric and radial vein orientations centred on the ring breccia zone. Most of the concentric veins have shallow dips, whereas the radial veins are subvertical. The LH veins have predominantly NEE and NW orientations in the deposit and are moderately inclined. We surmise that the veining was controlled by the local stress regime generated by the intrusion of a large, deep pluton that we interpreted to be the source of the granites, the breccia zone, and the molybdenum mineralization. Resurgence within the magma chamber reactivated the steep concentric structures in a reverse sense, and accumulation of magmatic and/or fluid pressure resulted in explosive brecciation, producing the ring breccia zone. A predominantly late set of NW-trending, post-ore felsic dikes, associated with the regional structures, are a consequence of far-field stresses exceeding local stresses in the deposit.  相似文献   

18.
Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO2, SO4), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO2, Ca, K, Ba, B and CO2 were leached from basalt and enriched in solution. SiO2 concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.  相似文献   

19.
The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a contact-type mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary ferritchromite in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.Editorial handling: P. Lightfoot  相似文献   

20.
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.  相似文献   

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