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1.
The Eastern Layered Series of the Rum Layered Suite, NW Scotland, comprises a sequence of sixteen (30–150 m thick) cyclic units. The upper troctolite–olivine gabbro parts of each of these units exhibit small-scale modal layering and a pervasive, layer-parallel mineral lamination that is often associated with ‘soft-sediment’ deformation structures. A sporadic, macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places. Anisotropy of magnetic susceptibility (AMS) fabrics were studied in three of these cyclic units, (8, 9, and 10) in the northern part of the Eastern Layered Series. Magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend NW–SE and plunge gently. Magnetic fabrics measured for two detailed traverses through Unit 10 on the northern side of Hallival also yield one dominant trend, similar to that measured elsewhere in the Eastern Layered Series. However, toward the centre of Unit 10 in each traverse, magnetic lineations sometimes plunge approximately downdip (SW) on the magnetic foliation planes. The implications of these results are discussed with reference to previous textural and fabric observations on Rum. A model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft-sediment deformation of unconsolidated crystal mushes during central sagging of the Rum Layered Suite.  相似文献   

2.
HOLNESS  M. B. 《Journal of Petrology》2005,46(8):1585-1601
The Eastern Layered Series of the Rum Layered Suite is formedof 16 macro-units each comprising a lower peridotite and anoverlying feldspar-rich layer (the local term is allivalite).The origin of the peridotite layers is unresolved, with twocontrasting models. The earlier of the two is based on repeatedreplenishment of an open-system magma chamber with depositionof fractionated material on the chamber floor. The second isbased on the early formation of a troctolitic complex, whichis then repeatedly intruded by sills of replenishing picriticmagma to form the peridotite horizons. The lack of resolutionof this fundamental problem is a consequence of the relianceof previously published studies on field observations. I presentevidence to show that the clinopyroxene in the allivalites preservesinformation about the distribution of the last melt to solidify,permitting determination of not only the extent of super-solidustextural equilibration but also the sub-solidus history of theallivalite horizons. Comparison of profiles of clinopyroxene–plagioclase–plagioclasemedian dihedral angle across allivalite units demonstrates thatit is possible to distinguish between those that were intrudedby later picrite sills and those adjacent to peridotite horizonsformed by replenishment and subsequent deposition of fractionatedcrystals above the pre-existing pile. In the region studied,only the main peridotite body of Unit 9 was intruded into apre-existing allivalitic mush. KEY WORDS: Rum Layered Intrusion; chamber replenishment; dihedral angles; cumulates  相似文献   

3.
Summary ?Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first time. The detrital platinum-group minerals (PGM) are dominated by Os–Ir–(Ru) species, which is typical for ophiolites or Alpine-type complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite–dunite massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are (1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly osmium. Dominant chromite, olivine and clinopyroxene inclusions recorded in Os–Ir–(Ru) alloys imply that they were derived from ultramafic sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation - multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (− 2.39 to − 2.07). These values are indicative of a common chondritic or subchondritic mantle source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from − 2.67 to − 1.30). The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing parent bedrock sources. Received July 12, 2001; revised version accepted December 27, 2001  相似文献   

4.
The Eastern Layered Intrusion of the Rum Layered Suite comprisespaired peridotite and allivalite (troctolite and gabbro) layersforming 16 macro-rhythmic units. Whereas the majority of thesemacro-units are believed to have formed by a process of crystal–liquiddifferentiation involving successive accumulation of crystalsfrom multiple picritic replenishments of the chamber, the Unit9 peridotite is interpreted as a layer-parallel picrite intrusion.Closely correlated with this discontinuous peridotite body isa distinctive feature generally known as the Wavy Horizon, whichdivides the overlying allivalite into a lower troctolite andan upper gabbro along a well-defined undulating contact. Wepropose that the Wavy Horizon is a metasomatic feature formedconsequent to the removal of clinopyroxene from an originalgabbroic mush. Foundering of the mush into the picritic sillresulted in the replacement of the original interstitial liquidby one saturated only in olivine (± plagioclase). Progressivethrough-flow of this liquid resulted in the stripping out ofclinopyroxene from the lower parts of the allivalite. We interpretthe Wavy Horizon as a reaction front, representing the pointat which the invading liquid became saturated in clinopyroxene.The distinctive pyroxene-enriched zone immediately above theWavy Horizon could have formed when mixing of the interstitialliquids on either side of the reaction front formed a supercooledliquid oversaturated in pyroxene, as a result of the curvatureof the olivine–plagioclase–clinopyroxene cotectic.The presence of many such approximately layer-parallel features,defined by differences in pyroxene content, in the Eastern LayeredIntrusion of Rum suggests that such an infiltration–reactionprocess was not unique to Unit 9. KEY WORDS: cumulate; infiltration metasomatism; Rum; Eastern Layered Intrusion  相似文献   

5.
Summary The study of platinum-group minerals (PGM) concentrates from the Nizhni Tagil placers related to the Soloviev Mountain (Gora Solovieva) Uralian-Alaskan-type intrusion revealed a predominance of (Pt, Fe) alloys over Ir-, and Os-bearing alloys. (Pt, Fe) alloys (“isoferroplatinum-type”) are interstitial with respect to chromite and show important variations in their chemical compositions, which are, however, falling within the experimentally determined stability field of isoferroplatinum. Tetraferroplatinum, enriched in Cu and Ni and tulameenite represent low-temperature mineral phases replacing (Pt, Fe) alloys. Alloys belonging to the Os–Ir–Ru ternary system have compositions corresponding to native osmium, iridium and ruthenium, respectively, and to rutheniridosmine. Osmium exsolutions appear in Ir-, and (Pt, Fe) alloys, and iridium exsolutions in (Pt, Fe) alloys. Laurite is a high-temperature phase included in native iridium and (Pt, Fe) alloys. Low-temperature PGM association comprises Ir-bearing sulpharsenides, including a phase (Ir, Os, Fe, Pt, Ru, Ni)3(As, Sb)0.85S, and a palladium antimonide Pd20Sb7. These two phases were previously unknown in nature. Furthermore, native palladium occurs in the studied concentrates. This low-temperature paragenesis indicates an interaction of Pt-, Os-, Ir- and Ru-bearing alloys with late fluids enriched in volatiles, As and Sb. The chromite composition is characterized by the predominance of Cr3+ → Fe3+ substitution like in other Uralian-Alaskan-type intrusions; that indicates a fO2 variation during the chromite precipitation. Monomineralic inclusions of euhedral clinopyroxene and chromite crystals in (Pt, Fe) alloys were observed. Furthermore, (Pt, Fe) alloys contain polyphase silicate inclusions, which occupy the alloy negative crystals. Two types of silicate inclusions were recognized: (1) Low-pressure inclusions composed of amphibole, biotite, Jd-poor clinopyroxene, magnetite, apatite and glass; (2) High-pressure inclusions include: omphacitic clinopyroxene (up to 56 mol.% Jd), tremolite, muscovite, apatite, titanite and glass. In this case, the clinopyroxene is strongly zoned, revealing a pressure drop from about 25 to 5 kbar. The chemical composition of glass is corundum-normative and its H2O content varies from about 12 to 15 wt.%. The composition of magmatic melts, from which the silicate inclusions have originated was estimated using EPMA and image analysis interpreted by stereology. Their compositions are close to those obtained experimentally by hydrous partial melting of upper mantle rocks. The interpretation of analytical data shows that magmatic melts entrapped by (Pt, Fe) alloys crystallized from about 1100 to 700 °C. The (Pt, Fe) alloys formed after the crystallization of chromite, clinopyroxene and albite. Consequently, the precipitation temperature of (Pt, Fe) alloys is estimated at about 900 °C. The significant pressure drop implies a decrease of volatile concentrations in the magmatic melt and the possible formation of a fluid phase, which might have generated, the precipitation of chromite and PGM.  相似文献   

6.
The primary source of the alluvial platinum-group minerals (PGM) of the Freetown Layered Intrusion, Sierra Leone, has been unknown since their discovery 70 years ago. This is a report on work in progress to establish a primary occurrence related to the igneous layering. New whole-rock major and minor element analyses and precious metal assays show that at key horizons there is a significant change in the silicate mineral compositions and in Cu, Cr and Ni, as well as in Pt and Pd, comparable to those encountered in other PGM-bearing layered intrusions. Anomalous (0.1 to 0.4 g/t) Pt levels occur in pyroxene troctolites, which maintain these levels along strike. Received: 30 July 1999 / Accepted: 2 December 1999  相似文献   

7.
The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by lherzolite–clinopyroxenite enveloped by olivine–plagioclase hornblendite and plagioclase hornblendite. Cu–Ni–PGE mineralization is closely associated with peridotite, especially in the inner, olivine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrhotite, pentlandite, chalcopyrite) are found in association with Al–Mg-rich spinel and Cr-magnetite. Primary magmatic textures are well preserved; however, deformation and mobilization due to shearing are locally observed. Platinum-group minerals (PGM) documented from the deposit are: merenskyite (PdTe2) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni,Pd) (Te,Bi)2. These minerals occur in intimate association with hessite (Ag2Te) and electrum (Au0.65Ag0.31Bi0.04) in two distinct textural positions: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite and (2) at sulfide–silicate grain boundaries and on microfractures in base-metal sulfides (BMS) and olivine associated with serpentine and secondary magnetite. Textural features suggest that PGM were exsolved from monosulfide solid solution over a wide range of temperatures. Late-stage, low-temperature hydrothermal solutions led to redistribution of PGE. Mineralized samples show Ni/Cu ratios ranging from 0.2 to 2 with an average of 1.0. The (Pt + Pd + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, and Ir are below the detection limit (2 ppb). The PGE contents show positive correlation with S only at low sulfur contents. The PGE patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with stratiform deposits, Gabbro Akarem is depleted in PGE. The consistently low PGE contents of the mineralization and their uniform distribution in the ultramafic rocks despite the high sulfur content of the rock is attributed to rapid crystallization of sulfides in a highly dynamic environment. Received: 3 November 1999 / Accepted: 29 July 2000  相似文献   

8.
In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides, sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2. This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously, they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide, Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and may possibly have been derived from the UG2 chromitite itself.  相似文献   

9.
Summary Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.  相似文献   

10.
An alkali basalt near Glen Innes, northeastern New South Wales, contains a suite of Cr-diopside group ultramafic xenoliths which includes some spinel peridotites but which is dominated by a diverse spinel pyroxenite assemblage. Pyroxenite xenoliths range from subcalcic clinopyroxenites (composed largely of unmixed prismatic subcalcic clinopyroxene megacrystals and lesser orthopyroxene megacrystals) to equant mosaic textured websterites (orthopyroxene and Ca-rich clinopyroxene ± spinel). Rare orthopyroxenite xenoliths also occur. The pyroxenite xenoliths are characterised by high 100Mg/(Mg + Fe2+) ratios (M˜ 90) and low concentrations of Ti, K, P, La, Ce and Zr. The websterites are mineralogically and chemically similar to many spinel pyroxenites occurring as layers or dykes in peridotite massifs such as those at Ronda in southern Spain and at Ariège (French Pyrénées). T / P estimates indicate crystallization temperatures of 1250–1350 °C for subcalcic clinopyroxene-orthopyroxene megacrystal pairs and 900–1000 °C for the equilibrated mosaic textured websterites and associated peridotites at pressures of 9–13 kbar. Subcalcic clinopyroxene megacrystals, websterites and orthopyroxenites have LREE-depleted chondrite-normalised REE abundances with (La/Yb)CN < 1 and their convex-upwards REE patterns are typical of subcalcic clinopyroxene-dominated cumulates. The pyroxenites are not residua from partially melted pyroxenite layers or dykes in mantle peridotites nor are they completely crystallized protobasaltic or protopicritic magmas. They are interpreted as high-pressure crystal segregations from basaltic magmas (probably mildly alkaline or transitional) flowing within narrow mantle conduits (the flow crystallization model of Irving, 1980). The parental magma(s) was Ti-poor (0.6–0.7% TiO2) and relatively Mg-rich (M˜ 74 − 70). Pyroxenite genesis was a two-stage process involving crystallization of tschermakitic subcalcic clinopyroxenes and orthopyroxenes  ±spinel as liquidus or near-liquidus phases at 1250–1350 °C and 9–13 kbar to yield “primary” subcalcic clinopyroxenites which then re-equilibrated at 900–1000 °C and similar pressures to produce the mosaic textured “secondary” websterites. The pyroxenites show a wide range of 143Nd/144Nd and 87Sr/86Sr values (0.513298–0.512473 and 0.702689–0.704659, respectively). Their isotopic ratios appear to have been variably modified by exchange with adjacent mantle peridotites or migrating basaltic melts. Received: 11 December 1995 / Accepted: 3 December 1996  相似文献   

11.
Scheelite-mineralized microtonalite sheets occur on the SE margin of the end-Caledonian Leinster Granite in SE Ireland. Scheelite, polymetallic sulphides and minor cassiterite occur in veins in the microtonalites, disseminated throughout the greisened microtonalite sheets and in the adjacent wallrocks. Two major mineralized vein types occur in the microtonalite sheets: (1) Scheelite ± arsenopyrite ± pyrrhotite occur in quartz-fluorite veins, generally without a muscovite selvage; (2) Sphalerite ± chalcopyrite ± pyrite ± galena ± cassiterite ± stannite occur in quartz + fluorite veins with a coarse muscovite selvage and are often intergrown with the muscovite. Quartz-hosted fluid inclusions were examined from representative samples of both vein types using petrographic, microthermometric and laser Raman spectroscopic techniques. Three distinct types of fluid inclusions have been recognized. Primary, vapour rich Type 1 inclusions in quartz from the scheelite-mineralized veins are of H2O-CO2-CH4-N2 ± H2S ± NaCl composition and formed between 360–530 °C. Primary and secondary, liquid-rich Type 2 fluid inclusions in the base metal sulphide-mineralized veins are of H2O-CH4-N2 ± H2S-NaCl composition and formed between 340–480 °C. They also occur as pseudosecondary and secondary inclusions in scheelite-mineralized veins. Late dilute, low temperature H2O-NaCl + KCl fluid inclusions may be related to late-Caledonian convection of meteoric waters around the cooling Leinster Granite batholith. Received: 4 September 1996 / Accepted: 23 May 1997  相似文献   

12.
The Hammamat molasse sediments of the Eastern Desert of Egypt were deposited in isolated basins formed during an initial stage of orogen parallel N–S extension (650–580 Ma) in the Neoproterozoic time. Supply of sediments to the molasse basins began after the eruption of Dokhan volcanics (602–593 Ma), exhumation of core complexes (650–550 Ma), and intrusion of late tectonic granites (610–550 Ma). The late Neoproterozoic structures in the molasse sediments include: (1) NNW-directed thrusts due to NNW–SSE shortening (650–640 Ma), indicated by the presence of NE-, ENE-, and WSW-trending folds and NNW-directed thrusts. (2) SW- and NE-directed thrusts due to ENE–WSW constriction during oblique convergence and arc accretion at around 640–620 Ma. Many of the map- and mesoscopic-scale NW-trending folds in the core complexes, the molasse sediments, and the Neoproterozoic nappes in the Eastern Desert are related to this event. Sinistral shearing along the Najd Fault System (650–540 Ma) resulted in the development of subvertical foliation, subhorizontal stretching lineation, and NW-trending tight folds overprinting earlier folds. Stretched pebbles are oriented NW–SE and WNW–ESE in the molasse basins localized within the Najd Fault System, and NE–SW in the basins outside the influence zone of this NW-trending fault system. Strain estimated using pebbles from nine molasse basins indicate that the amount of strain differs from one basin to another and from one place to another within the same basin. Weak tectonic strain (Rs = 2.16–2.24) is obtained from post-orogenic basins; moderate strains are reported from foreland basins (Rs = 2.37–3.18), whereas moderate to high tectonic strains are recorded from the intermontane basins (Rs = 2.40–4.36). The obtained tectonic strain and K values indicate that the flattening strain prevails in the post-orogenic and foreland basins, whereas as both constrictional and flattening strains are recorded in intermontane basins. Strain variation from one basin to another and within the individual basin is attributed to presence of thrust and sinistral shear zones. Away from the deformed zones, the pebbles show no significant stretching. Two phases of thrusting and an episode of transpressional sinistral shearing are the latest structure features of the East African orogeny in the Arabian–Nubian Shield.  相似文献   

13.
The Ferguson Lake Ni–Cu–Co–platinum-group element (PGE) deposit in Nunavut, Canada, occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites. Massive to semi-massive sulfide occurs toward the structural hanging wall of the metagabbro, and a low-sulfide, high-PGE style of mineralization (sulfide veins and disseminations) locally occurs ~30–50 m below the main massive sulfide. Water–rock interaction in the Ferguson Lake Ni–Cu–Co–PGE deposit is manifested mostly as widespread, post-metamorphic, epidote–chlorite–calcite veins, and replacement assemblages that contain variable amounts of sulfides and platinum-group minerals (PGM). PGM occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high-PGE zones, at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide, in undeformed sulfide veins and adjacent chlorite and/or epidote halos, in hornblende adjacent to hydrothermal veins, and in plagioclase–chlorite aggregates replacing garnet cemented by sulfide. The PGM are mostly represented by the kotulskite (PdTe)–sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi2), merenskyite (PdTe2), mertieite II (Pd8[Sb,As]3), and sperrylite (PtAs2) and occur in variety of textural settings. Those that occur in massive and interstitial sulfides, interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures <600°C, are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulfide/silicate veins and their epidote–chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite). The chemistry and textural setting of the various PGM supports a genetic model that links the magmatic and hydrothermal end-members of the sulfide–PGM mineralization. The association of PGM with magmatic sulfides in the massive sulfide and high-PGE zones has been interpreted to indicate that PGE mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization. However, the occurrence of PGM in undeformed sulfide-bearing veins and in their chlorite–epidote halos and differences in PGM chemistry indicate that hydrothermal fluids were responsible for post-metamorphic redistribution and dispersion of PGE.  相似文献   

14.
The clinopyroxene–plagioclase–plagioclase dihedralangle, cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle  相似文献   

15.
Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. 40Ar/39Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001  相似文献   

16.
Summary ?We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGE-sulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly laurite-erlichmanite) and alloys, are typical only of the unaltered podiform chromitite (assemblage I). Euhedral mono- and polyphase PGM grains in the submicron to 100 μm range show features of primary magmatic assemblages. The diversity of PGM in these assemblages is unusual for ophiolitic environments. In assemblage II, laurite-erlichmanite is intergrown with and overgrown by PGE-sulpharsenides; other minerals of assemblage I are missing. Potarite, stibiopalladinite, mertieite II, native gold and Ag-Au alloys, as well as PGE-sulpharsenides, sperrylite and base metal arsenides and sulphides are characteristic for the highly altered chromitite (assemblage III). They occur either interstitial to chromite in association with metamorphic silicates, in chromite rims or along cracks, and are thus interpreted as having formed by remobilization of PGE by hydrothermal processes during polyphase regional metamorphism. Received August 3, 2000;/revised version accepted December 28, 2000  相似文献   

17.
The Jupiter gold deposit in the northeastern Eastern Goldfields Province of the Yilgarn Craton of Western Australia is hosted in greenschist facies metamorphosed tholeiitic basalt, quartz–alkali-feldspar syenite, and quartz–feldspar porphyry. Syenite intrudes basalt as irregularly shaped dykes which radiate from a larger stock, whereas at least three E–W and NE–SW striking quartz–feldspar porphyries intrude both syenite and basalt. Brittle–ductile shear zones are shallow-dipping, NW to NE striking, or are steep-dipping to the south and west. Quartz ± carbonate veins that host gold at Jupiter occur in all lithologies and are divided into: (1) veins that are restricted to the shear zones, (2) discrete veins that are subparallel to shear zone-hosted veins, and (3) stockwork veins that form a network of randomly oriented microfractures in syenite wallrock proximal to shallow-dipping shear zones. The gold-bearing veins comprise mainly quartz, calcite, ankerite, and albite, with minor sericite, pyrite, chalcopyrite, galena, sphalerite, molybdenite, telluride minerals, and gold. Proximal hydrothermal alteration zones to the mineralised veins comprise quartz, calcite, ankerite, albite, and sericite. High gold grades (>2 g/t Au) occur mainly in syenite and in the hanging walls to shallow-dipping shear zones in syenite where there is a greater density of mineralised stockwork veins. The Jupiter deposit has structural and hydrothermal alteration styles that are similar to both granitoid-hosted, but post-magmatic Archaean lode-gold deposits in the Yilgarn Craton and intrusion-related, syn-magmatic, syenite-hosted gold deposits in the Superior Province of Canada. Based on field observations and petrologic data, the Jupiter deposit is considered to be a post-magmatic Archaean lode-gold deposit rather than a syn-intrusion deposit. Received: 5 January 1999 / Accepted: 24 December 1999  相似文献   

18.
 Volcan Popocatépetl is a Quaternary stratovolcano located 60 km southeast of Mexico City. The summit crater is the site of recent ash eruptions, excess degassing, and dacite dome growth. The modern cone comprises mainly pyroclastic flow deposits, airfall tephras, debris flows, and reworked deposits of andesitic composition; it is flanked by more mafic monogenetic vents. In least-degassed fallout tuffs and mafic scoria, transition metals are concentrated in phases formed before eruption, during eruption, and after eruption. Preeruptive minerals occur in both lavas and tephra, and include oxides and sulfides in glass and phenocrysts. The magmatic oxides consist of magnetite, ilmenite, and chromite; the sulfides consist of both (Fe,Ni)1-xS (MSS) and Cu–Fe sulfide (ISS). Syn- and posteruptive phases occur in vesicles in both lavas and tephra, and on surfaces of ash and along fractures. The mineral assemblages in lavas include Cu–Fe sulfide and Fe–Ti oxide in vesicles, and Fe sulfide and Cu–Fe sulfide in segregation vesicles. Assemblages in vesicles in scoria include Fe–Ti oxide and rare Fe–Cu–Sn sulfide. Vesicle fillings of Fe–Ti oxide, Ni-rich chromite, Fe sulfide, Cu sulfide, and barite are common to two pumice samples. The most coarse-grained of the vesicle fillings are Cu–Fe sulfide and Cu sulfide, which are as large as 50 μ in diameter. The youngest Plinian pumice also contains Zn(Fe) sulfide, as well as rare Ag–Cu sulfide, Ag–Fe sulfide, Ag bromide, Ag chloride, and Au–Cu telluride. The assemblage is similar to those typically observed in high-sulfidation epithermal mineralization. The fine-grained nature and abundance of syn- and/or posteruptive phases in porous rocks makes metals susceptible to mobilization by percolating fluids. The abundance of metal compounds in vesicles indicates that volatile exsolution prior to and/or during eruption played an important role in releasing metals to the atmosphere. Received: March 1997 · Accepted: 27 May 1997  相似文献   

19.
Summary The Jinchuan deposit is a platinum group element (PGE)-rich sulfide deposit in China. Drilling and surface sampling show that three categories of platinum group element (PGE) mineralization occur; type I formed at magmatic temperatures, type II occurs in hydrothermally altered zones of the intrusion, and type III in sheared dunite and lherzolite. All ore types were analyzed for Os, Ir, Ru, Rh, Pd, Pt and Au, as well as for Cu, Ni, Co and S. Type I ore has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios of <7 and relatively flat chondrite-normalized noble metal patterns; the platinum group minerals (PGM) are dominated by sperrylite and moncheite associated with chalcopyrite, pyrrhotite and pentlandite. Type II has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 40 to 330 and noble metal distribution patterns with a positive slope; the most common PGM are sperrylite and Pd bismuthotelluride phases concentrated mostly at the margins of base metal sulfides. Type III ores have the highest (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 240 to 710; the most abundant PGM are sperrylite and phases of the Pt–Pd–Te–Bi–As–Cl system. It is concluded that the Jinchuan deposit formed as a result of primary magmatic crystallization followed by hydrothermal remobilization, transport, and deposition of the PGE.  相似文献   

20.
The supply rates of basaltic magma to volcanoes restrict the flow rates within their mantle sources. Layered source regions consisting of alternating layers of melt and residuum have permeabilities that are orders of magnitude larger than percolative sources. Relevant supply rates for Hawaiian volcanoes are obtained for source permeabilities within the range 10−4–10−2 cm2. The results are within the range for layered sources, suggesting that a layered source is a physically viable model for Hawaiian plume sources. Received: 4 March 1997 / Accepted: 23 April 1998  相似文献   

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