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41.
Chromite is a ubiquitous accessory mineral in the olivine-pyroxene cumulate bodies that host massive and disseminated nickel sulphide mineralization in intrusions of the Kabanga-Musongati-Kapalagulu Alignment in East Africa. Its composition is related to the conditions of emplacement and petrologic evolution of its host magma in a spectrum of intrusions ranging from classical lopolithic layered intrusions to groups of smaller, discrete sill-like chonoliths.The Kapalagulu lopolithic intrusion, emplaced into polymetamorphosed Archæan-Palæoproterozoic crust, contains abundant chromite with relatively oxidized compositions, whereas chromites from the highly-mineralized Kabanga chonolith intrusions, emplaced into graphitic and sulphidic schists, are strongly reduced in terms of their Fe3+/Fetotal ratio. Ni in chromite correlates with Ni in olivine: Ni in both is depleted in the more strongly sulphide-mineralized intrusions. The Musongati intrusion, also emplaced through graphitic schists, but much larger and less-well mineralized in sulphides than Kabanga, has chromites intermediate in character. The compositions of the chromites can be used to determine the petrologic history of the intrusions, and may prove to be a useful exploration tool in such mineralized belts. 相似文献
42.
Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 2: Geology and genesis of the orebodies 总被引：1，自引：0，他引：1
S.?E.?Dowling S.?J.?BarnesEmail author R.?E.?T.?Hill J.?D.?Hicks 《Mineralium Deposita》2004,39(7):707-728
The Black Swan Ultramafic Succession hosts a number of magmatic Fe–Ni–Cu–PGE sulfide ore shoots, ranging from high grade massive ore to low grade disseminated sulfides. Of these, the most economically significant is the Silver Swan massive sulfide orebody, associated with the basal contact of the succession. The deposit varies in thickness between 5 and 20 m, reaches a N–S strike length of 75 m, extends for at least 1.2 km of vertical plunge and is open at depth. Overlying matrix (net-textured) ore is rare. Inclusions of dacite are abundant within the lower 5 m of the massive sulfide. They range from angular fragments through smooth sinuous and plumose morphologies to fine lace-like intergrowths with the sulfide matrix, and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts. Dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures, some well above their liquidus temperature. The composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30% komatiite. The consistent thickness of the inclusion-bearing basal layer within the massive sulphide is interpreted as the extent of 3-D physical connectivity between the inclusions and a partially molten underlying hybrid layer. Primary contacts between the Silver Swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse-grained chevron-textured chromites with skeletal textures. Compositions of these chromites match those from Kambalda, Perseverance and other localities, and are inconsistent with a metamorphic origin. They are interpreted as markers of primary magmatic contacts. The combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates. Taken together with observations on the internal fractionation of platinum group elements within the massive ores, these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube. The floor of the tube had been previously heated by passage of large volumes of lava, such that it had reached its melting range. The felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma. This constitutes strong evidence for the operation of thermo-mechanical erosion during ore emplacement. The disseminated Cygnet and Black Swan orebodies show a number of distinctive features. Cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip-up, transport and redeposition of sulfides from a pre-existing massive sulfide orebody, of which Black Duck may be a remnant. The Black Swan orebody, by contrast, does not show xenolithic features, but is characterised by an association of sulfide blebs with segregation vesicles, and by unusually coarse-grained olivine. The Black Swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals.Editorial handling: Peter Lightfoot 相似文献
43.
R.?E.?T.?Hill S.?J.?BarnesEmail author S.?E.?Dowling T.?Thordarson 《Mineralium Deposita》2004,39(7):684-706
The Black Swan Succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km NNE of Kalgoorlie, within the 2.7-Ga Eastern Goldfields greenstone province of the Yilgarn Craton. The komatiite stratigraphy comprises a steep dipping, east facing package about 700 m in maximum thickness and about 2.5 km in strike length (Fig. 1), which hosts a number of economically exploitable Ni sulphide orebodies including the Silver Swan massive ore shoot (approximately half a million tonnes at about 10.5% Ni). The sequence can be subdivided into a Lower Felsic Unit, comprising coherent and autobrecciated facies of multiple dacite lava flows; an upper Eastern and lower Western Ultramafic Unit, each showing marked lateral facies variation, and an Upper Felsic Unit coeval with the Eastern Ultramafic Unit. The komatiite sequence has been metamorphosed at sub-greenschist facies in the presence of high proportions of CO2-rich fluid, giving rise to pervasive talc–carbonate and talc–carbonate–quartz assemblages, with extensive preservation of pseudomorphed igneous textures. Cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession. The degree of penetrative deformation is generally very low, and original stratigraphic relationships are largely intact in much of the sequence. The Eastern Ultramafic Unit and Western Ultramafic Unit are interpreted as components of a single large komatiite flow field, representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows. The Western Ultramafic Unit which hosts the bulk of the high-grade massive and disseminated ores is a sequence dominated by coarse-grained olivine cumulates, 2 km wide and up to 500 m thick, with major magma pathways represented by thick, homogenous olivine mesocumulate piles at its northern and southern ends: respectively 400 and 200 m thick. The sequence between the two major pathways consists of olivine orthocumulates (oOC) with minor spinifex-textured intervals. The Unit is capped by a persistent spinifex-textured crust less than 1 m thick, and is locally vesicular. The Eastern Ultramafic Unit contains the Black Swan Cumulate Zone, a 500-m thick sequence of very coarse-grained hopper-textured, locally vesicular oOC containing disseminated sulphides in its lower 200 m. The zone is flanked to the north and south by complexly interdigitated sequence of highly irregular, spinifex-capped, olivine cumulate-rich flow lobes between 1 and 100 m thick, and dacitic lavas and tuffs. The complexity of the 3-D spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite, combined with thermal or thermomechanical erosion. The Eastern and Western Units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava, which developed localised thermo-mechanical erosion channels in the dacitic substrate. Komatiite and dacite eruption was synchronous, giving rise to complex interdigitation and extensive contamination and hybridisation.Editorial handling: Peter Lightfoot 相似文献
44.
This work aims to elucidate the factors governing the mobility of metals in soils of a lateritic nickel ore deposit in New Caledonia. The transfer of nickel and associated metals is determined along a topographic sequence ranging from a plateau to a thalweg. Mining exploration borehole data and soil pit data enabled us to define the general geochemical trends of the lateritic weathering. The homogeneous topsoil, which consists mainly of iron oxy-hydroxides, exhibits various amounts of plant available metals. Two interrelated factors control the differences in metal availability: (1) the presence of nickel-rich soil horizons underlying the topsoil and formed of silicate minerals, and (2) the biological cycling of metals from these horizons to the surface. These results have major implications for topsoil management during the mining activity where it is important to minimise metal inputs to water systems and to restore endemic vegetation after mining. 相似文献
45.
The ~260 Ma-old Baimazhai Ni–Cu–(PGE) sulfide deposit in the Jinping region, Yunnan, SW China, is hosted in a small mafic–ultramafic intrusion, which intruded Ordovician sandstone and slate. The intrusion is concentric with lens shape, about 530 m long, 190 m wide and 24 to 64 m thick, trends 296°, and dips 22°NE. The massive sulfide ore body forms the core of the intrusion and is surrounded by variably mineralized orthopyroxenite, websterite and barren gabbro. The proportion of gabbro, websterite, orthopyroxenite and massive ore is approximately 30, 30, 20 and 20 vol.%, respectively. Magmatic pyrrhotite, pentlandite and chalcopyrite make up more than 90% of the massive ores. The massive ores contain high Ni (1.6 to 4.2 wt%) and Cu (0.4 to 6.5 wt%) and low ∑PGE contents (85 to 524 ppb). They have Pd/Ir ratios ranging from 6.7 to 530, Pd/Pt ratios from 0.7 to 2.6 and Cu/(Pd×1,000) ratios from 31 to 400, which are comparable with those of the silicate rocks [Pd/Ir = 4 to 183, Pd/Pt = 0.7 to 3.5, and Cu/(Pd×1,000) = 100 to 400]. Similar Pd/Pt and Cu/Pd ratios of the silicate rocks and massive ores throughout the intrusion indicate a single sulfide segregation event. Excess sulfide melt segregation resulted from intensive crustal contamination that formed Si-rich and Mg-rich basaltic magmas in a deep-seated staging chamber before magma emplacement. The immiscible sulfide melts and the silicate melts were eventually evacuated from the staging magma chamber by compressive forces. Flow differentiation under high velocity concentrated the sulfide melts toward the middle of the magma flow, and consequently, formed a massive sulfide ore body in the central part of the intrusion. Low concentrations of PGEs and general absence of platinum-group minerals in the massive ores may have resulted from a relatively large mass fraction of the sulfide melts (e.g. R-factor = ~70) in Baimazhai compared with other intrusions elsewhere, such as Noril’sk-Talnakh with a R-factor of >10,000. 相似文献
46.
Bonang B.M. Nkoane Gerald M. Sawula Grethe Wibetoe Walter Lund 《Journal of Geochemical Exploration》2005,86(3):130-142
Plant species that accumulate high levels of metals in proportion to the metal content in the soil are of considerable interest in biogeochemical and biogeobotanical prospecting. This study was aimed at investigating copper and nickel accumulation in the plants Helichrysum candolleanum and Blepharis diversispina, to assess their potential use as mineral indicators in biogeochemical prospecting. Soils and plants were collected from copper–nickel mineralised areas in Botswana. Analyses of the soils and the respective plant parts (roots, stem, leaves and flowers) were carried out using ultrasonic slurry sampling electrothermal atomic absorption spectrometry (ETAAS), which allowed rapid determination of copper and nickel in small amounts of the samples.The metal concentration in the soil was in the range ≈ 40 μg/g–4% (w/w) for Cu and ≈ 60 μg/g–0.3% (w/w) for Ni. The concentration ranges of the elements in the plant parts were ≈ 6 μg/g–0.2% Cu and ≈ 3–210 μg/g Ni. At high soil metal content (greater than 2.5% (w/w) Cu and 0.1% (w/w) Ni), high levels of both nickel and copper were found in the shoots (leaves and flowers) of H. candolleanum. Concentrations as high as 0.2% (w/w) Cu were found in the leaves and flowers of H. candolleanum, indicating hyperaccumulation for this plant. For B. diversispina, the metal concentrations did not exceed 100 μg/g for any plant part, for both metals. Both plant species tolerate high concentrations of metals and should therefore be categorized as metallophytes. In order to evaluate metal translocation from the soil to the shoots, metal leaf transfer coefficients (ratio of metal concentration in the leaf to metal concentration in the soil) were calculated. Our data suggest that the two plant species have different metal uptake and transport mechanisms, which needs to be investigated further. The present work also suggests that H. candolleanum may be used as a copper/nickel indicator plant in biogeochemical or biogeobotanical prospecting. 相似文献
47.
The Santa Elena peninsula in the northwest of Costa Rica protrudes about 30 km westwards into the Pacific Ocean, and measures about 8–16 km in a north–south direction. Several geological studies have been carried out since 1953, showing that much of the peninsula is made up of peridotite, cut by mafic dykes. Only one previous brief examination appears to have been made of the vegetation in relation to the composition of the soils. We present here the results of a survey of some soils and plants of the eastern part of the peridotite massif, in which 73 plant specimens representing 51 identified species were collected and analyzed. The soils sampled all showed extreme ultramafic characteristics: Fe 10–16%, Mg 4–16%, Ca 0.5–1.4%; Ni 3000–7500 mg/kg, Cr 1400–3650 mg/kg, Co 150–325 mg/kg. The plants collected include several from genera such as Arrabidaea, Chamaesyce, Helicteres, Hyptis, Lippia, Oxalis, Polygala, Turnera and Waltheria that are also represented on ultramafics elsewhere in the Americas (e.g. Cuba, Puerto Rico, Brazil). Few of the species appear to be endemic to Costa Rica or to the ultramafics of Santa Elena. None of the specimens collected exhibited hyperaccumulation of nickel, the highest Ni concentration being 275 mg/kg in Buchnera pusilla. 相似文献
48.
M. L. Fiorentini N. Rosengren S. W. Beresford B. Grguric M. E. Barley 《Mineralium Deposita》2007,42(8):847-877
The Mount Keith (MKD5) nickel sulfide deposit is one of the largest komatiite-hosted nickel sulfide deposits in the world;
it is hosted by a distinctive spinifex-free, cumulate-rich, ultramafic horizon/unit termed the Mount Keith Ultramafic (MKU).
The Mount Keith Ultramafic shows significant variation along its lateral extent. The internal architecture is made up of adcumulate-textured
pods and lenses, which are flanked by thinner meso- and orthocumulate-textured units, overlain by pyroxenitic and gabbroic
horizons. The lateral and vertical changes in the geometry and internal architecture reflect variations in the lithological
association and emplacement conditions along the strike extent of the belt. The chilled margins of the Mount Keith Ultramafic
unit contain ∼1,200 ppm Ni. Olivine cumulates average ∼2,500–3,500 ppm Ni, with few exceptions (Ni > 4,500 ppm) reflecting
occurrence of minor nickel sulfides, whereas pyroxenites and gabbros generally contain, respectively, ∼1,500–2,000 and ∼100–1,000 ppm
Ni. Olivine cumulates generally contain low Cr concentrations (<2,500 ppm Cr), with the rare presence of chromite-rich intervals
containing anomalously high values (>5,000 ppm Cr). The internal stratigraphy of the Mount Keith Ultramafic unit may be subdivided
into two groups based on rare earth element distribution. The chilled margins and the internal units of the Main Adcumulate
domain display LREE-enriched patterns [(La/Sm)
n
> 1–3] and negative Eu, Hf, Zr, Nb, and Ti anomalies. The internal units in the Western Mineralized Zone generally display
flat chondrite-normalized REE patterns and only minor negative Nb anomalies. The pattern of platinum-group element (PGE) distribution
varies greatly along the strike extent of the Mount Keith Ultramafic unit. The chilled margins display relatively low absolute
concentrations [PGE (excl. Os) ∼16 ppb] and relatively fractionated patterns, with subchondritic Pt/Pd ratios (∼1.5), and
superchondritic Pd/Ir ratios (∼3). The PGE trends in the thick adcumulate-textured pods containing widespread nickel sulfide
mineralization display positive correlation with sulfide abundance, whereas fractionated pyroxenites and gabbros in the thinner
domains display highly depleted PGE concentrations and generally show compatible PGE trends. The nickel sulfide ore typology
and style vary greatly along the strike extension of the Mount Keith Ultramafic unit. Basal massive nickel sulfide mineralization
(e.g., Sarah’s Find) occurs in the thinner meso- and orthocumulate-textured units, whereas stratabound disseminated nickel
sulfide mineralization (e.g., MKD5 Ni Deposit) is hosted in the adcumulate-textured pods. We hypothesize that the very low
PGE content of the initial liquid of the Mount Keith Ultramafic unit indicates that the initial magma pulse that penetrated
through the dacite host-rock had already equilibrated with sulfides at depth and/or carried entrained immiscible sulfide blebs.
We argue that upon emplacement, the intruding magma experienced a significant thermal shock at the contact with water-saturated
volcaniclastic breccias. The sudden chilling would have increased the viscosity of the magma, possibly to the point where
it was no longer able to sustain the suspension of the immiscible sulfide liquid. As a result, the sulfide blebs coalesced
and formed the basal massive sulfide nickel sulfide mineralization at the base of the sill (i.e., Sarah’s Find). Prolonged
focused high volume magma flow within the sill resulted in the emplacement of a thick, lens-shaped accumulation of olivine
adcumulate. Local variations in intensive parameters other than crustal assimilation (e.g., T, fO2, fS2) may be principally responsible for sulfide supersaturation and controlled the local distribution of stratabound disseminated
nickel sulfide mineralization (e.g., MKD5 Ni Deposit), generally localized within the core of the thicker dunite lenses. 相似文献
49.
Empirical equations to predict the sulfur content of mafic magmas at sulfide saturation and applications to magmatic sulfide deposits 总被引：7，自引：0，他引：7
Empirical equations to predict the sulfur content of a mafic magma at the time of sulfide saturation have been developed based
on several sets of published experimental data. The S content at sulfide saturation (SCSS) can be expressed as:
where T is in degrees Kelvin, X is mole fraction and P is in kbar. The squared multiple correlation coefficient (r
2) for the equation is 0.88. Application of the equation to data from sulfide-saturated mid-ocean ridge basalts (MORB) samples
show that the SCSS is closely predicted for primitive MORBs, but that accuracy decreases for lower T (<1,130°C) and more evolved MORB samples. This suggests that because the calibrations are based on anhydrous experimental
runs done at temperatures of 1,200°C and above, it is not possible to extrapolate them to significantly lower temperatures
and hydrous conditions. Because the SCSS of a primitive MORB magma increases with decreasing P, sulfide saturation in MORB appears to be a function of the degree of en route assimilation of S from country rocks as well
as the degree of fractional crystallization in shallow staging chambers. Application of the equation to the high-T
impact melt sheet that produced the Sudbury Igneous Complex and associated Ni–Cu sulfide ores indicates that sulfide-saturation
was reached at ~1,500°C, well above the start of orthopyroxene crystallization at ~1,190°C. This would permit ample time for
the gravitational settling and collection of immiscible sulfide liquid that produced the high-grade ore bodies. The development
of a platinum group element (PGE)-enriched layer in the Sonju Lake Intrusion of the Duluth Complex is thought to be due to
the attainment of sulfide saturation in the magma after a period of fractional crystallization. Using the composition of the
parent magma of the Sonju Lake Intrusion the presented equation indicates that sulfide saturation would have been reached
at ~60% crystallization, when iron oxide was a liquidus mineral; the prediction is in agreement with field evidence which
indicates that PGE-enrichment occurs in the oxide-rich gabbro zone. Contamination and mixing processes that may be related
to the attainment of sulfide saturation in mafic magmas can also be evaluated. Mixing of a siliceous melt and a liquid of
olivine tholeiite composition, similar to that thought to be a reasonable parental composition for many Duluth Complex intrusions,
can induce sulfide saturation at mixing ratios in excess of ~0.1. If the contaminant contains low quantities of sulfur the
mixing ratio required to promote saturation is reduced. Mixing of mafic magmas at various stages of fractionation is evaluated
using magma compositions that are thought to be appropriate for the generation of the Merensky Reef in the Bushveld Complex.
Magma mixing is shown to be an effective process for the attainment of sulfide saturation, depending strongly on the sulfur
concentrations of the end-member magmas. 相似文献
50.
The metaturbidites of the Palaeoproterozoic Jormua–Outokumpu thrust belt in eastern Finland enclose m- to km-scale ultramafic massifs that are distributed over an area of more than 5000 km2. These bodies, which almost entirely consist of highly depleted mantle peridotites (now metaserpentinites and metaperidotites), are intimately associated with massive to semimassive, polymetallic Cu–Co–Zn–Ni–Ag–Au sulphide deposits that sustained mining in the region between 1913 and 1988. Currently, one deposit (Kylylahti) is proceeding into a definitive feasibility study emphasising the renewed economic interest for Outokumpu-type deposits.The origin of these Outokumpu-type Cu–Co–Zn–Ni–Ag–Au deposits is now re-interpreted to be polygenetic. First, their formation requires deposition of a Cu-rich proto-ore within peridotitic sea floor at 1950 Ma. Close modern analogues to the proto-ore setting include, for example, the Logatchev and Rainbow fields at the Mid-Atlantic Ridge, where venting of high-T–low-pH hydrothermal fluid resulted in accumulations of Cu–Zn–Co–Ag–Au sulphides on serpentinised ultramafic seafloor. Second, the Ni-rich composition of Outokumpu sulphide ores calls for a separate source for nickel: Some 40 Ma after the deposition of the Cu-rich proto-ore – concomitant with the obduction of the ultramafic massifs – disseminated Ni sulphides formed through chemical interaction between obducting peridotite massifs and adjacent black schists. This process was related to listwaenite–birbirite type carbonate–silica alteration at margins of the ultramafic massifs. Due to this alteration, silicate nickel was released from the primary Fe–Mg silicates and redeposited as Ni sulphides in the alteration fringes of the massifs.We propose that syntectonic mixing of these two “end-member” sulphides, i.e., the primary Cu-rich proto-ore and the secondary Ni-sulphide disseminations, resulted in the uncommon metal combination of the Outokumpu-type sulphides. Late tectonic solid-state re-mobilisation, related to the duplexing of the ore by isoclinal folding, upgraded the sulphides into economic deposits. 相似文献