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1.
The Nernst partition coefficient of nickel (DNi) between Cr-spinel and silicate melt in natural systems has been investigated using mid-ocean ridge basalts (MORB) and other volcanic rocks. The Cr-spinel/olivine DNi values in volcanic rocks are between 1.2 and 0.3, indicating that the Cr-spinel/liquid DNi values vary from slightly higher to significantly lower than the olivine/liquid DNi values in natural systems. The Cr-spinel/liquid DNi values from the MORB samples vary between 6 and 11, slightly higher than those from the S-bearing experiments of Satari et al. [Satari P., Brenan J. M., Horn I. and McDonough W. F. (2002) Experimental constraints on the sulfide- and chromite-silicate melt partitioning behavior of rhenium and platinum-group elements. Economic Geology97, 385-398]. The results of the MORB samples and the experiments of Satari et al. (2002) indicate a negative correlation between the Cr-spinel/liquid DNi and the XCr values in Cr-spinels (Cr cation number on the basis of 3 total cations in the spinel structure). Variations of Cr-spinel/liquid DNi values with Cr-spinel compositions can be estimated from an empirical equation based on the results of the MORB samples and the experiments by Satari et al. (2002). The choice of Cr-spinel/liquid DNi = 10 for numerical modeling by Righter et al. [Righter K., Leeman W. P. and Hervig R. L. (2006) Partitioning of Ni, Co, and V between spinel-structured oxides and silicate melts: importance of spinel composition. Chemical Geology227, 1-25] is reasonable for basaltic systems. For picritic and komatiitic systems a lower value of ∼5 is more appropriate. 相似文献
2.
含硫化物镁铁质-超镁铁质侵入体中橄榄石Mg、Ni含量的制约:原理、模式及Voisey's Bay侵入体样品研究(英文) 总被引：4，自引：0，他引：4
镁铁质-超镁铁质岩浆结晶分离早期形成镁铁矿物,镁铁矿物中的Ni和Mg是相容元素。随着结晶分离作用的进行,Ni、Mg在硅酸盐岩浆及后形成的硅酸盐物质中的丰度下降。橄榄石中Ni含量及硅酸盐物质MgO/FeO比值都与母岩浆的相关值相关,据此可推断母岩浆的信息,它们之间可由实验测得的系数相联系。当岩浆饱和硫化物时,在结晶分离过程中硫化物珠滴会与镁铁硅酸盐物质一道析出,同时,与硫化物非饱和岩浆相比,过多的Ni会随之析出。这也反映在Ni、Mg含量比无硫化物分离时有更迅速的降低上。Ni、Mg含量变化值可以在VoiseysBay侵入体的模式曲线上反映出,加拿大Labrador的这一侵入体赋存了一个世界级的Ni-Cu-Co硫化物矿床。过去的作法是将侵入体中橄榄石的Ni、Mg含量与Simkin和Smith得出的各种火成岩中橄榄石的Ni、Mg含量相比较以确定Ni亏损,进而假定橄榄石来自硫化物饱和、有经济价值的岩浆。现在的研究显示这种简单的对比会导致错误。将样品数据与模式曲线对比并反映出侵入体矿物结晶堆积特征是重要的方法。使用这一方法,样品数据能很好地被模式曲线拟合。以在VoiseysBay的研究为例,当硫化物液相与硅酸盐矿物被去除后,硫化物非饱和的分离作用期就会显现出来,随后是硅酸盐结晶作用期。 相似文献
3.
The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes 总被引：1，自引：0，他引：1
Wolfgang D. Maier Sarah-Jane Barnes Arindam Sarkar Ed Ripley Chusi Li Tim Livesey 《Mineralium Deposita》2010,45(5):419-441
The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with
current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008).
The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the
approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to
about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing
pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of
fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly
enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic
signatures (δ18O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic
pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ18O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine
orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions
in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions
and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses,
and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide
occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ34S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation
of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution
of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior
to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma
ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ18O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light
δ18O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively
assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides
deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny
and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones,
notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach
a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important
exploration target for additional nickel sulfide deposits. 相似文献
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The Eagle Ni–Cu–(PGE) deposit is hosted in mafic–ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in association with picritic magmatism in a dynamic magma conduit during the early stages in the development of the ~1.1?Ga Midcontinent Rift System. Four main types of sulfide mineralization have been recognized within the Eagle deposit: (1) disseminated sulfides in olivine-rich rocks; (2) rocks with semi-massive sulfides located both above and below the massive sulfide zone; (3) massive sulfides; and (4) sulfide veins in sedimentary country rocks. The disseminated, massive and lower semi-massive sulfide zones are typically composed of pyrrhotite, pentlandite and chalcopyrite, whereas the upper semi-massive sulfide ore zone also contains pyrrhotite, pentlandite, and chalcopyrite, but has higher cubanite content. Three distinct types of sulfide mineralization are present in the massive sulfide zone: IPGE-rich, PPGE-rich, and PGE-unfractioned. The lower and upper semi-massive sulfide zones have different PGE compositions. Samples from the lower semi-massive sulfide zone are characterized by unfractionated PGE patterns, whereas those from the upper semi-massive sulfide zone show moderate depletion in IPGE and moderate enrichment in PPGE. The mantle-normalized PGE patterns of unfractionated massive and lower semi-massive sulfides are subparallel to those of the disseminated sulfides. The results of numerical modeling using PGE concentrations recalculated to 100% sulfide (i.e., PGE tenors) and partition coefficients of PGE between sulfide liquid and magma indicate that the disseminated and unfractionated massive sulfide mineralization formed by the accumulation of primary sulfide liquids with similar R factors between 200 and 300. In contrast, the modeled R factor for the lower semi-massive sulfide zone is <100. The fractionated sulfide zones such as those of the IPGE-rich and PPGE-rich massive sulfides and the upper semi-massive sulfide zone can be explained by fractional crystallization of monosulfide solid solution from sulfide liquids. The results of numerical modeling indicate that the sulfide minerals in the upper semi-massive sulfide zone are the products of crystallization of fractionated sulfide liquids derived from a primary sulfide liquid with an R factor similar to that for the disseminated sulfide mineralization. Interestingly, the modeled parental sulfide liquid for the IPGE-rich and PPGE-rich massive sulfide zones has a higher R factor (~400) than that for the unfractionated massive sulfide mineralization. Except one sample which has unusually high IPGE and PPGE contents, all other samples from the Cu-rich sulfide veins in the footwall of the intrusion are highly depleted in IPGE and enriched in PPGE. These signatures are generally consistent with highly fractionated sulfide liquids expelled from crystallizing sulfide liquid. Collectively, our data suggest that at least four primary sulfide liquids with different R factors (<100, 200–300, ~400) were involved in the formation of the Eagle magmatic sulfide deposit. We envision that the immiscible sulfide liquids were transported from depth by multiple pulses of magma passing through the Eagle conduit system. The sulfide liquids were deposited in the widened part of the conduit system due to the decreasing velocity of magma flow. The presence of abundant olivine in some of the sulfide ore zones indicates that the ascending magma also carried olivine crystals. Sulfide saturation was attained in the parental magma due in large part to the assimilation of country rock sulfur at depth. 相似文献
6.
The Limahe Ni–Cu sulfide deposit is hosted by a small mafic–ultramafic intrusion (800 × 200 × 300 m) that is temporally associated
with the voluminous Permian flood basalts in SW China. The objective of this study is to better understand the origin of the
deposit in the context of regional magmatism which is important for the ongoing mineral exploration in the region. The Limahe
intrusion is a multiphase intrusion with an ultramafic unit at the base and a mafic unit at the top. The two rock units have
intrusive contacts and exhibit similar mantle-normalized trace element patterns and Sr–Nd isotopic compositions but significantly
different cumulus mineralogy and major element compositions. The similarities suggest that they are related to a common parental
liquid, whereas the differences point to magma differentiation by olivine crystallization at depth. Sulfide mineralization
is restricted to the ultramafic unit. The abundances of sulfides in the ultramafic unit generally increase towards the basal
contacts with sedimentary footwall. The δ
34S values of sulfide minerals from the Limahe deposit are elevated, ranging from +2.4 to +5.4‰. These values suggest the involvement
of external S with elevated δ
34S values. The mantle-normalized platinum-group element (PGE) patterns of bulk sulfide ores are similar to those of picrites
associated with flood basalts in the region. The abundances of PGE in the sulfide ores, however, are significantly lower than
that of sulfide liquid expected to segregate from undepleted picrite magma. Cr-spinel and olivine are present in the Limahe
ultramafic rocks as well as in the picrites. Mantle-normalized trace element patterns of the Limahe intrusion generally resemble
those of the picrites. However, negative Nb–Ta anomalies, common features of contamination with the lower or middle crust,
are present in the intrusion but absent in the picrites. Sr–Nd isotopes suggest that the Limahe intrusion experienced higher
degrees of contamination with the upper crust than did the picrites. The results of this study permit us to suggest that the
parental magma of the Limahe intrusion was derived from picritic magma by olivine fractionation and contamination in a staging
chamber at mid-crustal levels. Depletion of PGE in the sulfide ores in the Limahe intrusion is likely due to previous sulfide
segregation of the parental magmas in the staging chamber. Sulfide mineralization in the Limahe intrusion is related to second-stage
sulfide segregation after the fractionated magmas acquired external S from pyrite-bearing country rocks during magma ascent
to the Limahe chamber. The abrupt change in mineralogical and chemical compositions between the ultramafic unit and the overlying
unit suggests that at least two separate pulses of magma were involved in the development of the Limahe intrusion. We propose
that the Limahe intrusion was once a wider part of a dynamic conduit that fed magma to the overlying subvolcanic dykes/sills
or lavas. The ultramafic unit formed by the first, relatively more primitive magma, and the mafic unit formed by the second,
relatively more fractionated magma. Immiscible sulfide droplets that segregated from the first magma settled down with olivine
crystals to form the sulfide-bearing, olivine-rich rocks in the base of the intrusion. The overlying residual liquids were
then pushed out of the chamber by the second magma. Critical factors for the formation of an economic Ni–Cu sulfide deposit
in such a small intrusion include the dynamic petrologic processes involved and the availability of external sulfur. The Limahe
deposit reminds us that small, multiphase, mafic–ultramafic intrusions in the region should not be overlooked for the potential
of economic Ni–Cu sulfide deposits. 相似文献
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The coexistence of magmatic anhydrite and sulfide minerals in non-arc-related mafic magmas has only rarely been documented. Likewise the S isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured. In the Kharaelakh intrusion associated with the world-famous Noril’sk ore district in Siberia coexisting magmatic anhydrite and sulfide minerals have been identified. Sulfur isotope compositions of the anhydrite-sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite-sulfide pairs. Magmatic anhydrite and chalcopyrite are characterized by δ34S values between 18.8‰ and 22.8‰, and 9.3‰ and 13.2‰, respectfully. Coexisting anhydrite and chalcopyrite show Δ values that fall between 8.5‰ and 11.9‰. Anhydrite in the Kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks; partial reduction to sulfide led to mixing between sulfate-derived sulfide and sulfide of mantle origin. The variable anhydrite and sulfide δ34S values are a function of differing degrees of sulfate reduction, variable mixing of sulfate-derived and mantle sulfide, incomplete isotopic homogenization of the magma, and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling. The δ34S values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide/sulfate ratio. Variations in both sulfide and anhydrite δ34S values indicate that isotopically distinct domains existed on a centimeter scale. Late stage hydrothermal anhydrite and pyrite also occur associated with Ca-rich hydrous alteration assemblages (e.g., thomsonite, prehnite, pectolite, epidote, xonotlite). δ34S values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals. Anhydrite-pyrite Δ values are in the 9.1-10.1‰ range, and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages. The small Δ values are suggestive of either sulfate-sulfide isotopic disequilibrium or closure of the system to further exchange between ∼550 and 600 °C. Our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated δ34S values in the Kharaelakh intrusion, but highlight the sulfur isotopic variability that may occur in magmatic systems. In addition, our results confirm the need for more precise experimental determination of sulfate-sulfide sulfur isotope fractionation factors in high-T systems. 相似文献
10.
Yan Tao Chusi Li Ruizhong Hu Edward M. Ripley Andao Du Hong Zhong 《Contributions to Mineralogy and Petrology》2007,153(3):321-337
The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units
at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low
Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations
suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds
that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual
assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated
by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization.
Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively
passing through the conduit to achieve high PGE concentrations. 相似文献