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1.
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   

2.
Ore deposits associated with mafic magmas in the Kaapvaal craton   总被引：2，自引：0，他引：2  
Mafic and ultramafic magmatism played an important role in the 3.5 Ga long history of the Kaapvaal craton. The oldest (3.5 Ga) greenstone belts contain mafic and ultramafic volcanics that erupted in an oceanic environment, probably in oceanic plateaus. Then followed a series of continental flood basalts, from the ∼3.4 Ga old Commondale and Nondweni sequences, to the 180 Ma Karoo basalts. The history was dominated, however, by the emplacement, 2.1 Ga ago, of the Bushveld complex, an enormous layered ultramafic-mafic-felsic intrusion. Three types of ore deposits might be found in such a sequence: Ni-Cu-Fe sulfides in komatiites of the greenstone belts; “Noril'sk-type” Ni-Cu-PGE deposits in the Karoo and other flood basalts; and deposits of Cr, platinum-group elements (PGE) and V in the Bushveld and other layered intrusions. Only the latter are present. It is tempting to attribute the absence of komatiite-hosted deposits to the specific character of the ultramafic rocks in Kaapvaal greenstone belts, which are older that the 2.7 Ga komatiites that host deposits in Australia, Canada and Zimbabwe, and are of the less-common “Al-depleted” type. However, a review of mantle melting processes found no obvious connection between the character of the mantle melts and their capacity to form ore deposits. The lack of this type of deposit may be due to differences in the volcanic environment, or it may be fortuitous (the Barberton and other belts are small and could fit into deposit-free parts of the much larger Australian or Canadian belts). Still more puzzling is the absence of Noril'sk-type deposits. The Karoo and older flood basalt sequences appear to contain all the important elements of the volcanic sequences that host the Siberian deposits. It is now recognised that these deposits formed through the segregation of sulfide from magma flowing rapidly through conduits en route from deeper magma chambers to the surface. An exploration approach aimed at understanding the fluid dynamics of such systems seems warranted. Although the Bushveld intrusion has been studied for decades and its deposits are taken as type examples of magmatic mineralisation, the origin of its PGE deposits remains unclear. Opinion is divided on the relative importance of sulfide segregation from magma filling a large chamber at the time of emplacement, and the scavanging of PGE from fluids circulating through cumulates at a late magmatic stage. Answers to these questions may come from studies designed to gain a better understanding of the mechanisms through which the magma chamber filled and solidified. Received: 15 September 1996 / Accepted: 7 January 1997  相似文献   

3.
Although there are many mafic-ultramafic intrusions in the western and central regions of Mongolia, Central Asian Orogenic Belt (CAOB), no economic-grade Ni-Cu deposits have yet been discovered. To understand the economic Ni-Cu deposit potential of the intrusions in central Mongolia, the parental magma affinity and sulfide saturation of the Oortsog, Dulaan, and Nomgon Ni-Cu mineralized mafic-ultramafic intrusions are studied. These three intrusions are predominantly gabbroic in composition, while the Oortsog and Dulaan intrusions also contain small proportions of peridotites. The parental magmas of the Oortsog and Dulaan intrusions are tholeiitic, as indicated by their Cr-spinel and clinopyroxene compositions, whereas the parental magma of the Nomgon intrusions is likely calc-alkaline. The compositions of Cr-spinel and clinopyroxene, combined with the presence of significant Nb-Ta depletions, indicate that these rocks were most likely derived from modified mantle sources. Both the Oortsog and Nomgon intrusions form two clusters in terms of their olivine composition, suggesting that multiple magma surges were involved during their emplacement. The relatively low Fo values and Ni contents in olivine from the three intrusions compared to those from Ni-Cu deposits in NW China, as well as those in the Voisey’s Bay deposit in Canada, indicate that the three intrusions were crystallized from relatively evolved magmas. The Cu/Zr ratios of rocks of the Oortsog, Dulaan, and Nomgon intrusions are higher than 1, suggesting that these rocks contain cumulus sulfide. This, coupled with the presence of rounded sulfide inclusions in olivine of the Oortsog and Dulaan intrusions, suggests that sulfide saturation occurred before or during olivine crystallization. The distribution patterns of platinum group elements (PGEs) of the Dulaan and Oortsog intrusions record slight Rh, Pt, and Pd (PPGE) enrichment relative to Os, Ir, and Rh (IPGE). Furthermore, the Ni/Cu ratios of sulfide-bearing rocks from the Oortsog intrusion vary from 1.8 to 3.8, which are consistent with those of the Ni-Cu sulfide deposits in NW China. In contrast, the Ni/Cu ratios of sulfide-bearing rocks from the Nomgon intrusion are extremely low (0.03 to 0.07). This, together with the significant enrichment in PPGE relative to IPGE, suggests that these sulfides of the Nomgon intrusion were segregated from a magma that was extremely enriched in Cu and PPGE but depleted in Ni and IPGE. The characteristics of the chalcophile elements in these intrusions are attributed to the fact that the derivation of the Nomgon magma was significantly different from that of the Dulaan and Oortsog parental magmas. Overall, although the parental magmas of the intrusions in central Mongolia are more evolved than those in NW China, they are comparable in terms of the sizes of their intrusions, constituent minerals, and mineral chemistry. These similarities suggest that the intrusions in central Mongolia have economic Ni-Cu sulfide potential. Furthermore, intrusions similar to the Nomgon intrusion may feature PGE mineralization potential.  相似文献   

4.
The “Americano do Brasil” Complex (ABC) is part of a cluster of coeval synorogenic mafic–ultramafic intrusions emplaced during the Brasiliano/Pan-African Orogenic Cycle in Brazil. The medium-sized ABC consists of interlayered dunite, peridotite, websterite, and gabbronorite. High Fo values of olivine (up to Fo88) and the crystallization sequence of the ABC (Ol + Chr ≥ Ol + Opx + Chr ≥ Cpx + Opx ≥ Opx + Pl + Cpx ≥ Opx + Pl + Cpx + Ilm + Mag) suggest crystallization from tholeiitic high-MgO parental magmas. Light rare earth element (REE)-enriched mantle-normalized REE profiles and εNd(T) values of +2.4 for cumulate rocks from the ABC suggest a depleted mantle source for the parental magma. The ABC Ni–Cu sulfide deposit (3.1 Mt at 1.12 wt.% Ni and 1.02 wt.% Cu) consists of three distinctively different orebodies (S1, S2, and G2). The S2 orebody, an unusual occurrence of stratiform massive sulfide hosted by dunite and peridotite in the interior of a layered intrusion, results from sulfides accumulated at the transient base of the magma chamber following a new influx of parental magma. The G2 orebody has an irregular and roughly cylindrical shape, consisting mainly of net-textured sulfides. The G2 orebody is hosted by peridotite and pyroxenite and located stratigraphically below the S1 orebody. S2 and G2 orebodies are characterized by low Cu/Cu + Ni ratios (mainly below 0.4). The S1 orebody, hosted by websterite and gabbronorite in the more fractionated sequence of the ABC, is a cluster of several irregular discontinuous orebodies of Ni–Cu disseminated sulfides. The sulfides of the S1 orebody have high Cu/Cu + Ni ratios (mainly between 0.5 and 0.8) and are highly depleted in PGE. The S1 orebody is interpreted to result from a later event of sulfide segregation in the magma chamber, possibly following the event that originated the G2 orebody. The bulk of δ34S values for sulfides of the ABC orebodies and their host rocks fall in the range of 0 ± 2‰. Higher δ34S values (between 3‰ and 5‰) are restricted to pyrite from xenoliths of gneiss located close to the S1 orebody and sulfides from the S1 orebody. Crustal xenoliths and chemical data (lithogeochemistry and sulfur isotope composition) provide evidence of crustal contamination of the igneous rocks hosting the S1 orebody, suggesting that sulfur saturation was induced by contamination with sulfide-bearing crustal rocks. The ABC deposit is an example of Ni–Cu sulfide mineralization hosted by synorogenic mafic–ultramafic intrusions. The S2 orebody is the first documented example of an economic stratiform massive sulfide orebody located within layered intrusions, expanding the opportunities for exploration of Ni–Cu sulfides in orogenic regions worldwide.  相似文献   

5.
The Portneuf–Mauricie Domain (PMD), located in the south-central part of the Grenville province, contains Mesoproterozoic Ni–Cu ± platinum-group element (PGE) prospects hosted in a variety of plutonic intrusions (layered, with simple structures, or zoned) and emplaced in a mature island arc setting. A two-stage model is envisaged to explain the formation of magmatic sulfides. An early loss of a small amount of sulfides in the conduits of primitive, hydrous mantle-derived melts under high fO2, resulted in depletion of the magmas in chalcophile and precious metals (Cu/Pd ratios vary from initial mantle values up to 1.6 × 106). Then, nearer the mineralized zones, the magmas interacted with sulfide-bearing country rocks, resulting in felsification of the magmas, assimilation of crustal sulfur (δ 34S values up to +5.5‰), and the formation of an immiscible sulfide liquid. Liquid-sulfide formation was followed by variable interactions between the silicate and sulfide magmas, which were responsible for the enrichment of sulfides in Ni, Cu, and, locally, PGE. Indeed, low R factors are found for prospects hosted in intrusions with a simple internal structure and in layered intrusions whereas high R factors are found for prospects hosted in zoned intrusions. Finally, sulfide melt may have been partly incorporated into later pulses of magma and injected into shallow magma chambers to form the PMD prospects. The PMD prospects share common characteristics with other well-known deposits (Aguablanca, Vammala, Stormyrplunen, and deposits in Alaskan/Ural-type intrusions), attesting to the Ni, Cu, and PGE potential of deposits associated with subduction-zone settings.  相似文献   

6.
The Voisey's Bay nickel–copper deposit and the Pants Lake sulfide occurrences are the most important mineral systems discovered to date within the Nain Plutonic Suite in northern Labrador. There are many intriguing similarities at both locations. Both are hosted by relatively small troctolite/gabbro bodies that intrude the sulfide-bearing paragneiss of the Churchill Province, and these intrusions contain inclusions of the paragneiss. Similar chemical reactions of the gneissic inclusions with the host magmas are observed at both locations. The reactions resulted in the addition of SiO2, K2O, Na2O and sulfur to the magmas, and are responsible for sulfide-saturation and resultant segregation of immiscible sulfide liquids from the magmas. The initial sulfide liquids in both cases were relatively poor in metals, containing <2.5 wt% Ni and 2 wt% Cu. The sulfides at Pants Lake remained poor in metals because of a lack of subsequent interaction with new, chalcophile-undepleted magma. At Voisey's Bay, the initial sulfides segregated in a dynamic conduit, and were subsequently upgraded in metals to ∼6 wt% Ni and 3 wt% Cu by a new surge of undepleted magma using the same conduit. These sulfides were then concentrated to form large sulfide bodies in the wider parts of the conduit and its entry to an upper chamber in response to a sudden change of liquid velocity in these environments. This study confirms three of the most important factors for the formation of magmatic sulfide deposits in an evolved magma system: (1) contamination of magma with sulfide-bearing country rock to achieve sulfide saturation; (2) a dynamic magmatic system such as a magma conduit to transport large volume of sulfide liquid and to concentrate them in limited localities, and (3) upgrading of metals in the sulfide by new, chalcophile-undepleted magma. Received: 20 February 2000 / Accepted: 14 September 2000  相似文献   

7.
张照伟 《地质与勘探》2017,53(5):825-837
石头坑德镁铁-超镁铁质岩体位于东昆仑造山带东段五龙沟地区,邻近昆中大断裂,主要岩性为辉石岩、橄辉岩、橄榄岩、辉长岩等,铜镍矿体主要赋存于Ⅰ号岩体的辉石岩、橄辉岩和橄榄岩中。岩石地球化学特征表明,其镁铁比值(m/f)变化范围在2.77~6.01之间,属铁质系列的镁铁-超镁铁岩,有利于成铜镍矿。稀土总量总体较低,轻稀土元素之间分馏强,重稀土元素之间分馏弱。岩石总体富集LILE(Rb、Ba、Th、U),贫HFSE(Nb、Ta、Zr、Y),具有明显的Rb、U正异常及Nb、Ta的负异常。超镁铁岩类橄榄石中的Ni普遍亏损,指示深部存在硫化物的熔离作用,在橄榄石结晶之前,大量的Ni进入到硫化物中,极有利于形成富矿岩浆或矿浆。可见,伴随同化混染作用的发生,岩浆中S达到过饱和,进而硫化物发生不混溶作用,富含硫化物的岩浆运移至东昆仑造山带邻近昆中断裂构造薄弱的部位成岩成矿。铜镍矿体多赋存在含橄榄石的超镁铁质岩体内,指示石头坑德岩体深部存在高度富集铜镍的部位,是下一步的找矿方向。  相似文献   

8.
The Permian Huangshanxi Cu–Ni deposit is the second largest magmatic sulfide deposit discovered to date in a major Ni–Cu province related to protracted basaltic magmatism in eastern Xinjiang, China. It is hosted by a small mafic–ultramafic intrusion comprised predominantly of lherzolites, olivine websterites, gabbronorites, and gabbros. The Huangshanxi intrusion is coeval with Permian basalts of tholeiitic and alkaline affinities in the Tuha and Tarim basins, respectively. To evaluate a possible genetic relationship between the Huangshanxi intrusion and a specific type of coeval basalt in the region, as well as ore genesis in the intrusion, we have carried out an integrated mineralogical, petrological, and geochemical study. Our data reveal that the Huangshanxi intrusive rocks are characterized by relatively flat chondrite-normalized REE patterns, depletion in Nb and Ta, and elevated εNd values varying between 6 and 10. These features are similar to those of coeval tholeiitic basalts in the nearby Tuha basin, but are significantly different from those of coeval alkaline basalts in the relatively remote Tarim basin. The geochemical similarities and differences suggest that the Huangshanxi intrusion is genetically related to the tholeiitic basalts in the Tuha basin, not to the alkaline basalts in the Tarim basin, as suggested previously by some researchers. This implies that regional exploration for the Huangshanxi-type Cu–Ni deposits should be centered in the Tuha basin instead of the Tarim basin. More specifically, the uplifted areas around the Tuha basin where similar intrusions may have been brought close to the surface should be carefully examined for mineralization potential. Intrusive relations and mass balance constraints from incompatible trace elements and sulfide abundances suggest that the Huangshanxi intrusion represents a dynamic magma conduit through which multiple pulses of magma ascended to higher levels or to the surface. Numerical simulation of magma evolution and mixing calculations using Sr–Nd isotopes indicate that selective assimilation of S-bearing crustal materials is important for sulfide saturation during the early stages of magma evolution when lherzolites formed. Fractional crystallization may have also played a role in the attainment of sulfide saturation during the later stages of magma evolution when olivine websterites and gabbronorites formed. In both cases, immiscible sulfide droplets were retained in the conduit to form disseminated sulfide lenses while the fractionated silicate liquids and buoyant phases such as plagioclase continued to ascend. Extremely low PGE tenors in the sulfide ores of the Huangshanxi deposit suggest that the parental magma was highly depleted in chalcophile elements possibly due to previous sulfide segregation at depth.  相似文献   

9.
内容提要本文以金宝山为典型实例,根据元素地球化学特征探讨了西南暗色岩铜镍硫化物矿化岩体与峨眉山玄武岩的关系。分析表明,金宝山超镁铁岩与低钛峨眉山玄武岩在元素地球化学特征上具有一致的岩浆成因属性,两者在成岩机制上互补,低钛峨眉山玄武岩普遍经历了橄榄石结晶分异和硫化物熔离亏损作用,金宝山成矿岩体则与低钛峨眉山玄武岩同源岩浆深部分异的堆晶相相对应,由堆晶橄榄石及熔离硫化物和部分残余熔体构成的“晶-糊”侵位形成,因此认为铜镍硫化物矿床成矿岩体与低钛峨眉山玄武岩为同源异相产物。  相似文献   

10.
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   

11.
Re–Os isotope compositions of mantle-derived magmas are highly sensitive to crustal contamination because the crust and mantle have very different Os isotope compositions. Crustal contamination may trigger S saturation and thus the formation of magmatic Ni–Cu–(PGE) sulfide deposits. The ∼287-Ma Kalatongke norite intrusion of NW China are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization. The Re–Os isotope compositions in the intrusion are highly variable. Norite and massive sulfide ores have γ Os values ranging from +59 to +160 and a Re–Os isochron age of 239 ± 51 Ma, whereas disseminated sulfide ores have γ Os values from +117 to +198 and a Re–Os isochron age of 349 ± 34 Ma. The variability of Os isotope compositions can be explained as the emplacement of two distinct magma pulses. Massive sulfide ores and barren norite in the intrusion formed from the same magma pulse, whereas the disseminated sulfide ores with more radiogenic Os isotopes formed from another magma pulse which underwent different degrees of crustal contamination. Re–Os isotopes may not be suitable for dating sulfide-bearing intrusions that underwent variable degrees of crustal contamination to form magmatic sulfide deposits.  相似文献   

12.
毛亚晶  秦克章  唐冬梅 《岩石学报》2018,34(8):2410-2424
岩浆铜镍矿床100%硫化物中的Ni含量与赋矿岩石和成矿过程紧密相关,记录岩浆成分、分异程度与硫化物演化过程。硫化物异常高镍(高镍硫化物)往往被认为与科马提质岩浆或者后期热液作用密切相关。近年研究结合勘查证实,赋含高镍硫化物的矿床(高镍铜镍矿床)不仅限于科马提岩,还与苦橄质、玄武质岩浆有关,另外,热液富集作用并不是必要因素。本文总结了世界上高镍铜镍矿床的基本特征和形成机制,分析提出了不同机制的判别标志,并展望了其勘查前景。详细对比高镍铜镍矿床的产出环境、赋矿岩相、矿石特征、矿物组合等特征,该类矿床往往产于大陆裂谷和造山带环境,与基性程度较高的岩浆有关,以橄榄岩赋矿为主,含镍硫化物组合主要为镍黄铁矿-磁黄铁矿-黄铜矿组合,少数为针镍矿-镍黄铁矿-黄铁矿组合。科马提岩相关矿床可将Ni含量大于16%的硫化物定义为高镍硫化物,苦橄质-玄武质岩浆相关矿床的硫化物可分为高镍硫化物(Ni10%)、中镍硫化物(5%～10%)和富铜硫化物(Ni5%,CuNi)。原生高镍硫化物可由富镍岩浆熔离、硫化物从橄榄石中吸取Ni、硫化物结晶分异、硫化物与硫不饱和岩浆反应等机制形成。苦橄质-玄武质岩浆相关的矿床,硫化物与橄榄石的Fe-Ni交换反应是高镍硫化物形成的重要机制。辉石岩源区地幔部分熔融形成富镍岩浆是否为高镍硫化物形成的必要条件尚存争议。不同机制形成的高镍硫化物具有迥异的岩石-矿物组合和地化特征。硫化物矿物组合、橄榄石成分(Fo值、Ni含量、Fo值-Ni含量的相关性)、伴生元素(铜、铂族元素)丰度-配分模式等特征可作为区分不同高镍硫化物形成机制的有效指标。我国新疆黄山南、坡一和青海夏日哈木矿床(部分浸染状矿化橄榄岩)以赋含高镍硫化物为特征,新疆喀拉通克矿床的硫化物则以富铜为特征,中国其余矿床的硫化物均属中镍硫化物。目前研究指示中国的高镍铜镍矿床与母岩浆相对富镍、硫化物与橄榄石Fe-Ni交换作用密切相关,后者可使硫化物Ni含量提升3%～5%。在铜镍矿床勘查方面,稀疏-中等浸染状高镍硫化物矿石即可达到工业品位,稠密浸染状-块状高镍硫化物矿石可达到很高的Ni品位(10%),是高品位镍矿勘查的一个重要方向。造山带环境富水、相对高氧逸度(可高达QFM+1)的岩浆可能是形成高镍硫化物的有利条件,该环境橄榄石Fo值较高(87mol%)的岩体有利于形成高镍硫化物。  相似文献   

13.
Geology, tectonomagmatic reactivation of the Noril??sk district, as well as stratigraphy and geochemistry of the volcanic sequence are considered. Sources and formation mechanism of ore-bearing magma and the scope of ore formation are discussed. The Permian-Triassic flood-basalt magmatism of the Noril??sk district developed in part of the Siberian Platform with Archean-Paleoproterozoic basement broken into blocks and overlapped by a sedimentary cover up to 13 km thick and a volcanic sequence reaching 3.7 km in thickness. The geophysical data show that remnants of the subducted ancient oceanic crust exist in the mantle and fragments of transitional magma chambers and conduits are retained at different levels of the Earth??s crust. The cyclic tectonomagmatic evolution of the territory was characterized by alternation of extension with intense volcanic activity and compression accompanied by waning of volcanic eruptions. The early rifting, transitional stage, and late dispersed spreading are distinguished. The associations of volcanic (lavas and tuffs) and intrusive rocks were formed during each stage. The volcanic sequence is subdivided into 11 formations. The intrusions of the Talnakh and Noril??sk ore fields are distinguished by two-level structure with the Upper Noril??sk ore-bearing intrusions above and the Lower Noril??sk barren intrusions below. Two types of primary magmas differ in geochemistry of lavas and intrusions: (1) OIB-type high-Ti magma (iv, sv, gd formations of the first stage from bottom to top) and (2) low-Ti magma (hk, tk, nd formations of the second stage and mr-mk formations of the third stage). The nd formation depleted in ore elements and the ore-bearing cumulus composed of silicate and sulfide melts in combination with early silicate minerals and chromite are products of the fractionation of the primary low-Ti magma. As follows from geochemical parameters, intrusions of the Lower Noril??sk type are comagmatic to the evolved lavas of the nd3 subformation, whereas intrusions of the Upper Noril??sk type are comagmatic to the lavas of the mr-mk formations. Geochemical similarity with volcanic rocks provides evidence for the composition of the initial magma and the time of intrusion emplacement. The ore-bearing intrusions of the Upper Noril??sk type were formed at the onset of the third stage, when the primitive low-Ti magma similar to the lavas of mr-mk formations in composition was emplaced. When intruding, this melt captured and transported ore-bearing cumulus (drops of sulfide melt, early olivine and chromite grains) into the magma chamber. Separate portions of sulfide liquid were involved into movement as a self-dependent intrusive subphase during formation of the Talnakh and Kharaelakh intrusions. An extremal effect of pressure on sulfur concentration in fluid-bearing and sulfide-saturated mafic magmas has been established in experiments to be P = 1?2 GPa. In this interval of pressure, the S concentration in sulfide-saturated magmas increases in the following sequence: dry magma ??(H2O + CO2)-bearing magma <H2O-bearing magma. In the regions of low (<0.3 GPa) and high (>2.5 GPa) pressures, the S contents (0.1?C0.2 wt %) are commensurable. The extremal baric relationship of S concentration in fluid-bearing and sulfide-saturated mafic magmas may be important for the formation of ore-bearing magmas. The calculation results show that the amount of sulfides in the known deposits does not exceed 2% of geological resources of the sulfides separated from the flood basalts. Therefore, the chance of discovery of new deposits remains rather high. Proceeding from the conditions of ore-bearing magma formation and geological setting of the known deposits, criteria for recognition of potentially ore-bearing areas are proposed and such areas are outlined.  相似文献   

14.
喀拉通克铜镍矿床位于准噶尔板块北缘,矿区主要矿体赋存于Y1-Y3号岩体中。矿石构造类型为致密块状和浸染状两大类,其中前者与后者呈贯入接触,不同浸染状类型之间为过渡关系。岩石和矿石的PGE总量偏低,且以PPGE为主,IPGE含量较低。整体上岩石中的PGE含量显示随基性程度降低而变小。矿石中的PGE含量随硫化物含量增加增大,显示PGE主要分布于硫化物熔离形成的物相中。100%硫化物计算后,矿石PGE含量平均仅为573×10-9。各岩体中浸染状矿石PGE组成并无明显差异;岩石和矿石具有相似的PGE分配模式,均属于Pt-Pd配分型。岩石Ni/Cu-Pd/Ir关系以及岩石地球化学资料显示,形成喀拉通克岩体的初始岩浆为MgO含量较高的玄武质岩浆,属于PGE不亏损的岩浆。基于PGE不亏损的大陆拉斑玄武岩初始岩浆推算,喀拉通克矿床母岩浆明显亏损PGE,而深部硫化物熔离可能是导致母岩浆PGE亏损的主要原因。岩石和矿石Pd/Pt比值总体特征,岩石Cr与Ni、Ir、Ru和Rh相关性,以及硫同位素和岩石学资料分析表明,初始岩浆在地壳深部发生的橄榄石、铬铁矿等矿物的分离结晶作用,可能是促使硫过饱和与深部熔离的主要因素。IPGE与PPGE分异特征及其相关分析,结合矿床宏观地质特征,推断该矿床浸染状矿的成矿作用经历了初始岩浆（PGE不亏损）→橄榄石等矿物分离结晶→硫化物深部熔离→成矿母岩浆（PGE亏损）→上侵并结晶分异的成矿过程。块状矿则可能是这一过程中PGE亏损的成矿母岩浆相对滞后熔离形成的硫化物熔体贯入的结果。  相似文献   

15.
In models for the genesis of the Noril’sk Pt-Cu-Ni ore deposits, much importance is attached to the processes of assimilation of host rocks by basaltic melts. This idea is based on unusual relations between the silicate and sulfide constituents of this type of ore deposits and also on the heavy sulfur isotopic composition of the sulfide ores. The reason for this unusual composition is thought to be the assimilation of anhydrite from the host rocks. However, no other factors able to influence this process have ever been analyzed in the literature. We were the first to thoroughly analyze the inner structure of contact aureoles of the intrusions hosted in various rocks: the Maslovsky intrusion in Early Triassic basalts of the Ivakinsky and Nadezhdinsky formations and the Talnakh intrusion in Devonian anhydrite-bearing carbonate-terrigenous rocks. The distributions of trace elements, the 87Sr/86Sr isotopic ratio, and Sm and Nd isotopes indicate that host rocks were either not assimilated at all, or their effect is perceptible only within a very narrow (1 m) zone in the eastern apophyse in the southern portion of the Maslovsky intrusion. The Sr, Nd, and particularly, Pb isotopic composition indicate that the anhydrite could not be the source of isotopically heavy sulfur for sulfides at Noril’sk deposits. The ores of the Maslovsky and Talnakh intrusions have similar sulfur isotopic composition of their sulfides (the maximum δ34S values of these sulfides reach +10.8 and +14.2‰, respectively), in spite of the significant differences in the rocks hosting these intrusions. Our newly obtained data indicate that assimilation was insignificant and could not affect the origin of the ores.  相似文献   

16.
There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small uitramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-PI-OI-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and iow-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limabe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basaits. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.  相似文献   

17.
亲铜元素在岩浆演化和硫化物熔离过程中的行为是解释岩浆硫化物矿床形成过程的一个窗口,通过实验研究来探讨亲铜元素的地球化学行为,并用于岩浆硫化物矿床的定量化研究是此类矿床今后的一个发展方向。本文总结了硫和亲铜元素在岩浆演化过程中的行为规律,并阐明了在岩浆硫化物矿床中的应用,在如下五个方面分别做了讨论：① 通过实验对玄武质岩浆中S溶解度的研究,总结出引起硫化物饱和的4个控制因素: 岩浆混合、温度迅速降低、壳源混染、快速的结晶分异作用;② 通过Ni在橄榄石和硅酸盐熔浆中的分配,定量模拟了岩浆演化过程中,橄榄石中的Ni含量随着橄榄石成分（Fo）变化的规律;③ 总结了Ni—Cu—PGE—Au在液态硫化物和硅酸盐岩浆中的分配系数,总结了控制分配系数的因素,并探讨了“R因素”对亲铜元素富集的控制机理;④ 橄榄石被硫化物包围时,与硫化物发生交换反应,通过交换反应系数（KD）可以定量估算硫化物熔浆中Ni的含量;⑤ 通过实验得出的亲铜元素在单硫化物固溶体（MSS）和液态硫化物之间的分配,总结了岩浆铜镍硫化物矿床中的分带现象。最后探讨了岩浆硫化物矿床存在的问题和发展方向。  相似文献   

18.
Process models for ore formation in magmatic Ni–Cu–platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic–ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril’sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril’sk, the Ni–Cu–PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni–Cu–PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni–Cu–PGE sulfide deposits.  相似文献   

19.
The Huangshannan magmatic Ni-Cu sulfide deposit is one of a group of Permian magmatic Ni-Cu deposits located in the southern Central Asian Orogenic belt in the Eastern Tianshan, northwest China. It is characterized by elevated Ni tenor (concentrations in recalculated 100% sulfide) in sulfide within ultramafic rocks (9–19 wt%), with values much higher than other deposits in the region. Sulfides of the Huangshannan deposit are composed of pentlandite, chalcopyrite, and pyrrhotite and the host rock is relatively fresh, indicating that the high-Ni tenor is a primary magmatic feature rather than formed by alteration processes. It is shown that sulfides with high-Ni tenor can be generated by sulfide-olivine equilibrium at an oxygen fugacity of QFM +0.5, for magmas containing 450 ppm Ni and 20% olivine. Ores with >10 wt% sulfur have relatively low PGE and Ni tenors compared to other ores, R factor (mass ratio of silicate to sulfide liquid) modeling of Ni indicates that they formed at moderate R values (150–600). Based on this constraint on R values, ores with <10 wt% sulfides in the Huangshannan deposit can be segregated from a similar parental magma with 0.05 ppb Os, 0.023 ppb Ir, and 0.5 ppb Pd at R values between 600 and 3000. This, coupled with the supra-cotectic proportions of sulfide liquid to cumulus silicates in the Huangshannan ores imply mechanical transport and deposition of sulfide liquid in a magma pathway or conduit, in which sulfides must have interacted with large volumes of silicate magma. Platinum and Pd depletion relative to other platinum group elements (PGEs) are observed in fresh and sulfide-rich samples (S > 4.5 wt%). As sulfide-rich samples are also depleted in Cu, and as interstitial sulfides in those samples are physically interconnected at a scale of several cms, the low Pt and Pd anomalies are attributed to solid Pt and Pd phases crystallization and retention with the monosulfide solid solution (MSS) and Cu-rich sulfide liquid percolation during MSS fractionation. This finding indicates that Pt anomalies in sulfide-rich rocks from magmatic Ni-Cu deposits in the Eastern Tianshan are the result of sulfide fractionation rather than a hydrothermal effect. 187Os/188Os(278Ma) values of the lherzolite samples vary from 0.27 to 0.37 and γOs(278Ma) values vary from 110 to 189, indicating significant magma interaction with crustal sulfides, rich in radiogenic Os. Well constrained γOs values and δ34S values (−0.4 to 0.8‰) indicate that crustal contamination occurred at depth before the arrival of the magma in the Huangshannan chamber. Regionally, deposits with high-Ni tenor have not been reported other than the Huangshannan deposit; however, many intrusions with high-Ni contents in olivine are present in NW China, such as the Erhongwa, Poyi and Poshi intrusions. Those intrusions are capable of forming high-Ni tenor sulfides due to olivine-sulfide-silicate equilibrium and relative high-Ni content in parent magma, making them attractive exploration targets.  相似文献   

20.
The Ni–Cu–PGE sulfide deposits in the Yangliuping area, SW China, are hosted in mafic–ultramafic sills. The four mineralized sills are located in the Yangliuping tectonic dome and intrude Devonian carbonaceous marble, graphitic schist. The sills are 200–300 m thick and 1,000–2,000 m in strike length and now consist chiefly of serpentinite, talc schist, tremolite schist, and meta-gabbro. Disseminated Ni–Cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills. Massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite. Although the sulfide ores have been modified by hydrothermal activity, there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores. The Yangliuping Intrusions and the Dashibao Formation have similar primitive-mantle normalized trace element and platinum group element (PGE) patterns, indicating that they are derived from a common parental magma type. The positive correlation between Cu concentrations and Cu/Zr ratios of the Dashibao Formation basalts indicates that the chalcophile elements were removed before eruption. We propose that fractional crystallization of the Yangliuping magma accompanied by the introduction of S and CO2 from the wall rocks caused the magma to become S-saturated leading to the segregation of magmatic sulfides that became enriched in Ni–Cu–(PGE). The sills acted as conduits for the overlying Dashibao Formation basalts with the sulfide liquid, along with early crystallizing olivine and pyroxene, segregating from the magma as it passed through the conduits prior to eruption.Editorial handling: H.E. Frimmel  相似文献   

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