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51.
We have measured excesses of Pd, Rh, Ru, REE, Co, Sr, and Y in a peat column from the Northern peat bog of the 1908 Tunguska explosion site. Earlier, in this peat column the presence of an Ir anomaly at the event layers (30- depth) has been found (Planet Space Sci. 48 (1998) 179). In these layers, Pd, Rh, Ru, Co, Sr, and Y show pronounced anomalies of a factor 4-7 higher than the background value. In the event layers there are also good correlations between the siderophile platinum group elements (Pd, Rh, Ru) and Co, indicators of cosmic material, which imply they might have the same source, i.e. the Tunguska explosive body. The patterns of CI-chondrite-normalized REE in the event layers are much flatter than those in normal peat layers and different from those in the nearby traps. Furthermore, in these layers the patterns of CI-chondrite-normalized PGEs and the element ratios (e.g. C/Pd, C/Rh, and between some siderophile elements) give evidence that the Tunguska explosive body was more likely a comet, although we cannot exclude the possibility that the impactor could be a carbonaceous asteroid. We have estimated the total mass of a solid component of the explosive body up to 103-106 tons. 相似文献
52.
Platinum group elements zoning and mineralogy of chromitites from the cumulate sequence of the Nurali massif (Southern Urals, Russia) 总被引：1，自引：0，他引：1
The Nurali lherzolite massif is one of the dismembered ophiolite bodies associated with the Main Uralian Fault (Southern Urals, Russia). It comprises a mainly lherzolitic mantle section, an ultramafic clinopyroxene-rich cumulate sequence (Transition Zone), and an amphibole gabbro unit.The cumulate section hosts small chromitite bodies at different stratigraphic heights within the sequence. Chromitite bodies from three different levels along a full section of the cumulate sequence and two from other localities were investigated. They differ in the host lithology, chromitite texture and composition, and PGE content and mineralogy. Chromitites at the lowest level, which are hosted by clinopyroxenite, form cm-scale flattened lenses. They have high Cr# and low Mg# chromites and are enriched in Pt and Pd relative to Os and Ir. At a higher, intermediate level, the chromitites are hosted by dunite. They form meter thick lenses, contain low Cr# and high Mg# chromites, have high PGE contents (up to 26,700 ppb), and are enriched in Os, Ir and Ru relative to Pt and Pd, reflecting a mineralogy dominated by laurite–erlichmanite and PGE–Fe alloys. At the highest level are chromitites hosted by olivine–enstatite rocks. These chromitites have high Cr# and relatively low Mg# chromites and very low PGE content, with laurite as the dominant PGE mineral.The platinum group minerals (PGMs) show extreme zoning, with compositions ranging from erlichmanite to almost pure laurite and from Os-rich to Ru-rich alloys, with variable and irregular zoning patterns.Two chromitite bodies up to 6 km from the main sequence can be correlated with the latter based on geochemistry and mineralogy, implying that the variations in chromitite geochemistry are due to processes that operated on the scale of the massif rather than those that operated on the scale of the outcrop.Pertsev et al. [Pertsev, A.N., Spadea, P., Savelieva, G.N., Gaggero, L., 1997. Nature of the transition zone in the Nurali ophiolite, Southern Urals. Tectonophysics 276, 163–180.] propose that the Transition Zone formed by solidification of a series of small magma bodies that partially overlapped in time and space. The magmas formed by successive partial melting of the underlying mantle. We suggest that this process determined the changing PGE geochemistry of the successive batches of magma. The PGE distribution fits a model of selected extraction from the mantle, where monosulphide solid solution–sulphide liquid equilibrium was attained until complete melting of the monosulphide solid solution. Later and localized variations in fS2 resulted in the formation of different PGMs with complex zoning patterns. 相似文献
53.
Priyadarshini Sharma Birendra Kumar Mohapatra Prem Prakash Singh 《Resource Geology》2013,63(4):371-383
Potential chromite ore deposits of India are situated in Sukinda, Odisha, which may also be considered as a potential resource for platinum group elements (PGEs). This paper reports on PGE geochemistry in twenty six samples covering chromite ores, chromitites and associated ultramafic rocks of the Sukinda ultramafic complex. Platinum group element contents range from 213 to 487 ppb in the chromite ore body, from 63 to 538 ppb in rocks that have chromite dendrites or dissemination and from 38 to 389 ppb in associated olivine–peridotite, serpentinite, pyroxenite and brecciated rocks. The PGEs are divided into two sub‐groups: IPGE (Ir, Os, and Ru) and PPGE (Pd, Pt, and Rh) based on their chemical behaviour. The IPGE and PPGE in these three litho‐members show a contrasting relationship e.g. average IPGE content decreases from chromite to chromitite and associated rocks while PPGE increases in the same order. Appreciable Ag in chromitite (270–842 ppb) is recorded. Positive correlation between IPGE with Cr2O3 and with Al2O3 is observed while these are negatively correlated with MgO. Covariant relationships between Au and Mg in rocks devoid of chromite and between Ag and Fe in chromitite sample are observed. Chromite in all seams and some chromitite samples exhibit an IPGE‐enriched chondrite normalized pattern while PPGE are highly fractionated and show a steep negative slope, thereby indicating that PGE in the parental melt fractionates and IPGE‐compatible elements prefer to settle with chromite. The rocks devoid of chromite and rocks containing accessory chromite exhibit a nearly flat pattern in chondrite‐normalized PGE plots and this suggests a limited fractionation of PGE in these rocks. Variation in the distribution pattern of PGE and Ag in three typical litho‐members of the Sukinda Valley may be related to multiple intrusion of ultramafic magma, containing variable volume percentage of chromite. 相似文献
54.
泽当蛇绿岩位于雅鲁藏布江缝合带东段,岩体由地幔橄榄岩、辉长辉绿岩、辉石岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和二辉橄榄岩。在方辉橄榄岩中发现7处豆荚状铬铁矿,矿石类型主要有致密块状和浸染状。出露地表的长度0.5~3m,厚0.2~1m。矿体的延伸方向为北西向,与岩体展布的方向一致,铬铁矿的Cr~#=67.9~88.5,属于高铬型铬铁矿。泽当地幔橄榄岩岩相学特征以及矿物组合、矿物化学成份及岩相学特征,显示岩体至少存在两次的部分熔融过程,即为早期的MOR构造背景,以及后期SSZ环境的改造。铬铁矿的铂族元素(PGE)以富集Os、Rh、Pd,亏损Ir、Ru、Pt的负斜率分布模式,表明其形成过程中经受后期熔体/流体的改造。对比罗布莎岩体的矿物组合,矿物化学和地球化学等特征,显示泽当豆荚状铬铁矿矿体与典型高铬型具相似性,存在较大的找矿空间。 相似文献
55.
滇西金宝山铂钯矿床元素地球化学 总被引：2，自引：1，他引：1
金宝山铂钯矿床位于扬子板块西缘红河断裂东侧,宁蒗-弥渡镁铁-超镁铁岩带内,矿体呈似层状、透镜状产于辉石橄榄岩中。辉石橄榄岩和铂钯矿石均富集LREE,具有弱的Eu负异常和较强的Sr、Ba负异常;与N-MORB相比,辉石橄榄岩具有较低的(Nb/Th)PM比值和较高的(Th/Yb)PM比值,表明金宝山岩体受到了地壳物质混染;通过(Th/Yb)PM-(Nb/Yb)PM图解估算得到地壳混染程度在55%~70%之间,强烈地壳混染表明岩浆中的S达到饱和并使得硫化物发生大规模熔离。而利用硅酸盐岩浆/硫化物的质量比值(R因子)方程进行模拟计算,得到金宝山矿床R因子集中于5000~1000之间,明显大于金川、图拉尔根、白马寨等典型岩浆硫化物矿床,说明金宝山岩体形成时岩浆中的硫化物熔离程度较低。辉石橄榄岩和铂钯矿石的S/Se和Cu/Pd比值也同样反映了硫化物低程度熔离的特征。与Nb、Th等元素含量相对稳定的高场强元素相比,S、Se、Pd等元素在硫化物部分熔解以及热液作用过程中更容易发生迁移。类似于River Valley和Platreef矿床等大型层状PGE矿床,金宝山铂钯矿床的形成是一个两阶段的过程,早阶段在岩浆通道或深部岩浆房中,地壳混染使得硫化物发生强烈熔离并在有限的空间内大量聚集,产生富PGE岩浆;后由于硫化物的部分熔解,岩浆中硫化物熔体富集Se、Pd,亏损S、Fe,岩浆中的S由饱和变为不饱和。晚阶段在浅部岩浆房,少量地壳S的加入并未使得S饱和从而发生硫化物大规模熔离。金宝山岩体具有较低的Cu/Pd、Cu/Pt比值,即出现Cu/Pd比值较低的岩体也可能有较大的成矿潜力,这与传统意义上所认为的具有较高Cu/Pd、Cu/Pt比值的矿体不同。 相似文献