The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt

 The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated. The most abundant base metal sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite. The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS), tellurian arsenopalladinite (PdTeSbAs), potarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases. Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian gold shares boundaries with chromite. Potarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium is the most abundant PGE in the Gebel Lawi chromite. A paragenetic sequence of PGM formation is described. Textural evidence indicates that Os-, Ir- and Ru-bearing PGM formed early and were followed by Rh- and Pd-bearing PGM. The concentration of all five PGE could be magmatic, but much of the PGE mineralogy except for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes. At later stages, the environment became Te-, Sb-, As- and Hg-rich, which finally led to the formation of low-temperature alteration minerals. Received: 24 April 1995 / Accepted: 28 March 1996     

The Wadi Mubarak belt,Eastern Desert of Egypt: a Neoproterozoic conjugate shear system in the Arabian–Nubian Shield

The Wadi Mubarak belt in Egypt strikes west–east (and even northeast–southwest) and crosscuts the principal northwest–southeast trend of the Najd Fault System in the Central Eastern Desert of Egypt. The belt therefore appears to be a structural feature that formed postdate to the Najd Fault System. In contrast, it is shown here that the deformation in the Wadi Mubarak belt can be correlated with the accepted scheme of deformation events in the Eastern Desert of Egypt and that its geometry and apparently cross-cutting orientation is controlled by a large granite complex that intruded prior to the structural evolution. Structural correlation is facilitated by a series of intrusions that intrude the Wadi Mubarak belt and resemble other intrusions in the Eastern Desert. These intrusions include: (1) an older gabbro generation, (2) an older granite, (3) a younger gabbro and (4) a younger granite. The structural evolution is interpreted to be characterized by early northwest directed transport that formed several major thrusts in the belt. This event is correlated with the main deformation event in the Eastern Desert, elsewhere known as D2. During this event the regional fabric of the Wadi Mubarak belt was wrapped around the El Umra granite complex in a west–east orientation. The Wadi Mubarak belt was subsequently affected during D3 by west–east and northwest–southeast trending sinistral conjugate strike–slip shear zones. This event is related to the formation of the Najd Fault System. Detailed resolution of superimposed shear sense indicators suggest that D3 consisted of an older and a younger phase that reflect the change of transpression direction from east-southeast–west-northwest to eastnortheast–westouthwest. The El Umra granite complex is dated here with single zircon ages to consist of intrusion pulses at 654 and 690 my. These ages conform with the interpretation that it intruded prior to D2 and that the structural pattern of the Wadi Mubarak belt was initiated early during D2.     

The Mw 7.4 Colima, Mexico, Earthquake of 21 January 2003: The Observed Damage Matrix in Colima City and its Comparison with the Damage Probability Matrix

The Mw 7.4 earthquake of 21 January 2003 occurred within the Mexican subduction zone and produced many damages of masonry constructions in the towns of Colima state, México. The macroseismic investigation of damages produced by the earthquake in Colima city was realized for 3,332 constructions within the area of study representing about 20% of the total city area and covered with the different type of constructions. The 7-grade scale of damage was used to describe the damage distribution. The damage matrix, constructed for the area, showed that the damage distribution varied from 63% of constructions with relatively slight damages (grades 1–3) to 29% of constructions that had significative damages (grades 4–5) and 8% of completely destructed or demolished masonry. The damage matrices, constructed for 12 subzones of the area of study, reflected two tendencies in the damage distributions: the predominance of slight damages of the recent constructions situated in the northern and eastern parts of the area and the predominance of significant damages of the older constructions in the southern and western parts of the area. It was observed a significant dependence of damage index upon the age of constructions and the type of masonry. The comparison of the observed damage matrix with the damage probability matrix calculated for Colima masonry in 1999 gives MM intensity VII in Colima.     

The pleochroism of a gem-quality enstatite in the region of the OH stretching frequency,with a stereochemical interpretation

Summary The pleochroic behaviour of a gem-quality enstatite from Tanzania was investigated in the region =2500 to 4000 cm^–1. Two sharp absorption bands at =3410 cm^–1 and =3510 cm^–1 are interpreted to be caused by OH stretching vibrations. As their absorption coefficients are considerably larger parallel to [001] (=direction of the silicate chains) than perpendicular to this direction, the OH dipoles have to be oriented approximately parallel to [001]. On this basis, a stereochemical interpretation of the incorporation of the OH groups into the structure is given.

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Der Pleochroismus eines Enstatits von Edelsteinqualität im Gebiet der OH-Streckfrequenz und seine kristallchemische Interpretation

Zusammenfassung Das pleochroitische Verhalten eines Enstatits von Edelsteinqualität aus Tansania wurde im Bereich =2500 bis 4000 cm^–1 untersucht. Von zwei scharfen Banden bei =3410 cm^–1 und 3510 cm^–1 wird angenommen, daß sie durch OH-Streckschwingungen herrühren. Da ihre Absorptionskoeffizienten parallel zu [001] (=Richtung der Silikatketten) beträchtlich größer sind als senkrecht dazu, müssen die OH-Dipole ungefähr parallel zu [001] liegen. Auf dieser Basis wird der Einbau der OH-Gruppen in die Struktur diskutiert.

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With 3 Figures     

The impact of severe natural disasters on the livelihoods of farmers in mountainous areas: a case study of Qingping Township,Mianzhu City

The ‘5.12’ earthquake in 2008 and the ‘8.13’ debris flow in 2010 brought huge losses of life and property to Qingping Township. Farmers’ livelihoods assets were also seriously affected and damaged. These led to a change in farmers’ livelihood styles and reduction in their income. A study of the impact of natural disasters on farmers’ livelihoods in mountainous areas would have significant importance for policy making. Based on field survey data from sample households, this study comparatively analyzed the assets, livelihood styles and income of farmers over three periods—2007, 2009 after the ‘5.12’ earthquake, and 2 months after ‘8.13’ debris flows. It presents the following findings: (1) livelihood assets suffered heavy losses; (2) livelihood styles were significantly changed; and (3) the sustainability of farmer income was adversely influenced. These results suggest that, during the process of post-disaster reconstruction work, affected farmers’ security, the carrying capacity of livelihood assets, and the enhancement of employment and livelihood choice are worthy of much attention from policy makers.     

The statistics of counting clasts in rudites: a review, with examples from the upper Palaeogene of southern California, USA

The relative proportions of gravel sized particles of different lithology in rudaceous sedimentary rocks are generally determined in the field by clast counting. Clast counts are usually carried out qualitatively in order to assess sedimentary provenance. However, a review of the statistical aspects of clast counting suggests that this technique also can be applied quantitatively, and to investigate a variety of other objectives during basin analysis. Geographical and stratigraphical changes in the relative proportions of clasts can be quantified statistically and used to characterize sediment dispersal patterns in space and time, respectively. Statistical comparisons between clast assemblages can be used as a tool to match up rock units. This approach may help to constrain tectonic or suspect-terrane models, or to document sediment recycling. Both counting and sampling errors contribute to the total probable error of a clast count. Sampling error results from the uneven distribution of clasts in outcrop, perhaps caused by selective sorting. Counting error arises from a count of some number less than the total number of clasts in the population. Sampling and counting errors can be minimized by counting in closely spaced subsets, and by counting a total of at least 400 clasts, respectively. Thus, a useful procedure is to count four closely spaced subsets of 100 each, and combine the results for a total of 400. Point counting should not be used because differences in particle size produce biased results. A better method is to count all clasts above some minimum size within a specified area of outcrop. Analysis of upper Palaeogene non-marine conglomerates composing part of the Sespe Formation in California, using confidence intervals, hypothesis testing, analysis of variance, ratio analysis and varietal studies, demonstrates that useful statistics can be derived by counting clasts.     

The Proterozoic,albitite-hosted,Valhalla uranium deposit,Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39

The Valhalla uranium deposit, located 40 km north of Mount Isa, Queensland, Australia, is an albitite-hosted, Mesoproterozoic U deposit similar to albitite-hosted uranium deposits in the Ukraine, Sweden, Brazil and Guyana. Uranium mineralisation is hosted by a thick package of interbedded fine-grained sandstones, arkoses and gritty siltstones that are bound by metabasalts belonging to the ca. 1,780 Ma Eastern Creek Volcanics in the Western Succession of the Mount Isa basin. Alteration associated with U mineralisation can be divided into an early, main and late stage. The early stage is dominated by laminated and intensely altered rock comprising albite, reibeckite, calcite, (titano)magnetite ± brannerite. The main stage of mineralisation is dominated by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite, apatite, (uranoan)-zircon, uraninite, anatase, albite, reibeckite, calcite and hematite. The late stage of mineralisation comprises uraninite, red hematite, dolomite, calcite, chlorite, quartz and Pb-, Fe-, Cu-sulfides. Brannerite has U–Pb and Pb–Pb ages that indicate formation between 1,555 and 1,510 Ma, with significant Pb loss evident at ca. 1,200 Ma, coincident with the assemblage of Rodinia. The oldest ages of the brannerite overlap with ⁴⁰Ar/³⁹Ar ages of 1,533 ± 9 Ma and 1,551 ± 7 Ma from early and main-stage reibeckite and are interpreted to represent the timing of formation of the deposit. These ages coincide with the timing of peak metamorphism in the Mount Isa area during the Isan Orogeny. Lithogeochemical assessment of whole rock data that includes mineralised and unmineralised samples from the greater Mount Isa district reveals that mineralisation involved the removal of K, Ba and Si and the addition of Na, Ca, U, V, Zr, P, Sr, F and Y. U/Th ratios indicate that the ore-forming fluid was oxidised, whereas the crystal chemistry of apatite and reibeckite within the ore zone suggests that F⁻ and were important ore-transporting complexes. δ¹⁸O values of co-existing calcite and reibeckite indicate that mineralisation occurred between 340 and 380°C and involved a fluid having δ¹⁸O_fluid values between 6.5 and 8.6‰. Reibeckite δD values reveal that the ore fluid had a δD_fluid value between −98 and −54‰. The mineral assemblages associated with early and main stages of alteration, plus δ¹⁸O_fluid and δD_fluid values, and timing of the U mineralisation are all very similar to those associated with Na–Ca alteration in the Eastern Succession of the Mount Isa basin, where a magmatic fluid is favoured for this style of alteration. However, isotopic data from Valhalla is also consistent with that from the nearby Mount Isa Cu deposit where a basinal brine is proposed for the transport of metals to the deposit. Based on the evidence to hand, the source fluids could have been derived from either or both the metasediments that underlie the Eastern Creek Volcanics or magmatism that is manifest in the Mount Isa area as small pegmatite dykes that intruded during the Isan Orogeny.     

地质流体:多学科技术和概念的相互渗透--第二届国际沉积盆地和造山带流体演化、运移和相互作用大会简介

地质流体：多学科技术和概念的相互渗透——第二届国际沉积盆地和造山带流体演化、运移和相互作用大会简介陈红汉（中国地质大学武汉４３００７４）关键词地质流体流体演化、运移和相互作用定年和定时分类号Ｐ５１１会议概况“第二届国际沉积盆地和造山带流体演化、运移...     

东波超镁铁岩体:西藏雅鲁藏布江缝合带西段一个甚具铬铁矿前景的地幔橄榄岩体

东波超镁铁岩体产在雅鲁藏布江缝合带的西段,与周边白垩纪沉积岩地层和火山岩以断层接触.航磁资料显示该岩体约400km2规模,地表出露连续,地下有一定延深.超镁铁岩体由亏损的地幔橄榄岩组成,主要有高镁的方辉橄榄岩、纯橄岩和少量二辉橄榄岩.方辉橄榄岩和二辉橄榄岩中橄榄石和斜方辉石属高镁型,分别为Fo=89.5～91.5和Mg#=90～91.5.但二辉橄榄岩中的Al2O3和CaO含量明显高于方辉橄榄岩.方辉橄榄岩中单斜辉石Mg#=92～95,二辉橄榄岩的Mg#=92～93,两者的值也重叠.二辉橄榄岩中的Al2O3和CaO含量要明显高于方辉橄榄岩.这些均为阿尔卑斯型地幔橄榄岩的典型特征.纯橄岩中的橄榄石Fo=92～93.2,其斜方辉石和单斜辉石的Mg#=～93,但Al2O3和CaO的含量比方辉橄榄岩和二辉橄榄岩的低.三种岩石的成分变化规律,反映了地幔部分熔融程度的差异.二辉橄榄岩铬尖晶石的Cr#值20～30,反映为典型深海橄榄岩特征,指示MOR环境.与其不同的是,方辉橄榄岩的铬尖晶石的Cr#=20～75,指示MOR和SSZ两者兼有环境.岩石的原始地幔标准化的REE和微量元素蛛网图模式支持了上述的认识.东波地幔橄榄岩中的岩石学特征与产有大型铬铁矿床的罗布莎地幔橄榄岩可对比,岩体中已多处发现块状铬铁矿石,其铬铁矿的Cr2O3含量56％～59％,表明东波是寻找铬铁矿大矿和富矿甚具前景的一个超镁铁岩体.