The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration

The Lega Dembi deposit is the largest gold producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that follows the structural contact between underlying feldspathic gneisses and the volcanosedimentary sequence of the Megado belt. This contact also marks the northernmost extension of the regional-scale, sinistral strike-slip Lega Dembi-Aflata shear zone. Mineralization and intense quartz-veining is best developed in graphite-rich sediments within an area not more than 80 m away from this tectonic contact. Hydrothermal wall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with galena. The variable deformation of the gold-quartz veins suggests a syn-kinematic timing for the gold mineralization during transcurrent shearing in a dilational segment of the shear zone. In addition to the structural control, lithological control on gold deposition is indicated by the almost exclusive occurrence of the gold mineralization in graphite-rich metasediments. This close relationship suggests that gold precipitation was the result of chemical reduction of regional ore-bearing fluids. Temperature conditions of mineralization are constrained by the actinolite-biotite alteration assemblage and by arsenopyrite chemistry, which indicate that ore deposition occurred at or close to peak metamorphic conditions at upper-greenschist to lower-amphibolite metamorphic grades. Rb-Sr dating of sericite indicates an age of about 545 Ma. for hydrothermal alteration and, thus, for gold mineralization. The style of gold mineralization, structural pattern and lithological assemblages at Lega Dembi are very similar to lode-gold deposits most commonly reported from Archaean granite-greenstone terranes. These similarities may open new perspectives for the exploration of lode-gold deposits, which has previously primarily focused on Archaean greenstone belts rather than Proterozoic or even Phanerozoic meta-volcanosedimentary belts. Received: 26 July 1996 / Accepted: 8 January 1997     

藏南马扎拉地区玄武岩地球化学特征、成因及其地质意义

西藏东南部特提斯喜马拉雅带东段的遮拉组(J₂z)中出露有大量的玄武岩,其地球化学特征对了解该地区中侏罗世的构造背景具有重要意义。本文在详细的野外工作基础上,对发现于马扎拉地区遮拉组的玄武岩进行了地球化学研究。全岩主量元素及微量元素地球化学特征显示,玄武岩属钙碱性系列,SiO₂含量为47.50%~50.61%,具有高钾(K₂O平均为0.61%)、高钛(TiO₂平均为4.02%)、高磷(P₂O₅平均为0.52%)等特点;轻、重稀土元素分馏较为明显(∑LREE/∑HREE=7.30~7.58),(La/Yb)_N比值平均为7.94,富集大离子亲石元素(LILE)如Ba、Th和高场强元素(HFSE)如Nb、Ta、Zr、Hf,呈现出典型洋岛玄武岩(OIB)的地球化学特性。岩浆源于主要由石榴石橄榄岩组成的富集地幔发生10%左右部分熔融的产物,岩浆上升过程中未受到明显的地壳混染。结合区域地质资料,初步认为马扎拉玄武岩应为被动大陆边缘地幔柱或热点的产物。     

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.     

On the textures and paragenesis of the gold-quartz-tourmaline veins of Ondonoc,W.-Ethiopia

The gold-quartz-tourmaline veins of Ondonoc occur in diorites and schists and follow the non-distortion planes of the theoretical strain ellipsoid. Textural studies reveal a sequence of crystallization phases. In addition to an early prismatic tourmaline, a late phase metasomatic tourmalinization is recognized. The gold occurs mainly with pyrite which, in turn, is considered to belong to a late phase with ankerite and prochlorite. Micromylonitization and other tectonic influences are recognizable and bodies of the country rock, e.g. schists and tourmalinefels, are engulfed and tectonically mobilized within these veins, thus rendering the interpretation of the paragenesis more difficult.

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Zusammenfassung Die Gold-Quarz-Turmalingänge von Ondonoc treten in Dioriten und metamorphen Schiefern auf und folgen den nicht-deformierten Ebenen des theoretischen Strain-Ellipsoids. Die Textur weist auf eine bestimmte Kristallisationsfolge hin. Zusätzlich zu einer frühen prismatischen Turmalinbildung konnte eine späte metasomatische Turmalinisierung festgestellt werden. Das Gold tritt vor allem mit Pyrit zusammen auf, welcher seinerseits einer späten Phase zugeschrieben werden kann, die durch Ankerite und Prochlorit gekennzeichnet ist. Mikromylonitisation und andere tektonische Beanspruchungen sind verbreitet, wobei Teile des Nebengesteins umschlossen und innerhalb der Gänge tektonisch mobilisiert worden sind. Dadurch wird die Interpretation der Paragenese erschwert.

     

湘南矿集区长英质岩脉的特征及其成矿、找矿意义

南岭地区不仅广泛出露花岗岩,而且还广泛分布着长英质岩脉。本文选择湘南矿集区的奇古岭、香花岭、荷花坪、千里山、新田岭和瑶岗仙6个不同类型、不同演化程度的长英质（流纹质、微粒花岗岩质、细晶岩质）岩脉为研究对象,利用全岩分析、电子探针矿物化学成分分析等结果,对比总结了这些长英质岩脉的岩石学、矿物学、年代学和地球化学等方面的特征。奇古岭和香花岭是其中演化程度最高的2个岩脉,富含Li和F等挥发组分,含有黄玉和铁锂云母等特征矿物,全岩和锆石的Zr/Hf比值均较低,同时它们还富集成矿元素,并以不同产状氧化物（锡石、金红石和铌钽矿物）形式存在。其它4个岩体成矿信息不明显,仅见由于后期绿泥石化形成于绿泥石解理缝中的次生矿物金红石（富集Nb和W）。除了千里山以外,岩脉与寄主或相邻的花岗岩体大多属于同一时期的产物,但从年代学并不一定能够体现出它们与各自主岩体之间的相关性。这些岩脉都是花岗岩的次火山相或火山相的类似物,包括了斑晶矿物和基质矿物,快速的冷凝结晶的过程导致形成了特殊的岩石结构、矿物组合和成分特征,它们记录了岩浆-流体体系的活动过程。这些信息之间的相互印证,证实了奇古岭岩脉与骑田岭花岗岩来源于不同的源区,香花岭431岩脉可能是癞子岭花岗岩进一步高度分异结晶的产物,千里山岩脉与主体花岗岩也是不同成因的岩浆活动,荷花坪、新田岭和瑶岗仙岩脉与主体花岗岩未能判断明确的成因联系和成矿信息。因此,长英质岩脉,尤其是高演化的岩脉也能成为在南岭地区寻找成矿花岗岩的新窗口,它能够提供更直接的深部信息,特别是本身即为矿体的岩脉,可以指示深部或周围存在成矿作用。它们有可能与寄主岩体相关,也有可能来源于深部不同的岩浆房,这对与提供寻找隐伏岩体和矿体的证据、找矿勘探工作均有很重要的意义。     

Hydrogeochemical exploration for gold in the Osilo area,Sardinia, Italy

A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km² is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks.Forty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L⁻¹ Au). Gold concentrations in solution up to 3 ng L⁻¹ were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution.The best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.     

Pan-African deformations in the basement of the Negele area, southern Ethiopia

Polydeformed and metamorphosed Neoproterozoic rocks of the East African Orogen in the Negele area constituted three lithostructurally distinct and thrust-bounded terranes. These are, from west to east, the Kenticha, Alghe and Bulbul terranes. The Kenticha and Bulbul terranes are metavolcano-sedimentary and ultramafic sequences, representing parts of the Arabian-Nubian Shield (ANS), which are welded to the central Alghe gneissic terrane of the Mozambique Belt affinity along N-S-trending sheared thrust contacts. Structural data suggest that the Negele basement had evolved through three phases of deformation. During D1 (folding) deformation, north-south upright and inclined folds with north-trending axes were developed. East and west-verging thrusts, right-lateral shearing along the north-oriented Kenticha and Bulbul thrust contacts and related structural elements were developed during D2 (thrusting) deformation. The pervasive D1 event is interpreted to have occurred at 620-610 Ma and the D2 event ended prior to 554 Ma. Right-lateral strike-slips along thrust contacts are interpreted to have been initiated during late D2. During D3, left-lateral strike-slip along the Wadera Shear Zone and respective strike-slip movements along conjugate set of shear zones were developed in the Alghe terrane, and are interpreted to have occurred later than 557 Ma. The structural data suggest that eastward thrusting of the Kenticha and westward tectonic transport of the Bulbul sequences over the Alghe gneissic terrane of the Mozambique Belt, during D2, were accompanied by right-lateral strike-slip displacements along thrust contacts. Right-lateral strike-slip movements along the Kenticha thrust contact, further suggest northward movement of the Kenticha sequence during the Pan-African orogeny in the Neoproterozoic. Left-lateral strike-slip along the orogen-parallel NNE-SSW Wadera Shear Zone and strike-slip movements along a conjugate set of shear zones completed final terrane amalgamation between the Arabian-Nubian Shield and the Mozambique Belt in Neoproterozoic southern Ethiopia.     

Thermoluminescence of quartz from Shihu gold deposit, western Hebei province, China: some implications for gold exploration

Thermoluminescence (TL) of monomineralic separates have been widely used in various geosciences fields in order to trace the thermal history and aid in prospecting for gold deposits. Quartz is a ubiquitous mineral in the Shihu gold deposit, which is situated in the northern part of the Taihang orogenic belt in the North China craton (NCC). The deposit is hosted by ductile-brittle faults within an Archean metamorphic core complex of the Fuping Group. This deposit is characterized by gold-bearing quartz-polymetallic sulfides and quartz veins. New TL results have been obtained for quartz, in which four type-TL glow curves were identified. The gold-bearing quartz present type III glow curves that consist of two peak glow curves at the middle and high peak temperatures with the similar TL intensity. In addition, the cross-sections of peak temperatures and TL intensity highlight the valuable area where the Au-bearing quartz present weak TL intensity and low-middle peak temperatures. Our results significantly enhance the usefulness of quartz in metallogenic studies of the North China craton and as an indicator mineral in mineral exploration of the Taihang Mountain region.     

江西南部东坑盆地流纹岩地球化学特征及其地质意义

东坑盆地位于南岭构造带东段,其中的流纹岩为该带燕山期最早的“流纹岩—玄武岩”双峰式火山岩组合的酸性端元.主量元素、微量元素、Sr-Nd-Pb-O-Hf同位素研究表明,流纹岩富硅、钾,贫镁、钙、钛,属亚碱性弱过铝质岩石;稀土元素富集,轻重稀土分异和铕负异常明显,表现典型的M型稀土元素4分组效应,富集高场强元素Ta、Hf、Zr、Nb、Ce、Y和大离子亲石元素Rb、Th、U、Ba、Ga,亏损大离子亲石元素Sr,具有A型流纹岩和高Sr-Ba流纹岩的微量元素特征;(87Sr/86Sr)i较高,(206Pb/204Pb)i、(207Pb/204Pb)i和(208Pb/204Pb)i较低,εNd(t)、εHf(t)和δ18OV-SMOW较高,TDM2(Nd)和TDM2(Hf)较小.这些特征表明,东坑盆地流纹岩是拉张构造环境下源于新元古代亏损地幔和少量古老下地壳物质混合而成年轻下地壳部分熔融的产物,为早侏罗世早期南岭构造带东段处于拉张构造环境、地壳属正常厚度提供了岩石学证据.     

藏南折木朗造山型金矿成矿流体地球化学和成矿机制

折木朗金矿位于青藏高原雅鲁藏布江缝合带东段的南侧,矿体受大型脆-韧性剪切带的次级断裂控制。系统的显微测温和激光拉曼测定显示折木朗金矿矿石中存在3类流体包裹体: NaCl-H₂O溶液包裹体(类型Ⅰ);含CO₂盐水溶液包裹体(类型Ⅱ),此类包裹体又分为两相(Ⅱa)和三相(Ⅱb)2个小类;Ⅲ纯气相包裹体。折木朗金矿床中流体包裹体显微测温显示该矿成矿流体的盐度范围为2.31%~7.39% NaCleqv,平均值为5.33%% NaCleqv,峰值为4.0%~7.0% NaCleqv;均一温度的范围为164.5~273.1℃,峰值为220~240℃,平均值为221.0℃。相对应的密度范围为0.82~0.93g·cm^-3,峰值为0.84~0.90g·cm^-3,平均值为0.88g·cm^-3。折木朗金矿床成矿流体具有富含CO₂、低盐度、低密度、中低温度的特征,与造山型金矿成矿流体相似。此外,同位素测定显示成矿流体的氢氧碳同位素组成分别为δD_H2O=-36.7‰~-107.5‰,δ¹⁸O_H2O=4.1‰~5.5‰,δ¹³C=-9.6‰~-11.5‰,说明成矿流体主要为变质水,但有地幔流体的加入。综合成矿地质特征和成矿流体的证据,提出折木朗金矿为陆陆碰撞造山型金矿。     

青海南部沱沱河冻土水合物潜在区火山岩地球化学特征及地质意义

沱沱河盆地是冻土天然气水合物潜在分布区之一,其内发育下-中二叠统开心岭群九十道班组、上二叠统乌丽群那益雄组、上三叠统结扎群巴贡组、中-渐新统雅西措组4套烃源岩以及不同类型的火山岩。研究表明,火山岩岩石类型主要为玄武岩、玄武安山岩、安山岩、玄武质粗面安山岩和粗面安山岩。火山岩主量元素低TiO₂,Al₂O₃含量较高,K₂O含量较低,K₂O < Na₂O;火山岩的稀土元素配分模式为轻稀土富集型;微量元素配分模式呈锯齿状,Ta、Nb、P₂O₅、TiO₂、Y、Yb以及铁族元素Sc、Cr、Ni亏损,综合判断青海南部沱沱河地区火山岩形成于岛弧环境。结合火山岩的地球化学特征,推断火山活动可以加快烃源岩的热成熟,进而促使有机质裂解,产生热解气,为水合物的生成提供气源条件。同时,火山岩的气孔构造发育,连通性较好,有可能成为水合物的储层。火山热液具有的高压使周围的烃源岩产生裂隙,这些裂隙不仅是水合物气源的运移通道,还可以为水合物提供形成场所和储存空间。     

西藏南部绒布地区基性岩脉岩石地球化学、年代学特征及地质意义

绒布地区位于西藏南部特提斯喜马拉雅构造域中段北缘,夹持于北侧邛多江断裂和南侧的绒布-古堆断裂之间。该地区发育大量的基性岩脉。通过岩石学、年代学及岩石地球化学等方面的研究,认为区内出露的基性岩脉类型为辉绿玢岩、辉长辉绿岩及辉长岩等。辉绿玢岩结晶年龄为137. 3±1. 6Ma,具有E-MORB的特征,未遭受岩石圈地幔或地壳混染,主要形成于大洋板内环境,受洋中脊源区的影响明显。辉长辉绿岩、辉长岩结晶年龄为147. 3±3. 6Ma,与时代接近的拉康组、桑秀组基性火山岩具有相同OIB的地球化学特征,带有岩石圈地幔物质混染的痕迹,形成于强烈拉伸的大陆边缘裂谷环境。晚侏罗世-早白垩世特提斯喜马拉雅被动陆缘处于强烈拉伸、岩石圈减薄的构造背景之下,OIB型辉长辉绿岩及辉长岩与措美大火成岩省诸多OIB型基性岩具有相似岩石成因,是大陆裂谷背景下Kerguelen地幔柱与岩石圈地幔相互作用的产物;而E-MORB型辉绿玢岩则可能是靠近大陆边缘的热点以下地幔柱与软流圈地幔相互作用的产生的岩浆沿区域深大断裂运移至大陆边缘侵位的结果。OIB型辉长辉绿岩、辉长岩的结晶年龄明显早于Kerguelen地幔柱活动的峰期(132Ma),可能是地幔柱早期活动的产物; E-MORB型辉绿玢岩的存在可作为目前对于措美大火成岩省基性岩脉类型的补充,对认识措美大火成岩省具有一定的意义。     

Geochemistry of axial lavas from the mid- and southern Mariana Trough,and implications for back-arc magmatic processes

Mineralogy and Petrology - The Mariana Trough, a relatively simple intra-oceanic back-arc basin, is ideal for investigating magmatic processes and mantle-crust interaction in a subduction setting....     

Geochemistry and petrography of basalt grindstones from the Karak Plateau,central Jordan

Seventeen basalt grindstone fragments from central Jordan's Karak Plateau were studied. Most of these artifacts are vesicular or amygdaloidal with calcite as the dominant mineral filling the voids. The major minerals are olivine (with iddingsite rims), plagioclase, clinopyroxene, magnetite, and apatite. Glass is present in some samples. One basalt fragment is quite different in appearance and composition and may have come from flows closer to the Dead Sea. Grindstone fragment compositions plot in the tephrite‐basanite and basalt fields. A plot of the concentrations of niobium, zirconium, and yttrium reveal that the sample compositions plot in the “within‐plate alkali basalt” and “within‐plate tholeiite” fields. The acquisition of basalts for preparing such implements appears to have been random. Some may have been introduced through trade and migration. Archaeological and environmental studies on the Karak Plateau are urgently needed because Jordan's population growth and economic development are destroying many sites and their environmental contexts. © 2004 Wiley Periodicals, Inc.     

Electron petrography of shock-produced veins in the Tenham chondrite

Electron microscopy of the minerals in, and adjacent to the black veins of the Tenham meteorite reveals that the minerals have undergone varying degrees of shock. The orthopyroxenes of the bulk of the meteorite have been transformed to clinopyroxenes in areas adjacent to the veins, and to majorite in the black vein itself. The majorite, which occurs in both equant and dendritic habit, is associated with a glassy phase from which it crystallized, and with a microcrystalline clinopyroxene aggregate produced from the majorite upon the release of pressure. The olivines of the meteorite are also deformed, and some have been transformed to the high pressure spinel polymorph. The spinel has subsequently partially inverted to the -phase polymorph.     

西秦岭关子镇蛇绿岩地球化学及其大地构造意义

关子镇蛇绿岩是东秦岭商丹缝合带西延的标志,主要由蛇纹岩、变质辉长岩、斜长角闪片岩（变玄武岩）组成。变玄武岩总体均以高SiO2、MgO,低TiO2、Al2O3,Na2O>>K2O,LREE和LILE亏损,以及HFSE不分异为特征。在此基础上,变玄武岩又可以划分为A类（主要分布南部区段）和B类（主要分布北部区段）,B类相对于A类而言,具有更低的TiO2、Al2O3、MgO含量和高Σ Fe2O3含量特征。同时,B类岩石LREE亏损程度较A类岩石更为明显。综合主、微量元素地球化学特征分析,认为两类岩石均源于亏损地幔源区,形成于古大洋中脊构造环境,但成生于不同的岩浆演化阶段,是古洋盆扩张演化不同阶段的产物。西秦岭关子镇蛇绿岩性质的确定,为商丹古洋盆西延与演化,以及中国大陆古生代构造格局及其演化过程提供了重要的制约。     

Geochemistry,provenance, and tectonic setting of Neoproterozoic metavolcanic and metasedimentary units,Werri area,Northern Ethiopia

Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO₂ contents, high Al₂O₃, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with La_N/Yb_N values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd_(800 Ma) of +3.8 to +4.9 and mean T_DM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al₂O₃, TiO₂ and HFSE contents but variable and low Na₂O, CaO and K₂O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO₂ is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with La_N/Yb_N ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. ¹⁴⁷Sm/¹⁴⁴Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd_(800 Ma) of +3.4 to +5.0 and mean T_DM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.     

A geochemical survey of spring water from the main Ethiopian rift valley, southern Ethiopia: implications for well-head protection

The report discusses the stable isotope values and major solute compositions of 16 springs and river-water samples along a topographic gradient in the main rift valley of southern Ethiopia. Most of the springs used for drinking water supplies discharge from local flow systems at scales of only tens of meters. The δ¹⁸O and δD values of waters unaffected by shallow evaporation form a local meteoric water line of δD=8.1δ¹⁸O+19.0, almost identical to that for the eastern Mediterranean area. The δ¹⁸O values show an altitude effect of –0.5‰ per 100-m elevation rise. Total dissolved nitrogen concentrations locally exceed 6 mg/L (as N), and phosphate concentrations were elevated above background levels in some springs, exceeding 0.2 mg/L PO₄, probably due to agricultural practices upgradient of the springs. Modest well-head protection should be considered to protect public health from the effects of pollution by agricultural waste, given the very local scale of the flow systems providing spring water to Ethiopian villages. Electronic Publication     

Geochemistry of paleozoic basalts from the Juchatengo complex of southern Mexico: tectonic implications

Analyses of Lower Permian or older basalts and associated dykes of the Juchatengo sequence indicate that they are rift tholeiites that formed in a continental rift or back-arc tectonic setting. Age constraints include a Middle Permian fossil recovered from the tectonically overlying sediments and a cross-cutting, post-tectonic pluton dated by K/Ar on hornblende at 282±6 Ma. A location adjacent to the Oaxacan Complex or other old continental crust is suggested by (1) an Nd_i isotopic value of −8.95 and a T_DM age of 1487 Ma in the overlying sediments, which are similar to the Oaxacan Complex; (2) enrichment of incompatible elements in the lavas, suggesting old crustal contamination; and (3) the presence of Permian–Triassic calc-alkaline plutons that stitch the Juchatengo–Oaxaca boundary. The possible tectonic models depend on the age of the Juchatengo basalts, which requires future geochronological work. If the Juchatengo basalts are Permo-Carboniferous, they could have formed near the eastern edge of a back-arc basin: the contemporaneous arc would have rifted away to the west. Eastward migration of the arc magmatism indicated by the Permian–Triassic calc-alkaline plutonism may reflect shallowing of the dip of the subduction zone, which probably also produced the deformation of the Juchatengo sequence.     

云南鲁史取得找金突破

通过开展精准填图,在云南凤庆鲁史上三叠统三合洞组灰岩中发现金矿化体。并在黄铁矿化灰岩矿石的重砂矿物中发现自然金,呈不规则粒状和树枝状,粒度为0.04~0.2mm,成色在98%左右。另外,在方解石脉中也发现次圆状包裹金,金中含椭圆状黄铁矿包裹体。自然金的发现,表明金矿勘查取得了重大突破,有力支撑了找矿突破战略行动。