A major lineament in the Arabian Shield and its relationship to mineralization

The Al Amar Fault lies in a belt of Proterozoic, metamorphosed volcanic and sedimentary rocks, bounded by granitic batholiths. A string of metalliferous and industrial mineral deposits form a “mineral belt” which coincides with the volcanosedimentary belt. Orebodies of basic and ultra-basic association are directly related to the fault through its influence on intrusive activity. Cu-Zn-Au bearing veins of meta-volcanic affiliation are spatially related to the fault. A group of Pb-Ag bearing veins is associated with the granitic batholith which forms the western boundary of the volcano-sedimentary belt. Associations between ore minerals and particular igneous rocks indicate that granitic, basic and metamorphosed volcanic rocks were sources of Pb-Ag-W-Mo, Fe-Cr-Cu-Ni and Fe-Cu-Zn-Au-Ba, respectively. Hydrothermal activity in the fault zone promoted ore formation, and faulting provided sites for deposition. Al Amar Fault is a “copper-lead line” dividing a Pb-Ag subprovince (of sialic derivation?) from a Cu-Zn-Au sub-province (of plate margin/island arc derivation?). The fault is a useful empirical guide in exploration for ores of basic or ultra-basic plutonic and meta-volcanic affiliation and can be identified and traced, as a lineament, using ERTS satellite images and aeromagnetic maps.     

Volcanogenic mineralization and a rhyolite dome in the Arabian Shield

The district of Mahawiyah in the Proterozoic shield of Arabia contains a group of Zn-Cu-Au-Ag-Ba mineral prospects in folded meta-sedimentary, volcanoclastic and volcanic rocks, ranging in composition from basalt to rhyolite. The mineralization occurs in veins and as strata-bound, disseminated orebodies associated with intense argillic alteration of adjacent rocks. An intrusive rhyolite dome or laccolith is situated at the centre of an eight square kilometre area of slight but pervasive alteration whose outline can be traced from aerial photographs and within which many of the ore mineral occurrences lie. A model is proposed to explain the pattern of alteration in the volcano-sedimentary pile and formation of the volcanogenic mineralization, based on a concept of the dome acting as a heat source to drive a geothermal "cell". Circulating connate-hydrothermal fluids could have caused alteration and redistribution of trace metals within the volcanics and sediments which mantle the sub-volcanic, rhyolite intrusion. The ore genetic model implies that clusters of veins, disseminated strata-bound and stratiform massive sulphide orebodies occur in distinct areas of the shield, marked by tracts of pervasive alteration which can be identified in aerial photographs and satellite images.     

荣山滑覆构造与金矿化

江西临川市荣山Ｊ３－Ｔ３盆地与基底变质岩之间的滑覆构造，以地层减薄，缺失和产状变陡为特征，本文在分析滑覆构造特点的基础上，探讨了其成因机制，进而分析了滑覆构造与该区金矿化之间的关系，并作了富金矿可能存在部位的初步预测。     

Aeromagnetic map constrains cratonization of the Arabian Shield

A new aeromagnetic map together with new geological and geochronological data has led to a reinterpretation of the geological history of the Arabian Shield.
The magnetic anomalies outline an orogenic complex containing a network of mostly left-lateral strike-slip faults, including the Nabitah Belt and several peripheral mountain ranges. Oblique accretion resulted in obliteration of early volcanic-arc magnetic fabrics, which were almost completely replaced by a NW–SE magnetic fabric in the northern Shield; the southern Shield, however, reveals extensive E–W anomalies related to post-accretion magmatic intrusions. This complex web of orogenic zones is intimately associated with synchronous molasse basins that formed 680–610 Ma.
The distribution and chronology of orogenic zones, related to the closing of East and West Gondwana, brings into question several earlier assumptions, such as high continental growth rates, palaeogeodynamic reconstructions, the definitions of the Nabitah and Najd faults, and the significance of molasse basins.     

Mesozoic Large-scale Mineralization and Multiple Lithospheric Extensions in South China

South China is the most important polymetallic （tungsten, tin, bismuth, copper, silver, antimony, mercury, rare metals, heavy rare earth elements, gold and lead-zinc） province in China. This paper describes the basic characteristics of Mesozoic large-scale mineralization in South China. The large-scale mineralization mainly took place in three intervals： 170-150 Ma, 140-126 Ma and 110-80 Ma. Among these the first stage is mainly marked by copper, lead-zinc and tungsten mineralization and the third stage is mainly characterized by tin, gold, silver and uranium mineralization. The stage of 140-126 Ma mainly characterized by tungsten and tin mineralization is a transitional interval from the first to the third stage. In fight of the current research results of the regional tectonic evolution it is proposed that the large-scale mineralization in the three stages is related to post-collision between the South China block and the North China block, transfer of the principal stress-field of tectonic regimes from N-S to E-W direction, and multiple back-arc lithospheric extensions caused by subduction of the Paleo-Pacific plate.     

Ophiolite belts and the collision of island arcs in the Arabian Shield

The Arabian Shield is divided into several segments by ophiolite zones. The segments display features of island arcs with respect to their magmatic evolution as well as their mineralization.The northern part of the “Hulayfah—Hamdah ophiolite belt” which cuts the Arabian Shield in a north—southerly direction, has been sampled and described. Serpentinized ultramafics, gabbros, doleritic dike rocks and basalts are the most important members. The ophiolite belt is marked by magnetic anomalies with amplitudes of 200–500 gammas.In conclusion, the Arabian Shield is considered to be built up of several generations of juxtaposed volcanic arcs of Late Proterozoic age. The arcs have been closely swept together squeezing out the trench-fill sediments in the case of the Hulayfah—Hamdah belt. Cratonization was completed by the end of the Precambrian.     

Tectonic effects of sea-floor spreading on the Arabian Shield

Paleogeographic evidence shows that the series of broad E-W anticlines and synclines on the Arabian Shield (Southern Hadramawt Arch, Wadi Hadramawt Syncline, Northern Hadramawt Arch, Rub Al Khali Syncline, Tuwaiq Homocline, Nafud Basin) are not old, inherited structures, but were formed in late Eocene and Oligocene times, as indicated by the warping of Middle Eocene sediments. The fold axes of these structures trend parallel to the Gulf of Aden, and their separation increases from S to N, i. e., with increasing distance from the Gulf of Aden. The most pronounced orogenic phase of the Toros mountain belt and the folding of the foreland belt (Lebanon, Antilebanon, Palmyra Arch, Jebel Sinjar, etc.) took place simultaneously with the warping of the shield. Furthermore, the early Tertiary Trap Volcanism occurs only in the neighborhood of the Gulf of Aden (Yemen, W-Aden Protectorate, Eritrea, Ethiopia, Somaliland). Geophysical-oceanographic research in the Gulf of Aden suggests that emplacement of basic magmatic material forms a quasi-oceanic crust (sea-floor spreading) in that rift trough. This apparently causes the displacement of the continental blocks. The close connections in time as well as in directional trends of epirogenic, orogenic and volcanic activities on the Arabian Shield to the sea-floor spreading in the Gulf of Aden indicates tectonic interrelations.This impression is still emphasized, if one considers the younger tectonic development on the shield. The young Tertiary Aden-Volcanics Belt (Miocene-Recent) extends from the East African Rift system over the West Arabian Shield all the way up to Turkey, that is to say its trend is more or less parallel to the Red Sea. Warping effects (Ras en Naqb Uplift, Jafr Depression, Bayir Uplift, Wadi Sirhan Depression, Rutba Dome and the Mesopotanian Basin) on the northern part of the Arabian Shield, where the earlier developed (Aden Gulf-related) structures die out, can be related in time and direction to the rifting in the Red Sea. Faulting along the Aqaba-Dead Sea System is of the same age and cuts the foreland belt. Finally the folding of the Zagros mountain belt is of Miocene age too.The Arabian Shield, bounded by still-active rifting structures of different direction and age, provides a classical example of the effect of sea-floor spreading on a shield area itself, and on its surrounding instable belt. The correct interpretation of these tectonic connections eventually may allow far reaching, basic conclusions.

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Zusammenfassung Die känozoische tektonische und vulkanologische Entwicklung auf und um den Arabischen Schild ist relativ jung und im Vergleich zu anderen Gegenden in ihrer Gesamtheit noch verhältnismäßig gut überschaubar. Sie bietet daher ein Beispiel, dessen Verständnis möglicherweise von grundlegender Bedeutung für die Interpretation gebirgsbildender Vorgänge werden kann.Der Arabische Schild ist im Süden und Südwesten umrahmt von den Rift-Systemen des Golfes von Aden und Roten Meeres, deren zentrale Teile durch quasi-ozeanische Kruste gekennzeichnet sind. Die Einschübe basischen magmatischen Materials (sea-floor spreading) in die Rifttröge verursachten offenbar eine Verdrängung der kontinentalen Blöcke (Bewegungssinn senkrecht zum Streichen der Zonen des aktiven sea-floor spreading). Jedenfalls läßt sich ein solcher Beanspruchungsplan von den verschiedenen tektonischen Teilvorgängen auf dem Arabischen Schild ableiten.Die Entwicklung des Golfes von Aden ist älter als die des Roten Meeres, und das gilt dementsprechend für die Eo- bis Oligozänen vulkanischen, epirogenen und orogenen Vorgänge, die räumlich und zeitlich Beziehungen zum Geschehen im Golf von Aden aufweisen (Trap-Vulkanismus, Verbiegungen des Süd- und Zentral-Arabischen Schildes und Auffaltung des Taurusgebirges und Palmyra-bogens). Alle Ereignisse, die räumliche Beziehungen zur Rotcn-Meer-Streichrichtung zeigen (Aden-Vulkanismus, epirogene Verbiegung des Nord-Arabischen Schildes und Auffaltung des Zagrosgebirges) sind jünger, d. h. seit dem Miozän besonders aktiv.

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Résumé L'évidence paléogéographique montre que, comme l'indique le gauchissement des sédiments de l'Eocéne Moyen, les séries des grands géanticlinaux et synclinaux, orientés est-ouest, sur le craton arabique (S. Hadramawt Arc, Wadi Hadramawt syncline, N. Hadramawt géanticline, Rub Al Khali syncline, Tuwaiq homocline, Nafud bassin) ne sont pas des vieilles structures antérieures, mais ont été formés durant l'Eocéne et l'Oligocène. Les axes de plissement de ces structures ont tendance à être parallèles au Golfe d'Aden. Leur séparation augmente du sud vers le nord, c'est à dire de la même manière que leur distance du Golfe d'Aden augmente. La phase orogénique la plus prononcée de la ceinture montagneuse de Toros et les prémontagnes de l'Arabie septentrionale (Lebanon, Antilebanon, Palmyra Arc, Jebel Sinjar) eurent lieu au même moment que le gauchissement du craton arabique. Bien plus, le «Trap»-volcanisme du Tertiaire Inférieur n'a lieu que dans le voisinage du Golfe d'Aden (Yemen, W-Aden Protectorat, Éritrea, Ethiopia, Somali). Des recherches géophysiques et océanographiques dans le Golfe d'Aden et la Mer Rouge suggèrent que des emplacements de matériaux d'origine magmatique forment une croûte quasi-océanique («Sea-floor Spreading») dans le fossé d'effondrement. Ceci est apparement la cause du déplacement des blocs continentaux. Les proches coïncidences aussi bien en époque qu'en direction des activités épirogéniques, orogéniques et volcaniques entre le craton arabique et le «Sea-floor Spreading» du Golfe d'Aden indiquent des correspendances tectoniques.Cette impression est encore plus renforcée, si l'on considère les développements plus récents du craton arabique. La jeune ceinture Tertiaire d'«Aden»-Volcanisme (Miocène-Recent) s'étend depuid le rift d'Afrique de l'est jusqu'en Turquie au travers du craton arabique de l'ouest. Elle se trouve de ce fait être plus ou moins parallèle à la Mer Rouge. Les gauchissements (Ras En Naqb, Jafr dépression, Bayir hautes plaines, Wadi Sirhan bassin, Rutba dome, Bassin Mésopotanien) de la partie septentrionale du craton arabique, où disparaissent des structures (identiques à celles du Golfe d'Aden) dévelopées au paravent, peuvent être associées en temps et direction au « rifting » de la Mer Rouge. Les failles le long du système Aquaba-Mer Morte sont du même âge et coupent les prémontagnes de l'Arabie septentrionale. Finalement le plissement de la ceinture montagneuse de Zagros appartient aussi au Miocène.Le craton arabique délimité par des structures d'âge et de directions différentes et toujours en cours de séparation, est un example classique de l'effet de la «Sea-floor Spreading» sur un craton et sur sa ceinture instable environnante. L'interprétation correcte de ces effets tectoniques resultera, le cas échéant, en des conclusions fondamentales très importantes.

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Formerly Southwest Center for Advanced Studied P. O. Box 30 365 Dallas Texas 75 230(Contribution No.81).     

A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation.The low²⁰⁷Pb/²⁰⁴Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb characteristics.In the eastern part of the shield, east of longitude 44°20E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb relationship together with distinctly higher²⁰⁸Pb/²⁰⁴Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100±300 m.y. (2).The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions.     

PGE exploration in the Wadi Amarah complex,southwestern Arabian Shield

A stream sediment survey targeting PGE and their pathfinders was carried out at a gabbro/pyroxenite complex in Wadi Amarah in the SW Arabian Shield. Twenty-nine stream sediment samples were collected from wadi channels and analyzed for PGE and base metals. None of the samples contained detectable PGE except for WAS-3 and WAS-27 which returned values above 70 ppb of combined Pt and Pd; these two locations are also anomalous for Au. A follow-up survey was conducted to further investigate the anomalies at these locations. A total of 48 soil and rock samples were collected from the two anomalous localities. Unlike the initial survey, most samples contained detectable PGE and Au albeit not as high as the original anomalies. Factor analysis of the results from the initial survey returned five main factors, with the first four reflecting the effects of mineral detritus as well as adsorption onto Fe–Mn oxyhydroxides; however, the last factor is loaded only for Cu and Ni and is therefore considered an ore factor indicating the presence of Cu–Ni sulphides. Another five factors were obtained from the follow-up survey, and as was the case in the initial survey, the first four factors are detrital/adsorption-related, while factor 5 has high loadings for only Pd and Pt and is therefore interpreted as a PGE ore factor. These findings reveal different modes of dispersion of PGE and base metals in the secondary environment and, more importantly, demonstrate the effectiveness of factor analysis in detecting even faint anomalies from disseminated mineralization.     

Caldera-related volcanic rocks in the Shammar Group, northern Arabian Shield

Volcanic formations of the ca 630-620 Ma old Shammar Group in the Tuluhah area in the northern Arabian Shield occupy an oval area some 8×12 km. They overlie sedimentary rift-fill of the Kuara Formation and are interpreted as related to the formation of a caldera, here named the Awad Caldera. The earliest of the volcanic formations, the Dabsah Tuff, is more than 450 m thick in the south and wedges out in the north. It is composed of silicic, medial to proximal pyroclastic flow rocks that record an eruption during which an initial caldera is interpreted to have formed by probably trapdoor-style collapse. The Nijab Basalt, more than 200 m thick and present as flows overlying the Kuara Formation to the north of the caldera, is presumed to have originated outside the study area during an interval between periods of silicic volcanic activity, and to have flowed onto the Dabsah Tuff in the first-stage caldera. The succeeding Mindassa Megabreccia contains large rafts of the older Shammar rocks, mainly Nijab Basalt, in a tuff matrix, and is regarded as probably a caldera collapse and fallback megabreccia formed during a silicic eruption that led to the second stage of caldera development. The megabreccia is overlain by the post-collapse Sutayih Tuff, more than 450 m thick, composed of proximal pyroclastic flow units.     

Late Neoproterozoic adakitic magmatism of the eastern Arabian Nubian Shield

Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and greywacke along with ophiolitic melanges of boninitic affinity which underwent inversion and deformation by~620 Ma.This adakitic magmatism,while intruding parts of the Ad Dawadimi Basin,predates this deformation,but is possibly coincident with basin closure.As adakitic magmatism requires melting of an amphibolite or eclogitic source,empirical and experimental constraints require anomalously hot supra-subduction zone mantle.Considering that this magmatism immediately predates basin inversion,these magmas possibly pinpoint the timing of the slab breaking,marking the terminal stages of arc magmatism,terrane accretion and the influx of hot oceanic asthenospheric mantle.This influx of hot asthenospheric mantle may also be responsible for postcolltsional A-type magmatism.     

三江南段一中南半岛特提斯蛇绿岩大地构造与成矿

青藏高原东南部特提斯研究是世界地质研究的热点之一,而三江南段一中南半岛地区是该研究的薄弱而分散地区.因此在特提斯构造域背景下,结合地层学、岩石学、构造学及古生物学等前人成果资料,利用中小尺度构造解析,厘定蛇绿岩混杂带构造古地理并论述成矿规律具有重要意义.(1)在特提斯构造格局基础上,圈定出与蛇绿岩空间配套的构造岩浆带,...