Hydrocarbons and other volatile components in alkaline rocks from the Ukrainian shield and Kola Peninsula

Gas chromatography and other analytical techniques (EMR, PMR, and IR spectroscopy) were used to examine volatile components (CH₄, C₂-C₃, CO₂, CO, H₂, H₂O, and others) in alkaline rocks and minerals from the Ukrainian Shield (eight massifs and dikes of grorudites) and from the Khibina and Lovozero massifs in the Baltic Shield. The alkaline rocks from the Ukrainian Shield are mostly of Proterozoic (1.7–2.1 Ga) age. The alkaline rocks from the Kola Peninsula were confirmed to be rich in methane (21 ± 14 μl/g on average) and other hydrocarbons, whereas the analogous rocks from the Ukrainian Shield are poor in methane (2.1 ± 1.6 μl/g on average at a maximum of 14 μl/g). The latter rocks are richer in CO₂, which is one of the major volatile components of alkaline rocks, including agpaitic nepheline syenites from the Kola Peninsula. The rocks from the Ukrainian Shield often have elevated contents of nitrogen (up to 20 μl/g). The reasons for the differences in the composition of volatile components of rocks from the Kola Peninsula and Ukrainian Shield are as follows: the agpaitic crystallization trends of large massifs in the Kola Peninsula and much less clearly pronounced agpaitic trends in the small massifs in the Ukrainian Shield, the affiliation of these rocks with different complexes, the deeper erosion levels of the Ukrainian alkaline massifs, different ages of these rocks, etc.     

Multi-stage pyrite and hydrothermal mineral assemblage of the Hatu gold district (west Junggar,Xinjiang, NW China): Implications for metallogenic evolution

The Hatu, Qi-III, and Qi-V gold deposits in the Hatu–Baobei volcanic–sedimentary basin (west Junggar, Xinjiang) represent the proximal, middle, and distal parts of the Hatu gold district, respectively. Orebodies of these deposits mainly consist of Au-bearing quartz veins and altered host rocks with disseminated sulfide minerals. Six types of pyrite in these mines are studied here to illustrate ore-formation processes. Sedimentary pyrite, including framboidal and fine-grained pyrite, occurs in mudstone-bearing sedimentary rocks or altered volcanic–sedimentary rocks. Framboidal pyrite formed during redox changes in sedimentary layers. Hydrothermal pyrite contains five subgroups, from Py1 to Py5. Porous Py1 formed prior to gold mineralization, and is overgrown by Py2 that contains inclusions of sulfide minerals and native gold. Coarser Py3 coexists with arsenopyrite and native gold, and contains the greatest As concentrations. Gold and antimony are also preferentially concentrated in arsenian Py2 and Py3. The Au–As-deficient Py4 and Py5 formed during the post-ore process. There is a negative correlation between the As and S contents in Py1, Py2, and Py3, implying the substitution of sulfur by arsenic. Gold precipitated under relatively reducing condition in framboid- and graphite-bearing tuffaceous rocks. Cesium, Rb, Sr, La, Ce, Au, As, Sb, Cu, and Pb are concentrated in altered host rocks. The Au-bearing quartz veins and disseminated sulfide mineral orebodies were formed via a co-genetic hydrothermal fluid and formed during different stages. The variation of fO₂ during fluid/rock interactions, and crystallization of arsenian pyrite were major factors that controlled gold precipitation.     

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.     

Provenances of late Pliocene rare-metal-titanium placers in the Tamansky Peninsula

The comprehensive mineralogical analysis of the Taman nearshore Ti-Zr placers and their provenances in the adjoining late Pliocene sedimentation basin is presented. Taking into account paleogeographic reconstructions and contemporary mineral potential of sediments in the Sea of Azov, a contribution of specific feeding sources of terrigenous minerals to the formation of the late Pliocene placers have been estimated. These sources are crystalline and sedimentary rocks of the Caucasus, the southern Russian Plate, and the southeastern Ukrainian Shield (listed in the order of their contribution). The Miocene placers of the Stavropol Arch and Adygea Prominence, as well as the Cretaceous and younger placers of the Ukrainian Shield and the Voronezh Anteclise are suggested to be transitional reservoirs. Economic deposits are forecasted in the zone of paleostraits.     

The intermetallic compound Ni<Subscript>3</Subscript>Au and gold-nickel solid solutions in metalliferous sediments of the triassic chert formation in Sikhote-Alin in the Far East

The intermetallic compound Ni₃Au and Au-Ni solid solutions (native nickel Au and Au-bearing native Ni) were found in the contact metamorphosed metalliferous sediments of the Triassic chert formation in Sikhote-Alin. The metalliferous rocks are characterized by high contents of Au, Ag, and PGE, as well as the presence of diverse minerals of precious metals. Nickel gold (Au0._91-0.88 Ni_0.09-0.12) is found as grains and crystals (3 to 4 im in size) in Au-bearing cherts with hematite, which are conventionally defined as “itabir-ites” and in the altered siliceous rocks of the Dal’nerechensky district (the upper reaches of the Gornya River). The nickel gold is associated with copper Au, pure native Au (Au_1.00), and Au-Ag and Au-Ag-Pb solid solutions (Au_0.86/_0.84Ag_0.14__0.16 and Au_0.78Ag_0.19Pb_0.03, respectively). The Au-bearing Ni is found in the metamorphosed carbonaceous mudstones and Au-bearing “itabirites” of the Shirokopadninsky area (Olgin-sky district). The Au content varies from 6.09 wt % (Ni_0.98Au_0.02) to 11.30 wt % (Ni_0.96Au_0.04) in some Au-bearing Ni grains (about 10—15 im in diameter) taken from the metamorphosed mudstones. The grains of Au-bearing Ni (Co_0.001__0.00Au_00.2 - _0.17 Ni _0.98__0.83) in the “itabirites” are also characterized by their heterogeneous composition and the fine impregnation of the Ni₃ Au intermetallic compound (Ni_2.99Au_1.01 based on the microprobe analysis).     

Metasedimentary rocks of the paleoarchean Dniester-Bug Group,Ukrainian Shield: Composition,age, and sources

Paleoarchean granulite-facies metasedimentary rocks (quartzites, garnet quartzites, garnet-pyroxene gneisses, pyroxene-magnetite and magnetite quartzites) attributed to the Dniester-Bug Group of the Ukrainian Shield were studied. On the basis of geochemical data, including REE, the primary composition of these rocks was reconstructed as association of Fe-rich sandstones and sublitharenites, Fe-shales, and BIFs. This sedimentary association is similar to the rocks of other ancient greenstone belts and ascribed to the Algama-type iron formation. The sum of Al₂O₃, CaO, Na₂O, and TiO₂, high Zr contents (>100 ppm in quartzites), and the presence of detrital zircon grains of different ages are consistent with the terrigenous nature of sedimentary rocks. The Sm/Nd, Ti/Zr, Sc/Zr, and Ni/Zr ratios indicate the predominance of granitoid rocks in the source areas. The elevated Cr contents suggest that, in addition to granitoids, the source area contained ultramafic rocks. Geochemical characteristics, such as Fe/Mn ratio, low REE contents, and variations of REE versus the sum of Ni, Co, and Cu testify that sedimentation occurred under shallow-water conditions on the continent or its slope, similarly as the formation of ancient (3.5–3.2 Ga) basalt-komatiitic series intercalated with sedimentary rocks in the Pilbara Craton. The age of supracrustal rocks of the Dniester-Bug Group was constrained within the time interval of 3.4–3.2 Ga on the basis of U-Pb zircon dating and determination of Nd isotope composition. The DM model age of quartzites varies from 3.37 to 3.5 Ga. Sedimentary rocks together with volcanic rocks represent the oldest supracrustal association of the East European Platform.     

陕西略阳张家山金矿成矿作用及Au-Se矿物形成物理化学条件研究

陕西略阳煎茶岭金矿矿集区中的张家山金矿主要由破碎蚀变岩型、角砾岩型和含金石英黄铁矿脉型矿石组成。含金石英-黄铁矿脉型矿石产于断层下盘的石英菱镁岩中。黄铁矿发育富As黄铁矿边,环边受As含量的变化呈现一定的韵律变化,自然金赋存在富As黄铁矿中。在断裂发育形成断层角砾岩的过程中,流体充填破碎石英菱镁岩的裂隙中形成热液矿物,包括硫化物、硒化物以及自然金。石英菱镁岩发生破碎形成的网状裂隙被含金石英-方解石-黄铁矿脉充填。破碎蚀变岩型矿石中,自然金主要分布在含金石英-黄铁矿脉的石英之中或靠近热液脉的菱镁矿或石英间隙。随着大量方解石脉沿裂隙贯入,进一步促进石英菱镁岩的破碎及岩石角砾的分离,形成由石英菱镁岩碎屑、石英和褐铁矿组成的复成分角砾岩,自然金和硒化物呈浸染状分布在角砾岩中。笔者在角砾岩型矿石中发现了灰硒汞矿、直硒镍矿、硒铅矿等硒化物,这些硒化物往往与自然金密切共生。结合矿物组合以及相关化学反应关系,通过热力学计算,构建了该矿床在不同温度条件下的热力学相平衡关系图,限定了硒化物与其他相关矿物稳定存在的物理化学条件。硒化物一般与自然金和石英共生,高的f(Se2)值和f(Se2)/f(S2)比值是控制硒化物形成的关键因素。     

Geochemical features of ilmenites from the alkaline complexes of the Ukrainian Shield: LA-ICP MS data

LA-ICP MS data are presented for ilmenites from different rocks of the alkaline complexes of the Ukrainian Shield (Chernigovka carbonatite, Oktyabr’skii, Malaya Tersa, and Southern Kal’chinskii gabbrosyenite massifs). Ilmenites from the early intrusive phases (alkaline pyroxenites, gabbroids, and ultramafic rocks) have the elevated contents of Cr, Co, Ni, and V, while ilmenites from later alkaline and nepheline syenites, monzonites, and carbonatites are significantly enriched in Nb and Ta, which is caused by change in the alkalinity of the mineral-forming medium. Zr shows the more intrinsic behavior: its content is higher in the ilmenites from basic and ultrabasic rocks than in those from the nepheline syenites and carbonatites. This is mainly caused by temperature conditions of the formation of differentiated alkaline complexes. The carbonatites contain magnesian ilmenite (up to 22 mol % MgTiO₃). Variations of Mg contents in ilmentes are correlated with Mg number of mafic minerals and depend also on the iron oxidation state (amount of magnetite) in the carbonatites. In the alkaline massifs of the Ukrainian Shield, ilmenites usually have the low contents of hematite end member (3–7 mol %). Ilmenite serves as a sensitive indicator of temperature, oxygen fugacity, and alkalinity of the mineral-forming medium during crystallization.     

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO₂ or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between ?8.1‰ and ?6.8‰ for δ¹³C, +6.4‰ and +10.5‰ for δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO₂±CH₄±N₂) and aqueous-carbonic (H₂O-NaCl-CO₂±CH₄±N₂) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO₂-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.     

The typification of chemogenic allites

Analysis of terminology used for the aluminous rocks shows expediency of the application of term “allite” for the whole group of aluminous rocks. However, irregular behavior of Al in the natural setting calls for the identification of two principally different groups: typical bauxites of different genetic classes and chemogenic allites (chemallites). Taking into consideration the mineral and chemical compositions and geological constraints of these rocks, their typification is refined and some corrections are introduced into the terminology. Allites are divided into the following types: chemogenic bauxites of the Moscow region and Birilei area; supergene sulfate allites of Siberia and the Zhigulevsk area; volcanogenic sulfate allites of the Zaglik area; aluminous argillizites; and metamorphogenic allites. In terms of the chemical and mineral compositions and technological properties, chemogenic allites of the first two types are rather similar to sedimentary bauxites with a small boehmite admixture. In mineralogical diagrams, they mainly correspond to the clayey bauxite field.     

Initial Mineral Composition and Geochemical Signature of High-Alumina Gneisses in the Chupa Formation of the Belomorian Group,the Baltic Shield

The normative mineral composition of protolith for high-alumina gneisses in the Chupa Formation of the Belomorian Group (Baltic Shield) is presented. Based on protolith composition, index of maturity or chemical index of alteration (CIA), localization of data points of gneisses on discriminant diagrams, and several index ratios, it is suggested that the protolith of the studied rocks was composed of low-mature sediments (graywackes). The material delivered to the sedimentary paleobasin was derived from ultramafic, mafic, and intermediate rocks. Correlation between some elements (Rb versus K₂O, Ti and Ga versus Al, and Sr versus smectitic clay and plagioclase), typical of Phanerozoic sedimentary rocks, is also revealed in the studied gneisses.     

四川若尔盖阿西金矿岩石学及岩石地球化学特征

阿西金矿地处川甘陕“金三角”成矿集中区,中三叠统为本区内最重要的金矿源层和赋矿层。阿西金矿田的形成经历了一个从沉积－成岩－富集成矿的漫长复杂的由量的积累到质的飞跃过程,是多阶段、多层次、多因素作用有机结合的产物。赋矿岩系特征表明,阿西金矿田的主要成矿环境为半深海斜坡环境。区内岩石类型极为复杂,三大岩类均有产出。主要有细砂岩、杂砂岩、沉凝灰岩、碳酸盐岩、硅质岩;变粒岩、石英岩、角岩、大理岩、夕卡岩、角砾岩;岩浆岩类主要为英安斑岩（玢岩）、闪长岩等。其中最重要的赋矿岩石为石英岩,其主要特征为具块状、不规则条带状构造,呈褐黄色／浅灰色变余层状构造。由石英（75％－80％）、高龄石（10％－15％）、绢云母（2％－3％）、钛铁质（3％－4％）、玉髓（2％－3％）及少量碳质组成。其岩石化学成分与典型硅质岩相比,SiO2偏低,Al2O3总体偏高,出现较强的高岭土化。通过研究认为,该区主要赋矿岩石石英岩的原岩应属正常碎屑沉积岩。     

江南造山带中段正冲金矿矿床成因:来自流体包裹体和黄铁矿微量元素的证据

江南造山带上产出有大量金矿,但其成因存在争议,为厘清其成因,对该造山带上的正冲金矿床中的黄铁矿进行了LA-ICP-MS微量元素分析和流体包裹体研究.结果显示,正冲金矿的流体包裹体有含CO2包裹体、富CO2包裹体和水溶液包裹体,其中富CO2包裹体与水溶液包裹体共存,具有相似的均一温度,但盐度不同,说明流体不混溶可能导致了...     

北京怀柔崎峰茶-得田沟金矿田稀土元素地球化学特征

崎峰茶-得田沟金矿田位于华北地块东北部,是华北地块北缘巨型EW向金矿带的组成部分。本文研究了该矿田各类岩石的稀土元素特征。通过与含金石英脉和金属硫化物的对比分析,探讨了它们与成矿的关系。结果显示,金属硫化物、含金石英脉及蚀变岩石的稀土元素特征很相似,它们与矿田内的燕山期正长斑岩脉和花岗岩(脉)具有很大相似性,从而确定了它们之间的成因联系,为找矿预测提供了依据。      

Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits

Omai is a high tonnage, low-grade, world-class gold deposit located in the Paleoproterozoic Guiana Shield. It is the second most important gold deposit in the Guiana Shield (after Las Cristinas, Venezuela), and one of the largest in South America (4.0 million oz.). Sm-Nd and Sr isotope data are presented for host rocks and for scheelite from auriferous quartz-carbonate-scheelite-sulfide-telluride veins from the Omai deposit. Gold-bearing veins are hosted by the Paleoproterozoic Barama-Mazaruni Supergroup, a greenstone belt sequence consisting of mafic volcanic rocks interbedded with sedimentary rocks that are intruded by quartz-feldspar porphyry and rhyolite dikes. This lithologic sequence was folded and metamorphosed to lower greenschist facies during the Paleoproterozoic Trans-Amazonian orogeny. The volcano-sedimentary unit was intruded by a post-tectonic quartz monzodiorite-diorite-hornblendite stock. Initial Nd isotope ratios for the Omai volcanic rocks range from ɛ_Nd=+2.1 to +4.2. These values suggest that this part of the Guiana Shield was a site of new crust formation during the Paleoproterozoic and was not contaminated by older (Archean), reworked continental crust. Initial Nd isotope ratios for the Omai stock range between +0.5 and +2.3, which suggest limited contamination with previously formed continental crust. Although the Nd isotopic ratios of gold-related scheelites overlap with those of the host rocks, particularly the tholeiitic basalts at the interpreted time of vein emplacement, the lack of both isotopic mixing and significant Nd movement during the hydrothermal process suggest that the Nd isotope composition can be used to determine the isotopic characteristics of the ore fluid source area. At Omai, the ore fluid is largely derived from a radiogenic Nd source, represented by mantle or lower crustal reservoirs. Strontium isotope ratios for the scheelites cluster tightly between 0.7019 and 0.7021. The Sr isotope data suggest that unlike Nd, Sr was significantly mobile during the hydrothermal process. The fluids responsible for the Omai deposit may have picked up Sr along the flow path. The constant low Sr isotope values of scheelites probably reflect the key role that the local tholeiitic basalts played as the main source of Sr in the fluids. Whereas Nd isotopes identify the fluid source area, the Sr isotopes map the fluid flow paths. Received: 11 February 1999 / Accepted: 1 November 1999     

Geochemical Studies of Au—bearing Formation in Jiaodong Peninsula,Shandong Province

Geochemical characteristics of the Upper Archean-Lower Proterozoic Jiaodong Group-the host rocks of the famous Jiaodong gold deposit are described in this paper.It is shown that the Jiaodong Group is an important Au-bearing formation containing 19.5 ppb Au on average, about 5 times as much the abun-dance in the crust.It is also found that the distribution of gold in the strata is uneven and that plagioclase amphibolite is the prospective Au -bearing rock.In the processes of regional metamorphism, migmatization and fault -remelting ,gold in the source bed was remobilized and concentrated to from various types ofgold deposits in this province.     

The early crust beneath the Azov Domain of the Ukrainian Shield: Isotopic-geochronological and geochemical investigations of detrital zircons from metasedimentary rocks of the Fedorovka structure

The results of isotopic-geochronological investigations of detrital zircons from metasediments of the Fedorovka structure in the Azov Domain of the Ukrainian Shield by SIMS and LA-ICP-MS methods are considered. The data obtained are compared with the results of isotopic dating of zircons from metasediments of the Soroki greenstone structure of the Azov region. All the examined samples yielded zircons with ages exceeding 3600 Ma. The geochemical properties of the oldest zircons indicate that they originate immediately from tonalite rocks. The obtained data imply substantially wider development of the Paleoarchean crust in the Ukrainian Shield than was previously thought.     

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (～2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (～2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ～1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.     

Metallogeny of gold in relation to the evolution of the Nubian Shield in Egypt

Gold mineralization in the Eastern Desert of Egypt is confined, almost completely, to the basement rocks of the Nubian Shield that was cratonized during the Panafrican orogeny.Island-arc, orogenic and post-orogenic stages are indicated for the tectonic-magmatic evolution of the Nubian Shield in Late Proterozoic times. Different styles of gold mineralization recognised in the Eastern Desert are inferred to have developed during these stages.In the island-arc stage, which is characterized by volcanic and volcaniclastic rocks in an ensimatic environment, gold mineralization is hosted in stratiform to strata-bound Algoma-type BIF and associated tuffaceous sedimentary rocks. Both types represent exhalative deposits, formed during breaks in sub-marine basaltic and bastalic–andesite volcanic eruptions. The volcanic rocks have a tholeiitic affinity and reflect an immature arc stage. Gold hosted in massive-sulphide deposits within calc-alkaline rhyolites represents another style of gold mineralization connected with mature island arc stage.During the orogenic-stage, ophiolites and island arc volcanic and volcaniclastic rocks were thrust onto the Pre-Panafrican continental margin. Subduction was active beneath the continent while the thrusting was still operative. A phase of calc-alkaline magmatic activity developed during this stage and the compressional deformation event was synchronous with regional metamorphism (greenschist–amphibolite facies). Extensional shear fractures (brittle–ductile shear zones) were broadly contemporaneous with the intense compressional tectonic regime. These fractures opened spaces in which the mineralizing fluids penetrated.Gold mineralization associated with the orogenic-stage is represented by vein-type mineralization that constituted the main target for gold since Pharaonic times. Other styles of gold mineralization during this stage are represented by altered ophiolitic serpentinites (listwaenites), Gold mineralization associated with intrusion related deposits (possibly porphyry copper deposits), as well as, auriferous quartz veins at the contacts of younger gabbros and G-2 granites.The post-orogenic stage is characterized by the dominance of intra-plate magmatism. Small amounts of the element in disseminations, stockworks and quartz veins of Sn–W–Ta–Nb mineralization represent gold mineralization connected with this stage.The link between these tectonic–magmatic stages and gold mineralization can be used as a criterion at any exploration strategy for new targets of gold mineralization in Egypt.     

Principal stages in geological evolution of Ukrainian shield

The first attempt at coordination of the newest geochronological findings with identifiable petrographic complexes within the Ukrainian Shield, here seen as products of geosynclinal evolution and of granitization (in the modern sense), suggests the following characteristic traits of the Precambrian evolution of the earth's crust: an arrest of the upper Archean geosynclinal cycle (the second one of the three) in its initial or early stages; a general non-inversion of geosyncline; a very high geothermal gradient and hence the cardinal role of granitization and the "universality" - involvement of practically all kinds of rocks - of the regional metamorphism. The Shield has behaved as a platfomal entity ever since the end of its last (lower Proterozoic) geosynclinal cycle. The author's historical analysis of the Shield, as distinct from the still current stratigraphic representations, is based among other things on the recognized independence of metamorphic facies from stratigraphic as well as structural boundaries, identifications of formations corresponding to definite stages and zones of the geosynclinal evolution, correlations of geochronological data with petrographic varieties of rocks and with their spatial distribution, examination of migmatites and granitoids from the viewpoints of ultrametamorphism and granitization, proofs of the function of deep fissuring in the character of sedimentation and intrusive activities, and plane analysis of folded structures. Metallogenic analysis of the Shield is facilitated by such identifications and correlations, inasmuch as migrations and accumulations of ore constituents and others were definitely associated with granitization activities. — V. P. Sokoloff