New Insights into the Evolution and Age of the Neoproterozoic Jebel Ohier Porphyry Copper Deposit, Red Sea Hills, Northeastern Sudan
F. P. BierleinW PotmaF. CernuschiCarl W. BrauhartJamie A. RobinsonChris J. BargmannW. D. BullenJose F. HenriquezIan DaviesAllen Kennedy
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Abstract New SHRIMP U-Pb data from dioritic to granodioritic synmineral intrusions associated with the Jebel Ohier porphyry copper deposit (mineral inventory, including NI43-101-compliant total inferred and indicated resources, of 593 million tonnes [Mt] at 0.33% Cu and 0.05 ppm Au, for 1.953 Mt of contained Cu and 933,600 oz of Au at 0.15% Cu cutoff) in the Red Sea Hills of northeastern Sudan have bracketed the age of mineralization to ca. 816 to 812 Ma. This age range, as well as constraints from new and existing lithogeochemical data, is consistent with the deposit’s formation from a productive parental magma source during the early stages in the evolution of an intra-Mozambique Ocean island arc. The Jebel Ohier porphyry copper deposit bears many similarities to well-documented Phanerozoic analogues elsewhere in terms of (1) the mapped style and zonation of hydrothermal alteration (i.e., proximal K-silicate–dominated, to sericitic, to distal propylitic alteration), (2) the occurrence of intense Cu-bearing A- and B-type vein stockwork, as well as sulfide-only C-type veins, anhydrite veins, and younger, peripheral D-type veins, and (3) the geochemical fingerprint of the associated porphyry, which is akin to those of ore-related Tertiary porphyries in the Escondida area in northern Chile. The multiphase intrusion hosting the Jebel Ohier porphyry copper deposit has been intruded by several generations of mafic to felsic postmineralization dikes and voluminous plutons, with a peak in magmatic activity coinciding with the suturing of the Gebeit terrane at ca. 724 Ma. In spite of, or perhaps because of, the occurrence of extensive postmineralization magmatism, and regardless of subsequent deformation, regional metamorphism, uplift, and erosion, the deposit has remained remarkably intact. The discovery of a relatively ancient, yet well-preserved porphyry copper deposit in the Neoproterozoic Arabian-Nubian Shield has major implications for the exploration potential of this resource-rich geologic terrain.Keywords:
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The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber.
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The Cretaceous-Palaeogene alkaline province, mainly located in the Kırşehir Block, extends E-W for some 300 km in Central Eastern Anatolia. The Dumluca, Murmana, Karakeban and Çaltı plutons constitute the easternmost part of the province. These plutons have introsive contacts with the Cretaceous Divriği ophiolitic melange and, except Karakeban, are associated with huge iron deposits. The Ypresian-Lutetian sediments unconformubly overly them. The four studied plutons belong to two completely different types of magmatic assocsiation, based on petrographic and geochemical (mainly major elements and REE) fetures. The dominant association comprises three plutons (Dumluca, Murmana, Karakeban) and is rather uniformly represented within each of them. These three plutons have a bimodal character evidenced by the coexistence of two groups of rocks, one mafic and the other felsic. The dominant felsic group, mainly composed of Na-rich and Ca-poor quartz monzonites and monzonites (Murmana, Dumluca), may also include syenites, quartz syenites, adamellites and granites (Karakeban). This group displays a cafemic alkaline over staurated trend, usually magnesian (Dumluca and Murmana) and common (Karakeban). The mafic group is mainly made up Na-rich and Ca-poor gabbros/diorites, monzogabbros/monzodiorites, rather often silica-undersaturated. This group represents a cafemic alkaline saturated to undersaturated trend either ferriferous (Karakeban) or magnesian (Dumluca) or or common (Murmana). Mafic dykes, cutting through the felsic rocks, belong to the same mafic group. These three plutons represent a composite alkaline association and the same type of association characterizes also each individual pluton. Field, petrographic and geochemical data suggest that the felsic group has not been derived from the mafic one by crystal fractionation, but that the two coevial groups may have interacted with each other. The subordinate association is represented by Çaltı pluton. This homogenous pluton, tonalitic and gronodioritic in composition, corresponds to a cafemic calc-alkaline association,with slight magnesian affinity. Such a coexistence of two quite different magmatic associations in the Divriği region has already been reported in the western part of Kırşehir Block. It shows that the alkaline character is restricted to only part of the plutons located in this block and may suggest that several successive magmatic events, related to quite different geodynamic conditions, occured in this domain. In this framework, the Dumluca, Murmana and Karakeban plutons are thougth to be derived from two different magma sources. One of them is mantle related mafic magma generated in a post-collisional lithospheric attenuation environment which accordingly caused to melt the lower parts of the continental crust forming the felsic magma source. This collisional event is attributed to the juxtaposition of the Pontide and Anatolide plates in the pre-Maastrictian time. The Çaltı pluton, representing different mineralogy and chemistry, should be solidified from the collision related and calcalkaline another hybrid magma source due especially to intruding the already obducted Divriği ophiolitic melange. Isotopic and geochronological data would be particularly helpful to better constrain this magmatic history.
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Aztec Wash pluton, a 50 km[sup 2] intrusive complex in the northern Eldorado Mountains, was emplaced ca. 16 Ma (Faulds et al., 1990) during extension within the Colorado River Corridor. The pluton displays extreme compositional variability, ranging from olivine gabbro (ca. 50 wt% SiO[sub 2]) to highly evolved aplite (76% SiO[sub 2]). Most of the intrusion is medium grained, homogeneous granite (ca. 72% SiO[sub 2]), but 1/3 is highly heterogeneous and dominated by mafic to intermediate rocks; a 6 [times] 3km, N-S mafic zone almost bisects the pluton. Well-displayed magma mingling and late mafic and felsic dikes verify the coexistence of mafic and felsic melts. Hornblende barometry indicates that the entire exposed portion of Aztec Wash pluton was emplaced at very shallow depth (
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The Early Cretaceous Tabashine plutonic complex (TPC) in the southern Kitakami Mountains includes a range of intermediate to felsic rocks, and is divided into the North and South plutons on the basis of petrographical and petrochemical features. Rocks of the North pluton contain larger amounts of pyroxene and are slightly richer in K2O but poorer in Al2O3 than those of the South pluton. On the other hand, rocks of the South pluton scarcely contain pyroxene and are characterized by euhedral crystals of hornblende. A possible petrogenetic interpretation is that they were derived from a common original magma, but underwent different evolutional paths as a consequence of different water content in magma. The North pluton shows normal continuous zoning, having more felsic composition with decreasing amounts of mafic enclaves inward. Such a kind of compositional zoning may be explained by a combined effect of differentiation occurring in a deeper chamber and mixing between mafic and felsic magmas rather than in situ crystal fractionation. The TPC is accompanied by a gabbroic mass, quartz-dioritic and tonalitic intrusions, and the Bunatoge volcanics. The gabbroic mass and the TPC are characterized by high K2O content and their emplacement occurred within a short time period. On the other hand, the Bunatoge volcanics and the quartz-dioritic and tonalitic intrusions are distinctly lower in K2O contents. The Bunatoge volcanics erupted prior to the emplacement of the TPC, whereas the quartz-dioritic and tonalitic intrusions were emplaced later than the TPC. Thus, the Early Cretaceous magmatism in the study area is characterized by a complex variation in K2O content with respect to time.
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Field relations and geochemistry indicate that Aztec Wash pluton had a complex, open-system history. The tilted pluton represents a 2.5 km thick chamber that was recharged with both felsic and mafic magma. The lower portion is highly heterogeneous, with mafic sheets; cumulates; hybrid rocks; mafic, felsic, and composite dikes; and sheets and pods of granite (heterogeneous [H] zone). The upper part is granite that is generally homogeneous in texture and geochemistry (granite [G] zone). At the base of the G zone, a discontinuous zone (buffer [B] zone) records interaction between the G and H zones. Complexity of the H zone makes detailed reconstruction of magma chamber history difficult, and the relatively homogeneous G zone appears to offer few clues about the evolution of the pluton or the interaction between the felsic and underlying more mafic magmas. Accessory mineral textures, zoning, and assemblages in the G zone, however, are far from homogeneous and provide clear evidence for fluctuating conditions that elucidates magma chamber history.
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Abstract This chapter documents the fracture process associated with the early cooling stage of felsic magma. Characteristics of pre-exhumation joints include their spatial distribution in granite bodies, their fracture surface morphology, and geological and petrological evidence for the depth of fracture initiation. These characteristics allow inferences about the depth and the time of joint origin in the South Bohemian Pluton. The intrusion levels of currently exposed granites of the pluton were 7.4 km in the northern part and 14.3 km in the southern part. Within the northern Mrákotín Granite (Boršov) early NNE joints propagated while the granite was at a temperature near the solidus, and, in part, magma was still being injected, post-dated by thin granite dykes along NNE joints. Evidence for the pre-exhumation initiation of these joints comes from the geochronological dating of these late-granite dykes (1–2 cm thick) at 324.9 Ma in age, which were creating their own rupture in the rock. The timing of the pluton emplacement at 330–324 Ma and the cooling ages of 328–320 Ma have been given by previous studies. From fluid inclusions within the late-granite dykes that occupy joint surfaces, the trapping depth of the analysed inclusions was calculated to be 7.4 km. Near the solidus H 2 O separates during the crystallization of anhydrous phases. The associated increasing H 2 O pressure can initiate the first cracks and can generate a small portion of new granitic melt, which forces the cyclic fracture propagation together with mobile, low-viscosity ‘residual melt’ input into the fracture. The determination of the intrusion level and time at which the dykes began cooling provide evidence for the joint initiation at a depth of 7.4 km, which was connected with the level and process of final emplacement and early cooling of the Mrákotín Granite long before the main exhumation. At the earliest, the erosion of the upper rock pile, 7.4 km in thickness, started significantly after generation of the early joint sets. The NNE-trending joints are persistent in orientation throughout the South Bohemian Pluton, but the joint-surface morphology varies in all subplutons and occupies all sections of the stress intensity v. crack-propagation velocity curve (Wiederhorn-Bahat curve).
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The generation of pegmatite dikes during the cooling and crystallization of granitic plutons has been calculated using new models for the prediction of granitic melt viscosities and the propagation of dikes. These new models suggest that early in the cooling history of a modeled l0 x l0 x l0 km pluton, dikes cannot propagate, or will be short (on the order of I km), because dl surreunding country-rocks have not yet been significantly heated. However, dikes formed tens to hundreds of thousands of years after intrusion can propagate up to approximately 10 km. Because the far-propagating dikes form late in the magmatic history of the pluton, they will be composed of chemically more evolved magmas than the bulk of the pluton and will crystallize as pegmatites. The model predicts that pegmatites should only rarely be found more than ca. l0 km from their host pluton, that more-evolved pegmatites should be found at greater distances from their host pluton than less-evolved ones, and that pegmatites should not be associated with small plutons. All of these model results are consistent with field observations, and support the petrogenetic relationship between granitic plutons and the evolved pegmatites surrounding them.
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