Lead isotopic signatures of the polymetallic mineralization in the Ariab district, Red Sea Hills, northeastern Sudan (original) (raw)
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A new look at sulphide mineralisation of the northern limb, Bushveld Complex: a stable isotope study
Transactions of The Institution of Mining and Metallurgy Section B-applied Earth Science, 2005
The Platreef is the main platinum-group element (PGE)-bearing horizon in the northern limb of the Bushveld Complex, South Africa. It is considered to be richer in sulphides than other similar horizons within the Bushveld Complex, in particular the Merensky Reef. Previous work has indicated that assimilation of dolomite may be a mechanism to add sulphur to the magma from which the Platreef formed. Sulphur isotope data presented in this study indicates an additional sulphur source contributing to the Platreef. In the southern Platreef, the Duitschland Formation of the Transvaal Supergroup forms part of the direct footwall. Within this sequence are pyrite-rich shales, which we suggest contributed to the sulphur budget of the Platreef. Local variations in sulphur isotopes, as well as a decrease in crustal sulphur in the Platreef further away from the footwall, also indicate local rather than regional contamination processes. Platreef samples have δ 18 O values that are higher than expected in a mantle-derived magma, but there is no apparent systematic variation in δ 18 O with distance from the footwall. This may possibly indicate contamination of the Bushveld magma pre-intrusion, probably in a staging chamber. However, when compared to data from the central sector of the Platreef itself, analysis from this study have lower δ 18 O values indicating changes in the degree of contamination with varying footwall lithology. Also, differences between plagioclase and pyroxene δ 18 O values indicate exchange with fluids. It is possible that late-stage deuteric fluids may have caused this exchange.
Sulphur and lead isotope geochemistry of the Dursunbey (Balıkesir) lead–zinc deposit
Journal of African Earth Sciences, 2020
The Biga Peninsula (northwest Anatolia) hosts several Pb-Zn occurrences, and the Dursunbey Pb-Zn deposit is one such important Pb-Zn province of Turkey. The basement rocks in the study area comprise metamorphic rocks of the Menderes Massif (Simav Metamorphites, Sarıcasu Formation, and Budagan Limestone) and ophiolitic units of the İ zmir-Ankara Suture Zone, known as the Dagardı Mélange. These units were intruded by the Miocene Alaçam granite. Cover units start with the Early Miocene Yeniköy Formation (comprising alternating sandstone, claystone, marl, and clayey limestone, and tuff additives) and are terminated by Civanadag tuffs, Akdag volcanics, and the Toklargölü Formation, consisting of unattached grains and alluvium deposits. Mineralisation that is trending along N50 • W 30-35 • NE occurs between the Alaçam Granite and the carbonate rocks of the Menderes Massif Metamorphites. Microscopic analysis revealed that the mineralisation contains chalcopyrite, pyrite, sphalerite, galena, fahlerz (tetrahedrite, tennantite), pyrrhotite, pyrargyrite-proustite, magnetite, and haematite as major ore minerals. The δ 34 S values range from 2.4 to 4.1‰ in pyrite (n = 2, mean = 3.25‰), from 1.7 to 3.4‰ in chalcopyrite (n = 4, mean = 2.8‰), 2.2-3.5‰ in sphalerite (n = 6, mean = 3.05‰), but from only 0.2-1.8‰ (n = 8, mean = 1.12‰) in galena. The low δ 34 S values of the sulphide minerals (varying over a narrow range) may indicate that sulphur in the ore-depositing solutions had a magmatic origin.
Lithos, 2015
The peridotite xenoliths of the Letlhakane kimberlite (Botswana), which intrude the Proterozoic Magondi Belt on the western margin of the Zimbabwe craton, represent highly depleted melting residues. These residues suffered subsequent variable metasomatic overprinting, evidenced by cryptic trace element enrichments in the spinel peridotites to modal addition of phlogopite, clinopyroxene and spinel within the garnet peridotites. In order to assess the robustness of the Re-Os chronometer in such highly metasomatised peridotites, detailed investigations of base metal sulphide (BMS) petrography and single-BMS grain 187 Os/ 188 Os analyses have been undertaken in three representative peridotites. The BMS occur as b 10 μm-50 μm inclusions and interstitial grains that are associated with metasomatic phases or metasomatised rims of primary silicates or display melt-like morphology, all attesting of their metasomatic origin. Their 187 Os isotopic compositions vary from 0.1016 to 0.6109 yielding T RD ages from 3.75 ± 0.54 (2se) to future ones. They vary independently of the cryptic or modal silicate metasomatic overprinting on the peridotites and independently of the BMS-silicate textural habits (e.g. isolated inclusions, pseudo-inclusions, intergranular melt-like pools), contrary to what is commonly assumed. In such highly depleted peridotites that must have been sulphide-free after the partial melting event, the Eoarchean age is likely inherited from residual PGM (platinum group minerals; i.e. laurite and Os-alloys) that formed in response to the exhaustion of the primary BMS and were later redissolved within the metasomatic BMS. In contrast, the younger single grain T RD ages represents an increasing dilution of the residual PGM signals within the metasomatic BMS, with the single grain 187 Os/ 188 Os signatures becoming increasingly dominated by Os derived from metasomatic BMS. Taken as a whole, the single BMS grain Eoarchean age suggests a lithospheric stabilisation age in Letlhakane similar to the oldest crustal samples of the Zimbabwe craton, which thus supports the Letlhakane mantle root to be a westerly extension of the Zimbabwe cratonic mantle. The T RD ages at the single BMS grain scale show a much larger range than their respective whole-rock T RD age, most notably from BMS in the two metasomatic end members: the cryptically-metasomatised peridotite and the most modally overprinted peridotite containing BMS 1 to 2.5 Ga older than their whole-rock. Base metal sulphides are thus better time capsules than the whole-rock, regardless of the silicate metasomatism the whole-rocks have experienced. Furthermore, the ancient ages are obtained from interstitial Ni-rich BMS indicating that all possible hosts of Os, not only BMS which appear to be 'primary' (i.e. Fe-rich BMS isolated inclusions in olivine) should be investigated. Single BMS grain Re-Os dating should then be considered as the only reliable dating approach, across all tectonic contexts (orogenic, ophiolitic, cratonic), which have experienced extensive metasomatic overprinting or refertilisation.
The Balcılar (Çanakkale) barite-galena mineralization is a typical example of the vein type barite-lead-zinc deposits in the Biga Peninsula. The lithologic units in the study area are Akçaalan andesite, Eocene, Adadağı pyroclastics, Oligocene, Dededağ dacite, Miocene, Karaömerler basalt, Plio-Quaternary and alluvium Quaternary. Baritegalena veins occurred along the faults developed within the andesites. Barite, quartz and galena are main minerals and are accompanied by minor amounts of sphalerite, pyrite, chalcopyrite, covellite and marcasite. The earliest barite (barite I) occurs as coarse-grained subhedral-euhedral crystals and the later (barite II) as small tabular crystals in between the earlier coarse crystals. Quartz occurs as finegrained crystals with the later small barite crystals. Galena, sphalerite, chalcopyrite, pyrite, marcasite and covellite occur open spaces within the earlier barite (barite I) crystals. Sulfur isotopes indicate that in galena the reduced sulfur from bacterially or inorganically reduced sulphate, or from an isotopically light reduced S-source. The δ34SH2S values calculated from the barite-H2S and galena-H2S fractionation factors, in the same samples, indicate a temperature of between 200 and 300 °C. Relatively lower δ34S values of barites than dissolved sulphate in modern oceans or Eocene sea waters have been evaluated as the ore forming hydrothermal fluids were derived from interacting throughout the magmatic host-rocks. Balcılar (Çanakkale) barit-galenit cevherleşmesi, Biga yarımadasındaki damar tipi kurşun-çinko-barit yataklarının tipik örneklerinden birisidir. İnceleme alanındaki litolojik birimler; Eosen Akçaalan andeziti, Oligosen Adadağı piroklastikleri, Miyosen Dededağ dasiti, Pliyo-kuvaterner Karaömerler bazaltı ve Kuvaterner aluvyonlar olarak ayırtlanmıştır. Barit-galenit damarları andezitler içerisindeki faylar boyunca oluşmuşlardır. Barit, kuvars ve galenit ana mineraller olup, bu minerallere az miktarda sfalerit, pirit, kalkopirit, kovellin ve markazit eşlik etmektedir. Baritler erken evrede iri-taneli yarı özşekilli-özşekilli kristaller (barit I) ve geç evrede kaba kristallerle girift küçük çubuksu kristaller (barit II) biçiminde iki-evreli mineralizasyonla gelişmiştir. Kuvars küçük barit kristalleriyle birlikte geç evrede oluşmuştur. Galenit, sfalerit, kalkopirit, pirit, markazit ve kovellin geç evre ürünleri olup, önceki oluşan barit kristalleri içerisindeki boşluk zonları boyunca oluşmuşlardır. Sülfür izotop bulguları galenitteki sülfürün bakteriyel veya inorganik sülfattan (SO4=) indirgenmiş veya izotopik olarak hafif S-kökenle etkileşimi göstermektedir. Barit-H2S ve galenit-H2S ayrımlaşma faktörlerinden itibaren hesaplanan δ34SH2S değerleri, sıcaklık koşullarının 200-300 °C arasında olduğunu göstermiştir. Baritlerin güncel okyanus veya Eosen deniz suyu bileşiminden daha düşük δ34S değerleri, hidrotermal sıvıların magmatik yan kayaçlarla etkileşiminden kaynaklandığı biçiminde değerlendirilmiştir.
International Geology Review, 2011
The Chahgaz Zn–Pb–Cu volcanogenic massive sulphide (VMS) deposit occurs within a metamorphosed bimodal volcano–sedimentary sequence in the south Sanandaj–Sirjan Zone (SSZ) of southern Iran. This deposit is hosted by rhyodacitic volcaniclastics and is underlain and overlain by rhyodacitic flows, volcaniclastics, and pelites. Peperitic textures between rhyodacite flows and contact pelites indicate that emplacement of the rhyodacite occurred prior to the lithification of the pelites. The rhyodacitic flows are calc-alkaline, and show rare earth and trace elements features characteristic of arc magmatism. Zircons extracted from stratigraphic footwall and hanging-wall rhyodacitic flows of the Chahgaz deposit yield concordant U–Pb ages of 175.7 ± 1.7 and 172.9 ± 1.4 Ma, respectively, and a mean age of 174 ± 1.2 Ma. This time period is interpreted to represent the age of mineralization of the Chahgaz deposit. This Middle Jurassic age is suggested as a major time of VMS mineralization within pull-apart basins formed during Neo-Tethyan oblique subduction-related arc volcano-plutonism in the SSZ. Galena mineral separates from the layered massive sulphide have uniform lead isotope ratios of 206 Pb/ 204 Pb = 18.604–18.617, 207 Pb/ 204 Pb = 15.654–15.667, and 208 204 Pb/ Pb = 38.736–38.769; they show a model age of 200 Ma, consistent with the derivation of Pb from a Late Triassic, homogeneous upper crustal source.
Missing Lead and High 3He/4He in Ancient Sulfides Associated with Continental Crust Formation
Major terrestrial reservoirs have Pb isotopes more radiogenic than the bulk silicate Earth. This requires a missing unradiogenic Pb reservoir, which has been argued to reside in the lower continental crust or dissolved in the core. Chalcophile element studies indicate that continent formation requires the formation of sulfide-bearing mafic cumulates in arcs. Because Pb, but not U, partitions into sulfides, we show that continent formation must have simultaneously generated time-integrated unradiogenic Pb reservoirs composed of sulfide-bearing cumulates, now recycled back into the mantle or stored deep in the continental lithosphere. The generation of such cumulates could also lead to coupled He-Pb isotopic systematics because 4 He is also produced during U-Th-Pb decay. Here, we show that He may be soluble in sulfide melts, such that sulfide-bearing cumulates would be enriched in both Pb and He relative to U and Th, ''freezing'' in He and Pb isotopes of the ambient mantle at the time of sulfide formation. This implies that ancient sulfide-bearing cumulates would be characterized by unradiogenic Pb and He isotopes (high-3 He/ 4 He). These primitive signatures are usually attributed to primordial, undifferentiated mantle, but in this case, they are the very imprint of mantle differentiation via continent formation. T he geochemical behavior of Pb in the Earth is not fully understood. Most upper crustal and upper mantle reservoirs have Pb isotopes more radiogenic than bulk Earth estimates, manifested by silicate reservoirs plotting to the right of the geochron on a 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagram. This imbalance, known as the ''Pb paradox'', means that a missing reservoir with unradiogenic Pb is required to balance the radiogenic Pb 1. One solution is that the missing unradiogenic Pb resides in the present-day lower continental crust 2. However, most lower crustal xenoliths are not sufficiently unradiogenic to satisfy this mass balance 3,4,5. Another hypothesis, motivated by experimental studies, is that Pb dissolved into the Earth's core early in Earth's history 6,7. If Pb indeed segregated into the core, the U-Pb systematics of the bulk silicate Earth would indicate that the core formed 80-140 Myr after planetary accretion 6,7 , but this core formation age is much later than the ,30 Myr after accretion based on 182 Hf-182 W isotopic systematics, which is uniquely sensitive to core formation 8,9 (unlike the U-Pb system, which can be fractionated by processes unrelated to core formation). Alternatively, this young U-Pb age of the bulk silicate Earth, 80-140 Myr after Earth's accretion 6,7 , has been used to argue for a late accretion of volatile elements, such as Pb, to the Earth 10,11. Due to these inconsistencies, additional hypotheses should be explored. Sulfide minerals are another potentially important carrier of Pb 12. A recent study by Lee et al. 13 showed that pyroxenite cumulates formed during the differentiation of arc magmas are enriched in sulfides and therefore enriched in strongly chalcophile (sulfide-loving) elements like Cu. The bulk continental crust is felsic in composition because a large amount of mafic cumulates were formed and then removed from the continental crust by delamination 14,15. Lee et al. 13 showed that the Cu content of the continental crust is depleted relative to basaltic magmas and even the mantle, which implies that the complementary mafic cumulates should be sulfide-bearing. The amount of S, in the form of sulfide, which must be missing from the continental crust, can be evaluated by considering the compositional effects on sulfide solubility during magmatic differentiation. Assuming the initial building blocks of continents are basalts and that almost all basalts are sulfide-saturated in their mantle source regions, the initial S contents of these juvenile basalts will range from 1600-2000 ppm 16. Because sulfide solubility decreases substantially with increasing SiO 2 and decreasing FeO contents, by the time the basalts evolve to andesitic compositions typical of bulk continental crust, the S content at sulfide solubility has fallen to ,200 ppm 16 (solubility may rise with further magmatic evolution if oxygen fugacities rise, but it is the andesitic compositions that are of interest here as such compositions most closely match the average composition of the
Arabian Journal of Geosciences, 2018
At the Echo Bay mine, vein type U-Ni-Ag-Cu deposits occur along NE-SW-trending fracture zones in ca. 1,800 m.y. old andesitic tuffs. The mineralization stages within veins of the Echo Bay mine are: 1. Quartz-hematite stage. 2. Pitchblende stage. 3. Co-Ni arsenides with native silver and bismuth stage. 4. Early acanthite stage. 5. Main sulfide and dolomite stage. (Within this stage, acanthite, chalcopyrite, bornite, sphalerite, and galena occur in up to three phases of mineralization.) 6. Late native silver stage. 7. Mckinstryite stage. Both primary and pseudo-secondary fluid inclusions, in the dolomite and quartz of the later stages, contain NaCI crystals. The heating experiments on these samples and the analyses on the extracted fluids indicate a salt concentration of about 30 weight percent and K/Na atomic ratios of H0.4 to H0.2. The temperatures of mineralization suggested by the occurrence of native bismuth and mckinstryite, the fluid inclusion filling temperatures, the sulfur isotopic compositions of coexisting sphalerite and galena, and the oxygen isotopic compositions of coexisting quartz and hematite were: around 120 ø C for the first four stages, about 200 ø C for stages 5 and 6, and below about 95 ø C for the last stage. The original depth of ore deposition suggested from the temperature and the stratigraphic data was between 2.5 and 4.5 km. The oxygen isotopic compositions of quartz, hematite, calcite, and dolomite samples, together with the temperature data, suggest that the oxygen isotopic composition of hydrothermal water was nearly constant throughout the entire depositional stages with /•O •8 values of q-l.0 =l= 2.5 • (SMOW). These/•O•8a2o values suggest very little, if any, contribution of magmatic water in the Echo Bay ore-forming fluids. The/•C •a values of dolomite and calcite of different stages are similar and within the range of-4 to-1.5• (PDB). However, the/•S s4 values of the vein sulfides show a very large spread of H50•, with a definite trend of increasing/•S a4 values toward later stage sulfides: from ca.-22• for the earliest sulfide (acanthite), H-4 to for stage 5 sulfides, to ca. q-27• for the last stage sulfide (mckinstryite). This wide range in the/•S a4 values of the hydrothermal minerals contrasts with the uniform/•S values (+2 to +5 ff•) of the pyrite in the host rock tuffs. The data on the mineralogy, temperature, and the sulfur and carbon isotopic compositions of the hydrothermal minerals were used to evaluate the chemistry of hydrothermal fluids. Both the mineralogical and the isotopic data appear to be best explained if: (1) the oxidation state of the fluids decreased continuously with time from that within the hematite stability field, through near the pyrite-hematite-magnetite triple point, to near the pyrite-pyrrhotite-magnetite triple point (also near the quartzmagnetite-fayalite boundary), (2) the pH remained relatively constant at around 4.0, (3) 2S content decreased from •-,10-2 to H10-4 t6ward the later stages, (4) 2C content decreased from H1 m to H10-2 toward the later stages, (5) 2Ag concentration in the fluids decreased from H1 ppm to H0.1 ppm toward the latter stages, and (6) the mean isotopic compositions of sulfur and carbon in the fluids remained constant at = q-25 =l= 3 and/•C•a•c =-5 =l= 2•. The/•OlSa2o and/•Sa4•s values and the initial high oxidation state of the hydrothermal fluids possibly reflect an origin as surface water. A model which involves the circulation and boiling of sea water caused by the intrusion of diabase at about 1,450 m.y. ago is presented to explain the concentration of salts, enrichment of O •s, reduction of oxidation state, decreasing of pH of the fluids, and leaching of heavy metals from volcanic rocks. 635 636 1•. W. ROBINSON AND H. OHMOTO Introduction THE Echo Bay mine, one of Canada's richest silver producers, is situated adjacent to the old Eldorado mine at Port Radium .on the east shore of Great Bear Lake, Northwest Territories (Figs. 1 and 2). It is about 50 km south of the Arctic Circle, or about 1,400 km north of Edmonton at longitude 118ø02'W and latitude 66ø05'N. The Eldorado mine produced silver and uranium concentrates during the period of 1934 to 1960, and Cominco operated the Echo Bay property from 1930 to 1941. The current mining operation at Echo Bay (Echo Bay Mines Ltd.) commenced in 1964 as an extension of Nos. 1 and 2 adits driven by Cominco in the 1930s. In 1967 a third adit was opened for high-grade silver and uranium ore recovery. The sinking of a three-compartment shaft was completed in 1968, and the shaft is now being extended to service eight levels. During 1971, the mine produced approximately 75 tons of silver and 300 tons of copper. The present study is one of several geochemical projects aimed at understanding the genesis of ore deposits in the Northwest Territories. In this paper, data on the mineralogy, paragenesis, fluid inclusions, and stable isotopes of oxygen, carbon, VICTORIA BEAR [AI• PORT / I SLAVE PROVINCE I ()2390 m.y.) I YELLOWKNIFE GREAT SLAVE LAKE-. ,•BOUNDARY OF •' ß CANADIAN SHIELD •. ___BOUNDARY OF ' BEAVERLODGE Fro. 1. Location map of the Echo Bay mine and the Bear Province, Northwest Territories, Canada. The ages of the basement rocks are from Wanless et al., 1968. and sulfur are presented. These data together with recently developed concepts on isotopic behavior in hydrothermal ore deposits (Ohmoto, 1972)are used to define the chemistry and the origin of ore-forming fluids. Geologic Setting The east shore of Great Bear Lake constitutes part of the Bear Province which contrasts in age (2,390-1,640 m.y.) with the adjoining Slave Province (>2,390 m.y.) (see Fig. 1). It is also a discrete metallogenic province characterized by NE-SWtrending faults and by vein type deposits of uranium, native silver and bismuth, cobalt-nickel arsenides, and copper sulfides. A detailed description of the geology and the results of geochronologic studies of the east (or McTavish) Arm of Great Bear Lake are presented in Robinson (1971) and Robinson and Morton (1972). They are also summarized in Table 1. Three sedimentary-volcanic stratigraphic groups, the Echo Bay (oldest), Cameron Bay, and Hornby Bay Groups, have been distinguished in the McTavish Arm. The former two groups crop out primarily in the eastern part, and the last in the northern part of the area. Intrusive rocks, which range in ages from •-1,800 to •-1,400 m.y. and in composition from feldspar porphyry, granodiorite, granite, to diabase, are also the predominant rock types of the area. The structure of the area is relatively simple. The sedimentary and volcanic rocks occur as northerly elongated roof pendants within the intrusive granitoids. Folding is not common. Three main fault and fracture systems are recognized: the E-W system, oldest and characterized by intrusions of diabase dikes; the NE-SW system, characterized by mineralized veins; and the N-S system, the youngest. 638 TABLE 1. B. W. ROBINSON AND H. OHMOTO Sttatigraphy and Geologic Events within the East Arm of Great Bear Lake Stratigraphic Column Geologic Events Age (m.y.) Paleozoics (?) < 600 Uplift/Erosion Intrusions of quartz-diabase sills & dikes/U-Ag mineralization/ Intrusions of giant quartz veins Faulting (NE trending) Intrusions of diabase dikes Faulting (E trending) Hornby Bay Group (300+ m) Subsidence > 1,500 Uplift/Erosion Regional metamorphism (?) •1,650 (?) Intrusion of granites & granodiorites •1,800 Intrusions of porphyries Cameron Bay Group (300+ m) Subsidence N1,800 ? Uplift/Erosion Upper Echo Bay SubGroup (1,500+ m) Volcanism •1,800 Lower Echo Bay SubGroup (1,200+ m) Cliff Series (300 m) Hornblende Porphyry Series (180 m) Tuff Series (390 m) Mine Series (350+ m
Gondwana Research, 2023
Field geology, geochemical analyses and multi-sensor remotely sensed data of Landsat-8, ASTER and Sentinel-2 were used to detect various types of hydrothermal alteration and sulfide-gold mineralization in Neoproterozoic metavolcanic and gabbroic rocks from Darhib and El-Beida areas in the South Eastern Desert (SED) of Egypt. We also used RADAR data of Sentinel-1 using PCI Geomatica software to decipher the structural lineaments that control this mineralization. Different styles of disseminated to massive sulfide deposits include: 1) undeformed magmatic disseminated ores in Darhib gabbros, metavolcanic rocks and metabasic dykes; 2) deformed (remobilized) massive ores mostly of a magmatic-hydrothermal origin in Darhib tremolite talc rocks along E-W shear zones; 3) hydrothermal sulfide ores along El-Beida NW–SE shear zones. Cu-Zn massive ores represent the main sulfides in the Darhib silicified tremolite-talc rocks and meta-andesites, and occur as bands, veins and patches along Darhib E-W shear zones. Discontinuous occurrences of sulfide ores-bearing tremolite-talc rocks indicate that major shear zones act as structural controls on mineralization. Sulfide ores consist chiefly of chalcopyrite, sphalerite, pyrite and galena as well as minor covellite and bornite. These ores are highly concentrated along E-W striking Darhib shear zones and resulted from granite-derived hydrothermal fluids that form/and or accumulate sulfide minerals from the sulfide-bearing metavolcanic rocks around shear zones. Two stages of hydrothermal alterations are recorded in the Darhib talc mine including pre- (talcification, amphibolitization and carbonatization) and syn- (silicification and chloritization) sulfide ore alterations. Disseminated sulfide mineralization hosted in Darhib gabbros, metavolcanic rocks and metabasic dykes is of a magmatic origin, similar to Cu-Ni sulfide deposits hosted in gabbroic varieties in worldwide mafic–ultramafic intrusions. On the other hand, El-Beida gold-sulfide-bearing quartz and quartz carbonate veins are hosted in ferruginated and silicified metavolcanics/ metavolcaniclastic rocks along NW–SE shear zones (Najd Fault System trend). These are spatially associated with regional convergent structures (high-angle convergent wrench structures), suggesting orogenic gold type mineralization. The E-W with minor WNWESE shear zones in Darhib and NW-SE trending shear zones in El- Beida structurally controlled sulfides and gold mineralization in the SED of Egypt. El-Beida sulfide mineralization occurs as disseminated ores that consist mainly of pyrite, chalcopyrite, covelite and chalcocite with occasional occurrence of gold. The spatial occurrence of native gold in ferrigenous alteration zones indicates the role of iron in gold precipitation through buffering of gold-bearing complexes. Mineralogical and geochemical data of Darhib and El-Beida sulfide-bearing rocks indicate that they crystallized from depleted mantle sources in a back-arc basin setting, starting with the generation of arc-like volcanic rocks in a nascent BAB stage followed by the emplacement of plutonic equivalent of MORB/OIB-like rocks such as gabbroic intrusions in a mature BAB.