Multi-element association analysis of stream sediment geochemistry data for predicting gold deposits in Barramiya gold mine, Eastern Desert, Egypt

Journal of African Earth Sciences 68 (2012) 1–14

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Multi-element association analysis of stream sediment geochemistry data for predicting gold deposits in Barramiya gold mine, Eastern Desert, Egypt Hassan Z. Harraz a,⇑, Mohamed M. Hamdy a, Mohamed H. El-Mamoney b a b

Geology Department, Faculty of Science, Tanta University, 31527 Tanta, Egypt National Institute of Oceanography and Fisheries, Alexandria, Egypt

a r t i c l e

i n f o

Article history: Received 22 September 2011 Received in revised form 19 March 2012 Accepted 20 March 2012 Available online 13 April 2012 Keywords: Surficial dispersion of Au Stream sediment survey R-mode factor analysis Principal component analysis Multi-element association geochemistry Gold

a b s t r a c t The use of traditional statistical methods can provide suitable indicators of geochemical element dispersion, and aids in targeting potential areas for mineral exploration. Analyzes of stream sediments from an ophiolite suite of ophiolitic mélange matrix and metasediments belt are used for regional geochemical prospecting of gold in the Barramiya mining district, Eastern Desert, Egypt. The principal rocks exposed in the study area are Late-Proterozoic volcano-sedimentary sequences intruded by serpentinite, small bodies of Older and Younger Granitoids, all injected by dykes of various compositions. Gold production derived mainly from shear zone with Au-bearing quartz veins hosted by ultramafic schists and serpentinites at fault intersections or along the basal décollement of the major thrusts, especially where granitoid massifs and stocks are common. Orebodies are mainly sulfide-bearing quartz and quartz–carbonate lodes associated with graphite-schist, listvenite and marble exposures, showing signs of structural control expressed in preferable orientation and consistent meso- and microfabrics. The area has two known gold deposits where several chromite mines are present. Auriferous veins are confined along E and ENE fracture systems and zones in a passive tectonic contact between the serpentinites and the metasediments. Results of 425 stream sediment samples from an area of 73 km2 analyzed for 13 trace elements are presented using simple statistical and R-mode factor methods. The overall sample density achieved by the survey is 6 samples/km2. Significant variations in background metal contents are recorded near the known mineralized sites. Preliminary visual interpretation of individual spatial distribution patterns of Ag, As, Au, Cu, Mo, Pb, and W show clear-cut relationships with known gold mineralization in the study area. Geochemical patterns of these elements delineate drainage basins with anomalous concentration of elements genetically related to gold mineralization. Gold in analyzed samples ranges from <0.02 to 3.51 ppm with average 0.21 ppm. Most of the high element concentrations in stream sediments are found in the graphite-schist and serpentinized marble rocks. Application of R-mode factor analysis indicates significant components of the sample composition. These reflect lithological, environmental and mineralization controls. Preparation of factor score map for the association Ag–Au–As–Cu–Zn–Pb–Mo–W enables a more precise delineation of zones of known gold mineralization as well as areas that may contain (on geological grounds) primary gold mineralization. The exploration significance of some anomalies has not been established, but a number of these anomalies may be related to undiscovered mineralization while others may be of no economic significance. Groundwater pH influences the hydromorphic dispersion patterns of Ag, As, and Au in different ways and this requires consideration during data interpretation. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Conventionally, geochemical exploration for gold is based on the assumptions that (1) gold is chemically inert in surficial environments; (2) gold occurs mainly in discrete grains; and (3) gold is transferred by mechanical means to form elastic dispersion halos and dispersion trains. Consequently, the commonly adopted ⇑ Corresponding author. E-mail address: [email protected] (H.Z. Harraz). 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2012.03.009

methodology has been (1) to determine gold in heavy mineral concentrates; (2) to use large samples in order to improve the reproducibility of gold analyzes; (3) to use high detection limits and thresholds; and (4) to determine total gold contents and pathfinder elements. However, these methods are not always successful in locating gold deposits, and they have limited application in the search for buried or blind deposits (Cloke and Kelly, 1964; Lakin et al., 1974; Carver et al., 1987; Nichol et al., 1989; Fletcher and Wolcott, 1991; Zeegers and Leduc, 1993; Melo and Fletcher, 1999). In Egypt, studies of the distribution and migration of particulate and

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ultrafine gold indicate that (1) gold is active and mobile in surficial environments; (2) gold occurs not only as discrete grains, but also as ultrafine particles; and (3) regional low-concentration gold anomalies as well as local anomalies over buried gold deposits originate from ultrafine gold and other complex forms of gold (Bugrov, 1974; Salpeteur and Sabir, 1989; Harraz et al., 2001). This study interprets results of a geochemical drainage survey carried out during the period 2004–2005 covering most of the drainage system at the Barramiya gold mine area (73 km2, Fig. 1), where 425 samples of stream sediment were analyzed for Ag, As, Au, Co, Cr, Cu, Li, Mo, Ni, Pb, Sn, W and Zn. In this approach, the surficial dispersion of Au and various elements in wadi sediments in close proximity of shear zone with Au-bearing quartz veins is determined to define or refine exploration efforts. Moreover, drainage basins with anomalous metal contents are delineated (visualized) to recognize any additional subtle though important geochemical patterns that may exist in the area. The study area is a part of Barramiya region in central Eastern Desert of Egypt (Fig. 1). For several decades the Barramiya area was widely known for its substantial gold production, derived mainly from shear zone with Au-bearing quartz veins hosted by

ultramafic schists and serpentinites at fault intersections or along the basal décollement of the major thrusts, especially where granitoid massifs and stocks are common. The Barramiya district in central Eastern Desert of Egypt hosts several gold occurrences with goldbearing quartz veins mostly situated along the east–northeasttrending Barramiya–Um Salatit ophiolitic belt. The Barramiya deposit is considered as a vein-type gold–arsenic mineralization, close to chromite, magnesite and antimony ores (Sabet and Bondonosov, 1984). Worked orebodies are mainly sulfide-bearing quartz and quartz–carbonate lodes associated with graphite-schist, listvenite and marble exposures, showing signs of structural control expressed in preferable orientation and consistent meso- and micro-fabrics. Gold was probably produced in the pre-Dynastic and Roman periods and later. The Barramiya gold deposit comprises four veins namely: the Main Lode, the Taylor,s Reef, the Counter Lode and the new Counter Lode, all extensively worked for many years. They were not completely exhausted and still contain valuable reserves. Depth of the excavations varied between <40 and 76 m, and the Au content ranged from <0.1 to 31 g/t, with the Ag/ Au ratio 1:4.6 (Hume, 1937, p. 791). The mine was abandoned at the end 1918 due to increased costs of mining operation, transport

Fig. 1. General geologic map of Barramiya gold mine area (modified from Shukri and Lotfi, 1955; EGSMA, 1992; Gad and Kusky, 2006; Zoheir and Lehmann, 2011).

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problems, and low gold price (Gabbra, 1986). More than 1.8 million tons of gold ore grading 2.8 g/t Au reserves are estimated in the mine (Gabbra, 1986). In addition, 318–500 tons of tailings and dumps with 3.15 g/t gold are stock-piled in three sites (Centamin Egypt Ltd., 2000). According to Pharaoh Gold Mines, Centamin Egypt Limited Co. (2001), the Barramiya gold mine contains multi-million ounces of gold. 2. Regional geologic setting The surveyed area (73 km2, Figs. 1 and 2) comprises Late-Proterozoic metasediments intruded by serpentinized ultramafic bodies, small bodies of deformed gabbro-diorite, Older and Younger Granites, all injected by dykes of various compositions. The volcano-sedimentary sequence occupies 80% of the Barramiya gold mine area, and are composed dominantly of pelitic and semipelitic schists, quartzites and marbles (Shukri and Lotfi, 1955). These volcano-sedimentary sequences include the following varieties: graphite-, actinolite-, actinolite-tremolite, chlorite-sericite, and hematite-sericite-schists. Successions of finely banded quartz–sericite and graphite-bearing quartzo-feldspathic schists admixed with metagreywacke and –siltstone are locally intercalated with numerous lenticular bodies of massive and tectonized serpentinites (with harzburgite-dunite-matrices), talc-carbonate, tremolite-talc schist and listvenite (c.f. El-Bedawi et al., 1983; Osman, 1995; Zoheir and Lehmann, 2011). However, the volcanosedimentary sequence is seen fringing the serpentinite masses, and also as small roof pendants within the serpentinite masses. Serpentinites form huge hilly masses elongated in a general ENE trend concordant with that of foliation of the ophiolitic mélange matrix. Therefore, the Barramiya serpentinite belt is a part of an island arc system that is highly tectonized in an ENE direction.

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These masses are parts of an ophiolite suite and usually there is a passive tectonic contact between serpentinites and metasediments (Shackelton et al., 1980; Azer and Stern, 2007). Deformed gabbro-diorite rocks cut the ophiolitic mélange matrix and metasediments in the central part of the study area. On the other hand, several 10 s km-across intrusions of heterogeneously foliated, syn-orogenic quartz–diorite/granodiorite (i.e., Older Granitoids) cut through the ophiolitic rocks in the northwestern part of the study area. At the mine area, intensively weathered granite porphyry and elongate (E–W) granodiorite bodies cut the main foliation of the mélange matrix. Discrete elongate bodies of post-orogenic monzogranite and granite porphyry (i.e., Younger Granites) are bound to intersection zones of steeply dipping E–W and NW–SE trending faults. Post-granite dykes including quartz porphyry, rhyolite, dacite and less common basalt are oriented in different directions in the mine area. The tectonically admixed sedimentary/volcano-sedimentary and ophiolitic rocks form the ophiolitic mélange matrix in the area. Along the thrust zones, talc, magnesite, marble and chromite form veinlets, nodules or irregular pockets in the sheared ultramafic rocks and metabasalt. The serpentinite-metasediment zone around Barramiya gold mine area belongs to the older metasedimentary gold metallogenic province (Pohl, 1988; Botros, 2002). The serpentinite-schist zone is the most important rock unit from the point of view of its economic potential in this area. The economic interest arises from the presence of auriferous quartz in the central part of the area, chromite and magnesite in the northern and western parts, as well as talc in the most northeast part of the area. Auriferous quartz and magnesite veins cross-cut talc-carbonate rocks, in some places (Hamdy, 2007). Chromite deposits occur within the serpentinite and associated with the talc-carbonate rocks in lenticular, disseminated, thin veins, banded and nodular

Fig. 2. Drainage network and sample locations (n = 425) of stream sediment over the basement complex, Barramiya area.

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forms (Ahmed et al., 2001). The podiform nature of the Barramiya chromites is documented by geochemical and mineralogical features (Anwar et al., 1969; Azer and Stern, 2007) as well as by the common nodular type of ore. Marbles which are coarse grained metamorphosed calcitic or dolomitic rock are the well representatives of metasediments and hosted by sheared and altered ophiolitic serpentinized ultramafic rocks at the northern and northwestern parts of the area, where auriferous and uraniferous marbles (0.98–2.76 ppm Au, Hamdy and Aly, 2011) were recorded. Marble is usually gray to grayish white and occurs in pod-like and bedded shapes (5–8 m thick and up to 100 m long). Their country rocks are made principally of altered serpentinite and sometimes occur as fragments within marble. The contact between marble and serpentinite is usually not sharp. At this contact, the serpentinites are usually highly sheared, foliated, sometimes folded, and become rich in carbonates, graphite, magnesite, and chlorite, and their metamorphism (Pan-African) was retrograde. Peak of metamorphism was at greenschist facies to form metamorphic dolomite and calcite, and tremolite, then went down to the upper subgreenschist facies forming chlorite (Hamdy and Aly, 2011). 2.1. Gold mineralization The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Gold mineralization was worked in ancient times from several quartz veins scattered over an area of 24 km2 and mainly cut through graphite- and actinolite-schists, and highly sheared serpentinites. The auriferous quartz veins are composed of gray and dark bluish-gray quartz containing finely dispersed gold and are confined to ENE long regional fracture systems and schistosity planes in metasediments. Orebodies are mainly sulfide-bearing quartz and quartz–carbonate lodes associated with graphite-schist, listvenite and marble exposures, showing signs of structural control expressed in preferable orientation. Orebodies are made up of early bluish/gray quartz fractured and sealed with a late milky quartz phase, or mainly of milky quartz with appreciable amounts of Fe-carbonate (ankerite and magnesite) and wallrock materials. Less commonly, aggregates and fibers of magnesite–siderite ± calcite occur as replacement phases filling the open vugs in some veins, and are associated with comb quartz. Most veins exhibit boudinage structures along the strike and dip, and taper off at ends. Stylolitic and sheeted structures are common where slivers of carbonaceous wallrocks are associated with elongate quartz ribbons. Gold mineralization is closely associated with hydrothermally and metasomatically altered bands of ultramafic metasedimentary rocks (averaging 1 m thickness), almost entirely serpentinitized and later transformed into talc-carbonate rock. Most of the auriferous veins cut graphite- and actinolite-schists that are closely associated with listvenite, talc and highly ferruginous shear zone extending E–W (El-Ramly et al., 1970; El-Bedawi et al., 1983; Osman, 1995). The hydrothermally altered country rocks are important targets to exploited gold at Barramiya area. The main processes of the hydrothermal alteration are silicification, ferrugination, listvenitization, carbonatization, talcization ± sericitization ± chloritization. Listvenites and the graphite schist rocks enclosing the ore veins are often intensively fissured and filled with quartz, aggregates of chromite, chlorite, antigorite and sulfide minerals. All the hydrothermally altered zones contain gold (El-Bedawi et al., 1983; Harraz et al., 2001). The wallrock selvages comprise graphite-schist, carbonatized actinolite schist and listvenite, with or without carbonaceous material. These selvages are rich in disseminated pyrite–arsenopyrite–chalcopyrite and are characterized by microbrecciation, slip, and annealing textures. An assemblage of chlorite ± graphite–carbonate minerals occupies the serrate planes between deformed quartz crystals, outlining a

distinct stylolitic texture. Most gold-sulfide mineralization is confined to the stylolitic planes in recrystallized quartz veins, where disseminated sulfides are associated with a carbonaceous material. However, gold is almost always present in a free state in quartz gangue with a marked tendency to cluster in close proximity to alteration zones of carbonatization. The geochemical ability of carbonaceous materials to absorb sulfides and gold was discussed by Cameron (1979), Liu et al. (1999) and Likhoidov et al. (2007). Earlier investigations (Hume, 1937; Sabet et al., 1976) had demonstrated the association of pyrite, arsenopyrite, chalcopyrite, copper oxide and hydroxide, scheelite, molybdenite, and chromite with the Au-mineralization. Zoheir and Lehmann (2011) showed that ore minerals disseminated in quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. Hamdy and Aly (2011) recorded specks of gold and sulfide disseminations in marbles at the northern and northwestern parts of the area (Fig. 1). Native nuggets (20–35 lm) having globule, rod, crescent, and irregular streak shapes occur in pores, vugs, and fissures of marble. Other secondary phases include uranium minerals (i.e., autunite, uranophane, carnotite and uranothorite), covellite, chromite, hematite, goethite, limonite, bunsenite (NiO), and danbaite [(Cu–Zn) O], together with an assemblage of accessory minerals such as apatite, monazite, allanite, zircon, baddeleyite, halite, and sylvite. Other gangue minerals include sericite/mariposite, rutile, clinopyroxene, amphibole, talc, Fe-carbonate, chlorite, pyrophyllite, kaolinite, and graphite. 3. Geomorphology 3.1. Drainage features The drainage network of the Barramiya gold mine area is studied by the present authors to delineate its possible lithological and/ or structural controls. The present drainage network was mainly formed during early Quaternary humid periods. During the Quaternary, semi-arid to humid climatic periods alternated with arid periods. The drainage network is shown in Fig. 2, where the area is drained by a few main drainages, that possess numerous tributaries. The area is dissected at the northwestern and western parts by relatively large tributaries, that flow south-southwest to the west. While the northern, northeastern, and eastern parts of the area are drained by tributaries flowing to the north and south. Moreover, the southern and southeastern parts of the area are drained by tributaries flowing to the east and the west (Fig. 2). The network is a part of a vast watershed whose main wadis are directed to the southwest and run towards the River Nile following the general SW slope of the main road of Idfu-Mersa Alam (Fig. 1). The drainages are mature with broad gentle sloping valleys. In the eastern part of the area, there are high banks of ancient gravel through which later relatively large tributaries have cut their way, leaving terraces (Fig. 1). Therefore, the drainage network of the Barramiya area is radial, reflecting its dome-like structure. However, structural features affecting this area locally modify the drainage configuration to trellis, rectangular, subparallel or coarse dendritic patterns. 3.2. Physiography The topography of the Barramiya gold mine area is rugged and changes in altitude are abrupt and is for the most part undulating, with an average elevation of 500 m above sea-level, except in the northwest, where the landscape is quite well developed. Gentle

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mountain slopes and wadi floors are covered by recent eolian and alluvium. Alluvial sediments in upper parts of drainage systems and wadis are composed mainly of blocks and boulders, but downstream often comprise gravel and sands. Eolian (clay and sand) material is trapped in topographic hollows and wadi channels, where small bushes and grass grow. The study area has an extremely arid climate. Rainfall is usually scarce or is practically nil (<25 mm/y) leading to rare torrents (or dust storms) which fill the valleys with clast assemblages, ranging from boulders to fine silts and clays, weathered from source rocks in the drainage basins. Weathering and erosion are mainly by mechanical processes. Although diurnal temperature can be quite marked, there is little seasonal variation. Summer is very hot (but cools at night) and winter is mild and windy. The mean daily maximum temperature is between 35 and 45 °C, with annual mean evaporation 3500 mm/y. The nature of drainage sediments is largely a function of topography, local bedrock geochemistry and the influence of weathering as well as other secondary factors tending to modify the bedrocksoil-stream sediment relationships (Nichol et al., 1969; Melo and Fletcher, 1999; Halfpenny and Mazzucchelli, 1999). Sediments of streams draining out of the northern, western, southeastern and northeastern parts of the survey area are largely composed of clast assemblages derived from serpentinites and talc-carbonates. In the northwestern part of the area the stream sediment is largely siliceous derived from granitic and serpentinized marble rocks. In the southeastern and middle parts of the area the stream sediments are largely derived from metasediments.

3.3. Physical dispersion processes During the hot season hot ascending air-currents produce whirlwinds, which mobilize the surficial clay-sand material. During occasional rainstorms (once in four years) the upper silty-argillaceous layer is mobilized with the surficial water flowing on the reg pavement. This material is redeposited on the margins of the wadis. In flat sites the coarse material is not moved. In moderate to hilly relief if the rainfall lasts long enough (more than 1 h) gravity flows occur and the water content in the upper clayey layer increases creating mudflow conditions. The low permeability of the upper layer and lack of vegetation give way to a sudden influx of clayey water from the tributaries into the main channel or in temporary playas or in topographic hollows (playas and khabras). In wadis the water first erodes the upper eolian sand-silt deposits around the bushes. When the water influx climax is reached all the alluvial material is moved in a mechanism very similar to that of mudflows. When the rain stops, the flow energy decreases and the water lose its sand-clay load. Reworking of the top alluvium by braided streams creates crossbedded deposits on the bankets and black patches of heavy minerals on the surface. An understand this flow mechanism is of prime importance in the discussion of gold mobility in present-day drainage. This in particular explains the lack of sorting in the main alluvial sediments and the lack of enrichment of heavy minerals at the alluvium/bedrock interface as currently observed in temperate or tropical climates. A matter which led to bias of any optical determination of gold on a nonmagnetic fraction of heavy pan concentrated samples. In addition nugget growth is practically absent in the present arid conditions. Fig. 3 shows a typical alluvial sequence. The overall thickness of the sequence ranges from 50 to 100 cm. In some sites, one or two intermediate horizons are missing. The rapid evaporation of water (3500 mm/y) gives some nodular gypsum or carbonate precipitation in the brown intermediate sandy-gravel sediments.

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Fig. 3. Wadi-sediment profile at Barramiya gold mine area.

Reg pavement is equivalent of the Australian lag or gibber (Carver et al., 1987), but in Australia it is mainly composed of siliceous or ferruginous fragments of a pre-existing laterite cover. This is not the case in Eastern Desert of Egypt because most of the paleosoils were eroded during the Miocene. This is the main reason why gossans are exceptional on the Eastern Desert of Egypt (the present-day rainfall is too low; <25 mm/y; to induce severe oxidation of the sulfide ores).

4. Sampling and analytical techniques Some 425 samples were collected systematically from firstand second-order tributaries drainage channels over an area of 73 km2 (Fig. 2). The overall sample density achieved by the survey was 6 samples/km2. Samples were taken as close to the center of the wadi-channel as possible where small bushes and grass grow as well as debris of bedrock outcrops. About 10 kg of channel samples were taken from a depth of 15–30 cm below the surface and at intervals of 200–250 m along the drainage channels. Samples were air-dried and then split to obtain 3 kg of material, which was sieved in the field with retention of the 1 mm fraction which was re-sieved in the laboratory and the 1 to +0.25 mm fraction was pulverized for chemical analysis. Past experience of sampling in the Eastern Desert of Egypt (Bugrov, 1974; Levinson, 1980; El-Makky, 1981; Nichol et al., 1989) proved that these sampling methods are the most economical and useful for geochemical prospecting using stream sediments where clasts are being derived from local bedrocks. The remaining sample (7 kg) was panned by hand, then the heavy concentrates were separated with bromoform and the nonmagnetic fraction processed by a Frantz magnetic separator to select the enriched fraction for microscopic examination for gold. The pulverized samples were ground and digested in a concentrated acid mixture consisting of 2 ml HNO3, 2 ml HCl and 2.5 ml HF (Langmyhr and Paus, 1968, 1970). Solutions were analyzed for Ag, Co, Cr, Cu, Li, Mo, Ni, Pb, Sn, W and Zn using a Varian 10+ atomic absorption spectrophotometer (AAS). Arsenic was determined colourimetrically after KOH fusion. Gold was determined by AAS with a graphite furnace after organic extraction with methyl isobutyl ketone (MIBK). Analyzes were done by El-Mamoney at the National Institute of Oceanography and Fisheries, Alexandria-Egypt. Detection limits were Ag 0.1 ppm; As 30 ppm; Au 0.02 ppm; Co, Cr, Ni and W 3 ppm; Cu, Mo, Pb and Sn 1 ppm; Li and Zn 5 ppm. Due to duplicate analyzes of the above-mentioned elements systematic errors were avoided. The analytical precision for each element, determined from 46 duplicate samples using the method of Garrett (1969), expressed as the coefficient of variation, was ±7% for Ag, Cu, Li, Mo, Pb, W and Zn; ±12% for Au, Co, Cr, Ni and Sn; ±20% for As.

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5. Statistical treatment of data 5.1. Single-element distribution Results of chemical analyzes are summarized in Table 1 and illustrated in Fig. 4. They show variations in the distribution of the majority of elements with bedrock type. In particular Co, Cr, and Ni tend to be enriched over areas of serpentinites, talc-carbonates and metasediments, whereas Li, Mo, Sn and W are preferentially concentrated in the granitoids. Marked regional variations are noted in the mean contents of certain elements in the stream sediments associated with different sites of both rock varieties.

In contrast, patterns of Cu, Pb, and Zn do not appear to be related to a particular bedrock-type. Ag, Au, As, Cu and Mo; Cr; as well as Pb, Sn and W are predominantly associated with the distribution of auriferous quartz, chromite and mineralized greisens, respectively. Sediments derived from serpentinites are markedly enriched in Cr, Ni and Co relative to those from metasediments that are in turn somewhat higher than in sediments derived from granites (Table 1). Li, Pb, Sn, W and Mo show a significant enrichment in the latter (Fig. 4). Au, Ag, and As in sediments derived from both serpentinites and metasediments are relatively higher than in sediments derived from granite (Table 1). Classification of samples in terms

Table 1 Statistical parameters of some selected trace elements content of stream sediments associated with various geological units. Element

Range (ppm)

0

00

Mean (ppm)

Cb (ppm)

S

S

S (%)

t1 (ppm)

t2 (ppm)

Serpentinite (n = 173) Ag <0.10–5.85 As <15–1000 Au <0.02–2.74 Co 5–475 Cr 200–45,000 Cu 24–372 Li <5–22 Mo <1–37 Ni 50–3000 Pb 4.5–175 Sn <1–13 W <3–16 Zn 15–790

0.62 88 0.23 90 3129 99 7.00 2.77 957 26 3.35 2.81 162

0.22 22 0.05 71 1786 79 5.55 1.57 796 15 2.16 2.20 116

1.04 164 0.48 73 6012 71 4.73 4.05 545 33 2.53 2.57 131

3.936 4.732 5.508 1.914 2.477 1.910 1.999 2.710 1.936 2.655 2.877 1.854 2.355

167 186 207 81 192 72 67 146 57 128 76 91 81

2.7 415 1.2 237 15,152 241 16 11 2047 92 8.4 8.0 424

3.5 493 1.4 260 10,958 288 22 12 2983 106 18 7.6 643

Metasediment (n = 204) Ag <0.10–6.27 As <15–800 Au <0.02–3.51 Co 20–475 Cr 200–6000 Cu 21–320 Li <5–35 Mo <1–17 Ni 60–2500 Pb 4.5–138 Sn <1–19 W <3–65 Zn 30–885

0.59 81 0.21 77 1338 111 6.81 1.98 714 25 4.34 3.90 175

0.25 27 0.05 65 1128 96 5.21 1.21 567 17 3.16 2.45 134

0.96 127 0.43 57 830 64 5.83 2.62 479 26 3.09 6.91 124

3.758 4.385 5.675 1.718 1.820 1.722 2.009 2.489 2.028 2.203 2.483 2.133 2.138

162 156 203 74 62 57 86 132 67 105 71 177 71

2.5 334 1.1 191 2997 239 18 7.2 1673 76 11 18 423

3.5 519 1.6 192 3736 285 21 7.5 2332 83 19 11 613

Granite (n = 48) Ag As Au Co Cr Cu Li Mo Ni Pb Sn W Zn

<0.10–3.10 <15–215 <0.02–1.44 5–85 20–300 27–209 <5–89 <1–35 15–400 6–154 <1–57 <3–27 50–390

0.45 51 0.16 30 88 98 28 7.97 70 27 15 5.31 176

0.25 27 0.07 23 64 85 20 4.76 49 19 9.08 3.13 160

0.57 55 0.24 21 77 50 24 8.18 75 29 15 6.66 74

3.048 3.281 4.365 2.198 2.196 1.730 2.466 2.786 2.134 2.148 2.979 2.553 1.570

127 106 149 68 87 51 83 103 108 107 100 125 42

1.6 161 0.6 71 241 198 76 24 221 86 44 19 324

2.3 291 1.3 111 309 254 120 37 223 88 81 20 394

All data (n = 425) Ag As Au Co Cr Cu Li Mo Ni Pb Sn W Zn

<0.10–6.27 <15–1000 <0.02–3.51 5–475 20–45,000 21–372 <5–89 <1–37 15–3000 4.5–175 <1–57 <3–65 15–885

0.59 81 0.21 78 1967 105 9.32 2.98 760 25 5.12 3.62 170

0.24 25 0.05 61 1054 88 6.21 1.57 532 16 3.05 2.42 129

0.96 128 0.32 65 4006 66 12 4.56 534 27 6.60 5.59 123

3.758 4.106 5.047 2.042 3.034 1.816 2.301 2.851 2.016 2.193 2.915 2.084 2.138

163 158 150 83 204 63 129 153 70 108 129 154 72

2.5 337 0.9 208 9979 237 33 12 1828 79 18 15 416

3.3 421 1.2 254 9702 290 33 13 2162 77 26 11 590

0

Cb, local background; S, standard deviation; S , geometric standard deviation; S00 , coefficient of variation; t, threshold {t1 = (x + 2S); t2 = (Cb(S0 )2)}.

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Fig. 4. Bar diagrams showing the range and the arithmetic mean ( ) for stream sediments associated with various lithologic units at Barramiya gold mine area (Dotted line represents concentrations below the detection limits; solid line shows the total range).

of their bedrock type is therefore a critical matter. On the other hand, erratic distributions are displayed by threshold values of elements that occur in markedly varying concentrations in various lithologies (Table 1). In order to facilitate the assessment of geochemical patterns, maps are presented to show the distribution of selected elements of economic or exploration significance (Figs. 5 and 6). A preliminary visual interpretation of individual spatial distribution patterns of Ag, As, Au and W shows clear-cut relationships with known gold mineralization in the study area. Most of these elements are excess

the regional thresholds (Mean + 2 Standard deviation, Table 1) being confined to areas around the Barramiya gold mine (Figs. 5 and 6). In these areas the occurrence of Au-bearing quartz veins and veinlets and disseminations in graphite schists and serpentinized marbles have been reported and the enhancement of these elements may be related to them. Gold values in drainage sediments are generally low and erratic, only few samples have values exceeding 1 ppm (Fig. 5). The area around the Barramiya gold mine, which contains the highest density of old gold workings, is characterized by Au contents that generally exceed 0.2 ppm.

Fig. 5. Distributions of Au, Ag, Cu, As, Pb, and Zn in drainage sediment over the basement complex, Barramiya area.

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Fig. 6. Distributions of Mo, W, Sn, Co, Cr, and Ni in drainage sediment over the basement complex, Barramiya area.

Values greater than the regional threshold (1.2 ppm) are confined mainly to the northern and northwestern parts of the study area, nearly to Au- and U-bearing marbles (Fig. 6). Mo contents (Fig. 4) range from <1 to 37 ppm with a geometric mean concentration of 1.57 ppm. Values greater than the regional threshold (13 ppm) are confined mainly to the northwestern part of the study area (Fig. 6). Mo anomalies in stream sediments are somewhat more restricted geographically than Pb, Sn and W anomalies. This limited distribution is possibly due to the high solubility of Mo in the low-pH water (pH 4.5; Rose et al., 1979) of the study area where stream sediment derived from granites. Also, molybdenite is highly susceptible to mechanical weathering and most of the particulate molybdenite may be washed away in this terrain of moderate relief and fast-moving streams (Levinson, 1980; Harraz et al., 2001). Therefore, any particulate molybdenite found in the detrital material is usually near its source. These factors result in Mo anomalies clustering in the same areas as Pb, Sn and W, but closer to the source rocks. The abundant anomalies in the NW part of the area represent a greisen mineralization, which is in part related to the Younger Granites (Figs. 1 and 6). As expected the elements associated with the ores show high coefficients of variation in all samples draining out from all sources (Table 1). Other elements (e.g. Co, Cr, Li, Ni and Sn) in samples draining the different rock media often have smaller coefficients of variation. However, there is one exception, Cr, which has a high standard deviation for all samples draining the different rock media. In general, a belt of low metal values trends N–S through the eastern part of the area and gradually to the area with higher metal contents to the central, northern, and northwest (Figs. 5 and 6). The precise form of the regional distribution pattern of elements varies from one element to another and, for certain elements, local

patterns are superimposed on the regional distribution. For example, the N–S trough is shown very clearly by Ag, As, Au, Mo, Pb, Sn, W, and Zn, but in the cases of Co, Cr, Cu and Ni the overall pattern is not so clear (Figs. 5 and 6). The major factor controlling the distribution of trace elements in stream sediments is the composition of the bedrock, but in places variation in topography appears to have a modifying effect on the bedrock-stream sediment relationship. Therefore, it would be necessary to take fully into account the varying influence of local secondary factors related to weathering, transportation and deposition that serve to modify the rock-soilstream sediment relationship (Lakin et al., 1974; Nichol et al., 1989; Fletcher and Wolcott, 1991; Zeegers and Leduc, 1993; Melo and Fletcher, 1999; Hamdy and Aly, 2011). Problems of interpretation of regional pattern of elements in a drainage survey fall mainly into two categories: (i) discrimination of regional trends and relatively minor anomalies of regional data, and (ii) identification of variations related to bedrock mineralization and secondary environment. Application of factor analysis aids to improve interpretation of regional geochemical data. 5.2. R-mode factor analysis The compilation and objective interpretation of multielement geochemical data from a large number of samples is extremely tedious and difficult. Quantitative statistical treatment of geochemical data as a useful and even a necessary technique in geochemical interpretation is widely accepted and practiced (see for example Nichol et al., 1969; Closs and Nichol, 1975; Swan and Sandilands, 1995; Davis, 2002; Ali et al., 2006). The initial step in calculating R-mode factor analysis is to compute a correlation matrix. The latter was calculated using

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H.Z. Harraz et al. / Journal of African Earth Sciences 68 (2012) 1–14

chalcophile in nature and obviously related to gold mineralization (Fig. 7). Elements of this factor occur in pyrite, arsenopyrite, copper oxide and hydroxide, and sphalerite. Scheelite is widely distributed in wadi sediments, might also be associated with the elements of factor 1 (Hume, 1937). This metal occurs in the area underlain by the graphite-schist in contact with talc-carbonate at the central part of the study area. High factor scores, obtained at the northern and northwest parts of the study area, correlate spatially with the area underlain by Younger Granites at the contact with the serpentinized marble (Fig. 7). Moreover, specks of gold and sulfide disseminations were observed in quartz veins invading the Younger Granite samples as well as native nuggets (20–35 lm) having globule, rod, crescent, and irregular streak shapes, in pores, vugs, and fissures of serpentinized marble. A comprehensive review of the geochemistry of As and its prospective value as a pathfinder for Au has been given by Hale (1981) and Samal et al. (2008). Although the abundance of As in factor 1 is high, the element exhibits strong siderophile to chalcophile characteristics, which suggest a marked geochemical contrast between its concentrations in sulfide minerals as compared to silicate or carbonate rocks. On the other hand, carbonates have marked chemical favorability for infiltration of hydrothermal fluids (Liu et al., 1999). Thus, the metacarbonates (graphite and marble) are probably the host for many ore minerals. In addition, the post-collision magmatism and even surficial alteration and weathering could also be agents of enrichment and mineralizing the graphite schist and serpentinized marble rocks.

log-transformed values (Table 2). The majority of analyzed elements in stream sediment samples are positively correlated. The results demonstrate that Au is strongly correlated with Ag, As, Cu, Mo, Pb, W and Zn and weakly correlated with Co. Cr–Co–Ni and Li–Sn–Mo–Pb–W have significant inter-correlation. A preliminary examination reveals that three groups of the ore elements are moment correlated with positive correlations at r-values at 0.01% level of significance (Table 2). Therefore, the variables can be classified into three dispersion patterns: (1) Ag–As–Au–Cu– Pb–Zn–W–Mo–Co; (2) Co–Cr–Ni; and (3) Li–Sn–Mo–W–Pb. Although some elements (i.e. Co, Pb, Mo and W) are common in more than one dispersion pattern, others are characteristic of only one. These dispersion patterns are consistent with the three ore deposits well recorded in the Barramiya area (i.e. Gold, chromite and wolframite, see Gabbra, 1986). The low correlation between other trace elements perhaps could result from there being more than one group of elements, and is depending on dispersion of these trace elements in the three draining system. In order to investigate more subtle features in the data, factor analysis is employed. The R-mode factor multivariate analytical technique essentially measures correlation between variables on the basis of their mutual linear correlation coefficients. Closs and Nichol (1975) indicated that selection of the most appropriate model is based on the recognition of metal associations considered meaningful in terms of geological or surface processes. R-mode factor analysis is used to resolve the intercorrelations of the geochemical variables and to explain the observed relations among the numerous variables in terms of simpler relations. The Varimax matrix of the log-transformed data is shown in Table 3. All calculations of R-mode factor analysis were performed by SPSS-programme (SPSS 15.0 for windows: Norušis and SPSS Inc., 2006). After inspection of various factor models that were computed, a three-factor model that accounted for 74.10% of the data variability (Table 3) was considered to be the most consistent with the known geological and environmental processes. Only variables with loadings greater than 0.40 were considered as significant members of a particular factor. There is a general tendency for samples classified on geological basis as draining metasediment bedrock to have high scores of factor 1 (Fig. 7). In contrast, samples derived from serpentinites or granites tend to have low scores of factor 1. However, certain sediment samples classified geologically as derived from granites are shown statistically to have metasedimentary affinities and some metasedimentary samples have serpentinitic affinities, indicating the inadequacies of the classification based on the geological basis. To illustrate areas of high factor, factor scores were also computed and selected maps are shown in Fig. 7.

5.2.2. Factor 2 (Ni–Co–Cr) Factor 2 accounts for 30% of the data variability of this model and reflects, essentially, a mafic lithological control. The metal association map (Fig. 7) and the single element Co and Cr maps (Fig. 6) are also similar. The areal distribution of factor scores shows high values, lying dominantly within the area underlain by chromite lenses hosted by the serpentinite and talc-carbonate rocks at the central and northern parts of the area (Fig. 7). Isolated high scores in metasediment areas often correspond to streams that drain lenses of talc-carbonate (Fig. 1). The combination of this element association together with the areal disposition of factor scores is consistent with the interpretation that this factor is a strong indicator of chromite deposits, thus reflecting the close relation between them in the parent rocks and chromite lenses as a dominant influencing factor. 5.2.3. Factor 3 (Li–Sn–W) Factor 3 accounts for 9.5% of the data variability accounted for by three-factor model (Table 4). There is also some contribution from both Mo and Pb. This metal association coupled with strong positive correlation reflect their similar affinity. High scores for this

5.2.1. Factor 1 (Ag–Au–As–Cu–Zn–Pb–Mo–W) Factor 1 accounts for 61% of the variability of this model with some contribution from Co (Table 4). Factor 1 is definitely

Table 2 Correlation coefficient matrix for trace elements in the stream sediments of the Barramiya area.

Ag As Au Co Cr Cu Li Mo Ni Pb Sn W Zn

Ag

As

Au

Co

Cr

Cu

Li

Mo

Ni

Pb

Sn

W

Zn

1.00

0.81 1.00

0.88 0.81 1.00

0.39 0.38 0.33 1.00

0.15 0.12 0.07 0.59 1.00

0.72 0.71 0.68 0.30 0.12 1.00

0.29 0.27 0.29 0.10 0.24 0.27 1.00

0.56 0.56 0.60 0.02 0.21 0.48 0.50 1.00

0.07 0.10 0.12 0.57 0.75 0.15 0.24 0.23 1.00

0.68 0.66 0.62 0.33 0.05 0.68 0.40 0.42 0.13 1.00

0.24 0.22 0.26 0.15 0.24 0.29 0.58 0.40 0.24 0.39 1.00

0.54 0.60 0.58 0.31 0.09 0.50 0.44 0.52 0.19 0.57 0.35 1.00

0.67 0.67 0.66 0.25 0.07 0.60 0.26 0.48 0.12 0.58 0.27 0.40 1.00

Critical value of r = 0.321 for n = 425 samples at 0.01% confidence level.

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H.Z. Harraz et al. / Journal of African Earth Sciences 68 (2012) 1–14

Table 3 Metal associations, element loadings and data variability (%) of different factor models of trace element contents of stream sediments. Factor

Factor model

F1

Ag As Au Cu Pb Zn W Mo Co Li Cr Ni Co Li Sn Mo

2

F2

3

0.91 0.89 0.89 0.82 0.80 0.77 0.71 0.65 0.47 0.40 0.88 0.85 0.68 0.62 0.61 0.42

F3

4

5

8

Ag Au As Zn Cu Pb Mo W

0.89 0.87 0.86 0.84 0.82 0.70 0.57 0.45

Ag Cu As Zn Au Pb Mo W

0.87 0.85 0.83 0.83 0.83 0.75 0.47 0.41

Ag Cu As Zn Au Pb Mo W

0.87 0.84 0.83 0.83 0.83 0.76 0.48 0.42

Cu Pb Ag As Au Zn

0.90 0.79 0.69 0.64 0.60 0.45

Au Ag As Cu Zn Pb Mo

0.91 0.90 0.78 0.57 0.52 0.48 0.44

Ni Co Cr

0.88 0.83 0.82

Ni Cr Co

0.90 0.85 0.79

Ni Cr Co

0.92 0.86 0.77

Ni Cr Co

0.92 0.86 0.77

Ni Cr Co

0.92 0.88 0.75

Ni Cr Co

0.92 0.88 0.75

Li Sn W Mo

0.83 0.77 0.57 0.40

Sn Li

0.87 0.75

Sn Li

0.87 0.76

W

0.79

Mo Au

0.81 0.48

Cu Pb

0.70 0.62

W

0.77

W Pb

0.77 0.40

Li Pb

0.82 0.40

0.83

0.69

Sn

0.90

Li

0.84

Mo

0.69

0.83 0.43 0.40 0.40 0.80 0.44 0.83 0.91

W

Mo

Zn Au Ag As W Co Li Sn

F5

67.10

7

0.91 0.90 0.89 0.82 0.80 0.72 0.61 0.55

F4

F6 F7 F8 Data Variability

6

Ag Au As Cu Zn Pb Mo W

74.10

78.90

83.30

86.60

Fig. 7. Factor score maps in drainage sediment over the basement complex, Barramiya area.

89.70

Sn Zn Mo

0.91 0.81 0.76 92.30

H.Z. Harraz et al. / Journal of African Earth Sciences 68 (2012) 1–14 Table 4 R-mode varimax factor matrix. Element

Communality

F-1

F-2

F-3

Ag Au As Cu Zn Pb Mo

0.852 0.830 0.827 0.694 0.641 0.654 0.598

0.906 0.898 0.894 0.816 0.797 0.719 0.611

0.116 0.038 0.096 0.059 0.011 0.130 0.250

0.135 0.147 0.137 0.160 0.078 0.347 0.403

Ni Co Cr

0.846 0.802 0.802

0.178 0.326 0.118

0.878 0.832 0.820

0.208 0.059 0.341

Li Sn W Eigenvalue % of Var. Cum%

0.780 0.670 0.633

0.195 0.156 0.550 5.88 61.00 61.00

0.217 0.243 0.106 2.84 29.55 90.55

0.834 0.766 0.565 0.91 9.45 100.00

factor were scanty, but the areas of serpentinites and talc-carbonates are characterized by very low scores. The association of Li, Sn, W and Pb is typically of felsic rocks and is, therefore, considered as lithologically controlled. The distribution map of Factor 3 (Fig. 7) shows a few scattered clusters that correlate spatially with the area underlain by the highly weathered Older Granites, thus reflecting the parent rocks and greisen as dominant influencing factors. On the other hand, a few scattered cluster distributions of the factor scores, may reflect in part the irregular occurrence of greisens that contain cassiterite, wolframite and galena. 6. Discussion Application of R-mode factor analysis indicates that factor 1 gives the most interesting results, which associates Ag, As, Cu, Mo and W with Au and is considered indicative of the occurrence of arsenopyrite, copper minerals and scheelite (±sphalerite ± molybdenite) with gold. Although visible gold is rare, microscopic and submicroscopic free-milling gold grains occur as dispersed globule, rod, crescent, irregular streak, blebs and specks along fine ribbons of wallrock in quartz veins, and in the graphitic shear planes. These wallrock selvages are the locus of abundant fine-grained arsenopyrite that is frequently intimately associated with gold. Inclusions of gold (10 lm-across) have been observed in the As-bearing pyrite grains intergrown with arsenopyrite, or occur as free grains embedded in the quartz–sericite–ankerite selvage. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization was attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA–ICP–MS analyzes (n = 102, range 2193–116 ppm, average 667 ppm Au; Zoheir and Lehmann, 2011). As rocks weather and their content of sulfide minerals oxidize, Ag, As, Mo and W are likely to be carried into drainage systems in solution as well as included in clastic fragments. Therefore, in a geochemical exploration using drainage sediments, these elements can assume the role of pathfinder elements for potentially economic deposits. Their concentrations in this role depend upon many factors, but in particular upon the consistency of their association with the type of deposits sought, upon their geochemical dispersion characteristics and upon the ease with which geochemical analysis can be performed. Above the selected geochemical thresholds, Ag, As, Mo and W anomalies appear in general to be related to known or suspected mineralization (see Fig. 1). Electron microprobe analysis of

11

disseminated Ni–sulfides and Ni-arsenides and pentlandite revealed traces of Au (up to 0.64 wt.% Au; Takla and Suror, 1996). Accordingly, it is suggested that the serpentinized ultramafic rocks may have been an important source for gold (Takla and Suror, 1996). However, not all known sites of mineralization in the Barramiya area are represented by such anomalies. Analysis of common sulfide minerals suggests that As is most widely distributed in pyrite and chalcopyrite, while galena and sphalerite more readily accommodate Ag. Ag and As concentrations reported may occur as substitutions in lattice of the host sulfides, or as inclusions of Ag and As minerals in solid solution. On the other hand, Mo and W are mostly related to molybdenite and scheelite minerals, both elements are used as indicators since ancient times (Hume, 1937). The occurrence of significant quantities of sulfide minerals, either in the ore or in its associated minerals, is therefore an important source of Ag, As, Mo and W. These four elements can also form their own minerals, which occur in minor quantities in some mineralization sites. The work carried out here been not extended to the determination of Ag, As, Mo and W in sulfide minerals. However, the stream sediment data suggest that at least one of these elements, most usually Ag or As, appears to be present in the mineralization in quantities required for the application of pathfinder geochemistry in drainage surveys. Significant anomalies of Ag, As, Mo and W, related to oxidizing zones associated with gold mineralization, are likely to include a large hydromorphic component compared with background levels of these elements. Mobility in a surficial environment is dominated by transport in aqueous solutions; therefore greatly influence the occurrence of anomalies (see Hamdy and Aly, 2011). The elements Ag, As, Mo and W are soluble in aqueous phases and mobile in the vicinity of oxidizing sulfides, providing their host minerals are undergoing oxidation (Williams, 2003). Arsenic and Mo, are strongly scavenged from solution in Fe-rich environments, either by formation of Fe-compounds or by adsorption on Fe-oxides (Hem, 1977). Moreover, arsenic remains mobile under all conditions in natural environment, leading to the development of relatively long As dispersion trains and anomalies in a variety of host rocks at the Barramiya gold mine area.

6.1. Dispersion of gold in wadi sediments Gold contents in drainage sediments are generally low and erratic; only few values exceed 1 ppm (Fig. 5). Most of the high element concentrations in stream sediments are found in the graphiteschist and serpentinized marble rocks matched well with principal component images that represent multi-element associations related to gold mineralization. The area around the Barramiya gold mine, however, which contains the highest density of old gold workings, is characterized by Au contents that generally exceed 0.2 ppm (Fig. 5). Distribution of Au changes abruptly from the near source to 150 m downstream. Geochemical survey can be effective by using a sensitive analytical method for Au in the 1 + 0.25 mm fraction of the brown blocky gravel sediment. Geochemical dispersion patterns in the 0.5–1.0 ppm range indicate Au mineralization of 500–100 m upstream, depending on relief. Very strong anomalies of more than 1 ppm Au (Fig. 5) are mainly related to the surficial pattern due to the dumps of the ancient gold workings in the central part of the study area near the Barramiya gold mine. Despite the vicinity of very strong anomalies, Au contents in wadi alluvium fall very sharply from 0.75 ppm (100–300 m) downstream to 0.35 ppm (500–1200 m) far from their downstream (Fig. 5). Despite eolian contamination, superficial geochemical anomaly in the range 0.75–0.35 ppm is detectable at 1200 m downstream of the Au-vein systems that were artificially disintegrated by ancient workings.

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6.2. Supergene gold chemical mobility Dominant transport processes vary by environment (arid, tropical), age or type of cover, and structural/tectonic setting. In some areas water and electrochemical mechanisms may dominate metal transport. In others, movement of elements along faults by water, evapo-transpiration and/or gaseous diffusion dominate, such that the strongest signatures are evident around faults. In yet other areas hyper-saline groundwater contributes significantly to element mobility through formation of highly mobile metal anion complexes. Variable roles in metal transportation and fixation may be played by micro-organisms, cycling of metals by vegetation, seismic or barometric pumping of water and/or gases. This fact has major consequences on the chemical mobility of gold. In arid environments with abundant carbonate and prevailing soil pH of 7–9, most metals of interest to exploration do not travel far. The present-day desert conditions impede any nugget growth in soil and alluvium, a fact confirmed by the most recent failure to find gold placers in the Eastern Desert of Egypt. Scarcity of vegetation and lack of organic decomposition in soils hinder production of humic and cyanogenic complexes, which play an important role in tropical climates (Lakin et al., 1974; Harraz et al., 2001). During the more humid paleoclimatic periods, secondary enrichment must have been active but most of the resultant profiles were eroded by the Miocene period leaving only a ‘‘skeletal’’ residue of the adjacent rocks. The residence time of the Aubearing solutions in the present-day climate is too short to allow secondary growth of gold particles (Salpeteur and Sabir, 1989; Fletcher and Wolcott, 1991; Zeegers and Leduc, 1993). Gold particles larger than 100 lm are locally observed on the top of quartz veins and in pyritic limonitic boxworks hosted in granites at the contact with serpentinized marble at the northern and northwestern parts of the study area, but these are generally affected by sand and wind erosion. The average size of free gold particles recovered by hand panning is 45 lm in oxidized dump samples and 70–80 lm in the wadi-sediment samples (Fig. 8). These are maxima; most of the very fine gold is lost in sample desliming. Occurrence of free gold particles up to 0.5 mm on the oxidized outcrops of mineralized quartz veins and in listvenite may result from secondary enrichment by ascending oversaturated solutions (Bonnemaison, 1986; Osman, 1995; Likhoidov et al., 2007). Hamdy and Aly (2011) showed that specks of gold and sulfide disseminations are native nuggets (20–35 lm) in pores, vugs, and fissures of highly altered serpentinized marble and in the oxidation zone at the northern and northwestern parts of the studied area. The post-metamorphic gold mineralization in the marble took place mostly by surficial fluids. Mineralizing fluid was also responsible for leaching Au from its serpentinite source. The Au was most probably transported from its source to the marble rocks by pluvial periods-related meteoric and/or underground water (<1.5 Ma; Hamdy and Aly, 2011). Consequence, source of native nuggets gold in stream sediment samples at the northern and northwestern parts of the studied area is mostly the country ultramafic rocks and the mineralization timing, relative to the marblization and metamorphism of the source rocks (Harraz et al., 2001; Hamdy and Aly, 2011). A more accurate approach to the mineralogy of these secondary weathering products is needed to achieve a better understanding of the processes and their relative chronology with respect to the past pluvial climatic periods in the Eastern Desert of Egypt. Hydrogeochemical studies of water wells in the central Eastern Desert and Bir Barramiya especially indicate high Cl content and weak acidic conditions. Chemical transport of gold may then be  suspected since the AuCl4 and HAuO 3 complexes are stable in such conditions (Cloke and Kelly, 1964; Webster and Mann, 1984).

Fig. 8. Grain-size distribution of gold particles recovered by hand panning from drainage sediment samples at the Barramiya area.

During oxidation of sulfide minerals the ½SO2 2 content of the surficial water increases, thereby lowering the pH. Dissolution and hydrolysis of Ca- and Mg-bearing minerals increase the anionic content of the ground water. Subsequent evaporation in hot desert conditions produces oversaturated and slightly alkaline conditions.  The AuCl4 complex becomes unstable (Hamilton et al., 1983) and metallic gold is adsorbed on quartz silts. This mechanism has been experimentally demonstrated by Sakkarova et al. (1984). Oxidation of Fe2+ to Fe3+ may also reduce Au3+ to the metallic state (Williams, 2003), as suggested by the common observation of secondary gold nuggets in limonitic boxworks. Variability in chemistry of host rocks and their paleoclimatic history explain the difficulty of defining a general rule. 7. Conclusions The applicability of both simple statistical and R-mode factor multivariate analytical techniques for interpretation of regional stream sediment data from the study area has been shown. Single-element distribution plots, though useful, are both time consuming and inadequate for identifying subtle though significant patterns within the data. Single element maps of Au and As give starting point for further studies. These maps allow the location of the richest regions for these elements, and it is a very useful tool. However, the use of several numerical methods can help to identify specific areas of interest. In this way R-mode factor multivariate analytical techniques have been selected to show their benefits in interpreting stream sediment geochemical data from the study area. The three-factor model has been adequately explained in terms of geological and surface processes, which show great masking effects. The most interesting results are related to factor 1, which associates Ag, As, Cu, Mo and W with Au and is considered indicative of the occurrence of arsenopyrite, copper minerals and scheelite (±sphalerite ± molybdenite) with gold. Association of Ag

H.Z. Harraz et al. / Journal of African Earth Sciences 68 (2012) 1–14

with Au in this factor is well established since the mean purity values of ore samples are 702–794 Au and 117–171 Ag, with Ag: Au ratio 1:4.6 (Hume, 1937). In single-element distribution maps some elements show a clear-cut relationship with known gold mineralization or other potentially mineralized areas. In conclusion, the use of R-mode factor analysis proved effective for recognition of the metal association Ag–Au–As–Cu–Zn–Pb– Mo–W that is most probably indicative of potential mineralization in the Barramiya gold deposit. The most promising area for followup exploration is the district underlain by the graphite-schist in contact with talc-carbonate at the central part of the area and underlain also by serpentinized marble as well as Younger Granites at the northwest part of the area (Figs. 5 and 7). Gold specks and sulfide disseminations were observed in quartz veins invading the Younger Granite as documented by panning of stream sediments from the northwest part of the area as well as by detecting native gold nuggets in pores, vugs, and fissures of auriferous and uraniferous marbles. Acknowledgments The paper was benefited from the critical reviews and the constructive criticism of Prof. J. Roberts and anonymous reviewer, which helped to improve the manuscript. Prof. P. Eriksson is thanked for editorial handling the manuscript. References Ahmed, A.H., Arai, S., Attia, A.K., 2001. Petrological characteristics of podiform chromitites and associated peridotites of the Pan African Proterozoic ophiolite complexes of Egypt. Mineralium Deposita 36, 72–84. Ali, K., Cheng, Q., Li, W., Chen, Y., 2006. Multi-element association analysis of stream sediment geochemistry data for predicting gold deposits in south-central Yunnan Province, China. Geochemistry: Exploration, Environment, Analysis (Geological Society of London) 6 (4), 341–348. Anwar, Y., Kotb, H., Zohny, N., 1969. Geochemistry of Egyptian Chromites. Alexandria University, Bulletin Faculty Science, p. 1X. Azer, M.K., Stern, R.J., 2007. Neoproterozoic (835–720 Ma) serpentinites in the Eastern Desert, Egypt: Fragments of forearc mantle. Geology 115, 457–472. Bonnemaison, M., 1986. Les shear zones aurifères. Chron. Rech. Minilere, France 482 m, pp. 55–65. Botros, N.S., 2002. Metallogeny of gold in relation to the evolution of the Nubian Shield in Egypt. Ore Geology Reviews 19, 137–164. Bugrov, V., 1974. Geochemical sampling techniques in the Eastern Desert of Egypt. Journal of Geochemical Exploration 3, 67–75. Cameron, E.M., 1979. Effect of graphite on the enhancement of surficial geochemical anomalies originating from oxidation of sulfides. Journal of Geochemical Exploration 12, 35–43. Carver, R.N., Chenoweth, L.M., Mazzuchelli, R.H., Oates, C.J., Robbins, T.W., 1987. ‘‘Lag’’ A geochemical sampling medium for arid regions. Journal of Geochemical Exploration 28, 193–199. Cloke, P.L., Kelly, W.C., 1964. Solubility of gold under inorganic supergene conditions. Economic Geology 59, 259–270. Closs, L.G., Nichol, I., 1975. The role of factor and regression analysis in the interpretation of geochemical reconnaissance data. Canadian Journal of Earth Sciences 12, 1316–1330. Davis, J.C., 2002. Statistics and Data Analysis in Geology, third ed. John Wiley and Sons, NY. EGSMA, 1992. Geological map of Wadi El Barramiyah Quadrangle, Egypt, scale 1:250,000. Geological Survey of Egypt. El-Bedawi, M.A., Hassan, Y.M., Zaki, M.E., Arnous, M.M., 1983. Results of prospecting-evaluation work carried out at Barramiya gold ore deposit (1979–1981). Geological Survey Egypt, Internal Report Unpub, p. 206. El-Makky, A.M., 1981. Geochemical Prospecting at Mersa Alam Area, Eastern desert, Egypt. MSc thesis. Alexandria University, p. 159. El-Ramly, M.F., Ivanov, S.S., Kochin, G.G., Bassyouni, F.A., Abdel Aziz, A.T., Shalaby, I.M., El-Hammady, M.Y., 1970. The occurrence of gold in the Eastern Desert of Egypt. In: Moharram, O., Gachechiladze, D., El-Ramly, M.F., Ivanov, S.S., Amer, A.F. (Eds.), Studies on some Mineral Deposits of Egypt. Geological Survey of Egypt, Part I, Section A, pp. 53–64. Fletcher, W.K., Wolcott, J., 1991. Transport of magnetite and gold in Harris Creek, British Columbia, and implications for exploration. Journal of Geochemical Exploration 41, 253–274. Gabbra, S.Z., 1986. Gold in Egypt. A Commodity Package: Minerals, Petroleum and Groundwater Assessment program. USAID Project. 363–0105, Cairo, Egypt. Geological Survey of Egypt, p. 86.

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