Geochemistry and Petrography of Gold-Quartz-Tourmaline Veins of the Okote Area, Southern Ethiopia: Implications for Gold Exploration

Mineralogy and Petrology 2002) 75: 101±122

Geochemistry and petrography of gold-quartz-tourmaline veins of the Okote area, southern Ethiopia: implications for gold exploration

D. J. Deksissa and C. Koeberl

Institute of Geochemistry, University of Vienna, Austria

With 10 Figures

Received November 17, 1999; revised version accepted July 23, 2001

Summary Quartz-tourmaline vein-hosting rocks of the Okote area belong to the Neoproterozoic Adola Belt. Metasomatic auriferous quartz-tourmaline veins occur in ductile N±S trending, sinistral shear zones. These veins commonly contain quartz, carbonates, and tourmaline, with minor pyrite, and accessory chalcopyrite, pyrrhotite, and gold. Tour- maline forms isolated euhedral crystals in the fracture surfaces of quartz carbonate veins. Many of the tourmaline crystals are optically zoned with a bluish core and a bluish to brown rim. Electron microprobe analyses show that the tourmalines comprise an intermediate dravite-schorl solid solution with a mean FeO=FeO MgO) 0.47. Abrupt transitions between the colour zones within single tourmaline crystals are ac- companied by relative variations in the FeO=FeO MgO) ratios. The tourmaline separates indicate that the tourmalines contain highly variable average contents of trace elements. Chondrite-normalized rare earth element REE) abundances of tourmaline separates from auriferous veins show LREE-enriched to LREE-depleted patterns with negative to positive Eu anomalies and a ¯at, near-chondritic HREE pattern. The auriferous quartz-tourmaline veins have LREE-enriched patterns without a Eu anomaly and a ¯at HREE pattern, but tourmaline-free gold-quartz veins have very low REE contents and LREE-depleted patterns also without Eu anomalies. The FeO=FeO MgO) ratios, major and trace element compositions, and the types of wall-rock alteration are used to suggest that the sources of boron are dominantly metamorphic dehydration and devolatilization processes), but do not totally exclude the possibility of a magmatic source. The occurrences of high-grade gold associated with tourmaline make tourmaline a valuable prospecting guide for hydrothermal gold mineralization in the Adola Belt, southern Ethiopia. 102 D. J. Deksissa and C. Koeberl

Introduction The relationship between tourmaline-rich rocks and gold deposits has been documented in many metallogenic provinces, such as in the Precambrian basement of Ethiopia Augustithis, 1967), Brazil Fleischer and Routhier, 1973), Australia Plimer, 1986), in Caledonian rocks of southern Ireland McArdle et al., 1989), and the Archean Barberton greenstone sequence of South Africa Byerly and Palmer, 1991). King 1988) found that the REEs in tourmalines from the gold vein deposits of Lake Superior do not correlate with the REE levels in the host rock. He suggested that there must have been REE mobilization in hydrothermal solutions. Tourmaline is also common in hydrothermal gangue minerals in Archean to Phanerozoic mesothermal and intrusion-related gold deposits see, for example, King, 1988; Groves and Foster, 1991; Nesbitt, 1991; Garba, 1996). Garba 1996) and King 1988) indicated that tourmaline is a potential path®nder mineral for source region and vein gold mineralization. The composition of hydrothermal tourmalines is sensitive to variables, such as ¯uid chemistry, physicochemical conditions, and the nature of ¯uid rock interactions e.g. ¯uid rock ratios; Garba, 1996). Bone 1988) suggested that tourmalinites with tourmaline > 15 vol.%) are somewhat enigmatic rocks and highly signi®cant as ®ngerprints of unusual hydro- geochemical and paleogeographical conditions and should foster exploration interests. In this study we discuss the petrography and geochemistry of auriferous quartz- tourmaline veins and their signi®cance for gold exploration in the Adola Belt.

Geological setting The Okote gold mineralization is located in the southern part of the Adola Belt. The Adola Belt, covering an area of about 5000 km2 Billay et al., 1997), consists of two, N±S trending volcanosedimentary belts, namely the Kenticha Belt in the east and the Megado Belt in the west Fig. 1; Upper Complex). These two belts are separated and bounded in the east and west by high grade gneisses of the Middle complex and Lower complex according to the classi®cation of Kazmin, 1972). The study area is the southward extension of the Megado volcanosedimentary belt. The lithologic contacts, foliation surfaces, fold axes, and fault planes of the Megado volcanosedimentary belt change in strike at the Okote area, from N±S to almost E±W, due to a NW striking regional shear zone Fig. 1). The lithologic association, structural evolution, metamorphism, and geochemistry of the Megado volcano-sedimentary belt has been documented in detail already by Woldehaimanot and Behrmann 1995) and Worku and Schandelmeier 1996). The geology of the Okote area consists of amphibole- and biotite-quartzofeldspathic gneiss, amphibolite, meta-ultrama®c rocks, meta-gabbro, meta- volcanic rocks, chlorite-carbonate-amphibole schist, meta-tonalite, meta-granite, plagiogranite, and meta-sedimentary rocks graphitic quartzite and graphitic schist) Fig. 2). The meta-tonalite intruded the meta-volcanic rocks and meta-gabbro and contains amphibolite xenoliths. The peraluminous meta-granite intruded the biotite-quartzofeldspathic gneiss. The chlorite-carbonate-amphibole schist consti- tutes three hydrothermally altered, N±S trending, narrow, sinistral ductile shear Gold-quartz-tourmaline veins of southern Ethiopia 103

Fig. 1. Location of the study area on the geological map of southern Ethiopia. Inset shows the inter-®ngering relationships of the Arabian-Nubian Shield ANS) and the Mozambique Belt MB) and the location of Precambrian rocks of southern Ethiopia modi®ed from Mohammed, 1999; Kazmin, 1972; Vail, 1987) zones that cut across metagabbro and consist of chlorite-carbonate-biotite schist, epidote-amphibole-chlorite schist, and amphibole-carbonate-chlorite schist. The Okote gold mineralization is restricted to an anastomosing system of narrow high strain, N±S trending, ductile shear zones. Within these shear zones, gold occurs in quartz veins and quartz-carbonate-tourmaline veins, and their wall- rocks.

Petrography of the host rock The investigated quartz-tourmaline veins occur in chlorite-carbonate-amphibole schist and talc-tremolite schist. The chlorite-carbonate-amphibole schist is an alteration product of meta-gabbro and is sub-classi®ed into chlorite-carbonate-biotite schist, epidote-amphibole-chlorite schist, and amphibole-carbonate-chlorite schist, based on their mineralogical assemblages and intensity of deformation Fig. 3). 104 D. J. Deksissa and C. Koeberl

Fig. 2. Regional geological map of the Okote area, southern Ethiopia

Chlorite-carbonate-biotite schist: Chlorite-carbonate-biotite schist is ®ne- grained, light green to grey in colour, intensely sheared, and contains abundant N±S trending, foliation parallel, auriferous, quartz-carbonate- and quartz-carbonate- tourmaline veins. Chlorite, carbonate calcite and ankerite), quartz, biotite, epidote, and albite are the dominant minerals, whereas pyrite, muscovite, sericite, K-feldspar, apatite, ilmenite, and rutile are accessory. Chlorite and carbonates constitute up to 80 vol.% of the rock. Chlorite, carbonate, and quartz crystals are preferentially aligned parallel to the shear zone boundary. Biotite crystals appear to have formed by the break-down of chlorites and muscovite. Quartz grains occur predominantly in veins and veinlets that always enveloped by hydrothermal alteration minerals. Calcite is crystallizing at the expense of ankerite. Epidote-amphibole-chlorite schist: This schist subtype occurs next to chlorite- carbonate-biotite schist towards fresh meta-gabbro. The contacts of epidote- amphibole-chlorite schist with chlorite-carbonate-biotite schist and meta-gabbro are always gradational. The schist is ®ne- to medium-grained, well foliated, and consists of epidote, zoisite=clinozoisite, albite, actinolite-tremolite, hornblende, relict plagioclase, chlorite, and accessory pyrite, magnetite, carbonate, apatite, and relict Gold-quartz-tourmaline veins of southern Ethiopia 105

Fig. 3. Detailed geological map of the sheared, altered, and mineralized part of the Okote area, southern Ethiopia after Deksissa, 2000) pyroxene. Plagioclase porphyroclasts are almost completely altered to epidote, clinozoisite and albite, and contain chlorite inclusions. Amphibole-carbonate-chlorite schist: The amphibole-carbonate-chlorite schist is dominantly exposed in the eastern alteration zone Fig. 3). The schist is green in colour, medium-grained, well foliated, and composed mainly of tremolite-actinolite, carbonate, chlorite, albite, with accessory epidote zoisite=clinozoisite, iron oxides, and relict plagioclase. The intensity of shearing is comparable to that observed for the chlorite-carbonate-biotite schist.

Structural evolution of the Okote area Three main deformation episodes have been recognized in the ®eld, based on dif- fering structural styles and overprinting relationships. An earlier folding and thrusting event D1) was followed by N±S shearing and folding D2), and, ®nally, folding and NW-striking sinstral brittle-ductile shearing D3). D1 structures: The ®rst progressive deformation D1 was responsible for the formation of foliation and the oldest quartz veins. This deformation was followed by thrusting that resulted in the development of recumbent folds and boudinaged folds in quartz veins in biotite-quartzofeldspathic gneiss. D2 structures: The second deformation D2 formed a regional synform in the NW part of the area Fig. 2) and N±S trending shear zones. The D2 mesoscale structural elements in the ma®c-ultrama®c rocks are regional penetrative foliations that strike N20 E and dip steeply 60 to 80) to the west, quartz veins and quartz-carbonate- tourmaline veins concordant to foliation, asymmetrical folds, S-C fabrics, and anastomosing shear structures. 106 D. J. Deksissa and C. Koeberl

D3 structures: The mesoscale NW±SE trending brittle to ductile shear zones are very common in the study area. They clearly display sinistral sense of movement and are younger than the N±S trending structures. They contain concordant quartz veins that cut and displace all of the older auriferous quartz veins.

Quartz veins Three main groups of quartz veins associated with the above three deformation phases have been observed. These are E±W trending highly deformed quartz veins qv1), N±S trending quartz-carbonate-tourmaline veins and quartz-carbonate veins qv2), and NW±SE trending quartz veins qv3). The oldest qv1 veins are tightly folded, cut by the younger veins and shear foliations, and are barren of gold. The intermediate gold-bearing qv2 veins dip subvertically towards west and locally towards east, show pinch- and swell structure, and are massive to laminated. The youngest veins qv3) are very thin, barren of gold, and concordant to brittle shear zones that strike 30 to 40 NW dip at 70 towards NE. The present work con- centrates mainly on qv2 veins. Textures of auriferous quartz-tourmaline veins qv2): The veins are milky white to light gray in colour and strike sub-parallel to N±S trending chlorite-carbonate- amphibole units and N±S trending shear zones that are illustrated in Fig. 2. Vein widths range from less than 10 centimeters to 3 meters and commonly vary dra- matically over short distances along the strike and dip directions. Variation in thickness is commonly the result of progressive shearing during and after vein formation. The veins commonly display laminations on the cm-scale. Elongated tabular clasts of wall-rocks are occasionally included in the veins, generally close to the wall-rock contact, and the extremities of these clasts commonly display continuity with the vein laminations. The mineralogical composition of the laminae is dominated by chlorite, carbonate, sul®de, and tourmaline. Tourmaline is a minor to major gangue mineral in the gold-bearing veins at the Okote area. The major to accessory gangue minerals are quartz, carbonate ankerite and calcite), orthoclase, albite, and white mica=muscovite. The ore minerals are mainly pyrite with sub- ordinate amounts of chalcopyrite, and accessory pyrrhotite, covellite, chalcocite, gold, and galena. Tourmaline occurs in the form of black prismatic crystals visible in hand specimen and shows a brown to bluish pleochroism in thin section. The basal sections of most of the studied tourmalines show clear colour zoning from bluish-green cores to brownish rims Fig. 4A). The boundary between the core and rim is commonly sharp. A tourmaline sample from a talc-tremolite host rock shows os- cillatory zoning, with a wide bluish core and thin alternating bluish and dark blue to brown layers. Tourmaline crystals show two modes of occurrences. The ®rst type is ®ne- grained 20±300 mm in cross-section), and euhedral acicular tourmaline crystals are disseminated within the undeformed quartz grains Fig. 4B). The second type consist of tourmalines concentrated along the fractures within the quartz veins Fig. 4C, D). The latter tourmalines commonly ®ll cavities by developing comb-textured tourmaline crystals perpendicular to the wall of the cavity. The tourmalines that follow cracks and intergranular spaces of the quartz indicate a later paragenetic Gold-quartz-tourmaline veins of southern Ethiopia 107

Fig. 4. Photomicrograph plane polars) showing A tourmaline crystal with sharp boundary between core and rim, B ®ne-grained tourmaline crystals disseminated in quartz matrix, C tourmaline crystallization along fractures in auriferous quartz vein, and D tourmaline with radial growth pattern in fracture of quartz vein. Width of ®eld of view is 1.5 mm

sequence. The quartz from the older quartz veins is highly deformed, shows undulatory extinction and marginal recrystallization. These features generally indicate that the tourmaline fracture ®llings are younger than the disseminated tourmalines. In quartz-carbonate veins, calcite seems to be cogenetic with the second group of tourmalines. The crystallization sequence of vein materials can be deduced from patterns of mineral association. The ®rst generation of tourmaline is characterized by well- developed elongated prismatic crystals disseminated in the ®rst generation quartz matrix Fig. 4B). The crystallization of quartz was not restricted to this initial single stage. Calcite inclusions in the ®rst generation quartz-tourmaline vein contain ankerite. The second generation tourmaline with native gold occurs as inclusions in calcite Fig. 5). Very small grains of galena, commonly less than 10 mm in size, occur within grain boundaries of tourmalines. The occurrence of tourmaline in a calcite matrix indicates that calcite formed syn- to post genetically to tourmaline. Considering ®eld, thin section, and electron microprobe studies, we suggest that tourmaline, calcite, and gold formed cogenetically. Galena is the youngest mineral observed in the quartz-tourmaline veins. 108 D. J. Deksissa and C. Koeberl

Fig. 5. Back-scattered electron image showing relationships among tourmaline T), gold Au), calcite C) and quartz Q) within the quartz- tourmaline vein

Gold mineralization Gold occurs as native gold inclusions in chalcopyrite, pyrite, calcite both in veins and wall-rock), and as free gold in the quartz matrix. The auriferous veins are always surrounded by less than a meter to tens of meters wide intense hydrothermal alteration. The fabrics and primary minerals of the wall-rocks have commonly been replaced by chlorite, carbonate predominantly calcite and minor ankerite), pyrite, and biotite with minor chalcopyrite and pyrrhotite. Tourmaline occurs in quartz veins containing very high concentrations of gold e.g., sample DKR-75, Au 56.7 ppm, see Table 3). Although the gold concentration displays signi®cant temporal and spatial association with the lamination, gold particles tend to occur in fractures in the quartz associated with calcite and tourmaline.

Geochemistry

Sampling and analytical methods Representative bulk samples have been collected from auriferous quartz-carbonate- tourmaline veins in chlorite-carbonate-amphibole schist for the major and trace element analyses using electron microprobe, Inductively Coupled Plasma Mass Spectrometer ICP-MS), and Instrumental Neutron Activation Analysis INAA). The sample locations are shown in Figs. 2 and 3. Electron microprobe analyses: Polished sections have been prepared from tourmaline-rich samples and were carbon-coated before analysis. Tourmaline compositions were determined using a CAMECA SX 100 electron microprobe at the Institute of Petrology, University of Vienna Table 1). Natural standards were used for analyses of major and minor elements abundances. Measurement times per element were 20 s with the emission current of 15 nA and an acceleration voltage of 15 kV. The content of Na was measured ®rst for 10 s to avoid errors resulting from the volatilization. Table 1. Composition of representative tourmaline crystals from auriferous quartz-tourmaline veins of the Okote area, southern Ethiopia DKR-31 DKR-36 DKR-43 DKR-52 DKR-56 DKR-58 DKR-75Ã Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core

SiO2 36.16 37.58 36.77 36.87 36.91 37.41 37.05 37.54 35.80 37.30 36.12 37.29 37.29 37.17 TiO2 0.44 0.04 0.45 0.41 0.52 0.18 0.97 0.12 1.09 0.06 0.42 0.04 0.08 0.21 B2O3 Calc 10.59 10.80 10.78 10.73 10.63 10.85 10.72 10.81 10.38 10.68 10.58 10.85 10.80 10.82 Al2O3 31.86 33.80 33.11 32.84 32.54 33.86 30.58 32.24 30.53 33.02 32.39 34.13 33.68 33.75 FeO 8.08 6.82 5.89 5.69 7.89 4.47 6.71 6.24 9.11 7.33 7.59 6.61 4.40 5.54 MnO 0.02 0.04 0.01 0.03 0.01 0.00 0.03 0.05 0.00 0.00 0.02 0.00 109 0.00 0.02 Ethiopia southern of veins Gold-quartz-tourmaline MgO 6.69 6.37 7.60 7.62 5.69 7.82 8.29 7.98 5.86 6.19 6.69 6.49 7.74 7.37 CaO 1.01 0.24 0.85 0.69 0.41 0.24 0.74 0.26 0.89 0.23 0.69 0.21 0.55 0.61 Na2O 2.25 1.85 2.01 2.06 2.17 2.28 2.63 2.75 1.87 1.86 1.98 1.70 1.58 1.77 K2O 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.02 0.01 0.01 0.00 0.01 0.02 0.01 LiO2 Calc 0.16 0.21 0.15 0.16 0.30 0.18 0.13 0.18 0.17 0.25 0.09 0.10 0.19 0.13 Total 97.27 97.76 97.62 97.11 97.08 97.30 97.85 98.19 95.71 96.93 96.57 97.43 96.33 97.4 Structural formulae calculated on the basis of 29 oxygens Si 5.936 6.047 5.931 5.968 6.035 5.994 6.011 6.034 5.993 6.072 5.933 6.002 6.022 5.972 AlIV 0.064 0.069 0.032 0.006 0.007 0.067 0.028 B 2.999 3.000 3.001 2.999 3.000 3.001 3.002 2.999 2.999 3.001 3.000 3.000 3.011 3.001 Ti 0.054 0.004 0.054 0.050 0.064 0.022 0.118 0.015 0.138 0.007 0.052 0.005 0.010 0.025 Al 5.946 5.996 5.946 5.950 5.936 5.978 5.882 5.985 5.862 5.993 5.948 5.995 5.990 5.975 Al 0.150 0.414 0.279 0.286 0.335 0.409 0.123 0.152 0.342 0.256 0.485 0.420 0.389 Fe 1.108 0.917 0.794 0.170 1.079 0.599 0.910 0.839 1.275 0.997 1.043 0.891 0.594 0.744 Mn 0.003 0.006 0.002 0.004 0.001 0.000 0.003 0.006 0.001 0.000 0.003 0.000 0.000 0.003 Mg 1.635 1.528 1.827 1.840 1.388 1.868 2.006 1.912 1.462 1.501 1.638 1.558 1.863 1.765 Li 0.105 0.135 0.096 0.102 0.196 0.119 0.081 0.116 0.109 0.159 0.057 0.062 0.120 0.083 Y total 3.001 3.000 2.998 3.001 2.999 2.995 2.997 2.997 3.000 2.999 3.000 3.000 3.000 3.000 Ca 0.117 0.041 0.146 0.120 0.073 0.042 0.128 0.044 0.160 0.040 0.121 0.036 0.095 0.105 Na 0.714 0.578 0.629 0.647 0.687 0.070 0.826 0.856 0.606 0.586 0.631 0.531 0.495 0.551 K 0.001 0.001 0.001 0.002 0.002 0.002 0.001 0.004 0.003 0.001 0.000 0.002 0.004 0.002 X total 0.891 0.619 0.775 0.767 0.760 0.749 0.955 0.900 0.765 0.626 0.752 0.569 0.594 0.658 Fe=Fe Mg) 0.404 0.375 0.303 0.295 0.437 0.243 0.312 0.305 0.466 0.399 0.389 0.364 0.242 0.297 Na=Na Ca) 0.801 0.934 0.811 0.843 0.905 0.945 0.866 0.951 0.791 0.936 0.839 0.937 0.839 0.840 Oxides in wt %; total Fe as FeO; Calc calculated; Ã tourmaline sample without zoning 110 D. J. Deksissa and C. Koeberl

The calculation of the atomic proportions was made by assuming that B has a stoichiometric value of 3 atoms per formula unit apfu), OH ®lls the four O1) O3) sites i.e., normalization of the cations based on 29 oxygen), and Li ®lls the de®ciencies in the Y site, i.e., Li 3-Ysite total. Both the amount of B2O3 necessary to produce 3 B cations and the amount of Li2O required to ®ll the de®ciencies in Y were calculated from stoichiometry. After establishing the appropriate normalization scheme, the cations were allocated among the different sites of the tourmaline. Na, Ca, and K are assigned to the X site, with any site de®ciency assumed to represent an X-site vacancy. Si is assumed to be exclusively in the T site, with any de®ciency made up by Al. There is more Al than cations required to ®ll the Z site. Excess Al from T and Z sites and Fe, Mn, Mg, and Li were assigned to the Y site. All iron is assumed to be Fe2 ,as total charge calculation gave no charge excess or de®ciency. It is common practice to assume that all transition elements are in the divalent state e.g., all Fe as

Table 2. Trace element contents of tourmaline, Okote area, southern Ethiopia DKR-31 DKR-36 DKR-43 DKR-52 DKR-56 DKR-58 Sc 38.4 10.9 64.2 9.58 32.6 37.3 Cr 34.3 20.9 177 614 3.32 539 Co 21.4 21.3 55.7 29.9 52.4 21.4 Ni < 114 224 484 19.6 36.5 96 Zn 54 82 195 23 47 72 As 0.61 2.36 0.21 < 0.6 1.36 < 0.6 Rb 1.68 1.15 < 8.3 < 5.2 4.55 < 7.1 Sr 114 96 < 161 2009 138 168 Zr < 131 30 < 136 9.1 < 112 < 102 Sb 0.70 0.74 0.38 0.07 0.3 0.12 Cs 0.70 < 0.1 < 0.5 0.12 0.12 0.06 Ba < 84 < 43 36 18 < 82 < 70 La 3.96 0.13 0.17 0.08 0.13 0.36 Ce 5.88 1.75 < 1.2 0.48 < 1.1 < 1.0 Nd 2.92 1.67 < 2.7 < 1.7 < 2.6 < 2.3 Sm 0.95 0.07 0.09 0.02 0.06 0.40 Eu 0.19 0.03 0.07 0.21 0.07 0.04 Gd < 2.5 < 1.0 < 3.0 < 1.7 < 2.3 Tb 0.14 < 0.1 0.08 0.06 0.09 0.10 Tm < 0.1 < 0.01 0.51 < 0.1 < 0.1 0.10 Yd 0.33 0.18 0.17 0.14 0.31 0.19 Lu 0.05 < 0.01 0.02 0.01 0.05 0.03 Hf 0.42 0.08 0.18 0.07 < 0.3 0.8 Ta 0.29 0.02 1.10 0.13 0.42 0.39 W 0.13 0.2 1.29 0.68 0.54 0.52 Au ppb) 29 2.7 3.0 2.90 1.7 < 5.2 Th 0.13 0.02 0.11 0.03 < 0.2 < 0.2 U0.30 0.05 < 0.3 < 0.6 0.61 0.31 All contents are in ppm, except as noted Gold-quartz-tourmaline veins of southern Ethiopia 111

FeO), particularly if there is no de®ciency in the cation charge Henry and Dutrow, 1996). Trace element analyses: Bulk quartz vein samples of about 10 kg were collected from auriferous quartz-carbonate-tourmaline veins and quartz-carbonate veins of the similar host rock, strike and dip, and, possibly, the same age. The vein samples were crushed to less than 2 mm diameter in a stainless steel jaw crusher and 200 g sub-samples were taken by the cone and quartering method. The ®nal samples were powdered in an agate mill. Tourmaline-rich parts of the same quartz-tourmaline veins were sampled separately, coarsely crushed less than 2 mm), and pure tourmaline crystals were selected under a binocular microscope. These pure tourmaline crystals were pulverized in an agate mortar. The vein samples were analyzed for the contents of Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Rb, Sr, Ba, Au, Bi, and Pb using a Perkin-Elmer Sciex Elan 5000 ICP-MS at the Arsenal Laboratory, Vienna, Austria for see Table 3). For the analysis 100 mg samples were dissolved in high purity acids 1:1.5 ml HNO3±HClO4 [2.5 ml of each] and 10 ml HF). The solution was evaporated to near-dryness at 120 C. Then, 5 ml ultra pure HNO3 were added and the residue dissolved and cooled to room temperature. Ultrapure water was added and the contents were transferred into a 100 ml volumetric ¯ask. Finally, 2 ml of the sample solution were diluted with 8 ml ultrapure water. The analytical precision has been calculated based on duplicate analyses and was better than 10 rel.% for most of these elements. Rare earth elements REE) and other trace elements Sc, Co, Ni, Zn, As, Rb, Sr, Zr, Sb, Cs, Ba, Hf, Ta, W, Au, Th, and U) in bulk vein samples and tourmaline separates were analyzed using INAA Table 2), following procedures described by Koeberl 1993).

Results

Tourmaline chemistry The major and minor element compositions of hydrothermal tourmaline grains from the Okote area are reported in Tables 1, 2. Signi®cant compositional variations occur for TiO2, CaO, FeO, MgO, and Na2O. Tourmaline samples from the talc-tremolite schist host rock have the highest average abundances of TiO2, MgO, and Na2O, and the lowest abundance of Al2O3. The content of F was not directly determined, which may explain the low totals of the microprobe analyses. The average composition of the tourmaline samples varies from 0.01 to 1.10 wt% TiO2, 4.40 to 8.56 wt% FeO, 0.21 to 1.01 CaO, 5.52 to 8.63 wt% MgO, and 1.48 to 2.75 wt% Na2O; the FeO=FeO MgO) ratios range from 0.42 to 0.61 mean 0.47, n 7). The average Na2O=Na2O CaO) ratios of individual tourmaline samples vary from 0.68 to 0.91 mean 0.80, n 7). The FeO=FeO MgO) and Na2O=Na2O CaO) ratios are lower for auriferous veins compared to barren veins. Tourmaline grains from gold-enriched quartz-carbonate-tourmaline veins up to 56.7 ppm Au; DKR-75, Table 3) contain the highest amounts of SiO2, Al2O3, and the lowest contents of TiO2, FeO, and Na2O. The data indicate a decrease of total iron content as FeO) and an increase of the Al2O3 content is Table 3. Major and trace element contents of auriferous quartz-tourmaline veins of the Okote area, southern Ethiopia DKR-31 DKR-36 DKR-56 DKR-58 DKR-75 KC-318 KC-328 B8-72 KC-347 KC-353 KC-368 KC-389 KC-390 KC-400 KC-401 KC-466 Al 1.12 0.11 1.29 0.17 0.26 5.01 0.05 0.16 0.05 0.12 0.09 0.05 0.07 1.88 0.74 Fe 0.80 0.60 1.40 0.20 0.60 4.90 0.10 0.77 1.60 0.20 2.10 0.40 0.50 0.10 1.60 1.80 Mn ppm) 220 290 240 27 153 830 18 35 15 60 31 18 32 330 960 Mg 0.17 0.20 0.28 0.02 0.16 1.38 0.02 0.04 0.01 < 0.01 0.04 < 0.01 0.01 0.26 1.45 Ca 4.90 3.48 1.75 < 0.05 1.93 23.30 < 0.05 < 0.05 < 0.05 < 0.05 0.08 < 0.05 < 0.05 2.08 11.60 Na 0.21 < 0.01 0.16 < 0.01 0.01 0.06 < 0.01 0.13 < 0.01 < 0.01 0.01 < 0.01 < 0.01 < 0.01 0.01 0.04 K 0.08 < 0.01 0.04 0.04 < 0.01 0.14 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.01 < 0.01 < 0.01 0.02 V 17 13 33 5 6 108 4 12 3 8 8 6 4 49 52 Cr 20.0 16.0 19.0 37.0 61.0 15.0 44.0 55.6 17.0 15.0 43.0 43.0 24.0 68.0 28.0 39.0 Co 2.8 2.7 5.1 0.8 3.7 19.0 0.7 3.62 2.7 0.6 1.7 2.9 3.0 0.8 5.4 11 Ni 8 16 10 3 16 17 3 9.58 6 7 4 12 3 3 6 42 Cu 9.1 3.0 15 2.3 42 30 5.6 5.9 1.5 8.2 750 29 2.2 13 57 Zn 6 51 12 < 3652< 3 8.2 < 3 < 3 < 37< 3 < 31017 Ga 2.1 0.3 3.5 0.3 0.5 9.9 0.1 0.4 < 0.1 0.5 0.2 0.2 0.1 4.1 1.5 As 0.3 < 0.5 < 0.10 0.42 < 0.4 0.2 3.3 < 0.3 < 0.6 Rb 2.2 0.6 1.2 0.8 < 0.5 4.7 < 0.5 1.3 < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 0.8 Sr 67 13 46 4 4 200 < 125121 3 1< 1 2 61 24 Sb < 0.5 < 0.1 0.1 < 0.2 < 0.5 Ba 41 25 29 19 17 131 2 61 6 4 52 3 7 5 11 71 La 1.39 2.79 0.09 0.44 0.37 0.03 0.18 0.04 3.5 Ce 1.8 4.55 0.52 1.11 0.80 0.13 0.3 < 0.16 4.63 Nd 0.98 1.37 0.10 < 0.2 < 3.6 < 0.4 < 0.90 < 1.9 3.2 Sm 0.31 0.68 0.02 0.15 0.21 < 0.01 0.13 0.01 0.66 Eu 0.18 0.25 0.10 0.03 0.07 < 0.01 0.05 0.01 0.25 Gd 0.12 0.77 < 0.04 0.19 0.26 0.05 0.14 0.04 0.20 Tb 0.11 0.15 0.01 0.02 0.05 < 0.01 0.04 0.01 0.11 Tm 0.02 0.01 0.02 0.01 < 0.03 < 0.01 0.03 0.01 0.06 Yd 0.30 0.55 0.09 0.23 0.16 0.04 0.25 0.26 0.34 Lu 0.05 0.09 0.02 0.04 0.02 0.01 0.04 0.01 0.06 W < 0.3 < 0.3 0.16 0.45 0.13 < 0.2 0.08 13.9 Au ppb) 1600 < 10 2380 80 56700 270 20 11530 180360 2490 8160 350 47450 50 80 580 Pb 2.4 0.8 1.2 1.1 0.9 3.5 0.5 3.2 1.1 2.2 0.8 0.3 0.2 0.8 0.9 Bi 0.4 < 0.1 0.1 < 0.1 0.2 0.2 < 0.1 0.1 2.2 4.6 0.1 0.2 < 0.1 < 0.1 0.2 Measured by ICP-MS and INAA; major element data in wt% and trace element data in ppm, except as noted D. J. Deksissa and C. Koeberl: Gold-quartz-tourmaline veins 113

Fig. 6. Interelement and ratio plots, showing compositional variation among tourmalines from quartz-tourmaline veins qv2) from different localities of the Okote area. Note that all tourmaline samples have older cores with higher concentration of Al2O3, MgO, and SiO2, but have lower FeO and Na2O=Na2O CaO) than the rim noticeable from the rims to the cores of the tourmalines. Correlation diagrams show strong negative correlations between the contents of Al2O3 and FeO Fig. 6a), FeO and the SiO2 Fig. 6b), and FeO and MgO Fig. 6c). Generally, the cores of most tourmalines grains show MgO enrichments and FeO depletions, in contrast to their rims. The oxide ratios FeO=FeO MgO) and Na2O=Na2O=CaO) indicate that there is no signi®cant compositional difference between the samples Fig. 6d). Different cation substitutions have been recognized in the Okote hydrothermal tourmalines. The Fe and Mg contents of tourmaline show strong negative correlations within each separate zone, with the rim having higher Fe than the core e.g., DKR-52; Fig. 7a). The compositional variation is due to changes in ¯uid chemistry during tourmaline deposition. The strong negative correlation between Al and divalent cations Fe Mg Mn) e.g., DKR-58; Fig. 7b) can be ascribed to an alkali de®cient substitution NaX Fe Mg Mn)Y AlY X site vacancy), because of a strong negative correlation observed between NaX and AlY e.g., DKR-58; Fig. 7c), 114 D. J. Deksissa and C. Koeberl

Fig. 7. Intercation correlation diagram for 2 representative samples showing clear compositional variations between cores and rims of tourmaline crystals within a vein sample apfu atoms per formula unit)

but positive correlation observed between AlY and X site vacancy e.g. DKR-58; Fig. 7d). Tourmaline samples from auriferous quartz-carbonate-tourmaline veins e.g., DKR-31, DKR-56 and DKR-58) generally show Si and total Al enrichments at the core of the crystals as compared to the rim Table 1). Trace element compositions of tourmaline separates are shown in Table 2. The analytical results indicate low concentrations of most of the elements determined. Nickel and Cr contents show strong variations. Tourmaline samples from talc- tremolite host rock shows very high Cr and Sr but low Ni, Sc, and Zn. The gold contents of the tourmaline crystals vary between 1.7 and 29 ppb. The highest value is for tourmaline grains from the well-mineralized quartz vein DKR-31. The Gold-quartz-tourmaline veins of southern Ethiopia 115

Fig. 8. Chondrite-normalized rare earth element REE) abundance patterns for tourmaline separates. Open symbols samples: paragenetically older auriferous quartz-tourmaline veins in epidote-amphibole-chlorite schist; solid symbols: samples are from paragenetically younger auriferous quartz-tourmaline; normalization factors from Taylor and McLennan 1985) contents of the gold-path®nder elements Zr, La, Hf, Ta, W, As, and Sb are generally very low. Granites are typically depleted in the elements Zr, Hf, and La due to their high degrees of fractionation e.g., Blevin and Chappell, 1995). The rare earth element REE) contents of the tourmaline crystals are very low compared to chondritic abundances Fig. 8). Chondrite-normalized REE abundances show two different patterns. The ®rst group shows chondritic enriched LREEs, a negative Eu anomaly, and a ¯at and near-chondritic HREE pattern solid symbols, Fig. 8). The second group has subchondritic, but ¯at, LREE pattern, a positive Eu anomaly, and a ¯at to slightly depleted HREE pattern open symbols, Fig. 8).

Geochemistry of auriferous quartz-tourmaline veins The bulk vein samples have very high gold contents of 10 ppb to 180 ppm and low abundances of base metal elements, such as Cu, Co, Zn, Ni, and Pb Table 3). Most gold mineralized veins i.e., Au > 500 ppb) are quartz-carbonate-tourmaline veins. The copper contents of the veins are low except for a few samples that have > 10 ppm. The Bi contents of the veins are also low, but 0.1 ppm Bi was detected in samples containing high concentrations of gold. Generally, the analytical results imply that gold is not associated with base metals. The quartz-tourmaline vein samples display two different chondrite-normalized REE patterns, both without an Eu-anomaly Fig. 9). The ®rst group is enriched in the LREEs, with a ¯at, depleted chondrite-normalized HREE pattern solid symbols, Fig. 9). These veins contain signi®cant amounts of tourmaline and carbonates, 116 D. J. Deksissa and C. Koeberl

Fig. 9. Chondrite-normalized rare earth element REE) abundance patterns for quartz- tourmaline veins. Solid symbols are tourmaline-rich auriferous quartz veins and open symbols are samples from tourmaline poor quartz veins; normalization factors from Taylor and McLennan 1985) except for sample KC-347. The second group has very low total REE abundances, is LREE depleted and consists of quartz, gold, very low tourmaline, and hematite and iron oxide open symbols, Fig. 9). The REE patterns of quartz tourmaline veins indicate LREE mobility during vein formation see Hellman et al., 1979).

Discussion Tourmaline is a complex borosilicate mineral consisting essentially of Mg, Fe, Al, Na, Ca, B, Si, and O with the general formula X)Y3)Z6)T6O18BO3)3W)4. In the X position, Na may be partially replaced by K or Ca if the valance conditions are satis®ed. The Y site can be predominantly ®lled with Li ,Mg2 ,Fe2 , and Al3 , and minor Mn2 ,Cr3 ,V3 ,Fe3 ,Ti4 , Zn, Cu, and Ba. The Z position is occupied mainly by Al3 ,Fe3 ,Cr3 ,V3 , and some Mg2 and Fe2 . Si is assumed to be exclusively in the T-site with any de®ciency made up by Al. The W site consists of OH, F, and O Henry and Dutrow, 1996). The crystal structure of tourmaline allows the substitution of a variety of ions, depending on the size and charge, in three lattice sites, leading to a wide variety in compositions.

Tourmaline-gold mineralization The composition of hydrothermal tourmalinite is sensitive to ¯uid chemistry, physicochemical conditions, and nature of ¯uid-rock interactions ¯uid=rock ratios) Garba, 1996). Comparisons between the composition of tourmaline in the veins and their wall-rocks indicate active interaction between ¯uids and wall-rocks. Gold-quartz-tourmaline veins of southern Ethiopia 117

A tourmaline sample from the talc-tremolite wall-rock has higher MgO, TiO2, SiO2 and lower Al2O3 contents than those from the chlorite-carbonate-biotite schist and amphibole-carbonate-chlorite schist. This indicates that at least Mg is partitioned into the hydrothermal ¯uids in similar proportions to their relative abundances in the host rocks. The pronounced zoning of tourmaline, with a Mg-rich core and Fe- rich rim in most of the samples, indicates a variation in the composition of ¯uids during tourmaline deposition. During the early formation of the Mg-rich core, there is the possibility of Fe being partitioned into an iron oxide, such as magnetite. Magnetite is paragenetically older than pyrite in the host rocks of the veins Deksissa, 2000). The increase in the Fe contents in the rims of tourmalines may be due to high concentrations of Fe in the hydrothermal solutions. This enrichment of Fe is caused by increasing intensity of alteration and formation of chlorite after ferro-actinolite in the wall-rock, replacement of magnetite by pyrite, and ankerite by calcite veins Deksissa, 2000). The occurrence of gold as inclusions in pyrite and chalcopyrite has been reported by Deksissa 2000). The association of gold, calcite, and second generation tourmaline in a quartz vein is shown in Fig. 5. These observations suggest that gold deposition occurred during the growth of the iron- rich tourmaline rims. Nature and origin of ¯uids responsible for tourmalinization: The occurrence of tourmaline in the vein paragenesis in the Okote area indicates that boron is an important constituent in the hydrothermal systems responsible for gold mineralization. The typical comb-like growth structures, in which the long axes of tourmaline are oriented perpendicular to fracture surfaces in the quartz-tourmaline veins, indicates that the in¯ux of boron-rich ¯uids is slightly younger than the ¯uids responsible for deposition of early stage qv2 veins. The laminar textures of the quartz-tourmaline veins suggest the presence of alternating shearing and hydrothermal ¯uid in®ltration events within a major tectonic episode. The concentration of pyrite, carbonate calcite and relict ankerite), and tourmaline in the gold-quartz-tourmaline veins indicates that S-bearing species, such as H2S, as well as CO2 and B, were signi®cant volatile components in the mineralizing ¯uids. Similar mineral associations have been reported from hydrothermal alteration zones at Bin Yauri, Nigeria Garba, 1996). Ethier and Campbell 1977) indicate that tourmaline is stable in weakly acid to moderately alkaline solutions, but that it is unstable in strongly alkaline solutions. The typical schorl-dravite composition of tourmaline at the Okote area suggests that they were formed from hydrothermal ¯uids within the temperature range mentioned below. Temperatures calculated based on AlIV contents in the tetrahedral site of chlorite, according to the regression equation of Cathelineau 1988), range from 323 to 411C mean 366 Æ 37 C, n 80 crystals) Deksissa, 2000). The metasomatic mineral assemblages for example, chlorite, carbonate, biotite, epidote), which are typical of greenschist facies, suggest an equilibrium temperature within the stability ®eld of biotite 400 C). The presence of tourmaline in the gold carbonate paragenesis suggests a likely pH range of 6 to 8 for the mineralizing ¯uids. Source of boron and gold: The origin of hydrothermal B in ma®c-ultrama®c rocks is enigmatic, as these rocks have very low B contents. Unaltered ma®c igneous rocks mostly oceanic crust) contain < 10 ppm B Chaussidon and Jambon, 1994). Basaltic and ultrama®c components of oceanic lithosphere are known to contain 80 118 D. J. Deksissa and C. Koeberl to 110 ppm B, following hydrothermal alteration at mid ocean ridges Mottl and Holland, 1978). On the other hand, terrestrial sediments have about 100 ppm B, whereas values for pelagic marine) clays are mostly in the range of 100 to 1000 ppm. Marine evaporites represent potential sources of concentrated boron with representative values of 100 to 2000 ppm London et al., 1996). Granitic magmas, especially those derived from meta-sedimentary sources, are essential in the redistribution of B from deep to shallow levels of the Earth's crust London et al., 1996). The occurrence of various lithologic units and structural complexities, such as thrusting followed by intensive shearing and hydrothermal alteration, makes the source of the ¯uid and gold more complex. Fluid inclusion studies of samples DKR-31 and DKR-36 indicate that the ¯uid is of low salinity maximum 4.94 wt% NaCl equivalent), and H2O±CO2 ¯uid. The ¯uid inclusions are generally two phase liquid=vapor inclusions that homogenize into liquid and vapor phase. Trapping temperatures of the ¯uids range from 218 to 300 CDeksissa, 2000). Dewatering of the protolith during progressive metamorphism can effectively remove boron from the meta-sedimentary rocks Moran et al., 1992). The mean FeO=FeO MgO) ratios of tourmalines from the Okote auriferous quartz-tourmaline veins range from 0.41 to 0.56 mean 0.47, n 7 samples) Fig. 10). The FeO=FeO MgO) ratio of tourmaline has been suggested as an indictor of source environments. Ethier and Campbell 1977) compiled FeO= FeO MgO) ratios 0.41 to 0.67) of tourmaline from meta-sedimentary environments. Granitic tourmaline and tourmalines associated with sediment-hosted massive sul®de deposits are 0.86 to 0.96 and 0.21, respectively Taylor and Slack, 1984). Pirajino and Smithies 1992) have shown that the FeO=FeO MgO) ratio of tourmalines associated with granite-related Sn and Sn-W hydrothermal mineralization in South Africa, Namibia, and New Zealand vary from < 0.6 to > 0.8, depending on the distance from the granitic source. They proposed that FeO=FeO MgO) ratios < 0.6 indicate deposition environments distal to the granite source. Tourmalines in metamorphosed and hydrothermally altered ma®c- ultrama®c rocks tend to be dravitic, but can have a signi®cant schorl component Slack, 1996). Comparisons of our data with literature values indicate that the FeO=FeO MgO) ratios do not discriminate between distal granitic and meta- sedimentary source rocks. Based on ¯uid inclusion data, Deksissa 2000) suggested a metamorphic origin for the B-bearing ¯uids. The strong association of tourmaline and carbonate in quartz veins, with intense wall-rock alteration, indicates that tourmaline has crystallized from hydrothermal ¯uids. However, the sources of the hydrothermal ¯uids are still unknown. The plot of ternary Fe±Ca±Mg and Fe±Al±Mg) discriminant diagrams Fig. 10) favor meta-sedimentary rocks as the main source. The mean of carbon 13C) and oxygen 18O) stable isotope ratios of 6 samples from cogenetic calcite veins are À7.1 Æ 2.05ù and 8.1 Æ 1.19ù, respectively Deksissa, 2000), indicating either magmatic melts or metamorphic dehydration processes as the most probable sources of the hydrothermal ¯uids. The mean sulfur isotope ratio 34S) of pyrite from the hydrothermal alteration zones associated with quartz veins is 2.68 Æ 0.98ù n 10), suggesting a signi®cant contribution of magmatic sulfur Deksissa, 2000). Gold-quartz-tourmaline veins of southern Ethiopia 119

Fig. 10. Classi®cation of chemical composition of the quartz-tourmaline veins of the Okote area, southern Ethiopia, using ternary diagrams of Henry and Guidotti 1985). Diagram a Al±Fetot)±Mg diagram. Fields 1 Li-rich granitoids, pegmatites, and aplites; 2 Li- poor granitoids and their pegmatites and aplites; 3 Fe3 -rich quartz-tourmaline rocks hydrothermally altered granites); 4 Metapelites and metapsammites with Al-saturated phase; 5 Metapelites and meta-psammites without Al-saturated phase; 6 Fe3 -rich quartz-tourmaline rocks, calc-silicate rocks, and metapelites; 7 Low Ca meta-ultrama®c and Cr-, V-rich meta-sediments; 8 Metacarbonates and metapyroxenites. b Ca±Fetot)± Mg diagram: Fields 1 Li-rich granitoids, pegmatites, and aplites; 2 Li-poor granitoids and their pegmatites and aplites; 3 Ca-rich metapelites, metapsammites, and calc-silicate rocks; 4 Ca-poor metapelites, metapsammites, and calc-silicate rocks; 5 metacarbonates; 6 meta-ultrama®cs

The occurrence of tourmaline associated with pervasive hydrothermal alteration Deksissa, 2000) in ma®c-ultrama®c rocks of the study area can be related to the tectonic evolution of the Adola Belt Berhe, 1990; Worku and 120 D. J. Deksissa and C. Koeberl

Schandelmeier, 1996). The generation of hydrothermal boron and associated gold involves several crustal processes, such as ocean ¯oor spreading and deposition of boron-rich sediments, followed by subduction, greenschist to amphibolite facies metamorphism, and intrusion of granites Berhe, 1990; Woldehaimanot and Behrmann, 1995; Worku and Schandelmeier, 1996). Finally, N±S trending shear zones created favorable conditions for deep-seated hydrothermal ¯uids to leach boron and gold from wall-rocks and to precipitate them in veins Deksissa, 2000).

Metallogenic and exploration signi®cance The very high gold contents of some quartz-carbonate-tourmaline veins and textural relationships among gold, calcite, and tourmaline in the veins indicate that tourmalinization and gold mineralization are genetically linked. Signi®cant tourmaline contents are known to be associated with mesothermal, shear zone- hosted, Archean gold deposits of the Kolar Gold ®elds of India, and Au-quartz veins of the Superior Province of Ontario Canada) and the Kaapvaal Craton of South Africa Slack, 1996, and references therein). Tourmaline is also a local gangue mineral in the Proterozoic Au-quartz vein from Ondonoc, W. Ethiopia Augustithis, 1967). The giant Muruntau ore body in Uzbekistan, possibly the largest hydrothermal gold deposit in the world, consists of Au-bearing quartz veins and minor to abundant tourmaline Slack, 1996, and references therein). Tourmalinites may in some cases be interpreted as facies equivalents of exhalative sul®de deposits and banded iron formations Plimer, 1988), and their recognition should stimulate exploration interest in any area in which they occur. The high Mg-contents of the tourmalines are consistent with hydrothermal leaching of the ma®c-ultrama®c host rock, but they do not exclude the possibility that some of the hydrothermal ¯uids were derived from granitoid rocks.

Conclusions The following points are concluded from the present study: ± The mineralized zones of the Okote area were produced during intense alteration of ma®c-ultrama®c rocks by interaction with B-, CO2-, and S-bearing hydrothermal ¯uids that were channeled along high strain zones. ± Petrographic and geochemical data of auriferous quartz-tourmaline veins show that gold and tourmaline are genetically related. ± The strong correlation between the gold contents and the abundance of Mg-rich tourmalines in carbonate-rich quartz veins makes this mineral assemblage an important prospecting guide for gold exploration in the Adola Belt of southern Ethiopia.

Acknowledgments This work is part of a Ph.D. project by the ®rst author, which was funded by the Austrian Academic Exchange Service Osterreichischer Akademischer Austauschdienst, OAD). Laboratory expenses were covered by the Austrian Science Foundation Y58-GEO to C. Koeberl). The authors also thank the National Mining Corporation, Dawa-Digati gold Gold-quartz-tourmaline veins of southern Ethiopia 121 exploration project, for admittance to their project area, and for providing support during the ®eld work to D. J. Deksissa, who also acknowledges the project geologists in general and S. Demisse, T. Tadese, Y. Edossa, and A. Adugna in particular, for their stimulating discussions during the ®eld work. The authors would also like to thank J. Newton Univ. Vienna) for help with stable isotope analyses, and T. Nta¯os Univ. Vienna) for his help with the electron microprobe analyses. We further thank P. Spindler and H. Froschl Arsenal, Vienna) for their help with ICP-MS analyses.

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Authors' address: D. J. Deksissa and C. Koeberl corresponding author), Institute of Geo- chemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, e-mail: [email protected]