Preliminary Geological and Geochemical Study of Gold and Base Metals Mineralization in The Surupa Area, Borena Zone, Southern Ethiopia

Background: The southern Ethiopian Precambrian shield terrain largely of sutured medium to high-grades assemblages of Precambrian crystalline basement consists of metamorphic complex and associated plutonic rocks which belongs to northern and southern continuation of the Mozambique belt and the Arabian-Nubian Shield. The paper reported the information on geology, alteration pattern, ore minerals and geochemical investigation for gold and base metals and to delineate potential area of gold and base metals mineralization. The purpose of the study was to carry out a preliminary investigation on geological and geochemical study for gold and base metals mineral potentials around Surupa area. Results: The study area consists of heterogeneous sequences of medium to high-grade poly-deformed gneisses with igneous and sedimentary precursors and structurally affected by three phases of deformation like foliations, mesoscale folds, veins and fractures/joints respectively. The rocks of the area characterized by its medium to high-potassium calc-alkaline rocks of high silicon dioxide and displaying predominance of potassium oxide over sodium oxide. The positive and negative correlation with major oxides exhibited indicate the fractionation nature of the rocks during magmatic differentiation. Conclusions: Based on the result, nickel and cobalt are highly anomalous elements indicating the source for the mineralization controlled by different structures such as fractures and joints. Ore mineral assemblages of pyrite, goethite, magnetite and hematite characterizes the mineralization of the study area. Generally, all those integrated studies con�rmed that mineral potential of the study area is mainly controlled by the nature of the rock and associated structures.


Introduction
The Precambrian basement rocks of the southern Ethiopia are sources of most of the economic metallic mineral deposits that include primary and secondary enriched deposits of gold, platinum group elements (PGE), nickel, tantalum, base metals, industrial minerals such as phosphate, iron ore, gemstones and also decorative and dimension stones (Ministry of Mines and Energy, 2009).Based on geological set up of the country and possible occurrence of gold and base metal deposits, several studies have been carried out by the governmental and private companies reported their occurrence from different parts of the country in the low-grade basement rocks of greenschist facies (Tadesse, 2004).These includes; brittle-ductile shear-zone related mesothermal gold deposits in Lega Dembi, Megado and Sakaro area; Au, Ag Alluvialeluvial placers in Adola Belt to Bore; Brittle-ductile shear-zone related mesothermal gold deposits in Adi Dairo-Indallilo area(northern Ethiopia); Au-Cu-Pb-Zn in Ashashire; Au-Cu in Bomo; v) Au-Cu-Pb in Digati; ductile shear zone-hosted mesothermal gold mineralization in south Okote area; vii) mesothermal type gold in south-west Akebo area, and epithermal gold in the rift valley and Afar regions, among others (Tadesse, 2004).
The biggest developed mine in the country is that of southern Ethiopia's Lega Dembi gold mine which remains the only mesothermal gold deposit under mining operations.It is related to the shear-zone hosted hydrothermal quartz veins within the Neoproterozoic metamorphosed volcano-sedimentary succession of greenschist to amphibolites facies metamorphism (Billay et al., 1997;Tadesse, 2004).It is estimated to have a reserve of about 260 tons of gold with an average grade of 6ppm (Billay et al., 1997) and over 43 tones and 65.94 tones with average grade of 3.7ppm at nearby areas Sakaro and Tulu Kabi around Lega-Dembi respectively (Ministry of Mines and Energy, 2009).
Accordingly, the study area is located very close to the well-known Adola gold eld within the southern Ethiopia greenstone belt and there is an extensive artisanal mining activity by the local people for gold.
However, no systematic study were conducted in terms of their geology, alteration pattern, ore mineral assemblages, chemistry and petrography of the host rock responsible for gold and base metals mineralization.
The purpose of this paper is to provide information on geology, alteration pattern, ore minerals and geochemical investigation for gold and base metals and to delineate potential area of gold and base metals mineralization.

A. Study area
The study area covers an area of about 50 sq.km in Surupa Woreda of Borena zone, Oromia Region, Southern Ethiopia, which is bounded by, coordinates of 412000mE to 438000mE longitude and 560000mN to 586000mN latitude (Fig. 1).Topographically, it is characterized by very rugged to gentle sloping from northwest-southeast direction and ranges from 2000 to 1600 meters above mean sea level.The region experiences of a bi-modal monsoon rainfall type, where 60% of the 300-900mm annual rainfall occurs during March to May and 40% between September and November.
Heavy cloud cover, vapor and occasionally short showers characterize the period from June to September, while the main dry season occurs from November to March with high evaporation.The highland areas get the mean annual precipitation in the range of 600 to 900 mm whereas the lowland areas receive the lowest annual precipitation in the range of 300 to 500 mm.Furthermore, the minimum and maximum temperatures of the area fall within the range of 13.3 to 17 0 C and 18 to 25.1 0 C, respectively.Generally, the lowland areas tend to be relatively hotter than the highland areas.

B. Samples and Methods
Representative whole rock samples from different lithological units were collected and submitted to Geological Survey of Ethiopia, Addis Ababa for sample preparation by crushing to 70% less than 2mm, ri ed to split off 1kg and the splits were pulverized to 75 microns.The pulverized samples were shipped to ALS Services, Ireland laboratory for whole rock analysis to represent combination of ICP-MS and ICP-AES.Trace elements including Ba, Ce, Cr, Cs, Dy, Er, Eu, Ga, Gd, Hf, Ho, La, Lu, Nb, Nd, Pr, Rb, Sm, Sn, Sr, Ta, Tb, Th, Tm, U, V, W, Y, Yb and Zr were analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to understand the magma source and to discriminate tectonic setting of the rocks of the area.However, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) used for analyzing major elements in their oxide forms including SiO Additionally, about sixty stream sediment samples were collected mostly from rst order streams and analyzed for Au, Pb, Co, Cu, Zn and Ni by Atomic Absorption Spectrophotometer (AAS) at geochemical laboratory of Geological Survey of Ethiopia, Addis Ababa.About nine thin sections and ve polished sections have been prepared at Geological Survey of Ethiopia (GSE), Addis Ababa and analyzed at both Mekelle and Bahir Dar Universities for respectively.The fresh parts of the rock samples were sliced by the rock cutter and polished by DAP-U polishing machine to obtain the required size not greater than 4 cm to t the mould size, which has a maximum diameter of 4cm.The rock samples were ground mechanically using carbide abrasive until the slice reaches the conventional thickness of 30 microns.This is carried out using petro discs with 400-grit aluminum oxide powder.

Result And Discussion
Regionally, the basement rocks of Southern Ethiopia including the study area is found in the transitional zone with imprints of Pan-African tectonothermal events (Vail, 1976;Kazmin et al., 1978;Davidson, 1983).The Precambrian rocks of Southern Ethiopia are heterogeneous sequences of high-grade polydeformed gneisses with igneous and sedimentary origin and mainly represented by Moyale Belt, Adola Belt, and Bulbul Belt comprising the known placer and primary gold mineralization.These rocks are subjected to middle to upper amphibolite facies metamorphism with localized granitization and migmatization and pockets of granulites.The study area is the northern extension of the Moyale Belt and very close to the known Adola belt.
Based on the dominant mineral assemblages and their textures, the rocks are named as feldsparplagioclase-biotite gneiss, feldspar-plagioclase-biotite-muscovite gneiss, feldspar-quartz-plagioclasegranitic gneiss, feldspar-plagioclase gneiss, plagioclase-pyroxene schist and actinolitic hornblendequartz-plagioclase-schist.The nature of alignment, texture, size and type of minerals identi ed in the rocks implies that the rocks have experienced middle to high-grade amphibolite facies metamorphism (Fig. 2).The mineral assemblage from this rock indicates that the protolith is possibly intermediate rock and corresponding geochemistry (Appen1, Table 1, and MtR1) plotting in the intermediate eld (Fig. 3).

Univariate data analysis
The distributions of the element concentrations in the stream sediments are observed to be strongly related to speci c spatial and environmental factor like hydrothermal alteration zones, geological structures and lithologies drained by streams in the area (Figs. 4 and 5).Concentrations of nickel, cobalt and zinc are direct related and highly anomaly in the northwestern part of the study area mainly clustered on the metagranite and biotite gneiss rocks, which indicates the source for the mineralization controlled by different structures such as fractures and joints (Fig. 5B, E & F).However, the gold result from the stream sediment riches 0.116ppm indicates low concentration relative other base metals.The reason for the low content of Au in the stream sediment is, most of the stream sediments are not properly channelized, and hence accumulation of heavy elements like gold is randomly not stream channel controlled.

Multivariate data analysis
The Zn-Ni-Cu association represented by the rst principal component (PC1) is interpreted to be a multielement geochemical signature re ecting a possible combination of lithologic and chemical controls.Co has weak correlation in this principal component, which suggests these elements are depleted in the mineralized parts of the above stated lithological controls (Table 2).

Whole rock Geochemistry
Rock geochemical data of the study area show different variation in their geochemistry.Based on the analysis, most of major elements plotted against SiO2 show systematic negative correlation with TiO 2 , Fe 2 O 3 , MgO, CaO, and MnO, which can be explained by magma differentiation.The concentration of K 2 O and Na 2 O increase with increasing of SiO 2 concentration even if there are scattered and irregular distributions patterns (Fig. 6).This indicates normal fractionation igneous trends, which display a continuous gradation in chemical composition from ma c to felsic indicating the parental magma differentiation.The Na 2 O and K 2 O shows a wider data spread and small data scatter which are positively correlated with SiO 2 .Based on the idea of (Cox et al. 1979), biotite gneiss and metagranite rock units of the area are abundantly falls in the felsic elds and some analysis of amphibolite schist is anked by intermediate eld, which may be due to the compositional variation of the clasts (Fig. 3).The discrimination between tholeiitic and calc-alkaline series interpreted by AFM plot (Irvine and Baragar 1971), (Fig. 7) emphasis that the rocks the study area typically occupy the calc-alkaline a nity indicating their (Na 2 O + K 2 O) rich in nature.In addition, on SiO2-FeOt/MgO plot (Miyashiro 1974) and SiO 2 -K 2 O plot (Peccerillo and Taylor, 1976), (Fig. 7) most of the samples occupy the calc-alkaline series eld and high-K calc-alkaline series respectively.The Chondrite normalized REE abundances plot (Boynton, 1984) (Fig. 8),shows all the rock samples of the area has moderate to high negative Eu anomalies which indicate removal of feldspar from the melt by crystal fractionation or the partial melting of a rock in which feldspar is retained in the source (Rollinson, 1993).
According to the ideas of Rollinson (1993), the REE patterns of rocks show high enrichment in LREE and high depletion in HREE when the melting percentage decreases.However, the REE pattern of Surupa rocks shows only slight variation between the abundances of LREE and HREE, which indicates moderate to high percentage melting.
Most samples of the area shows enrichment in the incompatible large ion lithophile elements (LILE: Ce, Ba, Th and Rb) relative to high eld strength elements (HFSE: Zr, Y, Eu, and Yb).Large negative Ti anomaly indicates the fractionation of Ti-magnetite.The enrichment of Ba, K and depletion of Nb signifying contamination during fractional crystallization (Fig. 9).
Tectonic discrimination diagram suggest that the rocks have a tendency towards Calc-alkaline basalt (CAB) tectonic setting, which can be characteristic of subduction related magmatism.

Conclusion
The study area consists of heterogeneous sequences of medium to high-grade poly-deformed gneisses with igneous and sedimentary precursors and structurally affected by three phases of deformation (D1 to D3) like foliations, mesoscale folds, veins and fractures/joints respectively.Mineral assemblage, gneissose and some schistose texture properties suggest that the rocks of the study area have experienced medium to high grade amphibolite facies metamorphism and shows development of different type of alteration related to metamorphism and hydrothermal activity based on their abundance speci cally silici cation, oxidation, chloritization and epidotization.Based on the result, nickel and cobalt are highly anomalous elements indicating the source for the mineralization controlled by different structures such as fractures and joints.Ore mineral assemblages of pyrite, goethite, magnetite and hematite characterizes the mineralization of the study area.Generally, all those integrated studies con rmed that mineral potential of the study area is mainly controlled by the nature of the rock and associated structures.

Declarations
Dear Sir, I would like to submit the manuscript entitled "Preliminary Geological and Geochemical Study of Gold and Base Metals Mineralization in the Surupa Area, Borena Zone, Southern Ethiopia to be considered for a publication of original article in the Energy, Sustainability and Society Journal website.
The study area, which is located very close to the well-known Adola gold eld within the southern Ethiopia greenstone belt have recently found gold by artisanal mining activity and local people were experienced by crushing the quartz and country rock for primary gold mining.
Daniest mining PLC.( 2015) have also reported that they have identi ed occurrence of gold and base metals during rst year of systematic exploration, which involved geological mapping and geochemical sampling.However, there was no any systematic exploration conducted in the area except the Daniest Mining Company, which was collected soil sampling besides to the geological mapping and does not have analyze for their trace element.Therefore, this study has been designed to provide a better understanding of the geology, and geochemical investigation, and as well to search other possible targets of exploration activity.
I, Waktola Merera, a resident of Ambo town of Oromia state of Ethiopia, lecturer at Bule Hora University of Geology department, do hereby declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.
The data I used to accomplish this work were mostly primary data analyzed in different laboratory of Geological survey of Ethiopia as well as Ireland laboratory for rock geochemistry and all data are currently available with me.
I know of no con icts of interest associated with this publication, and there has been no signi cant nancial support for this work that could have in uenced its outcome.As corresponding Author, I con rm that the manuscript has been read and approved for submission by other people invited for their comments.
The second principal component (PC2) (Table2) represents antipathetic behavior between the negatively loaded Cu-Co and positively loaded Au-Pb.PC2 is possibly due to the leaching of elements from parent rocks enriched in Pb & Au and depleted in Cu & Co.Therefore, PC2 are more likely in the mineralized parts of the rocks of the study area.The variations explained by the PCs can be spatially outlined by calculating the principal component scores.The principal components are calculated according to the formula (George and Carter, 1989) as follow: Where Sci = scores for sample I on component c; Icj = loadings on element j on component c; Zij = concentration of element j for sample i.The geochemical investigation in the study area has proved successful in locating the base metal anomalies, associations and potential areas for mineral deposits.Most of the high anomalies recognized from the stream sediments correspond with metagranite and biotite gneiss rocks of north and southwestern part of the study area.The association in Zn-Ni-Cu more re ects to the underlying lithology of metagranite and biotite gneiss unit and the association of Au-Pb is related to base metal mineralization.

Figure 1 Location
Figure 1

Table 2
Na 2 O + K 2 O), F-(FeO + Fe), and M-(MgO), (B and C) SiO 2 vs. Fe 2 O 3 /MgO and K 2 O of the high K Calc-alkaline rock series further into low-K tholeiitic series of the study area.