Petrographic and Geochemical Study of Low Grade Metamorphic Rocks around Negash with Reference to Base Metal Mineralization and Groundwater Quality, Tigray, Northern Ethiopia

The paper presents preliminary petrography and geochemical data of Precambrian low grade metamorphic rocks from Tsaliet Group around Negash and discusses their mineralization potential and effects on groundwater quality. Petrographic data suggest that among the three types of metamorphic rocks( metavolcanic, metavolcaniclastic and metasediments present in the study area), metavolcaniclastics shows the presence of clasts set in a fine grained tuffaceous groundmass and indicates a significant compositional contrast between these, probably volcanic derived, groundmass and the clasts. The clasts vary in size and shape.They vary from angular, rounded to sub-rounded and elliptical shape (due to shearing). Metavolcanics are massive, non-foliated. They show the presence of relicts of plagioclase feldspar and pyroxenes which are set in a fine grained matrix. Phyllite, the predominant lithounit of metasediments is composed of fine grained muscovite, quartz and chlorite with incipient foliation and at places well developed crenulations. The presence of chlorite together with not well developed muscovite and biotite (rare) suggests low grade metamorphic conditions in the area. Geochemical data of metavolcanics indicate variation in their composition from basalt to andesite. Metavolcaniclastics are relatively enriched in alkalis and silica and deficient in MgO compared to metavolcanics. Phyllite, on the other hand, is enriched in K2O and silica compared to metavolcanics. Development of chlorite, sericite and others due to low grade metamorphism and hydrothermal alteration have modified the chemistry of the rocks particularly MgO by chlorite in phyllite. Shear zones are common in the area, trending N-S and showing the presence of clasts with nonideal tails, relatively higher amount of quartz veins, malachite stains, Cu anomalies and sericitisation. These stains and anomalies strongly suggest a shear zone-controlled copper mineralization. Island arc setting, bimodal volcanism, intrusive granitic plutons and similar type of shear zone –controlled Zn-mineralization in the nearby Abrha Atsbha area indicate the possible presence of a similar kind of mineralization elsewhere in the basement rocks around these plutons. Hydrogeochemical data indicate that groundwater is relatively fresh. Among major elements Na, Ca and Mg show relatively higher values compared to K. Water from metasediments is relatively harder among others. Na, though shows higher values compared to Ca and Mg, does not indicate any particular trend. Ca and Mg in water are related to the primary mafic and plagioclase feldspar minerals. Among trace elements, iron, nickel and lead show relatively higher values compared to other analyzed elements. Fe and Ni are related to metavolcanics and metavolcaniclast, and Pb to metapelite. K. Bheemalingeswara and Nata T (MEJS) Volume 1 (2):106-132, 2009 © CNCS Mekelle University 107


INTRODUCTION
The need for natural resources in the development of a country is an undisputable issue. The resources are available either on the surface or subsurface of the Earth. Search for these resources whether mineral or water thus mainly targets the Earth's crust. The resources are the result of a particular or combination of geological processes and occur in a suitable geological condition. A variety of exploration methods such as geological, geochemical and geophysical either single or in combination are employed in search of these resources, keeping in view the geology of the area and type of resource. The basement or Precambrian terrains which are invariably metamorphosed, tectonically disturbed and affected by intrusive plutons are some of the potential targets for a variety of mineral deposits e.g. base metal sulphides, native gold, skarn, greisen, pegmatite etc). Use of metallogenic/ deposit genetic models has become an important step in outlining prospective areas regionally in the search of mineral resources. A preliminary investigation related to geological mapping, petrography, alterations, and geochemistry provides the information whether to undertake a detailed survey or not. It is very common that in majority cases the ore deposits that we come across may be of low grade and uneconomical. Though the deposits would commercially be uneconomical, they could contribute high quantities of metals to the percolating meteoric water and circulating groundwater. The constant interaction of water with ores and gangue minerals facilitate leaching and release of metals into the aquifer system, depending on the nature of solubility of different minerals and geochemical condition, thus influencing the quality of groundwater and in turn causing problems to the end users in various ways.
The abundance of minor and trace elements, which will be generally below 1% of the total, can often get enriched to significant levels, depending on the geochemical conditions. For example, decrease of pH by one unit may lead to an increase of more than one order of magnitude in the concentration of certain metals like Al, Be (Edmunds and Smedley, 1996). Increase in the acidity of groundwater is a common feature particularly in non-carbonate areas (e.g. granite) or in areas where oxidation of sulphides (e.g. pyrite) is common. This increases the solubility of many metals like Al, Be, Cd, Pb, Cu etc. Such changes are also common when there is a change from Figure.1. Geological, rock, water sample location map around Negash, Tigray, Northern Ethiopia (modified after Arkin et al., 1971).   (Garland, 1980;Miller et al., 1967;Mock et al., 1999;Alemu, 1998;Tadesse, et al., 2000;Ayalew et al., 1990;Rogers et al., 1965;Jelene, 1996;Gilboy, 1970;Teklay et al., 1998). These ages are also considered to be related to the major structural and metamorphic events in different terrains in Ethiopia (Teklay et al., 1998;Ayalew et al., 1990;Tadesse, et al., 2000). The basement rocks in the study area are classified under Tsaliet and Tambien Groups (Kazmin et al., 1978). Tsaliet Group rocks are older and dominated by metavolcanics (MV), metavolcaniclastics (MVC) and metasediments/ metapelite (MP). MV is the oldest and MP is the youngest. The younger Tambien Group consists of only metasediments.
The area is also marked by the presence of two prominent faults: Negash fault trending N-NE and the younger Wukro fault trending E-W (Fig.1). Negash fault is a thrust fault where the younger Tambien Group rocks have thrusted over the older Tsaliet Group rocks (Fig.1). Since the fault is involved between the Tambien and Tsaliet Group rocks and also confined to the basement rocks, it may be related to the age of the Negash pluton. Wukro fault is a normal fault and much younger in age and involved in the upliftment of the basement rocks (Negash) with relation to the Mesozoic sedimentary rocks (Wukro) of the Mekelle basin. The intrusive granitic plutons and related tectonic activity has facilitated generation and mobility of ore-forming hydrothermal fluids within the basement rocks. Further, the fluid-wall rock interaction has produced alterations e.g. epidotisation, sericitisation by modifying the primary mineral assemblages.Among the rocks of Tsaliet and Tambien Groups, only the rocks of Tsaliet Group are considered for the present study and discussed in the paper. Brief description of the rocks is given below.

Metavolcanics
These rocks are dominated by metabasalts with subordinate meta-andesites and metarhyolites.
The metabasalts are massive, non-foliated, and fine grained. They typically show green color

Metavolcaniclastic
These rocks are relatively predominant in the area and occupy a large part of the study area.
MVC shows significant variation in the size, shape and chemical composition of the clasts and in turn overall rock composition. The clasts vary in size from very coarse in the east to fine in the west. They also vary in shape from angular, elongated (elliptical), sub-rounded to round. The clasts are mainly lithic and mineral fragments varying in composition from mafic (amphiboles/ pyroxenes) to felsic (feldspars and quartz). The matrix consists of fine grained material at places shows green color. The clasts in general show random orientation and at places show alignment indicating fluvial transport. On the basis of the field association, lack of proper sedimentary characteristics, fine grained matrix and clasts with varying composition and random orientation, the rock is considered to be volcanic-derived with limited amount of sedimentary input. Pyrite crystals are common in the rock but mostly are altered to limonite.

Metasediments
These are represented by phyllite and slate. Megascopically, the rocks are fine grained and show well developed foliation in phyllite and slaty cleavage in slate. These rocks are more prominent in the eastern and western parts of the study area and show a range of colors such as light grey, dark grey, green and brown/red. Development of crenulation cleavage is prominent at places.
Schistosity though is poorly developed, is seen along the contacts with the intrusive pluton where development of muscovite is quite prominent. Presence of pyrite crystals with cubic outline is quite conspicuous in these rocks. In majority cases, these are altered to limonite (pseudomorphs) and show red color.

Shear Zones
The zones trending N-NE are characterized by the presence of tails of non-ideal simple stress.
The rigid clasts (lithic fragments) show rotation and developed cracks during the shearing of the K. Bheemalingeswara and Nata T (MEJS) Volume 1 (2): 2009 © CNCS Mekelle University 114 matrix. The tails or the pressure shadows extend on either side of the clasts and the rotation of the clasts is marked by the layers/ tails being rolled around the rigid clasts (David, 1993). In the field, these zones are common in MVC and MP. The rigid clasts also show breakdown producing cracks possibly due to stretching and rotation. They are later filled by the fine grained matrix.
Relatively well developed foliation/ schistosity/crenulations, increased quartz vein activity, sericitisation and malachite stains are some of the common features associated with these zones.
Presence of green colored malachite is quite conspicuous in these zones indicating base metal mineralization. These zones vary in width from less than a meter to few meters and most common along the contacts between MV, MVC and MP. It is interesting to note that these zones are prominent only in Tsaliet Group rocks not in Tambien Group in the area.
Precambrian stratigraphic succession of the rocks in the study area from younger to older is as below (Kazmin et al., 1978;Miller et al., 2003).  in the Geochemistry Laboratory, Department of Earth Science, Mekelle University. Elements such as Ca, Mg, Na, K, Cu, Pb, Zn, Mn, Fe, Ni and Co were analyzed after setting the instrument, using the prescribed standard conditions. Chemical standards in the range of 1 to 10ppm for each element were used for constructing standard working curve and also as reference. The detection limits for all the elements except Pb was 0.1 ppm. For lead, it is 1.0 ppm. The precision range for these elements was within + 5-10%. The petrographic data is given in table.1 and whole rock and water sample data are given in tables 2 to 4. All rock and water sample locations are shown in figure.1.

Petrography
Among 30 samples studied for petrographic details, 15 are from MVC, 10 from MV and 5 from MP. On the basis of the petrographic investigation (Table. 1), the rocks are described below.
They are 1) metavolcanic -consists of plagioclase feldspar, hornblende, inequigranular quartz, relict and reaction textures with poorly developed schistosity indicated by chlorite. Green colored hornblende and colorless needle shaped tremolite showing random distribution and sometimes wrapping the relict pyroxenes indicate decussate texture; 2) metavolcaniclasticquartz, chlorite, muscovite, biotite, lithic fragments with different compositions, with well developed schistosity (at places), the fragments are being wrapped by the sheet silicates. The matrix comprises of fine grained material possibly volcanic derived tuff (glass)? The fragments sometimes show orientation indicating flow structure and also elongation due to stretching. The

K. Bheemalingeswara and Nata T (MEJS)
Volume 1 (2): 2009 © CNCS Mekelle University 116 fragments are irregularly distributed and vary in composition from mafic (pyroxenes) to felsic (feldspars); Mineral associations and field relations suggest that the rock is metavolcaniclastic rock. 3) phyllite-consists of fine grained quartz, chlorite and muscovite with well developed foliation and crenulations; and 4) slate-consists of similar mineralogy as in phyllite but with very fine and poorly developed foliation.  (40) Quartz (30) Calcite (15) Hornblende/ Chlorite (10) Opaque (5) The rock (plagioclase, quartz, calcite, hornblende/ chlorite and opaque shows medium grained plagioclase feldspar laths together with few coarse-grained rounded to sub-rounded quartz clasts in fine grained quartz dominated matrix. The mafic minerals though not very prominent seem to have undergone alteration to produce secondary chlorite. Younger, relatively more intense, calcite veins cut across the primary minerals (also incipient schistosity). Opaque minerals are randomly distributed in the rock and unrelated to these secondary veins and suggest that they are produced due to chemical break down of the primary mafic minerals.

MV (Metaandesite)
ND 71 Tremolite/ Hornblende (25) Pyroxene (10) Quartz (15) Plagioclase (20) Muscovite (10) Chlorite (15) Opaque (5) The section is of highly altered metabasalt. It shows presence of randomly oriented minerals (decussate texture) such as green color hornblende and colorless tremolite (?) needles and plagioclase laths and quartz with undulose extinction. The pyroxene (mostly relicts) and amphibole minerals show alteration to chlorite. Muscovite is also present together with quartz particularly along the veins. Opaques are mainly primary related to magnetite, ilmenite? Opaques related to alteration of mafic minerals are also present but in minor amounts.

MV (metabasalt)
ND 63 Muscovite (45) Quartz (25) Chlorite (20) Opaque (10) The rock consists of predominantly muscovite and followed in the order of abundance are quartz, chlorite, and opaque minerals. The minerals are fine grained and show a well developed foliation. Three sets of foliation trends are observed as indicated by the alignment of fine grained muscovite, chlorite and quartz. Minor amounts of opaque minerals are seen associated with later quartz veins probably indicating mobilized ore minerals. Due to crenulation, common in phyllite, preferential dissolution of quartz takes place at higher angle to the shortening direction; relict quartz grains become increasingly in-equidimentional. Precipitation of quartz is seen taking place predominantly in the fractures cutting across the crenulations. Opaque minerals do follow the trend of foliation.
Phyllite (Quartzmica schist?) ND 21 Quartz (60) Plagioclase (10) Muscovite (10) Chlorite (15) Opaque (5) Lithic fragments (20) The rock is composed of quartz, muscovite, chlorite, feldspar and opaque minerals. It is dominated by the presence of medium to coarse grained clasts of quartz and feldspar minerals. Quartz is irregular to sub-round in shape and feldspars occur as laths. The clasts show elongation due to deformation, parallel to the schistosity. Because they are competent, produce cracks which are later filled by the incompetent sheet silicates, secondary quartz, feldspar and opaque minerals. The lithic fragments are mostly mafic (mostly pyroxene) and hence are highly altered and produced iron oxides. Opaque minerals fine to coarse in size (probably pyrite) and are mainly in association with quartz veins. These opaques often show alteration to secondary iron hydroxides. The matrix is fine grained and consists of volcanic derived tuff (glass)? On the basis of the nature of the clasts and associated mineralogy and field association the rock is metavolcaniclast.

K. Bheemalingeswara and Nata T (MEJS)
Volume 1 (2): 2009 © CNCS Mekelle University 117 ND 58 Quartz (35) Chlorite (15) Muscovite (25) Calcite (10) Lithic fragments (15) The rock shows presence of lithic fragments (like lenses), comprising of fine grained aligned mica-rich mineral within quartzose matrix. The fragments show an alignment indicating a flow and also stretching indicating shearing effects and are wrapped by the quartzose matrix. The clasts are originally mafic rock fragments altered to secondary minerals like chlorite, biotite and muscovite. Equigranular quartz and calcite are also present in association with the clasts (mafic) as secondary products. They are produced after the chemical breakdown of the mafic minerals (clasts). Absence of feldspars in the section amply suggest that the parentage is sedimentary not igneous. The matrix is fine grained and consists of volcanic derived tuff (glass)? On the basis of the nature of the clasts and associated mineralogy and field association the rock is metavolcaniclast.      Fig.4 and Tables 1 and 2). Matrix varies from grey/ white colored volcanic tuff (+fine argillaceous) with fine clay/ash/muscovite to green colored fine grained mafic composition.

MVC
There are also indications of the presence of glassy material (glass shards?) in the section indicating volcanic source. The clasts, on the other hand, vary from dominant quartz, feldspar to rare green colored mafic rock fragments and show angular, rounded, sub-rounded and irregular shapes. The source for such variation in composition in clasts and matrix is assumed to be due to arc related volcanic activity with limited sedimentation. On the basis of the petrographic data, it may be said that the source for MVC is primarily volcanic with subordinate sediments. The clasts often show elongation/ stretching, indicating shearing effects and are typically related to the shear zones in the field. The size variation of the clasts from east (coarse) to west (fine) in the study area seems to indicate the direction of the source of supply to be in the east. However, these rocks being subjected to later folding and faulting, such relationship needs to be established only after regional correlation. Secondary quartz is more prominent in metavolcaniclastic rock as  Table 2). Absence of any indications of development of gossan, which is common in base metal sulfides, is quite conspicuous in the area. Anyone or more of the following reasons may explain the absence or non-development of gossan-i) the base metal mineralization is of very low grade, ii) pyrite shear zone-related and dispersed.
Finally, uplift of the basement rocks has facilitated meteoric water-rock interaction in addition to the hydrothermal solutions-rock interaction and has resulted in kaolinisation, sericitisation etc. So, the mineralogy and related textural properties suggest that the rocks have experienced low grade regional metamorphism and emplacement of granitic plutons have facilitated oreforming fluids that are dominant in the shear zones.

Geochemistry of Basement Rocks and Mineralisation
Tectonic discrimination diagram (Fig.6) drawn between TiO 2 and Zr for metavolcanic rocks suggests an island arc tectonic setting. Basic metavolcanic unit is the dominant unit among MV which is calc-alkaline in composition and related to volcanic arc setting environment (Alene et al., 2000). Major oxide data indicates lower values for K (Fig.4) suggesting that the metavolcanics are not only calc-alkaline in composition but also low -K type. Erratic behavior and low values of K seems to be a common feature in metamorphosed rocks due to the mobility of K either through diffusion, albitisation of plagioclase or loss due to mobility during metamorphism (Alene et al., 2000). Figure 6. Tectonic setting discrimination diagram, metavolcanics of Negash (Pearce, 1980 On the basis of the present and published petrological, geochemical and structural data, it is possible to construct a sequence of events that have taken place in the area. Sedimentation and development of linear sedimentary belts in intra oceanic basins (Tadesse, 1999)   Regarding tectonic setting, the rocks in the study area represent intra-oceanic arc sequences of northern Ethiopia (southern part of Arabian Nubian Shield) that have varied lithological and geochemical characteristics (Tadesse, 1999) and are consistent with the arc accretion models as suggested for Sudan, Egypt and Saudi Arabia (Alene et al., 2000). The arc related tectonic setting together with bimodal volcanism though supports VMS type base metal mineralization, a shear -controlled base metal mineralization is identified in the study area on the basis of petrographic, structural and geochemical data.

Geochemistry of Groundwater
Water hardness, one of the important parameters used to evaluate natural water quality is generally known to be influenced chemically by dissolution of calcite and dolomite. But it can also be influenced by other rocks which have significant amount of Ca and Mg -bearing soluble plagioclase feldspar and mafic minerals respectively. The proportion of Ca 2+ and Mg 2+ present in water can be used as an indicator of the geology of the aquifer. Calculated values for total hardness in 32 water samples range from 130 to 460 ppm. According to the hardness classification, the majority of the samples from MP come under hard water category (Todd, 1980 values about 20. It is known that higher amounts of sodium affects the soil by decreasing permeability and become toxic to plants (sodium poisoning). So, the soils either alkali (sodium with carbonate) or saline (sodium with chloride or sulphate) will not support proper plant growth. Hence the water is non-suitable for irrigation purpose (Fetter, 1994). According to TDS values, the water type is fresh water type and fit for domestic use.
Trace element data being limited, it is difficult to say whether the water particularly from shear zones is safe for drinking purposes. It is well known that the base metal mineralization, even low grade, can contribute trace elements Cu, Zn, Pb and others significantly to groundwater and influence their quality. The water geochemical data when compared with the water quality and lead to MP. Both Fe and Ni seem to be related to the primary mafic mineralogy such as olivine, pyroxenes and amphiboles ( Fig.2A-D ;Alene et al., 2000).The mafic and opaque minerals as mentioned above are quite susceptible to breakdown and capable of providing trace metals in significant quantities to groundwater.
Shear zones being good aquifers, have become sites for the construction of many hand dug wells.
Since these zones show incidence of base metal mineralization with varying metal contents (Cu,

CONCLUSION
• The presence of chlorite, muscovite, biotite ( where the source is related to basic metavolcanics. It is comparable with the nearby (Abrha Atsbha) shear zone-controlled Zn-mineralization where the source for Zn is related to metarhyolite.
• Groundwater geochemical data indicates that the water is safe for domestic purposes.
Trace elements such as Ni, and Fe are related to MV and MVC whereas Pb values to MP.
The major elements are Ca and Mg. Ca is related to plagioclase and epidote in MV and MVC, and Mg to chlorite in phyllite.
• Shear zones being mineralised and also form good targets for groundwater development, they need to be probed in detail to avoid long term human risks.

ACKNOWLEDGEMENTS
The study forms part of the NORAD-II funded project entitled "Qualitative and quantitative evaluation of metamorphites hosted base metal/ transition metal (Pb, Zn, Cu, Fe, Mn, Ni, Ag and Co) mineralization and its impact on groundwater and its chemistry in the area around Negash,