Abstract
Petrographic and geochemical data of Upper Proterozoic, low grade metamorphic rocks (Tsaliet Group) in and around Negash is interpreted in this paper with their mineralization potential and influence on groundwater quality. Among the three types of metamorphic rocks, metavolcanics (MV), metavolcaniclastics (MVC) and metasediments (metapelites) (MP) are present in the study area. Metavolcanics are massive, non-foliated and show presence of relicts of plagioclase feldspar and pyroxenes set in a fine grained matrix. Metavolcaniclastics show presence of clasts set in a fine grained tuffaceous groundmass and indicate a significant compositional contrast between these, probably volcanic derived, groundmass and the clasts. The clasts vary in size and shape from angular, subrounded to rounded and even elliptical (due to shearing). Phyllite, the predominant lithounit of metasediments is composed of fine grained muscovite, quartz and chlorite with incipient foliation and at places well developed crenulations. Presence of chlorite together with poorly developed muscovite and biotite (rare) suggests low grade metamorphic conditions that prevailed 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 other minerals 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 rocks of the study area, trending N-S and showing presence of clasts with non-ideal 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 possible presence of a similar kind of mineralization elsewhere in the basement rocks around these plutons. Hydrogeochemical data indicate that groundwater is relatively fresh and 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 concentrations are related to the 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. Key words: Low grade metamorphic rocks, Base metal mineralization, Petrography, Hydrogeochemistry, Negash, Ethiopia
Highlights
The need for natural resources in the development of a country is an undisputable issue
The fragments are irregularly distributed and vary in composition from mafic to felsic; 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
6.1 Petrography On the basis of the dominant mineral phases that are present in different rock types in the study area and their textures, the rocks are named as metavolcanic, metavolcaniclastic, phyllite and slate
Summary
The need for natural resources in the development of a country is an undisputable issue. The effects on health will be more intense in the case of metallic compared to the non-metallic deposits In this light, an area around Negash in Tigray region of northern Ethiopia, a Neoproterozoic metamorphic terrain was chosen for study with the purpose of evaluating the metamorphic rocks, their potential to host mineral deposits and influence on groundwater quality. An area around Negash in Tigray region of northern Ethiopia, a Neoproterozoic metamorphic terrain was chosen for study with the purpose of evaluating the metamorphic rocks, their potential to host mineral deposits and influence on groundwater quality To achieve this objective, the metamorphic rocks were probed for petrographic and geochemical characteristics and the information on (a) primary mineralogy; (b) textural relations; (c) alterations or changes in the primary mineralogy due to hydrothermal or meteoric water activity; (d) metal concentrations associated with shear zones and possible relation to mineralization; (e) presence of secondary minerals like limonite or malachite; and (f) trace metal geochemistry of groundwater to define its quality and utility value is obtained. Interconnecting cross roads though are of gravel, are accessible for vehicles even during rainy season
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