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

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