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

Abstract

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. Key words: Low grade metamorphites, Base metal mineralization, Petrography, Hydrogeochemistry, Negash, Ethiopia