Geology

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Browsing Geology by Author "A., Anatoli (PhD)"

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    Mineralogical and Geochemical Studies on the Sakaro Primary Gold Deposit (Sidamo, Southern Ethiopia): Zonation in Ore Bodies And Host Rocks
    (Addis Ababa University, 1993-06) Glrma, Mesfin; A., Anatoli (PhD)
    The Sakaro deposit is a vein-type fault-hosted gold deposit in the Adola area I southern Ethiopia. It is composed of qua rtz veins up to 9.3 m wide which occupy fault planes aloog foliation and petrographic contacts in precambrian low-grade metamorphic rocks. The veins are gold-rich and sulfidepoor, forming a mineralized zone of 760 m in strike length and up to at least 150 m in vertical extent. Veins Nos . 1 and 2 are the richest shoots with an estimated total ore reserves of 760,800 tons capable of producing 2,551 kg of gold at an average grade of 3.4 gft and 5.7 gft respectively. Geochemical and mineralogical studies of the ores and the bost rocks reveal that the Sakaro deposit might include tungsten other than gold as a potential exploration target. Tungsten mineralization is the earliest to be formed. Tungsten content increases up to > 1% in the ore veins and in the wall rocks (mainly in amphibolites proximal to the veins), forming a distinct zone that assumes W-As -Cu, a geochemical zone or wolframite-scheelite-iron sulfides zone, a mineralogi c al zone. Gold is mostly associated with Ag and Pb, showing asymmetrical dispersion pattern that starts in the hanging wall side of the vein and decays towards the foot wall rocks. Its tight relation with Ag and Pb is expressed by forming a Au-Ag-Pb zone though it i s a l so related at a lesser degree with other elements especially with Cu. Outward from this zone, Ag-PbCu, W- As-Cu, and Fe-Co-Ni-Mo-Cu follow in respective distances from the are veins toward enclosing rocks. Gold distribution in the ore veins is control led by thickness variation of the quartz fillings in both strike and dip directions, poor in swell s and rich in constrictions. In the plane of the ma in vein, five are columns are identified with marked contrast in gold mineralization: (1) obliquely running, EW dipping gold rich zone, (2) north e rly trending, vertically dipping gold rich zone, ( 3 ) intermediate low gold zone, (4) oxidation zone, and (5) surf i cial weathering zone. The first two zones are are s hoots representing the fronts of gold depositing mineralizing fluids . Gold contents follow a systematic pattern o f more or less regular course within and away from these front s which otherwise are perceived as very irregular and erratic distributions . Moreover, statistica l treatment on the meta l content shows that gold distributions in populations separated according to the rock types and environments obey lognormal law , with mean values highest in ore veins, less in amphibolites, and least in mica schists. Studies on the vein development reveal three distinct types of hypogene mineralization from veins margins to the center, of marcasite + chalcopyrite in the peripheral parts, galena + chalcopyrite + marcasite in the following portion, and sphalerite + h igh galena + c halcopyrite in the centra l part. Gold is present in all of the above associations, however, i t is highly enriched in sphalerite-high galena-chalcopyrite zone, followed by hanging wall side marcasite-chalcopyrite zone , and least in foot wall side marcasitecha l copyrite zone. The gold enrichment in spha lerite-high galena-chalcopyrite zone indicates the close association of Au, Ag, Pb , Cu and Zn . The selective enrichment of gold in the hanging wall side (marca s ite-chalcopyrite) zone is most likely due to intense shear remobiliz a tion effects. The ve ins di splay a crude ascending mineral sequence, from depth to the surface, of pyrrhotite, gold + galena + sphalerite + chalcopyrite + iron sulfides (productive zone), and marcasite. A des cending sequence in the zone of weathering shows that Fe-hydroxides a nd Fe-disulfides grade downward where go ld is en ri c hed with secondary minerals of Pb , Cu, Zn. I n the enclosing rocks, wolframite + sc heelite + Fe-sulfides form a zone adjacent to the ore veins a nd marcasite + pyrrhotite + chalcopyrite in di stal areas. These zones are in agreement with geochemical zones W-As-Cu and Fe-Co-Ni-Mo-Cu respectively.Disulfidization process of iron monosulfides increases in the upper levels of the deposit and is more intense a t the vein margin s especially in the hanging wa ll side of the ore veins due to shearing which is also responsible for recrysta ll ization of quartz a t the vein selvages. The vein mineralogy shows as the vein development is the result of multiple episodes of vein opening and filling characterized by wolframitescheelite- quartz, sulfide - gold-quartz and disulfide-carbonate stages. statistical parameters of distribution of the components, ore geochemistry, peculiarities of horizontal (along the strike and across the thickness), vertical and three dimensional geochemical and mineralogical zonation of the mineralized rocks and the general sequence of formation of minerals in ores of the Sakaro deposit considered and proved in the thesis are the features of its genetic model. These features are a lso of practical importance for are blending and further improvements of are dressing tec hnological schemes. They are of value and use in prospecting for similar mineral deposits in the region and their successfu l exp lo ration