Mineralogical and Geochemical Studies on the Sakaro Primary Gold Deposit (Sidamo, Southern Ethiopia): Zonation in Ore Bodies And Host Rocks
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Date
1993-06
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Addis Ababa University
Abstract
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