Mineralogical and Geochemical Studies on the Sakaro Primary Gold Deposit (Sidamo, Southern Ethiopia): Zonation in Ore Bodies and Host Rocks
No Thumbnail Available
Date
1993-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa Universty
Abstract
The Sakaro deposit is a vein-type fault-hosted gold deposit in the Adola
area, southern Ethiopia. It is composed of quartz veins up to 9.3 m wide
which occupy fault planes along 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 host rocks
reveal that the Sakaro deposit might include tungsten other than gold as a
potential exploration target. Tungsten minerqlization 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 mineralogical 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 is also related at a lesser
degree "dth 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 ore
veins toward enclosing rocks.
Gold distribution in the are veins is controlled by thickness variation
of the quartz fillings in both strike and dip directions, poor in swells and
rich in constrictions. In the plane of the main vein, five are columns are
identified with marked contrast in gold mineralization: (1) obliquely
running I EW dipping gold rich zone I (2) northerly trending, vertically
dipping gold rich zone, (3) intermediate low gold zone, (4) oxidation zone,
and (5) surficial weathering zone. The first two zones are ore shoots
representing the fronts of gold depositing mineralizing fluids. Gold contents
follow a systematic pattern of more or less regular course within and away
from these fronts which otherwise are perceived as very irregular and erratic
distributions. loloreover, statistical treatment on the metal 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 + high galena + chalcopyrite in the central part.
Gold is present in all of the above associations, however, it is highly
enriched in sphalerite-high galena-chalcopyrite zone, followed by hanging
wall side marcasite-chalcopyrite zone, and least in foot wall side marcasitechalcopyrite
zone. The gold enrichment in sphalerite-high galena-chalcopyrite
zone indicates the close association of Au, Ag, Pb, Cu and Zn. The selective
enrichment of gold in the hanging ''1all side (marcasite-chalcopyrite) zone is
most likely due to intense shear remobilization effects.
The veins display a crude ascending mineral sequence, from depth to the
surface, of pyrrhotite, gold + galena + sphalerite + chalcopyrite + iron
sulfides (productive zone) I and marcasite. A descending sequence in the zone
of weathering ShO"'S that Fe-hydroxides and Fe-disulfides grade downward where
gold is enriched "lith secondary minerals of Pb, Cu, Zn. In the enclosing
rocks, wolframite + scheelite + Fe-sulfides form a zone adjacent to the are
veins and marcasite + pyrrhotite + chalcopyrite in distal 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 alld is more intense at the vein margins especially in
the hanging wall side of the ore veins due to shearing which is also
responsible for recrystallization of quartz at 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) I vertical and three dimensional geochemical and mineralogical
zonation of the mineralized rocks and the general sequence of formation of
minerals in ores of the saka·ro deposit considered and proved in the thesis
are the features of its genetic model. These features are also of practical
importance for are blending and further improvements of are dressing
technological schemes. They are of value and use in prospecting for similar
mineral deposits in the region and their successful exploration.
Description
Keywords
Sakaro