Introduction.
Greenstones and gold prospects occur over a broad area, concentrated
around the Iramba Plateau (Fig.33). The plateau is well watered and rises
to an elevation of about 1,200 m. It is bounded by rift faults on three
sides. Gold was discovered here during the German colonial period. Total
production is reported to be 4,698.2 kg with 4,370 kg of this coming from
the Sekenke Mine. Since early 1990s, there has been a large number of
small scale miners in the area. Anmercosa Exploration Tanzania Limited
conducted exploration work between 1996 and 1998. During this period,
trenching, helicopter - borne magnetic, radiometric surveys and drilling
programmes were conducted. Gold assay values in samples collected from
Makaya, Missu,Binyoro, Wembwe, Sekenke and Kirondatal revealed that there
is potential for gold mining.
The assay
values range from less than 1 g/t gold to more than 15 g/t gold in the
mineralisation.
Geology.
The
oldest rocks in the Iramba Sekenke greenstone belt are the metavolcanic
and metasedimentary rocks. These rocks are intruded by a porphyritic
microcline - granite, which commonly exhibits hybridization phenomena at
its borders. In the vicinity of Sekenke, a band of quartz - diorite
separates the granite from the volcanic rocks. The volcanic series is
mainly altered basalts and dolerites, that occur mainly in the western and
central part of the area.
The
majority of the gold deposits lie in the Volcanic series near the granitic
contacts. Acid and basic dykes of many types cut the area. It has also
been much faulted and sheared, with extensive intrusion of granites and
fracturing by Cenozoic Rift Valley faults related to the East African Rift
Valley system. However, it is believed that the main tectonic movement
took place at a much earlier date.
Mineralisation.
Most of
the gold deposits are in quartz veins within fractures and shears. A few
cases are known which gold mineralisation appears to be controlled by
stratigraphy of the enclosing metasedimentary rocks, but none have been
found so far of ore grade. Numerous quartz reefs have been located within
the shear zone. About 3 km of gold bearing reefs have been identified in
the area. Mineralisation is largely confined to narrow pyretic quartz
lodes. The country rock shows little gold mineralisation. Geological
mapping and soil sampling carried out in the area identified a gold
anomaly measuring 2 km wide and 4 km long.
In the
past, Sekenke and Kirondatal were the only mines which achieved
significant production. Both deposits occur in steeply dipping vein
structures. Grades generally are high and it seems unlikely from the
records of surface work that the areas surrounding the mines were ever
fully prospected. There is no record of diamond drilling around the two
mines, but some holes were drilled for extensions of the veins. Both mines
extended to depths below the oxidation zone and were stopped at values of
interest (to the present investor) in primary mineralisation.The other
mines are all shallow with little reported production. At the kisimamba
prospect, small amounts of gold were recovered from what is described as a
quartzite, but what is probably a low - iron content BIF. This prospect
was drilled by the Geological Survey with disappointing results.
The
iramba - sekenke area is attractive because of its demonstrated ability to
support small tonnage mining operations on high - grade vein deposits.
Opportunities for discovery of deposits suitable for bulk mining and heap
leaching technology have yet to be identified.
Old
Sekenke Mine.
The Old
Sekenke Mine was discovered in 1907 and worked from 1909 to 1959, at an
average gold grade of 15.4 g/t gold and 2.5 g/t of silver. Several
parallel quartz veins contain gold values of interest close to the contact
between greenstone and diorite.
Economic
mineralisation has widths of 0.5 to 2.5 m. Mineralisation was detected
along a strike of 2.6 km, but mining was confined to a much shorter
length. The veins occupy shears, which strike northwest and dip steeply.
Intersections in diamond drill holes below the old mine returned values of
19.7 and 2.9 g/t gold, respectively, over 2.9 m in two veins at a depth of
335 m and 25.5 and 28.7 g/t gold, respectively, over 2.2 m containing the
same two veins at 244 m depth. Surface trenching by STAMICO between 1975
and 1977 found ore grade values in three separate veins. The tailings dump
here contains an estimated 200,400 tonnes of material averaging 2 g/t gold
which might be of interest to a small scale operator.
Kirondatal.
The old
kirondatal Mine is said to have been exploited by Arab Traders in
precolonial times. From 1939 to 1950, it produced 23,150 tonnes of ore
which yielded an average gold grade of 8.9 g/t gold. Ore was at the
intersection of shear with dykes of quartz - albite rock intruding
greenstones.Several ore shoots were developed. The deepest shaft reaches a
depth of only 40 m. Even at this shallow depth, the workings seems to have
penetrated the base of the zone of oxidation as sulphide minerals (Pyrite,pyrrhotite
and aresonopyrite) are mentioned in the descriptions of the ore.
Altogether 13 veins have been mined to a greater or lesser extent.
Opportunities.
Currently
activities in the area include gold mining by small - scale miners and
exploration activities by Barrick Exploration Limited around the Old
Sekenke and Kirondatal Mines. Exploration work conducted in the area
includes helicopter - borne magnetic, radiometric surveys, drilling,
geochemical surveys, drilling, geochemical surveys and trenching. The data
obtained indicate that the area has a high potential for gold mining. Work
is in progress to confirm some of the already obtained results.