Location
The Tsumeb Mine is located at long.
17042'E, lat. 19014'S in the northeastern part of the Republic of
Namibia in southwestern Africa. It lies some 430km from the capital
of Windhoek. The Tsumeb Township lies on the northern fringe of the
highlands of the Otavi Mountains at an elevation of 1300m in a
landscape of low hills. The climate is subtropical, an average
rainfall of some 572mm, and the vegetation is one of a bush covered landscape.
Geological Setting and Background
The deposit lies in the upper part of the
Otavi group of stomatolitic limestones of Proterozoic age, which were
once huge calcareous reefs. Over a period of some 100 million years
in these ancient seas this activity created a lime and dolomitic rock
some 5000m thick whilst in the deeper parts of the ocean clayey-sands
were deposited. Some 650 million years ago due to the movement of
ancient mainland rock (cratons) towards each other the sediments were
squeezed together and folded to form a high mountains, the Damara
Mountain Chain. Later erosion over a period of 500 million years wore
down the mountain chain until in places the deepest roots are exposed
of the Namibian basement rocks some 2100 million years old. During
the period of erosion some 650 million years ago the calcareous rocks
were subjected to karstification, that is a process of whereby the
rocks are dissolved by slightly acidic waters to form caves and sink holes.
During the uplift of the rocks during
the mountain building phase deep reaching cavities progressing
towards the surface were formed, in some cases by hot springs, and a
deep pipe-like cave was created at Tsumeb infilled with fragments
(solution breccia) of the local dolomitic rock. After the pipe
reached the surface of a seabed a collapse of the roof formed a
sinkhole. Later sands, which later became a feldspathic sandstone
rock, were washed into the cavity, and were mobilized further down
the pipe by hot spring activity. During the later stages of the
mountain building process, some 580 to 550 million years ago, these
ascending hot mineral springs were metal-bearing and the pipe was
enriched with ore minerals due to the chemical reaction between the
metalliferous hydrothermal waters and calcareous rocks.
During the Tertiary and Quaternary from 3
million to 10,000 years ago, during a period of more humid climatic
conditions, karstification was again in evidence. It is during this
time the downward moving waters oxidized and dissolved the primary
ore minerals in the Tsumeb orebody due to fluctuations in the water table.
The Tsumeb Orebody
The configuration and dimensions of the
orebody varies from a gently-dipping, narrow tabular lens, 130m long
and 10m thick, to a near vertical, elliptical, pipe-like orebody up
to 200m long and 100m across. The pipe is known to a depth of 1716m
and is vertically zoned in respect of the total and relative metal
abundance. The ore mineralogy and rock alteration is similar to the
Mississippi type of ore deposit. The pipe structure is defined by the
distribution of mineralization, dolomite breccia, feldspathic
sandstone, rock alteration and fracturing. The sandstone is
distributed throughout the known extent of the pipe-like structure.
The deposit contains a great diversity of ore
minerals of lead, copper, zinc, silver, arsenic, antimony, cadmium,
cobalt, germanium, gallium, gold, iron, mercury, molybdenum, nickel,
tin and tungsten as well as vanadium and contained around 11% lead,
5% copper and 4.3% zinc with economic concentrations of silver,
cadmium, germanium and arsenic. It was once the foremost producer of
lead in Africa and, over its life, has produced in excess of 2
million tonnes of lead, some 500,000 tonnes of zinc, and over 1
million tonnes of copper.
The orebody from surface to 44 level
fluctuates in plan dimension from 100x20m at 5 level, 25x6m at 6
level, 70x20m at 15 level, to 200mx100m at 24 level. From surface to
12L the orebody from outcrop to 6 level the dip is essentially
vertical, it then flattens out to about 450 south and parallel to the
bedding just below 6 level. Within the steep-dipping section are
auxiliary ore veins to the south and west of the main ore center.
These consist of a bedded hanging wall vein the
"Hangendertrum" and a steep vein known as the
'Bogemtrum" which is joined to the main orebody by a copper-rich
vein the 'Kupferglanzlise". A substantial portion of the metals
recovered above 12 level came from secondary minerals in the deeply
weathered zone. Below 12 level to 24 level the character of the
orebody is similar but below 20 level there is a change. There a
major change in dip takes place, with the mineralization breaking
through the dolomite into cherty dolomite and the orebody reverses
dip from south to 700 north with an enlargement of the orebody. Below
30 level there is a further dramatic change where the North Break, a
major fault and aquifer which introduced oxidizing waters into the
lower part of the orebody, intersects the orebody at 24 level and a
dolomitic breccia replaces the dark dolomite breccia and 'wing-like'
appendages of massive ore are around the main part of the orebody.
Below 30 level this has undergone alteration of calcitization which
further down becomes silicification and the dip flattens below 35 level.
Progressing down through the orebody it
varies in oxidization. From surface to 12 level up to 50% of the ore
is oxidized (upper oxidation zone), from 12 to 24 levels there is
little oxidation, but from the intersection with the North Break
Fault from 24 to 38 levels oxidation is again present with up to 20%
of the primary ore oxidized (lower oxidation zone), but below this
again there is little oxide ore. A substantial portion of the metals
recovered above 12 level came from secondary minerals in the deeply
weathered zone. In the second oxidation zone the most intense section
lies between 28 and 29 levels where the permeable North Break Zone
intersects the pipe. At this elevation there was a profusion of
perfectly developed secondary minerals. A third small oxide zone was
found at depth where there was a connection to the North Break Zone.
Although the ratios of lead, copper and
zinc vary throughout the deposit, lead is the dominant metal.
Vertical zoning is evident in the deeper levels and below 30 level
the overall metal content decreases with depth. From surface to 6
level high copper values, probably due to supergene enrichment were
encountered, whereas leaching may have caused the corresponding low
zinc content. Copper is about as abundant down to 10 level, and below
20 level the ratio of of copper:zinc averages 2.5:3. From there to 27
level, reaching peak concentrations on 29 level, where copper equals
lead due to the exceptionally high grade ore on the southern side of
the pipe. To 35 level, due to a constriction in the pipe the ratio of
copper:lead:zinc changes to roughly 0.5:2. The copper grade is
appreciably lower at 44 level but improves on 49 level. The
mineralization is of the replacement and fracture-fill type.
The various ore types at Tsumeb can be
classified into massive peripheral ores, manto ores, disseminated and
stringer ores and secondary ores. The massive peripheral ores are
complex lead, copper and zinc ores of up to 40% metal content. They
occur peripheral to the feldspathic sandstone and are prominent down
to 20 level, below which they become thin and pinch out at 34 level.
In places the massive peripheral ore progressively grades into less
feldspathic sandstone. The mato ores are extensions of the massive
ore into the wall rock of dolomite. They occur between 26 and 30
levels and were formed by replacement. The disseminated and stringer
ores are hosted by feldspathic sandstone, dolomite and dolomite
breccia and occur throughout the mine.
Throughout the pipe, tennantite is the
most persistent primary copper mineral. Chalcocite and bornite
arelocally important between 24 and 30 level, mainly in association
with massive orebodies. Enargite and digenite are also important
copper minerals. The complexity of the hypogene ores of the Tsumeb
deposit and the variety and abundance of secondary minerals have
attracted widespread attention. The principal hypogene ore minerals
are galena, tennantite, sphalerite, chalcocite, enargite, and
bornite, with lesser amounts of chalcopyrite, germanite and
renierite. Pyrite is widespread but in small amounts. Supergene
chalcocite, djurelite, digenite and covellite are important in the
upper and lower oxidation zones, both of these zones contain a great
diversity of oxidic minerals. The two oxidation zones have been
derived from the supergene aleration of the diverse primary sulphide
ores by the introduction of oxygen rich surface and later acidic
waters from the surface along aquifers. These dissolved the primary
sulphides and with a chemical reaction with the calcareous host rocks re-precipitated
the metal elements. The main oxide minerals, usually in solution
cavities, vugs and fractures in association with quartz, calcite, and
dolomite, are azurite, cerussite, mimetite, wulfenite, malachite,
native copper, cuprite, duftite, conichalcite, olivenite,
smithsonite, and mimetite. The concentration of vanadium ores,
chiefly mottramite, lies mainly at the periphery of the deposit
between outcrop and 4 level. Dioptase is present in the lower
oxidation zone within areas of intense silicification and associated
with drusy calcite and dolomite. Over 242 mineral species have been
identified with some 40 unique to the Tsumeb Mine and 38 are
currently under investigation.
Tsumeb Specimen mining plan to exploit ore
remnants left by the previous mining activity in the Upper Levels for
mineral specimens. The Upper Levels extend from 12 level to the
bottom of the open pit. At the beginning of mining operations in 1907
there was an immense oxidation zone with carbonate ores, the surface
outcrop of which had been exploited for copper by the Ovambos prior
to 1851. This, according to old records had a length of 180m by 40m
and stood 12m above the surface. Practically only carbonate ores were
present, mainly malachite. Apart from the malachite, azurite occurred
sparingly, however this increased in abundance with depth. In
character the orebody was similar from surface down to 3 level with
secondary ores of carbonates, sulfates, arsenates and vanadates,
which as well as malachite and azurite included atacamite, anglesite,
arsenotsumebite, aurichalcite, bayldonite, brochantite, caledonite,
cerussite, chalcanthite, conichalcite, crysocolla, cuprite, damite,
desclozite, dioptase, duftitie, greenockite, hemimorphite,
hydrozicite, leadhillite, linarite, mimetite, minium, mottramite,
olivenite, otavite, pharmacosiderite, phosgenite, pyromorphite,
rosasite, smithsonite, tsumebite, vanadinite, willemite, wulfenite,
with the copper content increasing with depth.
Native metals and oxides found in the
upper levels included cuprite, copper, minium, plattnerite and
silver. From 3 level down more primary ore was present and the rare
germanium sulphide germanite was encountered. Secondary minerals
found below 3 level and above 12 level included not only some of the
above but the rarities of schultenite and fleischenite. The central
high-grade core of the pipe was exploited by Otavi Minen-und
Eisenbahngesellschaft (OMEG) by stoping traversely and backfilling
with waste rock. During the later years of Tsumeb Corporation Ltd.
(TCL), later taken over by the Goldfields Group of South Africa,
lower-grade on-strike extensions and some peripheral ore was
recovered by stoping using a cut and fill method, generally in 10m panels.
Tsumeb Specimen Mining Pty Ltd (TSM) will
continue mining out remnant peripheral ore on the footwall and
hangingwall of the OMEG fill stopes, which in some cases are also
bounded by TCL cemented sandfill stopes. Access will be by the spiral
ramp and the utilization of either footwall or hangwall development
drives and sub-drives previously mined by TCL and attacking the ore
on a 2X2m heading and mucking out using a Wagner 2B scooptram. When
mining in mineralization TSM will advance at a rate of 1.5m per
round, checking the new exposure for vugs, and a diamond chain saw
will be used to remove the specimens encountered by mining. Advancing
updip by slashing out the stope hangingwall, backfill will replace
the mined out portion until the next level is reached. In phase 1
mining will commence in a substantial block of footwall ore extending
from 5 level to 3 No.1 sublevel and a second attack point will be in
a hangingwall remnant from 5 No.1 sublevel to 4 level. Both these
blocks have encouraging borehole intercepts showing azurite and
arsenate mineralization and have a history of producing specimens
both downdip and along strike.
Source of information.
Gebhard, G.1999. Tsumeb. Pub. G.G.
Publishing, Grossenseifen, Germany
Grunert, N. 2000. Namibia. Fascination of
Geology. A Travel Handbook. Pub.Klaus Hess Publishers, Germany.
Lombaard, A.F. et al. 1986. The Tsumeb Lead-Copper-Zinc-Silver
Deposit, South West Africa/Namibia. Mineral Deposits of South
Africa. pp.1761-1787
Mineralogical Record. 1977. Tsumeb.
Ed.Wilson, W.E. Pub. The Mineralogical Record Inc., Maryland,
USA.vol.8, no.3 (May-June 1977)
Ministry of Mines and Energy, Geological
Survey. 1992. The Mineral Resources of Namibia. pp. 2.3-52 to 2.3-69,
2.5-2 to 2.5-9
Sohnge,G. 1967. Tsumeb a historical sketch.
Pub. Committee of the S.W.A. Scientific Society, Windhoek
