Summary of Tsumeb Mi..

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Summary of Tsumeb Mine: Rocks and Minerals, Robert B.
Cook, Benjamin E. Nicolson, Ian R. Bruce, 2001
In 1996 the famous Tsumeb mine was shut down and allowed to flood,
seemingly ending almost a century as one of the world's great sources
of mineral specimens. Five years later, however, it had been reopened
as a specimen-mining venture that is already meeting with some
success.
Everyone who is involved with the collection of mineral specimens has
at least some familiarity with Tsumeb, Namibia. For most, it is the
source of the finest examples of many species and likely a name that
appears on the label of many of their favorite specimens. To curators
and scientists, it has been the source of a phenomenal suite of
mineralogical wonders, oddities, and associations that have been so
well chronicled in the technical literature that a compilation of major
Tsumeb references numbers well over three hundred. To economic
geologists, it has been a model, a target, a challenge, and a dream.
Unfortunately, every mine, no matter how great, has a finite life that
can be likened to a good book in that sooner or later a last chapter
must appear. With many mines, that final chapter is short and
uninspiring, relating nothing more than the exhaustion of ore, the
removal of equipment, the shutting off of power, and a final quiet wait
as the water slowly rises. Sometimes there are complicating factors
such as miners strikes, world economics, politics, and the like. No
matter, still the water rises. With respect to Tsumeb, what seemed to
be the final chapter began with a little of each. Unlike most mines,
however, the importance of mineral specimens encountered here and
the potential availability of additional valuable material in unmined
parts of the upper workings have extended that final chapter.
Had Tsumeb been the source of no specimens at all, it would still be
famous as an incredibly valuable mineral deposit, having produced
more than $5 billion in lead, zinc, copper, germanium, and other
metals from a relatively small, though quite rich, deposit that was
operated almost continuously for ninety-one years, ending in 1996 on
level 48 at a depth of 1,650 meters (Gebhard 1999; Wilson 1977;
Sohnge 1967). Thankfully, Tsumeb has been much more than a mere
cash cow for a series of mining companies. Because of an unusually
wide array of associated metals, the peculiar geologic history of the
site of ore deposition, and, most important, the presence of three
separate zones of oxidation, Tsumeb is one of the premier mineral
localities in the world. From this mine, 247 mineral species have been
identified, with 24 additional unknowns undergoing or awaiting study.
It is the type locality for 52 species and remains the only known
occurrence for 40 of these. Approximately 65 species are found here in
well-crystallized specimens, the quality of most of which is unequaled
in those from any other locality.
At the time of its closure, Tsumeb was operated by Gold Fields Group
of South Africa. Hundreds of miners worked each shift in myriad
stopes and headings, a modern system of internal initial ore
processing and decline access was in place, and a mill and smelter
were working at full capacity, handling some 2,000 tons of ore and its
concentrated products each day. Once the closure decision was made,
most equipment was removed, and the pumps were shut off. Some
ore was scavenged from pillars and other remnants higher in the mine
as the waters rose. Ultimately the mill was shut down, the smelter was
cut back to operate on ore from the nearby Kombat mine, and the
assets were purchased by a local group, Ongopolo Mining and Smelting.
After considering several proposals aimed at limited reopening of the
mine for mineral specimen production, Ongopolo entered into an
agreement with a new company, Tsumeb Specimen Mining (Pty) Ltd.
Principals in this venture are Ian Bruce of Crystal Classics and mineral
collectors Simon Brock and David Lloyd. An excellent review of the
deposit's history and geology and a series of detailed informative news
bulletins on Tsumeb Specimen Mining's progress can be found at their
Web site, www.MineralMining.com, to which the reader is referred. A
detailed review of the deposit's geology is available in Lombaard et al.
(1986).
It is one thing for a few miners out of hundreds to periodically recover
specimens in a mine from which thousands of tons of rock are
removed daily. A lot of mineralized rock is available to many eyes, and
there is a significant infrastructure in place: There is water for drilling,
compressed air at each heading, adequate ventilation, an internal
communications and electrical network, a technical support staff, and
ready access through a complex system of shafts, raises, and decline
entries. It is a completely different matter for a few enthusiastic
individuals thousands of miles from home to confront a flooded mine
into which mill tailings and related surface debris have flowed for half
a decade, with the primary objective of economically recovering
mineral specimens. There were problems, both those that were
anticipated and those that could not have been foreseen.
If anything, Tsumeb is a wet mine. Drill-core records, stope maps, and
recollections of former employees indicated that the most likely target
areas for good azurite, malachite, wulfenite, cerussite, and arsenate
minerals were between levels 5 and 8, some 100-150 meters below
the normal water table. A big pump would be required. Consequently,
a pump and supposedly seamless high-pressure piping were lowered
through shaft No. 1 to level 12, with the intention of initially pulling
the water level down to level 8. Did lightning strike the headframe,
shorting out the pump's electrical system and requiring that it be
removed, rewired, and reinstalled back down on 12? Yes, it did. Was
the supposedly seamless pipe really seamless, or did it burst when
confronted with hundreds of meters of hydraulic head? Burst it did.
Finally, however, the weather cleared, the pipe held, the pump did its
job, and access from the surface to level 8 was achieved after the
removal of more than 800 million gallons of water. The pump
continues to run at a rate of 1,800 gallons per minute.
The mine was accessible now, or was it? Access could only be achieved
through the decline, an inclined, spiral ramp of sorts with turnouts at
each level. Rubber-tired equipment would allow quick work of muck
removal and the transport of miners and equipment. The decline had
partially filled with tailings and other debris while underwater,
necessitating significant rehabilitation. After a great deal of work,
there was, at last, clear access down to level 5, but there were no
water and air lines, no ventilation, and no safe electrical system for
blasting. These were installed, and mining was set to commence based
on a plan to drive drifts into areas of remaining pillars and other
mineralized zones, generally on the periphery of the orebody. Two
two-man mining crews were employed along with a scooptram
operator. A foreman, former Tsumeb shift boss Kieviet Rust, would
oversee mining, with Ian Bruce in charge of the day-to-day operation.
A former Cornish miner, Vitek Urbanski, and Australian geologist Ben
Nicolson were added to complement the surface and underground staff.
Cool lubricating water flowed to the drills, voltage and amps were
there aplenty, ventilation was perfection itself, but the absolutely
necessary, consistent supply of compressed air to run the drills was
not there. There was a problem with the main compressor station at
the smelter, the only available source of compressed air. This was a
situation that could not be easily overcome and in the end required the
purchase of a compressor dedicated to the mining venture. Once the
new compressor was in place, the final mechanical hurdle had been
cleared, and the operation fell into a routine of drilling, loading,
blasting, mucking, and, periodically, removing specimens. In early
2001, Tsumeb was back in operation.
In order to understand the targets of the current Tsumeb effort, it is
necessary to first appreciate the geometry of the deposit itself. It is a
somewhat sinuously plunging, pipelike body of relatively small crosssectional area. Prior to mining, it cropped out as a green, malachiterich hill some 150 x 50 meters in area. The upper 50 meters or so of
the deposit were mined by surface methods that gave way to a
succession of deeper and deeper shafts. Relatively late in its history,
equipment and personnel access from the surface to level 12 was
changed from shaft to decline, a spiral rampway put down primarily in
the hanging wall of the deposit. Drifts were run off the decline at
appropriate intervals to allow access to those levels already
established. For engineering and geologic purposes, the mine was
divided into levels numbered relative to depth, and on each level
relative to position either east or west of the center of the deposit. So,
a position in the mine given as level 5, West-5 would be a stope or
other entry that would be in the fifth segment of the mine west of its
center and on the 5th level below the surface. This coordinate system
was critical in the understanding both of records for drill holes that
intersected vuggy, secondary mineralization and also of the detailed
initial specimen reconnaissance conducted primarily by David Lloyd.
This information pointed to prospective areas located on level 5 at
West-4, West-5, and East-5. These areas have received the greatest
attention.
Current mining follows a careful plan in which crosscuts are run at
discrete intervals off of the main level 5 access drift toward areas of
potentially productive ground. If specimen-bearing zones are
encountered, then mining is modified, based on the characteristics of
that zone. Normally, short 4-foot rounds are drilled and shot. An initial
inspection of muck (broken rock produced by the blast) and of the new
face resulting from each blast is made and any specimen material
recovered. Then muck is removed from the face and stockpiled
underground with the scooptram for possible future recovery as lowgrade ore. Five areas that are currently producing specimen-grade or,
in the case of nodular azurite, cutting material have been encountered.
One of these contains nodular and crystalline azurite in discontinuous
clay seams, a second has vuggy zones that locally contain cerussite
crystals on crystalline dolomite, a third is a zone of vuggy mottramite,
a fourth contains vugs of crystalline malachite in massive chalcocite,
and a fifth contains floater groups of reticulated cerussite. The
occurrences of specific mineral species within these areas are
described below.
Minerals
Aragonite, Ca(C[O.sub.3]). In the West-5 area of level 5, a short
distance from the mottramite-rich zone described below, is a vertical
water-bearing zone within which pale blue aragonite is precipitating as
botryoidal masses and crusts. The material is relatively abundant, and
platelike specimens to 25 cm across have been collected.
Azurite, [Cu.sub.3][(C[O.sub.3]).sub.2][(OH).sub.2], and Malachite,
[Cu.sub.2][(C[O.sub.3]).sub.2][(OH).sub.2]. Nodular and, less
frequently, rosettelike azurite crystal groups occur on level 5 in clay
seams that vary in width up to about 40 cm and appear to occupy late
jointlike fractures that dip from about 30-60 degrees, The azurite
content of clay seams can vary up to 50 percent azurite by volume,
with the largest nodules about 5 kilograms and 15 cm across.
Some are highly irregular to kidney shaped; others are almost
spherical. The most common nodules are pure azurite in radially
fibrous masses that exhibit a peculiar satiny sheen or chatoyancy on
broken surfaces. Others contain cores of malachite that constitute up
to about 30 percent of the nodule. In the largest, the associated
malachite is velvety and contains irregular central vugs. The largest of
these compound nodules is about 10 cm in diameter. All or parts of the
surfaces of some nodules are bounded by flat crystal faces reflecting
terminations of radially arranged tabular crystals, whereas on others
azurite occurs in groups of tabular, well-developed crystals that have
grown roughly tangential to the nodule surface. Locally within the clay
seams are attractive groups of medium-to-dark blue, etchedappearing azurite crystals to 8 x 6 cm with individual flat blades to 5
cm long. These crystals are commonly in a somewhat crude radial
arrangement, show only minor replacement by malachite, are often
deeply striated, and contain multiple, somewhat skeletal terminations.
Locally, however, these azurite groups are completely replaced by
malachite or have discrete masses of velvety malachite as a final
crystallization product on their surface.
Once out of the mine, nodular azurite is separated into three grades:
"A", "B", and "C". A-grade consists of competent nodules with no, or
only a small percentage of, inclusions other than malachite. B-grade
also is competent azurite with a somewhat higher clay content and
porosity. C-grade is generally country rock containing dense networks
of azurite-filled or healed fractures with more or less accompanying
malachite. A-and B-grades are weighed and laid out to dry. The
adhering clay cracks and separates as it contracts upon drying. Once
dry, the individual nodules are immersed in water, causing much of
the clay to fall away. The remainder is then removed easily with a
water jet. The nodules are then air-dried and resorted. A-and some Bgrade azurite that has no specimen potential is stored for shipment to
cutting houses. C-grade is retained for carving and lesser-quality
lapidary projects. The best mining day relative to nodular azurite was
29 March, when 95 kilograms of A-grade and 36 kilograms of B-grade
were recovered. On other good days, 69, 49, and 45 kilograms of Agrade material were recovered. Fine azurite particles are stored for a
potential pigment market.
Only a few sharp, perfectly formed azurite crystals of the sort for
which Tsumeb is so well known have been found to date. These occur
in fractures and small pockets in intensely brecciated, iron- and
copper-stained dolomite that locally contains nearby azurite-bearing
clay seams. Most of these are very sharp, dark inky-blue, flattened
crystals of Roman-sword habit, with the longest about 2 cm. Typically
only isolated crystals or small clusters occur in these pockets, with
individuals generally in the 1-cm range. Partial to complete alteration
of these crystals to velvety malachite is conspicuous in some pockets.
Other minerals of collector interest are generally absent, and many
such pockets are devoid even of azurite. Only a handful of high-quality
azurite in equant blocky crystals to 4 cm have been found in these
pockets so far.
Not all specimen-grade azurite occurs in or adjacent to clay seams.
Early in 2001, approximately 900 kilograms of botryoidal, cauliflowershaped azurite and malachite masses to about 30 cm across were
encountered in an iron-oxide-rich, crystalline cerussite-bearing zone
encountered on level 5 at West-4. These were carefully mined by hand
and, along with several hundred thumbnail to small cabinet-sized
azurite crystal specimens found in the general area of clay-seam
azurite mineralization, were shipped to London for cleaning,
preparation, and pricing. A cursory examination of accessible areas
between levels 2 and 5 indicates that additional azurite and malachite
of potential specimen grade remains in some stopes.
Malachite is ubiquitous in most of the accessible workings, occurring as
seams and as cavity and fracture fillings and coatings, commonly
associated with subordinate azurite. Malachite occurs with azurite in
clay seams as a local replacement in discrete, generally concentric
zones as well as in irregular portions of azurite nodules. In some
specimens, it occurs as a velvety replacement of the last phase of
azurite crystallization on the surfaces of associated rosettelike azurite
aggregates. Relatively sharp, elongate tabular azurite crystals
completely replaced by somewhat dull greenish-gray malachite occur
in fractures a short distance above and below the nodular azuritebearing clay seams. Pseudomorphs to about 8 cm have been
recovered, with most specimens having only one or two isolated
crystals, although a few nice groups with crystals to about 3 cm long
have been found. A single cavity in the clay-seam area contained pale
beige 8-mm wulfenite crystals on which were perched malachite
pseudomorphs after azurite to 5 mm long. The approximately 5 x 7 x
10-cm cavity was in relatively dense dolomitic limestone, suggesting
that recovery of undamaged specimens could present a problem.
In some fractures bordering the clay seams, azurite and malachite
occur as masses of alternating, crudely parallel bands. Although bands
of both minerals consist of crystal aggregates, malachite appears to
have replaced bands originally consisting of the most sharply
crystallized azurite. These bands are made up almost entirely of
subparallel malachite pseudomorphs after azurite, arranged
perpendicular to the band edge, and range to 2 cm thick. Individual
specimens to 8 x 14 cm across have been recovered in which plates of
malachite pseudomorphs commonly rest on, or occur with, irregular
masses or flat plates of blue, unreplaced azurite, making a striking
contrast.
On level 5 in the West-5 access drift is an area rich in massive
chalcocite that contains cavities to 3 cm from which project mats of
sparkling, apparently nonpseudomorphic malachite crystals. The
occurrence is in coarsely brecciated dolomite at the margin of a
previously mined-out area in pillars and on the walls of a stope where
the early work had ended.
Vuggy malachite-rich rock occurs locally within the open pit,
particularly in rubble along the north wall. Here velvety pseudomorphs
after crudely developed azurite crystals to 2 cm have been found.
Associated minerals are rosasite, mottramite, and minor smithsonite.
Cerussite, PbC[O.sub.3]. Good cerussite specimens have been
recovered from level 5 in the West-4 stope area. The occurrence is in
heavily oxidized, coarse dolomitic limestone breccia penetrated in a
heading that had been abandoned by one of the early operators. In
this zone of relatively bad ground (fig. 6), the surfaces of broken
limestone fragments are locally coated by sharp dolomite
rhombohedra to about 1 cm, among which are scattered cyclic
cerussite twins that range to about 2 cm across. The cerussite crystals
are bright and glassy and range from colorless to pale gray. Although
most are undamaged, some show points of contact with adjacent
breccia fragments, resulting in incompletely formed crystals. Breccia
fragments are loosely cemented by iron oxides and lesser secondary
copper minerals and have individual crystal-coated surfaces to 5 x 20
cm. A pale blue-green mineral tentatively identified as rosasite occurs
with the cerussite as botryoidal and stalactitic masses. About fifty
cerussite specimens have been collected. Good masses of botryoidal
malachite and azurite were collected from this zone when it was first
encountered (see above). A second cerussite occurrence encountered
on East-5 produced about forty floater specimens of reticulated
crystals in groups to 5 x 2.5 cm.
Chalcocite, [Cu.sub.2]S. Only massive chalcocite has been
encountered. Locally it occurs in rich masses that cement coarse
dolomitic limestone breccia, most commonly in remnant pillars near
the margin of the orebody in the West-5 area of level 5. Within these
zones occur irregular masses of acicular to somewhat tabular
malachite crystals that locally incompletely fill cavities in massive
chalcocite, producing vuggy specimens. Most such malachite pockets
are rimed by white calcite that separates the inner malachite from the
bright gray metallic chalcocite, resulting in attractive specimens.
Cuprite, [Cu.sub.2]O. Although Tsumeb is well known for fine cuprite
specimens, none have been found since the mine's reopening.
However, in the West-5 segment of level 5, massive fine-grained
cuprite ("tile ore") occurs in fractured dolomite in the vicinity of
chalcocite-cemented breccia. Some cuprite masses and veinlets are up
to 1 cm across and in a matrix that locally contains cavities lined with
sparkling calcite crystals, suggesting good specimen potential for that
area.
Duftite, PbCu(As[O.sub.4])(OH). Pale green to lime-green masses of
botryoidal duftite occur in local abundance in the East-5 access drift
with late crystalline calcite and in vuggy late quartz. The material is
similar to that found in the well-known dioptase occurrence at levels
30-32 of the mine. Somewhat earthy duftite occurs locally in parts of
the open pit characterized by relatively abundant malachite and
rosasite. Two of three green unknowns identified by X-ray diffraction
analysis of specimens collected since the mine reopened were duftite.
Mottramite, PbCu(V[O.sub.4])(OH). In the West-5 area of level 5,
approximately 50 meters along strike from the broken, oxide-rich
cerussite-bearing zone, a relatively large area of vuggy, dark greenishgray, botryoidal mottramite was encountered. The material forms
irregular masses to 30 cm that fill cavities in dark, oxide-rich clayey
fault gouge. Much of the mottramite exhibits satiny to sparkling drusy
surfaces that are attractive though quite fragile. Unusual associated
minerals have not been encountered with the mottramite.
Minium, [Pb.sub.3][O.sub.4]. Minium as orange fracture coatings was
recovered from boulders on the stockpiled low-grade ore near the
decline entrance.
Rosasite, [(Cu,Zn).sub.2](C[O.sub.3])[(OH).sub.2]. Pale-to-medium
blue mammillary rosasite aggregates occur with malachite in oxidized
rock fallen from the north face of the open pit near the decline access
drift. Specimens reminiscent of those characteristic of some Arizona
localities have been recovered. Minor rosasite occurs sparingly as
isolated spherical aggregates in the vuggy, cerussite-bearing zone in
the West-4 area of level 5.
Smithsonite, ZnC[O.sub.3]. Although famous for its phenomenally
variable smithsonite specimens, none have been found in the mine
since its reopening. However, core-drilling records indicate the
potential for vuggy smithsonite in an area of West-5 yet to be reached.
Masses of pale green botryoidal smithsonite in cavities to 5 cm have
been found in ore stockpiled on the dump immediately east of the
decline portal. Rosasite and an earthy yellow unknown are locally
associated.
Although the current venture is meeting with some success, specimens
equal to the best material for which the deposit is famous are yet to be
unearthed. Still, the promise of great things to be discovered in the
upper oxidized zone is certainly there, and despite the best efforts of
everyone involved in geology, engineering, and careful mining, the
impact of Lady Luck will surely be felt one way or the other during the
next year or so. This author (R. B. C.) is confident that he speaks for
collectors and mineralogists the world over in wishing everyone
involved all the best. This small enthusiastic group is to be
congratulated for undertaking a financially and rather physically
daunting series of tasks so that the rest of us might have a continued
supply of good Tsumeb specimens.
ACKNOWLEDGMENTS
The manuscript was improved considerably by the critical reviews of
Bob Ramik of the Royal Ontario Museum, Toronto, and John S. White
of Kustos, Stewartstown, Pennsylvania. Financial support for the site
visit was borne by Tsumeb Specimen Mining and is gratefully
acknowledged, as are the organizational skills of Simon Brock.
REFERENCES
Gebhard, G. 1999. Tsumeb: A unique mineral locality. Grossenseifen,
Germany: G. G. Publishing.
Lombaard, A. F., A. Gunzel, J. Innes, and T. L. Kruger. 1986. The
Tsumeb lead-copper-zinc-silver deposit, South West Africa/Namibia. In
Mineral deposits of southern Africa, ed. by C. R. Anhaeusser and S.
Maske, 1761-87. Geological Society of South Africa.
Sohnge, G. 1967. Tsumeb: A historical sketch. South West Africa
Scientific Society.
Wilson, W. E., ed. 1977. Tsumeb! The world's greatest mineral locality.
Mineralogical Record 8:1-128.
Dr. Robert B. Cook, an executive editor of Rocks & Minerals, is a
professor of geology and head of the Department of Geology and
Geography at Auburn University. He writes the Connoisseur's Choice
column for Rocks & Minerals.
Benjamin E. Nicolson, a geologist from Australia, has about ten years'
experience in the gold mines of Western Australia. He is a mineral
collector, successful electronic gold prospector, and owner of several
pegmatitic rare-earth mineral occurrences.
Ian R. Bruce is the owner of Crystal Classics, a well-known mineral
dealership in the United Kingdom.
ROBERT B. COOK Department of Geology and Geography Auburn
University Auburn, Alabama 36849 [email protected]
BENJAMIN E. NICOLSON Tsumeb Specimen Mining Ltd. P.O. Box 344
Tsumeb, Namibia [email protected]
IAN R. BRUCE Crystal Classics The Old Linhay, Woodcoxhayes
Halberton Road Willand, Devon, EX15 2QF United Kingdom
[email protected]
COPYRIGHT 2002 Heldref Publications
COPYRIGHT 2002 Gale Group
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