Sperrylite from the Tweefontein farm: Limpopo Province South Africa.
Until the discovery of large, well-formed sperrylite crystals at the Oktyabrsky mine in the Norilsk region of Siberia in the mid-1980s, the Tweefontein farm in the Transvaal, South Africa, was the undisputed premier locality for the species. Unfortunately the occurrence there produced very few specimens, and even most major museums do not own an example. Were it not for the entry in Bancroft's World's Finest Minerals and Crystals (1973), most mineral collectors today would probably still be unaware of this extraordinary occurrence.
The abandoned workings of the Tweefontein mine are located on Tweefontein (1) farm plot 238 KR (formerly 1033), about 14 km north-northwest of the town of Mokopane (formerly Potgietersrus (2) in the southern part of the Tweefontein farm, in the Potgietersrus district of the former Transvaal Province (subdivided in 1994; now called Limpopo Province), South Africa. Access is by highway north from Pretoria to Mokopane, and then by road north-northwest to the mine area. The Tweefontein farm 238 KR workings, in the northern limb of the Bushveld Complex, should not be confused with the Tweefontein Chrome mine located south of Steelpoort in the eastern limb of the Bushveld Complex.
Platinum was first discovered in South Africa in 1923 by Mr. A. Erasmus, a prospector who was panning the ground from termite mounds about 31 km southwest of the town of Potgietersrus. The results led him to the Waterberg lode deposits, which became the first platinum mine in South Africa, worked from 1924-1927. In September 1924, Hans Merensky (1871-1952) and Andries Lom-baard discovered an extensive platiniferous orebody (now known as the Merensky Reef (3)) on the Maandagshoek farm near Steelpoort in the Transvaal Province. The orebody runs along the eastern edge of the eastern limb of the mafic Bushveld Complex, an ancient igneous complex over 2 billion years old. Merensky and his colleagues traced the outcrop of the reef for some 80 km. Following the discovery a "platinum rush" to the area took place as miners scrambled for prospecting options in both areas.
In 1925 Merensky identified an even more promising occurrence in the northern limb of the Bushveld Complex, near Potgietersrus, which came to be called the Platreef. Pre-European copper workings were discovered on Tweefontein Hill, a bush-covered prominence rising about 140 meters above the surrounding plain on Tweefontein farm plot 1033, later redesignated 238 KR (Wagner, 1925,1926a-d), and prospecting was initially directed toward a pyroxenitic zone at the base of the Bushveld rocks which Merensky recognized as being similar to the rocks of the Merensky Reef. The pyroxenite zone was shown to have platinum group values of up to 28 grams/metric ton (28 ppm) (Wagner, 1929). It is known today as the world's third largest deposit of platinum-group elements, and it is here that some of the largest and finest sperrylite crystals ever seen were discovered.
[FIGURE 1 OMITTED]
Over 50 mining companies were established in 1925 in the Waterburg district and Bushveld Complex to exploit the platinum deposits. By far the largest claim-holder among these was Potgietersrust Platinums, Ltd. The deposits found in a breccia zone in ironstone on Tweefontein Hill were worked briefly in 1925-1926; 631 meters of underground development in the mineralized shear zones were excavated, plus another 1,012 meters farther down the hill along the contact between the Bushveld Complex rocks and the ironstone. The deposit was ultimately deemed uneconomical, though, and mining there ceased.
The large sperrylite crystals for which the mine is famous were encountered in 1925 or 1926 in the upper part of Tweefontein Hill, where an adit had been opened at a site showing evidence of early copper mining. The crystals were found embedded in limonite in a winze off of the main inclined shaft. Abandoned adits and the shaft can still be visited there today.
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Potgietersrust Platinums Limited, administered by the Johannesburg Consolidated Investment Company Limited, was formed in mid-1926 and commenced production in September 1926 on the nearby Zwartfontein and Vaalkop plots. During the period 1926 to 1930 a total of 111,000 tons of ore was milled to yield 25,000 ounces of platinum-group metals from the mining operations in the area. However, in May 1930 the price of platinum declined, and production in what was called the Potgietersrust district ceased. Mining activity shifted to the more profitable Merensky Reef deposit at Rustenburg, and it wasn't until the price of platinum increased again in the late 1960s and early 1970s that serious interest was again shown in the Potgietersrus area.
[FIGURE 5 OMITTED]
The Platreef is still being mined today by Potgietersrust Platinums Ltd., now a wholly owned subsidiary of Anglo American Platinum Corporation, which established the first viable mining operation on the Platreef. The old Tweefontein workings are now considered to be part of the Potgietersrust mine, which exploits the entire Platreef. At some time in the future an open pit mining operation may be initiated there, permanently obliterating the mineralogically historic site.
The Platreef is a 10-meter to 400-meter-thick pyroxenitic unit at the base of the northern limb of the Bushveld Complex. This famous (among geologists, at least) complex is an enormous and ancient layered intrusion of lenticular shape that is depressed in the middle--a configuration classifying it as a lopolith. Rich in copper, nickel and platinum-group elements, the Platreef strikes north-northwest for about 30 km and dips 40[degrees] to 45[degrees]W through the Tweefontein farm area. At Tweefontein Hill the Platreef ranges in thickness from 100 to 220 meters. There it is sandwiched between the overlying Main Zone gabbronorites and the 2.2 to 2.6-billion-year-old Transvaal Supergroup Metasedimentary Sequence (Cawthorn, 1999).
Typically, in the Platreef the proportions of platinum and palladium are about 44% and 46%, respectively, with the other platinum-group metals comprising the other 10%. Unlike the Merensky Reef, the Platreef mineralization does not have sharply defined boundaries. Current mining is from open pits, where the orebody dips at about 40[degrees]. Once the shallow part of the reef has been exhausted, it will be necessary to develop underground mining operations.
The discontinuous massive sulfide mineralization in the Platreef has been attributed to gravitational settling of a magmatic sulfide liquid within the reef structure, following a major pulse of magma. The amount of contamination by the country rock has been a subject of debate. Gain and Mostert (1982) believe that formation of the sulfide fluid in the magma was triggered by the absorption of dolomitic xenoliths that contributed [CO.sub.2], [H.sub.2]O, and S to the magma; this simultaneously lowered the solubility of sulfur in the magma while adding more sulfur, resulting in the separation of an immiscible sulfide liquid. But the most recent studies (Howell et al., 2007) suggest that the effect of contamination was relatively minor, and that the magma pulse carried with it the preformed platinum-group-element-rich sulfide liquid. The Tweefontein breccia deposit is believed (White, 1994) to represent a relatively low-temperature late-stage hydrothermal surge of platinum mineralization.
[FIGURE 6 OMITTED]
The distribution of the major sulfide minerals in the Platreef indicates a change from a sulfur-rich to a metal-rich environment during the crystallization. Pyrite, pentlandite, monoclinic pyrrhotite and chalcopyrite are dominant in the lower zone of the Platreef, whereas pentlandite, hexagonal pyrrhotite and chalcopyrite ([+ or -] cubanite) predominate in the upper zone (Gain and Mostert, 1982). Subsequent deformation can be correlated with regional fault structures which have had the effect of focusing hydrothermal mineralization and remobilization of platinum-group elements along north-south and north-northeast-trending faults (Nex, 2005).
Spencer(1926) described one of the British Museum's sperrylite specimens as embedded in copper-stained limonite from a brecciated shear zone in banded ironstone (Penge Formation of the Pretoria Series; Button, 1976). Included in these zones are "eyes" of a pegmatitic rock (a graphic intergrowth of quartz and grayish brown feldspar). Overlying the banded ironstone is a coarse-grained, black pegmatoid consisting of augite and inverted pigeonite with minor plagioclase and olivine. The rock is very well mineralized with disseminated sulfides, and has intruded into and reacted with the underlying banded ironstone. The limonite is obviously an alteration product of iron and copper sulfides (which, in their unaltered form, comprise the matrix of the Norilsk specimens).
Sperrylite was first discovered in South Africa in 1923, in gossan from the Vlakfontein mine in the Rustenburg district. In 1925 it was reported from the sulfidic platinum ore of the Sandsloot mine in the Potgietersrust district. The third discovery was at the Tweefontein mine, described by Wagner (1926c) as follows:
The sperrylite crystals are of a bright tin-white colour with splendent luster. Many of them look as though they have been polished, and are objects of great beauty. The crystals are mostly of cubic habit, but the cube and octahedron are sometimes equally developed. Other forms are invariably present, thus the edges of the cube and octahedron are generally beveled by narrow crystal faces, and in consequence reflect the light from numerous brilliant facets. The following forms were identified: the rhombic dodecahedron, the pyritohedron, the trapezohedron, several dyakis dodecahedra, and probably the triakis octahedron [= trisoctahedron],
Wagner stated that the Tweefontein sperrylite crystals are typically 1.3 mm to 1.7 cm in size.
[FIGURE 7 OMITTED]
Specimens first came to the attention of Leonard J. Spencer, Assistant Keeper in the Mineralogy Department of the British Museum (Natural History), in 1926. Spencer read a paper before the British Mineralogical Society on June 15 of that year, and it was published in the society's journal, The Mineralogical Magazine. Spencer described several specimens that had recently been acquired by the Museum:
[FIGURE 8 OMITTED]
(1) The first crystal was donated to the Museum by George H. Beatty of Johannesburg (at that time employed by the Johannesburg Consolidated Investment Company, Ltd., and acting as a consulting engineer for Potgietersrust Platinums, Ltd., the company operating the Tweefontein mine); it had been collected in "a new adit" on the Tweefontein farm, suggesting that underground mining had been going on there previously via other adits. Beatty had already given a brief preliminary account of Tweefontein sperrylite crystals at a meeting of the Chemical, Metallurgical and Mining Society of South Africa on February 20, 1926. The crystal Spencer described measures 5.3 X 5.5 mm, which at the time ranked it as larger than any sperrylite crystal ever found at any other locality worldwide. Spencer also mentioned seeing larger but imperfect and broken crystals from Tweefontein up to 1.3 cm.
[FIGURE 9 OMITTED]
According to Spencer, the crystal shows "brilliant metallic lustre with silver-white colour, much resembling the crystals of amalgam (silver-amalgam) from the Palatinate (Rhenish Bavaria)"--by which Spencer was probably referring to the mineral moschellandsbergite, which was not named until 1938. The sperrylite crystal is euhedral, without any point of attachment, and is cuboctahedral in habit with rounded edges that give curved outlines to the cuboctahedron faces, similar to the manner in which diamond crystals tend to be rounded. A close examination of the rounded edges reveals minute facets of the dodecahedron, tetrahexahedron and trapezohedron. One of the cube faces reveals terraced growth lines, a feature common to Norilsk sperrylite as well.
It is interesting to note that the Tweefontein sperrylite crystals are pristine and do not show the fine network of cracks that is typical of sperrylite crystals from Norilsk. This is probably because the Tweefontein crystals were found in soft gossan that had gradually altered from the sulfides that had originally encased them, whereas the Norilsk crystals are still embedded in unaltered sulfides. These solid sulfides must surely transmit the shock waves caused by blasting more effectively than would a soft gossan, thus resulting in cracks in the sperrylite.
(2) The second, much larger crystal (the largest that had been found at Tweefontein) was brought to the British Museum by Beatty as well, as a donation from the Chairman and Directors of the Pot-gietersrust Platinums, Ltd. This crystal measures 2.26 cm (nearly an inch). It is a cuboctahedron with rounded edges, silver-white in color and showing a brilliant metallic luster and mirror-smooth faces that under magnification show minute growth terraces. This crystal is accompanied by (though detached from) a 6-cm limonite matrix which has a sharply marked cavity into which the crystal fits perfectly. The inner surface of the cavity is smooth and glazed. This is the specimen illustrated in Bancroft (1973) and shown here (Fig. 9). Intriguingly, the matrix also shows two other similar cavities from which crystals had obviously been removed, but the ultimate fate of those crystals remains unknown.
[FIGURE 10 OMITTED]
Spencer notes that the crystal is "slightly broken" in two places; however, the largest damaged corner is not visible in the photo or on display in the museum. The curators have inserted the loose crystal up-side-down in its matrix socket so as to show instead the undamaged side (this can be detected in the photo because the crystal does not fit snugly down into the socket).
[FIGURE 11 OMITTED]
(3) A third specimen received by Spencer shows "minute specks and small splashes of sperrylite in a more compact matrix consisting of a mixture of limonite, malachite, quartz and feldspar ... no doubt representing a mineralized pegmatite."
(4) Spencer also mentions a "much smaller crystal of sperrylite from this locality, which was sent by Dr. A. W. Rogers for the Cambridge Mineralogical Museum." It shows a more perfect development of many small faces.
The Natural History Museum of Los Angeles County has one of the few Tweefontein sperrylite crystals in the United States; it is a 2.5-cm limonite matrix piece containing a well-formed 8-mm cuboctahedral sperrylite crystal with rounded edges-precisely like the British Museum matrix specimen but smaller.
A number of other rare-earth-element minerals have been found at the Tweefontein farm, mostly as small grains in ore concentrates and in polished sections. These include braggite (Pt,Pd,Ni)S, cooperite (Pt,Pd,Ni)S, laurite [RuS.sub.2], mertieite-II [Pd.sub.8][(Sb,As).sub.3], and native platinum Pt. Joints cutting the platinum-bearing veins are typically lined by botryoidal crusts of bright peacock-blue chrysocolla (Wagner, 1925).
The Tweefontein farm is also the type locality for stibiopalladinite. Adam (1927) described a new silver-white to dark steel-gray "[Pd.sub.3]Sb" mineral from the Tweefontein mine. He cited a description by Mr. E. L. Gay-Roberts, manager of the Tweefontein mine, stating that the new mineral is found as small crystals associated with sperrylite in the pegmatitic "eyes" in the brecciated ironstone. The formula was later refined to [Pd.sub.5][Sb.sub.2], and refined again to [(Pd,Cu).sub.5+x][(Sb.sub.2-x]_s (Cabri and Chen, 1976). It forms hexagonal, thin to thick tabular crystals to about 200 [micro]m.
My thanks to Bruce Cairncross and Anthony Kampf for reviewing the manuscript and offering helpful suggestions.
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WHITE, J. (1994) The Potgietersrus Project--geology and exploration history. In ANHAEUSSER, C. R. (editor), Proceedings of the XVth CMMI [Council of Mining and Metallurgical Institutes] Congress. Johannesburg. Geology, 3, 173-182.
Wendell E. Wilson
The Mineralogical Record
4631 Paseo Tubutama
Tucson, Arizona 85750
(1) The name Tweefontein = "Two springs."
(2) Spelled "Potgietersrust" in earlier literature.
(3) The term reef, as used by South African geologists, does not refer to an organic deposit like a coral reef but rather is a provincial term for a linear outcropping of a metalliferous deposit.
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|Author:||Wilson, Wendell E.|
|Publication:||The Mineralogical Record|
|Date:||Mar 1, 2010|
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