The West Waterberg tonstein,South Africa

What is thought to be the first recorded occurrence of a tonstein in the coalfields of South Africa is described from the Volkrust Shale Formation of the Permain Karoo Sequence in the West Waterberg Coalfield. Now, virtually pure kaolin, petrological, mineralogical and geochemical evidence points to its origin being an airfall tuff of acid composition. The concentrations of Ba, Sr, and Pb in the tonstein are unusually high. It is thought the elements were released from the ash and precipitated as phosphates of mixed composition. The phosphate distribution is not uniform and is apparently related to the fragmental texture of the rock.     

Primary anhydrite in Precambrian gneisses from the swakopmund district,South West Africa

Anhydrite occurs as a primary rock-forming mineral in late-Precambrian gneisses from the Rössing area, South West Africa. Studies on critical mineral associations in the area indicate that temperatures during metamorphism approached or exceeded 700° C. Textural relations in the anhydrite-bearing gneisses show that the mineral remained in equilibrium with its associated phases during metamorphism.     

The mineral chemistry of new titanates from the jagersfontein kimberlite,South Africa: Implications for metasomatism in the upper mantle

New members of the crichtonite mineral series are described in which K, Ba, Ca and REE are in significant concentrations (5 wt% oxides) filling the A formula position in AM₂₁O₃₈. These phases are chromium (16 wt% Cr₂O₃) titanates (58 wt% TiO₂) enriched in ZrO₂ (5 wt%) and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle. The mineral senes coexists with Mg-Cr-ilmenite, Nb-Cr-rutile, and Ca-Cr (NbZr) armalcolite that have equally unusual chemistries. Kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in LREE and HREE with chondrite normalized abundances of 10³ to 10⁵. Crichtonite, armalcolite, and Nb-Cr-rutile occupy a compositional range in TiO₂ contents bridging the gap between ilmenite and rutile, two minerals having a widespread distribution in kimberlites and mantle-derived nodule suites.In common with other associations, and based on similarities in mineral chemistry, it is concluded that these minerals formed at P = 20–30 kb, 900–1100°C by reaction of peridotite with metasomatizing fluids. Kimberlitic crichtonite may be expressed as spinel + Cr-ferropseudobrookite, and armalcolite is equivalent to Cr-geikielite + rutile in the system (FeMg)-TiO₂-Cr₂O₃. This system contains a number of Cr-Ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure αPbO₂-type crystallographic shear structures. The new minerals have exotic chemistries and the high K-affinities broaden the scope for the origin of alkalic rocks, the generation of highly potassic magmas in the upper mantle, and suggest that alkali metasomatism may be pervasive.     

Uranium-bearing phoscrete from Mali, West Africa

Many radioactive anomalies have been found in Mali. The occurrence of the continental U-bearing phoscrete differs considerably from that of marine sedimentary phoscrete which is being used as the raw material in the manufacturing of phosphoric acid. The results of some chemical and physical measurements on the U-bearing phoscrete will be briefly explained here and the genesis will be inferred.     

Granite intruding into Granodiorite: An example from the Damara Belt,South West Africa

Until now (Smith, 1965) it was thought that in the central Damara Belt, South West Africa, the Red Granite and the Salem Granodiorite are two magmatic bodies separated by (essentially) marbles. Three places were discovered, where the marbles are missing; in all three places the Red Granite intrudes the Salem Granodiorite. Experimental melting done with samples from two of these localities allows to appraise the likely conditions during the intrusion of the Red Granite (680° C, 4 kb H₂O-pressure).The White Granites — until now thought to be late or post kinematic — are a geologically heterogeneous group, at least in one place such an intrusion precedes that of the Red Granite.

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Zusammenfassung Bis jetzt dachte man (Smith, 1965) daß im Zentralbereich des Damara-Orogens, Südwest Afrika, der Rote Granit und der Salem Granodiorit zwei magmatische Körper wären, die überall von (im wesentlichen) Marmoren getrennt seien. Es wurden drei Gegenden entdeckt, an denen die Marmore fehlen; an allen drei Stellen intrudiert der Rote Granit in den Salem Granodiorit. Schmelz-Experimente mit Proben von zwei dieser Lokalitäten erlauben die Bedingungen abzuschätzen, die während der Intrusion des Roten Granites geherrscht haben (680° C, 4 kb H₂O-Druck).Die Weißen Granite — von denen man bis jetzt glaubte, sie seien spät-oder nachkinematisch — sind eine geologisch heterogene Gruppe. Mindestens an einer Stelle geht eine Intrusion der Weißen Granite derjenigen der Roten Granite voraus.

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Résumé Jusqu'à présent (Smith, 1965) on a pensé que dans la zone centrale de l'orogène de Damara (Sud-Ouest Africain) le Granite Rouge et la Granodiorite de Salem étaient deux plutonites séparées partout par des couches de marbres. On a découvert trois endroits où les marbres n'existent pas et où le Granite Rouge forme des intrusions recoupant la Granodiorite de Salem.Des essais de fusibilité sur des échantillons provenant de deux de ces localité permettent d'estimer les conditions d'intrusion du Granite Rouge (680° C avec des pressions H₂O de 4–5 kb).Les Granites blancs (acceptés jusqu'à présent comme tardi orogéniques) forment géologiquement une groupe hétérogène. En un endroit au moins, une intrusion de Granite blanc est recoupée par une intrusion du Granite Rouge, plus jeune.

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(SMITH, 1965), , , - , , . 3 , , . , , , (680° C, 4 ). , , -, , . , - , .

     

Althausite,a new mineral from Modum,Norway

Althausite occurs as cleavable masses in serpentine-magnesite deposits at Modum, Norway. The proposed formula in Mg₂PO₄ (OH_0.37F_0.25O_0.10)_0.81 with partly vacant halide sites. It is orthorhombic, space group Pna2₁, $\text{a}\text{= 8.258,}\text{b}\text{= 14.383,}\text{c}\text{= 6.054}\text{A}\text{?}\text{, Z}\text{= 8.}\text{D(meas)}\text{=2.97, ?(}\text{calc}\text{) = 2.91}\text{g/cm}{}^3\text{(}\text{X-ray}\text{), ?(}\text{calc}\text{) = 3.06}\text{g/cm}{}^3\text{(}\text{Gladstone-Dale}\text{),}\text{H}\text{= 3}\text{1}\text{2}\text{?4.}$ The strongest X-ray powder lines (41 given, FeKα radiation) with intensities and indices are 3.593 (100)(040), 3.316 (90)(211), 3.024 (80)(002), 2.786 (60)(112), 2641 (60)(122).The mineral is light grey with vitreous lustre, running brown on alteration to apatite. Non-fluorescent. Perfect cleavage {001}, distinct cleavage {101}. It is biaxial positive, α=1.588, β=1.592, γ=1.598, 2V_γ(calc)=78.5°, negative elongation, X=b, Y=c, Z=a. IR, DTA and TGA data are given.     

Preliminary notes on the Okonjeje igneous complex,South West Africa

Ohne Zusammenfassung     

New primary vanadium-, germanium-, gallium-, and tin-minerals from the Pb-Zn-Cu-deposit Tsumeb,South West Africa

Six unknown mineral species and subspecies are described. They were discovered in and nearby the Tsumeb main orebody and investigated under the ore microscope. Microprobe analysis revealed the presence of new mineral compounds with the paired elements: vanadium and gallium, vanadium and germanium, and tin and germanium, all of them sulfides. By means of diagrams their positions and their relations to minerals of known composition are shown and discussed.

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Zusammenfassung Es werden sechs unbekannte Mineralspezies und Subspezies beschrieben. Sie stammen aus dem Tsumeber Haupterzkörper und nahebei gelegenen Erzprospekten und wurden unter dem Erzmikroskop untersucht. Durch Untersuchung mit der Elektronen-Mikrosonde wurde das Vorhandensein neuer Mineralverbindungen nachgewiesen mit den Elementenpaaren: Vanadium und Gallium, Vanadium und Germanium und Zinn und Germanium sämtlich Sulfide. An Hand von Dreistoff-Diagrammen werden ihre Stellung und ihre Beziehungen zu Mineralien bekannter Art aufgezeigt und diskutiert.

     

Classification of five new ordinary chondrites from North West Africa

We classified five new ordinary chondrites from North West Africa. NWA 3010 is an L6(S5), NWA 3011 is an L5(S5), NWA 3012 is an LL4(S5), NWA 3013 is an L5(S5), and NWA 3014 is an H4(S1). The meteorites experienced a range of terrestrial alteration, with NWA 3010 equal to weathering grade W2, NWA 3011 equal to W3, NWA 3012 equal to W3, NWA 3013 equal to W2, and NWA 3014 equal to W4.     

Santite,a new mineral phase from Larderello,Tuscany

A new borate mineral has been found in a sample from Larderello, mixed with larderellite and lesser amounts of sassolite. The mineral is never well crystallized, but it constantly appears as very small, clear and colourless grains; its refractive indices are: =1.422, =1.435, =1.480. Single-crystal X-ray photographs show that the mineral is orthorombic with cell dimensions: =11.10 Å, b=11.18 Å, c=9.08 Å. X-ray powder diffraction and partial chemical analyses carried out on the whole sample (it being impossible to separate a sufficient quantity of pure material) confirm the foregoing data, according to which the mineral may be considered as the natural equivalent of the synthetic compound KB₅O₈ · 4H₂O. It is likely that it originated from the reaction between potassium-bearing hot solutions and primary deposits of larderellite. It is named for the Tuscan naturalist Giorgio Santi.This new mineral has been approved by the Commission on New Minerals and New Mineral Names of the International Mineralogical Association.This research was supported by Consiglio Nazionale delle Ricerche, Roma.     

The new hominid skeleton from Sterkfontein,South Africa: age and preliminary assessment

A new hominid skeleton from Sterkfontein Member 2 attaches to foot bones recovered from loose blocks during the 1980s and first described in 1995. Several flowstone horizons are present above and below the skeleton and have given clear palaeomagnetic signatures. Five changes in magnetic polarity have been identified; when constrained by the available biostratigraphy, this sequence can be placed confidently between 3.22 and 3.58 Ma. Interpolation of sedimentation rates over the small intervals between reversals allows this range to be reduced to 3.30–3.33 Ma. The skeleton is thus the oldest yet discovered and is considered to belong to a species of Australopithecus other than africanus. Copyright © 1999 John Wiley & Sons, Ltd.     

Zircon U-Pb ages of the Franzfontein granitic suite,northern South West Africa

U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed.     

Thaumasite from the Tschwinning mine,South Africa

Summary Thaumasite from the manganese mine of Tschwinning, South Africa, has the formula Ca_3.097 Si_1.062OH_6.372(CO₃)_0.994(SO₄)_1.000·11.596 H₂O. Unit cell parameters (a=11.013(2) Å,c=10.379(5) Å,V=1090.1(6) ų) and refraction indices (n_e=1.458(2),n _o=1.498(2)) compare well with the best ones available in the literature. The IR spectrum and DT-TG profiles are given. Contradictions between the IR spectrum, a recent structural refinement and the DT-TG are discussed, but cannot be explained satisfactorily.

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Thaumasit aus der Mangan-Mine von Tschwinning, Süd-Afrika

Zusammenfassung Thaumasit aus der Mangan-Mine von Tschwinning, Süd-Afrika, besitzt die Formel Ca_3.097 Si_1.062OH_6.372(CO₃)_0.994(SO₄)_1.000·11.596 H₂O. Die Parameter der Elementarzelle (a=11.013(2) Å,c=10.379(5) Å,V=1090.1(6) ų) und die Brechungsindizes (n _e=1.458(2),n _o=1.498(2)) zeigen gute Übereinstimmung mit verfügbaren Literaturdaten. Es werden ein IR-Spektrum und DT-TG-Profile präsentiert. Gegensätze zwischen dem IR-Spektrum, einer neueren Strukturverfeinerung und dem DT-TG werden diskutiert, können jedoch nicht zufriedenstellend erklärt werden.

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With 2 Figures     

Sedimentology of the Kuruman and Griquatown Iron-formations,Transvaal Supergroup,Griqualand West,South Africa

Iron-formation was deposited as a distal facies of ferruginous carbonate turbidites in an open shelf environment in front of a shallow-water carbonate platform at the time of deposition of the Campbellrand carbonate sequence. A subsequent transgression resulted in the deposition of open shelf iron-formation on top of the Campbellrand carbonate platform. Progradational sedimentation coupled with shoaling followed, and an iron-formation sequence represented by the Kuruman and Griquatown Iron-formations was deposited. This sequence consists from the base upwards of stacked open-shelf cycles of altered volcanic ash stilpnomelane lutite beds and autochthonous banded ferhythmite units; toe-of-slope greenalite—siderite rhythmites; platform slope greenalite—siderite rhythmites with grainflow bands; platform edge sideritic orthochemical and allochemical iron-formations; epeiric sea orthochemical and allochemical sideritic, hematitic and greenalitic iron-formations; supratidal disclutites and lacustrine banded greenalite lutite. Landwards, the lacustrine felutites were followed by deltaic chloritic claystone, siltstone and quartz wacke of the Koegas Subgroup.Autochthonous ferhythmites of the Kuruman Iron-formation reach a maximum development in a basin near Prieska, whereas orthochemical and allochemical units are more abundant on the Kaapvaal craton. Iron mineral and chert microbanding in the ferhythmites is attributed to seasonal changes in Eh and pH in the depository and may be related to biological activity. Chert mesobanding in the iron-formations is essentially of an early diagenetic origin.     

Oxide minerals in a layered kimberlite-carbonate sill from Benfontein,South Africa

The lower sill at Benfontein, South Africa, shows a high degree of magmatic sedimentation to kimberlite, oxide-carbonate, and carbonate layers. The iron-titanium oxide minerals are similar in the carbonate-rich and silicate-rich layers and are represented by titaniferous Mg-Al chromite, Mg-Al titanomagnetite, magnesian ilmenite, rutile, and perovskite. The spinel crystallization trend was toward enrichment in Mg and Ti and depletion in Cr; this trend is similar to that observed in many kimberlites. The ilmenite has Mg and Cr contents within the range observed in kimberlites and lacks the Mn enrichment observed in ilmenites from carbonatites. Perovskite in silicate-rich and carbonate-rich layers shows similar total REE contents and LREE enrichment and lacks the remarkable Nb enrichment observed in perovskite from carbonatites. These new data on the iron-titanium oxide minerals in the lower Benfontein sill do not support a genetic relationship between kimberlites and carbonatites.