Accessory mineralogy of orangeite from Swartruggens, South Africa

Summary ?Orangeite occurring as a complex series of dikes at Swartruggens (South Africa), is host to a diversity of accessory minerals, the most common of which are apatite, barite and calcite. Less common, but important phases are perovskite, wadeite, an unidentified Ca–Ti–Fe-silicate, strontianite, unidentified Ca-REE phosphate, zircon, rutile, titaniferous magnetite, quartz and diverse sulphides. The accessory minerals show wide variations in their mode in different segments of the dike suite as a consequence of crystal sorting during flow differentiation. Compositional data are given for apatite, barite, calcite, perovskite, wadeite and the unidentified Ca–Ti–Fe-silicate. The accessory mineral suite is similar to that found in lamproites but is sufficiently distinct in composition and paragenesis to preclude inclusion with that clan. Differences include the common presence of groundmass calcite, barite and serpentine in the orangeite and the absence of typomorphic minerals (leucite, sanidine, richterite) of the lamproite clan. Received January 15, 2001; revised version accepted October 15, 2001     

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     

Acid mine drainage at a coal mine in the eastern Transvaal, South Africa

 Several mines in the Witbank coalfield in South Africa are affected by acid mine drainage. This has led to a deterioration in the water quality in many surface streams. The Loubert Mine is one such mine. Hence, an initial investigation was carried out to determine the source of acid mine drainage pollution and the associated hydrogeological conditions. The investigation showed that most of the acid mine drainage is emanating from old opencast workings which have been backfilled. Most of the water from the backfilled area drains into control reservoirs. Unfortunately their capacity is limited, which means that water overspills and seeps from them. This water finds its way into a nearby stream, the water of which accordingly has an unacceptably low pH value and high sulphate content. The proposals advanced to control the problem basically involve inhibiting the amount of water infiltrating the backfilled opencast area on the one hand and reducing the amount of water entering the control reservoirs on the other. Received: 5 March 1997 · Accepted: 17 June 1997     

Implications for Neoarchaean ocean chemistry from primary carbonate mineralogy of the Campbellrand-Malmani Platform, South Africa

The precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform‐building process during deposition of the 2560–2520 Ma Campbellrand‐Malmani carbonate platform, South Africa. Aragonite fans and fibrous coatings are common in unrestricted, shallow subtidal to intertidal facies. They are also present in restricted facies, but are absent from deep subtidal facies. Decimetre‐thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies. The proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0% to > 65% depending on the facies. Subtidal facies commonly contain 20–35%in situ precipitated carbonate, demonstrating that Neoarchaean sea water was supersaturated with respect to aragonite, carbonate crystal growth rates were rapid compared with sediment influx rates, and the dynamics of carbonate precipitation were different from those in younger carbonate platforms. The abundance of aragonite pseudomorphs suggests that sea‐water pH was neutral to alkaline, whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans.     

The evolution of a grospydite from the Roberts Victor mine,South Africa

A grospydite from Roberts Victor contains the most Ca-rich garnets yet found in South African kimberlite xenoliths and also sub-micron sized sodic nepheline in melted and quenched clinopyroxene. Three stages can be recognised in the textural evolution of the grospydite. The first is the development of a layering of large kyanite laths. Kyanite together with complex aluminous clinopyroxene precipitated and accumulated from an evolved residual eclogitic liquid which has penetrated across the garnet join so that garnet no longer precipitated. Solidus conditions for the Roberts Victor grospydite are estimated as T = 1350–1550 ° C, P = 27–39 kbars. Adjacent layers in the grospydite have slightly different mineral compositions suggesting that the small-scale layering (1–5 cm) in this, and associated rocks, may be related to varying activities of R₂O₃ components and possibly to f _{o 2}.The second stage is represented by a necklace texture in which all the garnet and some kyanite developed along grain boundaries of clinopyroxenes with triple-point textures. This is interpreted as an example of incoherent, grain-boundary exsolution resulting from large subsolidus volume changes. The conditions for subsolidus equilibration are estimated to be T= 1120–1320 ° C, P = 42–56 kbars.Moderate Ca-contents in garnet and excess Al^[6] in clinopyroxene may be subsolidus indicators of eclogite samples evolving towards grospydite at the solidus.The third stage is represented by the melting of jadeite-rich clinopyroxenes and quenching to glass, nepheline and plagioclase. Most of the glass has a composition similar to clinopyroxene, except for K₂O, though local areas of different glass, possibly the result of phase separation, also occur. The melting process seems to be a low-pressure feature involving limited addition of H₂O at temperatures between 900–1000 ° C. Water-absent melting could indicate temperatures up to 1500 ° C.The temperatures and pressures assigned to the three-stage evolution of this grospydite imply formation at moderate pressures and subsolidus equilibration at higher pressures. This is equated with downgoing mantle/asthenosphere tectonic processes. After entrainment in a kimberlite magma the grospydite fragment apparently ascended rapidly, thus allowing low-pressure melting and quenching.     

Petrology and geochemistry of a diamondiferous lherzolite from the Premier diamond mine, South Africa

This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa.

The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr₂O₃ and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (<11 ppm), Zr (<18 ppm) and Sr (<0.5 ppm). Garnet rare earth element concentrations, when normalised against the C1 chondrite of McDonough and Sun (Chem. Geol. 120 (1995) 223), are characterised by a rare earth element pattern similar to garnet from fertile lherzolite.

All diamonds recovered are colourless. Most crystals are sharp-edged octahedra, some with minor development of the dodecahedral form. A number of crystals are twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions are rare. Where present they are predominantly small black rosettes believed to consist of sulfide. In one instance a polymineralic (presumably lherzolitic) assemblage of reddish garnet, green clinopyroxene and a colourless mineral is recognised.

Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and variable nitrogen aggregation state, from 20% to 80% of the ‘B’ form. When plotted on a nitrogen aggregation diagram a well defined trend of increasing nitrogen aggregation state with increasing nitrogen content is observed. Carbon isotopic compositions range from −3.6 ‰ to −1.3 ‰. These are broadly correlated with diamond nitrogen content as determined by infrared spectroscopy, with the most negative C-isotopic compositions correlating with the lowest nitrogen contents.

Xenolith mantle equilibration temperatures, calculated from nitrogen aggregation systematics as well as the Ni in garnet thermometer are on the order of 1100 to 1200 °C.

It is concluded that the xenolith is a fertile lherzolite, and that the lherzolitic character may have resulted from the total metasomatic overprinting of pre-existing harzburgite. Metasomatism occurred prior to, or accompanied, diamond growth.     

Kornerupine in Mg-Al-rich gneisses from Namaqualand,South Africa: mineralogy and evidence for late-metamorphic fluid activity

Three kornerupine occurrences are reported in distinctive SiO₂-poor, MgO- and Al₂O₃-rich paragneisses from the Namaqualand Metamorphic Complex in South Africa. Kornerupine coexists stably with phlogopite, cordierite, orthopyroxene, gedrite, sapphirine, sillimanite and plagioclase and, in sapphirine-free rocks, with spinel and corundum. Tourmaline of a texturally older generation than kornerupine is commonly present in the same samples.Ten analysed kornerupines show a variation in total Fe as FeO from 1.8 to 10.9 weight per cent. B₂O₃ contents are estimated from x-ray data and a few spectrochemical analyses to range from 0.9 to 3.5 weight per cent. There is a strong inverse correlation between B³⁺ and Al³⁺. Total iron content has a strong and systematic effect on refractive index, colour and dispersion. Fe and Mg are systematically partitioned with the other minerals, and Mg/(Mg+Fe) ratios increase as follows: spinel 相似文献   

Geochemistry and mineralogy of platinum-group elements at Hartley Platinum Mine,Zimbabwe

The behaviour of platinum-group elements (PGE) in the exogenic cycle was examined in profiles of oxidized Main Sulfide Zone (MSZ) ores, in which the general metal distribution patterns of the pristine MSZ are grossly preserved. However, at similar Pt grades, significant proportions of Pd have been lost from the system. This indicates that Pd is more mobile than Pt and is dispersed in the supergene environment. Sperrylite and cooperite/braggite are stable in the oxidized MSZ. In contrast, the (Pt,Pd)-bismuthotellurides, common in pristine MSZ ores, have disintegrated, and ill-defined (Pt,Pd)-oxides or (Pt,Pd)-hydroxides have formed. The assemblage of detrital PGM present in the Makwiro River close to the Hartley Platinum Mine indicates further mineralogical changes. Sperrylite largely remains stable whereas most cooperite/braggite grains have been partly altered or completely destroyed. Grains of Pt-Fe alloy are ubiquitous in the alluvial sediments. Most likely, these grains are neo-formations that formed either from pre-existing, unstable PGM or via a solution stage under low-temperature conditions.     

Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa

The Merensky Reef of the Bushveld Complex is one of the world'slargest resources of platinum group elements (PGE); however,mechanisms for its formation remain poorly understood, and manycontradictory theories have been proposed. We present precisecompositional data [major elements, trace elements, and platinumgroup elements (PGE)] for 370 samples from four borehole coresections of the Merensky Reef in one area of the western BushveldComplex. Trace element patterns (incompatible elements and rareearth elements) exhibit systematic variations, including small-scalecyclic changes indicative of the presence of cumulus crystalsand intercumulus liquid derived from different magmas. Ratiosof highly incompatible elements for the different sections areintermediate to those of the proposed parental magmas (CriticalZone and Main Zone types) that gave rise to the Bushveld Complex.Mingling, but not complete mixing of different magmas is suggestedto have occurred during the formation of the Merensky Reef.The trace element patterns are indicative of transient associationsbetween distinct magma layers. The porosity of the cumulatesis shown to affect significantly the distribution of sulphidesand PGE. A genetic link is made between the thickness of theMerensky pyroxenite, the total PGE and sulphide content, petrologicaland textural features, and the trace element signatures in thesections studied. The rare earth elements reveal the importantrole of plagioclase in the formation of the Merensky pyroxenite,and the distribution of sulphide. KEY WORDS: Merensky Reef; platinum group elements; trace elements     

Dating ultra-deep mine waters with noble gases and Cl, Witwatersrand Basin, South Africa

Concentrations and isotopic ratios of dissolved noble gases, ³⁶Cl, δD and δ¹⁸O in water samples from the ultra-deep gold mines (0.718 to 3.3 km below the surface) in the Witwatersrand Basin, South Africa, were investigated to quantify the dynamics of these ultra deep crustal fluids. The mining activity has a significant impact on the concentrations of dissolved gases, as the associated pressure release causes the degassing of the fissure water. The observed under saturation of the atmospheric noble gases in the fissure water samples (70-98%, normalized to ASW at 20°C and 1013 mbar) is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature. Corrections for degassing result in ⁴He concentrations as high as 1.55 · 10⁻¹cm³STP⁴He g⁻¹, ⁴⁰Ar/³⁶Ar ranging between 806 and 10331, and ¹³⁴Xe/¹³²Xe and ¹³⁶Xe/¹³²Xe ratios above 0.46 and 0.44, respectively. Corrected ^134(136)Xe/¹³²Xe and ^134(136)Xe/⁴He-ratios are consistent with their production ratios, whereas the nucleogenic ⁴He/⁴⁰Ar, and ^134(136)Xe/⁴⁰Ar ratios generally indicate that these gases are produced in an environment with an average [U + Th]/K-content 2-3 times above that of crustal average. In two scenarios, one considering only accumulation of in situ produced noble gases, the other additionally crustal flux components, the model ages for 14 individual water samples range from 13 to 168 Ma and from 1 to 23 Ma, respectively.The low ³⁶Cl-ratios of (4-37) · 10⁻¹⁵ and comparatively high ³⁶Cl-concentrations of (8-350) · 10⁻¹⁵ atoms ³⁶Cl l⁻¹ reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 times the half-life of ³⁶Cl.In combination, the results suggest residence times of the fluids in fissures in this region (up to 3.3 km depth) are of the order of 1-100 Ma. We cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk.     

Braunite mineralogy and paragenesis from the Kajlidongri mine,Madhya Pradesh,India

The Kajlidongri manganese deposit, Madhya Pradesh, India contains braunite belonging to at least two different parageneses, i.e. a fine-grained, metamorphic type and a younger, coarser-grained, hydrothermal type. Microanalyses of braunite from these paragenetic types indicate considerable variations in iron content from zero to 32.7% Fe₃O₃, which the writers consider is due to varying proportions of braunite I and bixbyite modules in polysomatic braunite.     

Some problems associated with past mining at a mine in the Witbank coalfield, South Africa

 Mining in the Witbank Coalfield commenced at the turn of the century. Initially there was little environmental degradation associated with mining activities; however, in the late 1930s and early 1940s a pillar-robbing programme commenced. At one particular mine this has had marked effects on the environment. Primary effects include subsidence, the appearance of tension cracks at the surface and crownhole development. Secondary effects include spontaneous combustion of the coal worked, as air has been provided with ready access to the mine, accelerated subsidence due to the strength of many pillars being reduced by burning, and a marked deterioration in groundwater quality in the area. Spoil heaps also form blemishes on the landscape. These contain significant amounts of coal and have undergone spontaneous combustion. The deterioration in the water quality has led to the decimation of vegetation in some areas and the eradication of aquatic flora and fauna in a nearby stream. Received: 5 March 1997 · Accepted: 1 July 1997     

On the relationship between cumulus mineralogy and trace and alkali element chemistry in an Archean granite from the Barberton region,South Africa

A change in the liquidus mineralogy from plagioclase-quartz-biotite to plagioclase-quartz-K feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of and interrelationships between Ba, Rb, Sr, K₂O and Na₂O. During plagioclase fractionation, Ba and Rb enrich in successive solids and melt, while Sr is depleted. The K₂O content of the solid phase (around 1%) is very different from that of the melt (greater than about 3%) while Na₂O contents are similar (about 4–5%) so that variations in the amount of intercumulus melt result in wide variation in the Na₂O/K₂O ratio (from about 5 in cumulus-rich to about 1 in intercumulus-rich samples). The incoming of K feldspar as a cumulus phase causes Ba to be depleted along with Sr in successively formed solids, while Rb continues to be enriched. A pronounced compositional hiatus with respect to trace elements therefore results. The K₂O and Na₂O contents of melt and cumulate are now similar (around 5% K₂O and 4% Na₂O), so that little scatter in Na₂O/K₂O ratio (around 1) occurs as a result of variation in the amount of intercumulus melt. In general, trace element data from a natural example in the Barberton Mountain Land, South Africa, fit the models well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions—possibly the result of the non-uniform distribution of biotite in the samples.     

Hydrochemistry,mineralogy and sulfur isotope geochemistry of acid mine drainage at the Mt. Morgan mine environment,Queensland, Australia

Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO₄ and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River.Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions (δ³⁴S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO₄ back to its source. The higher δ³⁴S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO₄ in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions (δ³⁴S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in ³⁴S (δ³⁴S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO₄ reduction in the weir pools, and in the water bodies within the river channel. The SO₄ and metals attenuate downstream by a combination of dilution with the receiving waters, SO₄ reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics.     

Structures at the base of the Upper Group 2 chromitite layer, Bushveld Complex, South Africa, on Karee Mine (Lonmin Platinum)

Exposures in a now-infilled pit mined for platiniferous UG2 chromitite in the Bushveld Complex, South Africa, are described. The layer of chromitite is underlain by anorthosite, providing a dramatic colour contrast. The interface between these two rock types shows evidence of various scales of irregularities. In plan view, small circular depressions, less than 3 cm across and 5 mm deep, occupy about 20% of the surface. Between them, the contact is planar. The anorthosite, immediately underlying the chromitite, has a planar fabric visible in thin sections that is not disturbed beneath these small depressions. Another set of depressions occurs, about 40 cm in diameter and with variable depth (< 40 cm). Again they are approximately circular. Larger structures, called potholes, reach several metres. No regular distribution pattern is apparent in any of these structures.

Several possible processes are reviewed for the origin of these irregularities, especially the small-scale structures, but none explains all the features noted. These processes include remelting, diapirism, impact-generated dimpling, gas escape, and interference rippling. We present a photographic record of these structures, but present no definitive model for their interpretation.     

The mineralogy and mineral associations of platinum group elements and gold in the Platreef at Zwartfontein, Akanani Project, Northern Bushveld Complex, South Africa

The mineralogy of the platinum-group elements (PGE), and gold, in the Platreef of the Bushveld Complex, was investigated using an FEI Mineral Liberation Analyser. Polished sections were prepared from 171 samples collected from two boreholes, for the in-situ examination of platinum group minerals (PGM). PGM and gold minerals encountered include maslovite (PtBiTe, 32 area% of total PGM), kotulskite (Pd(BiTe), 17?%), isoferroplatinum (Pt₃Fe, 15?%), sperrylite (PtAs₂, 11?%), cooperite (PtS, 5?%), moncheite (PtTe₂; 5?%), electrum (AuAg; 5?%), michenerite (PdBiTe; 3?%), Pd alloys (Pd, Sb, Sn; 3?%), hollingworthite ((Rh,Pt)AsS; 2?%), as well as minor (all <1 area% of total PGM) merenskyite (PdBiTe₂), laurite (RuS₂), rustenburgite (Pt_0.4Pd_0.4Sn_0.2), froodite (PdBi₂), atokite (Pd_0.5Pt_0.3Sn_0.2), stumpflite (PtSb), plumbopalladinite (Pd₃Pb₂), and zvyagintsevite (Pd₃Pb). An observed association of all PGM with base metal sulfides (BMS), and a pronounced association of PGE tellurides, arsenides and Pd&Pt alloys with secondary silicates, is consistent with the remobilisation and recrystallisation of some of the PGM’s during hydrothermal alteration and serpentinisation subsequent to their initial (primary) crystallisation from BMS (e.g. Godel et al. J Petrol 48:1569–1604, 2007; Hutchinson and McDonald Appl Earth Sci (Trans Inst Min Metall B) 114:B208–224, 2008).     

Pyroxenes of the Bushveld Intrusion, South Africa

New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca_45.4Mg_49.6Fe_5.0 to Ca_42.7Mg_0.6Fe_56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca_2.8Mg_85.0Fe_12.2)through pigeonites to ferropigeonites (approximately Ca₃Mg₂₇Fe₅₅).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system Wo—En—Fs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate     

Magnetic mineralogy of some ring complexes from the South Eastern Desert,Egypt

Ore microscopic study was carried out on a great number of samples collected from four well-dated alkaline rock masses (ring complexes), namely Gabal Mishbeh, Gabal Nigrub El Fogani, Gabal Nigrub El Tahtani, and Gabal El Gezira located in the Eastern Desert of Egypt. These rings consist mainly of wide variety of rock types ranging from basic to acidic and from under saturated to quartz bearing. The opaques of the basic rocks consist mainly of various ilmenite–magnetite intergrowths and differ essentially from those of the acidic rocks where minor amount of titanomagnetite is present. Numerous exsolution, replacement, and deformational textures were described. The distribution of opaque minerals and their intergrowths in the four well-defined ring complexes is described. Systematic rock magnetic measurements such as natural remanent magnetization intensity and initial susceptibility, Curie point, saturation magnetization, and coercive force were carried out, and their results were discussed in the light of opaque mineralogical studies.     

Texture, mineralogy, and rock strength in horizontal stress-related coal mine roof falls

Geologic structures can represent planes of preferential weakness that, by dismembering the roof beam, may contribute to the failure of roof spans. However, beam deflection and roof failure also occur in rocks where no visible geologic discontinuities are present. This suggests that roof failure may depend on rock strength, which in turn depends on intrinsic textural properties inherent to the rock. In this study, rock samples were collected from horizontal stress-related roof fall material in coal mines for petrographic characterization and compressive strength testing. Brittle, stress failure-prone rock types include thinly interlaminated siltstone and shale, and black shale that had been lightly recrystallized. Samples exhibit a narrow range of density values between approximately 2.5–3.0 g/cm³ but exhibit a wide range of unconfined compressive strength values, between approximately 20–70 MPa. Results of laboratory observations suggest that for samples of coal mine immediate roof shale, compressive strength is not well correlated with density, grain size, sutured grain boundaries, or quartz content. These results for shale are generally at odds with the results of similar studies for sandstone. The great variability of strength, texture, and mineralogy documented in these samples may be an indication of their complexity and the need for specialized methodology in the study of shale strength.     

Petrochemistry and mineralogy of early Precambrian anorthositic rocks of the Limpopo belt, southern Africa

Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An₃₁-An₈₀) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO₂, TiO₂ and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones.

The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California.