A study of eclogitic diamonds and their inclusions from the Finsch kimberlite pipe, South Africa

Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K₂O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses.     

Thrust-controlled gold mineralisation at the Elandshoogte Mine,Sabie-Pilgrim's Rest goldfield,South Africa

Stratiform quartz-sulphide-gold veins, locally termed reefs are hosted within the Proterozoic Transvaal Sequence sedimentary succession, in the Sabie-Pilgrim's Rest goldfield, eastern Transvaal. These deposits have produced about 180 tonnes of gold and share many characteristics with those of Telfer, Western Australia. Detailed examination of the Elandshoogte Mine shows that gold deposition occurred in two stages, both linked to bedding-parallel thrust faulting within the sedimentary pile. Deformation being concentrated within incompetent shale beds, interlayered within more competent units. The majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early-formed sulphide minerals. Deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone, from early shearing in shales to later duplex faulting of the quartz-reef. Fluids accompanying faulting are implied to have transported gold, and a magmatic source of mineralisation is suggested.     

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.     

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.     

Petrology, geochemistry and the mechanisms determining the distribution of platinum-group element and base metal sulphide mineralisation in the Platreef at Overysel, northern Bushveld Complex, South Africa

Platinum-group element (PGE) mineralisation within the Platreef at Overysel is controlled by the presence of base metal sulphides (BMS). The floor rocks at Overysel are Archean basement gneisses, and unlike other localities along the strike of the Platreef where the floor is comprised of Transvaal Supergroup sediments, the intimate PGE–BMS relationship holds strong into the footwall rocks. Decoupling of PGE from BMS is rare and the BMS and platinum-group mineral assemblages in the Platreef and the footwall are almost identical. There is minimal overprinting by hydrothermal fluids; therefore, the mineralisation style present at Overysel may represent the most ‘primary’ style of Platreef mineralisation preserved anywhere along the strike. Chondrite-normalised PGE profiles reveal a progressive fractionation of the PGE with depth into the footwall, with Ir, Ru and Rh dramatically depleted with depth compared to Pt, Pd and Au. This feature is not observed at Sandsloot and Zwartfontein, to the south of Overysel, where the footwall rocks are carbonates. There is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the Platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place. Textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable, inter-granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the Platreef magma. PGE were originally concentrated within a sulphide liquid in the Platreef magma, and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the IPGE and Rh from it whilst still within the mafic Platreef. Transport of PGE into the gneisses, via downward migration of the residual sulphide liquid, fractionated out the remaining IPGE and Rh in the upper parts of the gneisses leaving a ‘slick’ of disseminated sulphides in the gneiss, with the residual liquid becoming progressively more depleted in these elements relative to Pt, Pd and Au. Highly sulphide-rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall. This study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of PGE from the Platreef into the footwall rocks. Where the floor rocks are sediments, fluid activity related to metamorphism, assimilation and later serpentinisation has decoupled PGE from BMS in places, and transport of PGE into the footwall is via hydrothermal fluids. In contrast, where the floor is comprised of anhydrous gneiss, such as at Overysel, there is limited fluid activity and PGE behaviour is controlled by the behaviour of sulphide liquids, producing an intimate PGE–BMS association. Xenoliths and irregular bands of chromitite within the Platreef are described in detail for the first time. These are rich in the IPGE and Rh, and evidence from laurite inclusions indicates they must have crystallised from a PGE-saturated magma. The disturbed and xenolithic nature of the chromitites would suggest they are rip-up clasts, either disturbed by later pulses of Platreef magma in a multi-phase emplacement or transported into the Platreef from a pre-existing source in a deeper staging chamber or conduit.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Petrology and mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa

The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long, are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections. These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides, bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE grade may be decoupled from S and base metal abundance.     

Episodic diamond genesis at Jwaneng, Botswana, and implications for Kaapvaal craton evolution

Major element and Re–Os isotope analysis of single sulfide inclusions in diamonds from the 240 Ma Jwaneng kimberlite has revealed the presence of at least two generations of eclogitic diamonds at this locality, one Proterozoic (ca. 1.5 Ga) and the other late Archean (ca. 2.9 Ga). The former generation is considered to be the same as that of eclogitic garnet and clinopyroxene inclusion bearing diamonds from Jwaneng with a Sm–Nd isochron age of 1.54 Ga. The latter is coeval with the 2.89 Ga subduction-related generation of eclogitic sulfide inclusion bearing diamonds from Kimberley formed during amalgamation of the western and eastern Kaapvaal craton near the Colesberg magnetic lineament.

The Kimberley, Jwaneng, and Premier kimberlites are key localities for characterizing the relationship between episodic diamond genesis and Kaapvaal craton evolution. Kimberley has 3.2 Ga harzburgitic diamonds associated with creation of the western Kaapvaal cratonic nucleus, and 2.9 Ga eclogitic diamonds resulting from its accretion to the eastern Kaapvaal. Jwaneng has two main eclogitic diamond generations (2.9 and 1.5 Ga) reflecting both stabilization and subsequent modification of the craton. Premier has 1.9 Ga lherzolitic diamonds that postdate Bushveld–Molopo magmatism (but whose precursors have Archean Sm–Nd model ages), as well as 1.2 Ga eclogitic diamonds. Thus, Jwaneng provides the overlap between the dominantly Archean vs. Proterozoic diamond formation evident in the Kimberley and Premier diamond suites, respectively. In addition, the 1.5 Ga Jwaneng eclogitic diamond generation is represented by both sulfide and silicate inclusions, allowing for characterization of secular trends in diamond type and composition. Results for Jwaneng and Kimberley eclogitic sulfides indicate that Ni- and Os-rich end members are more common in Archean diamonds compared to Proterozoic diamonds. Similarly, published data for Kimberley and Premier peridotitic silicates show that Ca-rich (lherzolitic) end members are more likely to be found in Proterozoic diamonds than Archean diamonds. Thus, the available diamond distribution, composition, and age data support a multistage process to create, stabilize, and modify Archean craton keels on a billion-year time scale and global basis.     

Gold mineralisation and advanced argillic alteration at the Dobroyde prospect,central New South Wales

Gold mineralisation at the Dobroyde prospect in central New South Wales is hosted by a zoned alteration system characterised by peripheral propylitic alteration, grading inwards through argillic and advanced argillic alteration to a siliceous altered core. Overprinting textures indicate that propylitic, argillic, advanced argillic and siliceous assemblages were successively superimposed on each other. Au grades between 0.3–0.8 ppm are associated with siliceous alteration and cross‐cutting pyrite veinlets. Higher Au grades are associated with barite veins that cut the pyrite veinlets. Native Au, native Te, Au, Pb and Hg tellurides, Pb selenide, chalcopyrite, Zn‐sphalerite and tennantite‐tetrahedrite occur in the barite veins. Microscopic pyrophyllite shears cut the barite veins. The location of the Dobroyde prospect, the orientation of its internal alteration zonation and the orientation of auriferous barite veins in the core of the prospect are controlled by a 330°‐striking fault. Movement on this fault, synchronous with hydrothermal activity, at some time between the Late Ordovician and mid‐Devonian controlled the development of successive phases of brecciation, siliceous alteration, pyrite and later barite‐Au veining in the prospect core. The restricted distribution of auriferous barite veins within the siliceous altered core of the prospect is inferred to be controlled by the relatively brittle rheology of this assemblage during deformation, and its location on the fault that formed the main hydrothermal fluid conduit. Alteration zones distal from this fault remain unmineralised. The Dobroyde prospect may be a product of the same Early Devonian metallogenic epoch as the paragenetically similar Temora and Peak Hill deposits. All three deposits/prospects appear to be localised in splays of either the Gilmore Fault Zone or the Parkes Thrust.     

Geology and diamond distribution of the 140/141 kimberlite, Fort à la Corne, central Saskatchewan, Canada

The Cretaceous age Fort à la Corne (FALC) kimberlite province comprises at least 70 bodies, which were emplaced near the edge of the Western Canadian Interior Seaway during cycles of marine transgression and regression. Many of the bodies were formed during a marine regression by a two-stage process, firstly the excavation of shallow, but wide, craters and then subsequent infilling by xenolith-poor, crater-facies, subaerial, primary pyroclastic kimberlite. The bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin, but high volume, pyroclastic kimberlite deposits. Each body is composed of contrasting types of kimberlite reflecting different volcanic histories and, therefore, are considered separately.

The 140/141 kimberlite is the largest delineated body in the province, estimated to have an areal extent below glacial Quaternary sediments in excess of 200 ha. The infilling of the 140/141 crater is complex, resulting from multiple phases of kimberlite. The central part of the infill is dominated by several contrasting phases of kimberlite. One of these phases is a primary pyroclastic airfall mega-graded bed up to 130 m in thickness. The constituents grade in size from very fine to coarse macrocrystic kimberlite, through to a basal breccia. The mega-graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe. A second different phase of kimberlite is interpreted as representing a younger nested crater within the mega-graded bed. Centrally located thicker intersections (>450 m) of this younger kimberlite may indicate a vent for the kimberlite crater. The thickness of the mega-graded bed increases with proximity to the younger kimberlite in the study area.

Macrodiamond minibulk sample grades from the mega-graded bed have been obtained from nine large diameter drill holes, located within the northwest part of the body from an area of 20 ha, which represents approximately 10% of the currently modeled kimberlite outline. Diamond grade increases with depth within the mega-graded bed and also increases, within the same unit, towards the centrally positioned younger kimberlite. Macrodiamond sample grades vary from low at the top of the mega-graded bed, to considerably higher grades near the base. Total sample grade per drill hole varies from moderate near the vent feature to lower grades 200–300 m from the vent feature. Macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega-graded bed. There is a strong correlation between depth and increased stones per tonne, and a similar correlation between stones per tonne and proximity to the vent feature. The data supports the emplacement model of the mega-graded bed and, in turn, this information is useful in understanding the macrodiamond distribution within this bed.     

Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica

The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions.     

Trace element zoning of sulfides and quartz at Sheba and Fairview gold mines: Clues to Mesoarchean mineralisation in the Barberton Greenstone Belt,South Africa

The Fairview and Sheba mines are two of the major gold mines in the Paleoarchean Barberton Greenstone Belt of Southern Africa. At these mines, gold is associated with quartz–carbonate ± rutile veins and occurs both as “invisible” gold finely dispersed in sulfides (primarily pyrite and arsenopyrite), and as visible electrum grains hosted in pyrite. Up to approximately 1000 ppm Au are contained in pyrite, and up to approximately 1700 ppm in arsenopyrite. Mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and Au mineralisation. At Fairview mine, three stages of pyrite formation were identified, the last of which is associated with arsenopyrite, electrum and other sulfide minerals (sphalerite, chalcopyrite, galena, gersdorffite, and Sb-sulfides). At Sheba mine, pyrite was deposited in two stages, and electrum is associated with the second stage. At both mines, the last stage of sulfide formation is the main stage of Au deposition, and is associated with mobilisation of Au, As, Sb, Cu, Zn, and Ni. The host rock composition seems to have affected the composition of pyrite, since higher Ni and Co concentrations (up to 1.4 and 1.6 wt.%, respectively) have been measured in meta-(ultra)mafic host rocks in comparison with chert and metagreywacke. Arsenopyrite is chemically zoned, and has Sb- and S-rich cores and As- and Ni-rich rims. This zoning indicates variations in fluid compositions (decreasing Sb and increasing Ni), and crystallisation conditions (increasing As content for increasing temperature). Geothermometric estimates based on the As content of arsenopyrite (As ≤ 32 at.%) indicate temperatures up to ~ 420 °C for the crystal rims. Petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation, and no plastic deformation. This supports the notion that the ore-transporting veins were emplaced late in the deformation history. Variations of cathodoluminescence of quartz are correlated with changing Al contents (Al ≤ 0.16 wt.%), and can be related to fluctuations in the pH of the mineralising fluids.     

Causes of variations in morphology and impurities of diamonds from the Udachnaya Pipe eclogite

A unique xenolith of eclogite, 23×17×11 cm in size and 8 kg in weight, was found in the Udachnaya kimberlite pipe. One hundred twenty-four diamond crystals recovered from it were analyzed by a number of methods. The diamonds differ in morphology, internal structure, color, size, and composition of defects and impurities. The xenolith contains diamonds of octahedral and cubooctahedral habits. In cathodoluminescence, the octahedral crystals have a brightly glowing core with octahedral zones of growth and a weakly glowing rim. In the cores of these crystals the N impurity is mostly present in the B1 form (30 to 60%). At the same time, N in the rim is chiefly in the A form. The cubooctahedral crystals show a weak luminescence. The content of nitrogen and degree of its aggregation are close to those in the rim of octahedral crystals. The diversity of morphology and impurity composition of diamonds from the xenolith can be explained by their formation in two stages. At the first stage, the diamonds formed which became the cores of octahedra. After a long-time interruption, at the second stage of diamond formation crystals of cubooctahedral habit appeared and the octahedral crystals were overgrown. Wide variations in nitrogen contents in the xenolith crystals allowed their use to estimate the kinetics of aggregated nitrogen. The data obtained show that the aggregation of A centers into B1 centers in the diamonds is described by a kinetic reaction of an order of 1.5.     

冀北赤城地区古生代变质作用年代学研究及地质意义

赤城地区退变榴辉岩的围岩以黑云斜长片麻岩为主,其原岩是元古代红旗营子群和古生代的表壳岩;退变榴辉岩原岩则为其中的基性侵入体。本文通过赤城地区退变质榴辉岩及围岩的锆石U-Pb年代学研究,对华北北缘古生代以来的地质演化提出了新的认识。峰期榴辉岩相变质作用发生于355Ma,代表俯冲作用发生的年龄,在320~300Ma的抬升过程中发生角闪岩相退变质,随后在280~290Ma期间存在一期大规模强烈的热事件,影响范围大,流体丰富,锆石发生了强烈的重结晶,因此使得之前记录的年代学信息难以保存。西四道沟的变质闪长岩记录了240Ma左右的一次变质事件。由此反映了赤城地区在古亚洲洋闭合的过程中陆壳发生俯冲,并遭受榴辉岩相变质作用,之后整个华北北缘处于伸展环境,并遭受大规模热事件影响,一直到晚古生代—早中生代转为挤压环境,在晚古生代—早中生代最终完成闭合。     

Recent advances in the geology of Koffiefontein Mine, Free State Province, South Africa

The complex internal geology of the Koffiefontein pipe has contributed to the marginal nature of the mine. The key to this is the presence of a large zone dominated by down-rafted country rock Karoo sediment and dolerite xenoliths. Recent work indicates that the kimberlite pipe at Koffiefontein consists of precursor dykes (the West and East Fissures), and the main pipe, in which two main eruptive phases have been recognized. Groundmass spinel compositions have been used to provide a chemical fingerprint of each lithology. There is evidence for at least three magma batches, each with its own chemical signature. Cross-cutting contact relationships were used to determine the emplacement sequence. The characterization of the different internal geological units permitted the development of a three-dimensional (3D) model of the pipe. Both main eruptive phases, viz., the Speckled west kimberlite and the Speckled east kimberlite comprise volcaniclastic kimberlite. They are separated by a large irregular mass of kimberlite that contains abundant country rock xenoliths comprising varying proportions of Karoo mudstone and dolerite, as well as probable bedded crater–facies fragments. This zone of contamination dilutes the grade of the kimberlites, affects the geotechnical stability and adversely affects the economics of the mine.     

The relationship between the distribution of nitrogen impurity centres in diamond crystals and their internal structure and mechanism of growth

Fifty diamond crystals of different morphological types (octahedra, dodecahedroids, cubes and single tetrahexahedroid) with differing internal structures were examined using methods of cathodoluminescence (CL), anomalous birefringence and local infrared (IR) analysis. The main objective of the study was to examine the regularities of nitrogen impurity distribution in diamond with differing internal structures. Almost all the analyzed octahedra, as well as dodecahedroids with zonal structures and the blocky dodecahedroids, are characterized either by nearly isothermic growth conditions or by a decrease in formation temperature during the crystallization process. In contrast to zoned octahedra and dodecahedroids, dodecahedroids with zonal–sectorial and sectorial internal structures show a notably different distribution of nitrogen defects, with N_tot generally decreasing from crystal cores to marginal areas, and degree of nitrogen aggregation increasing in the same direction. From this, it would follow that in these crystals, the temperature of diamond formation of the outer crystal zones is approximately 40–50 °C higher than that of the inner zones. The same result (15 to 80 °C) was obtained for diamond crystals with cubic habit, which generally show a fibrous internal structure, reflecting normal mechanisms of growth. The anomalous distribution of nitrogen centres in diamond crystals that grew through the normal mechanism, with a high rate of growth and in an oversaturated medium, might point to non-equilibrium relationships between the concentrations of different nitrogen centres. It is likely that in crystals of this type, the rate of growth is higher than the rate of structural nitrogen aggregation. Thus, it appears that in these peculiar crystals of diamond we deal with non-equilibrium concentrations of nitrogen B centres and, consequently, with anomalous, non-actual diamond formation temperatures.     

Fate of carbonates within oceanic plates subducted to the lower mantle, and a possible mechanism of diamond formation

We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO₃ (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO₃ (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO₂ + perovskite via reactions with SiO₂. Magnesite is not the only host phase for subducted carbon—solid CO₂ also carries carbon in the lower mantle. Furthermore, CO₂ itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.     

Ore-structure relationships at Sishen Mine,Northern Cape,Republic of South Africa,based on fully-constrained implicit 3D modelling

A fully-constrained, implicit, 3D geological model of Sishen Mine reveals the original, pre-mining geometry of ore bodies, host rocks to mineralization and major structures. There are several overlapping controls, at a variety of scales, on the position, depth and geometry of laminated and conglomeratic ore. Most of these controls are structural or may be reconciled with the kinematic history of this part of the Maremane Dome. A series of near-horizontal sections, through the entire 3D model, demonstrates the manner in which these controls overlap and interact. First-order or large-scale controls comprise broad domes, which show preservation of laminated ore around their rims, outside of which conglomeratic ore occurs. Second-order controls comprise grabens and half-grabens, which are often bounded by strike-persistent normal faults, which show fault drag on their western flanks due to inversion, along with preservation of BIF-related supergene ore and conglomeratic ore. A type example is the thick, deep, linear ore to the west of the Sloep Fault. Third-order controls on the preservation of mineralization comprise downthrown blocks to the north of reactivated E-W, SE/ESE- or NE/ENE-trending conjugate faults. Upthrow to the south could be attributed to the 1.15–1.0 Ga NNW-directed Lomanian (Namaqua-Natal) Orogeny. Palaeosinkholes comprise fourth-order controls, which are superimposed on higher-order controls. Palaeosinkholes, which form the bulk of current mining, comprise deep, conical depressions with anomalous thicknesses of chert, chert breccia and haematite. Due to their limited size, the steepness of all units and the often chaotic nature of detached and slumped blocks in their centres, these volumes reflect longstanding models on palaeosinkhole development and very local ore control.     

Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry

Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). ⁸⁷Sr/⁸⁶Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Yb_n ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.