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The large Hebaoshan gold deposit (41.5 t Au, average grade: 3.5 g/t) is located in a hitherto poorly documented gold province in the northeastern part of the South China Block. It is hosted by Precambrian metasedimentary rocks that experienced Triassic greenschist- to amphibolite-facies metamorphism. Three hydrothermal stages can be distinguished: quartz + sericite + pyrite + pyrrhotite (stage I), quartz + magnetite + sulfide (stage II), and quartz + carbonate + pyrite + hematite (stage III). Auriferous pyrite samples yielded a Rb-Sr isochron age of 234 ± 3 Ma (2σ, MSWD = 0.24), and ore-related hydrothermal sericite yielded a 40Ar-39Ar plateau age of 226.4 ± 2.0 Ma (2σ, MSWD = 1.01), which are distinctly younger than ca. 430 Ma granitic rocks in the mining district and older than 170–130 Ma granites in the region, ruling out any genetic link between granite emplacement and gold mineralization. The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (2.2–10.1 wt% NaCl equiv.) and medium temperature of 290 to 350 °C (stage I) and 230 to 280 °C (stage II). The δ18O quartz ranges from 13.3 to 15.4 ‰, and the δD values for fluid inclusions in quartz range from − 97 to − 60 ‰. The calcite has C-isotopes ranging from − 6.1 to − 3.8 ‰ and O-isotopes from 2.8 to 13.6 ‰. H-O-C isotope data are consistent with a metamorphic fluid derived from devolatilization of Neoproterozoic basement rocks during regional metamorphism in the Late Triassic Indosinian period. The δ34S values of sulfides for stages I, II, and III are 0.1 to 7.8 ‰, − 10.6 to 5.1 ‰, and − 18.9 to − 14.1 ‰, respectively, indicating an increase in oxygen fugacity during fluid ascent which resulted in more negative sulfur isotope values of sulfides and precipitation of gold. Pyrite separates have 206Pb/204Pb ratios of 17.340 to 17.687, 207Pb/204Pb ratios of 15.539 to 15.604, and 208Pb/204Pb ratios of 37.749 to 38.094. The S-Pb isotope data suggest derivation of S and Pb from the Precambrian metasedimentary country rocks. Auriferous pyrite yielded elevated 3He/4He ratios (0.78–1.46 Ra), which suggest a mantle component as can be expected from fluids derived from a subduction setting. It is concluded that Hebaoshan is an orogenic gold deposit that formed during Triassic flat-slab subduction of the paleo-Pacific plate beneath the South China Block and thus constitutes the first Triassic deposit of this type recognized in the coastal area of Southeast China.

The Shuangqishan gold deposit (>20t gold) is located in a hitherto poorly-documented gold province in the southeastern Cathaysia Block, South China. It is hosted by Neoproterozoic amphibolite-facies metamorphic rocks and consists of quartz-sulfide veins with distinct alteration haloes. Three stages of hydrothermal activity have been identified, and gold is mainly associated with veins of Stage II (quartz-pyrite±chalcopyrite). Gold occurs both as refractory gold hosted by pyrite and free gold enclosed in, or filling, microfractures of pyrite and quartz. U-Pb isotope dating of hydrothermal apatite from the Substage IIa veins yielded an age of 436.8 ± 7.6 Ma. The ore fluid was of carbonic-aqueous composition and low oxygen fugacity. Total homogenization temperatures in Substage IIa and IIb range from 270° to 310°C and 250° to 295°C, respectively. Variable CO2/H2O ratios indicate fluid unmixing under fluctuating pressures. Stage I quartz veins show evidence of ductile deformation and have δ18Ofluid and δD values of 10.3 to 11.2‰ and −67to −66‰, respectively, consistent with a metamorphic fluid. Stage II veins have slightly higher δ18O values of 10.9 to 13.7‰, with calculated δ18Ofluid values of 4.8 to 8.0‰. The 3He/4He ratios of pyrite are elevated (0.06–0.23 Ra) relative to crustal He reservoirs (

Determination of highly siderophile element (HSE; Au, Pt, Pd, Ir, Os, Rh and Ru) concentrations in relatively unweathered and unaltered marine shales from the Barberton, Witwatersrand and Transvaal supergroups in the Kaapvaal Craton revealed systematic differences, interpreted to reflect secular changes in the HSE content of Mesoarchaean to Palaeoproterozoic seawater. Most of the studied marine shales have HSE concentrations in the range given for average Archaean crustal rocks (0.5–5 ppb), with the exception of the shales in the Witwatersrand Supergroup. These shales contain up to three times more HSE, independent of source rock lithology in the granitoid-greenstone-dominated hinterland. Although sedimentary pyrite incorporated gold from synsedimentary to early diagenetic waters, its modal proportion (

The Mesoproterozoic Aggeneys-Gamsberg ore district, South Africa, is one of the world´s largest sulfidic base metal concentrations and well-known as a prime example of Broken Hill-type base metal deposits, traditionally interpreted as metamorphosed SEDEX deposits. Within this district, the Gamsberg deposit stands out for its huge size and strongly Zn-dominated ore ( >14 Mt contained Zn). New electron microprobe analyses and element abundance maps of sulfides and silicates point to fluid-driven sulfidation during retrograde metamorphism. Differences in the chemistry of sulfide inclusions within zoned garnet grains reflect different degrees of interaction of sulfides with high metal/sulfur-ratio with a sulfur-rich metamorphic fluid. Independent evidence of sulfidation during retrograde metamorphism comes from graphic-textured sulfide aggregates that previously have been interpreted as quenched sulfidic melts, replacement of pyrrhotite by pyrite along micro-fractures, and sulfides in phyllic alteration zones. Limited availability of fluid under retrograde conditions caused locally different degrees of segregation of Fe-rich sphalerite into Zn-rich sphalerite and pyrite, and thus considerable heterogeneity in sphalerite chemistry. The invoked sulfur-rich metamorphic fluids would have been able to sulfidize base metal-rich zones in the whole deposit and thus camouflage a potential pre-metamorphic oxidation. These findings support the recently established hypothesis of a pre-Klondikean weathering-induced oxidation event and challenge the traditional explanation of Broken Hill-type deposits as merely metamorphosed SEDEX deposits. Instead, we suggest that the massive sulfide deposits experienced a complex history, starting with initial SEDEX-type mineralization, followed by near-surface oxidation with spatial metal separation, and then sulfidation of this oxidized ore during medium- to high-grade metamorphism., Julius-Maximilians-Universität Würzburg (3088)

Crystallisation of basic magmas can be accompanied by syngenetic concentration of ore minerals, which can lead to the formation of economic ore deposits of chromite, titanian magnetite, ilmenite, pentlandite, chalcopyrite and platinum group minerals (PGM).

The fundamental structural unit of the sulfate minerals is the anionic complex [SO4]2−. It forms a slightly distorted tetrahedron in which a central sulfur atom is connected with four oxygens by very strong covalent bonds. In contrast, the bonds between [SO4] and the cations are predominantly heteropolar. In the crystal structures of the H2O-free sulfate minerals baryte, celestine and anglesite, each of the relatively large cations Ba2+, Sr2+ and Pb2+ is coordinated with 12 oxygens at slightly different distances, forming [BaO12] polyhedra (Fig. 9.3). In the less distorted anhydrite structure, each of the smaller Ca2+ cations is surrounded by 8 oxygens only, at nearly the same distance.

The Wulong vein-type gold deposit, located along the northeastern margin of the North China Craton (NCC), contains > 80 tonnes of gold at an average grade of 5.35 g/t. A total of 380 auriferous quartz veins are hosted in Late Jurassic gneissic two-mica granite and Early Cretaceous granodiorite. A paragenetic sequence, representing three hydrothermal stages, has been identified: (1) quartz ± pyrite; (2) quartz-polymetallic sulfide; and (3) quartz-calcite ± pyrite. Petrography, fluid inclusion microthermometry, and laser Raman spectroscopy revealed three types of fluid inclusions: CO2 ± CH4 (type I), H2O-CO2 ± CH4 (type II), and H2O-NaCl (type III). Fluid inclusion data indicate that during mineralization, fluid temperatures evolved from 283 to 395 °C to 219–328 °C to 144–255 °C, and salinities varied from 0.70 to 8.95 wt% to 0.18–7.86 wt% to 0.18–4.96 wt% NaCl equivalent. Quartz yielded δ18OH2O-SMOW and δDSMOW values of 0.9–7.3‰ and −65‰ to −48‰, respectively, indicating that the ore-forming fluids were derived from magmatic water and mixed with meteoric water during the later stages of mineralization. Sulfides have δ34SCDT sulfur isotopic compositions ranging from 0.9‰ to 3.9‰, suggesting that the S was derived from a magmatic source. Pb isotopic compositions of sulfides (206Pb/204Pb = 17.488–17.718, 207Pb/204Pb = 15.565–15.631, and 208Pb/204Pb = 38.190–38.766) indicate that the Pb was derived primarily from the upper crust, with minor input from the mantle. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses of three generations of pyrite (Py1, Py2, and Py3) reveal that gold concentrations in Py2 are higher than those in Py1 and Py3, which indicates that gold mineralization occurred primarily during the middle stage. High Co/Ni ratios of pyrite are consistent with a hydrothermal origin. We suggest that the Wulong gold deposit formed during lithospheric thinning associated with destruction of the NCC, this process was triggered by subduction of the Paleo-Pacific plate during the Early Cretaceous.

We describe the presence of Witwatersrand-type gold mineralization in Paleoproterozoic strata indicating this style of deposit is not confined to the Archean. The discovery of basal and intraformational conglomerate-hosted gold mineralization in the Huronian Supergroup’s Mississagi Formation near Sudbury, Ontario, has indicated potential for Witwatersrand-type gold mineralization to occur also in early Paleoproterozoic basin fills on the Hearne, Wyoming and Karelian cratons. The Paleoproterozoic Hurwitz, Snowy Pass and Karelian supergroups, located on these cratons, are similar to the Huronian Supergroup on the Superior Craton, in terms of sequence and litho-stratigraphy, and maximum age of deposition. A review of the stratigraphy and past gold exploration from each supergroup has revealed each basin to contain similar fluvial poly- to oligomictic, pyritic conglomeratic strata with locally anomalous gold and/or uranium concentrations at stratigraphic levels that correlate to those of the Mississagi and Matinenda formations of the Huronian Supergroup. Examination of gold and gold-enriched pyrite from the Mississagi Formation conglomerates suggests that specific point sources existed for most of the gold therein. Each of the above basins forms part of a larger contiguous continental rift system prior to the breakup of the supercontinent Kenorland at around 2480 Ma. As each basin fill was displaced to different geographic positions following this continental breakup and then subjected to separate orogenic and metamorphic overprints, the gold and uranium mineralization in each of the basins is unlikely to be of a common, post-depositional hydrothermal origin, but in all likelihood of a paleoplacer origin. It is suggested that this period of gold-enrichment is the result of truncation and reworking of underlying stratigraphically older units and/or from former gold deposits/occurrences and gold-enriched greenstone-dominated hinterlands. The omnipresence of detrital and/or syn-depositional pyrite in conglomerates of each basin reinforces the idea that a lack of atmospheric oxygen at the time of Kenorland rifting existed during their deposition and prior to the “great oxidation event”. Our comparative analysis of Kenorland rift graben fills reveals that gold potential, which can be determined by the presence of detrital pyrite as observed at the Pardo project, should exist in all stratigraphic equivalents of the Mississagi and Matinenda formations where preferentially reworked fluvial conglomerates rest on an erosional unconformity and where a corresponding Au-enriched Archean hinterland existed.

The Jagtiyal section exposes a part of the Precambrian granulite – facies terrane in the northeastern Dharwar Craton, specifically charnockite, orthogneiss, mafic granulite, garnet-cordierite gneiss and silica-deficient Mg-Al granulite. The silica-deficient Mg-Al granulites exhibit well-preserved metamorphic reaction microstructures and were used to elucidate the metamorphic history of the study area. Textural evidence exists for the prograde break-down of biotite and corundum and formation of a peak metamorphic assemblage of spinel, sapphirine, orthopyroxene, cordierite and K-feldspar. KFMASHTO system based P-T pseudosections as well as the application of multi-equilibrium calculation for mineral core compositions, gave P-T conditions of 7.5–8 kbar and 800–840 °C. The thermal peak was followed by near isothermal decompression that led to a variety of coronal textures including, successive rims on corundum, consisting of spinel + sapphirine + cordierite ± orthopyroxene, and symplectitic intergrowth of sapphirine-cordierite, orthopyroxene-cordierite, spinel-cordierite, and biotite-cordierite. Modelled P-T conditions for this stage are 4.3–4.5 kbar and 750–800 °C. Few samples show textural evidence for further retrogression leading to the break-down of sapphirine and cordierite to form chlorite, corundum, spinel and andalusite. The proposed clockwise P-T path implies that granulites from the Jagtiyal section likely resulted from thickened continental crust undergoing decompression. New U-Pb zircon geochronologic data provide evidence for a Neoarchaean (2604 ± 25 Ma) age that likely corresponds to the timing of accretion of the terrane that was subjected to high temperature metamorphism at that time. Moreover, there is evidence of a Neoproterozoic thermal overprint in the area around 638 Ma. .

Sapphirine-bearing granulite from Usilampatti in the Madurai block of southern India preserves a variety of mineral textures and reactions that help in reconstructing a three-stage metamorphic evolution. Corroded biotite, sillimanite and quartz inclusions within garnet represent relics from the prograde history. Peak metamorphic conditions were attained with the development of sapphirine + quartz in textural equilibrium (Stage 1). This was followed by nearly isothermal decompression, leading to the formation of sapphirine + cordierite at Stage 2. Subsequent retrograde hydration (Stage 3) is only locally evident. Using the Perple_X software and the model system NCKFMASH, the peak P-T conditions were estimated from core compositions, and the retrograde evolution was deduced from rim or symplectite compositions of different minerals as computed by isopleths of XMg garnet, XCa garnet, XMg orthopyroxene, XMg sapphirine and XMg biotite. The P-T conditions for Stage 1 thus obtained, and supported by thermodynamic modelling using the winTWQ programme, is approximately 9 kbar and 940°C. Stage 2 conditions were constrained as 6.7 kbar and 900°C. Dating of zircon and monazite in the sapphirine-bearing granulite and associated gneisses by the U-Pb method using LA-ICP-MS indicates metamorphic overprint of zircon (lower intercept ages of discordant data arrays) at 546 ± 8 and 547 ± 11 Ma and metamorphic growth of monazite between 542 ± 3 and 551 ± 2 Ma. Upper intercept ages for zircon point to zircon growth at approximately 2514 ± 66 Ma. Although it remains unclear whether the metamorphic age data refer to Stage 1 or Stage 2 or, most likely, a continuum between both, they clearly document a late Ediacaran age for ultra-high temperature (UHT) metamorphism in the area, which, based on the obtained P-T path, was most likely the result of crustal thickening followed by uplift and erosion. Thus, it is concluded that the sapphirine-bearing granulites formed in response to Pan-African orogeny that led to the collision of the western and eastern Madurai domains, whereas initial zircon growth probably took place during late Neoarchaean arc magmatism that formed much of the western domain. Copyright © 2017 John Wiley & Sons, Ltd.

The stratiform Cu-Zn sulfide deposit at Kupferberg in Germany represents Bavaria’s largest historic base metal producer. The deposit is hosted by Early Paleozoic volcano-sedimentary strata at the margin of a high-grade allochthonous metamorphic complex. The present paper reports on the first Cu and S isotope data as well as trace element analyses of pyrite from this unusual deposit. The new data point to syn-orogenic mineralization that was driven by metamorphic fluids during nappe emplacement. Primary Cu ore occurs as texturally late chalcopyrite within stratiform laminated pyrite in black shale in two different tectonostratigraphic units of very low and low metamorphic grade, respectively, that were juxtaposed during the Variscan orogeny. Trace element contents of different pyrite types suggest the presence of at least one hydrothermal pyrite generation (mean Co/Ni = 35), with the other pyrite types being syn-sedimentary/early diagenetic (mean Co/Ni = 3.7). Copper isotope analyses yielded a narrow δ65Cu range of −0.26 to 0.36‰ for all ore types suggesting a hypogene origin for the principal chalcopyrite mineralization. The ore lenses in the two different tectonostratigraphic units differ with regard to their δ34S values, but little difference exists between poorly and strongly mineralized domains within a given locality. A genetic model is proposed in which syn-sedimentary/early diagenetic pyrite with subordinate chalcopyrite and sphalerite formed in black shale beds in the two different stratigraphic units, followed by late-tectonic strata-internal, hydrothermal mobilization of Fe, Cu, and Zn during syn-orogenic thrusting, which concentrated especially Cu to ore grade. In agreement with this model, Cu distribution in stream sediments in this region shows distinct enrichments bound to the margin of the allochthonous complex. Thus, Kupferberg can be considered a rare example of a syn-orogenic Cu deposit with the Cu probably being derived from syn-sedimentary/early diagenetic pyrite contained in Early Paleozoic shale units.

Petrographic and mineralogical studies of samples of the Normal (or undisturbed) Merensky Reef from Frank Shaft No.1 at Rustenburg Platinum Mine revealed the presence of a Pt-Fe-alloy, probably isoferroplatinum (58 vol.% of total precious metal minerals), arsenides (21 vol.%), bismuthotellurides (10 vol.%), electrum (9 vol.%) and platinum group element- (PGE-) sulfides and stannides (2 vol.%), associated predominantly with base-metal- and iron-sulfides. A Pt-Fe-alloy-dominated facies has been known for considerable time from potholes and discordant bodies and has been attributed to fluid activity with high fO2 and low fS2. Our petrographic results indicate that the normal thin reef has also undergone hydrothermal alteration. For the first time, the rare mineral troilite (stoichiometric FeS) was found as intergrowths with masses of Pt-Fe-alloy, together with Fe-rich pyrrhotite, secondary hydrous silicates, magnetite and calcite. The observed mineral assemblage and texture is interpreted as the product of partial desulfurization, caused by migrating S-undersaturated fluids, which led to the exsolution of Pt-Fe-alloy from pyrrhotite (Fex–1S) with the latter approaching a stoichiometric composition. Overall our new observations provide convincing support for the importance of metasomatism in the secondary modification of ore mineralogy and textures even in the undisturbed Merensky Reef.

The Dungash historic gold mine is located in the South Eastern Desert of Egypt. The gold-bearing quartz veins are hosted by the metavolcanic and metavolcaniclastic rocks along an ENE–WSW trending shear zone. Alteration types recorded in the wall rocks are sericitization, silicification, carbonatization, chloritization, sulfidization, ferruginization, and listwanitization. The ore mineral assemblage comprises arsenopyrite, pyrite, native gold, pyrrhotite, sphalerite, chalcopyrite, and galena. The primary sulfide mineral assemblage formed during a hypogene hydrothermal stage whereas anglesite and goethite occur as secondary supergene phases. Microthermometric fluid inclusion analysis revealed that the auriferous quartz precipitated from a moderately saline (5 to 11.22 wt% NaClequiv) solution at temperatures above the recorded homogenization temperatures (T h), which range from 380 to 177 °C. The minimum pressures of trapping are between 350 and 400 bars. The fluid evolution during mineralization is explained by mixing of a magmatic fluid with meteoric waters. Initially, the high temperature and moderately saline magmatic fluid dominated and progressively became diluted with meteoric waters. Highest gold content is recorded in the carbonatized zone and the quartz veins. However, gold content in the carbonatized zone of the footwall exceeds several times its content in the quartz veins and the carbonatized zone of the hanging wall.

The Diguva Sonaba area (Vishakhapatnam district, Andhra Pradesh, South India) represents part of the granulite-facies terrain of the Eastern Ghats Mobile Belt. The Precambrian metamorphic rocks of the area predominantly consist of mafic granulite (±garnet), khondalite, leptynite (±garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneisses and sapphirine–spinel-bearing granulite. The latter rock type occurs as lenticular bodies in khondalite, leptynite and calc-granulite. Textural relations, such as corroded inclusions of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse-grained laths of sillimanite, presumably pseudomorphs after kyanite, provide evidence of either an earlier episode of upper-amphibolite-facies metamorphism or they represent relics of the prograde path that led to granulite-facies metamorphism. In the sapphirine–spinel-bearing granulite, osumilite was stable in addition to sapphirine, spinel and quartz during the thermal peak of granulite-facies metamorphism but the assemblage was later replaced by Crd–Opx–Qtz–Kfs-symplectite and a variety of reaction coronas during retrograde overprint. Variable amounts of biotite or biotite+quartz symplectite replaced orthopyroxene, cordierite and Opx–Crd–Kfs–Qtz-symplectite at an even later retrograde stage. Peak metamorphic conditions of c. 1000°C and c. 12 kbar were computed by isopleths of XMg in garnet and XAl in orthopyroxene. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. The P–T path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust.

A lithogeochemical, mineral chemical, isotopic, and fluid inclusion study of barren, low-, and high-grade Au-mineralized samples from the shear zone-hosted Amantaytau gold deposit, Uzbekistan, shows that the local host rocks, Late Ordovician–Earlz Silurian carbonacous shales, are likely to have been an important source of Au, As, Ni, and S in the formation of the deposit. Syn-depositional pyrite in these shales contains on average 0.23 ppm Au, 1,083 ppm As, and 861 ppm Ni. The distribution of rare earth elements (REE) indicates a homogeneous source of light REE, whereas the heavy REE distribution reflects most likely primary variations in the sediments. The mineralized zone is marked by a positive Eu anomaly, which supports reducing conditions during the mineralization. A hydrothermal overprint by an aqueous–carbonic fluid is reflected in a high-grade Au-mineralized sample by δ13C values of −13.0 ‰ (V-PDB). The δ34S values in pyrite (−0.13 to +7.30 ‰ CDT) from barren and mineralized samples are consistent with marine sulfate being the principal source of the ore sulfur. Assuming a formation temperature of between 300 and 400 °C for the main stage of mineralization, as indicated by the alteration mineral assemblage, the calculated δ18Ofluid is between 9.5 and 13.4 ‰ V-SMOW, which points at a metamorphic origin of the ore fluid.

The first systematic analyses of the trace and rare earth element (REE) distribution in uraninite from various gold-bearing conglomerates of the Mesoarchaean Central Rand Group in South Africa’s Witwatersrand Basin by in situ laser ablation-inductively coupled plasma-mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof. The chemistry of commonly rounded to sub-rounded uraninite is highly variable from grain to grain but generally marked by elevated Th, W, Bi, Mo, Ta, Y, REE contents and unusually high Au concentrations. Especially, the high Th contents and the chondrite-normalised REE patterns are incompatible with post-sedimentary hydrothermal genetic models for the U mineralisation and point to derivation of the detrital uraninite from a high-temperature, magmatogenic, presumably granitic to pegmatitic source. The elevated Au concentrations (of as much as 67 ppm) in this uraninite are unique to the Witwatersrand and hint at a granitic hinterland that was enriched in both U and Au, thus presenting a potential source domain for some of the detrital gold in the Witwatersrand conglomerates. Minute fracture fills of brannerite in close proximity to the larger, rounded uraninite grains are devoid of detectable Bi, Mo, REE and Au and have only very low concentrations of Th, W, Ta and Y. This is explicable by crystallisation from a low-temperature hydrothermal fluid. Thus, Witwatersrand U phases show, analogous to many other ore constituents, such as pyrite and gold, clear evidence of partial, short-range mobilisation of originally detrital particles by post-sedimentary fluids.

Uranium and molybdenum mineralization at the Ekomedion prospect of southwestern Cameroon occurs within pegmatite pods in Pan-African two-mica granite. The two-mica granite and the associated pegmatite experienced brittle deformation. Mineralization occurs as veinlets following early fractures and as irregular bodies intimately associated with pegmatite pods. Syn-tectonic alteration comprises albitization, silicification, ferruginization, seriticitization and chloritization. Mica in the ore zone is less magnesian and F-rich relative to mica in the host two-mica granite. Uraninite crystals are anhedral to subhedral and molybdenite is commonly encapsulated by phengitic muscovite. The two-mica granite is a strongly fractionated high-K, low P ferroan granite. The pegmatite pods hosting U Mo mineralization are suggested to have been derived from the fractionation of magma parental to the two-mica granite as observed from Rare Earth Element (REE) patterns and the trend shown by Th/U ratios of the host granite and the pegmatite. This implies that uranium and molybdenum fractionated and concentrated preferentially into the late stage pegmatitic fluid that formed the pods and subsequently into the hydrothermal fluid phase. This therefore points to the two-mica granite as well as the metamorphic wall rocks as potential sources for U Mo mineralization in southwestern Cameroon.

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from 2 -rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ 18 O and δD data suggestive of mixing between metamorphic and meteoric waters; δ 34 S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.

A hitherto unnamed Bi–Pb tellurosulfide was discovered in the Wenyu quartz vein-type gold deposit within the Xiaoqinling gold province, in China. This is the first report of that rare mineral in China. This mineral is optically anisotropic and light gray, and it has a slightly higher reflectivity than galena. It occurs in close association with galena but not with any other tellurosulfide, Bi-bearing sulfide or telluride. Electron-microprobe analyses reveal an average composition of Pb 0.85 Bi 4.09 Te 4.06 S 3.01 (stoichiometric: PbBi 4 Te 4 S 3 ). The very small size of the mineral grains (usually 10–20 μm across) prevented structural analyses by X-ray diffraction. Tests by SEM revealed no heterogeneity on a micrometric scale, thus supporting the presence of a discrete phase rather than a mixture of submicrometric inclusions of PbS in tetradymite. By analogy with PbBi 4 Te 7 and evaluating the compositional data available, we speculate that PbBi 4 Te 4 S 3 has a layered structure, TeBiSPbSBiTe TeBiSBiTe, a seven-layer and a five-layer stacked together.

A petrological investigation of abyssal, plagioclase-free spinel peridotite drilled during ODP cruise 153 in the North Atlantic revealed that the peridotite represent refractory, partial residual mantle material that experienced depletion of incompatible trace elements during upper mantle melting. The degree of partial melting as estimated from spinel compositions was c. 12%. Fractionated middle and heavy rare earth elements imply polybaric melting, with c. 1–4% initial melting in the garnet peridotite stability field and subsequent partial melting of ~7–10% in the spinel peridotite stability field. Geothermobarometric investigations revealed that the solid-state equilibration of the spinel peridotite occurred at some 1,100–1,150°C and c. 20–23 kbar, corresponding to an equilibration depth of c. 70 ± 5 km and an unusually low thermal gradient of some 11–17°C/km. A thermal re-equilibration of the peridotite occurred at ~850–1,000°C at similar depths. Naturally, the initial mantle melting in the garnet-peridotite stability field must have commenced at depths greater than 70 ± 5 km. It is likely that the residual peridotite rose rapidly through the lithospheric cap towards the ridge axis. The exhumation of the abyssal peridotite occurred, at least in parts, via extensional detachment faulting. Given the shallow to moderate dip angles of the fault surfaces, the exhumation of the peridotite from its equilibration depth would imply an overall ridge-normal horizontal displacement of c. 50–160 km if tectonic stretching and detachment faulting were the sole exhumation mechanism.

A study of the platinum group element (PGE) distribution in various lowermost Cambrian rock types of the Kunyang phosphate deposit in south China revealed unusually high PGE concentrations (434 ppb Pt, 142 ppb Pd, 57.97 ppb Rh

A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic palaeoplacer deposits in the Ouro Fino Syncline (Quadrilatero Ferrifero; Brazil), the Elliot Lake area north of Lake Huron (SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite, which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice.

A geological study of the hitherto poorly described Neoproterozoic Gifberg Group, with emphasis on lithogeochemistry and O, C and Sr isotopic composition of the carbonate-dominated Widouw Formation (Vredendal Outlier, westernmost South Africa) revealed that the entire group is an equivalent of the relatively well constrained Port Nolloth Group in the external, paraautochthonous part of the Pan-African Gariep Belt further north. Thus, the Vredendal Outlier can be regarded as the southern extension of the Port Nolloth Zone. Two diamictite units are recognised in the Vredendal Outlier, which can be correlated respectively with the c. 750 Ma Kaigas Formation diamictite and the 583 Ma, syn-Gaskiers Numees Formation diamictite in the Gariep Belt proper. The dominating carbonate unit in the studied area is post-glacial with respect to the older of the two diamictite units. The combined textural, structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds. Sedimentation in a restricted, very shallow and proximal basin led to a wide range in C isotope ratios (δ13CPDB from − 4.2 to + 4.8‰), very high Sr concentrations (derived from original anhydrite) and initial 87Sr/86Sr ratios that are significantly higher (0.70785) than those of coeval seawater. As C and Sr isotopes are commonly used for chemostratigraphic correlation, and high Sr concentrations in Neoproterozic carbonates are often interpreted as evidence of former aragonite, the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition. Thus C and Sr isotope ratios alone in Neoproterozoic carbonates may be less powerful proxies of ancient seawater composition, and high Sr contents are not necessarily indicative of an “aragonite sea”, as previously inferred.

The Witwatersrand gold fields in South Africa, the world's largest gold-producing province, play a pivotal role in the reconstruction of the Archaean atmosphere and hydrosphere. Past uncertainties on the genetic model for the gold caused confusion in the debate on Archaean palaeoenvironmental conditions. The majority of Witwatersrand gold occurs together with pyrite, uraninite and locally bitumen, on degradational surfaces of fluvial conglomerates that were laid down between 2.90 and 2.84 Ga in the Central Rand Basin. Although most of the gold appears as a precipitate within, or associated with, post-depositional hydrothermal phases and along microfractures, available microtextural, mineralogical, geochemical and isotopic data all indicate that this hydrothermal gold, analogous to some pyrite and uraninite, was derived from the local mobilisation of detrital particles. Some of the key pieces of evidence are a significant correlation of the gold, pyrite and uraninite with other heavy minerals as well as sedimentary lithofacies, local preservation of in-situ gold micronuggets and abundant rounded forms of pyrite and uraninite, compositional heterogeneity on a microscale of the gold as well as the rounded pyrite and uraninte, and radiometric age data that indicate an age of the gold, pyrite and uraninite that is older than the maximum age of deposition for the host sediment. None of these observations/data is compatible with any of the suggested hydrothermal models, in which auriferous fluids were introduced from an external source into the host rock succession after sediment deposition. In contrast, those arguments, used in favour of hydrothermal models, emphasise the microtextural position of most of the gold, which highlights the undisputed hydrothermal nature of that gold in its present position, but does not explain its ultimate source. Furthermore, the macro-scale setting of the stratiform ore deposits is in stark contrast to any known type of epigenetic, hydrothermal gold deposit. Consequently, the best-fit genetic model involves post-depositional textural and mineralogical modification of original fluvial placer deposits. On the Kaapvaal Craton, Witwatersrand-type mineralisation is recorded over an extended period of time from 3074 to 2642 Ma. Rounded pyrite is common in the coarser grained fractions of the siliciclastic basin fill. A lack of sulphur isotope fractionation and typical magmatic δ34S values support its detrital origin. Together with rounded uraninite, which is particularly abundant in the older beds, it provides important constraints on the redox potential of the Meso- to Neoarchaean (3.1–2.6 Ga) atmosphere and hydrosphere. In combination with eukaryotic steroids documented from the Pilbara Craton, Australia, the ambient Neoarchaean oxygen fugacity is calculated as having been approximately 10−3, in equilibrium with a relatively acidic hydrosphere (pH=6). This is in agreement with the preservation of mass-independent S isotope fractionation, which provides independent support for an anoxic atmosphere and which has so far been recorded predominantly from sediments older than 2.3 Ga. An acidic meteoric palaeoenvironment is supported by intense chemical weathering below erosional unconformity surfaces in the Witwatersrand Basin. In contrast to the pyrite-bearing fluvial and near-shore shallow marine deposits, marine shale deposits contain magnetite. This supports the postulated reducing environment but also highlights total sulphur concentrations in the ancient ocean that were orders of magnitude lower than in modern ocean water.

In his comments on our paper (Depine et al. 2013) on the chemistry of uraninite grains from the auriferous conglomerates of the Mesoarchaean Witwatersrand Supergroup, Oberthur (2013) highlights a number of points, some of which had not been fully discussed by us due to the limitation on space in a short “letter”. We are therefore grateful for the opportunity to expand on some of the implications of our new data in this discussion here. The abundance of major and minor elements in uraninite from the Witwatersrand has been the subject of several previous studies. We could add to this knowledge, as acknowledged by Oberthur (2013), by presenting the first data on trace element concentrations in Witwatersrand uraninite as obtained by LA-ICPMS. The main message we tried to convey in our letter has been that the mineral chemistry of the uraninite grains, now also including a range of elements hitherto not or only poorly analysed for, provides unequivocal evidence of the detrital nature of these grains. We are satisfied, though not surprised, that Oberthur, an expert with livelong experience with the studied rocks, fully concurs with this principal conclusion. For many readers, this conclusion may not be worth receiving further attention after dozens of papers have been published over the past decades on the topic of the genesis of the world’s largest gold province and the U-ores therein, in which a strong case for a modified palaeoplacer model has been made. Since the last review (Frimmel et al. 2005), the discussion on the genesis of the Witwatersrand ores has been, however, by no means closed, and views opposing to those of Oberthur and many other workers have been presented more recently. The papers by Phillips and Powell (2011), who advocate a postdepositional syn-metamorphic introduction of the gold into the host conglomerates, or by Horscroft et al. (2011), who argue for a syngenetic, microbially mediated chemical precipitation of gold, uraninite and pyrite, may serve as examples. We therefore believe that it remains timely and opportune to continue this discussion on the Witwatersrand metallogeny with new data. Oberthur (2013) dismisses any model involving syngenetic or epigenetic-hydrothermal introduction of U into the host conglomerates because of the lack of oxidising conditions under an Archaean atmosphere. We could not agree more but want to point out that this has been a circular argument for a long time. After all, the evidence for a postulated reducing atmosphere in the Archaean has come primarily from the interpretation of the ore components, specifically rounded massive pyrite and uraninite, in the Witwatersrand goldfields (Krupp et al. 1994; Frimmel 2005). Only in the last decade, largely based on the discovery of mass-independent S isotope fractionation in Archaean rocks, independent evidence in favour of an O2-deficient Archaean atmosphere could be brought forward (Farquhar et al. 2007), thus supporting the original contention of the detrital Witwatersrand mineralogy. As effectively all points raised by Oberthur (2013) are discussed in greater detail in a full paper currently under review elsewhere (Frimmel et al. 2013), in which we present Editorial handling: B. Lehmann

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic–Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U–Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar–Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P–T–t evolution than previously assumed. A c. 540Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of PanAfrican age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet–omphacite assemblage within strainprotected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T ¼ 687–758 � C, P ¼ 9� 4–11� 3kbar) were attained subsequent to decompression from P >12� 9kbar. By analogy with limited U–Pb single zircon age data and on circumstantial textural grounds, this earlier eclogitefacies metamorphism is ascribed to subduction and accretion around 565Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of postorogenic granite at c. 480Ma. The recognition of extensive PanAfrican tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140Ma volcanic arc and 1072 � 10Ma granite.

Comparison of conglomerate-hosted, Witwatersrand-type gold deposits and /or occurrences worldwide reveals that this deposit type is by no means unique to the Kaapvaal craton but common to most Archean and/ or Paleoproterozoic cratons. The age of the variably mineralized fluvial to fluvio-deltaic conglomerates ranges from 3.1 to 1.9 Ga. They were deposited in tectonic settings ranging from continental rifts to passive margins and synorogenic foreland basins, and all of them are paleoplacers. Although several of them show evidence of local mobilization of ore components by postdepositional hydrothermal fluids, purely epigenetic hydrothermal models fail to explain the geometry of the orebodies as well as available lithogeochemical, mineral chemical, and isotope data. Conglomerates older than 2.4 Ga are characterized by an abundance of detrital (and secondary) pyrite, and in most cases also detrital uraninite, whereas most of the younger examples (

The auriferous conglomerate horizons (reefs) in the Witwatersrand Basin of South Africa are in many places cut by hydrothermal quartz veins that frequently contain sulphide, bitumen, and, less commonly, free gold. New Pb isotopic results for the Ventersdorp Contact Reef which has experienced particularly intense hydrothermal alteration, reaffirm the radiogenically enriched nature of the Pb in this reef and provide additional insight into its origin. This study focuses on analyses of galena, chalcopyrite, pyrrhotite, and bitumen from quartz veins, which presumably formed during the 2.020 Ga Vredefort meteorite impact event. The radiogenic, mainly uranogenic, component of the Pb appears to have been derived almost entirely from uraninite in the surrounding reef rock. Assigning a 2.02 Ga age of mineralization and constructing secondary isochrons for paragenetically early galena and chalcopyrite, ages of the source uraninite are calculated as 2.6–2.4 Ga. No special significance is ascribed to these source ages, which likely reflect extensive radiogenic Pb loss from originally somewhat older detrital uraninite during transport, sedimentation, and post-burial alteration. Analyses of detrital(?), syngenetic, and epigenetic pyrite from a reef conglomerate define a subsidiary linear array with a considerably shallower slope. Interpreted as a secondary isochron, the array gives an implausibly young mineralization and/or source age indicative of a superimposed isotopic disturbance.

Within the Pan-African Gariep Belt in southwestern Africa a para-autochthonous, predominantly sedimentary sequence, deposited in a rift graben and subsequently on a passive continental margin with a Kibaran basement, is distinguished from an allochthonous, predominantly oceanic terrane that was thrust on top of the former during collision of the Rio de la Plata and the Kalahari plates. 40Ar/39Ar mineral cooling ages presented here set the following constraints: after Kibaran high-grade metamorphism, the consolidated basement cooled through ∼300°C at 1006 Ma. Rifting between 781 and 741 Ma was followed by the formation of oceanic crust, the lower parts of which cooled through 500°C sometime between 630 and 600 Ma. After inversion from extension to compression, some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge—a process that is indirectly dated between 573–576 Ma by hornblende plateau ages. Continent–continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547–543 Ma as indicated by further hornblende plateau ages. Muscovite cooling ages for the internal zone are between 526 and 529 Ma, those for the tectonically deeper, external zone are 483 Ma in the north and between 495–506 Ma in the south of the belt. In conjunction with published data, these results suggest successive closure of first the northern Adamastor ocean (Kaoko Belt), followed by the Khomas sea (intracontinental Damara Belt), and finally the southern Adamastor ocean (Gariep Belt).

In the West Rand Group of the 3.07–2.71 Ga old Witwatersrand Supergroup, South Africa, a series of banded iron-formations occur. They are of chemical origin and were deposited in an offshore shelf environment. The coarser-grained, in places pyrite-bearing, and partly auriferous metasedimentary rocks forming the bulk of the Witwatersrand Supergroup are regressive. The iron-formations, however, were deposited during transgression. The presence of allogenic pyrite in the fluviatile metaconglomerates and that of magnetite and, in places, haematite in the marine iron-formations suggests a lower pH and higher sulfur activity for the Archaean meteoric environment than for recent hydrothermal fluids on the ocean floor. Post-depositional alteration of the Witwatersrand rocks includes burial metamorphism at temperatures between 300 and 350 °C and pressures around 2.5 kbar, and multiple hydrothermal inflitration events at slightly lower temperatures, coeval with the brittle deformation of the basin fill during the deposition of the Transvaal Supergroup and the Bushveld Vredefort events.

Micas with variable Ba content in the interlayer sites occur in silicate-rich bands within the high-grade, metamorphic banded iron formation enclosing massive sulfide bodies in the Broken Hill deposit, Namaqualand Metamorphic Complex, South Africa. In those layers that contain neither potassium feldspar nor muscovite, a green, barian (up to 17 wt°/o BaO), trioctahedral mica occurs that is similar to kinoshitalite. In contrast to the originally described kinoshitalite, however, this mica is Fe-rich (average XFe= 0.65), suggesting the existence of an Fe analogue of kinoshitalite not described to date. Ion chromatographic analysis indicates that about 8°1oof the total Fe in this mica is present as Fe3+, and a typical formula is (Ba1.2K0.4)(Fel1 Mno.2Mg2.oAlo.2Fe6.t Tio.2)(AI3.oSi5.o)02o(OH l.SF2.2). The unit-cell parameters were determined as follows: a = 5.383(2), b = 9.328(8), c = 10.055(8) A, {j = 100.44(5)0,with V = 496.5 A3, and Z = 2. The space group is C21m. The Ba-rich mica formed at the peak of metamorphism (T = 670 :t 20°C, P = 4.5 :t 1.0 kbar) at a pH below the muscovite + potassium feldspar buffer, 102buffered by quartz + fayalite + magnetite and fs2between 10-5 and 10-7.

Phase relations and mineral compositions indicate a polymetamorphic history of carbonate and siliciclastic sediments constituting the Kaaimans Group of the Pan-African Saldania Belt along the south coast of South Africa. Regional metamorphism during Pan-African orogeny did not exceed greenschist facies conditions but was superimposed locally onto low- to medium-grade contact metamorphism related to the intrusion of syn-orogenic granitoids. Petrogenetic grids, constructed for the System CaO-MgO Al2O3-SiO2-H2O-CO2 and based on available thermodynamic data, reveal that vesuvianite, grossular-rich garnet and wollastonite bearing minerai assemblages, found in cale-silicate rocks adjacent to the syn-orogenic Rooiklip granite gneiss sheet, were formed at temperatures possibly as low as 450°C and pressures between 2 and 3 kbar. These mineral assemblages were in equilibrium with an H2O-CO2 fluid that was characterized by a CO2/(CO2 + H2O) mole fraction of less than 0.02. The fluid composition in the cale-silicate rocks was buffered entirely externally through magmatic/hydrothermal fluids related to the granite intrusion. The fluid flux was enhanced by syn-intrusive bedding-parallel shearing in the sedimentary rocks. Vesuvianite, which is of highly variable but generally Fe-rich composition, contains up to 2.1 wt.% F and hence acted as sink for fluorine. A regional increase in Pan-African metamorphic grade from the tectonically lowest to highest formation suggests an overturned position of the whole Kaaimans Group. Later metamorphism during the Cape Orogeny caused only very low- to low-grade retrogression.

The metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan-African Gariep orogenic belt in south-western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro-) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio-riebeckite with little ferro-glaucophane component in a highly metasomatized albite-rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low-P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio-riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.

The different stages of the geodynamic evolution of Neoproterozoic basins in southwestern Africa were accompanied by a variety of mineralisation processes. These range from synsedimentary-exhalative base metal deposits in continental rift basins, Besshi-type cupriferous pyrite deposits in oceanic crustal rocks, syn-orogenic polymetallic Tsumeb-type mineralisation in foreland carbonate platforms, orogenic gold and magmatic U and Sn deposits, to REE-rich pegmatites and post-orogenic enrichment in the supergene zone. Of currently main economic significance amongst these are the Pb and Zn deposits around Rosh Pinah in the external Gariep Belt, the Tsumeb-type deposits at Kombat and the alaskite-hosted U deposits in the western part of the Central Zone in the Damara Belt.

The majority of gold within the Witwatersrand Basin, the world’s largest gold-producing province, in the Archaean Kaapvaal Craton, South Africa, occurs together with rounded pyrite and uraninite, and locally bitumen, on degradational surfaces of fluvial conglomerates that were laid down between 2.90 and 2.84 Ga. Although most of the gold takes a textural position late in a complex paragenetic sequence, available morphological, mineralogical, geochemical and isotopic data all indicate that this hydrothermal gold, analogous to some pyrite and uraninite, was derived from the local mobilisation of detrital particles. Rounded, detrital pyrite is common in the coarser grained fractions of the siliciclastic basin fill. Together with rounded uraninite, which is particularly abundant in the older beds, it provides important constraints on the redox potential of the Meso- to Neoarchaean atmosphere and hydrosphere. In combination with eukaryotic steroids previously documented from the Pilbara Craton, Australia, the ambient Neoarchaean oxygen fugacity is calculated as having been approximately 10-3, in equilibrium with a relatively acidic hydrosphere (pH = 6). An acidic meteoric palaeoenvironment is supported by intense chemical weathering below erosional unconformity surfaces in the Witwatersrand basin fill. In contrast, nearshore shallow marine deposits contain magnetite. This supports a postulated reducing hydrosphere/atmosphere and highlights that total sulphur concentrations in the ancient ocean were orders of magnitudes lower than in modern sea water.

In their discussion of our paper (Koglin et al. 2010), Reimer and Mossman criticise “flaws in the data, inadequate description of analytical methods, and misleading presentation of the data”. Although this is a severe accusation, most of which has to be rejected for reasons explained below, we welcome the opportunity to iron out some misunderstandings that might have arisen from our data presentation. The accusation of our data being flawed presumably refers to the very high Ni concentrations reported for some of the analysed spots (our original Table 1). The high values of Ni pointed out by Reimer and Mossman (229,415 and 809,904 ppm) are, of course, not reflecting the composition of pyrite (as incorrectly alluded to in the table caption) but reflect some Ni-sulphide phase that is associated with pyrite. This should have been noted in the table an omission we apologise for. These values, which clearly stem from phases other than pyrite, have not been considered any further in our study as is evident from our diagrams and box plots, and they have no bearing whatsoever on any of the conclusions. The criticism of “inadequate description of analytical methods” is not further elaborated upon by Reimer and Mossman. The analytical methods employed have been described in detail (Koglin et al. 2010, p. 268), with references provided that give further details on standards used and data reduction procedures. At the core of Reimer and Mossman's critique seems to be our presentation of data in terms of counts per second (cps) rather than absolute concentration. Of course, absolute concentrations (in ppm) would be preferable. The reliability of such values is, however, dependent on the standards used. We prefer to be honest and to use the actually measured numbers instead of inferred “absolute” concentrations for those elements for which no reliable standardisation has been available. A positive correlation between cps and absolute concentration (ppm) is beyond any doubt and has been documented by us in our original Fig. 7. We chose to plot Au/Ni ratios (in cps versus ppm) because for the purpose of comparing different types of pyrite from different areas, the ratio is more informative than absolute concentrations. Nevertheless, the picture would not be any different if one plots the measured cps versus absolute concentration as shown in Fig. 1. The “considerable spread”, highlighted by Reimer and Mossman, only applies to higher values and is a common feature for all standard geochemical methods that are based on the comparison of data obtained on a given sample with those for a known standard. If Reimer and Mossman want to challenge this relationship, they would have to question effectively all published geochemical or mineral chemical data obtained by, e.g. conventional XRF, ICPMS or EMP N. Koglin (*) :H. E. Frimmel Geodynamics and Geomaterials Research Division, Institute of Geography and Geology, University of Wurzburg, Am Hubland, 97074 Wurzburg, Germany e-mail: nikola.koglin@uni-wuerzburg.de