Detrital Heavy Minerals Research Articles

Detrital Heavy Mineral Constraints on the Triassic Tectonic Evolution of the West Qinling Terrane, NW China: Implications for Understanding Subduction of the Paleotethyan Ocean

We have examined the Triassic sediments in the west Qinling terrane, northeastern Tibet. These sediments consist mainly of flysch and shallow-sea and fluvial deposits with abundant lithic and heavy mineral detritus, sandwiched between and overlying Late Paleozoic and Early-Middle Triassic ophiolitic mélanges. Volcanic and metamorphic detritus dominates the lithic component of Lower Triassic sandstones accompanied by high Cr-spinel, pyroxene, and magnetite contents, indicating a mixed ophiolite and metamorphic source. Detrital mineral geochemistry further suggests that ophiolitic, high-grade metamorphic, basic, and intermediate-acidic igneous rocks must have been exposed and deeply eroded in their source area. Abundances of zircon, rutile, garnet, tourmaline, and epidote are greater in the Middle Triassic samples, and granitic and volcanic sources are the major contributors of detrital clasts. Considering these new observations on sedimentary petrography and detrital heavy mineral geochemistry, along with published data on paleocurrents, detrital zircon U-Pb ages, sedimentary facies, and regional magmatism, we suggest that these Triassic sediments represent the sedimentary fill of a forearc basin that overlies a late Paleozoic ophiolitic complex. A south-facing Andean-type convergent continental margin system developed along the southern margin of the North China block during the Triassic, in response to northward subduction of the Paleotethyan Ocean.

Continuous denudation and pediplanation of the Chinese Western Tianshan orogen during Triassic to Middle Jurassic: Integrated evidence from detrital zircon age and heavy mineral chemical data

The Mesozoic tectonic framework of the Chinese Tianshan orogen is ambiguous. However, thick Mesozoic sediments both inside and in front of the orogen can provide clues to solve this problem. LA-ICPMS U–Pb dating on detrital zircons and electron microprobe analysis on detrital heavy minerals are carried out on the Triassic to Middle Jurassic sediments along the Kuqa River section in the northern Tarim basin and in two intermountain basins (Bayanbulak basin and Gongnaisi valley), to reveal the change in provenance through time and space. Zircons from the Kuqa River section are mainly of magmatic origin and their ages have a wide range from 2800 to 234Ma, 70.5% of which are Palaeozoic. This age signature indicates a main provenance from the Tianshan orogen. Detrital heavy minerals (garnet and tourmaline) from the Kuqa River section suggest a complex provenance from granitic and metamorphic rocks of the South Tianshan belt, the South Tianshan suture and the Yili-Central Tianshan block. Detrital zircon U–Pb ages of the Early-Middle Jurassic sandstones from the Gongnaisi valley and Bayanbulak basin are predominantly Palaeozoic, without Precambrian, indicating a simple source from the Yili-Central Tianshan block. The different features of detrital zircon ages and heavy mineral chemical compositions in individual sample reflect provenance changes due to continuous denudation and pediplanation of the Chinese West Tianshan orogen in an extensional setting during Triassic to Middle Jurassic: (1) during Early to Middle Triassic, the South Tianshan belt towered to the north of Tarim basin, acting as a watershed and being the main source area for northern Tarim basin; (2) since Late Triassic to Early Jurassic, material from southern Yili-Central Tianshan block was largely transported to Tarim basin because of the continuous denudation of the South Tianshan belt; (3) in Middle Jurassic, the Chinese Western Tianshan orogen reached a peneplain state, resulting in the direct deposition in intermountain basins and the decrease of sediment supplying from Yili-Central Tianshan block to Tarim.

Tectonic constraints to Cretaceous magmatic arc deduced from detrital heavy minerals in northeastern Japan – evidence from detrital garnets, tourmalines and chromian spinels

Tectonic histories of sedimentary basins in the Cretaceous Japan arc have been assessed to understand the response of the Asian continental margin to the oblique subduction of the Paleo-Pacific (i.e. Kula or Izanagi) Plate beneath the Asian continent during the Early Cretaceous and that which subducted orthogonally in the Late Cretaceous. In the Lower Cretaceous Kuji Group (Santonian–Campanian) of the Kitakami Massif in northern Japan, sandstone petrography and chemistry of detrital heavy mineral grains were performed on sandstones to assess the tectonic environment on the basis of provenance analysis.Sandstone petrography results suggest that the material of the Kuji Group was derived mainly from areas of a Cretaceous volcanic belt (Rebun-Kabato Belt) and from a Jurassic accretionary complex (North Kitakami Terrane), which was intruded by Cretaceous granite, adjacent to the depositional basin. The chemical composition of detrital garnets suggests a North Kitakami Terrane origin, and detrital tourmalines are considered to have been derived mainly from meta-sedimentary rocks. The composition of detrital chromian spinels are compositionally diverse and mainly derived from tholeiitic and intra-plate basalts showing high TiO2 (>about 1.0 wt%) and island arc basalts with moderately low TiO2 (1.0 > TiO2 > 0.5 wt%) and high Cr#. Latter chromian spinels can be considered as a record of island arc activity including high Mg-andesite in Early Cretaceous time. Because adequate source rocks of the spinels are elusive near the basin compared with those of detrital garnets and tourmalines, these rocks are believed to have been disturbed by Cenozoic tectonics and eroded and covered by newly formed volcanic and sedimentary rocks.Comparison of chemical composition of the chromian spinels between Lower and Upper Cretaceous deposits in northern Japan indicates that chromian spinels with very low TiO2 (<0.5 wt%) prevail in the Lower Cretaceous (Aptian–Albian). In contrast, chromian spinels showing moderately low TiO2 predominated in the Upper Cretaceous (Santonian–Campanian). This clear difference suggests the change of oceanic plate motion around Japan arc promoted the change of source rock assemblage and the arc volcanic activity in mid-Cretaceous time. Thus the characteristics of detrital heavy mineral composition of the Kuji Group give the key to clarify the interaction between the swaying of young and hot plate and development of the Cretaceous island arc in eastern Asian margin.

Provenance of Late Triassic sediments in central Lhasa terrane, Tibet and its implication

In southern Tibet, Late Triassic sequences are especially important to understanding the assembly of the Lhasa terrane prior to Indo-Asian collision. We report new data relevant to the provenance of a Late Triassic clastic sequence from the Mailonggang Formation in the central Lhasa terrane, Tibet. Petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance, including volcanic, sedimentary and some ultramafic and metamorphic rocks. In situ detrital zircon Hf and U–Pb isotope data are consistent with derivation of these rocks from nearby Triassic magmatic rocks and basement that comprise part of the newly recognized Late Permian–Triassic Sumdo–Cuoqen orogenic belt. The new data suggests correlation with the Upper Triassic Langjiexue Group which lies on the opposing (southern) side of Indus–Yarlung ophiolite. Sediments from both the Mailonggang Formation and Langjiexue Group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the Sumdo–Cuoqen belt.

Tracing the origin of loess in Hungary with the help of heavy mineral composition data

Heavy mineral composition of the fine sand fraction in the Hungarian loess sediments provides important data about the closer sources of the loess in the Carpathian Basin. Quantitative detrital heavy mineral compositions of 216 loess, loess-like sediment, and paleosol samples were compared to each other, and to recent fluvial sands and different Cenozoic sands and sandstones of Hungary using statistical methods: cluster analysis, principal component analysis and discriminant analysis.Usually, garnet is the most frequent detrital heavy mineral in the studied loess sections, followed by chlorite, magnetite, epidote, and amphiboles. Old Loess and Young Loess have similar compositions, indicating uniform or similar sources. Based on the similarities, the garnet-rich loess sediments could originate from aeolian reworking of the floodplain sediments of the Danube and other rivers of Transdanubia, and the local Cenozoic sands of the uplifting Transdanubian Central Range and Transdanubian Hilly Region. The biotite- and tourmaline-rich loess in South Transdanubia, and the loess with high pyroxene content in the North Hungarian Range, had some material from nearby sources: weathered granitoids and volcanic rocks, respectively, or sediments of local rivers, which carried detritus of these rocks. The new results presented in this paper confirm numerous earlier ideas about the origin of loess in the Carpathian Basin, but also suggest newly identified sources of the loess particles at several locations.

Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil

Upper Cretaceous sandstones are the main clastic reservoirs of the Santos Basin, eastern Brazil, showing high porosity at depths greater than 4000 m. Porosity preservation in these deep sandstones is ascribed to the inhibition of quartz cementation and pressure dissolution by authigenic pore-lining chlorite. Santos sandstones are mostly fine-grained, lithic arkoses, rich in volcanic rock fragments (VRF). Chlorite, the most abundant diagenetic constituent, occurs as coatings, rims, rosettes and replacing grains. Chlorite precipitation was favoured by the presence of eodiagenetic smectite coatings and by the abundance of VRF. Detrital heavy minerals, biotite and mud intraclasts were also sources and/or substrates for chlorite authigenesis. The chloritization of VRF, biotite and heavy minerals resulted in precipitation of abundant TiO 2 minerals. Fluid flow patterns, burial and thermal history also played a role in chlorite authigenesis and reservoir quality evolution. Discontinuous and thin coatings and rims were not effective in inhibiting quartz cementation, while those too thick severely reduced the permeability. The rosette aggregates exerted no inhibition on quartz cementation, while reducing intergranular porosity. Systematic studies are necessary for modelling the diagenetic patterns of Santos Basin sandstones, in order to contribute to the reduction of risks involved in the exploration of these reservoirs.

Garnet and tourmaline as provenance indicators of terrigenous material in epicontinental carbonates (Middle Triassic, S Poland)

Heavy mineral grains contained in epicontinental carbonates are very rarely used for provenance analysis. This study deals with detrital heavy minerals separated from marly limestones and marlstones of the Gogolin Formation (GF; Middle Triassic), S Poland, to determine their source rocks and alimentary areas. The GF carbonates typically contain ≤1wt.% of heavy minerals within the very fine sand fraction. The heavy mineral assemblages consist predominantly of garnets (53.0–92.2%), and tourmalines (3.8–30.4%). EPMA analyses reveal four groups of garnets: low-Grs almandines with 30–50mol% Prp, almandines with 20–35mol% Prp and up to 35mol% Grs, almandines with up to 20mol% Sps, and rare pyrope grains (~60mol% Prp). The composition of the garnets indicates that HP felsic granulites were their main source rocks, but more mafic HP rocks (eclogites, retrograded eclogites, and mafic granulites), gneisses, mica schists, and garnet peridotites also contributed. The detrital tourmalines are represented mainly by dravite, and less frequently by schorl, which were derived mostly from paragneisses and mica schists, and in minor part from granitoids and pegmatites. The E–NE part of the Bohemian Massif is the most probable location of the source rocks. The presumed source rocks presently outcrop in the Moldanubian Zone, the Kutná Hora Unit, the Svratka Unit, and the Góry Sowie Massif. Nowadays, the bodies formed by the HP rocks within these units are insufficiently large to act as sources for delivering detrital material in the determined proportions. The high content of HP metamorphic grains thus indicates the past existence of extensively exposed and abundant high-pressure metamorphic rocks, eroded during and/or after the Middle Triassic.

Ruthenium Variation in Chromite from Komatiites and Komatiitic Basalts--A Potential Mineralogical Indicator for Nickel Sulfide Mineralization

More than 390 chromite grains from komatiites and komatiitic basalts from the Yilgarn craton of Western Australia and the Finnish part of the Fennoscandian Shield were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to identify ruthenium (Ru) signatures in chromite associated with nickel sulfide-bearing rocks. Results indicate a potential method to discriminate mineralized and barren komatiite and komatiitic basalt units based on Ru concentrations in chromite and indicate potential for chromite to be used as a resistate indicator mineral in exploration for komatiite-associated nickel sulfide deposits. Chromites from barren komatiites and komatiitic basalts display Ru concentrations mostly between ∼150 and 600 ppb. Chromites from mineralized units have distinctly lower Ru contents (<150 ppb). These results can be interpreted in terms of the much higher partition coefficient for Ru into sulfide liquid compared to that of Ru into chromite, resulting in much lower Ru concentrations in chromite where both chromite and sulfide liquid are present and competing for Ru. As a result, the Ru content of chromite can be used to determine if a komatiite melt equilibrated with a sulfide liquid during crystallization, and therefore, if a system and/or sequence is prospective for nickel sulfide mineralization. The strength of this method compared to previous whole-rock exploration techniques derives from combining (1) the geochemical properties of a chalcophile element that records an ore-forming process while being strongly immobile during postmagmatic processes, with (2) the in situ analysis of a mineral that is generally preserved even in highly altered and mildly weathered komatiites and that is a common constituent of detrital heavy mineral samples. Chromite Ru content has potential as a prospectivity indicator, applicable to a wide range of media including bedrock, laterites, and detrital resistates heavy mineral samples.

Provenance of Paleozoic very low- to low-grade metasedimentary rocks of South Tisia (Slavonian Mountains, Radlovac Complex, Croatia)

Abstract Monazite age dating, detrital heavy mineral content and whole-rock geochemistry provided insight into the provenance, depositional history and paleogeological setting of the Radlovac Complex very low- to low-grade metasedimentary rocks (South Tisia, Slavonian Mountains, Croatia). Electron microprobe based Th-U-Pb dating of detrital monazite indicates a Variscan age of the protolith (330 ± 10 Ma). The detrital heavy mineral assemblages of representative metasedimentary rocks are dominated by apatite, zircon, tourmaline and rutile accompanied by minor quantity of epidote/zoisite, monazite and titanite. Judging from the heavy mineral assemblage, felsic igneous rocks served as the source material. This is consistent with the major and trace element spectrum of studied metasedimentary rocks characterized by high concentration of Th, high L + MREEs and high ratios of La/Sc, Th/Sc, La/Co, Th/Co and Th/Cr. The occurrence of magmatic monazite, zircon and xenotime and the absence of metamorphic heavy minerals suggest that granitoids, migmatites and migmatitic gneisses served as one major source for the metapsammites. Such rock types are commonly exposed in the Papuk Complex of the older surrounding complexes, while the Psunj Complex also contains metamorphic rocks. This is in good correlation with the monazite ages presented here which fits better with ages of Papuk Complex representative rocks than with those of the Psunj Complex known from the literature. Overall, data show that the Radlovac Complex represents the detritus of the local Variscan crust characterized by granitoid bodies, migmatites and migmatitic gneisses typical for the Papuk Complex.

Constraining the provenance of Mu Us Desert (China) sands using U-Pb dating of detrital zircons and heavy mineral assemblages

Constraining the provenance of Mu Us Desert (China) sands using U-Pb dating of detrital zircons and heavy mineral assemblages

東北日本，上部白亜系久慈層群の砕屑性重鉱物.-特に砕屑性クロムスピネルの化学組成ついて－

東北日本，上部白亜系久慈層群の砕屑性重鉱物.-特に砕屑性クロムスピネルの化学組成ついて－

A detrital heavy mineral viewpoint on sediment provenance and tropical weathering in SE Asia

Understanding heavy mineral preservation is important for interpreting generation, pathways, provenance and geochemistry of sediments. Despite this, many assumptions about heavy mineral stability are based on ancient strata and few studies consider modern sediments, particularly those in tectonically active tropical areas such as SE Asia. We report new heavy mineral data on 69 river sand samples from the Malay Peninsula and Sumatra, in which one aim was to find provenance indicators specific to these areas. Identifications were performed using optical microscopy and confirmed with SEM-EDS. In the Malay Peninsula heavy minerals record granitic and contact metamorphic provenance. Variable amounts of zircon, tourmaline, hornblende, andalusite, epidote, monazite, rutile and titanite, and minor amounts of pyroxene, apatite, anatase, garnet, diaspore, colourless spinel, cassiterite and allanite are typical of this source area. The composition of assemblages from Sumatra indicates contributions from two major sources: the modern volcanic arc (I) and the basement (II). Abundant pyroxene, particularly hypersthene (up to 70%), is diagnostic of the volcanic arc source. Vesuvianite, garnet, andalusite, tourmaline, chrome spinel, rutile, anatase and corundum, are present only in small amounts (<3%), and are interpreted as recycled from the basement. Zircon, apatite, hornblende, epidote, and olivine are also common in Sumatra and are likely to have a mixed provenance. Abundance of ferromagnesian silicate minerals suggests mild weathering, possibly reflecting several processes: dilution of natural etching fluids by heavy rainfall, high erosion rates, rapid transport and short grain residence time in the river. The heavy mineral assemblages of modern rivers are very different from those recorded by the few previous studies of Cenozoic sediments of the Malay Peninsula and Sumatra. Assemblages in the Cenozoic basins are significantly more mature than those of modern rivers. The differences cannot be explained simply by dissolution of susceptible minerals during one sedimentary cycle and instead imply rapid source area unroofing.

Chemical composition of the detrital heavy mineral in the Upper Cretaceous Yezo Group in the Teshio-Nakagawa area, northern Hokkaido, Japan

Chemical composition of the detrital heavy mineral in the Upper Cretaceous Yezo Group in the Teshio-Nakagawa area, northern Hokkaido, Japan

Beach placer, a proxy for the average Nd and Hf isotopic composition of a continental area

Abstract Beach placer deposits concentrate detrital heavy minerals which are the erosion products of large areas of continental crust. Here, we report the first analyses of Nd–Hf isotopic ratios and trace element concentrations that we measured in a beach placer from Camargue, France and in its pure mineral separates. Both the bulk composition of the placer and those of its pure mineral separates were determined. We also report mineral proportions obtained using observations under a binocular microscope and micro X-ray fluorescence mapping. Our results indicate that monazite totally controls the placer's Nd isotopic composition ( e Nd = − 9.3) while zircon dominates its Hf isotopes ( e Hf = − 13.0) even though both mineral phases represent only a small proportion of the heavy mineral assemblage (3.5 and 10% respectively). We demonstrate that the Camargue placer provides a good estimate of the average Nd and Hf isotopic composition of the continental area drained by the Rhone River in Western Europe ( e Nd ≈ − 9 and e Hf ≈ − 13). Using these values, we calculate two stage model ages and show that almost all the placer minerals are derived from Proterozoic crustal protoliths. This provides valuable information on the history of the continental crust drained by the Rhone River. In particular, it suggests that little juvenile crust was created during the recent geological events that formed the Alps and the Massif Central, the two main massifs from which the placer minerals originate. More generally, we propose that similar measurements made on other worldwide beach placer deposits could provide estimates of the present-day Nd and Hf isotopic composition of large continental areas, values that are difficult to obtain due to the well-known heterogeneity of continental material but are essential to model the growth of continental crust through Earth history or to model the impact of crustal material when recycled into the mantle.

Detrital heavy minerals from Lower Jurassic clastic rocks in the Joetsu area, central Japan: Paleo‐Mesozoic tectonics in the East Asian continental margin constrained by limited chloritoid occurrences in Japan

AbstractDetrital chloritoids were extracted from the Lower Jurassic sandstones in the Joetsu area of central Japan. The discovery of detrital chloritoids in the Joetsu area, in addition to two previous reports, confirms their limited occurrence in the Jurassic strata of the Japanese islands. This finding emphasizes the importance of the denudation of chloritoid‐yielding metamorphic belts in Jurassic provenance evolution, in addition to a change from an active volcanic arc to a dissected arc that has already been described. Possible sources for the detrital chloritoids from the Jurassic sandstones are the Permo–Triassic chloritoid‐yielding metamorphic rocks distributed in dispersed tectonic zones (Hida, Unazuki, Ryuhozan and Hitachi Metamorphic Rocks), which are in fault contact with Permian to Jurassic accretionary complexes in the Japanese islands. This is because all of these pre‐Jurassic chloritoid‐yielding metamorphic rocks have a Carboniferous–Permian depositional age and a Permo–Triassic metamorphic age, whereas a Permian–Triassic metamorphic age on the Hitachi Metamorphic Rocks remains unreported. In addition, most metamorphic chloritoids imply a former stable land surface that has evolved into an unstable orogenic area. Therefore, the chloritoid‐yielding metamorphic rocks might form a continuous metamorphic belt originating from a passive continental margin in East Asia. Evidence from paleontological and petrological studies indicates that the Permo–Triassic metamorphic belt relates to a collision between the Central Asian Orogenic Belt and the North China Craton. The evolution of the Permian–Jurassic provenance of Japanese detrital rocks indicates that the temporal changes in detritus should result from sequences of collision‐related uplifting processes.

Geology, geochemistry and mineralogy of the lignite-hosted Ambassador palaeochannel uranium and multi-element deposit, Gunbarrel Basin, Western Australia

The Ambassador U and multi-element deposit occurs on the SW margin of the Gunbarrel Basin, Western Australia. Low-grade, flat-lying U mineralization averaging about 2 m thick at 0.03% U occurs in lignites at the redox front at the base of the weathering profile within a laterally extensive palaeochannel network. Uranium is principally associated with organic matter within the lignitic matrix, although rare discrete U minerals, such as coffinite and uraninite, are also present. The lignite is also enriched in a suite of other elements, principally base metals and sulphur, with concentrations of 0.3 ≥ 1% Cu, Pb, Ni, Co, Zn and total rare earth elements (REE) in some samples. Other element enrichments include: Cr, Cs, Sc, Se, Ta, Ti, Th, V and Zr as detrital heavy minerals of Zr, Ti and REE (oxides and silicates) or authigenic minerals of Cu, Bi, Pb, Zn, Ni, Se, Hg, Ti, Cr, Tl, V, U and REE (sulphides, vanadates, selenides, oxides, chlorides and native metals) and diffuse lignite impregnations. The Ambassador deposit probably formed from the convergence of redox-active weathering processes to unique source/host rocks, constrained within the palaeochannel. A proximal source of U and trace elements of lamproite/carbonatite origin is probable, as constrained by U–Pb isotope and U–Th disequilibria studies. Uranium and other metals were precipitated syngenetically with organic matter as it was deposited during a humid phase in the Late Eocene. Remobilization subsequently concentrated the metals in the upper 2 m of the lignite. This may have occurred during one or more periods of weathering and associated diagenesis, with the latest episode in the last 300,000 years.

In situ U–Pb rutile dating by LA-ICP-MS: 208Pb correction and prospects for geological applications

Rutile is a common accessory mineral that occurs in a wide spectrum of metamorphic rocks, such as in blueschists, eclogites, and granulites and as one of the most stable detrital heavy minerals in sedimentary rocks. The advent of rutile trace element thermometry has generated increased interest in a better understanding of rutile formation. This study documents important analytical advances in in situ LA-ICP-MS U/Pb geochronology of rutile: (1) Matrix matching, necessary for robust in situ dating is fulfilled by calibrating and testing several rutile standards (R10, R19, WH-1), including the presentation of new TIMS ages for the rutile standard R19 (489.5 ± 0.9 Ma; errors always stated as 2 s). (2) Initial common lead correction is routinely applied via 208Pb, which is possible due to extremely low Th/U ratios (usually <0.003) in most rutiles. Employing a 213 nm Nd:YAG laser coupled to a quadrupole ICP-MS and using R10 as a primary standard, rutile U/Pb concordia ages for the two other rutile standards (493 ± 10 Ma for R19; 2640 ± 50 Ma for WH-1) and four rutile-bearing metamorphic rocks (181 ± 4 Ma for Ivrea metapelitic granulite; 339 ± 7 Ma for Saidenbach coesite eclogite; 386 ± 8 Ma for Fjortoft UHP metapelite; 606 ± 12 Ma for Andrelandia metepelitic granulite) always agree within 2% with the reported TIMS ages and other dating studies from the same localities. The power of in situ U/Pb rutile dating is illustrated by comparing ages of detrital rutile and zircon from a recent sediment from the Christie Domain of the Gawler Craton, Australia. While the U/Pb age spectrum from zircons show several pronounced peaks that are correlated with magmatic episodes, rutile U/Pb ages are marked by only one pronounced peak (at ca 1,675 Ma) interpreted to represent cooling ages of this part of the craton. Rutile thermometry of the same detrital grains indicates former granulite-facies conditions. The methods outlined in this paper should find wide application in studies that require age information of single spots, e.g., provenance studies, single-crystal zoning and texturally controlled dating.

Karst bauxites in the Campania Apennines (southern Italy): a new approach

In this study new data on two bauxite districts in the Campania region (southern Italy) are reported: the Matese Mts. and the Caserta province, exploited in the first part of the XX century. The nature and distribution of diagenetic and detrital mineral phases were investigated by means of transmitted-light and scanning electron (SEM) microscopy, X-ray powder diffraction (XRPD) and energy dispersive spectroscopic microanalysis (EDS), whole-rock geochemical analyses (ICP-OES and ICP-MS), complemented by preliminary QEMSCAN ® investigation. The textures of the bauxite ore range between oolitic and oolitic-conglomeratic and arenitic. Boehmite is the most abundant phase, while other Al-bearing phases (gibbsite and diaspore) are very subordinate. The Fe-minerals hematite, goethite and lepidocrocite occur, and Ti-minerals anatase and rutile are ubiquitous. Among clay minerals, kaolinite is prevailing at the Regia Piana (Matese Mts.) and illite-montmorillonite in the Caserta province. Most recorded trace minerals are quartz, calcite, zircon and monazite. Small clusters of not previously detected qandilite [(Mg, Fe 2+ ) 2 (Ti, Fe 3+ , Al) O 4 ], and of hercynite-type spinels also occur. By means of QEMSCAN ® it has been possible to better recognize some diagenetic textures (e.g. mainly matrix- or ooids-supported bauxite, the clay/Al-hydroxides distribution etc.) and to detect other detrital minerals (feldspar, muscovite, olivine, titanite and possibly a serpentine/talc-like phase). A similar detrital mineral association has been found also in the Aptian “Orbitoline ” marls, which are marine sediments supposed to be the non-weathered equivalent of bauxites. Very detailed data about major, minor and trace element (comprising “bauxitophile ” and REEs) concentrations and some significant geochemical ratios are reported for the bauxite samples, and a new interpretation on the possible source of the parent material has been proposed. The detrital heavy mineral association may suggest not only a windblown volcanic source material (Dinarides explosive volcanism?), as hypothesized in literature, but also a partial origin from an exposed terrain. The exact nature and paleogeographic position of this source could not be determined by chemical analyses of major and trace elements (including REE), Eu/Eu* and TiO 2 /Al 2 O 3 ratios and Ni-Cr contents. However, due to the isolated position of the Appennine carbonate platform during the Cretaceous, the paleogeographic model precludes any possible fluviomarine transport for the source material (windblown?) of the bauxites.

Mineral chemical study of U-bearing minerals from the Dominion Reefs, South Africa

The Neo-Archean Dominion Reefs (~3.06 Ga) are thin meta-conglomerate layers with concentrations of U- and Th-bearing heavy minerals higher than in the overlying Witwatersrand Reefs. Ore samples from Uranium One Africa’s Rietkuil and Dominion exploration areas near Klerksdorp, South Africa, were investigated for their mineral paragenesis, texture and mineral chemical composition. The ore and heavy mineral assemblages consist of uraninite, other uraniferous minerals, Fe sulphides, Ni–Co sulfarsenides, garnet, pyrite, pyrrhotite, monazite, zircon, chromite, magnetite and minor gold. Sub-rounded uraninite grains occur associated with the primary detrital heavy mineral paragenesis. U–Ti, U–Th minerals, pitchblende (colloform uraninite) and coffinite are of secondary, re-mobilised origin as evidenced by crystal shape and texture. Most of the uranium mineralisation is represented by detrital uraninite with up to 70.2 wt.% UO2 and up to 9.3 wt.% ThO2. Re-crystallised phases such as secondary pitchblende (without Th), coffinite, U–Ti and U–Th phases are related to hydrothermal overprint during low-grade metamorphism and are of minor abundance.

Detrital heavy minerals, white mica and zircon geochronology in the Ordovician South Mayo Trough, western Ireland: signatures of the Laurentian basement and the Grampian orogeny

Abstract: The Ordovician strata of the South Mayo Trough in western Ireland were deposited as a conformable sequence, first in the forearc of a north-facing oceanic arc, then in a synorogenic basin above the arc–continent collisional Grampian orogen, then in an extensional hanging-wall basin above the exhuming Dalradian metamorphic complex and, finally, in an Andean-type margin successor basin. This depositional history, and its relationship to the Grampian orogeny, to the pre- and post-collision arcs, and to sources in the Laurentian foreland, is recorded in detrital heavy mineral suites, Ar/Ar ages of detrital white micas, and U–Pb ages of detrital zircons. Heavy minerals record the progressive extensional unroofing of an obducted ophiolite complex, then the rapid extensional unroofing of the Grampian Dalradian metamorphic complex. White mica ages give Neoproterozoic, Mesoproterozoic, and Neoarchaean ages in the lower part of the sequence then, at about 466 Ma, synchronously with the appearance of amphibolite-facies metamorphic detritus, a tightly unimodal Grampian metamorphic age. Zircons yield mainly Mesoproterozoic and Neoarchaean ages, indicating a Laurentian source, and a Grampian age of 460–468 Ma. Paralleling the appearance of Grampian-age micas, the colour of zircon changes from dominantly purple to dominantly colourless, reflecting the thermal annealing effect of metamorphism. However, the temperature of Grampian metamorphism was not high enough to reset the U–Pb isotopic systematics of detrital zircons.