Accumulation Of Heavy Minerals Research Articles

Mineral Composition and Provenance of Al-Chabbab Stream Sediments, Wasit, Southeastern Iraq

A mineralogical study was carried out on the sediments of Al-Chabbab stream, one of the seasonal tributaries of the Tigris River, southeast of Wasit, Iraq. This study includes the sediments of the Tigris River sited before and after the mouth of the Al-Chabbab stream. Light and heavy minerals are determined by using polarized microscope and X-Ray Diffraction technique. The light minerals are composed of quartz, rock fragments, and feldspar, which are the prevailing compositions in all samples. The quantity of monocrystalline quartz in the Tigris River was more than Al- Chabbab stream, which refered to the supplied of felsic sours rocks sediments to the Tigris River more than the sediments of Al-Chabbab stream. The average value of minerological maturity index and ZTR index of Al-Chabbab stream, Tigris and Al-Chabbab inflow samples indicates that clasts generally are sub-mature. The Tigris River clasts have high mineralogical maturity index, which refers to the long transportation. Heavy mineral accumulation of the recent sediments from Al-Chabbab inflow and the Tigris River is composed of opaque and transparent minerals including epidotes; pyroxenes; amphiboles (hornblende and glaucophane); zircon; garnet; tourmaline; rutile; kyanite; staurolite; flaky minerals (muscovite; biotite and chlorite) and celestite (glaucophane and celestite are absent in the sediment of Tigris River). Based on the mineralogical signatures, the light and heavy minerals reflect metamorphic, mafic and felsic igneous rocks, of the active margin of the unstable shelf, in addition to the carbonate, evaporite and mud rocks that could be sourced from the Euphrates, Fatha and Injana formations.

Coastal morphology and heavy mineral accumulation in an upper-macrotidal environment – A geological-mineralogical approach from source to trap site in a natural placer laboratory (Channel Islands, Great Britain)

The Isles of Jersey and Guernsey, GB, are situated in the Bight of St. Malo which ranks among the upper-macrotidal environments with one of the highest tidal ranges ever measured on the globe. They are built up of metasedimentary and –magmatic basement rocks followed by a suite of gabbro-diorite-granite intrusive rocks. Based on sedimentological, geomorphological and mineralogical investigations focused on heavy minerals (HM) the coastal environment has been subdivided into three principal coastal types headland-, headland-bay and bay types. Chemical and physical weathering, neotectonics, eustatic sea level changes, wave action, land-derived sediment supply and aeolian processes had not only a strong impact on the various coastal landform series (CLFS) established but also on the variegated HM assemblages casting them into the role of markers for the coastal environment and some kind of an ore guide for marine placer deposits. Tidal-dominated coastal environments under study sculptured exclusively out of crystalline bedrocks are conservative with regard to the economic geology and potential of HM placer-type deposits provided while physical weathering predominates over the chemical one. The tidal processes in context with the other land-forming processes are special importance for the diversity of coastal features within the environment hosting HM on a small- and large-scale. Neotectonics may accentuated or suppress tidally-controlled features in that strong uplift and provokes typical tidal features to get suppressed so that vast tidal flats are missing in a great deal of coastal areas and rocky coasts formed instead with consequences for the accumulation of HM. The “GMS tool (granulometry-morphology-situmetry) has methodologically proved to be very effective for the study of the coastal zones hosting HM in two principal ways. Granulometric and the morphological variation of HM can successfully be used for the host rock environment analysis (“ore guide”). Morphometric studies and situmetric measurements of gravel clasts in the coastal sediments bearing HM accompanied by similar measurement of joints and faults cutting through the bedrock can assist in the localization of HM trap sites in shoreline platforms and their rock veneer. The HM associations in upper-macrotidal environments are variegated in terms of morphology, mineralogical composition and their geomorphological trap sites so that they constitute a natural laboratory for environment analysis in terms of hydrodynamic and (paleo) climate along marine terrigenous shorelines and help better understand the way of HM from source to trap site in modern and paleoplacers.

Integrated mineralogical and geochemical study of the Rapel fluvial system, central Chile: An application of multidimensional analysis to river sedimentation

The mineralogical and chemical composition of the <180 μm fraction of 90 stream sediments collected along the Rapel, Cachapoal and Tinguiririca rivers were studied in order to assess the influence of natural and anthropogenic factors. In order to distinguish major and secondary chemical patterns the data was analyzed using an unsupervised artificial neural network technique. The studied rivers present different chemical signatures, which are associated with geological formations and anthropogenic factors such as mining, agro-industry and urbanization. The upper-middle reaches of the Cachapoal River are characterized by elevated Cu–Mo–As–Sb concentrations that reflect input of mining-derived material. This signature, which is further amplified in the middle reaches of this river due to the influence of the city of Rancagua, masks other signatures such as organic matter-related high B concentrations. The sediments of the Tinguiririca River are strongly depleted in heavy metals, but present high P2O5 concentrations, which reflects the extensive use of phosphates in its floodplain. At the upper reaches of the Rapel River the transition from granitic to sedimentary-dominated bedrock coincides with a marked decrease in P2O5–La–Ce–Y abundances. Similarly, the lower reaches of this river coincide with a marked CaO–MgO increase, which reflects an increase in the availability of CaO-rich detritus associated to a lithological transition. Enrichment of strongly correlated Fe2O3–TiO2–MnO–P2O5–Pb–V–Ni–Cr–Y–Ce–La in the middle-lower reaches of this river is interpreted as reflecting heavy mineral accumulation in point-bar deposits. This is further supported by the mineralogy of the associated samples. Most of the suspended load of the Cachapoal and Tinguiririca rivers settles in the Rapel Lake. Thus the quantity of metals and P2O5 contained in the sediments of this lake is considerable and, together with the very high Cu–Mo–As–Sb concentration of the sediments of the Cachapoal River constitute a potential hazard to the catchment environment.

Study of Heavy Minerals from the Vjosa and Mati river delta sediments in Albania

This research is focused on the determination of the heavy minerals (HM) load of the Vjosa and Mati river delta deposits along the Albanian coastline and it is based on X-ray Diffractometry. The Albanian coastline consists of sandy beaches at the north (Adriatic coastline) and rocky escarpments at the south (Ionian coastline). Several layers of heavy mineral deposits, up to 50 cm thick, with heavy mineral fraction up to 95% and 88% of total sample for Vjosa and Mati sediments respectively, are identified. The layers enriched in heavy minerals from Vjosa delta deposits are almost black in colour, while at Mati delta these layers are dark green coloured. Separation of the heavy from the light fraction was performed, in order to compare the different fractions between the two studied delta areas. The accumulation of HM occurs mainly in the fraction of 125-250 µm in the sediments of both deltas. The HM dominate in the magnetic field of 0.4-0.8 A/m. XRD analysis results show a great variety of minerals present in the delta samples which can be attributed to the wide variety of geological zones and lithologies that are intersected by the Vjosa and Mati rivers, respectively. In the 63-425 μm fraction rich in heavy minerals of both delta systems considerable amounts of magnetite (up to 39.4% in Vjosa samples), chromite (up to 20.2% in Vjosa samples), garnet (up to 13.6% in Vjosa samples), ilmenite (up to 8.3% in Mati samples), rutile (up to 4.7% in Mati samples), hematite (up to 2.2% in Mati samples), and zircon (up to 2.1% in Vjosa samples) are observed. Rock forming minerals such as pyroxene, amphibole, and epidote compose significant percentages of this fraction. In addition, the presence of gold grains in Vjosa delta sediments is remarkable. Both catchment areas consist to a great extent of similar formations such as the Mirdita Ophiolite Zone and the Pindos Ophiolite complex, providing thus a similar HM fingerprint at both delta areas. Minerals that occur in higher abundances reflect the extensive presence in the drained areas of related parent rocks which are rich in these minerals and which are often more vulnerable to weathering. The samples of Vjosa river delta show high percentage of carbonate constituents, which is related to the presence of carbonate rocks of the Ionian and Kruja tectonic zones within which the hydrographic network of the Vjosa River has been developed. The samples of Mati river delta show lower abundance of carbonate minerals, reflecting the limited presence of carbonate rocks at the Kruja Zone, which occur in the catchment area of the river near its mouth.

Mineralogical influence over the presence of primordial radionuclide along the industrial corridor of northern coastal region of Chennai

The present study focuses on the correspondence between the natural radioactivity and geological parameters such as the textural nature and mineralogical variation of the sediment. The activity concentration of the primordial radionuclide varied in correspondence with the variation in the heavy mineral accumulation along the coastal stretch and the coal ash deposition along the hinterland region. XRD analysis on heavy minerals confirms the presence of heavy mineral—Ilmenite to be present predominantly in the heavy minerals fraction of the samples. The magnetic susceptibility values confirm, the correspondence between the magnetic minerals and materials and the activity concentration of 238U and 232Th in the sediments.

Bauxite formation on Tertiary sediments in the coastal plain of Suriname

Lateritic bauxites in the coastal lowlands of Suriname form part of a belt along the northern margin of the Guiana Shield that has long been one of the world's major bauxite producing regions. The Surinamese deposits, many of which with an extensive mining history, originated on Tertiary siliciclastic sediments and were mostly buried under a layer of young sediments. The bauxite-bearing sequences are generally topped with an iron-rich layer largely made up of hematite and goethite. It covers a gibbsite-rich bauxite horizon that passes downward into a kaolinitic bottom section containing anatase and zircon as main accessory minerals. Weathering profiles across formerly mined deposits were analyzed for geochemical and mineralogical properties aimed at exploring compositional diversity, underlying controls of bauxite-formation and the nature of precursor sediments.Studied profiles in different parts of the coastal plain reveal overall similarities between individual deposits in showing significant depletion of Si, K, Na, Mg and Ca and strong, primarily residual, relative enrichment of Al, Ti, Zr, Nb, Hf, Ta and Th. In detail, however, there are distinct differences in major and trace-element signatures, accessory mineral assemblages, facies distribution and provenance of the terrigenous precursor sediments. Enrichments in high field-strength elements and heavy rare earth elements are largely attributable to accumulation of heavy minerals like zircon in the precursor. Petrological and trace-element evidence does not support a direct genetic relationship between bauxite and the underlying saprolitic clays. The complex petrologic characteristics and compositional heterogeneity of the coastal-plain deposits can essentially be explained by element fractionation, primarily through selective leaching, in combination with relative and absolute enrichment processes, erosion and reworking during two-stage, polycyclic bauxitization of a heterogeneous precursor.

Heavy mineral resource potential of Tista river sands, Northern Bangladesh

ABSTRACTSurface and borehole sampling along a ∼80 km section of the lower Tista river, northwestern Bangladesh, indicated that the river sands offer significant potential as a heavy mineral (HM) resource. Characterisation of sediments from the surface to 15 m depth showed that the sand-sized component was dominated by quartz, feldspar, mica, lithic fragments, amphibole and pyroxene group minerals. The most common particle size was between +125–500 μm with 84 wt-% of all material reporting to this size range. Laterally spaced sampling indicated slight grain size coarsening upstream. Heavy liquid separation studies revealed that HMs such as amphiboles, micas, garnets, aluminosilicate (Al2SiO5) phases, ilmenite and zircon made up ∼10.99% (on average). The percentage of valuable HMs (ilmenite, rutile, zircon, monazite, garnet) was 2.47% (average). Detailed borehole sampling and resource mapping of a large, mid-channel sand bar showed that placer-style HM accumulations occur upstream and along the margins of the bar.

Geochemistry of coastal sands of Eastern Mediterranean: The case of Nisyros volcanic materials

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920mg/kg) and Nb (245mg/kg), in the coastal sand. The low REE concentration (ΣREE+Y=240mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE.The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions.

Accumulation of heavy minerals in the eastern coast of the Gulf of Riga, south-western Estonia

Accumulation of heavy minerals in the eastern coast of the Gulf of Riga, south-western Estonia

Geochemical characteristics of stream sediments, sediment fractions, soils, and basement rocks from the Mahaweli River and its catchment, Sri Lanka

Geochemical variations in stream sediments (n=54) from the Mahaweli River of Sri Lanka have been evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Compositions of soils (n=22) and basement rocks (n=38) of the catchment and those of <180μm and 180–2000μm fractions of the stream sediments were also examined. The sediments, fractions, soils and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Fe2O3, TiO2, MnO, CaO, P2O5 and total sulfur contents. Abundances of high field strength and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals including zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180μm fractions. The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and overall have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and high values in the <180μm fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to the main channel. Al2O3/(K2O+Na2O) and K2O/Na2O ratios of the sediments and LOI values of the soils correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors (EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy mineral accumulation.

Glauconite Composition and Morphology, Shocked Quartz, and the Origin of the Cretaceous(?) Main Fossiliferous Layer (MFL) in Southern New Jersey, U.S.A.

Abstract The Main Fossiliferous Layer (MFL) is a concentration of vertebrate and invertebrate fossils, 20 to 30 cm thick, preserved in a sequence of glauconitic sand at or near the Cretaceous–Paleogene boundary in the New Jersey (USA) coastal plain. Several hypotheses have been proposed to explain the origin and age of the MFL, including: marine transgression and formation of lag deposits of reworked bones and shells, formation of a condensed section and attritional accumulation of fossil material, and catastrophic collapse of Late Cretaceous ecosystems following the end-Cretaceous bolide impact at Chicxulub. We use new data on glauconite morphology, concentrations, geochemistry, and presence of shocked quartz, coupled with previous data on sedimentology, taphonomy, and rare-earth-element geochemistry of fossil vertebrates to interpret the genesis of the MFL. Glauconite concentration and maturity steadily increases from latest Cretaceous sediments of the Navesink–New Egypt Formation, through the MFL and into the Paleogene upper Hornerstown Formation, suggesting that marine transgression and decreasing sedimentation rates were a factor in the formation of the glauconite, but not the MFL. Rare-earth-element patterns in fossil bones from the MFL, which are acquired during fossilization, are different from those of Cretaceous and Paleogene specimens, indicating that vertebrate remains in the MFL fossilized in situ and were not reworked from older, underlying units. Vertebrate fossils from the MFL are preserved as isolated to articulated specimens. While isolated specimens would be common in a transgressive lag, articulated specimens would not. Articulated specimens could be concentrated by attritional accumulation along a defined surface during a period of slow sedimentation, but the lack of a distinct increase in glauconite maturity or concentrations of elements associated with heavy-mineral accumulations at the Navesink–New Egypt–Hornerstown contact or MFL, which would be expected during a period of reduced sedimentation, hiatus, or unconformity, are absent at the localities studied. Shocked quartz was identified in a burrow fill directly beneath the MFL, at the contact of the Navesink and Hornerstown formations. Clay clasts with latest Cretaceous microfossils, along with reworked invertebrate fossils with infills of latest Cretaceous sediment, have been recovered from the MFL. The association of shocked quartz, mixture of isolated and articulated vertebrates with distinct rare-earth signatures, lack of a punctuated period of increased glauconite maturity, and presence of reworked Late Cretaceous invertebrates and clay clasts with Late Cretaceous microfossils suggests that the MFL may represent a thanatocoenosis that was the direct result of environmental disturbance associated with the end-Cretaceous impact event at Chicxulub.

Cenozoic Eucla Basin and associated palaeovalleys, southern Australia — Climatic and tectonic influences on landscape evolution, sedimentation and heavy mineral accumulation

The Eucla Basin including the vast Nullarbor Plain lies on the margins of the Yilgarn, Musgrave and Gawler cratons in southern Australia and owes its distinctive landscape to a unique set of interactions between eustatic, climatic and tectonic processes over the last ~ 50 Ma. Understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits, and the sediments derived from them contain remobilised gold, uranium, and heavy minerals. In particular, a remarkably preserved palaeoshoreline sequence along the north-eastern margin of the Eucla Basin is highly prospective for heavy mineral placer deposits. The record of marine, marginal marine, estuarine, fluvial and lacustrine environments, as constrained mainly by an extensive borehole dataset, reflects major depositional events during the Palaeocene–Early Eocene, Middle–Late Eocene, Oligocene–Early Miocene, Middle Miocene–Early Pliocene and Pliocene–Quaternary. These events reflect the key role of eustatic sea-level variation which, during highstands, inundated the craton margins, flooding palaeovalleys to up to 400 km inboard of the present coastline. However, a systematic eastward migration of the depocentre across the Eucla Basin during the Neogene, together with apparent flow reversals in a number of palaeovalley systems draining the Gawler Craton, suggest that the Eucla Basin has also been subject to differential vertical movements, expressed as a west-side up, east-side down tilting of ~ 100–200 m. This differential movement forms part of a broader north-down–southwest-up dynamic topographic tilting of the Australian continent associated with relatively fast (6–7 cm/yr) northward plate motion since fast spreading commenced in the Southern Ocean at ~ 43 Ma. We suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits, largely through multistage, eastward reworking of near-shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems.

Mineralogical and chemical distribution patterns of placers and ferricretes in Quaternary sediments in SE Germany: The impact of nature and man on the unroofing of pegmatites

Placer deposits mark the ultimate stage of mineral separation in clastic sediments by physical processes in an aquatic regime, whereas duricrusts are an example of extreme element separation by chemical processes in continental environments of deposition. In Quaternary sediments in SE Germany such contrasting types of sediments formed on the NE Bavarian Basement which is underlain by metamorphic and pegmatitic rocks reflecting intense primary element differentiation by themselves. The placer deposits were subdivided into trap and bed placers. Together with three types of encrustation they were put into a scheme illustrating six stages of land formation. The first-mentioned group of placer deposits formed from the Late Cretaceous to the Late Tertiary on a vast peneplain under subtropical conditions (stages I–II). Towards the Quaternary its more acidic meteoric fluids turned alkaline and linear erosion gradually replaced peneplanation as the main process of shaping the landscape (stage III). Evidence for these types of alluvial–fluvial Ti and Sn heavy mineral accumulations and the subtropical climatic conditions can only be found in the major placer mineral aggregates nigrine and cassiterite which preserved a great variety of marker minerals in fractures and vugs. Duricrust I consist of aluminum–phosphate (APS)-bearing phoscretes. Bed placer deposition started off with colluvial to alluvial placers passing eventually into fluvial placers (stages IV–V). Type II duricrusts evolved under humid temperate conditions in the aftermaths of the glacial period. Per-descensum fluids provoked the precipitation of these phosphate-bearing ferricretes. The clastic sediments were reworked in a low-sinuosity fluvial drainage system during stage VI. In addition to the ferricretes mentioned above another type of ferrous encrustations was added to the depositional system by man in form of fayalite-bearing slags from blast furnaces. These smelting artifacts were dumped on the floodplain of the modern river system and kilns which they derived from, were fed with phosphorous iron from duricrusts of type II. Age data for the latest stages of placer formation have been obtained from radio-carbon dating of drift wood and luminescence analyses of quartz and feldspar in placer deposits. These data reveal that morphological processes sparking placer deposition have obviously been influenced by the intensive clearing of settlers in the “Oberpfälzer Wald” (= Forest of Upper Palatinate). The results of the present study may be of assistance to geomorphologists dealing with continental placer deposits (Sn, Ti, Ce, Zr) and chemical sediments (ferricretes, phoscretes) which developed in-situ on ancient and modern peneplains. This is true for geomorphologists in academia and consulting companies alike. Archaeometallurgists may benefit from these results as they were investigating smelting residues scattered within alluvial and fluvial deposits of Quaternary age. Commonly they focus their attention on waste disposals and smelting sites and to a lesser extent on reworked smelting residues in drainage systems. The target group number one is among exploration geologists who are in search of continental placer deposits and want to know more about the clastic dispersion haloes around phosphate- and oxide-bearing pegmatites. Alluvial to fluvial placer deposits containing stable and ultrastable heavy minerals may well be preserved as to their mineralogical composition and chemical patterns even through long-lasting periods of shaping the landscape. These mineralogical and chemical patterns can be used to reconstruct the original morphology of a pegmatitic body which today is present only as “pegmatite ruin”.

05/01366 Representing energy technologies in top-down economic models using bottom-up information

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920 mg/kg) and Nb (245 mg/kg), in the coastal sand. The low REE concentration (ΣREE + Y = 240 mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE.The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions.

Depositional Placer Accumulations in Coarse-Grained Alluvial Braided River Systems

Depositional paleoplacer accumulations in coarse alluvial braided river systems form by the concentration of heavy minerals during sedimentation of the detritus and, where present, are usually associated with one or more subenvironments of gravel-dominated longitudinal bar complexes. Conditions necessary for the formation of this deposit type were investigated using three separate approaches. Huronian, uraniferous paleoplacers were studied in working faces of mines to gain insight into areas of heavy mineral accumulation. Primary data on processes responsible for the concentration of heavy minerals in the various subenvironments of longitudinal bars were obtained from examination and sampling of matrix and surficial sands of modern gravel bar systems in the North Saskatchewan River, Alberta, and in the Jackpine, Agawa, and Mississagi Rivers, Ontario. Concentration processes were further investigated through a series of experiments using a sediment-water recirculating flume. Sand composed of a mixture of quartz, pyroxene, magnetite, and lead was moved over a gravel bed; flow velocity, depth of flow, grain size of the quartz sand, and clast size systematically changed during 27 experiments. Data from these studies indicate that a number of conditions are necessary, or at least desirable, for heavy minerals to accumulate in coarse-grained alluvium. These are (1) a low proportion of granule to very small pebble-sized lithic fragments; (2) a very heavy mineral population whose hydraulic behavior more closely resembles that of the pebble population than that of the quartz sand; (3) flow velocities capable of creating a suspension cloud from the coarse-grained quartz sand population; (4) a change in the regional slope (inflection point) creating a gradient-parallel zone of energy loss; (5) infrequent major flood events; and possibly (6) preconcentration, i.e., an enriched, erosional lag upslope from the ultimate area of depositional placer accumulation. These conditions, plus the presence of economically significant minerals in the sediment load, control the formation of exploitable placer deposits in longitudinal bars of braided rivers.

Post-orogenic exhumation of an auriferous terrane: the paleoplacer potential of the Early Carboniferous St. Marys Basin, Canadian Appalachians

The St. Marys Basin of mainland Nova Scotia, Canada, consists of Late Devonian–Early Carboniferous clastic rocks of the Horton Group deposited in the waning stages of the ca. 420–360 Ma Acadian orogeny in the Canadian Appalachians. Clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the Meguma terrane that occurs to the south of the basin. The Meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca. 380 to 370 Ma and underwent rapid uplift and erosion between ca. 370 and 360 Ma. Within the St. Marys Basin, the contact between the lacustrine Little Stewiacke River Formation and the fluviatile Barrens Hills Formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold. Geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from Meguma terrane metasedimentary rocks, whereas the sandstones and conglomerates are predominantly derived from Meguma terrane granitoids. Geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold. Micron-scale (<150 μm) gold identified in the matrix of the conglomerates has a “nuggety” appearance and flakey microtexture indicative of a detrital origin. This observation indicates that the mesothermal deposits were exhumed by the latest Devonian, consistent with regional tectonic syntheses that invoke rapid uplift of the Meguma terrane following peak orogenic activity. This study suggests that favorable depositional environments for paleoplacer deposits may occur in Horton Group rocks that are derived from the Meguma terrane.

Stream sediment geochemistry of Sn, Au and associated elements in Southeast Asia

This paper summarises and reviews some detailed orientation surveys, case histories and experimental studies of the behaviour of cassiterite and gold in streams in SE Asia as a basis for recommendations on the design and interpretation of exploration geochemical surveys for these elements. In the tropical rain forests of SE Asia the frequent rain storms mobilize stream sediments and rapidly flush very fine sand, silt and clay from the stream bed in suspension. This flushing process causes fine heavy minerals such as cassiterite and gold to preferentially accumulate on the bed of the stream. The most consistent and longest geochemical dispersion trains for elements transported as heavy minerals are thus found in the very fine sand fractions « 100 l1m). Use of coarser fractions, collected at heavy mineral trap sites, gives more erratic geochemical anomalies that are often displaced downstream away from their source. During interpretation these displaced anomalies can be recognised by: (i) ratioing Au or Sn values to either abundance of a ubiquitous heavy mineral (e.g., magnetite) or the transport equivalent size of the stream sediment; (ii) by absence of mobile pathfinder elements (e.g., As and base metals); or (iii) from field observations of stream width, velocity, bed roughness and gradient that permit favourable sites for heavy mineral accumulation to be identified. Where removal of the rain forest and conversion ofland to agricultural production greatly increases rates of soil erosion, the accumulations of heavy minerals can be diluted to the extent that Au anomalies may go undetected. Possible answers to this problem are: (i) use of more sensitive analytical procedures; (ii) elimination of the silt-clay fraction and analysis offine sand (53-100 l1m) fraction; or (iii) use of heavy mineral concentrates. Once soil erosion is minimized by re-establishing a cover of ground vegetation, as in mature rubber plantations, fine sediments are again flushed from the stream bed and geochemical anomalies return to more natural conditions with accumulations of heavy minerals on the stream bed.

Transport equivalence of cassiterite and its application to stream sediment surveys for heavy minerals

Accumulation of heavy minerals, such as gold and cassiterite, on stream beds is a well-known phenomenon that results from differences in the rates of transport of the heavy minerals and the sediment as a whole. These accumulations can be both an advantage and a disadvantage to exploration geochemical surveys. Hence, it would be beneficial to be able to recognize such accumulations and, if necessary, correct for them. This, however, requires an improved understanding of the sedimentology of heavy minerals in fluvial channels. In this study we use pit-traps to determine relative rates of transport and estimate transport equivalent sizes of cassiterite and sediment on a coarse sand-gravel stream bed in Malaysia. Results show that for grain sizes finer than about 180 μm (80 mesh ASTM) cassiterite grains have the same average transport rate as sediment grains roughly three times larger. This information can be used to identify and correct geochemical data for Sn anomalies resulting from accumulations of cassiterite that develop on the stream bed in response to the high density of cassiterite and changing hydraulic conditions. In principle the same methodology should be applicable to other heavy minerals.

Transgressions of the Baltic Sea and the pecularities of the formation of transgressive coastal deposits

During the history of the Baltic Sea there have been several transgressive and regressive phases. Due to the uneven tectonic uplift of the Fennoscandian Shield and adjacent areas, the duration and culmination of transgressions in different Baltic Sea areas were not synchronous and the effect of transgressions on the sedimentation was different. Owing to the moderate uplift of the Earth's crust, the shoreline in the Middle Baltic (Estonia, northern Latvia, Central Sweden) was rather stable during the transgressions. Therefore, the transgressive coastal formations are characterized by a greater thickness of deposits and a greater variety and clarity of coastal relief-forms than the regressive ones. In the course of invasion of the sea, the migration of sediments up the shore slope caused more intensive mechanical and mineralogical differentiation in their composition, which, in its turn, brought about an accumulation of heavy minerals in a number of places.

Concentration of Magnetite and Gold at Bar and Reach Scales in a Gravel-Bed Stream, British Columbia, Canada

ABSTRACT A model is presented for placer formation at the grain, bar (10-100 m) and reach scales (> 1000 m) in gravel-bed streams. The model is based on the processes of erosion and re-deposition of the bed during annual flood events and uses Einstein's bedload function to estimate changes in equilibrium transport rates. Heavy mineral enrichment in a gravel bed is postulated to occur in the voids between framework clasts. It is demonstrated that increases in bed roughness (represented by the D65 diameter of material in the bed) result in preferential accumulation of heavy minerals, with enrichment increasing as their density increases. At the reach scale, decreasing channel slope from 0.04 to 0.02 also results in enrichment of heavy minerals. Distribution of gold and magnetite predicted by the model is in good agreement with field data for a gravel-bed stream in British Columbia. Samples were collected from counter-current eddies and other low velocity areas containing sands, at gravel bar mils, and from typical bar-gravel deposits. For the purpose of comparison with the model, the sands were assumed to represent the bulk composition of sediment transported by the stream under conditions of falling discharge after the seasonal snowmelt flood. Trends in magnetite and gold distribution along a 5-km reach can be directly related to the model: gravels are sites of heavy mineral accumulation with gold being enriched to a much greater extent than magnetite. As predicted by the model, density sorting is most obvious for the coars r sand fractions. Changes in channel slope along the reach also result in predicted changes in heavy-mineral distribution.