Heavy Mineral Fraction Research Articles

Rare earth element chemistry of zircon and its use as a provenance indicator

Sedimentary mineral assemblages commonly contain detrital zircon crystals as part of the heavy-mineral fraction. Age spectra determined by U-Pb isotopic analysis of single zircon crystals within a sample may directly image the age composition—but not the chemical composition—of the source region. Rare earth element (REE) abundances have been measured for zircons from a range of common crustal igneous rock types from different tectonic environments, as well as kimberlite, carbonatite, and high-grade metamorphic rocks, to assess the potential of using zircon REE characteristics to infer the rock types present in sediment source regions. Except for zircon with probable mantle affinities, zircon REE abundances and normalized patterns show little intersample and intrasample variation. To evaluate the actual variation in detrital zircon REE composition in a true sediment of known mixed provenance, zircons from a sandstone sample from the Statfjord Formation (North Sea) were analyzed. Despite a provenance including high-grade metasediment and granitoids and a range in zircon age of 2.82 b.y., the zircon REEs exhibit a narrow abundance range with no systematic differences in pattern shape. These evidences show zircon REE patterns and abundances are generally not useful as indicators of provenance.

Chemical and mineralogical forms of lead, zinc and cadmium in particle size fractions of some wastes, sediments and soils in Korea

Forms of Pb, Zn and Cd in the different size fractions (<2 μm, 2–53 μm and >53 μm) of waste dumps, stream sediments and surrounding soils from a former Au mine in Korea, were investigated chemically by sequential extraction analysis and mineralogically by XRD and analytical SEM, so as to clarify the relationships between chemical and mineralogical forms. Total concentrations for the waste dumps and the stream sediments range from 655 to 2920 mg/kg for Pb, 565 to 1191 mg/kg for Zn, and 24.4 to 71.4 mg/kg for Cd, while those for the surrounding soils do not exceed the natural background levels. Direct observations on the heavy mineral fractions of the waste dumps and the stream sediments indicates that the primary sphalerite is still the main pool of the Zn and Cd, while a large part of the primary galena has been changed into a carbonate-bound form. This is in a good agreement with the partitioning of chemical forms in the coarse fractions, in which most of the Zn (75.3 to 79.4% for the waste dumps) and Cd (54.8 to 60.1% for the waste dumps) are associated with the oxidizable form, while most of the Pb (68.8 to 71.0% for the waste dumps) is in the acid (NaOAc)-extractable form. On the other hand, the partitioning of metal forms in the clay fraction is characterised by the highest proportion of the reducible form for all metals (56.6 to 73.8% for Pb, 60.2 to 68.4% for Zn, and 27.1 to 36.8% for Cd in the waste dumps), suggesting precipitation of easily to moderately reducible oxides and hydroxides from the other forms during weathering. With the increase of pH, the dramatic changes of the acid-extractable Pb, the oxidizable Zn and Cd in the coarse fractions, and the exchangeable form, especially for Cd in the clay fraction indicate that pH is the prime factor controlling the partitioning of heavy metals.

Composition and trace-element geochemistry of detrital clay and heavy-mineral suites of the lowermost Amazon River; a provenance study

ABSTRACT Compositions of clays and heavy minerals as well as trace-element geochemistry of the lowermost Amazon sediments reflect source regions and the intense chemical weathering conditions. Illite and montmorillonite indicate mountainous sources under physical weathering conditions, whereas kaolinite is related to the soil composition under humid tropical conditions, where chemical weathering predominates. The suite of unstable heavy minerals, on the other hand, reflects some sediment transport from the Andes, more supply from tributaries draining the basic and metamorphic rocks from the Southeastern Guiana Shield, and strongest influence of the upstream Amazon and Solimoes areas. These source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain. The Brazilian Shield contribution (via the Xingu River) seems to be of minor importance, although the Alter do Chao and the Barreiras Formations, which are products of erosion of the shields, supply suites of stable heavy minerals that reflect recycling. Rare-earth and trace-element data from the clay and heavy-mineral fractions provide additional clues to the source areas: in the downstream direction, superimposed upon provenance of sediments from an undifferentiated arc, the Andes, is provenance from strongly weathered continental crust, the Precambrian shields, as well as sedimentary recycling. Small-scale provenance can be delineated by the most immobile elements. Heavy rare earth elements in the heavy-mineral fraction increase downstream toward Estreitos (to the Para River).

Provenance analysis of lower Paleozoic cratonic quartz arenites of the North American midcontinent region: U-Pb and Sm-Nd isotope geochemistry

Lower Paleozoic supermature quartz arenites in Wisconsin and Michigan were derived from several Proterozoic and Archean terranes. Single-grain, detrital zircon populations from the Galesville and St. Peter Sandstones in Wisconsin yield very similar, closely concordant age distributions (Cambrian Galesville Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 6]). In contrast, most of the nine detrital zircons that were analyzed from the St. Peter Sandstone in the Michigan basin have high U contents (450‐2500 ppm) and are strongly discordant (60%‐90%). Six zircons from the Michigan basin that have ca. 1.0 Ga 207 Pb*/ 206 Pb* ages define a regression line that has an upper intercept of about 1100 Ma and a lower intercept of 15 Ma; one zircon has a 2.7 Ga 207 Pb*/ 206 Pb* age. The zircon data indicate that although a number of different terranes contributed detrital material to the Paleozoic quartz arenites in Wisconsin, 1.1 and 2.7 Ga terranes were the dominant sources, and not the local basement, which primarily consists of the ♢2.7 Ga Marshfield terrane, the 1.8 Ga Penokean orogen, and the 1.4 Ga Wolf River batholith. A terrane that has a 1.1 Ga age is probably the main source for the St. Peter Sandstone in the Michigan basin. Quartz separates were also analyzed for PbPb and Sm-Nd isotope variations, and the data do not indicate significant source differences between the heavy mineral fraction (zircons) and the quartz framework grains. Pb-Pb isochrons and Sm-Nd isotope data for quartz separates reflect mixing of the age groups and approximate relative proportions that are identified from the single zircon results. All isotope data on quartz separates and UPb zircon data indicate that the detrital constituents (zircons and quartz framework grains) that compose the lower Paleozoic quartz arenites in Wisconsin were primarily derived from the 2.7 Ga granite-greenstone terrane of the southern Superior Province and a 1.1 Ga terrane. The latter terrane is either the silicic volcanic rocks associated with the Midcontinent rift system, or, more likely, the voluminous granitic rocks that are associated with the Grenville Province on the eastern margin of North America. The Middle Proterozoic Grenville Province was the most important ultimate source of quartz and zircons to the St. Peter Sandstone in the Michigan basin; lesser amounts of material were contributed from the Archean Superior Province.

Sobre a polêmica da origem do diamante na Serra do Espinhaço (Minas Gerais): um enfoque mineralógico

The origin of the diamond in the Serra do Espinhaco has been the target of exausted discussions for almost 200 years. The enigma about the primary diamondiferous source(s) conducted to several hypotheses, which can be resumed in two antagonic points of view: proximal, inside the sedimentation basin of the Espinhaco Supergroup (Mesoproterozoic), or distal, in the western lying cratonic area (Mesoproterozoic or earlier), peripherical to the sedimentation site, releasing the diamonds during the depositional record. Study of mineralogical features of the diamond and associated minerals, led to new data corroborating the second point of view concerning the origin of the gem. Analysis of the matrix of the diamond-bearing proterozoic conglomerates showed the absence of any mantle source indicators. Study of heavy mineral fractions of alluvial deposits in the same region, conduct to identical results; the analysed garnets are almandines of crustal origin. Mineralogy of representative diamond populations at several areas, show the following conclusive remarks: (1) Large crystals (>lct) are rare, totalizing less than 1%; (2) The predominance (90%) of intact crystals (not broken), outstanding rhombododecahedral, octahedral and transitional habits (>70%); (3) Polycrystaline aggregates are almost absent (<0,1%); (4) Diamond with macroinclusions (seen without a hand loup) are rare (<5%), and (5) The large predominance of gem quality diamonds (78-97%). These features point towards a distal lying source area for the Espinhaco diamonds, outside (extra basin) the depositional site, probably within the Sao Francisco cratonic area.

Geochemistry and provenance of the Altendorf feldspathic sandstone in the Middle Bunter of the Thuringian basin (Germany)

An integrated mineralogical and geochemical study of the Altendorf feldspathic sandstone in the Middle Bunter of the Thuringian basin (Germany) was carried out to obtain more information on its provenance, sedimentological history and geochemistry. The fine- to coarse-grained sandstone has an average composition of 66% (63–71%) quartz, 30% (27–35%) feldspar, and a fine-grained illitic matrix. The heavy-mineral assemblage consists mainly of tourmaline, zircon, rutile, apatite and opaque Fe–Ti minerals. It is inferred that this terrigenous material was laid down at the margin of the Bunter basin. High abundances of igneous quartz grains determined by cathodoluminescence (CL), the lack of metamorphic heavy minerals as well as alkali feldspar with an average K 2O/Na 2O ratio of 14.9 point to material of mostly granitic origin. Regional geologic and paleogeographic observations suggest that granites of the Older and Younger Intrusive Complex of the western Erzgebirge are possible source rocks of the feldspathic sandstone. The geochemical signature of the alkali feldspar (K 2O/Na 2O ratio, BaO content) points to the Bergen and Kirchberg Granites as possible parent rocks. Comparing the mineralogical and geochemical data of the presumed source granites with those of the Altendorf sandstone provides information on the sedimentation processes. Weathering, fluvial transport and sedimentation under arid climatic conditions have caused complete decomposition and removal of plagioclase and biotite, lowered the bulk REE content and led to the development of a positive Eu anomaly. The immobile trace elements (e.g., Ta, Th, Zr, Sc, Co, Ti) are best suited for provenance determination because of their relatively low mobility during sedimentary processes. Elemental ratios such as La/Sc, Th/Sc, Co/Th and Th/Ta have inherited the signature of the parent rocks and thus, may be particularly useful for provenance studies. To evaluate the control of specific minerals and grain-size fractions on the distribution of trace elements in the sandstone, separated fractions of quartz, feldspar, the <20-μm fraction and heavy minerals were additionally analyzed. The concentration densities (elemental concentration in the fraction/fraction weight) reveal that, with exception of Hf, the influence of the heavy-mineral fraction on the whole-rock geochemistry is insignificant although this fraction generally plays the most important role as carrier of trace elements. Quartz, feldspar and the <20-μm fraction have similar concentration densities (between 20 and 40%) for the REE with exception of Eu, which is mainly hosted in feldspar (strong positive Eu anomaly). The influence of the <20-μm fraction on the distribution of the REE and the elements U, Th, Sc, Ta strongly depends on the abundance of this fraction in the sediment. Alkali feldspar contains most of the Ba, Sr, Rb and Cs in the samples (concentration densities of up to 97%). Thus, differences in the abundances of these elements in whole-rock samples can be related to changes in the mineralogical composition (quartz/feldspar ratio) of the sediment.

The significance of various sediment magnetic mineral fractions for tracing sediment sources in Killimicat Creek

The mineral magnetic properties of sediment are increasingly being used to determine the sources of sediment, and associated nutrients and contaminants in drainage basins. This study was undertaken to measure the relative contributions of the magnetic mineral components in sediment (i.e., components associated with surface bound Fe, the heavy mineral fraction, and as inclusions in particles) to determine the extent to which they represent the bulk of the sediment. Deposited channel sediment samples from the tributaries and downstream reach at the first major confluence in the headwaters of Killimicat Creek, New South Wales, were sieved to separate 6 particle size fractions, and the mineral magnetic properties measured to determine the relative contribution of sediment from the smaller tributary basin. The finest sand component (63–125 μm) was then separated into light and heavy mineral fractions, and magnetic measurements made on each fraction. Magnetic parameter data from the 63–125 μm fraction, measured before density separation, show a dominance of sediment (70±12%) derived from the smaller tributary basin. Measurements of the light sediment fraction give a similar result (73±13%). The proportion of the heavy mineral fraction delivered from the smaller tributary is 78±38%. The light mineral fraction of all of the samples were treated with HCl to remove surface bound Fe, and subsequent magnetic measurements show that 41–94% of the mass specific magnetic susceptibility is attributable to magnetic minerals associated with surface bound Fe. The heavy mineral component, while <4% by weight of the 63–125 μm fraction, contributes 5–40% of the total magnetic susceptibility. The results show that most of the magnetic mineral component (>60%) is associated with sediment particles, rather than the discrete heavy mineral component.

Evaluating artifact burial by eolian versus bioturbation processes, South Carolina Sandhills, USA

Artifacts are commonly buried by approximately 50 cm of sediment at prehistoric archeological sites (early Archaic through Mississippian) on uplands of the Sandhills of the upper Coastal Plain of the southeastern United States. Bioturbation, eolian sedimentation, and colluviation are the primary processes that can explain artifact burial because of the upland position of the sites in an erosional landscape setting. Colluvial sedimentation is discounted at most of the sites because they occur on interstream divides and upper hillslope positions. Thus, the focus is on eolian sedimentation versus bioturbation as burial agents. Six sites in the midst of the Sandhills region along the corridor of South Carolina Highway 151 in Chesterfield County provide the data. The Sandhills consist primarily of Cretaceous and Tertiary marine, fluvial, and eolian sediments that are highly dissected and overlie crystalline rocks in the deep subsurface. Two of the sites are on high fluvial terrace remnants that predate 12 ka and serve as controls where bioturbation is the only reasonable burial process. Hillslope positions of the sites are on erosional elements of the landscape (crests, shoulder slopes, and upper backslopes) where sediment transfer operates (colluvial and overland flow), but where deposition is minimal. The sites occur on very sandy soils having a texture of loamy sand to sand. In some instances, a fine textured cover sand, which is about 1.5 m thick, overlies a clayey subsoil or Bt horizon. This cover sand has been interpreted by some as an eolian sand sheet that buries a second parent material and paleosol, but standard particle size and heavy mineral data indicate that it is simply a thick E horizon over a Bt horizon. Standard particle size fractionation at whole phi intervals, and particle size analysis of the heavy mineral fraction, indicate that eolian sedimentation is unlikely at five of the six sites. Heavy minerals were analyzed with respect to the sedimentological principle of hydraulic equivalence, which provides clear separation of eolian versus water-laid sediment. Results of particle size analysis suggest that the cover sands are water-laid (probably fluvial) at five of the six sites, which favors the bioturbation process of artifact burial. Heavy mineral analysis corroborates the standard particle size data, indicating that only one site, 38CT16, possibly is composed of eolian sediment. Soil profile development suggests that the age of the sediment at site 38CT16 probably is older than 12 ka and was in place prior to human occupation. Therefore, possible eolian sedimentation at that site is not relevant to artifact burial, which also suggests bioturbation is the primary process of artifact burial. Additional evidence favoring bioturbation as a vigorous artifact burial process in the Sandhills comes from the two sites on high elevation sandy fluvial terraces (38CT34, 38CT17) where artifacts are also buried. At these terraced sites bioturbation is the only possible burial process. Overall results suggest that bioturbation best explains the occurrence of buried artifacts and that eolian sedimentation processes are not readily apparent, and are not required, in explaining artifact burial. © 1998 John Wiley & Sons, Inc.

PROVENANCE FROM OPHIOLITES AND OCEANIC ALLOCHTONS: MODERN BEACH AND RIVER SANDS FROM LIGURIA AND THE NORTHERN APENNINES (ITALY)

The Northern Apennine arc consists of allochtonous oceanic units (Ligurids) and synorogenic terrigenous wedges, and includes both alpine (Voltri Massif) and apenninic (Bracco Unit) ophiolitic complexes and isolated slabs of oceanic lithosphere (Casanova Complex). Detritus transported by streams in this vast region, comprised between Bologna and Alessandria, and sand of Liguria beaches from Savona to La Spezia can be distinguished - through analysis of framework composition and transparent heavy mineral suites - in four provinces, further subdivided in sub-provinces and variants. The Apennine Province and the Antola Province are carbonaticlastic and characterized by recycling of Ligurid flysches, whereas the Voltri Province and the Bracco Province are dominated by ophioliticlastic detritus. Along the boundaries of major ophiolitic complexes various sub-provinces with mixed provenances characerized by significant ophioliticlastic detritus are recognized (Bormida di Spigno and Polcevera river sands, and Celle-Varazze beach sands for the Voltri Massif; Vara, Lavagna and Ghiaiaro river sands for the Bracco Unit). Ophiolite-derived material dominates the heavy mineral fraction also in the sand transported by major streams draining the Emilia Apennines (Trebbia, Nure, Taro). Framework composition and heavy mineral suites clearly differentiate the serpentineschist- and amphibole-rich (glaucophane, actinolite) detritus derived from alpine ophiolites involved in thick-skinned deformation and high-pressure metamorphism (Voltri Massif), from the serpentinite- and pyroxene-rich (diallage) detritus derived from oceanic lithosphere involved in thin-skinned deformation within the Apennine belt (Bracco Unit; Casanova Complex). In the study of ancient perisutural-basin clastic wedges, these criteria can help reconstructing subduction style and paleogeodynamic evolution of associated suture belts.

Genesis of podzols on coastal dunes in southern Queensland. V. Chemistry and mineralogy of the non-opaque heavy mineral fraction

Eight podzols on coastal dunes from the Cooloola chronosequence, and an associated pedon from North Stradbroke Island (Amity), were studied to establish (i) the degree of homogeneity of the parent material between and within profiles, (ii) the extent of heavy mineral weathering, and (iii) whether the parent sediments of each pedon had a common proximate source. The pedons are Quartzipsamments and Troporthods with ages ranging from Holocene to Pleistocene. On the basis of ratios zircon : rutile, zirconium : titanium, and non-magnetic : very magnetic heavy minerals in the fine sand fraction (53–125 µm), we concluded that the parent materials of the Cooloola pedons were mineralogically similar. By using zirconium in the non magnetic heavy mineral fraction as an index for zircon, it is evident that there has been considerable pedogenetic weathering of the heavy mineral fraction. There is a statistically insignificant difference in hafnium concentrations of zircons which implies that parent sediments of the soils at Cooloola and North Stradbroke Island were derived from a common immediate source.

Provenance of the Palaeocene‐Eocene clastic rocks of the Komandorsky Islands

The Palaeocene–Eocene sequence on the Komandorsky Islands is represented by sediments and minor volcanics with a total thickness of more than 3000 m. Sandstone, siltstone, mudstone, siliceous rocks and conglomerate, which have accumulated in the upper and middle parts of a deep‐water fan, prevail. All the debris was delivered from the north‐east, from the side of the contemporary Komandorsky basin in the Bering Sea. Products of volcanic denudation and fresh tephra dominate its composition. However, at some levels the sandstone contains fragments of jasper, quartzite, slate, schist, gabbro and granite as well as non‐volcanic quartz along with a volcanic component. The heavy minerals fraction of the sandstone contains considerable quantities of apatite, zircon, garnet and chromite, as well as pyroxenes, amphiboles and magnetite. The chemical composition of the garnet is specific to metamorphic rocks. A significant admixture of chromite is indicative of denudation from an ophiolite complex. The folded and partly metamorphosed pre‐Palaeogenic rocks were a source of these fragments. Their most likely source terrane was the Shirshov‐Bowers chain (Cretaceous palaeoarc). The Komandorsky block originally was the forearc part of the Aleutian arc where the latter was superimposed on the Shirshov‐Bauers chain. A subsequent north‐west displacement of forearc structures, in response to the transform fault at the boundary between the Pacific and North American plates, could have resulted in the separation of the Komandorsky block from its Early Palaeogene debris source area.

Geochemistry and petrology of recycled orogen-derived sediments: a case study from Upper Precambrian siliciclastic rocks of the Central Iberian Zone, Iberian Massif, Spain

Two areas of Upper Precambrian siliciclastic rocks separated from each other by 150 km were selected in the Central Iberian Zone (CIZ) for a petrological and geochemical study. The petrology of the coarser-grained rocks indicates the absence or scarcity of igneous clasts, the presence of extrabasinal and intrabasinal clasts and homogeneous features of sandstones ranging from very-fine to fine-grained. The modal data of these rocks plot close to the Qt-L and Qm-Lt sides of QtFL and QmFLt triangular diagrams, suggesting provenance from a recycled orogen. The geochemical data on the shales and sandstones from the two areas studied are very uniform. Both groups of rock show similar ranges of relevant major element ratios such as Al 2O 3 TiO 2 (17.46–21.51; 17.46–21.34, for shale and sandstone, respectively), Al 2O 3 FeO T (2.40–3.79; 2.18–3.33) and FeO T FeO T + MgO (0.71–0.77; 0.68–0.76), together with parallel average rare earth element (REE) patterns and similar values for some key trace element ratios ( La Th , Th Yb , Th U , Zr Hf , Sm Nd and Cr Sc ). Other trace element ratios ( La Sc , Th Sc , Cr Th and Cr Zr ) are uniform in the shales group and also in the sandstones group, although minor differences become apparent when both groups are compared. These differences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and, indirectly, the Sc and Cr contents, given their covariations with Al 2O 3. The Zr Yb , Zr Y and Zr Th ratios of the shales are uniform; the sandstones show higher and more variable values, and there is no correlation between Zr abundance and any of these ratios in the shales or sandstones. Together with the parallel REE patterns shown by both groups of rock, these results suggest the absence of heavy mineral fractionation during the transport of detrital material and/or the relative importance of other minerals, probably phyllosilicates, in the control of Yb, Y and Th abundances. Apparently, all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling areas. Thus, the petrological and chemical data are consistent and point to a very extensive, homogeneous and stable source region as the most plausible contributor to the Upper Precambrian siliciclastic rocks in the CIZ. The possible relationship between this zone and N Gondwana is also discussed.

Geochemistry of Spodosols formed in holocene till, Norra Storfjället Massif, northern Sweden

Silt and clay size fractions of soils, from a transect of six Spodosols formed in the Norra Storfjallet Massif, were analyzed by neutron activation to determine the degree to which pedogenic processes have influenced the distribution of macro, micro and trace elements. The distributions of Mg, Ca and Fe, together with Co, Cr and other trace elements in the profiles, suggest the presence of different parent materials, with A and E horizons arising from an influx of aeolian sediment. Translocation processes, both physical and chemical, occurred in the soil concentrating Fe and Br in the spodic (Bs) horizons of the profiles. The rare earth elements (REEs) are predominantly associated with the heavy mineral fraction of the soil material. The distributions of chondrite normalized REEs patterns of the profiles indicate that light rare earth element (LREE) concentrations increase with horizon depth. The depletion of LREEs in the upper soil horizons confirms the presence of material that is chemically different from that in the lower horizons, thus indicating a distinct chemical difference from the local glacial deposits.

Separation of heavy minerals from mudrocks

The non-clay-mineral fraction of mudrocks is receiving increased attention. The heavy-mineral fraction in fine-grained rocks, however, has largely been neglected, partially because of the difficulty in separating heavy minerals from the host mudrock. We propose a technique that will separate some heavy minerals from a clay matrix. This method was developed initially by soil scientists for separating quartz and feldspar from clays, and later modified for mudrocks. The technique involves fusing the rocks with sodium bisulfate, making clay minerals soluble in basic solutions. We have tested this procedure on nine common heavy minerals: andradite, almandine, grossular, zircon, magnetite, schorl, apatite, rutile, and a gem variety of tourmaline. Each mineral was fused and the mass reduction and resultant chemical changes observed. Physical grain alterations were examined by SEM and chemical changes analyzed by electron microprobe. Almandine, grossular, and zircon survived the fusion process in high percentages and showed little chemical change. Andradite, although it showed small mass loss, was highly etched by the precess. Schorl, gem tourmaline, magnetite, and ruffle showed a high mass loss with some chemical change to the residue. Apatite was totally dissolved. The chemical resistance of several heavy-mineral species makes this technique useful for provenance studies of mudrocks, especially when combined with chemical studies of heavy minerals.

Zincian staurolite in the Dry River South volcanic-hosted massive sulfide deposit, northern Queensland, Australia: an assessment of its usefulness in exploration

Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens. In the footwall alteration zone, staurolite ZnO levels vary between 0.4 and 1.3%, whereas in hanging wall metagraywackes and metapelites, the ZnO content is mainly below 1.0%. As staurolite from metapelitec rocks contains up to 7.9% ZnO, high ZnO levels in staurolite do not necessarily indicate a relationship to Zn-rich massive sulfide. Staurolite grains from the Dry River South amd other massive sulfide lenses have low TiO 2 concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO 2 in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO 2 values are indicative of staurolite associated with Zn-rich massive sulfide. Zincian staurolite is a potential exploration indicator at both prospect and reconnaissance scales. At the prospect scale, zinc levels have potential in distinguishing true from false gossans, distinguishing low grade portions of massive sulfide lenses from barren massive pyrite bodies, and characterizing alteration zones. At the reconnaissance scale, staurolite can be collected in the heavy mineral fraction of stream sediments, and multiple grains can be analyzed rapidly using modern electron microprobes.

Sulphide speciation in surface sediments of the Lagoon of Venice: A geochemical and mineralogical study

Three 1-m cores of recent sediments from the Lagoon of Venice were sampled to obtain the vertical distribution of Fe, Zn, Cu, Pb and Cd, and to study authigenic iron sulphides from the geochemical and mineralogical viewpoints. Zn, Cu, Pb and Cd occur in superficial sediments in concentrations many times higher than those of their natural background; the existence of ZnCd and CuPb couples were revealed by their very similar depth profiles. The latter couple is also closely correlated with sulphide-sulphur. Iron is characterized by quite a different behaviour, showing a sharp increase at a depth of 35 cm. From a geochemical viewpoint, authigenic sulphides were distinguished into pyrite and Acid Volatile Sulphides (greigite and mackinawite). Their distribution is characterized by high concentrations of AVS in the first 25 cm of sediments, dramatically decreasing with depth. Pyrite, present in lower amounts, shows the opposite trend, although in one core both species were present in comparable amounts. Mineralogical study of the heavy mineral fraction revealed that the iron sulphides are composed of pyrite (organised both in the classic framboidal growth and in unordered micro-crystal swarms) and iron “mixed sulphide” aggregates, with variable optical patterns and colloform, botroidal or massive growth. No heavy metals were detected in the crystalline lattice of the sulphides. Down to 25 cm all types of sulphides occur, and with increasing depth pyrite clearly prevails. This behaviour may have various causes, e.g. chemico-physical control by specific microniches, temporal and spatial variations favouring the formation of different types, evolution of iron mixed sulphides into more stable forms.

Silt fraction heavy-mineral distributions in a lateritic environment: the rivers and insular shelf of north-central Puerto Rico

Mineralogical studies of sediments from the rivers and insular shelf in north-central Puerto Rico examine the effects of lateritic weathering and assess the silt fraction for economically important heavy minerals. This fraction, which is enriched in heavy minerals relative to the sand fraction, is mainly detrital but contains a strong authigenic component. The detrital silt heavy-mineral fraction in the rivers is dominated by an amphibole-garnet-pyroxene-epidote assemblage. Amphiboles are more abundant in the Rio Cibuco; pyroxenes are more abundant in the Rio de la Plata; and epidote and ilmenite are more abundant in the Rio Grande de Manati. The authigenic silt heavy-mineral fraction is largely a product of the lateritic weathering and dominated by iron oxides and alterites. Grains of bladed rutile and leached ilmenite are common. Spatial variability in silt-fraction mineralogy is considerable. Within the Rio Cibuco system variability is related to compositional differences in rapidly eroding source rocks. On the shelf, silt heavy-mineral abundances are greatest at the river mouths and decrease seaward. Variability in the shelf samples is controlled primarily by source rivers and shelf sorting processes. Compositional differences exist between the silt heavy-mineral assemblages in the rivers and the shelf, and between the heavy-mineral assemblages in the silt- and sand-sized fractions from these areas. Minerals of economic importance that occur in the silt fraction within the study area include cerargyrite, chromite, gold, magnetite, and rutile.

Radiometric fingerprinting of sediments on the Dutch, German and Danish coasts

Concentrations of natural radionuclides in the heavy-mineral fraction of coastal sands have been shown, in earlier work, to be distinctly different for two regions along the Dutch coast. Because these two regions have different source areas, it was concluded that concentrations of natural radionuclides in the heavy-mineral fraction of coastal sediments represent a radiometric fingerprint. This fingerprint is considered to be a sensitive tool for identifying sediments of different provenance. To further investigate this assumption, samples were collected along the North Sea and Baltic coasts of the Netherlands, Germany and Denmark, as an extension of the earlier work. In the present investigation, we find that two additional source areas may be identified using the radiometric fingerprint. The four regions correspond with glacial terranes of Saalian and Weichsellian age, and the present-day watersheds of the Elbe and Rhine river systems.

The Occurrence of Vitric Tuff in Quaternary Deposits of Gljev in Dalmatia (Southern Croatia)

For the first time in the Dinaric region of Dalmatia vitric tuff has been discovered in the clayey Quaternary deposits. Petrographic, heavy mineral and X-ray analyses revealed fine volcanic ash, illitic material (as an alteration product of glass shards), rare feldspar grains and an abundance of monoclinic pyroxene in the heavy mineral fraction. Pyroclastic quartz grains were not observed. It is postulated that explosive volcanism of basic or neutro-basic character occured and the finest particles were transported over a large distance by wind. From the heavy mineral composition the investigated tuff differs from the underlying and overlying Quaternary clay sediments, as well as from the tuffs found in Neogene deposits of Central Dalmatia.

Platinum distribution in soil profiles of the Tulameen ultramafic complex, southern British Columbia

Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium associated with the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occurrences consist of massive-to-discontinuous segregations of platiniferous chromitite. Platinum content of the −70 mesh fraction of soils and sediments was determined by lead fire assay-inductively coupled plasma emission spectrometry. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Platinum concentrations in the −70 mesh fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Platinum content of heavy mineral (SG > 3.3) fractions is 10–20× greater than in light mineral fractions. Platinum is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. Scanning electron microscopy and microprobe studies of heavy fractions from C-horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter are relatively more abundant in the magnetic fraction and are interpreted as Pt-poor grains disseminated throughout the dunite. The Mg-Cr-rich chromite fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The −270 mesh fraction or the magnetic heavy fraction of C-horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast in Pt concentrations, more limited dispersal and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable medium for detailed geochemical sampling.