The identification of various types of geochemical stream sediment anomalies in Northern Ireland

A regional geochemical stream sediment survey of part of Northern Ireland led to the recognition of three types of base metal anomaly of exploration interest. These were significant anomalies of both elastic and hydromorphic origin, and non-significant hydromorphic anomalies in several upland areas. The significant anomalies are derived from Pb-Zn-Cu mineralization, whereas the non-significant anomalies are due to the leaching of Zn and other metals from acidic and/or gleyed soils and subsequent concentration in stream channels by coprecipitation with manganese oxides. Significant hydromorphic anomalies can be identified on the basis of abnormal Zn/Mn ratios and significant elastic anomalies on the basis of anomalous Pb contents. It is of the utmost importance to distinguish the dispersion mechanisms. Regional variations in bedrock geochemistry may only be identified when consideration is given to local dispersion mechanisms and account taken of the effects of variations in the surface environment on stream sediment composition. Changes in vegetation and land use may lead to rapid alterations in the nature of the dominant dispersion mechanisms.     

Geochemical exploration at yandera porphyry copper prospect, Papua New Guinea

A case history is presented describing geochemical exploration of the porphyry Cu-Mo system at Yandera in the Bismark Ranges of Papua New Guinea. Three phases of geochemistry are discussed: (1) stream sediment, (2) ridge and spur, and (3) detailed rock and soil from contour trails. Results of each phase are presented and their relationship to drill-indicated mineralisation discussed. The effectiveness of the three techniques and various elements as geochemical guides to primary mineralisation is compared.Copper in stream sediment samples at a density of one sample per 1–2 km² effectively delineated the porphyry Cu system. The extent of ridge and spur sampling was limited, therefore its effectiveness is uncertain. Detailed sampling along contour trails indicated that Au and Mo are the most effective geochemical tools. Copper geochemistry is of limited use as its distribution is largely a function of recent processes and dispersion from supergene mineralisation. No clear relationship exists between Ag geochemistry and mineralisation. Lead and Zn are distributed peripheral to the porphyry Cu system.The effectiveness of Au and, to a lesser extent, Mo as geochemical guides to ore in the steep mountainous terrain of the prospect area where high rainfall, deep weathering and rapid erosion prevail is due to the relatively high stability of these metals in the soils and the oxide zone.     

Contrasting dispersion patterns for gold in stream sediments at Timbarra,NSW, Australia

Regional geochemical exploration for granite-hosted Au mineralisation on the Timbarra Plateau has identified poor correlation between bulk cyanide leach (BCL) Au contents in the wet-screened <2000-μm fraction and aqua regia (AR) extractable Au in the wet-screened <180-μm fraction of stream sediments across most parts of the plateau.At James East, there is a weak response to mineralisation in both the BCL and <180-μm fraction Au contents of stream sediments and poor correlation between these fractions. The total Au content of the sediments is primarily contained within the organic-rich <63-μm fraction. There is little visible Au in panned concentrates from stream sediments or adjacent alluvium. Dispersion of Au appears to be predominantly hydromorphic with minor mechanical contributions from a more diffuse palaeo-colluvial source.At the nearby Surface Hill, there is a strong response to mineralisation in stream sediments with contributions to total Au from a wide range of size fractions. The BCL and <180-μm fraction Au contents are closely correlated. Visible Au is present close to bedrock sources in eluvial and alluvial accumulations. Particle size and Au distribution in the stream sediments indicates dispersion of Au to be dominantly by mechanical processes from the proximal part of a colluvial fan on the margin of the plateau.Variations in the concentration, form and distribution of Au in stream sediments at the two sites can be attributed to variations in surficial conditions and the effects of varying amounts of organic material; however, a complimentary factor is the potential loss of colloidal or organic-adsorbed Au through decanting of sieve water during wet-screening of sediments in the field.     

Statistical evaluation of stream sediment geochemistry in interpreting the river catchment of high-grade metamorphic terrains

Stream sediment geochemistry is extensively used in mineral exploration and environmental studies. However, quantitative assessments of the effectiveness of stream sediment geochemistry for describing upstream lithologies are rare, especially in high-grade metamorphic terrains. This study statistically evaluates whether stream sediment geochemistry can aid in recognizing variations in upstream geology in several high-grade metamorphic lithotectonic units having different metamorphic and tectonic histories, including the Highland Complex, Vijayan Complex, Wanni Complex, and Kadugannawa Complex of Sri Lanka. For this study, concentrations of 21 elements were measured in 2080 stream sediment samples collected from the Walawe River, Maha Oya, Gala Oya basins located on above lithotectonic units and Uma Oya, Belihul Oya, Badulu Oya basins situated adjacent to each other on the Highland Complex. These rivers flow across dry, intermediate and wet zones of Sri Lanka, with river courses having both slope (> 20°) and flat (< 20°) areas. Elemental concentrations, averaged over each river basin, show patterns of enrichment and depletion which may relate to localized mineralization conditions, local lithological changes, anthropogenic activities and environmental factors such as local variations in climate and morphology among river basins. Comparison of element concentrations in sediments from the four different lithotectonic units shows that enrichment – depletion patterns can be partly related to rock geochemistry of the associated lithotectonic unit. However, climate and basin morphology also seem to play an important role. Results of Kruskal–Wallis H tests show that both major and trace element levels in sediments from the four different lithotectonic units, as well as from adjacent Uma Oya, Badulu Oya and Belihul Oya basins, are significantly different. Discriminant function analysis appropriately classifies sediments into the four different lithotectonic units with an accuracy of 91.9%. This method also classifies sediments into river basins which share common lithology being situated adjacent to each other in the same lithotectonic unit with an accuracy of 89.5%. This strongly suggests that stream sediment geochemistry is capable of describing the upstream regional scale as well as local scale lithological changes at a great accuracy in complex high-grade metamorphic terrains. In both cases use of channel slope and basin climatic zone as additional variables does not significantly increase overall or individual accuracy in classification.     

A comparison of selective extraction soil geochemistry and biogeochemistry in the Cobar area, New South Wales

In parts of the deeply weathered and semi-arid environments of the Cobar area (NSW, Australia), detection of mineralisation using conventional soil sampling and total metal analysis is impeded. This is due to the intense leaching of trace elements within the weathered profile, discontinuous coverage of transported materials and the existence of diffuse regional geochemical anomalies of ill-defined source. Selective chemical extractions, applied to various regolith components, and biogeochemistry offer a means of isolating localised geochemical patterns related to recent dispersion of trace elements through the overburden. Lag geochemical patterns across the McKinnons deposit (Au) and Mrangelli prospect (Pb–Zn–As) reflect mechanical dispersion processes and minor hydromorphic effects. Concentrations of more mobile elements tend to be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematite than in the magnetic fraction. The subdued soil geochemical responses for metals extractable by cold 40% hydrochloric acid (CHX) and for total element concentration reflect the leached nature of the residual profile, low grade of mineralisation, dilution by aeolian components and disequilibrium of fine fractions with coarser, relict Fe-oxides. The stronger contrast for CHX for most metals, compared with total extraction, indicates surface accumulation of trace elements derived from underlying mineralisation. Enzyme leach element anomalies are intense but generally located directly over bedrock sources or major structural breaks, irrespective of the nature of the overburden. Though mechanisms for the dispersion of trace elements extracted by enzyme leaching are not well established, the lack of lateral transport suggests vertical migration of volatile metal species (atmimorphic dispersion). The strong, multi-element response to mineralisation in cypress pine needles indicates significant metal recycling during the present erosional cycle. However, a comparison of the trace element concentrations in vegetation (cypress pine needles) and selective extractions of soils indicates that recycling by the plants is not the dominant mechanism for transportation of metals through the overburden. The vegetation may be responding to hydromorphic dispersion patterns at depth. The use of selective extractions may be useful in detecting mineralisation through deeply leached profiles, but offers even greater potential when integrated with biogeochemistry to detect targets buried by significant thickness of transported cover.     

Small-scale heterogeneity in the geochemistry of seagrass vegetated and non-vegetated estuarine sediments: causes and consequences

In addition to nutrient and light availability, sedimentary biogeochemical processes can play an essential role in seagrass productivity. Previous investigations of the interactions between seagrasses and their underlying sediments have failed to clearly identify the spatio-temporal variability of the major geochemical parameters involved. Dissolved and solid phase chemical parameters in eelgrass vegetated and nearby non-vegetated sediments were investigated in this study to determine their vertical, lateral, and temporal distributions. Solid-state microelectrodes were used to investigate dissolved O₂, ΣH₂S, Fe²⁺, and Mn²⁺ on mm space scales. In this study, spatial heterogeneity was assessed and diurnal “ventilation” by seagrass productivity (i.e., the translocation of photosynthetically produced oxygen to the anoxic sedimentary environment) was not observed probably because benthic infaunal activity (bioturabation and bioirrigation) and microzones established by microbial processes led to highly heterogeneous sediment geochemistry where temporal variability was obscured by small-scale spatial variability. Non-vegetated sediments were less geochemically variable laterally than vegetated sediments, however, in some cases, they had similar vertical variability, possibly because they had been vegetated at an earlier time. This study demonstrates that in vegetated sediments where there is also substantial benthic macrofaunal activity it is difficult to separate the impacts of the two types of biota on sediment geochemistry and their spatial patterns, and it also raises the question of the applicability of traditional one-dimensional diagenetic models for such spatially–temporally complex sediments.     

The geochemistry of the sedimentary rocks in the vicinity of the Tom Pb-Zn-Ba deposit, Yukon Territory, Canada

A total of 138 samples of the Devonian sediments in the vicinity of the Tom stratiform Pb-Zn-Ba deposit were analysed for major elements and 16 minor and trace elements by X-ray fluorescence.The geochemistry of the footwall argillites is characterised by a concentration of elements that are typically associated with the detrital resistate minerals and feldspars (e.g. Al₂O₃, Na₂O, K₂O, TiO₂, Ce, Nb, Zr), which are contained within the interbedded silty layers of probable distal turbidite origin.The hanging-wall shales are characterised by high V concentrations. The C-organic data and the V/Cr ratios suggest that sapropelic conditions may have been locally developed in the vicinity of the West zone mineralisation and in the hanging-wall shales. Very high concentrations of Ba were found to be present in the hanging-wall shales (>0.5% Ba).Zn is more widely dispersed than Pb in the sediments around the mineralisation. There is no marked enrichment of Fe, Mn or Cu in the sediments close to the mineralisation.     

Pollen taphonomy in a canyon stream

Surface soil samples from the forested Chuska Mountains to the arid steppe of the Chinle Valley, Northeastern Arizona, show close correlation between modern pollen rain and vegetation. In contrast, modern alluvium is dominated by Pinus pollen throughout the canyon; it reflects neither the surrounding floodplain nor plateau vegetation. Pollen in surface soils is deposited by wind; pollen grains in alluvium are deposited by a stream as sedimentary particles. Clay-size particles correlate significantly with Pinus, Quercus, and Populus pollen. These pollen types settle, as clay does, in slack water. Chenopodiaceae-Amaranthus, Artemisia, other Tubuliflorae, and indeterminate pollen types correlate with sand-size particles, and are deposited by more turbulent water. Fluctuating pollen frequencies in alluvial deposits are related to sedimentology and do not reflect the local or regional vegetation where the sediments were deposited. Alluvial pollen is unreliable for reconstruction of paleoenvironments.     

Seasonal Influence of the Needle Rush <Emphasis Type="Italic">Juncus roemarianus</Emphasis> on Saltmarsh Pore Water Geochemistry

Previous studies have shown that saltmarsh macrophytes have a significant influence on sediment biogeochemistry, both through radial release of oxygen from roots and also via primary production and release of labile organic exudates from roots. To assess the seasonal influence of the needle rush, Juncus roemarianus, on saltmarsh sediment geochemistry, pore waters and sediments were collected from the upper 50 cm of two adjacent sites, one unvegetated and the other vegetated by Juncus roemarianus, in a Georgia saltmarsh during winter and summer. Pore waters collected at 1- to 2-cm intervals were analyzed for pH, alkalinity, dissolved phosphate, ammonium, Fe(II), Fe(III), Mn(II), sulfide, sulfate, and organic carbon. Sediments were collected at 5-cm intervals and analyzed for iron distribution in the solid phase using a two-step sequential extraction. The upper 50 cm of the sediment pore waters are mostly sulfidic during both winter and summer. The pore water and sediment geochemistry suggest organic matter degradation is coupled mostly to Fe(III) and sulfate reduction. In summer, there is greater accumulation of alkalinity, sulfide, ammonium, and phosphate in the pore waters and lower levels of ascorbate extractable Fe, which is presumed to be comprised primarily of readily reducible Fe(III) oxides, in the sediments, consistent with higher organic matter degradation rates in summer compared to winter. Lower pH, alkalinity, ammonium, and sulfide concentrations in sediments with Juncus, compared to nearby unvegetated sediments, is consistent with release of oxygen into the Juncus rhizosphere, especially during summer.     

Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

Distinguishing between natural and anthropogenic sources of metals entering the Irish Sea

International agreements (e.g. OSPAR) on the release of hazardous substances into the marine environment and environmental assessments of shelf seas require that concentrations and bioavailability of metals from anthropogenic sources can be distinguished from those originating as a result of natural geological processes. The development of a methodology for distinguishing between anthropogenic and natural sources of metals entering the Irish Sea through river inputs is described. The geochemistry of stream, river and estuarine sediments has been used to identify background geochemical signatures, related to geology, and modifications to these signatures by anthropogenic activities. The British Geological Survey (BGS) geochemical database, based on stream sediments from 1 to 2 km² catchments, was used to derive the background signatures. Where mining activity was present, the impact on the signature was estimated by comparison with the geochemistry of sediments from a geologically similar, but mining free, area. River sediment samples taken upstream and downstream of major towns were used respectively to test the validity of using stream sediments to estimate the chemistry of the major river sediment and to provide an indication of the anthropogenic impact related to urban and industrial development. The geochemistry of estuarine sediments from surface samples and cores was then compared with river and offshore sediment chemistry to assess the importance of riverine inputs to the Irish Sea. Studies were undertaken in the Solway, Ribble, Wyre and Mersey estuaries. The results verify that catchment averages of stream sediments and major river samples have comparable chemistry where anthropogenic influences are small. Major urban and industrial (including mining) development causes easily recognised departures from the natural multi-element geochemical signature in river sediment samples downstream of the development and enhanced metal levels are observed in sediments from estuaries with industrial catchments. Stream sediment chemistry coupled with limited river and estuarine sampling provides a cost-effective means of identifying anthropogenic metal inputs to the marine environment. Investigations of field and laboratory protocols to characterise biological impact (bioaccumulation) of metals in sediments of the Irish Sea and its estuaries show that useful assessments can be made by a combination of surveys with bioindicator species such as clams Scrobicularia plana, selective sediment measurements that mimic the ‘biologically available’ fractions, and laboratory (mesocosm) studies.     

In-channel woody vegetation controls on sedimentary processes and the sedimentary record within alluvial environments: a modern example of an anabranch of the River Loire, France

A multidisciplinary approach was used to analyse the role of pioneer trees (Populus nigra) on bar formation in the modern fluvial system of the River Loire (France). Data were collected on seven plots located on a secondary channel, chosen for their vegetation cover and their sedimentary context. Physical features of vegetation, topographical evolution, sediment grain size and flow velocity were investigated during and after floods between 2001 and 2003. Woody vegetation responds morphologically to fast‐flowing waters and sediment deposition. These adaptations influence the role played by trees on local sedimentary processes. Some morphological types of trees exert a strong influence during flood events by deflecting, reducing or increasing the flow energy. As a consequence, the evolution of bedforms, distribution of sediment grain size and slopes are controlled by the flood type, the morphological context (i.e. high‐ or low‐energy zones), the morphology of the vegetated bar and the characteristics of vegetation. Conceptual models are proposed to account for local processes and evolution of tree groves. The first model, which describes the behaviour of groves during a single flood, underlines the sediment deposition downstream of the trees and the trapping of bedload sediments. During the falling stage of the flood, the reorientation of current streams, constrained by both local bed topography and woody vegetation, induces transverse sediment fluxes. For these water levels, the control exerted by trees on small‐scale sedimentary processes becomes significant. The second model shows the evolution of vegetation bands over several flood events. In particular, it describes the influence of vegetation on the fixing and reworking of bedload sediments during the falling limb of the hydrograph. The coalescence of vegetation‐induced bedforms and the resulting morphological changes increase the deflection power of the vegetated bar, inducing the deposition of finer sediments.     

Geochemical prospecting for stratabound mineralization in late Precambrian sediments of East Greenland (72–74°N)

A geochemical investigation was carried out mainly in a 14-km thick sedimentary sequence of late Precambrian age. The project included analysis of rock samples and drainage samples for Cu, Co, and Zn. The drainage samples were heavy-mineral concentrates and stream sediment samples.The results for the drainage sediments distinguish three well defined cupriferous horizons in the sedimentary column. Lithogeochemical studies of quartzites, pelites, and carbonates confirm that the anomalous values obtained in the drainage sediments arise from several Cu-mineralized horizons.The study has shown that geochemical prospecting appears to be well suited to Cu prospecting in Central East Greenland under arctic climate conditions even with a low sampling density of one sample per 5–10 km² for the drainage samples, and one sample per 10–100 m of the sedimentary stratigraphic column for the rock samples.     

Lipid geochemistry of Lake Kinneret

Lake Kinneret, a relict lake from the Neogene, is characterised by the dominance among its phytoplankton of the dinoflagellate Peridinium cinctum. The lipid geochemistry of Lake Kinneret is discussed herein in terms of the biology, chemistry and hydrology of the lake. Lipids isolated from two sediment sections (surface and 15 cm deep), obtained from the deepest point of Lake Kinneret, include: (1) 4α-methyl-5α (H)-stanols and related derivatives characteristic of P. cinctum, the novel sterol 4a-methylgorgosterol, and peridinosterol and 4α-methylgorgostanol, not previously reported to occur in lacustrine sediments; (2) C₃₀ and C₃₂ alkane-1,15-diols, not previously reported to occur in contemporary lacustrine deposits, and (3) products of early diagenesis. Many similarities were observed with the more widely studied marine dinoflagellates and marine sediments with dinoflagellate input.     

Spinifex biogeochemistry across arid Australia: Mineral exploration potential and chromium accumulation

Spinifex (Triodia spp.) grasslands cover vast areas of arid Australia, across a variety of soils and landscapes. These grasses are deep rooted and long lived, hence have great potential as a biogeochemical sampling medium for mineral exploration. This study discusses the results of analyses of Triodia pungens and Triodia scariosa from field sites over buried Au mineralisation (Coyote, Oberon and Tunkillia Prospects). At each site there is a multi-element anomaly in the vegetation over the projected mineralisation, the haloes are of different scales depending on the local landscape setting and dispersion potential of each element associated with mineralisation. The magnitude of the anomalies is similar for each site independent of underlying substrate. Overall, spinifex chemical composition has the potential to act as a point indicator of substrate geochemistry with very minimal dispersion (hundreds of metres only) that can delineate the extent of a potential ore deposit.This study also discusses the Cr accumulation potential of T. pungens and T. scariosa, discovered during the mineral exploration studies, from several field sites (Coyote, Oberon, Tunkillia and North Miitel Prospects). Triodia species are shown to be able to accumulate Cr up to potentially toxic levels independent of substrate concentration. This could be due to accumulation (active transport) or the lack of a barrier mechanism (passive uptake) within the plant.     

Carbon Geochemistry of Gas Hydrate‐associated Sediments in the Southwestern Taiwan Basin

Marine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate-bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973-3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon(DIC) and high total organic carbon(TOC)(mean = 0.46%) originated from terrigenous organic matter(mean δ~(13)C_(TOC) value of-23.6‰) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon(IC), and the depleted δ~(13)C_(IC), coupled with the increase of TOC and the decrease of δ~(13)C_(TOC) values owing to the methanotroph ‘Methanosarcinales/ANME' in 430–840 cm. Environmental factors and archaeal communities in core 973-3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate-associated sediments.     

Triterpenols from sediments of Santa Monica Basin, Southern California Bight, U.S.A.

The triterpenol geochemistry of the Santa Monica Basin from the Southern California Borderland, off the U.S.A., is described from the study of two sets of trap deployments, five box cores (≈30 cm) and a hydroplastic core (≈1 m). The biogenic sources and diagenetic stability of the triterpenols are discussed.The 17β(H), 21β(H)-hopanols (22R isomer) occur in the carbon number range from 30 to 32 and their abundance is nearly uniform in the shallow sediment sections. However, the three hopanols follow the order of abundance, C₃₂ > C₃₁ C₃₀, in deeper sections. Their concentrations spans from trace levels to 156 μg/g organic carbon (<15 ng to 7 μg/g dry sediment). Tetrahymanol (gammaceran-3β-ol) has been identified in all the samples except in one set of trap particles collected at 100 m water depth, from trace level (<1 μg) to 215 μg/g organic carbon (<20 ng to 9 μg/g dry sediment). Diplopterol is also detected in trace amounts in some samples. The triterpenols in the trap material generally increase with the water column depth and decrease with the subbottom depth in the sediment cores.The extended hopanols are either degradation products of polyhydroxybacteriohopanes or are biosynthesized by bacteria. Tetrahymanol is probably the only suggested biological precursor of gammacerane (the reduced counterpart of tetrahymanol), which has been recognized in numerous crude oils and lithified sediments. Although it has been reported earlier from Green River Shale and from a residual Pleistocene lake sediment, tetrahymanol has so far been positively identified from recent marine sediments only in two recent studies. The decreasing content of tetrahymanol in sedimentary depth profiles in the Santa Monica Basin would favor an origin for this compound in the water column or at the sediment surface. The ubiquitous occurrence of this compound throughout the study area suggests that this triterpenol most probably originates from primitive organisms (protozoa, bacteria?), hitherto not identified or, more likely, not yet analyzed for their lipid composition.     

Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits, 1. Lady Loretta Deposit, Queensland

Stratiform sediment hosted Zn–Pb–Ag deposits, often referred to as SEDEX deposits, represent an economically important class of ore, that have received relatively little attention in terms of defining lithochemical halos and geochemical vectors useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. We examined the major and trace element chemistry of carbonate-bearing sediments surrounding the deposit and defined a series of halos which extend for several hundred metres across strike and up to 1.5 km along strike. The stratiform ore lens is surrounded by an inner sideritic halo [Carr, G.R., 1984. Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn–Pb–Ag deposit, North Queensland. J. Geochem. Expl. 22, 217–238], followed by an outer ankerite/ferroan dolomite halo which merges with low iron dolomitic sediments representative of the regional background compositions. Carbonate within the inner siderite halo varies in composition from siderite to pistomesite (Fe_0.6Mg_0.4CO₃), whereas carbonate in the outer ankerite halo varies from ferroan dolomite to ankerite (Ca_0.5Mg_0.3Fe_0.2CO₃). Element dispersion around the stratiform ore lens is variable with Pb, Cu, Ba and Sr showing very little dispersion (<50 m across strike), Zn and Fe showing moderate dispersion (<100 m) and Mn and Tl showing broad dispersion (<200 m). Within the siderite halo Cu, Mg and Na show marked depletion compared to the surrounding sediments. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta orebody has enabled the development of three geochemical vectors applicable to exploration. Whole rock analyses are used to calculate the three vector quantities as follows: (1) SEDEX metal index = Zn + 100Pb + 100Tl; (2) SEDEX alteration index = (FeO + 10MnO)100/(FeO + 10MnO + MgO); (3) manganese content of dolomite: MnO_d = (MnO × 30.41)/CaO. All three vectors increase to ore both across strike and along strike. The manganese content of dolomite (MnO_d) exhibits the most systematic pattern increasing from background values of about 0.2 wt% to a maximum of around 0.6 wt% at the boundary between the ankerite and siderite halos. Siderite within the inner halo contains considerably more Mn with MnO values of 0.4 to 4.0 wt%. It is suggested here that the basket of indices defined at Lady Loretta (Zn, Tl, metal index, alteration index, MnO_d and MnO_s) is applicable in the exploration for stratiform Zn–Pb–Ag deposits in dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units favourable for SEDEX mineralisation, and secondly provide vectors along these units to ore. The alteration index and MnO_d, however, should only be used for exploration dolomitic sequences; they are not recommended for exploration in clastic sequences devoid of carbonates.     

A comparative study of stream water and stream sediment as geochemical exploration media in the Rio Tanama porphyry copper district,Puerto Rico

To test the relative effectiveness of stream water and sediment as geochemical exploration media in the Rio Tanama porphyry copper district of Puerto Rico, we collected and subsequently analyzed samples of water and sediment from 29 sites in the rivers and tributaries of the district. Copper, Mo, Pb, Zn, SO₄^2?, and pH were determined in the waters; Cu, Mo, Pb, and Zn were determined in the sediments. In addition, copper in five partial extractions from the sediments was determined. Geochemical contrast (anomaly-to-background quotient) was the principal criterion by which the effectiveness of the two media and the five extractions were judged.Among the distribution patterns of metals in stream water, that of copper most clearly delineates the known porphyry copper deposits and yields the longest discernable dispersion train. The distribution patterns of Mo, Pb, and Zn in water show little relationship to the known mineralization. The distribution of SO₄^2? in water delineates the copper deposits and also the more extensive pyrite alteration in the district; its recognizable downstream dispersion train is substantially longer than those of the metals, either in water or sediment. Low pH values in small tributaries delineate areas of known sulfide mineralization.The distribution patterns of copper in sediments clearly delineate the known deposits, and the dispersion trains are longer than those of copper in water.The partial determinations of copper related to secondary iron and manganese oxides yield the strongest geochemical contrasts and longest recognizable dispersion trains. Significantly high concentrations of molybdenum in sediments were found at only three sites, all within one-half km downstream of the known copper deposits. The distribution patterns of lead and zinc in sediments are clearly related to the known primary lead-zinc haloes around the copper deposits. The recognizable downstream dispersion trains of lead and zinc are shorter than those of copper.     

A contribution to geochemical prospecting for fluorite

With fluorine as indicator, eight different extraction methods were tested for their suitability in geochemical prospecting for fluorite in stream sediments. Since this material can be regarded as coarse-grained, clastic fragments derived from the bedrock - i.e. granites, gneisses, and fluorite veins as well - it was concluded that the fluorine content of the stream sediments comes mostly from fluorine-bearing bedrock minerals. Anomalous samples contain fluorine predominantly as fluorite from fluorspar mineralization.Five of the commonest fluorine-bearing minerals - fluorite, mica, apatite, amphibole and tourmaline - were treated with the different extractants and with various times of reaction and mineral grain size. Fluorine analysis was by ion-selective electrode. The ratio cx-F (fluorite) vs. cx-F (mica, apatite) was found to be a useful measure of the applicability of the respective method.In order to test this conclusion 41 stream sediment samples (−177 microns −80 mesh) from the area under investigation were treated with eight different extractants.The most suitable extraction solution - boric acid solution - was used for the reconnaissance stream sediment survey in an area of 400 km² in the southeastern part of the Black Forest (F.R. Germany).