Geochemical Dispersion Research Articles

Pisolitic laterite geochemistry in the golden grove massive sulphide district, Western Australia

Abstract The two known massive Cu-Zn sulphide deposits in the steeply dipping strata of the Golden Grove District of Western Australia — the Gossan Hill Deposit and Scuddles Prospect — both have a multi-element secondary geochemical dispersion halo in the pisolitic layer of laterite of probable Tertiary age. The Gossan Hill geochemical anomaly is about 1.5 × 2 km and has a Bi-Sn-Mo-In-Sb-As-Cu-Au association. Mean compositions of pisolitic laterite samples range from 4 to 18 times background, with maxima for Bi, Sn and Mo reaching 90 to 100 times background. Relief of Gossan Hill, which rises some 80 m above the laterite plain, would have favoured dispersion during lateritization. Nevertheless at Scuddles, where the laterite surface has only a gentle slope across strike, an anomaly in pisolitic laterite measures 1 to 1.5 by 1.5 to 2.0 km. The anomalous association, Bi-Sn-Mo-Sb-As, is similar to that at Gossan Hill; the mean contents in pisolitic laterite samples from the Scuddles anomaly range from 2.3 to 8 times background, with maxima for Bi being 12 times background. At Gossan Hill, individual pisolites and the concretionary skins contain gossan fragments and cassiterite, indicating mechanical dispersion during interitization. However, anomalous Cu, Zn and As in goethites of the concretionary skins of pisolites and nodules from the centre of the anomaly indicate that there has also been hydromorphic dispersion. The geochemical characteristics of pisolitic laterite samples have been represented by empirically derived additive indices based on up to eleven chalcophile elements. One index is suggested as a method to increase anomaly size and simplify interpretation; a more specific index based on Bi, Sn and Se is suggested for defining more closely the actual centres of mineralization. For routine exploration of lateritic terrain where the dips of the host sequence are steep, 320-m spacing of samples on a triangular grid appears suitable for locating anomalies and generally defining their source. However, for reconnaissance exploration, the density of sampling could be significantly reduced for example to 1 km spacing.

The secondary geochemical dispersion of some selected elements around the Cravezes scheelite deposit, northeastern Portugal

Selected results of a geochemical survey using stream-sediment sampling for ore minerals are presented. The study was undertaken in the Mogadouro (Cravezes) region, northeastern Portugal in order to investigate prospecting parameters for scheelite (tungsten) mineralization within calc-silicate rocks (skarnoids). Statistical and mathematical treatment of the data obtained during prospecting aided the interpretation, particularly, factor analysis. Five factors explaining about 85% of the variability in the data, were extracted; some were characteristic lithological and mineralization factors which are still imprinted in the secondary environment. Two apparently non-correlated types of mineralization are demonstrated: a scheelite (tungsten)-tin mineralization, and a copper-lead-zinc mineralization. Anomalous patterns delineating the scheelite ore deposit are revealed by the spatial distribution of various chemical elements. This highlights the importance of some pathfinder elements which can be used for tungsten prospecting in this type of geological environment. Copper, Pb, Zn, Ni, Co, V, and Cr are suitable pathfinders for W and Sn in the Mogadouro region.

Mineralogy and geochemical dispersion in lateritic soil profiles of northern Nigeria

Six lateritic soil profiles, overlying metasedimentary and crystalline basement rocks of northern Nigeria have been investigated in order to study the mineralogical and geochemical relationships between bedrock and weathering products. A total of 96 samples have been collected including 26 from exposed quarry and road-cut faces and 70 from auger-holes to a depth of 8 m. Two selective leaching methods (cold extraction with 0.1 M HCI and hot extraction with aqua regia) have been applied to the < 75- μm fraction and up to sixteen elements determined by AAS. Conclusions are based largely on the auger-hole samples where element distributions have not been modified by subaerial exposure. Quartz is the dominant mineral in the upper part of the soil profile while kaolinite increases strongly with depth. At the base of some profiles mica is also abundant. Elements are distributed through the soil profile according to their association with clays, iron oxides and to a lesser extent, carbonates. While intercalations of lateritic crusts obscure general trends with depth, dispersion and accumulation processes appear to be similar within each profile. In spite of the intensive weathering and secondary accumulation processes characteristic of the savanna belt, the lateritic soils within surface sampling depth appear to retain a geochemical imprint of the underlying bedrock. Criteria and potential problems in the application of soil geochemistry to bedrock mapping are discussed.

Biogeochemistry and pedogeochemistry of the White Spur area, near Rosebery, Tasmania

A comparative study of element distribution in soils and plants was conducted over an area in western Tasmania. Analyses of the trees indicated that in general Cu, Pb, Zn, Fe, Ni, Ba, and Cd are preferentially concentrated in the wood, followed in order by the bark, old and young twigs, and finally the leaves. Manganese is preferentially concentrated in the bark followed by the leaves, twigs, and wood. Of the five plant organs sampled, only the leaves proved to be of any use in reflecting the Pb, Fe, and Mn soil concentrations. Reflection for Cu, Zn, and Ni is poor. Chemical analyses of decayed organic material in the top soil correlate well with soil and bedrock concentrations. The downslope soil geochemical dispersion pattern is similar to a chromatogram: iron precipitates first where ground water surfaces, followed by manganese, and later, lead precipitation. Zinc travels farther, and copper and nickel generally escape the system. Vertical distribution in the soils shows the same relative elemental mobilities.

Geochemical dispersion in wallrocks associated with the Norbec deposit, Noranda, Quebec

Increasing emphasis is being given to the search for blind mineral deposits and the presence of geochemical dispersion halos in wallrocks may significantly enhance the chances of exploration success. Accordingly, the nature of wallrock dispersion associated with the Lake Dufault Cu-Zn-Au-Ag massive sulphide deposit, Noranda, Quebec, was examined to see if any such patterns existed. On the basis of relatively well defined geology, the area offered an opportunity to establish the relation between geochemical alteration and metallogeny, texture, structure and mineralogy. Anomalous element contents spatially related to mineralization are confined to the stratigraphic footwall (Waite Rhyolite) where Na 2O, SiO 2 and possibly CaO depletions and total Fe 1 Mn and MgO enrichments can be related to the hydrothermal vent. Dispersion halos 150–500 m in diameter are at least twice as wide as the obvious mineralogical alteration halo. Cu, Zn, Ag and S distributions show roughly similar patterns which are not obviously related to the main alteration pipe but do reveal broad enrichment zones in the general area of mineralization. The dispersion appears to be partly controlled by texture. Because of greater primary permeability, the halos produced in rhyolite breccia are wider and better defined than those in more massive rhyolites. On a wider scale (10 km 2) dispersion patterns are related to the fault and fracture systems of the area. As many as five stages of metamorphism have affected the area, altering all rocks to some extent. Samples can be classified petrographically into alteration groups, both related and unrelated to mineralization. Comparison of these groups reveals the inadequacy of certain elements for outlining alteration related to mineralization. For example, the range of K 2O in relatively unaltered rhyolite overlaps the composition of all other groups, including those related to mineralization. Similarly apparent is that the distribution of certain elements, e.g., Na 2O, may adequately define one part of an alteration zone such as the sericite-rich zone, but not another. It is concluded that careful use of lithogeochemistry in conjunction with detailed geological mapping and core logging can provide information otherwise obtainable only by time-consuming petrographic analysis. Thus it can be a potent weapon in the search for blind ore deposits.

Metal transport, partition and fixation in drainage waters and sediments in carbonate terrain in Southeastern Ontario

Lake sediment composition as an indicator of mineralization within the catchment area has found widespread application in recent years, particularly in Canada. Results have indicated, however, the existence of varying relationships between lake sediment composition and mineralization resulting from local features of the limnological environment. Accordingly it was considered appropriate to examine the nature of metal transport in the lake and stream environment, the partitioning of metal between the stream waters and stream sediments and between lake waters and lake sediments to obtain some understanding of the factors that affect the lake sediment-mineralization relationship. This investigation was carried out over an area containing Pb-Zn occurrences of supposed “Mississippi-Valley type” in Grenville and Paleozoic bedrock in southeastern Ontario. The headwater drainage systems comprise active streams, swamps, beaver ponds and small lake-bog systems giving way downstream to open lakes. The beaver swamps and seasonal swamps act as drainage sinks for metals, restricting the extent of geochemical dispersion in drainage systems adjacent to mineralization. Selective extraction analysis of bog, stream and lake sediments indicates that metals are preferentially concentrated with amorphous iron oxides, which readily adsorb and complex lead and zinc and are stable in the alkaline environment common in swamps adjacent to carbonate-hosted lead-zinc mineralization. The accumulation of lead and zinc with amorphous iron oxides combined with the adsorbing and chelating action of organic matter on lead and zinc makes organic-rich sediments from these small swampy areas an excellent sample medium for reflecting local mineralization. Down drainage anomalies of these elements can be accentuated by selective analysis for the amorphous iron oxide-held metal, involving selective extraction techniques. In contrast, within larger lake systems, the analysis of water samples indicates that geochemical dispersion in surface waters in the high pH environment (pH = 8.0) associated with the carbonate-hosted lead-zinc deposits is extremely restricted. In this environment, anomalous metal contents in lake water were not evident in lakes adjacent to mineralization, while anomalous lake sediment compositions exist only in lakes immediately adjacent to Pb-Zn mineralization and do not extend down the drainage system. The restricted dispersion necessitates basing geochemical reconnaissance surveys on collection and analysis of samples from the headwater organic-rich swamps at a higher sample density and resulting higher cost than in areas where a lower sample density is acceptable due to a wider dispersion.

Trace-element patterns in lateritic soils applied to geochemical exploration

Geochemical soil surveys in areas underlain by Precambrian volcano-metasedimentary sequences and around rare-metal-bearing pegmatites of southwestern Nigeria demonstrate that residual geochemical soil patterns reflect a wide range of potential source rocks adequately. The secondary geochemical dispersion processes in these typically tropical weathering environments adjust the trace-element distribution during lateritic soil development to narrow fluctuation ranges in comparison to the differing Clarke values of various source rock units. The sample density in these soil surveys, averaging at one sample per square kilometer, favours geochemical inventories even at regional scale and shows great potentials to predict bedrock composition of tropical terrain where rock outcrops are rather scarce.

Movement of radon in overburden

An analytical solution for simple one‐dimensional geometry establishes the basic theory of the movement of [Formula: see text] (radon) in overburden, involving diffusion and convection. The computer‐adapted finite‐difference method is then used to determine radon concentrations for the following more complex configurations: a two‐dimensional source, a vertical fault, a three‐dimensional source, and multilayered overburden. The key parameters are the radon concentration at the source, the diffusion coefficient of the overburden, and the geometry. This analysis indicates that if diffusion is the only transport process considered, the maximum depth at which uranium mineralization can be detected by the usual types of field equipment is limited to a few tens of meters. However, if convective transfer is also considered, radon attenuation is significantly decreased, e.g., by as much as a factor of 800 for a one‐dimensional configuration considered. It appears that an upward velocity component for the movement of radon, or geochemical dispersion of uranium and radium, are needed for long‐distance detection of uranium mineralization.

Geochemical dispersion from the Gamsberg Orebody, northwestern Cape, South Africa

The geochemical dispersion, in an arid environment, of Pb and Zn in stream sediments is examined with respect to size fractions and depth of sampling in a stream draining the Gamsberg Zn orebody. Sampling results indicate that there is a well-developed drainage train for approximately 8 km from the orebody for both Pb and Zn. The length of the drainage train is similar for both the minus 80 and minus 200-mesh size fractions but it is less for the minus 10, plus 40-mesh fraction. Higher metal values were obtained from samples taken from the surface of the stream bed. Metal values are generally lower in samples collected below the surface of the stream bed. The sampling position within the stream channel is not important. Sampling of the magnetite fraction from the stream sediments produced a well-defined Zn train for a distance of 14 km from the orebody. This train is more significant than the train found by normal sediment sampling.

Geochemical dispersion of heavy metals via organic complexing: A laboratory study of copper, lead, zinc, and nickel behaviour at a simulated sediment-water boundary

Abstract Data are presented in this study from laboratory model experiments describing the behaviour of Cu, Pb, Zn and Ni at a simulated sediment-water boundary. The interactions involved are sorption by kaolin and by bentonite, organic complexing in solution by fulvic acid and by humic acid, carbonate reactions, hydrolysis, and desorption of the cations from a clay-bound phase and from their metal hydroxides by the organic acids. The organic acids increase the solubility of Cu, Zn and Ni in the presence of clay. The Pb solubility is variable and can even decrease, particularly at acidic pH, with organic complexing likely due to colloidal coagulation. Both Zn and Ni are influenced by hydrolysis at basic pH. When carbonate was added to the metal-organic acid-clay mixtures, a further decrease in solubility was observed for Ni and, to a lesser extent, for Zn. The organic acids prove capable of remobilizing Cu, Pb, Zn and Ni from the solid phases examined. However, there is a general kinetic hindrance to the desorption particularly at basic pH. Copper desorption appears to be the most kinetically hindered. Conclusions pertinent to the geochemical dispersion of these metals are drawn.

The nature of metals—sediment—water interactions in freshwater bodies, with emphasis on the role of organic matter

A review with 227 references of the title subject is presented. It is divided into two main sections, viz., nature and properties of humic matter, and water—metal—sediment interactions. The first section deals with the essential properties of organic matter which occurs naturally in drainage sediments and waters. Discussion of the basic molecular structure of humic and fulvic acids is followed by some details of the chemical nature of functional groups within these structures which are important in metal-ion adsorption and complexing reactions which these materials can undergo. Information is also presented for colloidal and polyelectrolyte properties, complexation properties, and finally a summary discussion of metal-ion—humic-acid, metal-ion—fulvic-acid stability constants for both single ligand and mixed ligand systems completes the section. The second section comprises discussions of some specific aspects of interactions between metals, sediments and waters, including metal and organic speciation studies; sorption interactions between organic matter, clays and humic acids; chemical reaction between humic acids, heavy-metal minerals, clays and other silicate minerals; metal-ion adsorption—desorption studies, oxidation—reduction reactions between metal ions and humic acids; effects of sulphide ion on some of the above interactions and finally a summary of some relevant field geochemical dispersion studies. This second section describes both laboratory and field studies for each aspect.

Superficial geochemical dispersion around sulphide deposits: Some examples in France

Abstract Examination of geochemical exploration data accumulated in France over a period of almost 20 years has led to detailed consideration of the relationship between anomalies and mineral deposits. Geochemical anomalies associated with sulphide mineralizations seem to follow several general rules i.e. Ag and Pb are almost systematically present, As is very frequent, while Cu and Sb may also be present. The behaviour of Zn seems to be controlled by the lithological character of the host rock with anomalies in carbonate media even for occurrences without economic interest and an absence of anomalies in non-carbonate media, even for Zn deposits of considerable size. Moreover, the contents of this element are greatly amplified by contamination from old workings. The abundance (or scarcity) of supergene minerals in the occurrences directly influences the nature of the anomalies. Lead anomalies frequently correspond to the presence of Pb oxides such as pyromorphite while Zn anomalies derive from Zn oxides which are much more common in carbonate media (smithsonite-hydrozincite) than in schistose host rock. High Cu and As contents are most frequently linked to the presence of “ferruginous fragments” (gossans) derived from sulphides. As a general rule, the various geochemical peculiarities or associations encountered in soils are very much similar to those of the oxidized part of the deposits. We are therefore led to assume that, contrary to the usual conceptions, superficial dispersion is basically mechanical and that the anomalies detected near mineralizations are due to the presence in the soils of supergene minerals, gossan microfragments and more or less mineralised rock debris, etc. Mechanical migration of this kind can be explained by the existence in France during the Pleistocene of cold climates of the tundra type with associated permafrost. The thawing in summer of the superficial part of the frozen overburden resulted in massive downslope solifluction favoring mechanical migration over distances in the order of 100 m and contributing to the surface dispersal of the mineralizations. These phenomena are extremely important for geochemical exploration because they allow and justify the use of large grids (200 m × 200 m or even 400 m × 400 m) during soil surveys at least at the initial stage of prospecting on areas of several square kilometers. These conclusions are valid not only for France but for all regions with the same paleoclimate. These form a belt bounded to the north by glacial formations linked to ancient inland ice and to the south by the maximum extension of Pleistocene permafrost, including, in particular, the north of the United States and Central Europe. This paleo-climate belt characterized by traces of solifluction constitutes a well-defined unit in which geochemical prospecting has proved itself extremely effective.

Primary and secondary geochemical dispersion of Y, Yb, Nb, and La in granites and alluvial sediments of Wadi Baroud, eastern desert, Egypt

Primary and secondary geochemical dispersion of Y, Yb, Nb, and La in granites and alluvial sediments of Wadi Baroud, eastern desert, Egypt

Geochemical dispersion patterns associated with the Lake Yindarlgooda sulphide mineralization, Western Australia

Geochemical investigations have been carried out in the Lake Yindarlgooda area, 40 km east of Kalgoorlie, Western Australia, where sulphide deposits and black shales occur in a sequence of metavolcanics and metasediments of the Eastern Goldfields greenstone belt. The studies were made in order to understand the processes of element migration and the formation of dispersion patterns in semi-arid regions having partly lateritic weathering conditions. Outcrop exposure in the vicinity of the mineralized zones is limited to the siliceous caprock and gossans. Mineralogical data, and geochemical data on the distribution of Cu, Ni, Co, Zn, Pb and other elements, have been obtained from unweathered and weathered rocks, and different horizons of the overlying soil in the area surrounding the mineralized zones of Lake Yindarlgooda, Scotia and Ringlock. The results indicate, in principle, two different kinds of weathering environment: (1) playa lakes, with the development of solonchaks; and (2) Tertiary peneplains outside the playa lakes, with lateritic soils, solonized brown soils or desert loams. The trace element distribution patterns in the solonchaks are determined only by the initial element content of the alluvial sediments. Therefore, sampling of solonchaks generally cannot be recommended for geochemical exploration surveys, but sampling of weathered bedrock below the solonchaks is highly recommended in the playa lake environment. By sampling of the bedrock, the non-outcropping continuation of the Lake Yindarlgooda sulphide body has been precisely delineated. In areas of the Tertiary peneplains outside the playa lakes, lateritic soils, solonized brown soils or desert loams cover the weathered bedrock. Comparative geochemical investigations in these areas in the vicinity of the nickel sulphide deposits at Scotia and Ringlock have shown that the migration of elements in solution is restricted, a result of the very low water table. In this area the main process of dispersion is mechanical. Therefore, the secondary dispersion haloes of Scotia and Ringlock are narrow, restricted to a few tens of metres, and sampling at closer intervals is needed to locate the orebodies.

Geochemical dispersion in mineralized soils of a permafrost environment

Abstract The Agricola Lake massive sulphide body occurs in metavolcanic rocks of Archean age in the northwestern Canadian Shield. The mineralization contains Zn, Cu, Pb, Ag and Au and, in addition, there are elevated contents of As and Hg. The area lies within the zone of continuous permafrost. The distribution of elements in soils in the vicinity of the mineralization shows the influence of three independent dispersion processes — glacial, solifluction and hydromorphic — superimposed on the primary bedrock distribution. Oxidation of sulphides has been active in the past and continues to this day. In the absence of large amounts of carbonate minerals, this has produced an acidic weathering environment. The most mobile of the elements studied — Zn — has been largely removed from the mineralized soils and dispersed a considerable distance down drainage. Au, Hg, Pb and Ag are relatively immobile and are retained in the soils overlying mineralization or in nearby, glacially dispersed detritus. Fe, Cu and As are of intermediate mobility and are deposited in the proximal portion of the drainage system. The estimated order of mobility is: Zn > Cu > Fe > As > Ag > Pb > Hg > Au. Glacial action dispersed oxidized detritus, already depleted in mobile elements. Material precipitated along the proximal portion of the drainage system shows an association of As and Cu with iron oxides and Ag and Hg with manganese oxides. In addition to the elements discussed above, Mg in soils indicates the alteration zone underlying the massive sulphide and Ca outlines the soils subject to acidic leaching.

Geochemical dispersion in lake waters and sediments from massive sulphide mineralization, Agricola Lake area, Northwest Territories

Abstract The Agricola Lake area lies within the tundra and is underlain by continuous permafrost. Archean metavolcanic rocks are host to massive sulphide mineralization, which contains Zn, Cu, Pb, Ag, Au, As, Cd and Hg. This mineralization is being actively oxidized, producing acidic waters enriched in a number of metals. The elements Pb, Ag and Hg are immobile in the surface environment and are largely retained in the soils near the mineralization. Zn, Cd and Cu are mobile and are dispersed in quantity far along the lake-stream system draining the mineralization. Arsenic is relatively immobile, but it has so wide a primary distribution, particularly in metasedimentary rocks overlying the volcanics, that it too is anomalous in sediments throughout the drainage. Ni and Co, derived from these metasedimentary rocks, and as mobile as Zn and Cd, are also strongly anomalous in lake sediments. Fixing of the mobile elements in sediments appears to be determined by their adsorption on iron oxides, Cu being adsorbed at lower pH values and closer to the mineralized source, than Zn, Cd, Ni or Co. The use of nearshore and centre-lake sediments for reconnaissance-level geochemical exploration is compared. The latter are a more homogeneous, finer-grained sampling medium. Dispersion of mobile indicator elements from their source in centre-lake sediment is greater than for nearshore sediment. This is because nearshore sediments are essentially subaqueous soils, that are not produced by lacustrine sedimentation. Adsorption of metals on nearshore material takes place in situ, so that dispersion trains can be no longer than that of the waters in contact. In the case of centre-lake sediments, it is suggested that metals are adsorbed on fine-grained particulates, which travel downstream before settling in the centre-lake bottoms. Waters are the most convenient sampling medium for more detailed levels of exploration. Surface waters are homogeneous within any one lake and show only moderate variation in composition throughout the summer ice-free season. Lake waters that are not derived from mineralized areas are very pure, with dissolved solids contents of 10 ppm or less.

Major element halos in granitic wall rocks of porphyry copper deposits, Guichon Creek batholith, British Columbia

Abstract Geochemical dispersion patterns of major elements in altered granitic wall rocks of two Highland Valley porphyry copper deposits are documented. K2O and K2O/Na2O ratios consistently increase from the outer margins of the deposits to the inner zones of intense hydrothermal alteration and metallization. In contrast, concentrations of Na2O, CaO, MgO and total Fe (as Fe2O3) decrease inward. These geochemical variations are explained in relation to prevalent alteration assemblage and primary lithology. It is concluded that the use of major element halos is more advantageous and less time-consuming than quantitative mineralogical techniques in delineating epigenetic wall-rock anomalies and proximity to mineralization.

Modern submarine hydrothermal mineralization: examples from Santorini and the Red Sea

Summary Submarine hydrothermal mineralization is known to occur on parts of the World Mid-Ocean Ridge System and in association with island arc volcanism. Two areas of submarine hydrothermal activity have been examined in detail — one in the Atlantis II Deep of the Red Sea and the other off the volcano of Santorini in the Cyclades Volcanic Arc, eastern Mediterranean. In each case hydrothermal solutions are mixing with sea water, resulting in the fractionation and precipitation of a variety of metal-bearing phases as the physico-chemical properties of the solution change. Sulphides occur locally as a minor component of the newly precipitated hydrothermal sediments off Santorini, which are principally composed of iron oxides. Iron precipitates close to the hydrothermal outlets and decreases in concentration away from the volcanic source. By contrast, manganese is low in the sediments richest in iron, but increases in concentration more than twenty-fold away from the hydrothermal outlets and iron-rich zone. The distribution of certain minor elements in the sediments follows that of Mn (e.g. Zn) and Fe (e.g. Mo), indicating that scavenging may be a control on their distribution. Early-formed precipitates from the Atlantis II Deep brines also include sulphides, principally of Cu, Zn and Fe. Subsequent precipitates include iron silicates, after which the remaining iron is oxidized and precipitated as ferric oxide and hydroxide. Finally, Mn 2+ is oxidized to Mn 4+ and can scavenge considerable quantities of trace metals out of solution on precipitation. Manganese precipitated in the waters over the Deep is probably redissolved on falling back into the brine. By contrast, manganese and those minor elements that escape from the Deep can precipitate up to 10 km from it, giving rise to well-developed geochemical dispersion haloes. The selective fractionation and precipitation of hydrothermally introduced metals on mixing with sea water in these two environments results in a geochemical zonation away from the metal source in each case. The principal fractionation, other than the early precipitation of metal sulphides, is between Fe and Mn, but some other metals also show distinctive concentration gradients. These observations may be of great potential value in geochemical exploration for submarine hydrothermal orebodies, as the extent of the dispersion of the most mobile elements can be many times the extent of the potential orebody itself. The significance of these observations in regard to metallogenesis on mid-ocean ridges in general has been discussed elsewhere. Metalliferous sediments found to date on the World Mid-Ocean Ridge System are of much lower grade than those in the Red Sea, but these may, in part, represent the analogues of the widely dispersed Fe and Mn oxides around the Atlantis II Deep. On this basis, Cronan concluded that higher-grade deposits, including metal-rich sulphides, might occur within the upper part of the oceanic crust and within the deep fracture zones that cut mid-ocean ridges in all the oceans. Furthermore, studies on the geochemical relationship between metalliferous sediments and ferromanganese oxides on mid-ocean ridges suggest that the two are genetically related in that they receive their metals from the same sources, but in different proportions and at different rates. Ancient volcanogenic orebodies sometimes show features similar to those described here, indicating that in the modern environment we may be observing a not uncommon volcanic ore-forming process in operation.

Geochemical dispersion patterns associated with submarine geothermal activity in the Bay of Plenty, New Zealand.

Trace element analyses of marine sediments from the Bay of Plenty indicate that submarine geothermal activity does not contribute significantly to the trace element geochemistry of sediments in this locality. The terrigenous sediments are similar in composition to North Island greywackes and are probably derived principally from the erosion of Mesozoic greywackes on the western flanks of the Huiarau Ranges rather than from the Quaternary acid volcanics of the Taupo region. There is no evidence for the diagenetic remobilisation of manganese or other trace elements in the sediments of this locality.

Secondary geochemical dispersion patterns associated with the nickel sulphide deposits at Kambalda, Western Australia

Geochemical investigations have shown that the nickel sulphide mineralisation at Kambalda is detectable by sampling and analysing the minus 80-mesh fraction of near-surface soils. Over unmineralised mafic and ultramafic rocks the distribution of Ni in soils accurately reflects bedrock Ni distribution. In the environment of sulphide mineralisation Ni is preferentially concentrated in the coarse fractions of the soils, with a corresponding depletion in the minus 80-mesh fraction. Both Ni and Cu nevertheless display strong anomalies in minus 80-mesh fraction soils in the immediate vicinity of sub-outcropping mineralisation. The distribution of Ni, and to a lesser extent Cu, in the minus 80-mesh soil fractions appears to be controlled by a strong association with secondary iron oxides which is most marked in the silt fraction. Clay mineral abundance and type appears to have little influence on the secondary dispersion of Ni in the Kambalda environment.