Geochemical dispersion about the Western Tharsis Cu–Au deposit, Mt Lyell, Tasmania

The Western Tharsis disseminated Cu–Au orebody, which occurs within the Cambrian Mt Read Volcanics of Western Tasmania, is surrounded by a pyritic halo that extends 100–200 m stratigraphically above and below the ore zone. Although this halo extends laterally along the same stratigraphic position to the south, it probably closes off to the north based on limited surface and drill hole data. The ore zone is characterized by extreme enrichment (the enrichments and depletions referred to herein are relative to background; these have not been established using mass balance techniques) in As, Bi, Ce, Cu, Mo, Ni, S and Se; with the exception of Mo, these elements are also enriched, but at a much lower level, in the pyrite halo.Pronounced depletion in K, Cs and Mg occurs in 20–30 m wide stratiform zones that flank the orebody on both sides within the pyritic halo. These anomalies and depletions in Be, Ga, Rb, Y, MREE and HREE are associated with a pyrophyllite-bearing alteration zone that wraps around the main pyrite–chalcopyrite-bearing ore zone. This zone is also characterized by positive Eu anomalies which persist up to 150 m both into the hanging wall and footwall of the orebody. The depletion of these elements is consistent with the advanced argillic alteration assemblage developed about acid-sulfate Cu–Au deposits.The pyrite halo is surrounded by a peripheral carbonate halo which is highly enriched in C, CaO and MnO, and weakly enriched in Zn and Tl. Zinc and Tl are most enriched in the upper 100–150 m of the stratigraphically lower halo. In the stratigraphically upper halo, Zn and Tl values are anomalously high but erratic.Barium and Sr enrichment, although mainly restricted to the pyrite halo, extends into the stratigraphically lower carbonate halo by up to 100 m. A Na depletion anomaly extends from 150 m below the orebody and to at least the Owen contact (i.e. ≥400 m)in the hanging wall.The dispersion patterns observed at Western Tharsis are quite unlike those of Zn–Pb-rich volcanic-hosted massive sulfide (VHMS) deposits in western Tasmania. Rather, the dispersion patterns observed at Western Tharsis are more akin to those surrounding porphyry Cu deposits and related acid-sulfate Cu–Au deposits.     

Electron paramagnetic resonance (EPR) spectroscopy in massive sulphide exploration, Rosebery mine area, western Tasmania, Australia

Electron paramagnetic resonance (EPR) spectroscopy of hot HNO₃ insoluble residues of rock powders is used as a new exploration technique for the volcanic-hosted massive sulphide (VHMS) deposit in the Rosebery mine area. The EPR signal intensities measured in 326.5±5 mT sweeps are strong in the altered rocks, and show a negative correlation with Ca, Na and Sr, and a positive correlation with K/Na, Rb/Sr and (K × Rb)/(Ca × Na × Sr). The EPR intensities measured in 326.5±100 mT sweeps show high values in the footwall pyroclastics, host rocks and hanging wall pyroclastics near and around the Rosebery deposit, and correlate positively with K, Fe, Mn, Ba, F, Rb, Zn, Pb and Zr. The Rosebery deposit and associated footwall alteration zone are located at the intersection of two elongated paramagnetic halos. The first is characterized by strong intensities of [AlO₄]° signals measured at magnetic flux density sweeps over 326.5±5 mT, trends NE–SW, and passes discordantly from the west to the east the White Spur Formation, altered footwall (footwall alteration zone), host rock of the Rosebery deposit, hanging wall and Mount Black Volcanics. The second, largely stratabound, halo is defined by strong intensities of Mn²⁺ sextets observed at magnetic flux density sweeps over 326.5±100 mT, runs N–S following the stratigraphic trend, and outlines the mineralized host rock and footwall alteration zone. It also extends toward the south into the unaltered footwall and hanging wall rocks. The first type of halo is considered to be related to wall rock alteration due to the VHMS mineralization processes as well to later Devonian metamorphism, and the second is thought to be related to massive sulphide mineralization alone.     

Geochemical and mineralogical haloes about the Elura Zn-Pb-Ag orebody, western New South Wales

The Elura Zn-Pb-Ag deposit, situated 43 km NNW of Cobar, western New South Wales, is hosted by the C.S.A. Siltstone, a distal turbidite sequence. Deep weathering has given rise to a bleached quartz-muscovite-kaolinite rock to a depth of approximately 80 m. Weathered bedrock is mantled by a thin (0.3–2 m) layer of soil and transported overburden which contains thin layers of maghemite-bearing gravels. Outcrop in the area is extremely poor with the insignificant gossan subcrop covered by 0.2 m of soil. The water table is presently at a depth of about 80 m. Groundwater is saline with up to 2.5% total dissolved solids.Oxidation of the orebody has resulted in the formation of a gossan and ferruginization of wall rocks. Elements associated with ore and retained at high concentration in the gossan are Ag, As, Ba, Cu, Hg, Mo, Pb, Sb, (Se) and Sn; much of the Zn has been leached whilst Cd and Tl are below the detection limits. Silver, Cu and Hg have been partially leached and concentrated in the supergene zone. Ferruginous wall rock contains substantial amounts of Pb, As and Ba but other element contents are substantially lower than in the gossan.Secondary dispersion from the Elura orebody is largely restricted to an interpreted paleodrainage channel SW of the orebody and has occurred in two distinct periods. Mobile elements, particularly Zn, leached during gossan formation, occur in anomalous concentrations at or near the water table. More recently, the less mobile elements Pb, As, Bi, Hg and Sb have been leached from mechanically transported fragments of ferruginized wall rock and gossan by the saline groundwaters and occur as anomalies up to 150 m from the gossan in near-surface bedrock. Copper and Zn form broad low-contrast anomalies whilst Sn is retained within the gossan.Iron-rich bands, 50–1200 mm in thickness, which are common in the weathered zone about the Elura orebody, were formed by precipitation from groundwater passing along bedding planes, shears, fractures and cracks. They have higher As, Bi, Co, Cu, Mn, Ni, Zn, lower values of Ba and Sr, and similar Pb, Sb and Sn contents to the weathered siltstones. Iron may be derived from Fe-rich carbonates in the siltstones and be redeposited as goethite and minor hematite. These Fe-rich bands have trapped target and pathfinder elements which are believed to be from primary haloes rather than from the orebody or gossan.A two-stage mechanism for the formation of some secondary minerals within the weathered zone has been confirmed by stable light isotope studies. These studies have also shown that fractionation of S isotopes is minimal during gossan formation, and that a S isotopic halo in weathered bedrock may be used as an exploration tool.     

The geochemistry, mineralogy and maturity of gossans derived from volcanogenic Zn–Pb–Cu deposits of the eastern Lachlan Fold Belt, NSW, Australia

The Eastern Highlands of Australia have probably been in existence since the Late Cretaceous or earlier and so there has been ample time for mature gossan profiles to form over outcropping volcanogenic Zn–Pb–Cu mineralisation in the eastern Lachlan Fold Belt. The mature gossan profiles are characterised by the upward progression from supergene sulfides to secondary sulfates, carbonates and phosphates into a Fe-oxide dominated surficial capping which may contain boxwork textures after the original sulfides (as at the Woodlawn massive sulfide deposit). However, the region has locally been subjected to severe erosion and the weathering profile over many deposits is incomplete (immature) with carbonate and phosphate minerals, especially malachite, being found in surficial material. These immature gossans contain more Cu, Pb and Zn but lower As, Sn (and probably Au) than the mature gossans. Although Pb is probably the best single pathfinder for Zn–Pb–Cu VHMS deposits of the eastern Lachlan Fold Belt, Ag, As, Au, Bi, Mo, Sb and Sn are also useful, with most of these elements able to be concentrated in substantial amounts in Fe oxides and alunite–jarosite minerals.     

Sulfur isotope characteristics of metamorphosed Zn–Cu volcanogenic massive sulfides in the Areachap Group, Northern Cape Province, South Africa

Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ³⁴S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ³⁴S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ³⁴S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase in δ³⁴S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.     

Do Stable Isotope Data from Calcrete Record Late Pleistocene Monsoonal Climate Variation in the Thar Desert of India?

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal¹⁸O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ¹³C is 1.7‰ (0.0–1.7‰), and δ¹³C covaries positively and linearly with δ¹⁸O. δ¹³C values are highest when δ¹⁸O values indicate the most arid climatic conditions. This is best explained by expansion of C₄grasses at the expense of C₃plants at low latitudes during glacial periods when atmosphericpCO₂was lowered. C₄dominance was overridingly influenced by global change in atmosphericpCO₂despite the lowered summer rainfall.     

Biomediation of calcrete at the gold anomaly of the Barns prospect,Gawler Craton,South Australia

Anomalously high Au concentrations (2.5 to 50 ppb) in regolith carbonate accumulations, such as calcrete and calcareous sands in aeolian sand dunes overlying Au mineralisation of the Gawler Craton, South Australia, show a marked covariance of Au with K, Mg and most notably Ca. This relationship appears to be linked to the authigenic formation of smectites and carbonates within the aeolian dunes in the region. However, little is known about the processes involved in the formation of carbonates under semi-arid and arid conditions. In this study the geochemical properties of aeolian dunes along several depth profiles of 2 to 4 m are investigated in order to assess the relationship between Au mobility and calcrete formation. In the profiles a strongly systematic relationship between Au and the increasing Ca–Mg contents at depth highlights the close association between the enrichment of Au in the calcrete and the underlying hydrothermal mineralisation. Intense calcrete formation and concurrent Au enrichment also occurs in the vicinity of roots penetrating the dune. Thin section petrography and cathodoluminescence show that most of the calcrete in the regolith profiles is micritic; some sparic crystallites have also been identified. To demonstrate the presence of microbial processes that may mediate the formation of calcrete, samples from a depth profile in the dune were taken under sterile conditions. After amendment with urea and incubation of up to 24 h, up to 18 mg/l of NH₄⁺ were detected in near surface samples. At depth of 2.3 m 1 mg/l NH₄⁺ were detected compared to a control that contained below 0.05 mg/l NH₄⁺. These results suggest that the genesis of calcrete and pedogenic carbonate in the area may be partly biologically mediated via processes such as the metabolic breakdown of urea by resident microbiota which generates a pH and pCO₂ environment conducive to the precipitation of carbonate. In the process of urea breakdown organic Au complexes such as Au-amino acid complexes may become destabilised in solution and Au may be co-precipitated, resulting in the fine, non-particulate distribution of Au throughout the micritic calcrete carbonate. In conclusion, this study suggests a coupled mechanism of biologically mediated and inorganic mechanisms that lead to the formation of Au-in-calcrete anomalies.     

Distribution and speciation of gold in biogenic and abiogenic calcium carbonates - Implications for the formation of gold anomalous calcrete

Calcrete (pedogenic Ca carbonate) is an important sampling medium for geochemical gold (Au) exploration in semi-arid and arid regions of Australia, because it is widespread, easy to sample and calcium (Ca) shows a strong positive correlation with Au, but not with base metals, in calcrete overlying buried Au mineralization. In this study we show that the formation of Au-anomalous calcrete can be biomediated through the activity of resident microorganisms, and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes. Calcified microfossils are highly abundant in calcrete from the Barns Au-prospect in South Australia. These microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from Au-anomalous calcareous sand from the Barns prospect. Calcium carbonates precipitated by these cultures consisted mostly of calcite, which is the main carbonate mineral in calcrete. Synchrotron micro-X-ray fluorescence (S-μXRF) mapping was used to assess the distribution of Au, Zn, Ca and other metals in Ca carbonates precipitated by active bacterial cultures. On a μm-scale the distribution of Au was heterogeneous in these Ca carbonates and differed from base metal distribution, thus mimicking the spatial separation of these metals observed in calcrete. The speciation of Au in Ca carbonates precipitated by active bacteria was measured using micro-X-ray absorption near edge structure spectroscopy (μ-XANES) and resembled that observed in Au-anomalous calcrete closely. While metallic Au was observed in Au ‘hotpots’, ionic Au was detected in the halo surrounding the ‘hotspot’. In contrast, the precipitates produced in the presence of dead bacterial cells or by raising solution pH or pCO₂, i.e., hydroxylapatite, portlandite and vaterite, respectively, did not reflect the mineralogy of calcrete. Gold distribution and speciation in vaterite, formed by raising pCO₂, were homogenous and did not reproduce the variation observed in calcrete and Ca carbonates precipitated by active cells. Increasing the supersaturation with respect to Ca in solution by incremental drying of the medium produced only X-ray amorphous precipitates, or hydroxylapatite in the presence heat-killed cells. In conclusion, this study shows that active microbial processes that combine biogenic Ca carbonatogenesis with Au precipitation are likely to drive the formation of Au-anomalous calcrete.     

干旱戈壁区金属元素的垂向变化与分散——以新疆金窝子金矿为例

本文选择被风化物覆盖(4米至几十米厚)的金窝子金矿作为研究对象。分析矿体上方风化层和非矿上方的垂直剖面中的各种金属元素的总量及活动态含量,以获得金属元素在风化层中的活动规律,为化探找矿提供方法依据。结果发现金等的某种活动态的分布模式在矿体上方呈现双层分布,即他们趋向于在垂直剖面的底部与顶部富集,而在中部降低。可以解释为元素在剖面底部风化层中的富集是由于金继承了矿体的风化产物,而在地表或近地表的富集是由于从矿体迁移至地表的元素被粘土物质和铁锰氧化物膜所吸附。     

Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster,Botswana

This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100–200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10–20 m cover at 20–50 l, 20–30 m cover at 50–100 l and 30–80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.

     

Late Pleistocene eolian features in southeastern Maryland and Chesapeake Bay region indicate strong WNW–NW winds accompanied growth of the Laurentide Ice Sheet

Inactive parabolic dunes are present in southeastern Maryland, USA, along the east bank of the Potomac River. More elongate and finer-grained eolian deposits and paha-like ridges characterize the Potomac River–Patuxent River upland and the west side of Chesapeake Bay. These ridges are streamlined erosional features, veneered with eolian sediment and interspersed with dunes in the low-relief headwaters of Potomac- and Patuxent-river tributaries. Axis data for the dunes and ridges indicate formation by WNW–NW winds. Optically stimulated luminescence and radiocarbon age data suggest dune formation from  33–15 ka, agreeing with the 30–13 ka ages Denny, C.S., Owens, J.P., Sirkin, L., Rubin, M., 1979. The Parsonburg Sand in the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Surv. Prof. Pap. 1067-B, 16 pp. suggested for eolian deposits east of Chesapeake Bay. Age range and paleowind direction(s) for eolian features in the Bay region approximate those for late Wisconsin loess in the North American midcontinent. Formation of midcontinent loess and Bay-region eolian features was coeval with rapid growth of the Laurentide Ice Sheet and strong cooling episodes (δ¹⁸O minima) evident in Greenland ice cores. Age and paleowind-direction coincidence, for eolian features in the midcontinent and Bay region, indicates strong mid-latitude WNW–NW winds for several hundred kilometers south of the Laurentide glacial terminus that were oblique to previously simulated anticyclonic winds for the last glacial maximum.     

The Teutonic Bore deposit,Western Australia: a lead isotope study of an ore and its gossan

The Teutonic Bore deposit occurs in an Archaean greenstone belt within the Eastern Goldfields Province of the Yilgarn Block in Western Australia. The ore is hosted by basaltic rocks and consists of a conformable massive sulfide lens underlain by a thick zone of pyritic stringer ore. The zone of oxidation reaches a depth of 90–100 m with the development of secondary copper sulfides. The lead isotopic compositions of six samples of massive sulfide, three mineral separates from the ore and eight gossan samples collected from the open cut were determined by standard mass-spectrometric techniques. Four of the massive sulfide samples, all three mineral separates and seven of the eight gossan samples have lead isotopic compositions identical to each other, within experimental error. These results confirm the findings of earlier studies that the lead isotopic signature of a massive sulfide ore is transferred to its gossan, and provide additional data suggesting the usefulness of lead isotopic determinations in ore prospect evaluation. The Teutonic Bore leads plot below the average global lead evolution curves for the uranogenic isotopes ²⁰⁶Pb and ²⁰⁷Pb, suggesting that the lead in the ore contains a significant mantle component. This feature of the isotopic data is consistent with the idea of a mantle plume origin of the Eastern Goldfields greenstone belts.     

Geochemical exploration challenges in the regolith dominated Igarapé Bahia gold deposit,Carajás,Brazil

The Igarapé Bahia, situated in the Carajás Mineral Province, is a world-class example of a lateritic gold deposit. It has developed under tropical weathering conditions since at least the Eocene and resulted in a regolith cover of at least 100 m thickness. The regolith is dominated by ~ 80 m thick ferruginous saprolite containing gossan bodies that constitute the main Au ore. Above saprolite the regolith stratigraphy has been established considering two distinct domains. One composed of residual materials and the other transported materials deposited over palaeochannels. In the residual domain the ferruginous saprolite grades upwards into a fragmental duricrust, interpreted as a collapsed zone, and then into different types of ferruginous duricrusts. Over palaeochannel the ferruginous saprolite is truncated by poorly sorted ferruginous sediment of variable composition that grades upwards into the ferruginous duricrusts formed over transported materials. Lateritization took place during a marked period that transformed the colluvium of the residual domain, and the transported materials accumulated in the channel depressions, into the ferruginous duricrust units. A later bauxitization event has overprinted all duricrust types but has mostly affected the duricrusts over the palaeochannel forming gibbsitic nodules. All duricrusts were finally covered by a transported layer of latosol which flattened the whole landscape in the Carajás region. Gold shows a depletion trend across the regolith but is enriched in the fragmental duricrust below the ferruginous duricrust from which gold is leached. Gold is also chemically dispersed laterally into the fragmental duricrust, but lateral Au dispersion in the ferruginous duricrusts of the residual domain is probably also influenced by colluvial transport. Metals associated with Au mineralization (Cu, U, Mo, Pb, Ag, LREE, Sn, W, Bi, Sb and P) are generally depleted in the saprolite but most of them are still anomalous. The fragmental and ferruginous duricrusts are more leached but the tests performed to estimate the dispersion potential of metals contained in the ferruginous duricrust show that some metals are still significantly anomalous especially Au, Ag and Cu. However, if ferruginous duricrusts are used as an exploration sample media their environment of formation must be considered. Metal depletion is generally more advanced in the ferruginous duricrusts developed in the vicinities of palaeochannels as oppose to those developed in residual domain. On the contrary, Au over palaeochannel areas is enriched in the upper bauxitized ferruginous duricrusts and in their gibbsitic nodules as a result of lateral chemical transport that is more widespread than in the colluvium over residual domain. The latosol is highly depleted in most metals due to its transported nature. However, the nodular fractions of the latosol show the greatest dispersion potential especially for Au, Ag, W, U, Bi and Sn. It can incorporate magnetic nodules that bring a rich suit of metals associated to the magnetic gossans, and non-magnetic nodules, classified as concretion and pisolites, which bring metals enriched or dispersed in the ferruginous duricrusts. This suggests that Lag constitutes a promising sample medium for geochemical exploration in the lateritic terrains of the Carajás region.     

Surface geochemical and biogeochemical expression of base‐metal mineralization at Woodlawn,New South Wales,Australia

The presence of base‐metal mineralization at Woodlawn was first recognised early in 1968 when a roadside reconnaissance geochemical sampling survey, conducted over felsic volcanic rocks in the Goulburn‐Tarago area, encountered anomalous B horizon soils containing up to 200 ppm Cu, 800 ppm Pb and 300 ppm Zn. Regional soil thresholds have been determined at 50 ppm Cu, 90 ppm Pb and 50 ppm Zn. Chip samples from the subsequently located gossan revealed up to 2000 ppm Cu, 8000 ppm Pb and 2000 ppm Zn, 500 ppm Sn, 25 ppm Ag and 3000 ppm As.

The first grid B horizon soil geochemical survey was conducted in 1968 over the gossan and surrounding area, and repeated with closer spaced sampling in the first half of 1970. The first survey delineated strong Cu (to 1000 ppm) and Pb (to 2500 ppm) anomalies coincident with the gossan zone, and intense hydromorphic zinc anomalies (to 3000 ppm) located down slope from the gossan in residual clay‐soils derived from dolerite bedrock. Threshold values have been determined at 140 ppm Cu, 700 ppm Pb and 580 ppm Zn. Ag and Sn in B horizon soils show pronounced anomalies coincident with the gossan and are suitable metals for geochemical target definition. Of fourteen trace elements determined in 1974 from B and C horizon soils on two lines across the ore zone Cu, Pb, Zn, Se, Ba, Sn and Ag show direct correlation with the mineralization, whereas Cd and Mn show moderate hydromorphic dispersion, having accumulated principally in clay soils derived from dolerite weathering. As, Sb and Bi, whilst responding over the ore zone, show elevated values in soils over hanging‐wall units; Ni and Co show maximum levels in soils over dolerite bedrock.

Bark and leaves of Acacia mearnsii, collected from a line across the gossan, contain anomalous levels of Cu, Pb, Zn, Sn and Ti near the ore zone, and weaker, but clearly anomalous Mn and Ni levels over dolerite bedrock. Both bark and leaves of Acacia mearnsii reflect the presence of concealed mineralization. The shrub Solanum linearifolium grows preferentially over and close to the Woodlawn ore zone, where it contains up to 840 ppm Cu, 250 ppm Pb, 7300 ppm Zn, 6 ppm Sn and 250 ppm Ti in leaf ash compared with levels of 200 ppm Cu, 2 ppm Pb, 400 ppm Zn, 0.8 ppm Sn and 60 ppm Ti in plants growing 1.5 km from the ore zone. This shrub has potential as an indicator of base‐metal mineralization.     

Distribution of trace and major elements in the −180 + 75 μm and −75 μm fractions of the sandveld regolith in northwest Ngamiland,Botswana

An orientation survey was undertaken in northwest Ngamiland to evaluate, among other criteria, an optimum size fraction for trace element analysis in the Kalahari sandveld regolith samples for a regional geochemical mapping project in the area. The most prominent geomorphological features of the region are the longitudinal rolling sand dunes and floodplain sediments adjacent to the unique, mid-continent Okavango Delta. Most of the dune sands were deposited in the Late Quaternary, and they show evidence of both pluvial and and environments. Presently the a lab dunes support varied shrub/grass vegetation with intermittent tree cover. Extensive forest fires have affected the region for generations, and the occurrences of remnants of large trees indicate that the region may have also supported large forests at some stage.The sandveld regolith comprises 70% quartz sand (−1000 + 63 μm) and 10–20% silt/clay (< 63 μm) material. There are no size variations between material from dune crests, dune depressions and down the soil profile to a depth of a metre. The process of regolith formation is relatively young and dynamic, being widely churned by a variety of boring animals, termites, ants and other insects. Hence no distinct zonation of soil horizons in the top 100 cm of the soil catena were observed. Remarkably, the significant proportion of the silt/clay fraction in the regolith indicates progressive weathering and possible deposition of precipitates.Trace element contents are invariably enhanced in the fine fractions, a characteristic widely observed elsewhere in residual soils. On the Xaxa-Cheracheraha traverse Zn values range from 2 to 98 ppm (mean 9 ppm); Pb varies from 1 to 101 ppm (mean 27 ppm) and Cu from 0 to 18 ppm (mean 5 ppm) in the −75 μm fraction. Contents in the −180 + 75 μm are, however, significantly lower than in the fine fractions. The variations in element concentrations along each of the orientation survey traverses at Qangwa-Nokaneng, Nxaunxau-Qurube and Nxaunxau-Ghane, in the coarse and fine fractions, generally remain sympathetic, showing that the enhanced values in the fines are a result of secondary dispersion, rather than mere compositional differences in the sandveld regolith.     

内蒙古巴彦宝力道金矿区物化探综合找矿方法及其应用

巴彦宝力道金矿区位于内蒙古苏尼特左旗,区内发育与金矿化有关的破碎蚀变带和糜棱岩化带。通过研究地质及物化探工作成果,建立本区地球物理、地球化学成矿模式,采用相关分析和"格氏法"等方法,确定了矿床元素轴向分带序列为:As Sb Hg Bi Li Sr Ba Ag Pb Au Cu Zn Sn V Co Cr Ni Ti Mo W,前缘晕元素为As、Sb、Hg、Bi,主成矿元素为Au、Cu、Zn,尾晕元素为Ti、Mo、W。据轴向分带特征,确定金矿体剥蚀程度的判别指标为w(As)×w(Sb)×w(Bi)/w(Ti)×w(Mo)×w(W)。建立了找矿标志,为下一步地质找矿提供了依据。     

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.     

Volty?ov gold deposit: Case history from the Bohemian Massif, Czechoslovakia

The Voltýov gold deposit in Central Bohemia is situated in the contact zone of granitic rocks of Variscan age and the Upper Proterozoic and Lower Paleozoic metasediments of the Krásná Hora Metamorphic Islet — the relict of an original sedimentary cover. Goldbearing quartz mineralization of vein and stockwork types is developed in rocks of both geological units, its form depending upon lithology. While ore shoots in metasediments and orthogneiss form generally regular bodies several tens of meters thick, those developed in granodiorite are rather irregular, without any indication of the tectonic control of the mineralization. The mineralized zone peters out extremely rapidly downwards, with a marked decrease both in extent and in gold contents. The gold-bearing mineralization is formed by two generations of gold, both of high fineness (over 900) and very fine-grained (5–20 m). The gold is accompanied by accessory arsenopyrite, maldonite and other Bi-Te minerals, scheelite, and exceptionally chalcopyrite, pyrite, and sulphosalts. Gold contents are 2–3 g/t on average. In addition, late quartz-carbonate mineralization, with U and Se minerals and remobilized gold, is also present. Detailed soil prospecting (25 × 50 m grid) of the deposit detected a large and constrasting gold halo with gold contents exceeding 2 ppm (–200 mesh fraction), accompanied by higher concentrations of characteristic pathfinders — above all As and Bi. The halo extent exceeds the area of the ore outcrop by several times. The origin of gold mineralization in the deposit is explained by remobilization of gold from orthogneiss by the intrusion of the Central Bohemian Pluton.     

Applying the box plot to the recognition of footwall alteration zones related to VMS deposits in a high-grade metamorphic terrain, South Africa, a lithogeochemical exploration application

Alteration zones (more commonly foot wall alteration zones) are related to volcanic-hosted massive sulfide (VMS) deposits and represent unique features that may be targeted during exploration. Of these, the chloritic foot wall alteration pipe is the most extensive and characteristic of VMS deposits. This feature is geochemically identified by a strong relative enrichment in aluminium and magnesium and a coupled depletion in calcium and sodium, giving rise to chloritic rocks in the primary environment of formation. During high grade regional metamorphism such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing magnesium rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium rich minerals such as sillimanite and corundum. This suggests that the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, should be recognisable in lithogeochemical exploration.The massive sulfide deposit in the eastern part of the metamorphic Namaqua Province, South Africa, at Areachap, Kantienpan and the defunct Prieska Cu–Zn Mine are hosted by a Mid-Proterozoic volcano sedimentary succession known as the Areachap Group. These deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed VMS deposits.The application of the known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure of complexly deformed and metamorphosed areas, such as in the eastern part of the Namaqua Province.The box plot presents a more readily applicable lithogeochemical method to characterize and identify the alteration process, but it was designed for relatively un-metamorphosed environments. It is demonstrated here that the box plot may also be applied to high-grade metamorphic terrains and that the mineral phases used in defining the boxplot in low grade metamorphic environments may be replaced by their equivalents in high grade metamorphic terrains. The compositional trends of the metamorphic minerals themselves may be used in defining the boxplot for high grade metamorphic terrains. These include the transition of: annite to phlogopite; grossular to almandine or pyrope, augite to enstatite or clinoenstatite, and hornblende to gedrite or cummingtonite. Close to the ore zone, the relative Mg content of pyroxene, cordierite and biotite are higher than further away from this zone. It could be demonstrated that the changes in the mineral compositions are gradational when comparing unaffected rocks with progressively more altered wall rocks.Conclusions based on an application of the isocon method demonstrate that primary footwall alteration zones in the Areachap Group's VMS deposits are characterized by elemental depletion of Na₂O, CaO, Sr, Ni, V and La and enrichment of MgO, Fe₂O_3(total), S, Zn, SiO₂, Co and F. It is shown that the whole rock compositions of rocks that were independently identified as the metamorphic equivalents of altered rocks, using the isocon method, plot in the correct place in the box plot for high grade regionally metamorphosed terrains. This establishes the box plot as an effective and practical tool for lithogeochemical exploration for VMS deposits in complexly deformed high grade metamorphosed terrains.