Au Dispersion Research Articles

Dispersion into the Tertiary Southern Cross Formation sedimentsfrom the Scott and Cindy Lodes, Pajingo, NE Queensland,Australia

Auriferous epithermal quartz veins occur at Pajingo, in NEQueensland, Australia, within a basement of relatively flat-lying Devonian– Carboniferous andesitic lava, tuff and volcaniclastic sediments. The host rocks are relatively fresh on the Mt. Janet Range but are weathered and mottled on the pediments, where they are partly covered by the immature, coarse-grained Tertiary Southern Cross Formation sediments and by more recent colluvia and alluvia. The Tertiary sediments were derived from saprock, saprolite and pisolitic detritus from the volcanic basement and were then mottled by later weathering. Dispersion of Au from the basement into the thick (20–80 m) Southern Cross Formation sediments was investigated at the Scott and Cindy Lode deposits. Within the Southern Cross Formation, regional background Au concentrations are at 5–10 ppb but, at just over 1 km from known Au mineralization, they are elevated to >30 ppb. The sediments proximal to the Scott Lode, exposed by mining, are Au-rich (100 ppb locally reaching 700 ppb) as they were largely derived from the Scott Lode and its environs. Here, local Au anomalies (500–700 ppb) occur both near the base and well above the base of the sedimentary column. In contrast, sediments proximal to the Cindy Lode mineralization contain significantly less Au, as their detritus was derived from up-slope. A very localized Au anomaly (150–500 ppb) occurs near the base of a palaeochannel that drained the eastern side of the Cindy Lode. All the evidence from the Scott and Cindy lodes suggests mechanical Au dispersion. However, partial extractions of sediments over the Scott Lode indicate that about 18% of the Au is soluble in 0.1 M KI (a weak Au extractant) so there may have been minor relocation of Au by weathering. Compared with total Au analysis (instrumental neutron activation analysis), only about half the Au in the Southern Cross Formation at Scott Lode can be dissolved by KCN. Analysis of exploration data confirmed that Au is dispersed into the Southern Cross Formation around Scott Lode at several levels. Dispersions of 100–300 m in these sediments occur not only at Scott Lode but also near zones of numerous sub-economic auriferous quartz veins. Implications for exploration are as follows. (1) Dispersion from buried mineralization into the Southern Cross Formation was initially mechanical but did not necessarily occur at the base of the sediments. There was minor hydromorphic relocation of Au during subsequent weathering. (2) Dispersion may be traceable for 100–300 m down-slope from mineralization, or from areas of numerous sub-economic auriferous quartz veins. (3) Large dispersion patterns are most likely where mineralization was exposed for a long period during sedimentation (Scott Lode) and less likely where mineralization was covered by early sediments or quickly smothered by material from up-slope (Cindy Lode) so an understanding of palaeotopography is essential. (4) Elevated Au at 25–35 ppb may indicate distal mineralization, whereas anomalies of 35–70 ppb and, more specifically, >70 ppb Au may indicate proximal mineralization. (5) Total Au analysis of the Southern Cross Formation is important as about half of its Au is occluded from cyanide attack.

Structured Au/FeO x/C Catalysts for Low-Temperature CO Oxidation

Innovative structured catalysts based on nanoparticles of gold supported on activated carbon fibers (ACF) in the form of woven fabrics are presented for low-temperature CO oxidation. Gold was deposited by adsorption from aqueous solution of ethylenediamine complex [Au(en) 2]Cl 3 followed by reduction in hydrogen. The catalysts were studied under transient reaction conditions and characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray energy dispersive analysis (EDS). HRTEM-EDS shows that gold is present on the surface of Au/ACF catalyst in the form of metallic particles with sizes of ∼2.5–5 and ∼30–50 nm. A predeposition of iron oxide on the ACF was beneficial for the Au dispersion and catalytic activity in CO oxidation. Gold particles in the Au/FeO x /ACF samples were not in direct contact with the Fe 2O 3 phase and their size was smaller than without doping by iron oxide. The mechanism of catalyst formation, its morphology, and the influence of preparative conditions are discussed. The activity of Au/FeO x /ACF was substantially higher as compared to Au/Al 2O 3 and Au/FeO x /Al 2O 3 catalysts. A reductive pretreatment with H 2 was necessary to activate the catalyst, but the activity decreased rapidly in CO/O 2 atmosphere. Addition of hydrogen or water vapors to the reaction mixture increases the catalyst activity.

Geochemical exploration around the Harmony gold deposit, Peak Hill, Western Australia

Primary Au mineralization at Harmony is associated with sulphide-poor quartz veins in mafic and ultramafic rocks. Gold and W are the most significant indicators of mineralization in fresh rock. Elements associated with the phyllic alteration halo are K, Rb and Ba, and Eu, Lu and Yb; these increase in abundance and range near mineralization. Although As and Sb are elevated, their abundances are far lower than in sulphide-rich mineralization styles. The Harmony gold deposit was completely covered by a blanket of soil and colluvium. Drilling beneath this revealed a complex, partly eroded residual regolith, clay-rich valley fill sediments and colluvium. Sampling and analysis of the top of the basement (ferruginous saprolite, lateritic residuum and mottles washed from the mottled zone) on a 250 m sample spacing showed significant Au and W anomalies in the vicinity of the deposit. Gold dispersion in the ferruginous saprolite is restricted and requires close-spaced sampling (50 m). The unconformity between the stripped basement and colluvium was sampled at a 150 m spacing to capitalize on any mechanical or hydromorphic dispersion along the contact. Dispersion of Au, W, Ta and Nb along this interface, indicating mechanical down-slope migration, produced better and more consistent anomalies than the basement sampling in this stripped environment. There are some very weak Au anomalies in the fine soil fraction (<75 μm), where the colluvial cover over the Harmony deposit was extremely thin (<2 m). However, this cannot be relied upon to locate mineralization.

Evolution, classification and use of ferruginous regolith materials in gold exploration, Yilgarn Craton, Western Australia

Ferruginous regolith materials are abundant and widespread in the Yilgarn Craton of Western Australia and have been successfully used as sample media in Au exploration. However, their formation has been complex. They are developed in residual and transported materials of various ages. Four general types are recognized: ferruginous duricrust and gravel, ferruginous mottles, ferruginous saprolite and iron segregations. Ferruginous duricrusts include lateritic residuum and ferricrete. Lateritic residuum has evolved by partial collapse of mottled or ferruginous regolith, involving local vertical and lateral (generally 10–50 m) movements after chemical wasting. Ferricretes are ferruginized sediments. Some are detrital clasts cemented by Fe oxides and others are authigenic pisoliths and nodules in sandy or clayey sediments. Ferruginous mottles are formed by accumulation of hematite and goethite in saprolite, residual clays or sediments. Ferruginous saprolite is formed by the uniform ferruginization of saprolite. Iron segregations form by the replacement and/or modification of sulphide-rich lithologies or as exotic accumulation of Fe oxides along preferred pathways, such as fractures, faults and lithological contacts within saprolite. Gold dispersion in the ferruginous zone of the profile is commonly considered to have a ‘mushroom’ shape formed by downward weathering, where the surficial Au halo is derived from the mineralization by a combination of chemical, residual and mechanical processes. In this study, appreciation of regolith–landform history provides an understanding of the nature of Au dispersion (or lack of it) in ferruginous materials. It has been possible to relate the mechanisms of dispersion of Au to the material being ferruginized, the environment of ferruginization and its position within a weathering profile. The distribution of Au at the micro-morphological scale provides evidence of a mobility related to the formation of particular facies of ferruginous duricrust. Accordingly, it is essential that careful attention is paid to characterization of potential ferruginous sample media and their location in the landscape in order to deduce their relationship to bedrock.

Form and distribution of gold mobilized into surface waters and sediments from a gossan tailings pile, Murray Brook massive sulphide deposit, New Brunswick, Canada

Stream waters and sediments draining a gossan tailings pile at the Murray Brook massive sulphide deposit were collected to investigate Au mobility. Weathering of the massive sulphides at Murray Brook during the Late Tertiary period resulted in the concentration of Au in the gossan cap overlying the supergene Cu and unoxidized massive sulphide zones of the deposit. The gossan was mined between 1989 and 1992, and Au and Ag were extracted using a cyanide vat leach process. Although stream sediments prior to mining had Au<5 ppb (the detection limit), sediments collected in 1997 had Au contents ranging up to 256 ppm with values up to 6 ppm more than 3 km downstream from the deposit. Dissolved Au contents were similarly anomalous, up to 19 μg/L and in excess of 3 μg/L 3 km downstream. The elevated Au contents in the waters and sediments are interpreted to reflect complexation of Au (as Au(CN) 2 −) by cyanide hosted within the gossan tailings pile. Precipitation recharges through the tailings pile with groundwater flow exiting to Gossan Creek. Degradation of cyanide along the flow path and within Gossan Creek allows colloidal Au to form via reduction of Au(I) by Fe 2+, consistent with SEM observations of Au as <1 μm subrounded particles. In the surface waters, the majority of the Au must be in a form <0.45 μm in size to account for the similarity in Au contents between the <0.45 μm and unfiltered samples. The very elevated stream sediment Au values close to the headwaters of Gossan Creek near the tailings indicate that upon exiting to the surface environment, Au(CN) 2 − complexes are rapidly destroyed and Au removed from solution. However, the high Au <0.004 μm/Au total in the headwaters and the extended Au dispersion in Gossan Creek waters and sediments suggest that Au(CN) 2 − complexes persist for the full length of Gossan Creek. The decrease in aqueous Au which is less than 0.004 μm indicates that Au is converted from a complexed form to a colloidal form with increasing distance downstream, consistent with dissolved NO 3 − contents which decrease from 5210 μg/L near the headwaters to 1350 μg/L at the lower end of the stream.

Gold dispersion in Andean desert environments (Atacama, Chile)

The mechanism of Au dispersion in sediments of mountainous desert environments has been studied in two different areas of the Chilean Andes. The San Pedro de Cachiyuyo placer (ca. 1800 m a.s.l.) consists of alluvial fans and ephemeral stream sediments deposited in a stable piedmont. The primary Au source is Cu–Au-bearing hydrothermal tourmaline breccia. The relief of this area is low (1850 m) and the average annual rainfall is ca. 20 mm. The La Coipa area (ca. 4000 m a.s.l.) is characterized by a rugged relief. The annual average rainfall is ca. 100 mm. The sedimentary deposits are less sorted than in San Pedro de Cachiyuyo and consist of ephemeral stream alluvium and slope deposits formed in a periglacial environment. The primary Au sources are two volcanic-hosted epithermal precious metal deposits. At San Pedro de Cachiyuyo, the halo is less than 1 km in length and the highest Au concentration are observed at the break in slope between the hillside and the piedmont. In the La Coipa area, Au was only detected by chemical analysis in the 125–63 μm and <63 μm fraction; however, the dispersion halo extends for over 10 km from the source. In both cases the geochemical signal of gold was strongest in the fraction <63 μm.

High-activity Au/CuO–ZnO catalysts for the oxidation of carbon monoxide at ambient temperature

Ambient temperature CO oxidation has been studied with Au/ZnO, Au/CuO and Au/CuO–ZnO catalysts prepared by a coprecipitation method. The catalysts are highly active for CO oxidation and are capable of sustained activity at 20°C. The highest activity is observed with Au/ZnO and this catalyst is shown, using transmission electron microscopy, to be comprised of smaller Au metal particles (2–5 nm) and a higher Au dispersion when compared with the other catalysts.

Gold, wolframite, tourmaline-bearing lateritized gossans in the Amazon region, Brazil

Several important supergene Au occurrences have recently been discovered in the Amazon region, representing a great impulse for mineral exploration in north Brazil (Costa, 1993). This follows what has already happened in the 1980's in lateritic environments of West Africa and Australia. These occurrences present ubiquitously the following important characteristics: (1) primary related sulfide mineralization associated with quartz veins that cut Proterozoic volcano-sedimentary sequences; (2) an upper oxidation zone (gossanous) altered by at least two main lateritization events — lateritized gossans and latosols; and (3) wolframite and tourmaline occurrences as resistate minerals (in addition to elevated As, Cu and Sn values) in the topmost horizons. The general structure of the upper part of these profiles comprises an upper latosol cover, a brecciated lateritic iron crust and ferruginous gossan. In the Águas Claras area, several studies have been conducted in order to characterize the Au distribution in the different supergene horizons as well as their mineralogical and geochemical associations. Gold mineralization is closely associated with the Fe oxy-hydroxides in the gossans, with a great range of composition in the different parts of the profile. SEM-EDS analysis of isolated grains from the gossans reveals highly heterogeneous Au compositions, especially in Ag contents, representing the presence of both primary grains and supergene/lateritic gold (higher fineness). Present understanding indicates that lateritization processes that took place in the Amazon region since early Tertiary times was superimposed on pre-existing gossans and gave rise to breccia-like Fe crusts, with a further dispersion of Au and other elements. The final products of lateritization are the widely distributed yellow, loose-clay latosols that invariably cover and mask these profiles and weaken Au signals. The ferruginous horizons (gossans, lateritic crusts, stone lines, etc.) are thus the better sampling media for Au geochemical exploration. The occurrence of coarse-grained tourmaline and wolframite can be an important auxiliary tool in the exploration, acting as true pathfinder minerals and also in distinguishing different Fe crusts and their probable primary parent rocks.

Secondary dispersion from gold deposits in west Turkey

Orientation studies over the Sarpdaǧ prospect in the Biga peninsula and the Arapdagi deposit near Izmir have provided elear evidence for elemental dispersion around west Turkish gold prospects. Although these deposits are of different types, silicification associated with the deposits results in the main part of both deposits forming topographic highs. At Sarpdaǧ gold mineralisation is relatively weak compared with nearby prospects and associated with a silicified cap on the main hill. Comparison of coarse and fine fractions, based on a 190 μm size split of 8 kg of -2 mm material, suggests that gold disperses clastically on the steep slopes, probably within silica, but coarse grains break down giving Au concentrations in the finer fractions at the base of the main slope. Discrete gold grains, that can be panned, only occur 1–2 km downstream within the streams and heavy mineral concentrations are very limited. This interpretation of Au dispersion is consistent with the data from 1 kg samples collected at the higher primary grade, but more contaminated, Arapdaǧi prospect. Antimony is the most consistent pathfinder both for the silicified cap at Sarpdaǧ and for gold-rich veins at Arpadaǧi. It gives high contrast anomalies. Arsenic is useful being more mobile than Sb, although contrast may be low. High resolution Ag data can be useful but base metal enrichments are also often associated with Ag anomalies. Most prospects have some base metal enrichments although they can be displaced from the main gold-rich parts of the deposit and anomalies may be weak. Lead and Cu are the more consistently useful elements. The use of large (> 8 kg of -2 mm material) samples produces consistent stream sediment data that can be used to reliably interpret single samples and quantify Au anomalies. A survey around the Halíköy Hg and Emirli Sb mines, using these large samples, confirmed the extension of the known gold-bearing Emirli structure. In contrast the major Hg-bearing Halíköy Fault is gold poor although a structure parallel to it is auriferous.

Highly Selective Catalytic Reduction of NO by H2over Au0and Au(I) Impregnated in NaY Zeolite Catalysts

The NO–H2reaction has been studied over Au0/NaY and Au(I)/NaY catalysts, which were prepared by the mechanical mixing of AuCl3with NaY zeolite and by the monolayer dispersion of Au(I) species into NaY zeolite, respectively. The precursor samples were reduced in flowing hydrogen at 423 K. The NO–H2reaction over these catalysts took place at relatively low temperatures under isothermal conditions from 373 to 673 K. The Au(I)/NaY catalysts were more active toward the above reaction compared with the Au0/NaY catalysts. The NO selectivity was pointed to the N2and N2O formation, with the former catalysts being more selective to N2.In situFT–IR study of the NO–H2reaction on the gold catalysts demonstrated that NH3was detected in the adsorbed NH4+form at 1440 cm−1, along with N2O at 2240 (2220) cm−1and H2O at 1645 cm−1. Two characteristic bands due to the vibrations of N–O bond in adsorbed NO were identified. The bands at 1880 cm−1in the spectra of Au0/NaY were assigned to Aun+–NO (n≈0) complexes, and the corresponding bands at 1910 cm−1in the spectra of Au(I)/NaY were assigned to Aun+NO (n=1) complexes in cages of zeolite. It was proposed that N2O and NH3formed simultaneously at low to moderate temperatures (<500 K) by the addition of N atom to NO and H atoms, respectively. The presence of adsorbed N atoms was confirmed by the coadsorption of NO/CO/H2mixture onto Au(I)/NaY at 423 K, where a band at 2280 cm−1ascribed to isocyanate [NCO] intermediate was developed intensively with time. This band did not deliver when a mixture of NO/CO was admitted alone to the IR cell. Therefore, the H atom promoted the N–O bond fission to form Nadand OHad. The [NCO] species is the result of a reaction between the adsorbed N atom and gaseous CO.

Hydrogeochemical exploration for gold in the Osilo area, Sardinia, Italy

A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km 2 is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks. Forty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L −1 Au). Gold concentrations in solution up to 3 ng L −1 were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution. The best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.

Dispersion and Optical Absorption of Au and Ag Particles Supported on an Amorphous SiO 2 Substrate

Optical absorption spectra were measured on samples of Au and Ag supported on amorphous SiO 2 substrates that had a great variation in the state of metal dispersion. The degree of metal dispersion was evaluated with a packing factor determined by transmission electron microscopy. It was found that the spectral peak wavelength (λ p) red-shifts with an increase of the packing factor ( q) for either Au or Ag. The observed λ p - q) relation was shown to be well simulated with a simple model based on the Maxwell Garnett theory.

The oxidation of amorphous Zr-base binary alloys in air

The oxidation tendency of amorphous Zr 65M 35 (M = Fe, Co, Ni, Cu, Rh, Pd or Au) binary alloys at 573 K in air was found to have a close relation with the atomic size of Zr and M elements. The amorphous Zr 65M 35 alloys containing Au or Pd with atomic sizes larger than 0.135 nm have a very high oxidation tendency and the resulting oxidized structure consists of finely mixed phases consisting of ZrO 2 + Au or ZrO 2 + Pd. The oxidation of the other amorphous Zr-M alloys containing Fe, Co, Ni, Cu or Rh with atomic sizes smaller than 0.135 nm is very sluggish and the amorphous phase remains almost unchanged. The oxidation at room temperature takes place only for M - Cu and Au and no distinct oxidation is recognized for the other M elements even after aging for several years. Furthermore, the oxidation tendency for the Au- or Cu-containing alloys has a strong compositional dependence and becomes significant with increasing M content. Production of finely mixed materials consisting of ZrO 2 + Au, ZrO 2 + Cu or ZrO 2 + Pd produced by oxidation is very attractive as a new synthesis method because of the achievement of uniform dispersion of ultrafine Au, Cu or Pd particles.

Improvement of photocatalytic activity of titanium (IV) oxide by dispersion of Au on TiO 2

TiO 2 samples were prepared by two procedures and their catalytic activities were compared to samples of commercial TiO 2 (P25). The addition of gold to all of these samples increased their photocatalytic activity.

Gold dispersion in a tropical rainforest weathering profile at Dondo Mobi, Gabon

Lecomte, P. and Colin, F., 1989. Gold dispersion in a tropical rainforest weathering profile at Dondo Mobi, Gabon. J. Geochem. Explor., 34 285-301. At Dondo Mobi in the gold district of Eteke, south Gabon, gold mineralization was studied in a tropical forest setting. The mineralization occurs in quartz veins within amphibolites of an Archaean gneiss-amphibolite series. Gold distribution patterns were studied in the different horizons of the weathering profile and in the different grain-size fractions of the materials sampled from three pits sunk in the weathered zone: upslope (pit Pl), directly overlying (pit P2) and downslope (pit P3) of the mineralization. The weathering profile consists of an upper, thick, loose sandy argillaceous horizon (Hl), an irregular nodular horizon with laterite nodules or blocks (H2) and a saprolite layer (H3) up to 70 m thick. In each pit, the specific geochemical signature of the bedrock is recognized in the three horizons of the weathering profile. Some goups of elements, e.g., Cr-Ni or Ba-V-P205, characterize amphibolite or black shale, respectively. Signal attenuations in the upper horizon can be explained by a homogenization effect. That can be related to a mushroom supergene dispersion, simultaneously inducing vertical decrease and lateral enrichment in element contents. Gold also mushrooms extensively in the different horizons of the weathering profile. The dispersion pattern is somewhat anisotropic, a strong enlargement being observed in the upper (Hl) and intermediate nodular (H2) horizons alike. However, the evolution of Au distribution is not the same for all grain-size fractions: (a) in the finest fraction, Au is regularly distributed in the weathering profile at the anomalous top and tends to preserve an equivalent level of concentration laterally; (b) in the coarsest fraction, the maximum Au content is found in the upper horizon just above the mineralized structure; it is rapidly decreases laterally, in the surface halo. The Au geochemical dispersion halo consists mainly of fine-grained gold developed from rather coarse-grained mineralization. Its characteristics suggest that Au dispersion was produced as a result of Au dissolution together with collapse of the weathering profile causing gold migration over short distances. The minimal volumetric reduction of saprolite in the upper horizon can be estimated to be in the range of 60%, on the basis of the content of the different grain-size fractions and the density variations in the various horizons. In exploration, geochemical techniques, even using wide sample spacing, are proving quite adequate to prospect for Au in tropical rainforest weathering environment. Indeed, significant, wellcontrasted dispersion haloes facilitate detection of rather confined mineralization. f 1 -Y 4 c

Dispersion of gold and base metals in the Mborguéné lateritic profile, east Cameroun

Abstract In lateritic profiles, the behavior of trace elements is difficult to predict and may vary significantly from one element to another. As this behavior directly controls the formation of the secondary dispersion halo around mineralization, it has a dramatic influence on the effectiveness of geochemical techniques in such environments. In the Betare Oya region, in east Cameroun, gold mineralization was indicated by contents of As, Bi, Ag, Mo, W, Pb, and Au. The dispersion of Au and associated trace elements was studied in detail from the fresh ore in deep drill hole samples to soils in surface materials. The local geology consists of Proterozoic (Birrimian) volcanosediments dipping northwest at 35 degrees. Bedrock is covered by a deep (25 m) weathered mantle with ferruginous induration at the top. The mineralization is related to silicified structures and is completely oxidized and weathered at the surface. Trace elements have been dispersed in the lateritic profile. Four main chemical dispersion modes have been defined here for base metals: Bi is residual and is little dispersed; W, Mo, Pb, and Au are progressively remobilized and well dispersed; As is strongly associated with Fe and widely distributed, while Ag is completely leached. An attempt is made to evaluate and compare the size of the geochemical dispersion haloes in the different horizons of a laterite profile and to define an order for halo size at the top of the profile: Ag Little dependent on the hydromorphic processes, a new model for the formation of secondary haloes in laterites is proposed. Haloes could also be produced during lateritization by downward progress of the weathering front. Mineralized veins are dissolved in the top horizons, and trace elements are dispersed and concentrated in the surrounding ferruginous horizons.

Effects of valley and local channel morphology on the distribution of gold in stream sediments from Harris Creek, British Columbia, Canada

Abstract Large (60 kg, minus 2 mm) sediment samples collected from a 5-km reach of a mature highland stream were used to investigate the effects of bar morphology and channel slope on the downstream dispersion of Au. Results are compared with those for an abundant heavy mineral (magnetite) and are discussed in terms of differential entrapment of light and heavy minerals by gravels and differences in supply of magnetite and Au to the study reach. Gold is selectively trapped in gravels; therefore, highest Au concentrations, a long dispersion train and lowest between-sampling-location variability were obtained with heavy-mineral concentrates of the minus 105+74 μm fraction from sandy gravels. In contrast, sands in bar-tail eddy pools gave the least reliable Au concentrations, due to random (Poisson) sampling errors and local post-depositional winnowing, and show rapid downstream anomaly decay.

The 1 × 1 to 1 × 2 phase transition of Au(110): An inverse photoemission study

We have measured the energy versus momentum dispersion of Au(110) in the (1×2) and the (1×1) phase with inverse photoemission using the isochromate mode. Our results are compared with theoretical bulk band structure calculations. The temperature dependent behaviour for the (1×2)-order-(1×1)-disorder phase transition is also presented.

Distribution and dispersion of gold in glacial till associated with gold mineralization in the Canadian shield

The results described relate to an investigation into the nature of Au dispersion in glacial till, undertaken to identify optimum search techniques for use in exploration for Au mineralization. The diversity of Au mineralization, in terms of the host rock lithologies, mineralogy and grain size of the Au, would be expected to give rise to differences in the secondary response in the associated overburden. Common exploration procedures involve the analyses of the heavy-mineral fraction or a particular size fraction of the tills. However, having regard to the expected variable response of Au in associated glacial till, attributed to variations in primary mineralization, effective exploration requires that the methodology employed is capable of locating all types of Au mineralization. Bulk till samples were collected from various sites associated with the Owl Creek deposit near Timmins and the Hemlo deposits. Grain size analyses were carried out on the till samples and on the heavy-mineral concentrates. The concentration of the Au in the various fractions was determined by Instrumental Neutron Activation Analysis. Preliminary results allow a number of provisional conclusions to be drawn: 1. (1) Grain size analysis of the −2 mm fraction of tills indicates that the silt and clay fraction constitutes 20–50%, whereas, in contrast, the equivalent heavy-mineral concentrates are dominantly composed of the coarser −500 + 63 μm material. 2. (2) The amount of Au present in the heavy-mineral concentrates of tills represents only a minor proportion of the total Au in the original till samples. In addition, the proportion of the total Au recovered in the heavy-mineral concentrate varies from 4 to 15%. Both factors indicate that caution is necessary in interpreting the significance of heavy-mineral Au data. 3. (3) Examination of the size distribution of Au within the heavy-mineral concentrate indicates that the majority of the Au is contained in the −125 μm fraction. 4. (4) The concentration factor (the original sample weight divided by the heavy-mineral concentrate weight) varies up to 7-fold between samples due presumably to the differing proportions of heavy minerals. Hence, in Au deposits of equivalent economic significance this gives rise to varying Au concentrations in heavy-mineral concentrates according to the quantity of heavy minerals present. Significant interpretation can only be achieved by re-expressing the Au contents of heavy-mineral concentrates in terms of the absolute amount of Au in heavy-mineral concentrates. 5. (5) A comparison of the heavy-mineral concentrates produced by different laboratories indicates marked differences in the weight of the heavy-mineral concentrate, the Au concentration of the heavy-mineral concentrate, the total weight of Au in the heavy-mineral concentrate and the size distribution of the Au in the heavy-mineral concentrate. 6. (6) Analysis of the −63 μm silt and clay size fraction indicates anomalous Au contents within this fraction of the tills collected from Owl Creek and Hemlo, extending over 500 m down-ice from mineralization at Hemlo. 7. (7) Analysis of the −63 μm silt and clay size fraction is suitable for the detection of fine-grained Au deposits that are not amenable to detection on the basis of heavy-mineral concentrate analyses. 8. (8) The analysis of the silt and clay fraction reduces the sample representativity problems associated with analyzing coarser fractions. 9. (9) A comparison of the Au distribution in heavy-mineral concentrates and the −63 μm fraction of till down-ice from the Owl Creek deposit indicates broadly similar dispersion patterns. In conclusion, although the results are based on relatively few samples, their consistency permits some general conclusions to be drawn. The silt and the heavy-mineral concentrate analyses provide different information and in view of the diversity of exploration targets and surface environments exploration reliability can be increased by analyzing both the −63 μm silt and clay fraction and the heavy-mineral concentrate.

A comparison of the effects of Cu and Au on the surface reactivity of Ru(0001)

A programme of work is described directed towards an understanding of the influence of deposits of copper and gold on the surface reactivity of Ru(0001). A combination of Auger electron spectroscopy, static secondary-ion mass spectroscopy, work-function measurements and thermal desorption studies have been used to devise surface preparation procedures which allow the concentration and dispersion of Cu or Au at the Ru surface to be controlled. Then using static secondary-ion mass spectroscopy low-energy electron diffraction, thermal desorption studies and electron energy loss spectroscopy the adsorption of CO and C2H4 have been studied on a range of surfaces having differing coverages and dispersions of Cu or Au. In CO adsorption at 150 and 300 K the main function of Au is to reduce coverage by physically blocking the adsorption sites. As would be expected, the highly dispersed Au is far more effective in this respect than Au clusters. Cu on the other hand, provides new adsorption sites at low temperature. Even at 300 K, although CO coverage is reduced by the Cu atoms CO is moved from the linear bonding invariably found on Ru into higher coordination sites some of which seem to involve Cu. At 130 K surface Cu and Au result in significant quantities of π-bonded ethene as compared to the di-σ-bonded ethene normally observed on Ru(0001). Whilst it is evident that Cu and Au influence adsorption on the Ru, Ru also influences adsorption on Cu and Au. π-Adsorption of C2H4 is observed on 1 monolayer Au on Ru. When the layer is thicker ethene will not adsorb on Au at 130 K. Where di-σ-adsorbed ethene is present, annealing the surface to 230 K produces adsorbed ethylidyne, whereas π-adsorbed ethene just desorbs. The ‘promoter’ action of Cu and Au is discussed in terms of the relative influence of ensemble and ligand effects.