Transport Of Gold Research Articles

Transport of magnetite and gold in Harris Creek, British Columbia, and implications for exploration

Earlier studies have suggested that difficulties sometimes encountered in following up anomalies can result partly from large seasonal variations in gold content of stream sediments. This possibility has been investigated using pit traps installed in a bar on Harris Creek, British Columbia. Sediments caught in the traps in 1988 and 1989 provide data on variations in concentrations of gold and magnetite in bedload as stream discharge changes. These show that anomalous concentrations of gold are only transported for brief periods when increased discharge, caused by snowmelt floods, disrupts the cobble-gravel pavement. For the purposes of exploration geochemistry, these flood sediments, trapped in the voids of the newly formed pavement as the flood peak passes, provide the best chance of recognizing the presence of anomalous concentrations of gold in a catchment. However, their subsequent burial by post-flood gold-poor sediments may cause seasonal variations in gold content of near-surface sediments.

Mechanism of gold transfer and deposition in a supergene environment

The possibility of gold mobility in supergene environments is suggested by numerous indirect observations of gold particles. However, presently there are no dissolution or transport data based on the chemistry of actual weathering solutions. We have investigated models of gold transport by studying the water chemistry of a stream where weathering of auriferous minerals is occurring under supergene conditions in a temperate climate. Dissolved gold concentrations in the auriferous zone are forty times higher (200 pmolL −1) than in non-auriferous zones (5 pmolL −1). We have examined the potential gold ligands: thiosulfate, hydroxide, and chloride. Speciation calculations using the field data show that gold solubility is controlled by a gold thiosulfate complex: Au(S 2O 3) 2 3−. The oxidation of the thiosulfate ligand induces the destabilization of the aurothiosulfate complex. Gold is then reduced to the metallic state as submicronsized suspended particles. This conclusion is supported by the measured increase in gold content of suspended particulates. Mass balance calculations performed with both dissolved and paniculate gold data permit the quantification of gold deposition along the stream.

Magmatic Oxidation in the Syenitic Murdock Creek Intrusion, Kirkland Lake, Ontario: Evidence from the Ferromagnesian Silicates

The syenitic Murdock Creek intrusion, of late Archean age, lies within one of the major lode gold camps of the world, at Kirkland Lake, Ontario. There is a strong spatial association between syenitic intrusions, gold mineralization, and carbonatization. Hydrothermal fluids that carried the gold had an unusually high oxygen fugacity ($$f_{o_{2}}$$). Notwithstanding a wide range in rock compositions within the intrusion (from clino-pyroxenite to alkali-feldspar syenite), clinopyroxene and biotite have consistently low Fe/(Fe + Mg) and high $$Fe^{3+}/(Fe^{2+} + Fe^{3+})$$ ratios. These compositional features indicate a deficiency of $$Fe^{2+}$$ in the magma during crystallization. Together with the presence of abundant, early-formed magnetite and titanite, this is interpreted to result from an intrinsically high magmatic $$f_{o_{2}}$$ that remained constant during pluton evolution ($$f_{o_{2}} \approx 10^{-12} bars$$). Mineral compositions, proportions of crystallizing mineral phases, and textural features indicate that the pluton formed from a single pulse of already-evolved syenitic magma that fractionally crystallized in situ under low water pressure at mid-crustal depths. The intrinsically oxidized nature of the pluton implies an oxidized source region such as metasomatized upper mantle where partial melting occurred. $$CO_{2}$$-rich fluid or melt was the likely agent for metasomatism. Dissolution and transport of gold is favored by high $$f_{o_{2}}$$ in the fluid phase. The high $$f_{o_{2}}$$ of the syenitic magma suggests a common genetic link between magma generation, $$CO_{2}$$ represented by "carbonatization, and gold-bearing fluids.

Theories on the transport and deposition of gold and PGM minerals in offshore placers near Goodnews Bay, Alaska

The U.S. Bureau of Mines conducted a mineralogical study of placer grains from the Bering Sea near Goodnews Bay, Alaska, using scanning-electron microscopy and energy-dispersive X-ray analysis. The objectives of the study were to define the Au and platinum-group-metal (PGM) mineralogy and to examine the placer grains in the context of the theories of mechanical deposition and accretion. Native Au and fourteen PGM-mineral species were identified. The placer grains are distinguished from typical in-place, on-land, magmatically formed PGM-mineral grains by their large grain size, chemical compositions, and unique zoning textures. Inclusions of PGM arsenides, sulfides and tellurides; exsolution phenomena; and grain border occurrences indicate that the placer grains are products of mechanically weathered primary ore. However, micro-crystalline PGM-mineral assemblages present in grain rim cavities suggest that leaching and recrystallization, two components of the accretion theory, also occurred. Results of this study indicate that both long-term chemical reactions and mechanical processes change the size, chemistry, and textures of primary Au and PGM minerals. This type of characterization provides information that helps geologists to determine age and depositional history of such placers and aids them in targeting promising geological environments where Au and PGM may be found.

Analysis of calcitonin gene‐related peptide‐like immunoreactivity in the cat dorsal spinal cord and dorsal root ganglia provide evidence for a multisegmental projection of nociceptive C‐fiber primary afferents

Recent studies have suggested that calcitonin gene-related peptide (CGRP) can be used as a marker for a subpopulation of nociceptive primary afferents. Consequently, CGRP-immunoreactive (CGRP-IR) primary afferents have been reported to project many segments rostral to their segment of entry and to send collaterals into the superficial and deep laminae of the dorsal horn. This study reports that some CGRP-IR primary afferents of sacral origin project rostral through the ipsilateral lumbar enlargement in the cat. The ultrastructure of these multisegmentally projecting primary afferent axons and terminals identified in a partially denervated cat was examined and compared to the ultrastructure of CGRP-IR afferents from an intact cat. Retrograde transport of wheatgerm agglutinin-colloidal gold injected into the cat L4 spinal cord resulted in labeling of primary afferent cell bodies in the ipsilateral L4-S1 dorsal root ganglia (DRG). Analysis of every fourth section through the ipsilateral S1 DRG revealed as many as 1,000 retrogradely labeled neuronal cell bodies. One third of these cell bodies were double labeled for CGRP-like immunoreactivity. The number of single- and double-labeled cells increased in ganglia closer to the injection site (L4-L7). At the ultrastructural level, in the lumbosacral dorsal spinal cord of a normal cat, most CGRP-IR axons were unmyelinated, while the rest were small myelinated axons. In both the superficial dorsal horn and lamina V, CGRP-IR varicosities were dome shaped, scallop shaped, or elongated. The CGRP-IR varicosities contained small agranular vesicles and frequently a few dense core vesicles. These labeled varicosities formed asymmetric synapses on unlabeled dendritic spines, shafts, or neuronal somata. One cat received multiple unilateral dorsal rhizotomies (S1-L4) and an ipsilateral hemisection (mid L4). CGRP-IR axons and terminals were found within each of the rhizotomized segments, although their density was greatly reduced compared to that in the intact animals. In Lissauer's tract the majority (greater than 90%) of CGRP-IR fibers were unmyelinated. In laminae I and V, the remaining CGRP-IR varicosities were mainly the dome-shaped varicosities morphologically similar to those observed in the normal spinal cords. They contained small agranular vesicles and a few dense core vesicles and formed asymmetric synapses on unlabeled dendritic shafts and spines. These data demonstrate that unmyelinated, presumably C-fiber nociceptive primary afferents and some small myelinated A-delta nociceptive primary afferents of sacral origin project rostral through the cat lumbar enlargement and make synaptic connections in both the superficial and deep laminae of the cat dorsal spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

The nature of the fluids associated with the Monte Rosa gold district, NW Alps, Italy

Recent O-isotope and fluid inclusion studies have provided evidence on the nature of the fluids associated with the late-Alpine quartz-gold deposits of the Monte Rosa district. The most abundant inclusions in quartz from these deposits contain a low salinity aqueous fluid (about 2% to 10% wt. NaCl eq.), and a CO2 phase (usually less than 20% mol), in places with minor methane. CO2 densities and total homogenization temperatures vary widely throughout the district, reflecting diverse conditions of trapping (P = 1 to 3 kb, T = 300° to 450°C). At Miniera dei Cani, unmixing between CO2-rich and H2O-rich fluids possibly occurred. A second type of inclusion contains an aqueous brine without recognizable CO2, and is especially abundant at Val Toppa. O-isotope studies suggest that fluids were largely equilibrated in a metamorphic environment. It is concluded that the gold-related fluids in the district were mainly of a metamorphic nature; at Val Toppa, both isotopic and fluid inclusion data point to contributions of unexchanged meteoric waters. Mechanisms of gold transport and precipitation are less contrained. A possible model involves transport of gold as bisulfide complexes, and precipitation due to one or more of the following processes: decrease of sulfur activity due to precipitation of sulfides, wall-rock reaction, cooling/dilution, and/or fluid unmixing.

Transfer and deposition of gold in the Congo watershed

Gold has been measured in water and suspended particulate matter samples coming from the Oubangui-Congo system. Gold anomalies up to 800 pico M/l were measured at two different locations. These are interpreted as the result of gold transport by dissolved organic complexes. Everywhere else the gold content is in the range of 10 −11 M/l and gold is present as an aurous hydroxichloride complex. The knowledge of both discharge rates and gold content allows a quantification of the gold input into the Atlantic ocean. Respective values for dissolved and particulate gold input are equal to 2.86 t/a and 2.95 t/a. Both chemical denudation and physical weathering rates were computed from this new set of data.

Distribution of septohippocampal neurons containing parvalbumin or choline acetyltransferase in the rat brain

A combination of retrograde transport of horseradish peroxidase or wheat germ agglutinin-colloidal gold with either single or double-label immunohistochemistry is used to describe the comparative topographic distribution of parvalbumin- and choline acetyltransferase-immunoreactive septal neurons that project to the hippocampal formation of the rat. The morphometric parameters of the retrogradely labelled, parvalbumin-containing neurons were very similar, if not identical, to those neurons of the midline and medial part of the medial septum and the diagonal band regions that had previously been shown to be immunoreactive for gamma-aminobutyric acid or for glutamate decarboxylase following colchicine treatment. The total number of parvalbumin-immunoreactive and choline acetyltransferase-positive retrogradely labelled cells was counted at 9 representative levels through the rostrocaudal extension (from 2.4 mm anterior to the level of bregma) of the medial septal-diagonal band complex. In the whole medial septum-vertical limb of the diagonal band region, about 33% of the total retrogradely labelled neurons showed immunoreactivity to parvalbumin, whereas the parvalbumin-negative cells were mainly choline acetyltransferase-immunopositive. In comparison with the average figure, the proportion of the retrogradely labelled parvalbumin-containing neurons was higher in the middle part (around 1.5 mm anterior to the bregma) than in either the rostral or caudal ends. The reverse was true for the distribution of the cholinergic septohippocampal neurons. At the maximum levels the parvalbumin-immunoreactive neurons accounted for more than half of the total retrogradely labelled cells in 4 out of 6 rats. Moreover, within the complexity of the septal neurons, a marked regularity of topographic organisation was observed in the distribution of retrogradely labelled parvalbumin-containing GABAergic and choline acetyltransferase-positive cholinergic neurons as if they were subdivided cytoarchitectonically.

Progressive alteration associated with auriferous massive sulfide bodies at the Dumagami Mine, Abitibi greenstone belt, Quebec

Pyritite and massive sphalerite-galena bodies are surrounded by two distinct concentric alteration zones within the metafelsites of the late Archean Blake River Group at the Dumagami Au-Ag-Cu deposit, southern Abitibi belt, northwestern Quebec. The inner alteration zone is characterized by andalusite-bearing rocks and is interpreted as a metamorphosed advanced argillic alteration zone; the outer zone is represented by white mica phyllonite and phyllonitic breccia and is interpreted as a metamorphosed sericitic alteration zone. Surrounding least altered rocks, represented by massive quartziferous augen rocks and ribboned micaceous augen schists, are composed of blue quartz and albite phenocrysts, quartz, biotite, ilmenite, and chlorite. The sericite zone is characterized by the metamorphic mineral assemblage quartz-sericite-rutile-pyrite + or - andesine and by the absence of albite phenocrysts, biotite, and ilmenite. The andalusite zone is characterized by the metamorphic mineral assemblage quartz-andalusite-sericite-rutile-pyrite + or - kyanite and by the absence of albite phenocrysts, andesine porphyro-blasts, biotite, and ilmenite.Petrographic and geochemical features of least altered, sericitic, and andalusite-bearing rocks indicate that felsic effusive rocks were the principal protolith. Strong leaching of alkali and alkaline earths (especially Mg) within the inner alteration zone has resulted in a notable absence of chlorite, a feature atypical of the alteration associated with Archean massive sulfide deposits. Chemical and mineralogical changes from least altered to altered rocks suggest that highly acidic hydrothermal fluids were responsible for the premetamorphic formation of alteration assemblages at the Dumagami deposit. However, gold transport in these fluids would be minimal, and gold emplacement is linked instead to a late, structural event.

Colloidal transport of gold and silica in epithermal precious-metal systems: Evidence from the Sleeper deposit, Nevada

Research Article| August 01, 1990 Colloidal transport of gold and silica in epithermal precious-metal systems: Evidence from the Sleeper deposit, Nevada James A. Saunders James A. Saunders 1Department of Geology and Geological Engineering, University of Mississippi, University, Mississippi 38677 Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (8): 757–760. https://doi.org/10.1130/0091-7613(1990)018<0757:CTOGAS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James A. Saunders; Colloidal transport of gold and silica in epithermal precious-metal systems: Evidence from the Sleeper deposit, Nevada. Geology 1990;; 18 (8): 757–760. doi: https://doi.org/10.1130/0091-7613(1990)018<0757:CTOGAS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Bonanza-vein gold ore from the Sleeper deposit, Nevada, was deposited at a depth of less than 500 m and exhibits a characteristic texture of alternating colloform bands of fine-grained quartz and bands of opaline silica containing abundant electrum. Textural evidence indicates that the quartz bands recrystallized from a gel-like amorphous precursor, whereas the gold-rich bands were deposited originally as amorphous precipitate formed by the coagulation of colloidal silica and gold particles. Gold and silica precipitated initially as colloidal particles at deeper levels in the hydrothermal system and were mechanically transported upward by the hydraulic action of the ore-forming fluids. Coagulation of the colloidal particles may have been caused by solution cooling or boiling, and catalyzed by hydrated polycationic Al3+ aqueous species. Transport of gold as colloidal particles in epithermal systems indicates that, at least in some instances, higher gold content is possible than in true solution. A gold-enriched ore fluid appears necessary to explain the origin of the bonanza-grade ore at the Sleeper mine, and the colloidal gold transport process locally might have played a role in the genesis of the more-common disseminated epithermal precious-metal deposits. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Demonstration of extensive brainstem projections to medial and lateral thalamus and hypothalamus in the rat

We use the method of retrograde transport of colloidal gold with silver intensification to map locations of brainstem neurons projecting to various nuclei of the medial and lateral thalamus (Menetrey, Histochemistry 83, 391–395, 1985; Seeley and Field, Brain Res. 449, 177–191, 1988). In rats injections of < 1.0μl of a gold-wheatgerm agglutinin-horseradish peroxidase complex were restricted to the centrum medianum, centralis lateralis, medialis dorsalis, zona incerta, ventrobasal thalamic complex, or medial or lateral hypothalamus. Injections in the centralis lateralis, with some involvement of centrum medianum and medial medialis dorsalis, labeled numerous neurons throughout the rostrocaudal extent of the midbrain periaqueductal gray, and adjacent reticular formation, dorsal raphe nucleus and substantia nigra. These labeled neurons were distributed bilaterally, but with a preponderance ipsilaterally. Numerous neurons in the deep layers of the ipsilateral superior colliculus and in the anterior pretectal nucleus were also labeled. Many neurons in the pontomedullary raphe nuclei (including nucleus raphe magnus), locus coeruleus and dorsolateral parabrachial nuclei, and in the pontomedullary reticular formation, were labeled bilaterally. Fewer were seen bilaterally in the lateral reticular nucleus and nucleus of the solitary tract, with relatively few in the dorsal aspect of the trigeminal nucleus caudalis. Injections restricted largely to the centrum medianum labeled fewer cells in the brainstem, but with similar distributions. An injection restricted to the centralis lateralis also resulted in a similar distribution of labeled neurons in midbrain and nucleus raphe magnus but did not label neurons at more caudal medullary levels. An injection restricted to zona incerta labeled numerous neurons in a distribution similar to that following medial thalamic injections, except that many more were observed in the contralateral sensory trigeminal nucleus and in the dorsal column nuclei. Injections of the ventrobasal thalamic nucleus labeled many neurons in the trigeminal and dorsal column nuclei, but many fewer neurons in the midbrain, periaqueductal gray and reticular formation compared with medial thalamic injections. Labeled neurons were also seen in the superior and inferior colliculi, due presumably to involvement by the injection of the lateral posterior and magnocellular medial geniculate nuclei, respectively. Injections in the lateral hypothalamus labeled numerous neurons in a distribution similar to that seen following medial thalamic injections. A restricted medial hypothalamus injection labeled neurons in a pattern similar to that seen with lateral hypothalamus injections. A very restricted, caudal injection in the premammillary region labeled a small number of cells scattered throughout ventral periaqueductal gray, but revealed a strikingly dense, bilateral projection from the ventral and laterodorsal tegmental nuclei (Gudden) in the caudal midbrain. These results largely confirm the origins of brainstem projections to the diencephalon as delineated previously, and additionally reveal larger numbers of labeled neurons compared with some previous studies using other retrograde tracers. The relationship of these brainstem areas to mechanisms of pain and analgesia is emphasized.

Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides; evidence from sulfide mineralogy and the composition of sphalerite

Hydrothermal fluids responsible for the formation of volcanogenic massive sulfides may transport gold in solution as Au(HS) 2 (super -) at low temperatures ( or =350 degrees C), low pH, and elevated salinities (> or =1 m NaCl). The documented fluid chemistry of active hydrothermal vents on the modern seafloor confirms that gold transport is mainly due to Au(HS) 2 (super -) , and the calculated solubility of gold as Au(HS) 2 (super -) is highest in low-temperature (150 degrees -250 degrees C) vent fluids at elevated oxygen and sulfur activities. Sulfides which coprecipitate with gold commonly have properties which can be related to the temperature and sulfidation state of the hydrothermal fluids and therefore reflect conditions which were favorable for the transport or deposition of gold as Au(HS) 2 (super -) .A comparison of gold grades with sulfide mineral equilibria and the FeS contents in sphalerite from a variety of deposits indicates that gold enrichment is closely related to the temperature-a (sub S 2 ) conditions in the ore-forming fluids. Sea-floor sulfides range from sulfur-rich, pyrite-marcasite assemblages to relatively sulfur-deficient, pyrrhotite-isocubanite assemblages, with coexisting sphalerite containing 0 to 55 mole percent FeS. Significant gold enrichment occurs exclusively in low-temperature ( 300 degrees C contain Ferich sphalerite (1-5 mole % FeS) and gold grades < 1 ppm Au. Archean Cu-Zn deposits at Noranda, Quebec, consist mainly ofpyrite-pyrrhotite ores containing 0.5 to 1 ppm Au together with Fe-rich sphalerite (10-12 mole % FeS). However, pyritic ores which occur stratigraphically up-section commonly contain 1 to 3 ppm Au and relatively Fe-poor sphalerite (5 mole % FeS). Different sulfide mineral equilibria and FeS contents in sphalerite are interpreted to reflect the same physical and chemical conditions which influence gold grades and suggest that petrologic indicators of the sulfidation state may be useful guides to gold mineralization in volcanogenic massive sulfides.

Le transport chimique de l'or dans les environnements de surface: formation d'un colloïde et complexation organique

Two different categories of experiments were undertaken to determine the significance of humic substances in the chemical transport of gold. The experiments of the first category were designed to study Au(Cl)4− reduction by humic acid at pH 3.5, which is consistent with the formation of this ion in sulfide oxidation zones. These experiments were repeated with denaturated humic acid (obtained by heating in an acid medium). The results show that Au(Cl)4− is reduced by humic acid to an unstable colloid, but not by denaturated humic acid. The purpose of the second category of experiments was to study gold solubility in various organic materials: humic acid, fulvic acid, and potassium humate. Solutions of these substances and demineralized water (the latter served as the reference point) were placed in contact with metallic gold for 1 month at pH 7, the common pH in surface environments. The data indicate an increase in gold solubility according to the following order: humic acid &lt; fulvic acid &lt; potassium humate. A two-step gold chemical-transport mechanism for surface environments is proposed on the basis of our results. In the first step, unstable colloidal gold is formed from inorganic complexes produced in the sulfide oxidation zones (zones with acidic pH). After full oxidization, the pH moves to the normal value, causing the dissociation of functional groups in humic substances. In the second step, these humic substances, through the dissociated functional groups, could form organo-metallic complexes with gold, ensuring gold dispersion in surface environments. [Journal Translation]

Concentrations of reduced sulphur in inclusion fluids associated with black shale hosted quartz vein gold deposits: implications for mechanisms of transport and deposition of gold and a possible exploration tool

Analyses of reduced S and other inclusion volatiles in vein quartz from the black shale hosted Clogau-St. Davids Au deposit (N. Wales) show that a relation exists between elevated inclusion S content and Au grade. This relation is better at discriminating between high and low grade vein material than criteria based on the other inclusion volatiles. Analyses of material from a similar but subeconomic vein system yields a small number of samples with an “auriferous signature” on the basis of either S 2− content or CO 2/CH 4 ratio, but application of both criteria would reject all samples The concentration of reduced S species in metamorphic fluids thought to have transported Au suggests that Au(HS) 2 − was the dominant Au complex; thermodynamic calculations indicate that up to 8 mg Au/kg H 2O may have been transported in this manner. Gold precipitation in the ore body resulted from destabilisation of the Au(HS) 2 − complex, probably either through reduction by the black shales or as a result of “flashing” of H 2S into the CH 4-rich phase which was generated during the mineralising event.

A chemical model for the concentration of gold in volcanogenic massive sulphide deposits

Study and review of the gold distribution in selected volcanogenic massive sulphide deposits (Rosebery and Hellyer in Tasmania, Mt. Chalmers in Queensland and Millenbach in Quebec) indicates that gold has two distinct occurrences in these deposits: (1) a gold-zinc-lead-silver association which is commonly found in the upper part of zinc-rich massive sulphide lenses (e.g. Rosebery and Hellyer), and (2) a gold-copper association which is commonly present in the stringers and lower portions of copper-rich deposits (e.g. Mt. Chalmers and Millenbach). In each deposit one of these occurrence dominates almost to the exclusion of the other. Thermodynamic modelling of the volcanogenic fluid, which is based on previous work by Large (1977), Eldridge et al. (1983), and Pisutha-Arnond and Ohmoto (1983), indicates that gold in the upper part of the zinc-rich massive sulphide lenses was transported as Au(HS) 2 − while gold in the stringers and bases of copper-rich massive sulphide deposits was transported as AuCl 2 −. The pH and ƒ O 2 of the ore forming fluid have an important control on gold deposition and therefore gold grades. High gold grades in zinc-rich massive sulphide are promoted by near neutral, oxidised and low temperature (150°C to 275°C) fluids as indicated by the presence of barite-carbonate-(feldspar) gangue. High gold grades in the copper-rich stringers and base of the massive sulphide are promoted by higher temperatures (275°C to 350°C), relatively oxidised fluids as indicated by bornite, or by barite or hematite gangue, and relatively acid fluids as indicated by kaolinite gangue. In ƒ O 2 −T space significant gold transport by Au(HS) 2 − occurs only in the pyrite stability field at relatively low temperatures ( < 300°C). On the other hand, gold transport by AuCl 2 − is not restricted by iron mineral equilibria, but is greatest in the hematite, magnetite and pyrite fields at high temperatures ( > 300°C) and high ƒ O 2 ( > 10 −30). Under conditions of Au(HS) 2 − transport, “bonanza” type gold deposition may be found in areas where the hydrothermal fluids passed from the field of pyrite stability to that of hematite stability. Oxidation of hydrothermal fluids by seawater mixing and the rapid precipitation of sulphide minerals at the top of the massive sulphide lens may cause the precipitation of gold via the mechanisms of either H 2S oxidation or sulphide-gold coprecipitation. Other types of gold deposits may be classified into either the gold-zinc or gold-copper association. The gold-copper-(bismuth) association is developed in higher temperature systems of moderate to high salinity where AuCl 2 − is the dominant complex (e.g. porphyry CuAu deposits and Au-rich skarns), whereas the gold-(zinc-lead-silver) association is developed in systems of low salinity and high α H 2S where Au(HS) 2 − is the dominant complex (e.g. epithermal AuAg and Archean greenstone hosted Au-quartz).

Characterization and evolution of ore-forming fluids at Fazenda Brasileiro Gold Mine, Rio Itapicuru Greenstone Belt, BA

Fluid inclusion studies by microthermometry and Raman spectroscopy carried out on the main hosts of the gold mineralizations at Fazenda Brasileiro Mine, southern part of Rio Itapicuru greenstone belt define the mineralizing fluid s as relatively dense (0.85-0.95 g/cm3 ) and hot (&gt; 400°C) solutions composed essentially of CO2 (89.7-85.3 mole%) and minor amounts of CH4 and N2, which gradually evolved to more a90ueous fluids (40 to 62.5 moles% H20) of low salinity(&lt; 10 eq. wt% NaCl) at lower temperatures (250-300°C). The isochores of the carbonic and H20-C02 fluids and the temperature of total homogeneization combined with geothermometric data on the are paragenesis, point to at least two periods of gold deposition: 1. 380-419° C and 2.2-3.2 kb in the quartzofeldspathic and arsenopyrite. Pyrite quartz veins and 2. 270-300°C and 1.2-1.4 kb in the massive quartz vein. Based on experimental work on the solubility of gold complexes, chemical characteristics of mineralizing fluids, ore paragenesis and Au/Ag ratio, it was possible to speculate that the transport of gold occurred mainly by reducing (high ΣH2S/ ΣS04), slightly neutral to alkaline solutions, as thio-complexes such as Au(HS)2, HAu (HS)2 or AU2(HS)2S-2. In the earliest stage of mineralization conditions of gold deposition were probably attained due to fluid/ ock reactions which led to a decrease in the activity of S-2 by precipitating sulphides (arsenopyrite, pyrite, pyrrhotite, etc). The predominance of the H20 regime over the C02 regime in the latest stage might have been the cause of CO2 dilution, decrease in pH and oxidation and, as a consequence, the precipitation of gold. The fluid inclusion data also suggest a metamorphic origin for the mineralizing fluids, most likely through the devolatilization of the basal sequences of the volcano-sedimentary pile. The devolatilization process would be able produce low salinity, H20-C02 fluids which would later migrate through favourable structural sites and deposit their metal content.

Use of colloidal gold complexes of wheat germ agglutinin as a label for neural cells

We have made stable complexes between wheat germ agglutinin and either 5 or 10 nm particles of colloidal gold. These complexes were phagocytosed by neuronal and glial cells in embryonic rat hippocampal cultures and the incorporated gold gave intense, low-background staining in the light microscope either directly, for the most heavily labelled cells, or after intensification by physical development of silver. Cells were labelled in a punctate fashion over perikarya and processes. In the electron microscope, particles of gold were observed in lysosomal vesicles, frequently in an aggregated form. Gold complex incorporated into cells in culture was retained by those cells over periods up to 20 days. Embryonic hippocampal cells were labelled in suspension culture by incorporation of wheat germ agglutinin-gold complexes and transplanted into the brains of syngeneic adult host rats. Grafted neurons and glia were observed in the electron microscope to retain high levels of gold label over periods up to 30 days. Receipt of synaptic connections by transplanted neurones was observed. Complexes of wheat germ agglutinin with 10 nm gold particles were injected unilaterally into field CA3 of the hippocampus of adult rats. Specific retrograde transport of gold was observed in the light and electron microscopes to pyramidal and hilar neurones of the contralateral hippocampus and to neurones of the medial septal nucleus. Colloidal gold-wheat germ agglutinin complexes appear to be useful cellular markers that can be visualized at both light and electron microscope levels.

Gold in mantle peridotites from upper Proterozoic ophiolites in Arabia, Mali, and Morocco

An ultramafic rock source has been suggested for many Archean gold deposits related to carbonatized komatiites. Similar gold mineralization and alteration mineralogy are known in gold-bearing carbonatized peridotites (i.e., listwaenites) from various ophiolites. Carbonatized ultramafic rocks are abundant in upper Proterozoic ophiolites in three areas (Arabia, Mali, and Morocco) that were investigated. There, the average gold contents in serpentinized mantle peridotites range between 3 to 5 ppb. Most of the gold is located in accessory magnetites (10 ppb). Magnetite crystallization and dissolution during serpentinization and shearing, followed by carbonatization of the ultramafic rocks, resulted in concentration of gold in carbonatized serpentinites. Fluid inclusions, trace elements, and isotopic investigations indicate that gold was mobilized at low temperature (150 degrees -250 degrees C), by seawater-derived heterogeneous H 2 O-CO 2 -CH 4 hydrothermal fluids during serpentinization and carbonatization. Gold concentration in listwaenites, which may reach economic grades of 1 to 10 ppm, are closely associated with sulfides, sulfarsenides, or arsenides, suggesting gold transport by sulfur or arsenic complexes in the hydrothermal fluids. Gold was precipitated with these minerals and with silica by an increase in pH as the acidic gold-bearing hydrothermal solutions interacted with the carbonate lenses. Thus, an upper mantle rock source is suggested for gold concentrations in ophiolite complexes.

Fluid-wall rock interaction in an Archean hydrothermal gold deposit; a thermodynamic model for the Hunt Mine, Kambalda

Gold mineralization at the Hunt mine, Kambalda, is situated in a 20-m-thick, steeply dipping shear zone within a sequence of rather uniform metabasalts. Zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization. Limited major element mobility ensured assemblages of low variance over most of the alteration zone, providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic, geochemical (electron microprobe analysis of mineral compositions), and fluid inclusion studies (microthermometry).Equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event, and in particular, activity gradients of components. Profiles across the schist zone show increasing f (sub H 2 S) around auriferous quartz veins (log f (sub H 2 S) = -0.6) but constant f (sub O 2 ) (log f (sub O 2 ) = -29.7). These results are near the magnetite-pyrrhotite-pyrite triple point for the inferred temperature of 350 degrees C and explain the pyrite-pyrrhotite-magnetite zonation sequence outward from veins. The profile of X (sub (CO 2 )) is very close to the value of 0.25 determined independently from fluid inclusions. The K (super +) /H (super +) profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability. Using a value for fluid salinity of 2 equiv wt percent NaCl, pH was calculated to be around 6.9.The solubility of gold in solution was estimated for the calculated conditions: for the complex Au(HS) (super -) 2 the maximum solubility is around 0.5 mg kg (super -1) (i.e., 0.5 ppm), for Au(Cl) (super -) 2 it is close to 10 (super -6) mg kg (super -1) , indicating the efficacy of sulfide transport of gold, compared to chloride transport, under these conditions. The calculated decrease in f (sub H 2 S) away from the auriferous quartz veins results in an 80 percent (or greater) decrease in gold solubility, suggesting hat the sulfidation of Fe-rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures.

Genesis of the Lass vein system, Beaverdell silver camp, south-central British Columbia

The Lass vein is in the Beaverdell silver, lead, zinc (gold) vein camp in south-central British Columbia. Veins in this camp are generally hosted within propylitized Westkettle granodiorite of Jurassic age, but mineralization is related to the Beaverdell quartz monzonite stock of Late Paleocene age (based on a K–Ar biotite date of 58.8 ± 2.0 Ma). Galena lead isotopes, interpreted using the recent "shale," "Bluebell," and "mixing-line isochron" models for the Canadian Cordillera, confirm a Tertiary age for all major vein mineralization in the Beaverdell camp.Examination of metal zoning, mineralogy, fluid inclusions, and sulfur isotopes indicates that the Lass vein system can be divided into two distinctly different parts, an upper western portion and a lower eastern portion. Differences between the two parts are related to the dominance of one of two mineralizing events.Event 1, the earlier, was most dominant in the lower portion of the Lass vein system. By comparison with event 2, event 1 is characterized by (i) relatively gold-, zinc-, and lead-rich but silver-poor ore (Au, 3163 ppb; Zn, 5.34%; Pb, 2.34%; Ag, 208 ppm); (ii) thicker veins (20 cm); (iii) sulfides with abundant pyrite, arsenopyrite, and dark sphalerite with exsolved chalcopyrite; and (iv) fluid inclusions with higher salinities (15 wt. equiv. wt.% NaCl), local CO2 phases, and higher temperatures of homogenization (287 °C) with matching equilibrium temperatures indicated by sulfur-isotope geothermometry from galena and sphalerite (294 °C).The younger event 2 is responsible for most of the mineralization in the upper portion of the Lass vein. Many characteristics of this event are statistically different from those associated with event 1. Namely, event 2 (i) is relatively silver rich and gold, zinc, and lead poor (Ag, 291 ppm; Au, 764 ppb; Zn, 3.07%; Pb, 1.27%); (ii) has narrower veins (10 cm); (iii) has sulfides characterized by silver–sulfosalt-bearing galena and pale sphalerite; and (iv) is represented by fluid inclusions that are variable but on average lower in salinity (7 equiv. wt.% NaCl), have lower temperatures of fluid homogenization (225 °C), and do not contain CO2 phases.Estimates from fluid inclusions indicate that event 1 could have occurred at depths equivalent to those of event 2 if the former was under lithostatic pressure at temperatures near the boiling point and if the latter formed at hydrostatic pressures near boiling temperatures. Sulfur-isotope data indicate that event 1 was nearly boiling; no similar definition is available for event 2. Thus, only minimum depth estimates are available for event 2.The most likely scenario for formation of the Lass vein system starts with event 1 and is followed by event 2. During event 1, minerals were deposited in a confined system under lithostatic pressures from high-salinity fluids where chloride complexing could have been important in the transportation of gold. Fracturing of the vein system to the surface changed the pressure regime to hydrostatic, and CO2 was released. Consequently, temperatures and salinities of the fluids dropped, presumably mainly in response to mixing with cooler and less saline groundwater. Thus, gold solubilites concomitantly decreased, and silver deposition became more significant.Fluid-inclusion and lead-isotope analyses appear to be useful exploration procedures in the Beaverdell area in the identification of gold-rich systems. A further practical observation is that gold should continue to depth in the Lass system, if the vein can be followed.