Placer ore minerals are commonly intergrown with "relict" phases that coexisted with the ore mineral in the original lode deposit. Studying these relict phases can yield important information about the nature, and formation of the lode deposit. This type of study can be useful in areas with poor exposure, areas that are remote, or areas where discretion is important. Analysis of the heavy mineral suite of stream sediments from the Brush Creek area shows no correlation between the heavy minerals and the gold. However, analysis of the relict phases intergrown with the gold grains yields important results. Placer gold grains recovered from streams draining the Brush Creek deposit, in southwestern Virginia, contain relict quartz, orthoclase, ilmenite and mica. Textures, and fluid inclusion composition and character in the relict quartz, indicate that the gold mineralization post dated the mylonitization associated with the Fries ductile deformation zone, which hosts the gold mineralization. The relict orthoclase is interpreted to be adularia, which is common in low-temperature, hydrothermal environments. The intergrowth textures of the gold and ilmenite show that the ilmenite was present in the country rocks prior to gold mineralization, and was not, therefore, cogenetic with the gold. The relict mica was not positively identified, but is believed to be chlorite, which is consistent with the proposed low temperature mineralization. The textures of the relict phases indicate that gold mineralization occurs in late, brittle fractures, with little or no significant alteration. The study of the relict phases intergrown with the alluvial gold grains has yielded information that otherwise could only have been obtained by more advanced, but also much more expensive, exploration techniques. / Master of Science

Thesis (PhD)--Stellenbosch University, 1992. / ENGLISH ABSTRACT: The dissolution of gold from refractory ores is a complex kinetic problem involving a number of chemical, mass transport and mineralogical factors. In most Witwatersrand ores in South Africa more than 97 % of the gold is dissolved in cyanide medium after a residence time of about 16 hours in pachuca tanks. This high percentage may be the reason why so little fundamental research has been done into the mechanism and kinetics of the leaching process. With the increasingly lower grades of ore mined, the introduction of backfill mining, and the reduction of profit margins, it has become imperative to increase the efficiency of gold dissolution. The effects of the chemistry and particle size on the dissolution of gold in each sample of ore were studied in detail. The emphasis in this study is on the effect of the leaching behaviour of various ore constituents on the rate of gold dissolution. Interferences with the leaching of gold in contact with other minerals or metals could be attributed to the galvanic interaction (electrical conductivity) between the gold and the mineral and to the formation of a surface film on the gold surface. Sulphide minerals and their oxidation products cause the largest decrease in gold dissolution rate. Galena enhances the rate of gold dissolution owing to dissolved Pb(II)-ions. Gold in contact with conductive minerals passivates as a result of the enhanced magnitude of the cathodic cu1Tent. In all experiments the rotating disc of gold passivated so that the rate of dissolution was much slower than that predicted by a mass-transport limiting model. The various films that form on the surface of the gold and associated minerals, as well as the galvanic interaction, depend largely on the pretreatment of the ore. Pre-elimination of host minerals from the gold bearing ore increases the dissolution rate of gold, and explains the kinetics of reaction on the gold surface to a large extent. The selective destruction of the various minerals with oxidative acid leaches destroys and/or decomposes certain minerals which may form films on the gold surface by precipitation. The chemical composition of these films and precipitates depends on the mineralogy of the sample. These films may be oxides, sulphides, carbonates and cyanide complexes. The complexes can be destroyed, depending on the nature of the film, by interstage dilute acid and/or cyanide washes in an agitated vessel. The destruction of the films exposes the gold surface for cyanidation. A simple distribution function similar to the King liberation model is proposed and tested to describe the dissolution step in the multi-step leaching mechanism. For the King model, good agreement is shown with experimental results. For the liberation results obtained by leaching in this study, the trend is co1Tect, but calibration is required for a close fit. A potentially important use for the liberation model by leaching is to predict the leachable or free gold in an ore from the free gold in the complete sample. This approach for studying the leaching behaviour of different gold bearing minerals has provided reasons why some ores leach better than others. / AFRIKAANSE OPSOMMING: Die loging van goud vanuit weerbarstige ertse is 'n komplekse kinetiese probleem wat verskeie faktore soos massa-oordrag, chemiese aspekte en mineralogiese ingeweefdheid insluit. Goud ekstraksies so hoog as 97 % in sianied oplossings in Pachuca reaktore na ongeveer 16 uur logingstyd word behaal in die meeste Witwatersrand ertse in Suid-Afrika. Hierdie hoë ekstraksies mag dalk die rede wees vir die min fundamentele navorsing oor die ekstraksie van goud vanuit minerale in 'n spesifieke erts. Die dalende erts grade, die verlaging van winsgrense en die terugplaas van geloogde erts in die myn noodsaak verbeterde goud ekstraksie. Verkeie faktore nl., chemie, diffusie, partikelgrootte en oplosbaarheid van goud in elke monster erts is in hierdie studie ondersoek. Die sentrale tema was om die logingsgedrag van goud vanuit verskeie minerale in 'n erts te bepaal. Galvaniese interaksie (hou verband met elektriese geleidingsvermoeë) en film vorming is die belangrikste faktore wat die loging van goud in kontak met minerale nadelig beïnvloed. Sulfied minerale en hul oksidasie produkte speel die grootste rol in die verlaging van die tempo van goudloging. Galena verhoog die tempo van goudloging as gevolg van die Pb (II)- ione in oplossing. Goud in kontak met geleidende minerale passiveer as gevolg van die verhoogde katodiese stroomdigtheid. In alle eksperimente met die roterende skyf (goudskyf) apparaat, passiveer die goudskyf in so 'n mate dat die logingstempo baie stadiger is as wat voorspel word met die massa-oordrags model. Die onderskeie films wat vorm op die goud- en geassosieerde minerale se oppervlaktes, asook die galvaniese interaksies, is 'n funksie van die voorafbehandeling van die erts. Die selektiewe eliminering van minerale vanuit 'n gouddraende erts verhoog die tempo van goudloging drasties en dit beskryf die kinetika van goudloging op die goudoppervlak in 'n groot mate. Die selektiewe eliminering van minerale deur gebruik te maak van oksiderende suurlogings, vernietig sekere van die minerale wat films op die goudoppervlakte kan veroorsaak deur middel van presipitasie. Die chemiese samestelling van hierdie films hang af van die mineralogie van die monster. Dit bestaan meestal uit oksiedes, sulfiedes, karbonate en sianiedkomplekse en hulle kan vernietig word deur middel van inter-stadia verdunde suur-en/of sianied wasse. 'n Eenvoudige distribusiefunksie, soortgelyk aan die King bevrydingsmodel word voorgestel en eksperimenteel getoets om die logingstap in die multi-stadia logingsmeganisme te beskryf. Vir bevryding deur loging, is die neiging van King se model korrek, maar kalibrasie word benodig vir goeie passing. 'n Potensiele gebruik van die aangepaste model is om vrye of loogbare goud in 'n spesi fieke partikel grootte fraksie van 'n erts te voorspel as die vry goud in die totale fraksie bekend is. Die resultate uit hierdie studie kan gebruik word om die logingsgedrag van gouddraende minerale te voorspel en te beskryf, en dus veduidelik hoekom goud uit sekere ertse beter loog as uit ander.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: In this study an economical, environmentally friendly, selective and efficient process for the extraction and recovery of [AuCl4]- from aqueous acidic chloride-rich solutions, particularly those aqueous solutions having low concentrations of the precious metal complexes has been investigated. Functionalized poly(styrene-co-maleimide) (PSMI) nanoparticles were synthesized by thermal imidization of the poly(styrene-co-maleic anhydride) (PSMA) copolymer with 3-N,N-dimethylaminopropylamine. Stable water-based dispersions were obtained containing spherical, monodisperse PSMI nanoparticles with a narrow size-distribution and average diameters of 50 ± 5 nm. The specific surface area of the bulk PSMI nanoparticles is 88.1 ± 2.2 m2/g with an average pore diameter of 82.3 Å. 13C NMR, FTIR and elemental analyses confirmed the successful and complete conversion of PSMA into the PSMI derivative. The functionalized PSMI nanoparticles synthesized were investigated as a novel anion-exchange material for the extraction of [AuCl4]- ions from aqueous acidic solutions. Batch sorption studies were carried out as a function of various parameters, such as initial gold concentration, PSMI mass, contact time and agitation rate. The [AuCl4]- extraction occurred with extremely fast sorption kinetics and is dependent on the rate of agitation during the batch sorption process. The functionalized PSMI nanoparticles show a maximum gold loading capacity of 1.76 mmol/g (347.7 mg/g). Langmuir and Freundlich isotherm models were applied to analyze the experimental sorption data. The best model describing the sorption process is given by the Langmuir model. Desorption efficiencies of about 80 % and 93 % were obtained using acidified thiourea (0.25 M thiourea/2 M HCl) and a mixture of 10 M HNO3/0.5 HCl as elutant solutions, respectively. X-ray photoelectron spectroscopy (XPS) analysis unambiguously confirms that the immobilized gold species exists in several oxidation states of 0, +I and +III on the PSMI nanoparticles. This proves that the [AuCl4]- ions initially present in the gold feed solutions unfortunately are subject to a reduction phenomenon on the surface of the functionalized PSMI nanoparticles. The existence of the various gold species contributed significantly to poor desorption efficiencies. In addition to PSMI nanoparticles, micro- to millimeter size PSMI resin beads was prepared by an electrospray methodology. This allows for a wide range of PSMI spherical and quasi-spherical bead diameters of shape to be prepared by manipulation of the experimental conditions employed during the electrospray process, such as the concentration of the PSMI in solution, the capillary tip-to-collector distance, flow rate and the applied voltage. 13C NMR and FTIR spectroscopy analyses show that the electrospray methodology allows PSMI resin beads preparation without any change in chemical composition of the PSMI material. Surface area and porosity analysis shows that 450 μm and 1620 μm PSMI beads selected for use in the gold extraction experiments are microporous and have BET specific surface areas of 2.8 ± 0.4 m2/g and 2.0 ± 0.1 m2/g, respectively. Micro- to millimeter size PSMI resin beads of 450 μm and 1620 μm diameter were tested as potential anion-exchange resins for the extraction of [AuCl4]- from aqueous acidic solutions. The time-dependent studies reveal that the extent of gold uptake increases with an increase in contact time and is dependent on the gold concentration in the [AuCl4]- feed solutions. A maximum loading capacity of 120.7 mg/g and 98.16 mg/g was attained for the 450 μm and 1620 μm resin beads, respectively. The experimental sorption data followed a linear trend consistent with a Freundlich sorption model. This sorption trend for [AuCl4]- suggests that a multi-layer sorption process predominates. Desorption of immobilized gold species from the loaded PSMI resin beads was investigated using various elutants such as HCl, HNO3, thiourea, NaCN and NaOH solutions. The best results were obtained using a mixture of 10 M HNO3/0.5 M HCl as elutant with a desorption efficiency of about 97%. Finally, superparamagnetic magnetite (Fe3O4) nanoparticles with a high degree of crystallinity and phase purity were synthesized by a chemical co-precipitation of Fe2+ and Fe3+ salts. The average diameters of the obtained Fe3O4 nanoparticles were about 7 – 8 nm. The Fe3O4 nanoparticles were coated with oleic acid surfactant molecules and used as seed particles for the preparation of 50 nm diameter magnetic PSMI nanoparticles via an in situ imidization reaction. TEM analysis confirmed that the magnetically responsive PSMI nanoparticles consist of magnetite core-polymer shell structure, although more work is required to perfect such materials. / AFRIKAANSE OPSOMMING: In hierdie studie is ‘n ekonomiese, omgewings vriendelike, seleketiewe en effektiewe proses vir die ekstraksie en herwinning van [AuCl4]- uit suur chloried-ryke oplossings, spesifiek oplossings van lae konsentrasies van edel metal komplekse was bestudeer. Gefunksionaliseerde poli(stireen-ko-maleïmied) (PSMI) nanopartikels was gesintetiseer deur middel van termiese imidisasie van die poli(stireen-ko-maleïk anhidried) kopolimeer met 3-N,N-dimetielaminopropielamien. Stabiele dispersies in water was gevind wat soos sweriese mono-disperse PSMI nanopartikels met ‘n noue partikel-grootte verspreiding met ‘n gemiddelde deursnee van 50 ± 5 nm. Die spesifieke oppervlak area van die massa PSMI nanopartikels is 88.1 ± 2.2 m2/g met ‘n gemiddelde porie-grootte van 82.3 Å. 13C NMR, FTIR en elementêre analiese bevestig die suksesvolle en volledige omskakeling van PSMA na PSMI. Die gefunksionaliseerde PSMI nanopartikels was bestudeer as ‘n nuwe anion-uitruil material vir die ekstraksie van [AuCl4]- ione uit suur oplossings. Stel sorpsie studies was uitgevoer as ‘n funksie van verskeie parameters soos onder andere die goud konentrasie in oplossing, PSMI massa, kontak tyd en ‘n mengings tempo. Die [AuCl4]- ekstraksie gebeur met ‘n geweldige sorpsie kinetika en is afhanklik van die mengings tempo gedurende die stel sorpsie proses. Die gefunksionaliseerde PSMI nanopartikels het ‘n maksimum goud sorpsie kapsiteit van 1.76 mmol/g (347.7 mg/g). Langmuir en Freundlich isoterm modelle was gepas en geanaliseer op die experimentele sorpsie data waarvan die Langmuir isoterm model die data die beste gepas het. De-sorpsie effektiwiteit van ongeveer 80 % en 93% was vekry vir die aangesuurde thiourea (0.25 M thiourea/2 M HCl) en ‘n mengsel van 10 M HNO3/0.5 M HCl as elueer oplossings, onderskeidelik. X-straal foto-elektron spektroskopie (XPS) analiese bevestig ongetwydeld die geimmobileerde goud spesies in oksidasietoestande van 0, +I, en +III op die PSMI nanopartikels. Hierdie is bewyse dat die [AuCl4]- oorspronklik teenwoordig in die goud oplossings is onderhewe auto-reduksie fenomeen op die oppervlak van die gefunksionalieerde PSMI nanopartikels. Die bestaan van verskeie goud spesies dra by tot die power de-sorpsie effektiwiteit van ge-immobiliseerde goud. Bykomend tot die nanopartikels is mikro- tot millimeter grootte PSMI partikels voorberei met ‘n elektro-sproei proses. Hierdie metode stel ons instaat om ‘n wye reeks sferiese en quasi-sferiese PSMI partikel se deersnee voorteberei. Deur die manupulasie van die eksperimentele kondisies gedurende die elektro-sproei proses, soos die konsentrasie van die PSMI in oplossing, die kapilêre punt-tot-ontvanger afstant, vloeispoed en die toegepasde potensiaal. 13C KMR en FTIR spectroskopiese analiese wys dat die elektro-sproei proses die PSMI partikel bereiding toelaat sonder enige veranderinge in die chemiese samestelling van die PSMI materiaal. Oppervlak area en porie-grootte analise wys dat 450lksdfhld en dskl jmm partikels gebruik in die goud ekstraksie eksperimente is mikro-porieës en het spesifieke oppervlak-areas van 2.8 ± 0.4 m2/g en 2.0 ± 0.1 m2/g onderskeidelik. Mikro- tot millimeter grootte poli(stireen-ko-maleimied) (PSMI) partikels van 450 μm en 1620 μm deursnee was getoets as potensieele anion-uitruilings-hars vir die ekstraksie van [AuCl4]- vanuit suur oplossings. Die tyd afhanklike studies gee aanduiding dat die mate van goud opname toeneem met ‘n toename in kontak-tyd en is afhanklik van die goud konsentrasie in die [AuCl4]- oplossings. ‘n Maksimum opname-kapasiteit van 120.7 mg/g en 98.2 mg/g was verkry vir die 450 μm en 1620 μm hars partikels onderskeidelik. Die eksperimentele sorpsie-data volg ‘n lineêre neiging in ooreenstemming met die Freundlich model. Die sorpsie neiging van [AuCl4]- dui aan dat ‘n meervuldige laag sorpsie proses domineer. De-sorpsie van die geimobiliseerde goud spesies vanaf die PSMI hars partikels was bestudeer deur gebruik te maak van verskeie elueermiddels soos HCl, HNO3, thiourea, NaCN en NaOH oplossings. Die beste resultate is verkry deur ‘n mengsel te gebruik van 10M HNO3/0.5M HCl as elueermiddel met n de-sorpsie effektiviteit van ongeveer 97%. Superparamagnetiese magnetiet (Fe3O4) nanopartikels met ‘n hoë graad van kristaliniteit en fase-reinheid was voorberei deur ‘n chemiese ko-neerslagvorming van Fe2+ en Fe3+ soute. Die gemiddelde deursnee van die Fe3O4 nanopartikels was ongeveer 7 – 8 nm. Die Fe3O4 nanopartikels was omhul met oleic suur benatter molekules wat gebruik word as saadjies vir voorbereiding van 50 nm deursnee-magnetiese PSMI nanopartikels deur middel van ‘n imidisasie reaksie. TEM analiese bevestig dat die magnetiese PSMI partikels nanopartikels bestaan uit ‘n magnetiet-kern polimeer-skil struktuur.

The gold metallogeny of Australia is predominantly confined to the Archaean and Palaeozoic Provinces. The Archaean gold occurrences are predominantly hosted in ultramafic-mafic dominated greenstone belts, with less associated tofelsic-volcanic and sedimentary sequences. Most gold occurrences are confined to shear zones or faults, and adjacent discoveries of economic laterite-hosted deposits, host rocks. Recent are presently under investigation and will supply a significant proportion of production in the future. The Proterozoic gold deposits of Australia , are confined to geosyncinal sequences, commonly turbidites (eg: Telfer), with other hydrothermal deposits associated directly to granites. An important feature of the North Australian Craton deposits, is the spatial association of most deposits to granite bodies, although a genetic link has not been established conclusively. The Roxby Downs deposit in South Australia is a unique occurrence of gold in association to copper, uranium and R.E.E. This deposit is tentatively related to intraplate alkaline-magmatism, with further work necessary. The most significant recent discovery of gold mineralization in Australia is in the Drummond Basin in Queensland. This epithermal is tentatively related to mineralization within the Georgetown Inlier. The latter mineralization is Permo-Carboniferous, in a Proterozoic (and possibly Archaean) sequence of schists. It is tentatively suggested that all the gold mineralization in northern Queensland may be related to single tectonic event, a feature which requires further study . Other mineralization in the Phanerozoic includes the turbidite-hosted metamorphogenic deposits of Victoria, the rift related deposits in New South Wales and magmatic related deposits in Queensland. The gold deposits in Australia may in the future be classified in a tectonogeological framework, similiar to the layout of this dissertation, particularly once further data becomes available on recent discoveries.

M.Sc. / This study focuses on the mineralogical variability of the A1, A5 and UE1A Elsburg reefs, obtained at Rand Uranium’s underground mining areas. A total of 133 reef samples, consisting of the Elsburg UE1A, A1 and A5-reefs have been obtained from Cooke 2 and 3 (two of the three Rand Uranium Mines) using the conventional chip sampling method. One of the challenges faced by Rand Uranium Gold Mines in the Cooke section area is the difficulty in differentiating between the various reef types by means of their macroscopic characteristics (colour, pebble types/sizes/shapes, sorting, matrix type, visible sulphide mineralization etc.). This difficulty led to this study which is aimed at utilizing mineral liberation analyzer (MLA)-based automated mineralogy to distinguish between the various reefs and to assess the mineralogical variation within the A1, A5 and UE1A-reefs. The mineralization in this area is hosted by the upper Central Rand Group of the Witwatersrand Supergroup. The main orebodies that are exploited at the mines occur within the Gemsbokfontein Member of the Elsburg Formation. These orebodies have been deformed into an east-west trending anticline at Cooke 3. The present study also attempts to prove or disprove the equivalence of the UE1A-reef on the western limb of the anticline to the A1 or A5-reefs on the eastern limb of the anticline on the basis of mineralogy. Representative splits of the samples were subjected to mineralogical abundance quantification as possible through quantitative MLA-based modal abundance protocols such as XMOD. A standard file on the various mineralogical phases encountered, was created on the 600F MLA and complemented by quantitative XRD (X-ray diffraction) data. Mineral abundances were quantified by MLA, based on integrated backscatter electron (BSE) images and energy dispersive spectrometry (EDS) analyses. Thirty one minerals have been detected using the MLA and they include phases such as quartz, pyrophyllite, chlorite, brannerite, gold, monazite and pyrite as well as minor unknown minerals. Only a few of the minerals are relatively more abundant within the reefs while the majority occurs in very low abundance. Albite, chlorite, muscovite, pyrite, pyrophyllite, quartz, uraninite and zircon are relatively more abundant than the rest of the minerals.

Thesis (Ph.D. (Physics)) --University of Limpopo, 2011 / The importance of gold for scientific uses is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters. It is therefore imperative that the structure of gold nanomaterials is understood for better applications in catalysis and other developments. Molecular dynamics and the density functional theory have proven to be good tools in computational material science and have thus been used to greater lengths. Molecular dynamics simulations on different gold nanoclusters and nanotubes were successfully carried out at different thermodynamic conditions. The effect of size on the melting of materials was duly tested and our results to some extend agree with what has already been reported. Gold nanoclusters show melting below the bulk and the melting temperatures increase with cluster size. However, the Au55 cluster shows different results in that it melts above the bulk due to structural reconstruction. The structure of the clusters changes from spherical shapes to tetragonal or face centred cubic (fcc) structures. Gold nanotubes show no resistance to temperature and different configurations are obtained in different ensembles. Single wall nanotubes form spherical clusters in the NVT while the NPT conditions give patches of clusters at elevated temperatures. The multi wall nanotubes also form spherical clusters in the NVT but fcc structures are obtained in the NPT Berendsen ensemble towards melting. Ab initio calculations in DMOL3 code on different gold nanoclusters show the stability of the clusters to increase with size and the Au3 and Au8 clusters contain the most stable structures. The Au-Au bond length in the dimer was obtained to within reasonable agreement with experiments and other theoretical works. Doping of the clusters further improved their stability although different impurities give different observations. The QMERA code calculations show that a gold atom on top of the surface causes slanting of the outer MD layers. The morphology of the quantum atoms also changes as compared to the neutral surface and the results are compared by the DMOL3 code which confirms the QMERA results. / Mintek, and the National Research Foundation

Although bacterial leaching of sulphidic minerals is a well-known phenomenon, it is only in the last ten years that full-scale bacterial leaching plants have been commissioned for gold processing. In order for bacterial leaching to compete successfully with other pretreatment processes for refractory ores, particularly with established technologies such as roasting and pressure leaching, it needs to be efficient. This requires the optimization of the parameters affecting the leaching reaction and the growth of bacteria. The entire biotreatment process is agitation leaching, carried out in stirred reactors or Pachuca type reactors. The bacterial oxidation is a complex reaction involving gaseous, liquid and solid phases. The interactions are highly complex, and analysis is complicated by the presence of solids in the leaching medium. Inspite of the amount of research that has been performed, kinetic and process models are underdeveloped. Since kinetic data varies widely with the type and source of concentrate, experimental data should be generated before doing the full-scale reactor design. In sizing reactors for a commercial scale process, it would be useful to have a mathematical model that one could use to predict the amount and rate of release of metal, as a function of the various operating parameters of the system. G.R.Halli arsenical gold sulphide concentrate obtained from Hutti Gold Mines Ltd., Karnataka, was chosen for our study, because of its high refractoriness. An indegenous strain of Thiobacillus ferrooxidans was used for biooxidation. The experiments were conducted in a well-agitated stirred tank reactor under controlled conditions. Sparged air was supplemented with carbon-dioxide for optimized growth. In this work, more than 90% gold and 95% silver could be recovered from the sulphidic gold concentrate when bioleaching was used ahead of cyanidation, compared to 40% and 50% by direct cyanidation. A generalized model, which accounts for both direct bacterial attack and indirect chemical leaching, has been proposed for the biooxidation of refractory gold concentrates. The bacterial balance, therefore, accounts for its growth both on solid substrate and in solution, and for the attachment to and detachment from the surface. The overall process is considered to consist of several sub-processes, each of which can be described in terms of a mechanism and related rate expressions. These sub-processes were studied seperately under kinetically controlled conditions. The key parameters appearing in the rate equations were evaluated using the experimental data. Since the refractory concentrate contains pyrite and arsenopyrite as the major leachable entities, leaching studies have been done on pure pyrite and arsenopyrite as test minerals and the key parameters in the rate equations are evaluated using this data. The model so developed is tested with the leaching kinetics of the concentrate. The growth of bacteria is dependent on the availability of the substrate, ferrous iron, and the dependence is modelled by the widely accepted Monod equation. The effect of carbon dioxide supplementation on the bacterial activity was studied and the optimal concentration for growth was found to be l%(v/v). Studies on indirect chemical leaching showed that the rate is sensitive to surface area of concentrate. Indirect rate constant of arsenopyrite was found to be greater than that of pyrite, since pyrite is more nobler than arsenopyrite. Conditions of direct leaching alone was obtained at high pulp density and using substrate adapted bacteria. The rate constant of arsenopyrite was found to be greater than that of pyrite. The parameters obtained were tested with the overall batch leaching data of the concentrate and favourable comparision was obtained. Thus, it has been possible to isolate the various simultaneous sub-processes occurring during the leaching and propose useful models to describe these processes in some detail. The model has been extended successfully to predict the continuous leaching behaviour using the parameters obtained from the batch data. Studies on the effect of residence time and pulp density on steady state behaviour showed that there is a critical residence time and pulp density below which washout conditions occur. The critical residence time at 10% pulp density was found to be 11 hrs. Operation at pulp densities lower than 5% and residence times lower than 72 hrs is not favourable for efficient leaching. Studies on the effect of initial ferric iron concentration showed that there exists an optimum concentration of ferric iron at which the time required to reach steady state is minimum.