Secondary Gold Research Articles

Angular Arenediethynyl Complexes of Gold(I)

Alkynylgold(I) complexes have been prepared from the angular bis(alkyne) MeC6H3-3,5-(CCH)2, in which the alkyne units are rigidly fixed at an angle of 120° to each other. Reaction with [AuCl(SMe2)] and base yields the shock-sensitive, insoluble compound [{MeC6H3-3,5-(CCAu)2}n] (2), which reacts with isocyanide, phosphine, or phosphite ligands (L) to give [MeC6H3-3,5-(CCAuL)2] (3; L = P(OPh)3, P(OMe)3, PPh3, PMePh2, t-BuNC, MeNC). The complexes with L = P(OMe)3, PMePh2 exist in the solid state as infinite one-dimensional ribbon structures in which neighboring molecules are associated through secondary gold−gold bonding. Complexes with diisocyanide ligands 1,4-C6R4(NC)2 (R = H, Me) have the formula [{MeC6H3-3,5-(CCAu)2(μ-LL)}n] and are insoluble in organic solvents; they are suggested to have a zigzag polymeric structure. In contrast, complexes with diphosphine ligands (CH2)n(PPh2)2 (n = 1−6) are soluble and are suggested to be the cyclic tetragold(I) complexes of formula [{MeC6H3-3,5-(CCAu)2(μ-LL)}2]; the ...

Oxidised gold deposits: Relationships between oxidation and relative position of the water‐table

Many oxidised gold deposits exhibit features which strongly suggest an interplay between geo‐chemical processes and movement of the water‐table. Geochemical processes involving the dissolution of gold‐silver and gold‐tellurium alloys, and precipitation of high‐fineness gold (low Ag) in transition zones, neck‐oxide and lateral oxide zones can take place both above and below the water‐table. In general the lower availability of oxygen below water‐tables ensures that these processes occur at a slower rate in the saturated zone than in the overlying water‐undersaturated weathering profile. Mass‐balance calculations on the Golden Web deposit, Coolgardie, indicate that dissolution and reprecipitation occur above the water‐table. Transfer of gold downwards from the leached zone created to the transition zone located just above the water‐table, is quantitative. Variations in the rate of downward movement of the water‐table(s), relative to the rate of oxidation of the primary gold and sulfides contained in the system, can result in up to eight different types of oxidised gold deposits. Three of these occur in relict profiles (R1‐R3), three are related to erosional profiles (E1‐E3), and two are representative of depositional profiles (D1, D2). Type R1, relict profile with one (high) water‐table, is representative of new systems in which oxidation and downward leaching are incipient. Type R2, relict profile with recently receded water‐table, is representative of a more mature system in which there has been late‐stage stream capture or climate change resulting in water‐table recession, whereas type R3, relict profiles with multiple stillstands, occurs in yet more mature profiles with a succession of such events. When such relict profiles are eroded, three related profiles are developed with very different surface expression: type E1, eroded profile with recently receded water‐table; type E2, eroded profile with receding water‐table and very active weathering; and type E3, eroded profile with stillstands deep in the profile. In depositional profiles two distinct types can be recognised: type D1, depositional profiles with lateral flow; and type D2, depositional profiles with no lateral flow (for which salt‐lake environments provide a classic example). Examples for all but the first type are known in Western Australia. Gradual, rather than episodic reductions in water‐table can result in secondary gold in oxide remnants in the upper parts of weathered profiles. These styles of oxidation have considerable impact on exploration in two ways: they affect the methods of exploration, and the economics of mining the deposit. Deeply leached profiles clearly have large amounts of overburden; enriched transition zones can have a positive impact on profitability. Knowledge of the oxidation processes and styles of gold oxidation can be used to direct exploration to the optimum strategy.

Quantification of sulfur and phosphorus within secondary gold rims on Yukon placer gold

An examination of Yukon placer gold nuggets using ultrathin-section (70 nm) transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed three distinct morphological and compositional regions. The outer surface of the nuggets possessed silicate minerals, which included electron dense particles containing gold (13.6 to 27.2 at%) and silver (6.7 to 13.2 at%). The organic composition of these particles ranged from 7.3 to 12.3 at% S and 0.53 to 1.5 at% P (4.4 to 5.4 at% PO). The periphery of the nuggets also possessed crystalline gold-bearing material that was enriched with gold (81 to 93.12 at%; 30Au:1Ag) compared to the particulate material (2Au:1Ag) present within the silicate minerals annealed to the nuggets' surface. The organic composition of this peripheral secondary gold ranged from 0.29 to 5.9 at% S and from 0.63 to 5.9 at% P (4.5 to 9.5 at% PO). The internal structure of the secondary gold around the periphery of the nuggets was highlighted by the occurrence of compositional banding caused by these lighter, electron-translucent compounds. Internal regions of the nuggets were composed of gold or gold and silver, indicating the presence of an original unweathered grain that served as a nucleating site. These findings suggest that sulfur and phosphorus, presumably of biological origin, are important to placer gold formation.

Detrital origin of hydrothermal Witwatersrand gold—a review

The Witwatersrand ‘basin’ is the largest known gold province in the world. The gold deposits have been worked for moren than 100 years but there is still controversy about the ore forming process. Detailed petrographic studies often reveal that the gold is late in the paragenetic sequence, which has led many researchers to propose a hydrothermal origin for the gold. However, observations, such as the occurrence of rounded, disc‐like gold particles next to irregularly shaped or idiomorphic secondary gold particles in the same sample, suggest an initial detrital gold source within the Witwatersrand strata. Mineral chemical and isotopic data, together with SEM cathodoluminescence imaging and fluid inclusion studies, provide evidence for small‐scale variations in the fluid chemistry – a requirement for the short‐range mobilization of the gold. The existing data and observations on the Witwatersrand rocks support a model of hydrothermally altered, metamorphosed placer deposits, with at least two subsequent gold mobilization events: hydrothermal infiltration in early Transvaal time (2.6–2.5 Ga) and during the 2.020 Ga Vredefort impact event.

Deposition and transport of gold by thiosulphates, Veitsch, Austria

AbstractTwo types of gold can be distinguished in tetrahedrite from the Veitsch magnesite deposit, Austria. Primary gold present in unfractured massive tetrahedrite, has a grain size up to 18 μm and contains, on average, Cu 8.73, Ag 7.01, Au 78.63 and Hg 2.35 (wt.%). Secondary gold is present in fractures and is directly intergrown with digenite, covellite, Cu-thiosulphate, cuprite and chrysocolla but not with malachite or azurite. The secondary gold is up to 200 µm in size with an average composition of Cu 3.06, Ag 6.82, Au 86.41 and Hg 3.51 (wt.%). It is usually closely intergrown with thiosulphates containing up to 0.21 wt.% Au. This, together with the presence of ‘dirty’ gold with cloudy thiosulphate inclusions, directly indicates the transport and deposition of Au by a thiosulphate ligand. We believe this is the first reported direct evidence of gold transport and deposition by thiosulphate complexes in a natural environment.

PLACERS: Geophysical characteristics of the Tertiary palaeochannels in the Yilgarn Block, Western Australia

The precise geometric definition of the Tertiary palaeochannels in the Yilgarn Block of Western Australia is important in the exploration for uranium and for secondary and placer gold which occur in the channel sediments. The positions of the palaeochannels normally show considerable displacement from the positions of the present-day drainages. Physical property contrasts which exist between the channel sediments and the underlying Archaean bedrock can be differentiated by geophysical methods to locate the best parts of the channels for follow-up drilling. The low densities of the channel-fill sediments in many areas give rise to 1 to 2 mGal gravity lows and the gravity technique can be used to define the broad shape and depth of the channel. The deepest parts of the channel, which correspond to the zones of highest salinity, can then be delineated using time-domain electromagnetics. Downhole gamma logs are useful in identifying and quantifying prospective uranium zones.

Use of the polymerase chain reaction for the detection of Chlamydia trachomatis from endocervical and urine specimens in an asymptomatic low-prevalence population of women

The Amplicor Chlamydia trachomatis test is a polymerase chain reaction (PCR)-based methodology used for the detection of a cryptic plasmid found in C. trachomatis. It was evaluated in comparison with cell culture and the Microtrak II Chlamydia enzyme immunoassay (EIA) for the detection of C. trachomatis in urogenital specimens from women. Endocervical swabs were collected from 993 women attending the women's unit at the Mount Sinai Hospital in Toronto. In addition, concomitant first void urine specimens were collected from 394 of these women for PCR testing only. As compared with culture of the endocervical specimens, PCR and EIA had a sensitivity, specificity, positive predictive value and negative predictive value of 84.6%, 99.2%, 57.9%, and 99.8% and 61.5%, 99.7%, 72.7%, and 99.5%, respectively. As compared with the secondary gold standard of a positive culture and/or a positive PCR using a primer to the major outer membrane protein the sensitivity, specificity, positive, and negative predictive values for culture were 72.2%, 100%, 100%, and 99.5%, respectively. For the Amplicor PCR and EIA the results were 88.9%, 99.7%, 84.2%, and 99.9% and 61.1%, 99.9%, 91.7%, and 99.6%, respectively. When the urine PCR was compared with the same standard, the test had a sensitivity of 91.7% and a specificity of 99.5%. Based on this study the Amplicor C. trachomatis test was found to be sensitive and specific for the detection of C. trachomatis in both endocervical and urine specimens.

The in vitro formation of placer gold by bacteria

A laboratory simulation was developed to provide mechanistic information about placer (nugget) gold development in the natural environment. To initiate the simulation, ionic gold was immobilized to a high capacity by Bacillus subtilis 168 (116.2 μg/mg dry weight bacteria) as fine-grained intracellular colloids (5–50 nm). During the low-temperature diagenesis experiment (60°C), the release of organics due to bacterial autolysis coincided with the in vitro formation of hexagonal-octahedral gold crystals (20 μm). This octahedral gold was observed to aggregate, forming fine-grained placer gold (50 μm). In addition to achieving a fundamental understanding into secondary gold deposition, a significant economic benefit could be realized by employing this environmentally safe procedure to concentrate widely dispersed gold in placer deposits to facilitate mining by conventional methodologies.

Mineralogy of the near-surface expression of Au-As-Cu mineralization in an arid environment

AbstractIn the arid, Late Precambrian terrain of southern Israel, a complex suite of minerals and amorphous species were deposited in host gneiss from fluids under near-neutral conditions within 1 m of the surface. The morphology of secondary gold appears to relate to its host mineral (skeletal-dendritic with quartz; multi-faceted crystals with arsenates; spherical droplets with iron oxide). The gold is very fine-grained, and was most likely complexed as a thiosulphate.Three amorphous phases are present (iron oxide, chrysocolla, Cu-Mn-(Fe-As) silicate). At least in part, gold and baryte appear to have crystallized out of a metal-Fe-oxide gel. Other minerals, including apatite, anglesite, and conichalcite, may have grown from appropriate crystallites present in the gel.The conichalcite occurs mainly as bladed to acicular radial spherulites. In the presence of lead, a solid solution phase between duftite and conichalcite (‘Pb-conichalcite’) was formed.

Gold mass balance in lateritic profiles from savanna and rain forest zones

The gold mass balance was studied in two areas gold mineralization; one in a savanna zone in South Mali, the other in a rain forest zone in French Guiana. In the ferricrete profile of the savanna zone, the gold mass balance is strongly negative within the ferricrete itself, where almost all the primary gold stock is leached. Beneath the ferricrete, an absolute accumulation of fine secondary gold is found in the mottled zone. The gold cycle is controlled by the migration of Fe and associated with the neogenesis of iron oxides, particularly hematite. The lateritic profile in the rain forest zone is truncated, and a soil is developing from the underlying saprolite in equilibrium with the equatorial conditions. An absolute accumulation of gold is found in the soil, with gold migration and reprecipitation being controlled by the evolution of the macroporosity. The global mass balance of gold in the Guiana deposit shows a leaching rate of 40 % in the weathered part of the mineralization. This process occurs mostly at the weathering front. In the two examples, gold shows opposite behaviors due to different pedogenic and paleoclimatic conditions. This might influence the interpretation of geochemical soil surveys in various intertropical environments.

Gold Enrichment in the Supergene Zone of a Southern Urals Pyrite Deposit, Russia

The Gay deposit, situated in the Orenburg region, is identified with one of Russia's principal occurrences of pyrite (pyrite deposits are an important source of Russia's gold). It belongs to the west subzone of the Magnitogorsk synclinorium and occurs in Devonian rhyolite-basaltic volcanic rocks. The deposit comprises five large pyrite-chalcopyrite, pyrite-chalcopyrite- sphalerite, and pyrite orebodies. The supergene zone extends to 120-240 m below surface and consists of the following three subhorizontal zones (from bottom to top): the secondary sulfide enrichment, the leaching, and the oxidation zone (where ores are enriched in gold). There are two levels of secondary gold enrichment in the weathering profile. The lower level, located in the leaching zone, corresponds with the level of water table fluctuations. The rich, flat-lying horizon (1.5-10.0 m) is composed of bedded, friable native sulfur-quartz ores; it contains 19.0-52.2 ppm Au and up to 389 ppm Ag. Native gold and silver halides (chlorargyrit...

Gold abundance and an aluminosilicate-rich alteration zone in the Val D'Or mining district, abitibi, Quebec: Examples of the fossil geothermal fields

Investigation of gold abundance in a gold-rich mining district by accurate analytical techniques (e.g., Instrumental Neutron Activation and/or Radiochemical Neutron Activation) shows that gold is anomalously distributed in some parts of the Val d'Or mining district of Quebec. It is possible to distinguish: (1) background values (1.4–3.5 ppb Au); (2) zones of primarily anomalous gold values around the Lamaque-Sigma mines (median: 15 ppb Au); (3) enrichment halos around gold orebodies (median: 70 ppb Au); (4) secondary gold enrichment in shear zones; and (5) high background values for the East Sullivan Monzonitic Stock. The Bourlamaque batholith is considered to be a gold-enriched intrusion because of areas (shear zones) that are anomalously enriched in gold (median: 10 ppb Au). It is concluded that there are auriferous geological units in the Val d'Or mining district, such as the Bourlamaque batholith and Val d'Or Formation. The latter contains the Lamaque-Sigma gold mines, representing 49.6% of the total gold production in the district. It is interpreted that the Val d'Or Formation is part of a central volcanic complex within an island arc system, that the centre of this complex is located in the main Lamaque plug, and that this environment may be compared to high-temperature active geothermal systems which are commonly responsible for the formation of epithermal gold deposits. The Lamaque-Sigma area is considered to be a fossil geothermal field, potentially explaining the anomalous gold values. In addition, a dumortierite-bearing, aluminosilicate-rich zone of hydrothermal alteration in the Val d'Or mining district is considered, as for other similar occurrences, to be the result of hydrothermal alteration within a geothermal field producing an Al- and Si-rich leached protholith which has subsequently been metamorphosed and deformed. A model is proposed for the formation of the gold deposits within the primary gold-rich environment, by mobilisation of gold in the volcanic rocks due to metamorphism and deformation. Therefore, two distinct and successive events are postulated: (1) gold-rich synvolcanic geothermal activity; and (2) late remobilisation from the host rocks, followed by deposition of gold ore within favourable structures.

Morphology of Witwatersrand gold grains from the Basal Reef; evidence for their detrital origin

Thousands of rounded particles of fine-grained gold, released from a crossbedded gold specimen of the Basal reef placer in the Welkom gold field after acid digestion of the host quartz arenite, have revealed morphological features and abrasion textures that are characteristic of detrital gold. The size range, which is between 0.038 and 0.473 mm, has a lognormal distribution with a mean of 2.88 phi units (0.136 mm) and a standard deviation of 0.48. This is a characteristic of well-sorted sediment. The shapes are predominantly disclike with toroidal forms being most common. A Corey shape factor of 0.08 indicates extreme flattening. The preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport.A knobby surface texture similar to placer grains recovered from both recent and Devonian placers, and produced during laboratory-controlled tumbling with sand and cobbles in water, is considered distinctive of sediment abrasion. Secondary gold, comprising 25 percent of the total gold content, is finer grained, has an interstitial jagged form, and displays unabraded smooth to crystalline surfaces.Study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive Basal reef placer and led to research concerning its metamorphic overprints.

A case study of the postdepositional alteration of the Witwatersrand Basal Reef gold placer

An investigation of the mineralogy, quartz texture, fluid inclusions, and gold composition of a crossbedded auriferous specimen of the Archcan Basal reef placer from the Welkom gold field, Witwatersrand, South Africa, has established the degree of postdepositional alteration to which the detrital gold particles were subjected. Three different postdepositional events can be distinguished: (1) regional static recrystallization at a peak metamorphic temperature between 290 degrees and 350 degrees C and a pressure between 2 and 3 kbars, (2) local fluid infiltration along fractures and sedimentary interfaces at a temperature between 240 degrees and 300 degrees C and a pressure similar to the above, and (3) local K (super +) metasomatism during a later fluid infiltration event along the preexisting zones of weakness. Intracrystalline diffusion during these events led to homogenization of the gold on a hand specimen scale with respect to Ag (8.9 wt %) and with respect to Hg (1.0 wt %). Furthermore, during the second event (2), fluid infiltration caused not only homogenization but also partial mobilization of some of the detrital gold with transport over distances of not more than a few millimeters to centimeters. These processes were accompanied by microscopic silicification, calcitization, and possibly Hg metasomatism. The pH value of this infiltrating fluid was around 4. Primary fluid inclusions associated with secondary gold in authigenic quartz consist of CaCl 2 -rich aqueous solutions whereas secondary fluid inclusions within such authigenic quartz are dilute aqueous solutions, having a very low salinity that is attributed to NaCl and KCl (event 3). It is believed that gold mobilization was coeval with a fracturing and shearing event after deposition of at least parts of the Transvaal Supergroup on top of the Witwatersrand and Ventersdorp Supergroups. The last event (3), the infiltration of K (super +) -enriched fluid after the partial mobilization of the gold, is tentatively correlated with the Bushveld or Vredefort events.

Supergene gold mineralogy at Ashanti, Ghana: Implications for the supergene behaviour of gold

AbstractAt the Ashanti concession, Ghana, gold-bearing quartz veins and disseminated sulphide lodes occur in narrow (1-3 m) shear zones with altered argillites and metatholeiite host rocks. The mineralisation is concealed by up to 10 m of kaolinite-mica forest ochrosol soils, beneath which is a saprolitic zone of leached rock extending down 60-70 m to the hypogene ore zone. In the unweathered hypogene orebody, gold occurs as free grains in quartz, as sub-microscopic inclusions in the disseminated arsenopyrite, as gold tellurides and as aurostibite. The gold is released from the hypogene orebody by physical dissaggregation and chemical dissolution, the latter involving hydroxyl, thiosulphate, cyanide, and fulvate complexing. Dissolution and reprecipitation of the gold appears to have taken place largely in situ with little evidence of supergene enrichment. Consequently, the gold mineralogy of the soils is complex with residual and secondary gold grains exhibiting widely different textural and compositional characteristics. Residually enriched grains display pitted, rounded surfaces and have silver-depleted rims, while supergene gold grains are compositionally homogenous and have unpitted surfaces. The supergene grains display platelet, dendritic, irregular and octahedral habit. A fine grained spongy form of gold has also been observed from weathered telluride-bearing quartz veins. Much of the secondary gold is intergrown with iron oxides and hydroxides. The gold mineralogy of the Ashanti soils appears to be controlled by physico-chemical processes active during the lateritic pedogenesis producing residual and supergene enrichment of gold.

Fluid-inclusion studies of high-grade metamorphic rocks of the Ashuanipi complex, eastern Superior Province: constraints on the retrograde P–T path and implications for gold metallogeny

The high-grade metamorphic rocks of the Ashuanipi complex have been the subject of a microthermometric fluid-inclusion study. Four types of fluid inclusions were observed: CO2-rich fluids; low-temperature, high-salinity H2O fluids; CH4 ± N2-rich fluids; and high-temperature, low-salinity H2O fluids. The regionally distributed CO2-rich fluids are the earliest fluids, and their calculated isochores indicate a clockwise post-peak metamorphic P–T–t path for the Ashuanipi complex. The low-temperature, high-salinity aqueous fluid inclusions are also distributed regionally and can be interpreted as late brines, retrograde metamorphic fluids, or the wicked-off aqueous component of H2O–CO2 fluid inclusions. Both CH4 ± N2-rich fluids and the high-temperature, low-salinity aqueous fluid inclusions were found only locally in gold-bearing metamorphosed banded iron formations. Fluid-inclusion microthermometry, arsenopyrite thermometry, and metamorphic petrologic study at Lac Lilois, one of the principal gold showings, suggest that some gold deposition may have occurred during regional post-peak metamorphic exhumation and cooling at P–T conditions near the amphibolite–greenschist transition. However, it is possible that gold deposition began at higher near-peak metamorphic P–T conditions. Another major gold showing, Arsène, is characterized by CH4 ± N2-rich fluid inclusions, tentatively inferred to be either directly related to gold deposition or responsible for secondary gold enrichment. The association of CH4 ± N2-rich fluids with gold occurrences in the Ashuanipi complex is comparable to gold deposits of the Abitibi greenstone belt and of Wales, Finland, and Brazil.

Near‐surface secondary gold mobility and grain‐size enhancement, Barewood Mine, east Otago, New Zealand

Abstract Primary gold at Barewood, east Otago, New Zealand, occurs in quartz veins in the Otago Schist. The gold typically occurs as 1–3 pm grains enclosed in arsenopyrite, pyrite, or chalcopyrite. Secondary gold mobility occurred during formation of a zone of kaolinitisation (up to 20 m deep) in the Otago Schist immediately beneath an early Tertiary unconformity. Oxidation of the sulphides released the gold and allowed local migration, possibly as thiosulphate or reduced sulphur complexes. Silver was slightly more soluble than gold, and secondary gold has higher atomic fineness (930) than primary gold (870). Secondary gold is coarse grained (up to 1 mm), and was deposited in fractures, shears, and cavities in quartz veins and associated sheared schist. The secondary gold was precipitated with goethite and kaolinite, and locally with scorodite and/or pharmacosiderite. Dissolution, migration, and redeposition of gold is due to fluid composition evolution from local moderately reduced, sulphur rich, acid conditions at the site of decomposition of sulphides, to highly oxidised neutral pH conditions which prevailed elsewhere. There is no evidence for enrichment of gold in the alteration zone. Grain‐size enhancement of gold by secondary migration near the unconformity was important for formation of alluvial gold deposits in Otago when the unconformity was uplifted and eroded in the late Tertiary.

Gold-bearing iron duricrust in Central Brazil

In the Cuiabá region—State of Mato Grosso, Central Brazil—primary gold mineralization is hosted by two generations of quartz veins in Precambrian metamorphic terrains of the Cuiabá Group. Gold is mined from the veins and mainly from the eluvial horizons that cover the deeply altered basement. In the lodes gold occurs as small particles (less than 1 mm) associated with pyrite and contains up to 5% Ag. Larger particles and nuggets of almost pure gold are found in the iron duricrust which caps the upper levels of the weathering profile. It is difficult to determine the average grade of this kind of deposit but some prospects in the Cuiabá region produce up to 2 g gold per ton of ore. Lateritization is responsible for both the formation of the iron crust and the concentration of gold within the regolith. Under a tropical climate, the supergene alteration of phyllites of the Cuiabá Group has led to the formation of a weathering profile consisting typically of saprolite, mottled clay zone and duricrust, from bottom to top. The duricrust is directly derived form the in situ weathering of phyllites. Geochemical balance calculations indicate that in the transition from the saprolite to the duricrust lateritization has promoted a progressive loss of Si, Al and K, and more than 500% of absolute Fe enrichment. Gold underwent a supergene evolution related to the development of the weathering profile. In the saprolite and mottled clay zone, associated with quartz and oxidized sulfides, gold dissolves as demonstrated by corrosion features at the surface of the particles. The formation of secondary gold in the duricrust is indicated by the larger size of the nuggets, their higher fineness and the close relationship between gold and the neoformed iron oxy-hydroxides.

Supergene geochemistry and crystal morphology of gold in a semiarid weathering environment: application to gold exploration

Oxidation of sulfide- and carbonate-rich vein gold deposits under semiarid conditions can be represented as a three-stage process, each creating supergene environments conducive to dissolution and reprecipitation of gold-silver alloys. The three-stage weathering process of sulfide-carbonate gold veins is depth-dependent, and develops from the relatively young, lowermost weathering horizon just below the water table, through an intermediate weathering horizon in the oxidation zone above the water table, and culminating in the oxide-rich upper saprolite and oxisol. Neoformed gold crystals in the weathering profile have distinct composition and morphologic characteristics from the hypogene gold crystals associated with the sulfide- and carbonate-rich ores. Two distinct types of secondary gold are present in the weathering profile: (1) gold crystals associated with sulfates and arsenates; and (2) gold crystals associated with iron and aluminum oxides/hydroxides, or with kaolinite. The distinct crystal morphologies and mineralogical associations of primary and secondary gold are useful in prospecting for gold deposits in weathered terrains.

Geochemical evolution of laterites from two areas of the semiarid region in Bahia State, Brazil

Lateritic covers developed during the Pleistocene in two distinct areas (Gentio do Ouro District and Sento Sé) of the State of Bahia, Brazil, have been investigated geochemically. Samples of the bedrock (gabbros, from the Gentio do Ouro District, and silicate facies iron formation and associated iron-rich banded carbonates, from the Sento Sé Area) and the lateritic products (latosols, cuirasses and pisolites) were analyzed for major and selected trace elements (Co, Cu, Cr, Ni, V, Pb, Ba, Au and ree). The purpose of the research was to study the geochemical evolution of the laterites and the relationship of weathering processes to secondary gold mineralization. Four types of element mobilities are recognized, defined by progressive depletion (type A) or enrichment (type C) in the sequence latosol-cuirasse-pisolite or by extreme enrichment (type B) or depletion (type D) in the cuirasse. Cr and V have similar behavior (type C), whatever the type of parent rock: however, when the original rocks are gabbros, Co, Cu and Ni demonstrate behavior A, B and A, respectively. Also Ti has the same type of mobility (type A), whatever the type of parent rock. The concentrations of some residual elements, e.g., Ti, V, Mn, Fe and Au, are inherited from the original rocks, that is, they exhibit higher values in the lateritic materials on parent rocks in which their contents are high. The enrichment of ree in the lateritic materials on iron-rich carbonates is attributed to relatively basic and reducing conditions at depth. This environment allowed the ions of these elements (Ce, Sm, Eu and Yb in the reduced ionic form) to stay in solution along with Fe 2+. At shallow depht, under oxidizing conditions, the iron oxyhydroxides adsorbed the ree ions. This adsorption was more effective for La, Ce and Yb ions. The ree contents in the lateritic products from gabbros indicate a depletion of hree plus Eu, Nd and Sm are also depleted in cuirasses and Ce is depleted in pisolites. The loss of these ree and the slight concentration of lree in the red latosol of the Gentio do Ouro District are attributed to partial leaching of ree by a relatively acidic solution. The adsorption of ree by iron oxyhydroxides was less intense here than in the weathering of the iron-rich carbonates at Sento Sé Area. It was also more effective for La and Ce (ions). This type of weathering solution may be generated from oxidation of the accessory pyrite present in the gabbros of the Gentio do Ouro District. It may dissolve primary gold which then precipitates on encountering basic conditions at depth.