Lode Gold Mineralization Research Articles

Gold grade variation and particle microchemistry in exploration pits of the Batouri gold district, SE Cameroon

The Batouri area hosts lode-gold mineralization under several-m-thick lateritic cover. Pitting to bed rock on a geochemical Au anomaly defined from previous reconnaissance soil sampling identified five horizons ranging from saprock at the base to laterite at the top. Analysis of bulk samples from each horizon by fire assay shows that most of the horizons are barren although 119 ppb and 48 ppb Au values were obtained from one laterite horizon and one saprolite horizon, respectively, from two separate pits. All the horizons were panned and particulate gold was also recovered only from these two horizons. The gold grains from both horizons are morphologically and compositionally indistinguishable with rare quartz, pyrite and galena inclusions. The grains have irregular, sub-rounded, bean to elongated shapes and they show a remarkable core–rim zonation. Electron microprobe analysis of the grains recorded high gold content in the rims (86.3–100 wt%) and along fissures within the grains (95.1–100 wt%). The cores are relatively Ag rich (11.8–14 wt% Ag) while the rims (0.63–13.7 wt% Ag, most of the values fall within the lower limit of this range) and fissures (0.03–5.02 wt% Ag) are poor in Ag. The low Ag concentration in the rims and along fissures is attributed to preferential leaching of Ag; a process recognized in gold grains and platiniferous alloys from alluvia. The core composition of the grains is similar to that of primary gold composition in the bedrock. These results show that gold in the soil is relic particulate gold derived from the primary source with no evidence of secondary gold precipitation in the weathering cycle. In all the pits no horizon was systematically enriched in gold suggesting there has been no chemical remobilization of gold in this environment. Rather the dispersion of gold here is in the particulate form. Therefore combining particulate gold features with assay data is relevant to exploration in such tropical environments.

A Giant Gold System, Geita Greenstone Belt, Tanzania

[Extract] Archean greenstone belts are a well-known source of gold deposits. If we ignore the giant Witwatersrand gold system we can confidently say that the majority of Archean gold comes from greenstone belts. Even though as a general rule greenstone belts are highly prospective for gold some greenstone belts have proved to be much more productive than others. For example most of the gold in Canada's Superior Province comes from Abitibi Greenstone Belt, particularly from Timmins area (e.g. Wyman, 2003). Similarly, most of the gold mined from Australia's Yilgarn Craton comes from the Kalgoorlie camp which accounts for more than half of the gold mined so far (Philips et al., 1996) in the entire craton. Interestingly, although far apart, the geology and mineralization styles of gold deposits in both areas are very much alike suggesting that the processes leading to the formation of giant ore systems must be similar starting from the ground preparation to the mineralization process. In the Tanzania Craton the giant Bulyanhulu gold deposit is also similar to the giant deposits found in the Timmins and Kalgoorlie areas, respectively. That is, lode gold mineralization style, mafic host rocks, ages, map scale patterns, deformation styles and the short premineralization crustal history are all largely similar (e.g. Bateman and Bierlein, 2007). Bulyanhulu is much more similar to the Kalgoorlie camp in the sense that it consists of one major gold deposit, while the Timmins camp is made of a large number of smaller gold deposits. These similarities between three giant gold camps situated on three different continents may indicate that in order to form giant gold deposits certain conditions need to be satisfied. However, that is not always the case and the gold mineralization found in Geita Greenstone Belt does not follow the typical pattern of Archean lode gold deposits. Geita Greenstone belt contains a minimum of fifteen gold deposits which may resemble the Timmins camp, but the mineralization style, alteration, structures and host rocks are different. The mineralization in Geita Greenstone belt is preferentially hosted within deformation zones developed along the contact of banded ironstones and porphyries of various compositions rather than along some major shear systems. The structures associated with the mineralised system are minor, the alteration zone is restricted to the mineralised zone, quartz veins are rare or missing although silicification is common.

Metal and fluid sources in a potential world-class gold deposit: El-Sid mine, Egypt

Lode gold mineralization at the El-Sid mine area is associated with the ca. 600 Ma Fawakhir granite intrusion, which cuts the ~737 Ma ophiolite nappes in the Central Eastern Desert of Egypt. The mineralized quartz veins are hosted by ~E- and NE-trending fault/fracture sets cutting the western boundary of the intrusion and sheared ophiolites. The results of electron microprobe analyses of gold-associated hydrothermal sulfide and silicate minerals suggest that Au was mobilized alongside Ni, Co, Cr and As from the adjacent ophiolitic serpentinite. After granite emplacement, hydrothermal fluids interacted with the sheared serpentinite, leaching metals and re-depositing them in the faults/fractures and adjacent wall rock in a cyclic process. Low-salinity aqueous-carbonic fluids with significant quantities of volatile species (CO2, CH4, and N2 ± H2S) leached and transported Au from deep to shallow crustal levels. Carbon dioxide had a buffering effect on the Au-bearing hydrothermal solution, maintaining its pH within a narrow near-neutral range, where elevated gold concentration was transported by complexation with reduced magmatic sulfur in a reducing environment. Gold deposition along fault/fracture conduits in the Fawakhir granite and adjacent serpentinite resulted from interplay of pressure drop, fluctuations in oxygen and sulfur fugacities, and exsolution of the volatile phases. Infiltration of meteoric water may have contributed to the formation of the late stage gold-sulfide mineralization that formed at shallower levels during terrane uplift. Sulfidation of the Fe-rich magmatic minerals was, on the other hand, the overriding process in the wall rock as evidenced by abundant disseminated sulfides with gold inclusions. Considering the structural control by regional shear zones (fluid conduits) and the voluminous granitic and ophiolitic rocks (metal sources), a high tonnage gold deposit amenable to open pit mining at the El-Sid mine area is very likely.

Fluid-rock interaction during high-grade metamorphism: Instructive examples from the Southern Marginal Zone of the Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex documents strong evidence that CO2-rich (XCO2fluid=0.7–0.9, XH2Ofluid=0.1–0.3) and brine fluids of greatly reduced water activity interacted with cooling metapelitic granulite during the thrust-controlled emplacement at 2.69–2.62Ga onto the granite-greenstone terrain of the northern Kaapvaal Craton. Interaction of cooling metapelitic granulite with CO2-rich fluids at T<600–630°C and P<∼6kbar is recognized by the presence of a regional retrograde Opx-out/Ath-in isograd and an associated zone of retrograde hydrated granulites that occupies ∼4500km2 of retrogressed crust located in the hanging wall section of the shallow north-dipping Hout River Shear Zone that bounds the Southern Marginal Zone in the south. On the other hand, brine fluids are considered to have triggered the main pulse of anatexis that resulted in production of large volumes of granodioritic-trondhjemitic melts that intruded and started to interact with metapelitic granulite in the deep crust at T>∼900°C, P>∼7.5kbar. Interaction of hot melt with metapelitic granulite continued until final emplacement in the middle crust (P=∼6kbar, T=∼630°C). Brine fluids also initiated shear zone-hosted metasomatism of quartzo-feldspathic gneisses at T between ∼600 and ∼900°C and amphibolite-facies lode-gold mineralization. Available data implicate devolatilization of underthrusted greenstone material as the dominant deep crustal source for infiltrating CO2-rich and brine fluids.

Fluid-rock interaction in retrograde granulites of the Southern Marginal Zone, Limpopo high grade terrain, South Africa

Fluid infiltration into retrograde granulites of the Southern Marginal Zone (Limpopo high grade terrain) is exemplified by hydration reactions, shear zone hosted metasomatism, and lode gold mineralisation. Hydration reactions include the breakdown of cordierite and orthopyroxene to gedrite + kyanite, and anthophyllite, respectively. Metamorphic petrology, fluid inclusions, and field data indicate that a low H2O-activity carbon-saturated CO2-rich and a saline aqueous fluid infiltrated the Southern Marginal Zone during exhumation. The formation of anthophyllite after orthopyroxene established a regional retrograde anthophyllite-in isograd and occurred at P-T conditions of ∼6 kbar and 610 °C, which fixes the minimum mole fraction of H2O in the CO2-rich fluid phase at ∼0.1. The maximum H2O mole fraction is fixed by the lower temperature limit (∼800 °C) for partial melting at ∼0.3. C-O-H fluid calculations show that the CO2-rich fluid had an oxygen fugacity that was 0.6 log10 units higher than that of the fayalite-magnetite-quartz buffer and that the CO2/(CO2+CH4) mole ratio of this fluid was 1. The presence of dominantly relatively low density CO2-rich fluid inclusions in the hydrated granulites indicates that the fluid pressure was less than the lithostatic pressure. This can be explained by strike slip faulting and/or an increase of the rock permeability caused by hydration reactions.

Greenstone-hosted lode-gold mineralization at Dungash mine, Eastern Desert, Egypt

The auriferous quartz±carbonate veins at Dungash mine, central Eastern Desert of Egypt, are confined to ∼E-trending dilation zones within variably foliated/sheared metavolcanic/volcaniclastic rocks. The vein morphology and internal structures demonstrate formation concurrent with a dextral shear system. The latter is attributed to flexural displacement of folded, heterogeneous rock blocks through transpression increment, late in the Neoproterozoic deformation history of the area. Geochemistry of the host metavolcanic/metavolcaniclastic rocks from the mine area suggests derivation from a low-K, calc-alkaline magma in a subduction-related, volcanic arc setting. In addition, chemistry of disseminated Cr-spinels further constrain on the back-arc basin setting and low-grade metamorphism, typical of gold-hosting greenstone belts elsewhere.Mineralogy of the mineralized veins includes an early assemblage of arsenopyrite–As–pyrite–gersdorffite±pyrrhotite, a transitional pyrite–Sb–arsenopyrite±gersdorffite assemblage, and a late tetrahedrite–chalcopyrite–sphalerite–galena–gold assemblage. Based on arsenopyrite and chlorite geothermometers, formation of gold-sulfide mineralization occurred between ∼365 and 280°C. LA-ICP–MS measurements indicate the presence of refractory Au in arsenian pyrite (up to 53ppm) and Sb-bearing arsenopyrite (up to 974ppm). Abundant free-milling gold associated with the late sulfide assemblage may have been mobilized and re-distributed by circulating, lower temperature ore fluids in the waning stages of the hydrothermal system.Based on the isotopic values of vein quartz and carbonate, the calculated average δ18OH2O values of the ore fluids are 5.0±1.4‰ SMOW for quartz, and 3.3±1.4‰ for vein carbonate. The measured carbonate δ13C values correspond to ore fluids with δ13CCO2=−6.7±0.7‰ PDB. These results suggest a mainly metamorphic source for ore fluids, in good agreement with the vein morphology, textures and hydrothermal alteration. The calculated δ34SH2S values for early, transitional, and late sulfide assemblages define three distinct ranges (∼1.5–3.6‰), (∼0.4–1.0‰), and (−3.7‰ to −1.9‰), respectively. The systematic evolution towards lighter δ34S values may be attributed to recrystallization, or to ore fluid buffering under variable physicochemical conditions.The shear zone-related setting, mineralogy and isotopic characteristics of gold mineralization in Dungash mine are comparable with other orogenic gold deposits in the region (e.g., Barramiya deposit), which may suggest a regional setting controlling gold metallogeny of the region. This setting should guide future exploration programs in the central Eastern Desert province.

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.

Controls on lode gold mineralization, Romite deposit, South Eastern Desert, Egypt

Field and remote sensing studies reveal that Au-bearing quartz ± carbonate lodes in Romite deposit, in the extreme South Eastern Desert of Egypt, are controlled by NNE-striking shear zones that splay from the ca. 660–550 Ma Hamisana Zone. Quartz in releasing bends with sinistral shear geometry and abundant boudinaged quartz-carbonate lodes with serrate ribboned fabrics suggest vein formation throughout a transpressive wrench system. Ubiquitous hydrothermal quartz, carbonate, and subordinate chlorite and sericite within the shear zones and as slivers in veins, indicate that gold deposition and hydrothermal alteration occurred under greenschist facies conditions. The Al (IV) in chlorite indicates a formation temperature of ∼300 °C, comparable with temperatures estimated from arsenopyrite composition for grains intimately associated with gold in quartz veins.The new geological and geochemical data indicate that splays off the Hamisana Zone are potential gold exploration targets. Quartz veins along the high order (2nd or 3rd) structures of this crustal-scale shear zone are favorable targets. In the Romite deposit and in surrounding areas, a Au-As-Cu-Sb-Co-Zn geochemical signature characterizes mineralized zones, and particularly rock chips with >1000 ppm As and high contents of Cu, Zn, and Co target the better mineralized areas.The carbonate δ13CPDB and δ18OSMOW isotope signatures preclude an organic source of the ore fluid, but metamorphic and magmatic sources are still valid candidates. The intense deformation and lack of magmatism in the deposit area argue for metamorphic dewatering of greenstone rocks as the most likely fluid source. The narrow ranges of δ13C (−4.6‰ to −3.1‰) and δ18O (11.9‰–13.7‰) in carbonate minerals in lodes imply a corresponding uniformity to the ambient temperature and δ13CCO2 (δ13CΣC) of the ore fluids. The calculated δ18OH2O values of 6.9‰–7.9‰ for ore fluids, based on δ18O values of vein quartz further suggest a likely metamorphic origin.

Lode-gold mineralization in convergent wrench structures: Examples from South Eastern Desert, Egypt

Data from regional- and mine-scale mapping, structural, geochemical and fluid inclusion studies reveal a syn-kinematic genesis of gold in convergent wrench structures that cut the Neoproterozoic greenstone belt in the south Eastern Desert of Egypt. Au-quartz lodes in the Wadi El Beida–Wadi Khashab area are associated with NNW-trending shear zones in pervasively silicified, ferruginated volcanic/volcaniclastic rocks, or along steeply dipping thrust segments bounding allochthonous ophiolitic blocks. Development of the mineralized shear zones is attributed to a wrench-dominated transpression assigned to D2 deformation throughout the Pan-African evolution of the South Eastern Desert (620–540Ma?).Hydrothermal alteration associated with Au-quartz lodes comprises an inner quartz–sericite–pyrite assemblage progressed outwards into an outer quartz–chlorite–calcite assemblage. Mass balance calculations reveal a systematic volume- and mass-increase, addition of SiO2, K2O, Fe2O3t, S and L.O.I. and slight depletion in Na2O in the altered wallrocks approaching the Au-quartz lodes. Erratic concentrations of S, MgO and CaO in the altered host rocks, however, suggest selective carbonatization, and sulfidation of Fe-rich host rocks proximal to the ore bodies.Ore bodies include quartz-only and quartz–carbonate lodes with disseminated pyrite, chalcopyrite, chalcocite, covellite, marcasite, subordinate pyrrhotite, sphalerite and gold. In addition to free gold inclusions in As-poor pyrite, microprobe and LA-ICP-MS spot analyses reveal the presence of traces of Au, Ag (10s to 100s ppm-levels), positively correlated with Cu contents (1000s ppm-levels). Analyses of pyrrhotite and marcasite indicate a comparable relationship. Chalcopyrite intimately associated with pyrite contains lower levels of refractory gold and silver (av. 12ppm Au and 3ppm Ag). Solid solution may have been responsible for invisible gold, whereas, free gold deposition is a function of remobilization, reconstitution and concentration of the earlier phase.Primary and pseudosecondary fluid inclusions in the auriferous quartz veins comprise low salinity aqueous–carbonic fluids (2–12eq. wt.% NaCl). Homogenization temperatures of synchronous aqueous-dominant and carbonic-dominant (H2O–CO2–NaCl±CH4±N2) fluid inclusions (258–343°C) correspond to 0.8–2.3kbars and depths of 3 and 9km (mesothermal conditions). Decreasing gold solubility and segregation from bi-sulfide complexes most probably resulted from interplay of dilution and mixing of an evolved carbonic-rich fluid with a more oxidized aqueous fluid, pressure fluctuation and wallrock-sulfidation.New geochemical data combined with available geophysical information indicate viable gold ore bodies in the study area, and suggest similar situation for zones with discernible signs of hydrothermal alteration along the major convergent wrench (shear) zones in the Eastern Desert of Egypt.

The Barry Gold Deposit, Abitibi Subprovince, Canada: A Greenstone Belt-Hosted Gold Deposit Coeval with Late Archean Deformation and Magmatism

The Barry gold deposit is an example of an Archean greenstone-hosted lode gold deposit located in the Urban-Barry greenstone belt in the Abitibi subprovince of Quebec, Canada. The host mafic volcanic rocks are part of the 2717 Ma Macho Formation of the Northern Volcanic zone. They are cut by a series of weakly foliated preore diorite, pre- and postore quartz-feldspar porphyry (QFP), and quartz monzonite dikes and plugs, providing a unique opportunity to evaluate the age of the gold mineralization. Auriferous zones are spatially associated with northeast-trending ductile shear zones with moderate south-easterly dip. Gold mineralization occurs within straight N64° E/64° SE and folded N20° E/60° SE, on average, albite-carbonate-quartz veins and within the surrounding carbonate-quartz-pyrite, and locally within biotite-carbonate alteration zones of the host mafic volcanic rocks. The auriferous veins comprise 5 to 15 vol percent of the mafic volcanic rocks, are 1 to 5 cm wide, and are locally boudinaged. Although the volcanic units strike N55–60° E and dip 40° SE, the ore envelope (>2 g/t Au) is constrained from surface to a depth of 30 m in an antiformal shape. Gold generally occurs as microinclusions in pyrite but is also present as free gold in albite-carbonate-quartz veins, synmineralization altered host rocks, and locally within quartz veins cutting early QFP dikes. The deposit has an indicated resource of 309,500 oz Au (7,701,000 t at 1.25 g/t Au) and an inferred gold resource of 471,950 oz Au (10,411,000 t at 1.41 g/t Au). U-Pb dating of single zircon grains from the premineralization diorite and postmineralization QFP yielded indistinguishable ages, averaging 2697 ± 0.6 Ma, which is interpreted as the age of gold mineralization at the Barry deposit. This age and the field relationships reveal that lode gold mineralization was coeval with regional deformation and magmatism, similar to that documented at the Kiena, Norlartic, and Siscoe (Main zone) deposits of the Val d’Or district. These deposits represent the earlier orogenic gold event in the Abitibi greenstone belt, which preceded the postmagmatic, quartz-tourmaline auriferous veins such as those present at Sigma. Barry, Kiena, Norlartic, and Siscoe share many similarities with other orogenic gold deposits worldwide that are spatially associated with felsic and intermediate intrusions. The genesis for these deposits is not well constrained, as it is not obvious to distinguish whether the auriferous fluids were derived from the coeval magmas or whether both magmas and fluids were derived from a thermal event from the deep crust, or possibly the mantle.

Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek, Yukon Territory, Canada as deduced from microchemical characterization of placer gold grains

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration.

Archaean gold mineralization synchronous with late cratonization of the Western Dharwar Craton, India: 2.52 Ga U–Pb ages of hydrothermal monazite and xenotime in gold deposits

New zircon U–Pb ages for a felsic volcanic rock (2,588 ± 10 Ma) and an intrusive granite (≥2,555 ± 6 Ma) in the Gadag greenstone belt in the Western Dharwar Craton, southern India, are similar to dates for equivalent rocks in the Eastern Dharwar Craton and indicates docking of the two cratons prior to this time. The zircons in the intrusive granite are strongly overprinted, and coexisting titanites yielded two different age populations: the dominant group gives an age of 2,566 ± 7 Ma, interpreted as the emplacement age, whereas the minor group gives an age of 2,516 ± 10 Ma, reflecting a hydrothermal overprint. In situ U–Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522 ± 6 Ma) and Ajjanahalli (2,520 ± 9 Ma) gold deposits reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit in the eastern Dharwar Craton. The new dates confirm that both the younger greenstone belts and lode gold mineralization in the Dharwar Craton are about 100–120 My, younger than in other well-dated Archaean cratons. Although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking of the lower-middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India.

COMMENTS ON THE PAPER "CRATONIC EXTENSION AND ARCHAEAN GOLD MINERALISATION IN THE SHEBA-FAIRVIEW MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA" BY P.H.G.M. DIRKS, E.G. CHARLESWORTH AND M.R. MUNYAI (SAJG, 112, 291-316, 2009)

The authors are to be commended on this meticulous addition to the data-base and interpretation of the structural controls of hydrothermal lode gold mineralisation in the Barberton Greenstone Belt. However the conclusion that all mineralised structures resulted from extension rather than compression may …

Mapping of prospectivity and estimation of number of undiscovered prospects for lode gold, southwestern Ashanti Belt, Ghana

In the southwestern part of the Ashanti Belt, the results of fractal and Fry analyses of the spatial pattern of 51 known mines/prospects of (mostly lode) gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales. Intersections of NNE- and NW-trending faults were likely predominantly involved in local-scale structural controls on lode gold mineralisation, whilst NNE-trending faults were likely predominantly involved in district-scale structural controls on lode gold mineralisation. The results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area. The applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines/prospects results in a one-level prediction of 37 undiscovered lode gold prospects. The applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines/prospects result in an estimate of 40 undiscovered lode gold prospects. The study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits.

Automatic Analysis of Aeromagnetic Images for Gold Exploration

Archaean lode gold mineralisation is known to occur near major crustal breaks manifesting as large-scale shear zones; which act as conduits for mineralising fluids. Mineralisation occurs in regions of structural complexity adjacent to the shear zones. This paper reports on-going image processing research that aims to automatically detect such geologically significant characteristics from aeromagnetic data for the prospectivity analysis. The current system finds the regions of magnetic discontinuity that are related to shear zones and lithological boundaries, and then identifies prospective regions nearby. The regions of magnetic discontinuity are detected using a combination of texture analysis and bilateral symmetric feature detection, where line-like features representing high local magnetic variations are identified. Then the areas adjacent to the regions of magnetic discontinuity are examined using a 2D fractal surface analysis technique to search for areas with complex magentic expressions that are related to structural complexity. A preliminary experiment was conducted using aeromagnetic data from the Yilgarn Craton in Western Australia and the regions selected by the proposed system contained over 76% of all known deposit locations and 82% of the greater than 1 tonne-deposit locations. Our on-going research also includes identifying the major shear zones from the regions of discontinuity to examine their spatial and geometrical characteristics.

Geochemistry of komatiites in the Eastern Goldfields Superterrane, Western Australia and the Abitibi Greenstone Belt, Canada, and implications for the distribution of associated Ni–Cu–PGE deposits

The Abitibi Greenstone Belt (AGB) of the Superior Province in Canada, and the extensive greenstone sequences of the Eastern Goldfields Superterrane (EGS) of the east Yilgarn Craton in Western Australia, both contain widespread and well-studied komatiite sequences. Both of these 2˙7 Ga belts contain broadly similar assemblages of volcanic rocks ranging from komatiites to rhyolites, and both contain abundant lode gold mineralisation. Despite this similarity, the two belts have widely differing metal endowments. The EGS is heavily endowed with magmatic Ni–Cu–PGE deposits, but contains only minor volcanogenic massive Cu–Zn sulphide occurrences, whereas the situation in the AGB is the reverse. The komatiite assemblages of the two belts are compared using a locality-weighted analysis of large whole-rock geochemical datasets. The EGS contains a higher relative proportion of highly olivine-rich cumulate rocks relative to the AGB, and shows more extensive evidence for crustal contamination of komatiite magmas. The most highly mineralised region of the AGB, the Shaw and Bartlett Dome area, is the most similar to the EGS in the distribution of komatiite rock types and also the presence of a crustal contamination signature. The bulk of samples in both data sets show no evidence for extensive PGE depletion attributable to sulphide liquid segregation, indicating that the komatiite suites in both belts were undersaturated in sulphide during partial melting, ascent and eruption. The combination of abundant thick cumulate-rich komatiite units and widespread crustal contamination in the EGS can be attributed to a predominance of rapid, high volume eruptions, relative to generally lower eruption rates in the AGB. Differences in lithospheric structure are likely to be the cause.

Palaeozoic arc magmatism in the Central Asian Orogenic Belt of Kazakhstan: SHRIMP zircon ages and whole-rock Nd isotopic systematics

Early Palaeozoic tonalite to granodiorite intrusions in northern Kazakhstan are associated with lode gold mineralization and have SHRIMP zircon ages of 457.3 ± 6.6 Ma (Aksu), 452.9 ± 5.6 Ma and 447.4 ± 5.4 Ma (both Zholymbet). The Stepnyak intrusion contains large xenoliths with an age of 480.6 ± 5.0 Ma. One early Palaeozoic zircon from a porphyritic diorite at Stepnyak has a core with a near-concordant 207Pb/ 206Pb age of 3888 ± 1.5 Ma, whereas other xenocrystic grains are between 983 and 2698 Ma old. The early Archaean age is probably inherited from unexposed basement of the Kokchetav Massif and represents the oldest crustal material so far known from the Asian continent. It appears that the Aksu, Zholimbet and Stepnyak granitoids were emplaced in the late Ordovician in an Andean- or Japan-type continental arc environment on the margin of the Kokchetav Massif. Late Palaeozoic granitoids in central Kazakhstan have Devonian zircon ages of 407.1 ± 3.9, 381.1 ± 3.1 and 369.2 ± 4.9 Ma, whereas the youngest sample from the Topar Massif has a late Carboniferous emplacement age of 314.1 ± 5.1 Ma. Initial ε Nd values range from +5 to −1 corresponding to Nd-model ages of 1.1–0.6 Ga. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly juvenile crustal protoliths. Our data also confirm that Devonian and Carboniferous arc magmatism in central Kazakhstan assimilated older and already accreted island arc systems, yet is derived from primitive sources, probably in a subduction setting. The range of ε Nd values with corresponding Nd mean crustal residence ages in the range of 0.6–1.1 Ga for most samples indicates variable recycling of late Proterozoic largely juvenile continental crust in the formation of the Palaeozoic magmatic arcs of central Kazakhstan. The orogenic evolution terminated with Permian anorogenic, rift-related alkaline granite magmatism ( ε Nd = +2 to +8) which reflects melting of juvenile, mantle-derived lower crust.

The golden transformation of the Cretaceous plate subduction in the west Pacific

Long-term couplings between the subducting and overlying plates are very important to understanding plate tectonics, in particular intraplate evolutions. Geological records of this coupling however, are usually not well preserved. Here we show a good example in eastern China where Cretaceous tectonic evolution matches remarkably well with the drifting history of the Pacific plate. The most pronounced phenomenon is that the eastern China large-scale orogenic lode gold (Au) mineralization occurred contemporaneously with an abrupt change of ~ 80° in the drifting direction of the subducting Pacific plate, concurrent with the formation of the Ontong Java Plateau. Given that lode Au deposits usually form at the onset of compressional or transpressional deformations, the Au deposits dated the major tectonic change from extension to transpression in eastern China, coherent with the subduction regime. The Cretaceous drifting history of the Pacific plate also tallies with other major geological events in eastern China, e.g., the evolution of the Tan-Lu fault and magmatic activities, suggesting that the major geological events in eastern China in the Cretaceous were mainly controlled by the subduction of the Pacific plate, and that plate interactions during subduction are important driving forces for geological evolution in eastern China and intraplate tectonics in general.

Ore controls in the Charters Towers goldfield, NE Australia: Constraints from geological, geophysical and numerical analyses

The approach taken in this paper, namely synthesising a wealth of previous information with new data and a genetic model, in combination with integrated numerical analyses, led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the Charters Towers goldfield, NE Australia. The method also has implications for the assessment of other “mature” goldfields worldwide. Despite a number of different ore controls having operated within the Charters Towers goldfield, the controlling factors can be linked to a single genetic model for orogenic, granitoid-hosted lode-gold mineralisation in a brittle deformation regime (D 4) of NE–SW to NNE–SSW shortening, under conditions of supralithostatic fluid pressure and low stress difference. Spatial autocorrelation results suggest district-scale alignment of the auriferous veins parallel to and overlapping with the ESE–WNW- to E–W-striking Charters Towers–Ravenswood lineament, a major crustal boundary in the basement to the Ravenswood batholith. At the camp-scale, auriferous veins have abundance and proximity relationships with NW–SE-, NNW–SSE-, NE–SW- and ENE–WSW-oriented lineaments, suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps. Fractal dimensions obtained with the box-counting method range from 1.02 to 1.10, whereas veins in the Charters Towers City camp are characterised by a significantly higher fractal dimension of 1.28. This discrepancy may be taken to imply that most or all outcropping and near-surface deposits within the Charters Towers City camp have been found and that new discoveries are more likely to occur at greater levels of depth, or outside the boundaries of this camp. The new understanding has implications for the assessment of the residual prospectivity of the Charters Towers goldfield, where large areas of prospective rock types and structures (e.g., approximately 40% of the Charters Towers–Ravenswood lineament) are hidden under cover. This parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers. The following steps are suggested for the development of a targeting strategy for lode-gold exploration in areas of the goldfield under cover: (1) identify from geological and geophysical data the ENE–WSW (± 15°) and NNW–SSE (± 15°) striking structures and geological boundaries within a 20-km-wide corridor parallel to and centred upon the Charters Towers–Ravenswood lineament, the potential control on gold deposit distribution at the regional- to district-scale, (2) interpret from geological and geophysical data the distribution of pre-Middle Devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits, (3) deduce from geophysical data the ENE–WSW (± 15°) and NNW–SSE (± 15°) striking structures that cut or bound the intrusions identified in step 2, (4) locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line (e.g., potential bends or splays) and/or intersect other structures or geological contacts, or both, and (5) define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones.

Lode–gold mineralization in the Tanami region, northern Australia

The Tanami region of northern Australia has emerged over the last two decades as the largest gold-producing region in the Northern Territory. Gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks, and by sulfide-rich replacement zones within iron formation. Although limited, geochronological data suggest that most mineralization occurred at about 1,805–1,790 Ma, during a period of extensive granite intrusion, although structural relationships suggest that some deposits predate this period. There are three main goldfields in the Tanami region: the Dead Bullock Soak goldfield, which hosts the world-class Callie deposit; The Granites goldfield; and the Tanami goldfield. In the Dead Bullock Soak goldfield, deposits are hosted by carbonaceous siltstone and iron formation where a late (D5) structural corridor intersects an early F1 anticlinorium. In The Granites goldfield, deposits are hosted by highly sheared iron formation and are interpreted to predate D5. The Tanami goldfield consists of a large number of small, mostly basalt-hosted deposits that probably formed at a high structural level during D5. The D5 structures that host most deposits formed in a convergent structural regime with σ 1 oriented between E–W and ENE–WSW. Structures active during D5 include NE-trending oblique thrust (dextral) faults and ESE-trending (sinistral) faults that curve into N- to NNW-trending reverse faults localized in supracrustal belts between and around granite complexes. Granite intrusions also locally perturbed the stress field, possibly localizing structures and deposits. Forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid-crust. In areas characterized by the N- to NW-trending faults, orebodies also tend to be N- to NW-trending, localized in dilational jogs or in fractured, competent rock units. In areas characterized by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during E–W- to ENE–WSW-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation. Ore deposition occurred at depths ranging from 1.5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430°C, similar to lode–gold fluids elsewhere in the world. These fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granites. Lead isotope data suggest that lead in the ore fluids had multiple sources. Hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids. Gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks. Fluid unmixing is caused by three different processes: (1) CO2 unmixing caused by interaction of ore fluids with carbonaceous siltstone; (2) depressurization caused by pressure cycling in shear zones; and (3) boiling as ore fluids move to shallow levels. Deposits in the Tanami region may illustrate the continuum model of lode–gold deposition suggested by Groves (Mineralium Deposita 28:366–374, 1993) for Archean districts.