Quartz-pebble Conglomerates Research Articles

Factors responsible for Witwatersrand gold mineralisation

Abstract The exceptional gold mineralisation in quartz pebble conglomerates of the Witwatersrand Basin is attributed to a combination of factors. These factors are linked to the co-evolution of the atmosphere, hydrosphere, lithosphere and biosphere, at a very specific time in Archaean geological history and the evolution of the Kaapvaal Craton. Following craton stabilisation and its subaerial emergence, intense chemical weathering and erosion of large volumes of granitoid-greenstone basement released detrital and dissolved gold. Shallow-marine reworking in a long-lived and slowly subsiding basin subjected to episodic compressional deformation and relative sea-level oscillations led to sedimentary concentration of detrital gold. The interaction between acidic, anoxic, and sulfurous surface runoff and more oxidizing marine waters in a near-coastal oxygen oasis supported microbially mediated diagenetic pyrite formation and incorporation of dissolved gold in the pyrite crystal lattice. Erosion and reworking of diagenetic pyrite gave rise to detrital pyrite that characterise most reefs. Abundance of detrital uraninite in conglomerates, derived from erosion of Mesoarchaean granites, and episodes of hydrocarbon migration through sedimentary strata during deep burial set the scene for further enhancement of gold grades in the reefs. Granular and seam pyro-bitumen formed by radiation-induced polymerisation of hydrocarbons around detrital uraninite. Gold dissolved in migrating hydrothermal fluids was then reduced and precipitated upon interaction with the reef pyro-bitumen during hydrothermal placer modification.

Sulphide petrology and ore genesis of the stratabound Sheep Creek sediment-hosted Zn–Pb–Ag–Sn prospect, and U–Pb zircon constraints on the timing of magmatism in the northern Alaska Range

The Sheep Creek prospect is a stratabound Zn–Pb–Ag–Sn massive sulfide occurrence in the Bonnifield mining district, northern Alaska Range. The prospect is within a quartz–sericite–graphite–chlorite schist unit associated with Devonian carbonaceous and siliceous metasedimentary rocks. Volcanogenic massive sulfide (VMS) deposits in the district are hosted in felsic metavolcanic rocks (362 ± 2 Ma) associated with siliciclastic and carbonaceous sedimentary rocks that overlie the stratigraphic sequence hosting the Sheep Creek prospect. Felsic metaigneous rocks in underlying units are 372 ± 4 to 366 ± 4 Ma. Sheep Creek is atypical of the other sulfide deposits in the district in (1) having Sn grades up to 1.2%; (2) being contained in fine-grained, quartz-rich rocks and quartz–pebble conglomerate that likely originated as chert and chert-clast sediment, respectively; and (3) showing minimal evidence of volcanic components in the host rocks. Comparison of immobile trace-element proportions for graphitic and siliceous rocks from the Sheep Creek area with those for argillite associated with the Bonnifield VMS deposits indicates a continental volcanic-arc provenance for the former and a within-plate and passive margin provenance for the latter. In contrast to previously published interpretations, our data analysis supports a clastic-dominated (CD) rather than a VMS affinity for the Sheep Creek prospect. In our model, Zn–Pb–Ag–Sn mineralization formed by syngenetic or early diagenetic processes on or beneath the seafloor, possibly in the shallow-water environment of an outer continental shelf setting. Potential analogues are the Paleozoic CD deposits in the Canadian Selwyn Basin outboard of the Laurentian continental margin.

Detrital gold morphology and recycling around the early Miocene St Bathans paleovalley, Central Otago, New Zealand

ABSTRACT The St Bathans paleovalley developed as a major southwest-draining fluvial system during early Miocene rejuvenation of the Central Otago landscape. Rounded quartz pebbles and detrital gold were recycled from Eocene quartz pebble conglomerates into the headwaters of this Miocene paleodrainage system on the northern margin of the Otago Schist belt. Gold particle morphology is mostly irregular and rough, locally with relict gold crystal shapes, plus a minor component of well-defined flakes. Incremental rounding and surface deformation at the 10 µm scale has occurred through several recycling events in Central Otago, hindering linkage of Miocene and younger placer gold to ultimate basement sources. More intense deformation of the gold, forming thin flakes, occurred in the lower reaches of the Miocene paleodrainage system as gold was transported >100 km to eastern Southland. Hence, the St Bathans paleovalley has been an important source of gold for historic mining in Central Otago and Southland.

Structural Architecture and Mineralogical Relationship in Chitradurga Schist Belt, Dharwar Cration, South India

Abstract: The succession in the late Archaean supracrustal belts in the Chitradurga region begins with basal quartz-pebble conglomerates and quartzites which were deposited on a basement of tonalitic--granitic gneisses, ca. 3000 Ma, containing tracts and enclaves of older supracrustal rocks. Thebasal beds are overlain by basic metalavas and amphibolites interbeddedwith crossbedded quartzites and siliceous phyllites. This series passes up into siliceous phyllites, locally containing a large lenticular mass of polymict conglomerate (Talya Conglomerate), overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates. Greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks, the latter being more abundant at higher levels. With progressive deepening of the whole basin, finegrained, graded greywackes and local polymict conglomerates (Aimangala Conglomerate) were deposited ; the turbidite sequence forms the youngest sedimentary rocks exposed.Estimated total stratigraphic thickness is at least 10 km indeeper parts of the basin. Deformation led to a series of upright anticlines and synclines and relatedLS fabrics with extreme variations in plunge of the coaxial fold axes and Lfabrics within axial surfaces which maintain a steep orientation. Low-grademetamorphism outlasted the deformation. Sedimentation and volcanism in the Chitradurga supracrustal belt is linkedto variable subsidence of the basement foundation, probably as a result of rifting. The structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history. Variable displacement of the basement during theselate Archaean events in the Karnataka craton took place on shear zones and related fractures that acted as channels for water and CO: asindicated by vein minerals and alteration products. Uplift of basement segments and their distension by granitic diapirs led to deformation of thesupracrustal cover made ductile by its high water content

Linking Archaean climate change with gold metallogeny

First large-scale concentration of gold to ore grade in Earth’s crust took place at around 2.9 Ga in quartz-pebble conglomerates, well before endogenous deposits, such as porphyry and epithermal systems or orogenic-type deposits, started to play a significant role from ca. 2.75 Ga onwards. The conglomerate-hosted gold placers, typified by those in the Mesoarchaeaen Witwatersrand Basin in South Africa, are thought to have been sourced from the leaching of background levels of gold in the Archaean continental crust, promoted by deep chemical weathering under a reducing acidic atmosphere. Gold dissolved in meteoric waters was trapped by possibly acidophile microbes, fossil remnants of which are preserved as kerogen layers in the 2.9 Ga lower Central Rand Group of the Witwatersrand Supergroup. Mechanical reworking of the delicate microbial mat-bound gold led to rich gold placers from 2.9 Ga onwards. Well-endowed gold placers older than 2.9 Ga are conspicuously missing, although suitable siliciclastic host rocks, even containing relics of former microbes, are known from continental sediment sequences as old as 3.22 Ga. A review of climate-sensitive rock types and geochemical data on the extent of chemical weathering reveals that the period from 2.96–2.91 Ga was cold, with repeated glaciations as evidenced by glaciogenic diamicite, whereas warmer and (or) wetter climates reigned from 2.90 to 2.78 Ga. In concert with the temporal distribution of placer gold accumulation, the conclusion is reached that chemical weathering rate, dictated by climate, was a key determining factor in the exogenous binding of Au into sedimentary deposits.

Mesoarchean diamonds formed in thickened lithosphere, caused by slab-stacking

When and how Earth's ancient crust – the cratons – became underpinned by cool, thick lithospheric mantle roots capable of hosting diamonds are among the most controversial aspects of Archean geology. Alluvial diamonds in cratonic sedimentary cover rocks, whose minimum age is determined by detrital-zircon geochronology, provide a unique perspective on this topic. A new discovery of a diamond-bearing quartz-pebble conglomerate from the northern Slave craton, Canada contains detrital zircon with a restricted U-Pb age distribution that has a dominant peak at ∼2.94 Ga and depositional age of ∼2.83 Ga. Pressure-temperature constraints derived from an olivine-diamond host pair lie on a conductive Mesoarchean geotherm of ∼36–38 mW/m2, comparable to the coolest modern lithospheric geotherms. This result is at odds with a hotter geothermal gradient related to nearby Mesoarchean komatiites. We propose a model whereby early building blocks for cratons were small but with deep cool roots that formed by slab-stacking, and were subsequently juxtaposed with regions of thinner, hotter lithosphere. This heterogeneous initial architecture later amalgamated and thickened through lateral accretion forming the more uniformly thick cratonic lithosphere observed today. Thermal modelling indicates that stacking/thickening of cool initial lithosphere into a lithospheric keel thick enough to stabilise diamonds is the most likely way of generating the observed geotherm by Mesoarchean times.

Seismic delineation of the gold‐bearing structures in the South Rand goldfield (South Africa): A comparison of pre‐stack time and depth migration approaches on legacy seismic data

ABSTRACTTo better image complex geological structures and explore for the gold‐bearing quartz pebble conglomerate (locally termed Kimberley Reef) near Burnstone mine in the South Rand goldfield (South Africa), we reprocessed legacy 2D reflection seismic data using up‐to‐date seismic processing algorithms. The mining region is dominated by steeply dipping normal and reverse faults, as well as dolerite intrusions that crosscut the tabular gold mineralization and complicate the extraction of the ore. The legacy seismic data were acquired in 1993 in the South Rand goldfield as part of the gold exploration programme by the Gold Division of Anglo‐American Corporation. The pre‐stack data quality was improved by cautiously removing noise and applying static corrections to enhance the continuity of the reflection events. Improved structural imaging was then achieved using pre‐stack time migration and pre‐stack depth migration through careful velocity analysis. In particular, pre‐stack depth migration provided better imaging of near‐surface stratigraphic units and deep‐seated complex structures, relative to older post‐stack migrated images produced in 1993. Other reprocessing improvements included accurate imaging of key seismic reflective interfaces that can be used as proxies to 2D map the gold deposit, its dip variations, and geological structures (faults/dykes) crosscutting it. Synthetic shot gathers data computed using a 2D finite‐difference algorithm provided some insight into the source of seismic reflectivity observed on the reprocessed seismic data. The new structural information from reprocessing is essential for enhancing ore resource evaluation and guiding future exploration projects in the region.

Detrital zircon ages from Archaean conglomerates in the Singhbhum Craton, eastern India: implications on economic Au-U potential

New U–Pb age and Hf isotope data obtained on detrital zircon grains from Au- and U-bearing Archaean quartz-pebble conglomerates in the Singhbhum Craton, eastern India, specifically the Upper Iron Ore Group in the Badampahar Greenstone Belt and the Phuljhari Formation below the Dhanjori Group provide insights into the zircon provenance and maximum age of sediment deposition. The most concordant, least disturbed 207Pb/206Pb ages cover the entire range of known magmatic and higher grade metamorphic events in the craton from 3.48 to 3.06 Ga and show a broad maximum between 3.38 and 3.18 Ga. This overlap is also mimicked by Lu–Hf isotope analyses, which returned a wide range in εHf(t) values from + 6 to − 5, in agreement with the range known from zircon grains in igneous and metamorphic rocks in the Singhbhum Craton. A smaller but distinct age peak centred at 3.06 Ga corresponds to the age of the last major magmatic intrusive event, the emplacement of the Mayurbhanj Granite and associated gabbro, picrite and anorthosite. Thus, these intrusive rocks must form a basement rather than being intrusive into the studied conglomerates as previously interpreted. The corresponding detrital zircon grains all have a subchondritic Hf isotopic composition. The youngest reliable zircon ages of 3.03 Ga in the case of the basal Upper Iron Ore Group in the east of the craton and 3.00 Ga for the Phuljhari Formation set an upper limit on the age of conglomerate sedimentation. Previously published detrital zircon age data from similarly Au-bearing conglomerates in the Mahagiri Quartzite in the Upper Iron Ore Group in the south of the craton gave a somewhat younger maximum age of sedimentation of 2.91 Ga. There, the lower limit on sedimentation is given by an intrusive relationship with a c. 2.8 Ga granite. The time window thus defined for conglomerate deposition on the Singhbhum Craton is almost identical to the age span established for the, in places, Au- and U-rich conglomerates in the Kaapvaal Craton of South Africa: the 2.98–2.78 Ga Dominion Group and Witwatersrand Supergroup in South Africa. Since the recognition of first major concentration of gold on Earth’s surface by microbial activity having taken place at around 2.9 Ga, independent of the nature of the hinterland, the above similarity in age substantially increases the potential for discovering Witwatersrand-type gold and/or uranium deposits on the Singhbhum Craton. Further age constraints are needed there, however, to distinguish between supposedly less fertile (with respect to Au) > 2.9 Ga and more fertile < 2.9 Ga successions.

Schroeder Hill, central Transantarctic Mountains, Antarctica: Triassic stratigraphy and Sirius Group glacigenic deposits

AbstractThe geology of the Schroeder Hill region near the head of the Shackleton Glacier, central Transantarctic Mountains, consists of Triassic Fremouw Formation and overlying Falla Formation strata intruded by Jurassic Ferrar Dolerite sills. At ‘Alfie’s Elbow', south-east of Schroeder Hill, upper Fremouw strata are overlain by Upper Cenozoic Sirius Group deposits. These upper Fremouw beds differ from all other examined upper Fremouw strata in the Shackleton Glacier region in being carbonaceous. Quartz-pebble conglomerate characterizes the basal Falla beds, emphasizing a change in provenance. Sirius Group beds occur as a stratigraphic succession draped on modern topography and as structureless sand wedged in modern microtopography. Fremouw beds locally are arched with the fold axis approximately parallel to regional normal faulting related to the uplift and formation of the Transantarctic Mountains.

Stratigraphy and palaeogeography of the Cretaceous Blue Spur Conglomerate, Otago, New Zealand

ABSTRACT The non-marine Blue Spur Conglomerate in southeast Otago hosts the largest gold paleoplacer in New Zealand, but its formal stratigraphic relationships have thus far not been clear. The name reflects the extensive formation of ferrous iron-bearing diagenetic clay. We present a reference section for the unit, which is in part late Haumurian in age (c. 81–66 Ma). The unit is defined as a formal Member of the nearby, but not contiguous, Late Cretaceous Taratu Formation, Onekakara Group, Haerenga Supergroup. The Blue Spur Conglomerate contains three principal clast types: proximal schist debris, quartz pebbles and recycled distal greywacke cobbles, whereas the wider Taratu Formation is dominated by quartz pebble conglomerate with subordinate lithic clasts. Rare clasts of silicified quartz pebble conglomerate (silcrete) in both units attest to the recycling of older mature quartz sediments. The Blue Spur Conglomerate formed locally at the base of active normal fault scarps that controlled a broad valley in which the regionally extensive Taratu Formation accumulated. Detrital gold in the Blue Spur Conglomerate had both proximal and distal sources, whereas the Taratu Formation elsewhere received only distal gold.

Geochemistry of detrital zinc-rich chromite in conglomerates from eastern India

AbstractOn the western part of the eastern Indian shield, Archaean basement (Bonai granite) is overlain by radioactive conglomerate. The conglomerate contains well-rounded, fractured Mg-poor (&lt;0.18 wt.% MgO), Al-rich (up to 21.14 wt.% Al2O3) Zn-rich chromite (&gt;35.0 wt.% Cr2O3) containing up to 15.5 wt.% ZnO. This is the first reported occurrence of detrital zinc-rich chromite having such unusually high ZnO from India, and to our knowledge, the third reported occurrence in radioactive quartz-pebble conglomerate after Witwatersrand, South Africa and Tarkwa, Ghana. Zinc-rich chromite grains are either clean or contain exsolution blebs/lamella of rutile. The conglomerates show evidence for post-depositional hydrothermal fluid influx and fluid-induced mineral alteration. The strong negative correlation between Zn and Fe2+, Al and Cr, and Al and Fe3+, and strong positive correlation between Zn and Al suggest secondary incorporation of Zn and Al by substitution of Fe2+ and Cr (and Fe3+), respectively, leading to partial transformation of (Fe)(Cr,Al,Fe3+)2O4 towards ZnAl2O4 composition. The chromite grains were possibly derived from komatiite. The timing of Zn enrichment, either at the provenance prior to sedimentation, or at the depositional site post-dating sedimentation, remains unresolved.

Geochemistry and tectonic setting for the deposition of IOG siliciclastics at the western margin of Bonai Granite, Singhbhum-Orissa Craton, India

Bagiyabahal and Birtola areas are located in the south-western extension of the Noamundi-Koira Iron Ore Group (IOG) basin. Rock types exposed in the area comprises of siliciclastics and volcanics which occurs unconformably over the basement tonalite-trondhjemite granite-gneiss (Bonai Granite Phase-I). The cover rocks show sheared contact with the porphyritic Bonai Granite Phase-II. The IOG basin margin is suggested to be a part of a ‘volcanic passive margin’ as indicated by the geochemical behaviour of the siliciclastics as well as massive emplacements of mafic intrusives (doleritic sill, dyke and gabbro) and extrusives (basaltic lava flow) along faulted continental blocks. The siliciclastics comprise of U and Au bearing quartz-pebble conglomerate (QPC) and quartzite succession. It was deposited along the western margin of the Bonai granite (phase I) in anoxic conditions as indicated by their low Th/U ratios and presence of detrital uraninite grains. Repeated cycles of sedimentation and volcanism led to the formation of alternate layers of siliciclastics and basic bodies in the area. Major, trace and rare earth elements (REE) geochemical data suggests a semi-humid to humid palaeo-climatic environment of during the deposition in the passive continental margin setting characterized by fault-controlled sedimentation over a rift related faulted continental crust and shelf. Geochemical data suggests chemically weathered provenance dominated by clay minerals. Higher content of U, Th, Au, Cr, REE, platinum group of elements (PGE) and other geochemical ratios suggest a mixed provenance for the deposition of the siliciclastics comprising a predominantly acidic/granitic source possibly from the Bonai Granitic Complex (BGC) along with granite derived reworked quartzose sediments, minor basic and ultrabasic sources of Older Metamorphic Group (OMG). This paper attempts to characterize the geochemical behaviour, tectonic setting and provenance of the siliciclastics of Birtola and Bagiyabahal areas by analyzing drill core and surface samples.

Delineation of sulfide mineralization over a part of western margin of greenstone belt of the Dhanjori Basin, Singhbhum Craton from petrophysical, petrographic and geophysical studies

The Archean greenstone belt occupying the Dhanjori basin is a potential target for exploring resources of various precious metals and sulfide minerals like other Precambrian greenstone belts. Thick alluvium cover and limited outcrops in the survey area pose challenges in mineral exploration strategies. This work presents an integrated study of petrophysics, petrography and geophysics (gravity and magnetic survey) over an identified conductor in the western margin of the Dhanjori basin, which aims to delineate anomalous sulfide-bearing zones. The present analysis of petrophysical parameters (bulk density, and volume magnetic susceptibility) of various litho-units including dolerite, metabasalt, magnetite, chert and metasediments (quartzite and quartz pebble conglomerate (QPC)) indicate the occurrences of sulfide minerals. Petrographic studies of the smoky and fuchsitic quartzites also suggest sulfide mineral phases (chalcopyrite, and pyrite). EPMA studies on these metasediments reveal the presence of sulfide minerals (pyrite, chalcopyrite, millerite, bornite and pyrrhotite) and sulfosalts (gersdorffite, and cobaltite), along with traces of gold and silver. The 3D inversion of gravity and magnetic data identified significant geophysical signatures corresponding to possible subsurface sulfide mineralization and have helped in the reduction of the target area. This work also demonstrates the utility of integrated investigations in identifying deep-seated mineralization, which may be helpful in exploring analogous prospective Precambrian greenstone belts.

Ulungarat Basin: Record of a major Middle Devonian to Mississippian syn-rift to post-rift tectonic transition, eastern Brooks Range, Arctic Alaska

Abstract The Ulungarat Basin of Arctic Alaska is a unique exposed stratigraphic record of the mid-Paleozoic transition from the Romanzof orogeny to post-orogenic rifting and Ellesmerian passive margin subsidence. The Ulungarat Basin succession is composed of both syn-rift and post-rift deposits recording this mid-Paleozoic transition. The syn-rift deposits unconformably overlie highly deformed Romanzof orogenic basement on the mid-Paleozoic regional angular unconformity and are unconformably overlain by post-rift Endicott Group deposits of the Ellesmerian passive margin. Shallow marine strata of Eifelian age at the base of the Ulungarat Formation record onset of rifting and limit age of the Romanzof orogeny to late Early Devonian. Abrupt thickness and facies changes within the Ulungarat Formation and disconformably overlying syn-rift Mangaqtaaq Formation suggest active normal faulting during deposition. The Mangaqtaaq Formation records lacustrine deposition in a restricted down-faulted structural low. The unconformity between syn-rift deposits and overlying post-rift Endicott Group is interpreted to be the result of sediment bypass during deposition of the outboard allochthonous Endicott Group. Within Ulungarat Basin, transgressive post-rift Lower Mississippian Kekiktuk Conglomerate and Kayak Shale (Endicott Group) are older and thicker than equivalents to the north. North of Ulungarat Basin, deformed pre-Middle Devonian rocks were exposed to erosion at the mid-Paleozoic regional unconformity for ∼50 m.y., supplying sediments to the rift basin and broader Arctic Alaska rifted margin beyond. Although Middle Devonian to Lower Mississippian chert- and quartz-pebble conglomerates and sandstones across Arctic Alaska share a common provenance from the eroding ancestral Romanzof highlands, they were deposited in different tectonic settings.

Geochemical Characteristics of Quartzite in Parts of Paleoproterozoic Dhanjori Group, Singhbhum Craton, Eastern India: Implications for Provenance and Paleoweathering

Abstract Singhbhum craton preserves the records of sedimentation, magmatism, and tectono-thermal events from Mesoarchaean to Neoproterozoic. The Dhanjori Group (DG), located on the northeast margin of the Singhbhum craton, comprises of a variety of rocks including quartz pebble conglomerate, quartzite, schist, interlayered with mafic to intermediate and rarely acidic lava flows, tuff and agglomerate. Petrography and geochemical characteristics of the Dhanjori quartzite have been undertaken to interpret paleoweathering and provenance characteristics. Studied quartzites are made up of abundant quartz, feldspar, mica, and lithic fragments. With the help of geochemical data, the studied quartzites are classified as quartz-arenites and sub-arkoses. In addition to the A-CN-K plot, various chemical indices such as the chemical index of weathering (CIW), chemical index of alteration (CIA), and plagioclase index of alteration (PIA) indicate moderate to intense chemical weathering of the provenance under semi-humid climatic conditions. Rare earth element (REE) chondrite normalized patterns of the studied samples illustrate enriched light rare earth elements (LREEs) (LaN/SmN = 3.11 – 6.19), depleted heavy rare earth elements (HREEs) (GdN/YbN = 0.35 – 3.12) and negative Eu anomaly (Eu/Eu* = 0.55 - 0.87, ~ 0.68). Such patterns are similar to that of the upper continental crust (UCC). In the discrimination function diagram, studied samples of the Dhanjori Group fall in quartzose sedimentary field. In addition, La/Sc vs. Th/Co and La-Th-Sc diagrams, various ratios like Th/Sc (~0.61), La/Sc (~4.42), Th/Co (~0.03), La/Co (~0.11), and Cr/Th (~141.62) imply a mixed provenance for the studied samples. Hence, it has been inferred that the meta-sediments and orthoamphibolites of the older metamorphic group, Singhbhum granitoid complex, as well as associated Archaean metavolcanic suites, would have acted as the probable source rocks for the studied Dhanjori quartzites.

A mineralogical perspective on the recovery of uranium from brannerite-rich ore at Cooke Section, West Rand Goldfield, South Africa

Abstract The recovery of uranium from quartz-pebble conglomerates of the Witwatersrand Basin is accomplished through sulphuric acid dissolution under oxidising conditions. At Cooke Section on the West Rand Goldfield, the extraction process has been plagued by low to moderate yields on the order of 40 to 75%, as opposed to a target recovery of 80%. This has been ascribed to the high abundance of brannerite in the ore, which has traditionally been more problematic to leach. In addition to brannerite, poor metallurgical recoveries may also be associated with processing inefficiencies related to comminution, residence time, acid dosage and leach temperature. In view of this, a range of ore samples (channel samples) were collected from four uranium-bearing conglomerate horizons at Cooke Section (the A1, A5, E9EC and UE1A reefs) for detailed mineralogical and metallurgical characterisation, involving automated mineralogical analysis, and laboratory-scale leach testwork. The mineralogical results show that the major uranium-bearing minerals of uraninite, coffinite and brannerite are fine-grained (~80% passing 32 micron) and exhibit high degrees of mineral exposure to the lixiviant (~99%). Despite these favourable attributes, the elemental deportment data indicate that brannerite accounts for approximately 43% of the combined uranium budget. Further inspection shows that brannerite can be subdivided into three compositional subtypes: uraniferous brannerite (~13% U deportment), brannerite (~25% U deportment) and titaniferous brannerite (~5% U deportment). Baseline laboratory leach tests, which replicated plant leach conditions of 30 kg/ton acid, 4 kg/t oxidant, 24 hour residence time and 60°C leach temperature, yielded elevated dissolutions between ~77% and ~96%, with a combined overall uranium recovery of ~94%. These results are not consistent with the low yields obtained at the processing plant, and suggest that the high level of uranium recovery can be attributed to the effective leaching of brannerite (most likely uraniferous brannerite and brannerite). In view of prevailing market conditions, variability tests were carried out on a representative bulk composite sample to investigate the potential to achieve similar yields under more cost-effective leaching conditions. In these tests, a single parameter was varied (e.g. acid dosage), while the remaining parameters remained at baseline conditions. The results demonstrate that uranium recoveries of ~80% can be achieved on Cooke Section ores at low acid dosages and high temperatures (18 kg/t, 60°C) or at moderate acid dosages and low temperatures (23 kg/t, 30°C). The associated reduction of input costs would represent a significant cost-saving for the Ezulwini gold and uranium recovery plant. It is concluded that the poor uranium yields encountered during commercial processing of the ore are most likely related to undiagnosed inefficiencies in the treatment plant, such as excessive acid consumption related to elevated temperatures/oxidant addition and/or insufficient leach residence times, especially when recirculating, continuous flow-through leaching systems are in use. The broader implication of this study is that uranium processing operations beyond Cooke Section may be able to optimise their process designs and reduce input costs by quantifying the different types of brannerite within their ores through automated mineralogical analyses. The present study thus demonstrates the value of a geometallurgical approach in enhancing the understanding of uranium recovery through acid leaching.

Investigation of auriferous mineralisation over greenstone schist belt of Dhanjori Basin, Eastern India using 2D electrical resistivity tomography and induced polarisation surveys

Dipole–dipole resistivity and induced polarisation (IP) surveys were conducted over a conductor identified from an audiomagnetotelluric (AMT) survey run in a greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum, India. The shallow conductors identified in the AMT were not well resolved due to insufficient high-frequency data. The geology of the area suggests that gold mineralisation is associated with quartz pebble conglomerates (QPC) or sulfides. The resistivity and IP surveys were conducted over an area of 1.2 × 0.4 km with an electrode spacing of 10 m and a line spacing of 50 m. This data set was inverted using Gauss–Newton optimisation. Based on the resistivity and IP results, responses were categorised into five distinct geological/geophysical signatures defining local lithology and associated mineralisation. Low resistivity and low to moderate chargeability at depths up to 20 m from the surface indicate the weathered zone (which is not a strong feature in some of the IP models). High resistivity and high chargeability zones are identified as quartz vein/QPC with disseminated sulfide ± auriferous concentrations. Moderate resistivities and chargeabilities are interpreted as being related to the greenstone schists that make up the major part of the geology in the Dhanjori Basin. Zones with high resistivity and low chargeability are interpreted as Proterozoic gabbroic anorthosite ultramafics (PGAU) and/or younger dolerites. PGAU is intrusive to the local greenstone schist/phyllites; we have observed that its resistivity is considerably high and there is no significant gain in its chargeability due to the hindrance in ionic and electronic movement in the formation. Low resistivity and low to moderate chargeability zones below the weathered layer are attributed to mineralisation associated with massive sulfides. The integrated geophysical study indicates the presence of quartz vein/QPC and sulfide hosting auriferous mineralisation over the greenstone schist belt of the Dhanjori Basin, Eastern Singhbhum.

3D Inversion of Audio Magnetotelluric Data for Sulphide Mineralization over Dhanjori Basin

Abstract Quartz-pebble conglomerate (QPC) in the Dhanjori basin, a rifted greenstone belt, is known for its gold mineralization. Audio-magnetotelluric (AMT) soundings have been acquired for 32 sites spread in grid fashion for frequency 10kHz to 10 Hz along four parallel profiles. Due to noise conditions in the data only 26 sites have been used for further studies. The site spacing between adjacent AMT sites were ~250 m. The grid spreads over QPCD and Mayurbhang Granite (MG) of rocks. The dimensionality and the directionality analysis hinted towards a complex structure which tempted to perform a 3D inversion of AMT data for full impedance data. To obtain the optimum value of initial uniform half space resistivity, 3D inversion of off-diagonal component of inversion data was performed with various values of resistivity. The optimized value of resistivity was then used to perform 3D inversion for full impedance component with different set of error floor for the impedance component. The final 3D model showed the presence of a continuous conducting feature all along the three profile. Whereas, the southernmost profile showed three well developed conductors. The model corroborates well with electrical resistivity tomography (ERT) sections and GSI borehole which lies within the same geological domain. The borehole intercepted sulphide mineralization with varying concentration from the near surface to ~ 100m. The near surface conducting heterogeneity thus is attributed to sulphide mineralization.

Imaging Regional Geology and Au – Sulphide mineralization over Dhanjori greenstone belt: Implications from 3-D Inversion of Audio Magnetotelluric data and Petrophysical Characterization

The 2.1-Ga-old Paleoproterozoic Dhanjori Group of rocks covers 800 sq. km area within the Singhbhum Crustal province of eastern India, and is composed of siliciclastic metasedimentary sequence interlayered with mafic to ultramafic rocks. The meta-volcanosedimentary sequence is deformed and is of low-grade greenschist metamorphic facies. The gold occurrences within the study area are confined to the sulphide hosted mafic/ultramafic volcanic rocks and also in the basal quartz pebble conglomerates (QPC) within the Dhanjori Formation. Gold mineralization within the mafic/ultramafic meta-volcanics formations occurs within the quartz ± carbonate vein i.e. lode gold and as detrital gold within the QPC (0.5−7 ppm) of Phuljhari Formations. Regional Audio Magnetotelluric (AMT) measurements across the geological domains of Dhanjori Group and Iron Ore Group (IOG) were carried out in the frequency range of 10 kHz–10 Hz. This work discusses the conductivity model from 3-D inversion along with geology and petrophysical properties in mapping the regional geological features, associated sulphide mineralization and influence of texture, grain and bulk properties on mineralization. Dimensionality and strike analysis of the AMT data indicate complex conductivity structures. 3-D inversion of off-diagonal component of impedance was performed for obtaining an optimum value of uniform half space initial resistivity and error floor for off-diagonal component of impedance data. These were used to perform 3-D inversion of full impedance to produce a conductivity model of the study area. The conductivity model delineated major geological boundaries and the sulphide bearing mineralized zone. A major conductor is observed which extends to a depth of ∼250 m within meta-volcanics. On comparison with the available borehole adjacent to this, it is confirmed that the conductor corresponds to sulphide mineralization. The resistivity low at a depth of 150 m coincides with metabasalts (pyrite present which extends to 159 m with arsenopyrite). Another mineralized zone is observed at the boundary of iron ore group volcanics and Proterozoic-gabbro-anorthosite-ultramafics (PGAU), it starts appearing from a depth of ∼400 m and can be seen which extends to a depth of ∼800 m. It is worth mentioning that there lies Kundarkocha gold mine in the same geological domain. In addition, near surface conductor to a maximum depth of ∼50 m is observed all along the AMT profile. The sediments and fluids could account for this conducting feature. This is in agreement with available Electrical Resistivity Tomography section. The borehole within Dhanjori Volcanics also detected sulphide mineralization for ∼50 m. Thus, it can be inferred that the near surface conductor indicates the presence of gold in association with other sulphide minerals. The petrophysical studies helped to estimate the concentration of minerals within the rocks. High density and low susceptibilities in the area is an indicative of low-grade metamorphism and low level of serpentenization. Combined density and susceptibility scatter diagram and component proportion phase diagram suggests 20 vol%, 78 vol% and 2 vol% of haematite/sulphides, barren rocks and magnetite respectively. The high value of chargeability and negligible value of susceptibility indicates the presence of disseminated sulphides rather than haematites and/or massive sulphides in the rocks. Ternary plot suggest that the mineralization is controlled by texture and grain properties. Petrophysical analysis suggests that the lithotypes of the study area are similar to those of greenstone terrain in the Abitibi Subprovince. Integrating inverted section, geology and petrophysical properties has a substantial impact on the study area in terms of predicting the mineralization prospect and at the same time substantially reduces the drilling cost. Thus, the study provides a new orientation and advancement to the exploration strategy for deeper deposits.

Gold and associated minerals in the Waikaia placer gold mine, Northern Southland, New Zealand

ABSTRACTThe Waikaia alluvial gold mine is hosted in a Pleistocene paleochannel that is buried beneath younger sediments beside the Waikaia River. The paleochannel sediments were derived primarily from local basement, with a minor component of recycled Miocene quartz pebble conglomerate. The detrital gold in the paleochannel has a wide range of textures, from angular nugget-like particles, through equant rounded and abraded particles, to highly flattened thin flakes that have commonly been folded. The internal grain structures of these various particles reflect differing degrees of particle deformation during transport, followed by variable amounts of post-depositional recrystallisation and associated leaching of Ag from the gold. The diverse gold textures represent passage through different numbers of sedimentary recycling events and associated deformation since the first particles were eroded from the adjacent Otago Schist source area in the Cretaceous. Minor authigenic gold overgrowths occur on the gold particles, typically with authigenic clay. However, the finest sediment component (<10 µm) of mine-related waters is dominated by quartz and albite rather than clay minerals.