Archaean Gold Deposits Research Articles

Targeting nickel sulfide deposits from 3D seismicreflection data at Kambalda, Australia

The greenstone belts of the Yilgarn Craton, Western Australia, host numerous Archaean gold, nickel, and iron ore deposits. These deposits typically are found in complex geologic structures hidden by a deep, heterogeneous, and often conductive regolith profile. This added complexity limits the depth of penetration for the potential field methods, but at the same time opens new revenue possibilities through the application of seismic methods. To explore this opportunity, we acquired high-resolution, experimental, 3D seismic data over Lake Lefroy in Kambalda, Western Australia. The main objective was to map exceptionally complex, deep structures associated with Kambalda dome. Survey design used 3D ray tracing to improve the distribution of the common reflection points across ultramafic-basalt contacts which host numerous small, high-grade nickel sulfide deposits. A combination of small explosive sources, high-shot/receiver density, and exceptionally good coupling over the ultrasalty lake surface produced seismic data of very high quality. Processing focused on computation of accurate static and dynamic corrections, whereas imaging was helped by the existing geologic model. Advanced volumetric interpretation supported by seismic forward modeling was used to guide mapping of the main lithological interfaces and structures. Forward modeling was carried out using rock properties obtained from ultrasonic measurements and one borehole, drilled in the proximity of the 3D seismic volume. Using this information, geometric constraints based on the typical size of ore bodies found in this mine and a simple window-based seismic attribute, several new targets were proposed. Three of these targets subsequently have been drilled and new zones of mineralization were intercepted. The case study presented demonstrates that high-quality, high-resolution, 3D seismic data combined with volumetric seismic interpretation could become a primary methodology for exploration of deep, small, massive sulfide deposits distributed across the Kambalda area.

Exploration of the Yandal gold province, Yilgarn Craton, Western Australia

The opening of the Yandal gold province during the 1990s provides a detailed case history of repeated exploration successes. First highlighted in 1992 with the discovery of the Bronzewing gold deposit, the resources of the Yandal gold province grew rapidly through the 1990s with a series of discoveries. By 2000, the gold endowment of the province was 470 tonnes (t), and it was producing 25 t Au p.a. from the Jundee, Bronzewing and Darlot mines. The exploration activities in the deeply weathered and mostly covered Archaean Yandal greenstone belt during this period set the standard for Australian gold exploration in deeply weathered terrains through a twofold approach that involved drilling 100 000 holes, and at the same time making a strong commitment to ore deposit research. Confidence came from an understanding of Archaean gold deposits and the regolith, including the effects of gold dispersion in the weathered zone and the necessity to properly penetrate the Cainozoic cover for effective drill testing. Special emphasis was placed on gold distribution, alteration, lithologies and structural geology, and these soon became the focus of both data collection and drill targeting during the exploration process. Aeromagnetic data provided the platform for interpolation between geologically constrained areas in this province of less than 10% outcrop. Rapid and effective communications and data handling were critical issues for managing large exploration programs involving many geologists in remote areas.

Invisible gold in arsenian pyrite and arsenopyrite from a multistage Archaean gold deposit: Sunrise Dam, Eastern Goldfields Province, Western Australia

Invisible gold in arsenian pyrite and arsenopyrite from a multistage Archaean gold deposit: Sunrise Dam, Eastern Goldfields Province, Western Australia

Invisible gold in arsenian pyrite and arsenopyrite from a multistage Archaean gold deposit: Sunrise Dam, Eastern Goldfields Province, Western Australia

The Sunrise Dam gold mine (11.1 Moz Au) is the largest deposit in the Archaean Laverton Greenstone Belt (Eastern Goldfields Province, Yilgarn Craton, Western Australia). The deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal-scale structure. The Au content of arsenian pyrite and arsenopyrite from four mineralizing stages (D1, D3, D4a, and D4b) and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry. Pyrite contains up to 3,067 ppm Au (n = 224), whereas arsenopyrite contains up to 5,767 ppm (n = 19). Gold in arsenopyrite (D4a stage) was coprecipitated and remained as “invisible gold” (nanoparticles and/or lattice-bound) during subsequent deformation events. In contrast, gold in pyrite is present not only as “invisible gold” but also as micrometer-size inclusions of native gold, electrum, and Au(Ag)–tellurides. Pristine D1 and D3 arsenian pyrite contains relatively low Au concentrations (≤26 ppm). The highest Au concentrations occur in D4a arsenian-rich pyrite that has recrystallized from D3 pyrite. Textures show that this recrystallization proceeded via a coupled dissolution–reprecipitation process, and this process may have contributed to upgrading Au grades during D4a. In contrast, Au in D4b pyrite shows grain-scale redistribution of “invisible” gold resulting in the formation of micrometer-scale inclusions of Au minerals. The speciation of Au at Sunrise Dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple Au-precipitating mechanisms within a single plumbing system.

Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits) and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface. Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions. The δ 34S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred where fluids of contrasting redox state mixed.

Distribution of Precambrian iron and gold deposits on the southwestern East European Platform reflected in underlying transcrustal structure and current river systems

The East European Platform is underlain by Archaean and Proterozoic complexes of the East European Craton. In the southwest these are locally exposed in the Ukrainian Shield and the Voronezh Massif on either side of the ca. 2000 km long ESE-striking late Palaeozoic Pripyat–Dniepr–Donets rift. Evaluation with Landsat imagery of 1 : 1,000,000 scale published maps of the Precambrian complexes [Zaritsky, A.I., Galetsky, L.S. (Eds.), 1992. Geology and Metallogeny of the Southwest of the East-European Platform Map Series, 1 : 1,000,000, Ukrainian State Committee on Geology and Utilization of Mineral Resources, Kiev.] is largely obstructed by a cover of post-Palaeozoic sediments and soils of variable thickness. This obstruction is aggravated by an almost continuous patchwork of farmlands. However, analysis of the current drainage patterns in the Dniepr River basin and surrounding regions reveals a spatial coincidence of numerous stream courses and watersheds with previously inferred steep, transcrustal discontinuities of most probably Precambrian age. Transcrustal dislocations constituted important pathways for heat and fluids as is indicated by the distribution of a large proportion of assumed Early Proterozoic hydrothermal iron and gold deposits along them. This distribution is underpinned by the spatial coincidence of mineralization and elongate areas of highly irregular magnetization attributed to uneven distribution of hydrothermal magnetite in banded iron formation. In view of the extent of these dislocations, both vertically and laterally, the generation of hydrothermal fluid flow, emplacement of mantle-sourced magma and associated mineral potential away from banded iron formation complexes is likely. A second group of gold deposits, of Archaean age, is known to occur in association with still recognizable volcanic edifices in greenstone complexes. It is not known if and to what extent such Archaean gold deposits are related to these major transcrustal discontinuities. The kinematics and dynamics of these dislocations and pathways appear largely unknown and deserve high-priority investigation. The geological longevity of the transcrustal dislocation framework till the present day inferred from the current drainage systems is corroborated, however, by repeated regional topographical levelling surveys.

Sub-audio magnetic survey experiments for high-resolution, subsurface mapping of regolith and mineralisation over a blind gold discovery near Agnew in Western Australia

Experimentation with sub-audio magnetic (SAM) survey parameters over a recently discovered Archaean mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The horizontal total field magnetometric resistivity (TFMMR) response produced by current channelling during SAM surveying is shown to have a very similar pattern to gradient array apparent resistivity results using 50 m dipole spacing or less. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and 2 m along-line sample density provides much greater resolution. TFMMR anomaly trends were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The main ore zone forms an elongated, shallow plunging pod of weakly chargeable ore that shows up in gradient array and dipole-dipole IP surveys. SAM total field magnetometric induced polarisation (TFMMIP) was ineffective at imaging this ore zone using surface transmitter electrodes and 1 Hz transmitter frequency. However, the TFMMIP response of the main ore zone was later imaged in great detail by placing transmitter electrodes down boreholes into a shallow part of the ore body, and then down-plunge 700 m to the south. SAM surveying using optimal survey parameters identified shallow conductive shears and deeper chargeability anomalies within the main ore zone that correlated to economic gold mineralisation, and the SAM results helped to target resource definition drilling.

Middle Archaean gold deposits in the North Shaw–Coongan Greenstone Belt, Australia: the relative timing of events responsible for gold mineralisation

Gold deposits of the Mid-Archaean East Pilbara Granitoid–Greenstone Terrane, Australia, are unique because they characterise metallogenic evolution of the Early Earth. In the North Shaw–Coongan Greenstone Belt, quartz–sulphide lodes within shear and fault zones are host to gold mineralisation. Detailed field mapping of crosscutting structures at the Sharks Well Mining Centre and Blue Bar Mine in the Coongan Greenstone Belt, and at the North Shaw Mining Centre in the North Shaw Greenstone Belt, have established a relative chronology for gold deposits. The results suggest that the accepted classification of regional gold mineralisation, in terms of two events, may need modification. The Sharks Well Mining Centre is hosted by meta-basalt to andesite and carbonatised quartz–chlorite schist of the Mt. Ada Basalt (Warrawoona Group, ca. 3.49–3.31 Ga). The gold-bearing quartz–sulphide lodes are principally situated in the Shark Gully Shear Zone. The lodes and veins formed early in D1 (local) and prior to development of cleavage and folds. Gold occurs in NW-trending fractures that cut across the quartz lodes and veins, suggesting syn- to post-D1 gold mineralisation. The maximum relative age of gold mineralisation at the mining centre is constrained by the age of the Duffer Formation, which is stratigraphically younger than the Mt. Ada Basalt and dated at 3471–3463 Ma. The Blue Bar Mine is hosted by quartz–fuchsite schist of the Central Coongan Shear Zone, which trends northerly and dips steeply eastward. The mine consists of a single, brecciated, gold-bearing quartz lode that is unconformably overlain by scree breccia, and in turn, by the Mount Roe Basalt of the Fortescue Group (ca. 2.7 Ga). The maximum relative age of gold mineralisation at the mine is constrained by the age of the Euro Basalt at 3395–3325 Ma, while the minimum relative age is constrained by the Mount Roe Basalt at ca. 2.7 Ga. The North Shaw Mining Centre consists of several gold-bearing quartz lodes (including the Auroria and Eldorado lodes), as well as a number of historic gold workings (alluvial and shaft mining) on the plains adjacent to the Shaw River. The mining centre is located on the contact margin of the North Shaw Suite of the Shaw Granitoid Complex. The host rocks consist of contact metamorphosed basalt–andesite and hornblende amphibolites. Five stages of (local) deformation are recognised in the rocks of the mining centre: D1—macroscopic folding of the North Shaw Greenstone Belt about a northerly trending fold axis; D2—NNE-trending foliation and folds; D3—development of a NE-trending disjunctive cleavage, folds and conjugate kinks; D4—development of faults and master joints during emplacement of dykes of the Black Range Suite; D5—formation of the WNW-trending Callina Fault Zone. The fault zone crosscuts mafic dykes of the Black Range Suite, which places constraints on the minimum relative age of gold mineralisation in the fault zone. Based on crosscutting structural relationships, the relative and absolute chronology of the gold deposits in the North Shaw–Coongan Greenstone Belt indicates they are not all of the same age. The crosscutting structural relationships justify a review of the currently accepted ‘Event 1 or 2’ classification of Huston et al. [Econ. Geol. 97 (2002) 691]. Two new events, one at 3325–2774 Ma, and one younger than 2.7 Ga, also affected the mafic and ultramafic rocks of the Warrawoona Group.

Origins of Nd–Sr–Pb isotopic variations in single scheelite grains from Archaean gold deposits, Western Australia

Accessory gangue scheelite (CaWO 4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique. Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/ 86Sr and ε Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/ 86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone. The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/ 86Sr ratios and ε Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/ 86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite. A positive correlation between Na and REE suggests that REE 3+ are accommodated by the coupled substitution REE 3++Na +=2 Ca 2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration. The very high- ε Nd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnäsite were found within micro-fractures of one scheelite grain containing an extremely high- ε Nd signature. The hydroxyl-bastnäsite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high- ε Nd i values observed in scheelite from Western Australia.

Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositions can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescence imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations in the relative growth velocities of crystallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does fluorapatite, which is locally abundant and has REE contents similar to that of scheelite. In this context, the systematic increase of the Eu-anomaly between type I and type II patterns is produced by the difference between the partition coefficients of Eu2+ and Eu3+, and not by fluid mixing or redox reactions. Consequently, the high positive Eu-anomaly typical of scheelite from gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concentrations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be highly sensitive to the dynamics of the hydrothermal system.

Gold mineralisation at Eqi, north-east Disko Bugt, West Greenland

Gold mineralisation at Eqi, north-east Disko Bugt, West Greenland, is hosted in Archaean (c. 2800 Ma old) supracrustal rocks; the latter are divided by a thrust into a lower volcanic unit and an upper sedimentary and volcaniclastic unit. The lower volcanic unit comprises three parts: a basal pillowed greenstone sequence, an acid volcanic complex, and an upper mafic igneous complex. Intensive hydrothermal activity resulted in extensive carbonatisation and sericitisation, which is most intense just above a system of acid feeder dykes within the basal greenstone sequence. Primary enrichment in gold took place during pervasive hydrothermal alteration, and the gold is mainly located in carbonate-altered rocks. Remobilisation of gold occurred during formation of later quartz veins in the altered zone; these quartz veins have gold contents of up to 60 ppm. The geological setting, geochemistry and formation of the gold mineralisation at Eqi is similar to many Archaean gold deposits in the Abitibi belt of Canada.

Review of the application of isotopic studies to the genesis of Cu‐Au mineralisation at Olympic Dam and Au mineralisation at Porgera, the Tennant Creek district and Yilgarn Craton

This paper reviews the application of radiogenic isotopes to the study of four Cu‐U and Au deposits or deposit types in the Australasian region: Olympic Dam, Porgera, the Proterozoic Au deposits of the Tennant Creek district and the Archaean gold deposits of the Yilgarn Craton. In each case it has been possible to date the mineralisation and to correlate ore formation with a specific igneous event or stage in crustal evolution. In three cases it was also possible to use radiogenic isotopes to trace the source of the metal(s) or to constrain the fluid pathway. The results illustrate the power of radiogenic isotopes in ore‐genesis studies. At the Porgera gold deposit in Papua New Guinea K‐Ar and 39Ar/40Ar dating has shown that both the mineralisation, and the spatially associated Porgera Igneous Complex, have an age of ca 6 Ma. Sr and Pb isotopic tracing of the ore fluids indicate that these elements were derived from a mixed sedimentary‐igneous source, as might be expected for a large hydrothermal system developing around a hypabyssal igneous complex. SHRIMP U‐Pb dating of zircons from the Olympic Dam Cu‐U‐Au deposit in South Australia has constrained the age of mineralisation to be ca 1590 Ma, the same as the age of the Gawler Range thermal event in South Australia. Nd isotopic tracing of the ore fluid has shown that it contains a significant mantle component. The most likely source of mantle Nd is a suite of hydrothermally altered alkali ultramafic dykes that cut, but are coeval with, the Olympic Dam breccias. A possible common link between Porgera and Olympic Dam is that both were derived from oxidised alkali mafic/ultramafic bodies. It is suggested that these magmas had unusually high fo2 so that Cu and Au concentrated in these magmas as they fractionated to produce the Au‐ or Cu‐Au‐rich magmatic‐hydrothermal fluids that gave rise to the mineralisation. SHRIMP U‐Pb dating of zircons from felsic intrusives that cut, or are cut by, Au mineralisation constrain the age of most of the gold mineralisation in the Yilgarn Craton to lie between 2660 and 2630 Ma. Direct 39Ar/40Ar dating of hydrothermal minerals associated with the mineralisation gave ages of 2627 for Victory (Kambalda), 2629 for the Golden Mile (Kalgoorlie) and 2623 for Matilda M1 (Wiluna) that are consistent with the above constraint. An age of ca 2630 Ma correlates with late felsic magma‐tism in the Yilgarn Craton and with metamorphism in the lower crust. Mt Charlotte (Kalgoorlie), Wiluna East Lode and Lady Bountiful give younger ages of 2602, 2565 and 2590 respectively. The young age at Mt Charlotte is consistent with the field relationships and is believed to be the age of mineralisation but the Wiluna and Lady Bountiful dates may have been reset by later thermal events. Isotopic tracing at Mt Charlotte, using Nd in scheelites, showed that most of that element came from a komatiitic source. 39Ar/40Ar dating of the gold mineralisation in the Tennant Creek Block place the age of this event at 1825 Ma, towards the end of the felsic volcanism of the Barramundi Orogeny. Therefore the gold mineralisation in both the Yilgarn Craton and Tennant Creek Block can be related to the waning stages of a major period of felsic magmatism. This can be interpreted to mean that gold originated from late felsic magmas or that it was derived from the supracrustal rocks, buried deep in the crust, during the peak metamorphism in the lower crust.

Inhomogeneous Distribution of REE and Initial Nd Isotopic Compositions in Scheelite Crystals from an Archaean Gold Deposit (Mt Charlotte, Western Australia)

Scheelite (CaWO4) is a widespread accessory mineral in mesothermal Au deposits of various ages. Scheelite can host relatively large amounts of Sr and R E E (up to > 1 wt.% REE), and has been used to date mineralisation, and to constrain the nature and origin of the ore-forming fluids (Anglin et al., 1996; Eichhorn et al., 1997; Kent et al., 1996; Mfiller et al., 1991). The Archaean Yilgarn Craton (Western Australia) consists of large volumes of felsic basement supporting relatively thin (< 8 km) greenstone belts. Many large Au deposits were formed in the greenstone belts of the Eastern Goldfields Province during a major crustal scale mineralising event at around 2.63 Ga. The Mt Charlotte deposit is a large quartz stockwork embedded in a differentiated gabbroic sill (locally known as 'Golden Mile Dolerite') within the Norseman-Wiluna greenstone belt, and is spatially associated with the giant shearhosted Golden Mile deposit (Clout et al., 1990). Previous Sm-Nd isotopic studies on scheelite from Mt Charlotte (Kent et al., 1996) have revealed large variations in R E E concentrations and in 1478m/144Nd ratios (~ 0.2 to 0.4). The Sm-Nd data define an errorchron at 2772 +_ 86 Ma, older than the mineralising event (2.63 Ga). We present in situ trace element data obtained by UV laser ablation high-resolution inductivelycoupled plasma mass-spectrometry (LA-HR-ICPMS, using a Merchantek LUV266 laser microprobe and a Finnigan MAT E L E M E N T ) on scheelite from Mt Charlotte, with the aim to decipher the nature of the inhomogeneous distribution of R E E in scheelite. The morphology of the R E E zoning in scheelite was VIEPS, Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia

GOLD: Magnetic Susceptibilities of Rocks Associated with some Archaean Gold Deposits in Western Australia

Magnetic susceptibility measurements of rocks obtained from a number of Archaean mesothermal gold deposits have aided the interpretation of their geological and geophysical settings. Five deposits within the Yilgarn Craton were chosen on the basis of their magnetic host rocks: Youanmi (tholeiitic basalts), Greenfields (layered differentiated gabbro), Mount Martin (strongly sheared komatiitic sequence), Queen Margaret (serpentinised komatiitic peridotite) and Bounty (banded iron-formation). Iron-rich minerals associated with the gold ores include: weakly magnetic pyrrhotite (Greenfields and Bounty), magnetite (Mount Martin and Bounty) and non-magnetic pyrite (Youanmi and Queen Margaret). The existence of two styles of mineralisation, whose apparent susceptibilities are either less than, or greater than, the host rocks, is a consequence of the geochemical interaction between the hydrothermal fluids and the wallrocks. This can have important implications for exploration since the target magnetic anomalies will be different in each case. Where apparent susceptibility values in the ore are greater than the host rocks, the mineralisation would be expected to be represented by a secondary positive anomaly on the flanks of a larger regional anomaly; where they are less, any local magnetic minima could be highly significant in terms of targeting drillholes. A correlation between gold grades and apparent susceptibility at Bounty and Mount Martin suggests that, in some cases, susceptibility measurements can be used as a guide to ore. In deposits containing komatiitic volcanic rocks, such as Queen Margaret and Mount Martin, the susceptibility meter can be used as an effective mapping tool, particularly where facing evidence is required to interpret complex structures.

GOLD: Archaean BIF-hosted gold, Mount Morgans, Western Australia: a geophysical case history

The Mount Morgans Archaean gold deposit is hosted by banded iron-formation and has produced over 25 t of gold to date. The gold is associated with sulphides which have replaced magnetite. Aeromagnetic data covering the Mount Morgans deposit show a variably depleted magnetic effect caused by the replacement of magnetite by non-magnetic minerals, chiefly pyrite. Coincident with the magnetic effect is a conductive response recorded using airborne electromagnetics. The likely source of this conductivity anomaly is the weathering profile. The main ground geophysical surveys over the deposit are magnetics and 50 m dipole-dipole induced polarisation. The induced polarisation detected a chargeability anomaly associated with the sulphide mineralisation and a conductive response associated with the weathering of sheared rocks. Surveys of magnetometric resistivity and very low-frequency electromagnetics have been conducted over the Mount Morgans North resource, a similar but smaller gold resource located on the BIF ridge north of the main Mount Morgans deposit. The responses observed over this resource confirm the conductive and chargeable nature of the sulphide mineralisation associated with the gold.

Magnetic Susceptibilities of Rocks Associated with Some Archaean Gold Deposits in Western Australia

Magnetic Susceptibilities of Rocks Associated with Some Archaean Gold Deposits in Western Australia

Archaean BIF-Hosted Gold, Mount Morgans, Western Australia: A Geophysical Case History

The Mount Morgans Archaean gold deposit is hosted by banded iron-formation and has produced over 251 of gold to date. The gold is associated with sulphides which have replaced magnetite.Aeromagnetic data covering the Mount Morgans deposit show a variably depleted magnetic effect caused by the replacement of magnetite by non-magnetic minerals, chiefly pyrite. Coincident with the magnetic effect is a conductive response recorded using airborne electromagnetics. The likely source of this conductivity anomaly is the weathering profile.The main ground geophysical surveys over the deposit are magnetics and 50 m dipole-dipole induced polarisation. The induced polarisation detected a chargeability anomaly associated with the sulphide mineralisation and a conductive response associated with the weathering of sheared rocks. Surveys of magnetometric resistivity and very low-frequency electromagnetics have been conducted over the Mount Morgans North resource, a similar but smaller gold resource located on the BIF ridge north of the main Mount Morgans deposit. The responses observed over this resource confirm the conductive and chargeable nature of the sulphide mineralisation associated with the gold.

Sediment and shear-hosted gold mineralization of the Tartoq Group supracrustals, southwest Greenland

Abstract Gold mineralization occurs in the Archaean Tartoq Group supracrustals, which are dominated by greenschist-amphibolite facies metamorphosed mafic rocks. Although deformed, recognizable features of the stratigraphy (pillowed lavas, intercalated sills and interflow sediments), suggest shallow to deep sea-floor deposition. Large, intensely carbonatized structural zones cut through the greenstones, subparallel to regional layering, and postdating major folding. They are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones. Erratic high-grade gold mineralization is associated with quartz-pyrite lenses and veins that occur subparallel to layering within the shear zones. Moderate, but more consistent values are found in more extensive pyrite-arsenopyrite-quartz layers, also subparallel to layering. These lenses and layers occur adjacent to, or within the major carbonatized shear zones. Microscopically, gold is found as small (10 μm) grains in both pyrite and arsenopyrite. Low, but consistently anomalous grades of gold are also found in strongly contorted banded, muscovite-rich, quart-zpyrite rocks. These are thought tobe thin, chemically deposited interflow sediments within the greenstone pile, analogous to oxide-facies “banded iron formation” which is found nearby. Metamorphism has obscured critical textures, but the gold was either chemically deposited along with exhalative silica and sulphides, or may have been introduced later, during deformation and alteration of primary BIF. The Tartoq Group rocks are similar in most respects to those of other Archaean greenstone belts, being distinguished only by their excellent outcrop exposure. Current models for the origin of Archaean gold deposits are adequate to explain the Tartoq gold mineralization. A multistage model of gold concentration is proposed. Gold is initially leached from mafic volcanics (and possibly underlying gneisses) and redeposited at anomalous levels in chemical sediments. These may be further enriched by sulphidation reactions. High-grade gold mineralization is typically epigenetic in style, where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation.

A post-deformational, post-peak metamorphic timing for mineralization at the Archaean Big Bell gold deposit, Western Australia

Previous workers were unable to find any suitable criteria to time gold mineralization at Big Bell mine — an enigmatic, low-grade, large-tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the Murchison Province of the Archaean Yilgarn Craton, Western Australia. Earlier studies suggested that the gold mineralization occurred by hydrothermal alteration, either synchronous with volcanism or prior to peak metamorphism ∗. The following conclusions have been drawn from a detailed meso- and microstructural analysis of the mine sequence and greenstone belt hosting the orebody: 1. (1) Prior to mineralizaton, the Big Bell area underwent five phases of deformation and M1 regional metamorphism to the staurolite-andalusite zone of the amphibolite facies for metasediments. 2. (2) Granite emplacement post-dated D5 folding causing local, M2, thermal metamorphism. This peaked at the Ms+Qtz breakdown reaction for pelites, on or close to the And-Sil transition, but below the onset of partial melting, characterized by the narrow non-degenerate divariant field involving seven KFMASH phases: Qtz+Ms+Bt+Crd+Kfs+Als+H 2O This hornfelsing consumed, cross-cut and post-dated S5 cleavage. Prograde Qtz+And+Sil veins replaced this hornfels assemblage (and S5 cleavage) and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post-deformational hornfelsing. 3. (3) Retrograde M3 assemblages and two stages of gold-related mineralization, cross-cut and replaced all previous structures. The retrograde gold-bearing assemblages are both underformed. Stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations: Py-Po-Ccp-Ilm-Mag-Au-Tur-Sch-Rt-Spn-Zrn-Ms(Bt)-Ep-Czo-Qtz The second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre-existing mineralogies, including quartz, are totally destroyed within sericitic domains. The stage 2 mineral associations are: Apy-Sbt-Sb-Mo-Sp-Au-Py-Po-Ilm-Mag-Ccp-sericite-Chl-carbonate (Cal)-Ep-Czo-Qtz 4. (4) Gold mineralization at Big Bell was post-deformational and post-peak metamorphic. Unlike many Archaean gold deposits there are no obvious shear zones controlling the formation of the Big Bell gold deposit at the structural level exposed either on the surface or in drill core. Big Bell Mine was in a low-strain domain during the formation of late-stage sinistral sub-vertical D6 shear zones, that are typically concentrated at the granitoid-greenstone contacts. The retrograde character of the mineralization has been deduced from detailed meso- and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the Murchison Province.

Diverse metal sources of Archaean gold deposits: evidence from in situ lead-isotope analysis of individual grains of galena and altaite in the Ross and Kirkland Lake deposits, Abitibi Greenstone belt, Canada

Lead isotope compositions for individual grains of galena and altaite (PbTe) were determined in situ using a secondary ion mass spectrometer (SIMS). Galena was collected from the Ross deposit and altaite from the Kirkland Lake (KL) deposits in the southern Abitibi greenstone belt, Superior Province of Canada. The samples from KL are more radiogenic than those from the Ross deposit. Isotopic compositions vary significantly between different grains in each deposit and form broad linear arrays in 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb diagrams. The linear arrays of Pb-isotope data are attributed to mixing of Pb from different sources. At least two sources are required for individual deposits: one with low U/Pb and Th/Pb ratios and the other with high ratios. Lead minerals occurring with Au are less radiogenic than those that are not obviously associated with Au, suggesting that Au was supplied from low U/Pb sources such as sulphides or older ultramafic-mafic rocks. While most data are consistent with the derivation from local rocks, highly radiogenic Pb with relatively low 207Pb/206Pb ratios recorded at KL require post-Archaean mineralization or derivation of the Pb from an unusual crustal source with low μ. The latter interpretation is favored because of the lack of textural evidence and because it is difficult to dissolve and precipitate altaite at low temperatures. The presence of a Pb reservoir with low μ is also inferred from the data of Archaean banded iron formations and volcanogenic massive sulphide deposits. Different isotopic patterns of the two deposits suggest different sources of metals in the two deposits. While this conclusion does not reject the derivation of fluids from the lower crust or mantle, the data are not in accord with conceptual models invoking a common source reservoir for metals. The study suggests that fluids, which may have a common origin, leached metals and other constituents from the upper crustal rocks during their ascent. The proposed model, different origins for different constituents, explains much of the conflicting evidence presented by Archaean Au deposits, including provinciality of mineralogy and relatively uniform fluid inclusion and C-isotope data from many Au deposits.