Norseman-Wiluna Greenstone Belt Research Articles

Rank statistical analysis of nickel sulphide resources of the Norseman-Wiluna Greenstone Belt, Western Australia

Estimating the undiscovered mineral resources of a terrane is a challenging, yet essential, task in mineral exploration. We apply Zipf’s law rank statistical analysis to estimate the undiscovered nickel sulphide resources in the Norseman-Wiluna Greenstone Belt, Western Australia. The analysis suggests that about 3.0 to 10.0 Mt of nickel sulphide resources are yet to be discovered in this belt, compared to the currently known total nickel sulphide endowment of 10.8 Mt. This undiscovered nickel sulphide endowment is likely to be hosted by incompletely delineated deposits and undiscovered deposits in less explored komatiites in the belt. Using the more detailed data subset of the Kambalda domain, this study manipulates Zipf’s law to estimate the sizes of undiscovered deposits, in addition to the domain’s total nickel sulphide endowment estimate. Importantly, regression analysis shows that the gradient of the line of best fit through the logarithmic rank-size plot for the detailed Kambalda data subset is −1. This gradient, which is the key Zipf’s law constant k, has the value of −0.92 for the Norseman-Wiluna Greenstone Belt which is collectively less mature than the Kambalda domain. This result corroborates the use of k = −1 in Zipf’s law predictive analyses of mineral resources for deposit populations for which the value of k = −1 has not yet been attained due to exploration immaturity.

The Archean Chalice gold deposit: a record of complex, multistage, high-temperature hydrothermal activity and gold mineralisation associated with granitic rocks in the Yilgarn Craton, Western Australia

The Chalice gold deposit (∼20 t Au produced), in the southwestern portion of the Late-Archean Norseman–Wiluna greenstone belt of the Yilgarn Craton, Western Australia, is hosted both in a sequence of intercalated mafic and ultramafic amphibolites and a post-peak metamorphism monzogranite dike. The deposit is flanked on the western side by calc-alkaline plutonic rocks, and to the east by a predominantly monzogranitic pluton, with at least four generations of monzogranitic dikes that intrude the local mine stratigraphy. Locally, four deformation events (D 1–D 4) have affected this sequence of amphibolites, and two stages of gold mineralisation are recognised, controlled by the progressive D 2 event. Main Stage gold mineralisation (95% of the resource), which is demonstrably later than all granitic bodies except second generation and subsequent monzogranite dikes, is controlled by localised D 2 asymmetric folds developed in the mafic amphibolite. It is expressed as veins and wall-rock replacement, characterised by a prograde assemblage of quartz–diopside–plagioclase–K-feldspar–titanite–pyrrhotite–pyrite–magnetite±garnet, hornblende, scheelite and biotite. Second Stage gold mineralisation (5% of the resource) is associated with the intrusion of a monzogranite dike and is expressed as disseminated gold in the dike, as well as foliation-discordant veins of quartz–gold, quartz–diopside–gold, actinolite–gold and molybdenite–tellurobismuthite–gold. Detailed field mapping and petrography suggest that Main Stage gold mineralisation at Chalice is equivalent to hypozonal orogenic lode–gold mineralisation in other Archean greenstone belts of the Yilgarn Craton, whereas monzogranite-associated Second Stage gold mineralisation is atypical of such deposits. The Main Stage mineralisation cannot be correlated to any particular granitoid in the local environment, although temporally, it is bracketed by granitic magmatism. A deep magmatic or metamorphic source cannot be resolved from the field data. It is also uncertain as to whether Second Stage mineralisation represents a true magmatic-gold event, or is a product of the assimilation and remobilisation of Main Stage gold, during the intrusion of the monzogranite dike. However, its distinctive metal association of Au–Bi–Mo–Te–W suggests the former. Irrespective of this uncertainty, Chalice remains as a deposit which is more intimately related both spatially and temporally to granitic magmatism than other Yilgarn deposits.

Nitrogen isotope systematics of mesothermal lode gold deposits: Metamorphic, granitic, meteoric water, or mantle origin?

Research Article| November 01, 1999 Nitrogen isotope systematics of mesothermal lode gold deposits: Metamorphic, granitic, meteoric water, or mantle origin? Yiefei Jia; Yiefei Jia 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Search for other works by this author on: GSW Google Scholar Robert Kerrich Robert Kerrich 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (11): 1051–1054. https://doi.org/10.1130/0091-7613(1999)027<1051:NISOML>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yiefei Jia, Robert Kerrich; Nitrogen isotope systematics of mesothermal lode gold deposits: Metamorphic, granitic, meteoric water, or mantle origin?. Geology 1999;; 27 (11): 1051–1054. doi: https://doi.org/10.1130/0091-7613(1999)027<1051:NISOML>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Archean gold-bearing quartz vein systems constitute one of the most important classes of precious metal deposits; however, their origin remains controversial. Mantle, granitoid, meteoric water, and metamorphic related models do not resolve this controversy. New N and nitrogen isotope data on hydrothermal muscovite from six gold camps from the Late Archean Superior Province of Canada, and biotite from nine Au-mineralized locations from the Norseman-Wiluna greenstone belt, Western Australia, are reported. The majority of muscovites and biotites have N contents of 20–190 ppm and δ15N of 10‰–20‰, and 10–70 ppm and 10‰–24‰, respectively. The δ15N of the mantle is −5‰, and of granitoids is 0‰–10‰, both with generally low N contents; organic N is 0‰–10‰; meteoric water is 1.9‰–9.4‰; and metamorphic rocks are 3‰–18‰, increasing with higher grade. Accordingly, the data appear to rule out a mantle hypothesis, granitoid-related magmatic processes, or meteoric water for the deposits, but are consistent with ore fluids derived by metamorphic dehydration reactions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

Chromite in Komatiites, 1. Magmatic Controls on Crystallization and Composition

Chromite is a widespread accessory mineral in komatiites, ranging Zhou & Kerrich, 1992). This is the first in a series of papers which addresses the occurrence of chromite in from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition komatiites and its chemical variability. This paper looks at the factors controlling the crystallization of chromite of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in from komatiites, and at the textural and compositional variation within chromites which have apparently restrongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite tained some or all of their igneous characteristics. Textural and compositional features of chromites are compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus interrelated in complex and interesting ways, and show systematic relationships to host rock chemistry and crysFe–Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe contents, whereas tallization regime. Future papers in the series will deal with superimposed metamorphic effects. comparable thin flow chromites contain higher Fe. This is attributed to an initial reduced state of the magma, thin flow lavas A large body of new data on chromite compositions, including high-precision wavelength-dispersive analyses being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing of minor and trace elements Ti, V, Mn, Zn, Ni and Co, is presented from the Norseman–Wiluna Greenstone Belt Mg/(Mg+ Fe), and enrichment in Fe, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These in Western Australia, and synthesized with a compilation of literature data taken mainly from Roeder (1994) and trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency various unpublished sources. Whole-rock geochemical data are similarly compiled from the literature and some of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is unpublished sources, and include a large body of data collected over the last 15 years at CSIRO. Details of mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling. data sources are listed in Appendix A, and details of electron microprobe analytical techniques in Appendix B.

Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

The Bulletin lode-gold deposit is within the northernmost part of the Norseman–Wiluna greenstone belt in the Archaean Yilgarn Block, Western Australia. It is located within a brittle–ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite–calcite, intermediate calcite–dolomite, outer proximal sericite and inner proximal dolomite–sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, CO 2, K, Rb, S, Sb, Te and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for >150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Te (>10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1–35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, though stepwise, vector towards ore.

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

Chronology of the Mount Magnet granite-greenstone terrain, Yilgarn Craton, Western Australia: implications for field based predictions of the relative timing of granitoid emplacement

In this paper we report ion-probe UPb ages for zircons from thirteen granitoids and five supracrustal rocks from the Mount Magnet area of the Archaean Murchison Province of Western Australia. The oldest granitoids in the area are two porphyry dykes and three small, apparrently underformed bodies that intrude the greenstones. The porphyries have ages of 2715 and 2752 Ma and the undeformed granitoids range in age between 2696 and 2716 Ma. The external monzogranites that surround the greenstone belt are larger than the internal granitoids and show variable evidence of deformation from a foliated texture to recrystallization at grain boundaries. With the exception of a gneissic sample, which has an age of 2710 Ma, they are younger than the undeformed granites with ages between 2640 and 2694 Ma. A late leucocratic vein in one sample is ∼ 20 Ma younger than the mesocratic foliated rock it cuts. The young age for the external monzogranite is unexpected and it implies that the supracrustal rocks away from the margins of the Mount Magnet greenstone belt, and the small granitoids that intrude them, were cold and rigid at the time of emplacement of the monzogranites so that they behaved as a coherent block. We suggest that the stress associated with the deformation even that produced foliation, gneissic textures and recrystallization in the external granitoids was dissipated at the margins of the greenstone belts, so that the cores of these belts were little affected by this event. The age difference of ∼ 20 Ma between the melanocratic and leucocratic phases in the same rock suggest slow cooling and crystallization in the lower or middle crust, followed by tectonic emplacement into the upper crust. The Mount Magnet granitoids contain abundant xenocrystic zircons that show three pronounced peaks in their age distribution: a broad peak between 2910 and 2950 Ma and sharp peaks at 2800 Ma and 2730 Ma. These ages correlate with eruption of volcanic rocks in the Murchison. We suggest that the thermal events that produced these periods of volcanism were also recorded in the lower crust which later melted to produce the Mount Magnet granitoids. Two felsic volcanics from near the top of the stratigraphy yield ages of 2704 and 2727 Ma, and a chert, the Warramboo-Galtee Moore Chert of the Windaning Formation, an age of 2798 Ma, not 3.0 Ga as had previously been thought. The 2727 Ma felsic volcanic unit unconformably overlies the Warramboo-Galtee Moore Chert and is in turn overlain by the Wattle Creek Basalt of the Mount Farmer Group. This suggests that the Mount Farmer Group has an age of ∼ 2.7 Ga and that 2.7 Ga mafic volcanism is present in the Murchison. The younger external monzogranites, which make up the bulk of the granitoids in the Mount Magnet region, may have derived their heat from the mantle plume head that is believed to be responsible for basaltic and komatiitic volcanism of the Norseman-Wiluna greenstone belt. However, the smaller internal granitoids are older than the plume-related volcanic activity of the eastern Yilgarn and require an additional heat source to have affected the northern part of the Yilgarn Craton.

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia

Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite. Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment of the Norseman-Wiluna Greenstone Belt. In the Yakabindie and Mt. Keith areas, thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet-like sequences of finer grained olivine orthocumulates. They show fine-scale internal layering, broad-scale cryptic layering and upper fractionated sequences containing harrisites, pyroxene bearing cumulates and in some cases gabbroic derivatives. Low grade disseminated sulphide mineralisation occurs within the dunite lenses. In the stratigraphically equivalent Kathleen East area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows. A similar relationship is also seen further south, where the Perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows. These field relationships indicate an extrusive origin for the adcumulate dunite bodies. The Walter Williams Formation, in the west-central part of the Norseman-Wiluna Belt, is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent. It consists of a central sheet-like body of adcumulate dunite, underlain and overlain by olivine orthocumulates. The adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite. At its northernmost extent, the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros, dolerites and pyroxene spinifex-textured material, interpreted as a periodically replenished lava lake sequence. Both the lenticular dunite bodies of the Agnew-Wiluna Belt and the sheet-like dunite body of the Walter Williams Formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates. The adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows. Flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates. Field evidence from Perseverance, and the general geometry of the dunite lenses, suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low-melting felsic volcanic substrates. The textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion. Dunite sheets form close to the eruption sites, and dunite lenses as a result of channellisation further away from the vent, or close to the vent in the situation where the substrate is non-refractory and thermal erosion can take place readily. Kambalda-type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic, and Munro-type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions, or close to the vent of very small ones. Waning eruption rates leads to proximal facies being overridden by distal ones, a common observation in komatiite sequences. The size of thermal erosion channels requires very rapid eruption rates comparable to those in Phanerozoic flood basalt terrains.

Two stages of CO2 metasomatism at the Munro mine, Munro Township, Ontario: evidence from fluid-inclusion, stable-isotope, and mineralogical studies

The Munro asbestos mine is hosted by a differentiated ultramafic sill of Archean age. Localized carbonate alteration at the mine has resulted from two separate episodes of CO2 metasomatism, and the fluids were unrelated. The first episode affected only the serpentinized peridotite and occurred at 250 °C. The fluid was a saline brine (up to 24 wt.% NaCl–CaCl2), and had an oxygen isotopic composition of −3‰, and δ13C was equal to −7.8‰. Calcite veins were emplaced into the overlying, fractured pyroxenite at approximately 300–400 °C during the second episode. The salinity of this fluid was only 1–5 equiv. wt.% NaCl, the oxygen isotopic composition was +7.5‰, and δ13C equaled −3 to −5‰. The first episode was probably associated with burial metamorphism (diagenesis) and the second episode with regional metamorphism. The widespread occurrence of two separate stages of CO2 metasomatism in the Abitibi belt and in other well-documented Archean terranes, such as the Norseman–Wiluna greenstone belt in Western Australia, suggests that this may be an important factor in the tectonic evolution and metamorphic history of Archean greensone belts.

A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States

A comparison of high-alumina mineral assemblages spatially associated with the Haile and Brewer gold deposits in the Paleozoic Carolina slate belt (CSB) of the southeast United States and the Sons of Gwalia and Mount Celia gold deposits in the Archean Norseman–Wiluna greenstone belt (NWGB) in Western Australia suggests a similar hydrothermal origin and subsequent metamorphic and deformational history. A common hydrothermal origin is supported by the striking similarity in whole-rock chemistry, even though there were probably significant variations in original unaltered protoliths. An analysis of rocks from each of the deposits that contain high-alumina minerals demonstrates that the protoliths were leached of alkalis with respect to aluminum and have a ratio of Al2O3/Na2O + CaO + K2O greater than three. Although the rocks contain abundant high-alumina minerals, the aluminum content in these rocks is not unusually high, and it does not appear that there has been any significant transport of aluminum either into or out of the rocks that were altered.The most common high-alumina minerals found in rocks affected by the feldspar destructive alteration event include andalusite, kyanite, pyrophyllite, kaolinite, and sericite. Other minerals present in variable amounts include diaspore, chloritoid, alunite, natroalunite, paragonite, and topaz. It is possible that some of these minerals formed during the early hydrothermal alteration event and were recrystallized during metamorphism. Regardless of when the high-alumina minerals formed, textural relations suggest that the rocks had been hydrothermally altered by the time the metamorphic minerals formed. A comparison of alteration from the deposits studied in the CSB and the NWGB suggests there are many similarities to acid-sulfate alteration associated with geothermal areas, such as Yellowstone Park, Wyoming, and with acid-sulfate gold deposits, such as Goldfield, Nevada. Thus, it is possible that the protolith of the metamorphosed rocks in the CSB and NWGB contained an alteration assemblage that included alunite, pyrophyllite, and kaolinite.A generalized paragenetic sequence determined from petrographic and field observations, beginning with regional metamorphism, follows: (i) formation of andalusite, kyanite, chloritoid, and topaz during prograde metamorphism, depending on whole rock chemistry, (ii) formation of pyrophyllite and quartz-rich pods during silicification of aluminosilicate-bearing rocks, (iii) bedding parallel schistosity and fracturing produced by a deformational event, (iv) fractures filled by quartz, sericite, pyrophyllite, or calcite, (v) folding of early layering in the rocks to form a crenulation cleavage accompanied by introduction of quartz veins at high angles to foliation; and (vi) retrograde metamorphism of andalusite–kyanite-pyrophyllite to produce an assemblage of kaolinite ± diaspore.The spatial association of the acid-sulfate alteration with gold mineralization, together with comparison of analogous alteration associated with younger unmetamorphosed acid-sulfate gold deposits, suggests that at least some of the gold was introduced during the early premetamorphic alteration event. The present location of gold in each deposit is a result of local changes brought about by later metamorphic and deformational events.

Structural setting and geochemistry of Archaean I-type granites in the Bardoc-Coolgardie area of the Norseman-Wiluna belt, Western Australia

Abstract Regional structures in the Bardoc-Coolgardie area of the Archaean Norseman-Wiluna greenstone belt in Western Australia developed during early, large-scale thrusting followed by a transpressional regime characterized by ENEWSW shortening and large-scale sinistral strike-slip faults. Structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history. Emplacement mechanisms include solid- or liquid-state diapirism, forceful dilation of country rocks, and passive stoping. I-type granites are common, and include early- and late-tectonic intrusions. The widespread occurrence of I-type granodiorite and monzogranite, probably derived by partial melting of lower continental crust or subducted oceanic crust, suggests a continental margin setting for the belt. Alkalic quartz-diorite occurs as rounded enclaves in granodiorite, and is enriched in Ba, Rb, Sr, Ce, La, P and F, which suggests a mantle-derived volatile component. These quartz-diorite melts may have formed by partial melting of metasomatized mantle or, alternatively, mantle degassing may have lowered solidus temperatures in the source region, and thus induced melting. Emplacement of hot, volatilerich quartz-diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite. Recent models for Archaean gold mineralization suggest that auriferous fluids contained CO 2 derived from lower crustal or mantle sources. In the Bardoc-Coolgardie area, CO 2 may have been flushed out during partial melting of the lower continental crust. However, the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late-stage event, and took place over a considerably shorter time interval than the formation and emplacement of I-type granites.

Gold exploration in the callion area, eastern goldfields, western Australia

Abstract The Callion gold-quartz veins are hosted by metabasalts, interflow sediments and acid metatuffs within the Archaean Norseman-Wiluna greenstone belt. The area has been subjected to deep lateritic weathering. Gold prospecting using soil geochemistry led to the discovery of a laterite Au deposit (geological reserves of 103,000 t with 1.86 g/t Au at 0.7 g/t cut-off) and subsequently to the discovery of the underlying quartz vein mineralization (geological reserves of 104,000 t with 7.9 g/t Au at 2 g/t cut-off). The quartz vein mineralization is blind, beneath 2–10 m of laterite cover. The vein, which can be traced along strike for 1 km, is mineralized over a strike length of 300 m. Weathering around the vein extends to a depth of 70 m below surface. Ore microscopy shows that the Au in the weathered vein was partially mobilized and deposited contemporaneously with Fe-hydroxides. Mining of the laterite and the top of the quartz vein enabled a detailed reconstruction to be made of the Au-grade distribution from the top of the laterite down to the saprolite. The grade distribution is mushroom-shaped, widening at the top of the mottled zone or base of the pisolithic zone. From the vein in the saprolite, the “mushroom”, defined by the 2.5 ppm Au contour, widens to 40 m, at least 5 times the original width. This zone represents the transition to the overlying wide soil anomaly which has peak Au values of 2 ppm, which are about 50% of those in the laterite in the vicinity of the vein. Magnetic methods were also used during the course of exploration and led to the discovery of another small mineralized vein.

Structural control of greenstone-hosted Gold mineralization by transcurrent shearing: A new interpretation of the Kalgoorlie mining district, Western Australia

The Kalgoorlie mining district encloses the Golden Mile and Mt. Charlotte mining areas, and has produced 1200 metric tons of gold. It is classed as one of the largest Archaean gold deposits in the world. The district is located in the Norseman-Wiluna greenstone belt of the Yilgarn craton, Western Australia. This greenstone belt is characterized by NNW-trending transcurrent shear zones, some of which are traceable over more than 500 km strike length. Mineralization in the district is controlled by second-, third- and fourth-order structures located between principal shear zones of regional extent. An early deformation phase of regional folding (D1), associated with prograde regional metamorphism of low grade, preceded two phases of wrenching. The geometry of mineralized shear zones in both mining areas, as well as field and microstructural studies, allow us to distinguish an early phase of transpressional sinistral shearing (D2) from a later phase of transcurrent dextral shearing (D3). The earlier sinistral shear system in the Golden Mile area was in part reactivated, but its gross original geometry was preserved. These observations are consistent with clay model experiments carried out by the authors. Hydrothermal carbonate-sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event. Mineralization within individual shear zones forms tabular orebodies, whereas pipe-shaped ore shoots occur at the intersection of shear zones. The ore shoots contain high-grade telluride mineralization locally. Relative timing relationships show that mineralization postdates the peak of regional metamorphism. Hydrothermal activity in the district was preceded by the intrusion of calc-alkaline porphyry dykes of granodioritic to tonalitic composition, and accompanied by the intrusion of lamprophyre dykes.

Archaean carbon reservoirs and their relevance to the fluid source for gold deposits

It is commonly assumed1,2 that seawater-derived carbon in altered volcanic rocks is the only major pre-metamorphic carbon-in-carbonate reservoir in Archaean greenstone belts. Thus carbonates from Archaean gold deposits with stable isotope ratios too negative (median δ13C ≈ −3‰) to have been derived from such carbon (δ13C usually assumed to be near 0‰) have been interpreted as being derived from a local felsic magmatic source1. Barley and Groves3, however, recognize two carbonate-alteration styles that predate regional metamorphism and gold mineralization in the Norseman–Wiluna greenstone belt of Western Australia: evidence is summarized below. These are seafloor alteration and fault-controlled regional alteration, which, as shown below, have completely different carbon isotope compositions. The latter (median δ13C = −4.8‰) implies a major juvenile carbon (CO2) flux from the mantle during greenstone-belt evolution, and is thus of fundamental importance in itself. However, here we show why the mere existence of two carbon-in-carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area, negate some of the fundamental assumptions used to support magmatic-fluid models1.

Revised Sm–Nd systematics of Kambalda greenstones, Western Australia

Sm–Nd isotope systematics in mafic–ultramafic lavas from the Norseman–Wiluna greenstone belt, Western Australia, have been reinvestigated. A previous age based on mixing lavas and granites is not confirmed: instead, a Sm–Nd age of 3,262 ± 44 Myr is indicated for the lavas, 400 Myr older than intruding granites, and predating gneisses that have been proposed as basement rocks.