Murchison Province Research Articles

Gold mineralisation in the Yalgoo Singleton Greenstone Belt, Western Australia

The Yalgoo-Singleton Greenstone Belt (YSGB) is an ancient Archaean sequence that lies in the Murchison Province of Western Australia and like many of the greenstone belts of the area, is host to si...

Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence, Murchison Province: Oxygenated marine bottom waters

Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce + 3 in an oxic water column.

Rutile compositions at the Big Bell Au deposit as a guide for exploration

The compositions of 277 grains of rutile from 24 samples from the Big Bell Au deposit and nearby smaller deposits, in the Murchison Province of Western Australia, reveal that rutiles associated with ore at Big Bell, and for up to 200 m into the footwall rocks, are V+Fe+W+Sb-rich – a characteristic of rutile that has previously only ever been documented at the Hemlo Au deposit in Canada. Rutile from the nearby Fender and Indicator deposits share this Big Bell-type signature, but that from North Fender contains Nb±Ta, with W and Fe, suggesting formation from a quite different fluid and at a different period. The preservation of the different types of rutile in regolith samples in the Big Bell region indicates that it is possible to discriminate between the V+Fe+W+Sb-rich and Fe+Nb±Ta-rich alteration in rutile using regolith samples.

The Mount Gibson Banded Iron Formation-Hosted Magnetite Deposit: Two Distinct Processes for the Origin of High-Grade Iron Ore

The Mount Gibson banded iron formation lies within the Windanning Formation of the Luke Creek Group, which is found in almost all greenstone belts throughout the Murchison province. The banded iron formation (BIF) typically consists of alternating bands of magnetite and microcrystalline quartz (chert) with rare carbonaceous and iron silicate-rich shale partings and layers and rare, thin fine-grained tuff bands. Owing to repetition by isoclinal folding and attenuation by faulting, the true thickness of the Mount Gibson BIF is unknown but appears to be on the order of 100 m. Although the Mount Gibson BIF is typical of many Algoma-type iron formations, situated within a greenstone belt on an Archean craton, it is also similar to Hamersley-type BIF in petrology, areal extent, and ore genesis. High-grade hematite deposits formed within BIF were thought to have formed by the supergene leaching of chert from typical cherty BIF. Recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite. Magnetite-carbonate BIF, in which there is clear evidence of hydrothermal replacement of chert by carbonate, forms distinctive magnetite-goethite ore with magnetite locally persisting to the surface. Mount Gibson shows clear evidence of the formation of high-grade ore by this process but also contains high-grade hematite ore and chert-free BIF that show no evidence of the hypogene replacement of chert. High-grade hematite occurrences, up to 1 km in strike length, are found within the weathered zone overlying the magnetite BIF at Mount Gibson and continue into unweathered chert-free BIF at depth that show no evidence of hydrothermal carbonate or supergene enrichment. The cherty BIF shows sharp contacts against chert-free BIF and high-grade ore, even when strongly weathered. This suggests that deep saprolitic in situ high-grade ore may be produced by different processes, including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert-free BIF, in which chert bands either never developed or were apparently removed during diagenesis. Neither model requires supergene selective leaching of quartz (chert) during deep weathering.

Archean crustal evolution in the northern Yilgarn Craton: U–Pb and Hf-isotope evidence from detrital zircons

The integrated application of U–Pb dating, Hf-isotope analysis and trace-element analysis to detrital zircon populations offers a rapid means of assessing the geochronology and crustal evolution history of different terranes within a composite craton. In situ U–Pb and Hf-isotope analyses of 550 zircons from 21 modern drainages across the northern part of the Yilgarn Craton and the adjacent Capricorn Orogen provide a broad view of crustal evolution in Archean and Proterozoic time.The oldest crustal components (3.7Ga) are identified in the Yeelirrie geophysical domain [A.J. Whitaker, Proceedings of Fourth International Archaean Symposium Ext. Abstracts AGSO-Geoscience Australia Record, vol. 37, 2001, p. 536] that runs N–S down the middle of the craton; these components are represented by ancient zircons and also are reflected in the Hf model ages of younger magmas. Ancient (>3.4Ga) crust contributed to the generation of younger magmas in the Narryer Province, and the proportion of ancient recycled material increases from east to west across the Murchison Province. In contrast, the Hf-isotope data provide no evidence for crust older than 2.9–3.0Ga in the Southern Cross or Eastern Goldfields (including the Marymia Inlier) domains. The Yeelirrie domain and the composite Narryer–Murchison block are interpreted as ancient microcontinents, sandwiched with the juvenile terranes of the Southern Cross and Eastern Goldfields domains.There is little evidence for the existence of a Depleted Mantle reservoir beneath the Yilgarn Craton prior to 3.1–3.2Ga, but this reservoir is a major contributor to crustal generation from 3.1 to 2.6Ga; this suggests that much of the continental crust in the craton was generated after ca. 3.2Ga. 1.8–2.3Ga magmatism, associated with the Capricorn Orogen, involved the recycling of older crust with little obvious contribution from the Depleted Mantle. A significant (and previously unrecognised) 540Ma episode in the NE part of the craton involved metamorphism or remelting of the 2.7–3.0Ga crust of the Eastern Goldfields Province.

Fluid Chemical Evolution as a Factor in Controlling the Distribution of Gold at the Archean Golden Crown Lode Gold Deposit, Murchison Province, Western Australia

The Golden Crown lode gold deposit is one of a number of northwest-southeast– to north-south–trending quartz veins hosted by a steeply dipping, 500-m–thick, differentiated dolerite sill (the Great Fingall sill) and adjacent graphitic shale in the Day Dawn district of the Murchison province in the Archean Yilgarn craton, Western Australia. Quartz veins of the Golden Crown deposit are shear zone-hosted, crosscut the dominant host-rock stratigraphy at a high angle, and are laminated, partly boudinaged, and generally bifurcate at their extremities. Veins are dominated by dark gray or milky white quartz, with subordinate calcite and ankerite, and, in order of abundance, arsenopyrite, pyrrhotite, pyrite, galena, chalcopyrite, and minor sphalerite. The vein sulfides coexist with native gold and are most abundant at vein margins. Textural evidence indicates that precipitation of the majority of these minerals overlapped, and that most phases can be considered to have been in equilibrium during vein genesis. Mineral chemistry, fluid inclusions, alteration, and ore assemblages show that there are significant systematic gradients in intensive variables within the Golden Crown deposit. These are recorded over the approximately 450 m of vertical exposure of the vein system, and over up to 30 m into the wall rocks. Based on proximal alteration assemblages and zoning patterns, zoned wall-rock alteration haloes are divisible into four sequences, three of which are closely related to gold mineralization. Specific alteration sequences occur within different parts of the vein system. Biotite alteration predominates in the deeper portions, whereas muscovite alteration characterizes the upper and southern portions of the system. Higher-grade gold zones within the Golden Crown deposit are limited to the northern end of the system in the deeper portion of the veins, and expand along strike to the south in the upper portions. Fluid inclusions, interpreted to be trapped during gold mineralization, are low-salinity (<10 wt % NaCl equiv) H2O-CO2 ± CH4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility. Inferred methane concentrations in the carbonic phase vary significantly from 19 ± 7 mole percent in the vicinity of the shale contact to 5 ± 2 mole percent in the shallower, southern end of the deposit. Carbon and oxygen isotope compositions of carbonates ( δ 13Ccalcite = –10.5 to –1.9‰; δ 13Cankerite = –5.3 to –4.1‰; δ 18Ocalcite = 7.0–18.3‰; and δ 18Oankerite = 7.7–9.2‰) and oxygen isotope compositions of vein quartz ( δ 18Oquartz = 10.2–12.5‰) are well within the range of known Archean orogenic lode gold deposits. Combining the results of all geothermometers and geobarometers, a temperature of 300° to 400°C and pressure of 2.5 ± 0.5 kbars are determined, with temperature variations over the exposed quartz vein of less than 40°C. The As content (at. %) of arsenopyrite and Au/Ag ratios of native gold show significant compositional variation, which appears to be controlled by parameters such as a H2S(aq) and f O2, not temperature gradients. On the basis of the variations in fluid composition determined in the reefs, the two major processes responsible for high-grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore-forming system (approximately below –400-m level) and, above these levels, mixing between two modified ore fluids, one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite.

Fluid Chemical Evolution as a Factor in Controlling the Distribution of Gold at the Archean Golden Crown Lode Gold Deposit, Murchison Province, Western Australia

Fluid Chemical Evolution as a Factor in Controlling the Distribution of Gold at the Archean Golden Crown Lode Gold Deposit, Murchison Province, Western Australia

Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO (<0.05 wt.%). This type of tourmaline has δ 11B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.

Early tectonic history of the Marymia Inlier and correlation with the Archaean Yilgarn Craton, Western Australia

The Archaean granite‐greenstone rocks of the Marymia Inlier outcrop within Proterozoic rocks forming the Capricorn Orogen. Five major deformation events are recognised in the rocks of the Plutonic Well and Baumgarten greenstone belts. The first two events were Late Archaean and synchronous with major epithermal gold mineralisation in the belts. Palaeoproterozoic extensional faulting was probably related to the early stages of the Capricorn Orogeny. The fourth event records a compressional phase of the Capricorn Orogeny associated with greenschist‐facies metamorphism, whereas the last major event involved wrench faulting associated with minor folding. The Archaean tectonic history, rock types and timing of mineralisation strongly suggest that the Marymia Inlier is part of the Yilgarn Craton, and that each of the provinces in the craton experienced the same geological history since 2.72 Ga. The inlier is now interpreted to include two components; one is the eastern or northern extension of either the Narryer Terrane, Murchison Province or Southern Cross Province, and the other is the northwestern extension of the Eastern Goldfields Province. The Jenkin Fault, which was active in Proterozoic times, separates these two components.

Geochronology of supracrustal rocks from the Golden Grove area, Murchison Province, Yilgarn Craton, Western Australia*

SHRIMP U‐Pb zircon dating of four supracrustal rocks from the Golden Grove area of the Murchison Province shows the need for reassessment of the accepted stratigraphy of the greenstone sequence in this area. Three samples from the Gabanintha Formation give dates of 2960 ± 6 Ma, 2953 ± 7 Ma and 2945 ± 4 Ma. A sample from the overlying Windaning Formation yielded a date of 2809 ± 5 Ma which correlates with similar dates for the same formation in the Mt Magnet and Meekatharra areas within the province. The results indicate that there is a gap of ∼ 140 million years between the time of deposition of the Windaning and Gabanintha Formations making it inappropriate to assign both of these formations to the same group (Luke Creek Group). The data also constrain the age of the stratabound Cu‐Zn deposits in the Golden Grove area to lie between 2953 ± 7 Ma and 2945 ± 4 Ma.

The Archaean Mount Gibson gold deposits, Yilgarn Craton, Western Australia: Products of combined synvolcanic and syntectonic alteration and mineralisation

The Mount Gibson gold deposits are sited at the southern tip of the Archaean Yalgoo–Singleton greenstone belt, in the southern portion of the Murchison Province, Yilgarn Craton, Western Australia. Primary gold mineralisation is hosted within an eastward facing, dominantly tholeiitic metabasalt sequence which has been metamorphosed to amphibolite facies conditions. Alteration and mineralisation occurs within the north to north-northeast striking, steeply east-dipping, Mount Gibson shear zone, paralleling primary layering within the tholeiitic mine sequence. Two main styles of wallrock alteration and associated sulphide mineralisation are present. The first of these is represented by garnet (spessartine–almandine)–gahnite and cordierite–muscovite-bearing schists. These lithologies are unusual for lode-gold mineralisation, and, despite their apparently pelitic nature, have mafic precursors and reflect pre-metamorphic alteration of tholeiitic basalts. The garnet-bearing schists form a coherent stratiform horizon which overlies the main ore zone at the Orion Two deposit and are associated with recrystallised, metamorphosed, polymetallic sphalerite–galena–pyrite–(pyrrhotite–chalcopyrite–tetrahedrite) mineralisation. A more typical (for Archaean lode-gold), pervasive, syn-peak metamorphic, syn-shearing quartz–biotite±sulphide alteration envelope is associated with pyrrhotite–pyrite±chalcopyrite mineralisation at Mount Gibson. Although free gold is rare, there is a strong empirical relationship between enhanced gold grades and the pyrrhotite–pyrite±chalcopyrite assemblage, suggesting that gold was introduced as part of this event. Mass balance calculations at Orion Two show enrichment of Au, Ag, K, Rb, Ba and W and depletion of Na, Ca and Sr over mineralised zones, which is typical of lode-gold mineralisation. However, the deposit also exhibits strong base-metal enrichment, particularly within the garnet-bearing schists, and zonation from the Au–Cu enrichment of the main ore zone to the overlying, stratiform Pb–Zn–Ag–(Mn) enrichment, which is dominant in the garnet-bearing schist horizon. These features are typical of VHMS mineralisation. Hornet exhibits similar overall elemental enrichment and depletion to Orion Two. Lead-isotope data for sulphides from Orion Two and Hornet provide evidence of two discrete mineralising events at Mount Gibson, separated by approximately 300 m.y. A two-stage model best explains the features of the Mount Gibson gold deposits. Metamorphism of the aluminous alteration and weak mineralisation associated with an early synvolcanic event resulted in the formation of cordierite–muscovite and garnet (spessartine–almandine)–gahnite bearing assemblages, respectively, which were less competent than the surrounding tholeiitic metabasalts and consequently tended to localise shearing. Shear-hosted lode-gold mineralisation was synchronous with peak metamorphism and shearing. Like the shearing, hydrothermal alteration was mostly concentrated in and around the less competent rocks which were produced by metamorphism of synvolcanically altered precursors, resulting in a fundamental stratigraphic control on gold mineralisation at Mount Gibson.

Taking Downhole EM Underground, at Hill 50 Decline, Mount Magnet, Western Australia

Hill 50 Decline (part of the Hill 50 Gold Mine group of deposits) is located at Mount Magnet, 560 km north-northeast of Perth, in the Murchison Province of the Yilgarn Craton. The Hill 50 orebody is hosted by banded iron formation (BIF) "bars" of the Sirdar Formation, generally having a width greater than 20 m. Gold mineralisation is localised along the north-northeast to northeast striking faults, known as Boogardie Breaks, and accompanying sub-horizontal fractures. Sulphide replacement of the adjacent oxide facies BIF has resulted in steeply plunging stratabound shoots ("pencils") of massive gold-bearing pyrite-pyrrhotite ore, with pyrrhotite being the dominant sulphide at depth and at the centre of the ore zones. Previously, pyrrhotite-rich BIFs have been shown to be characterised by significantly higher conductivities than their unmineralised counterparts.Downhole electromagnetic (DHEM) surveying carried out in the Hill 50 underground was aimed at detecting the massive, pyrrhotite conductors. A 65 m x 80 m transmitter loop was laid out underground and drillholes were surveyed using the three-component, Crone pulse EM system. Logistical problems experienced during data collection were largely related to firing times and data quality was greatly influenced by the numerous noise sources encountered in an operating underground mine (eg, drill rigs, trucks, etc.).Modelling and interpretation of the resulting DHEM data has identified several conductors. All the interpreted conductors are positioned within the Hill 50 BIF Bar, except for one conductor, lying in the Outer BIF Bar 1. Most conductors are modelled as steeply dipping, while those modelled as shallow-dipping are considered to be related to mineralisation within the sub-horizontal fractures. Of the models tested so far, all have been shown to be directly related to massive sulphide mineralisation, with the majority being associated with economic gold grades, demonstrating that the technique has been effective in delineating the pyrrhotite-rich BIF target underground.

SHRIMP U-Pb geochronological constraints on Archean volcanic-hosted massive sulfide and lode gold mineralization at Mount Gibson, Yilgarn Craton, Western Australia

The base metal-rich Mount Gibson gold deposits are hosted within the ductile Mount Gibson shear zone in the Retaliation greenstone belt of the Murchison province of the Archean Yilgarn craton. The deposits are unique in the context of the Yilgarn, as they exhibit two main styles of hydrothermal alteration and sulfide mineralization, consistent with overprinting of synvolcanic, volcanic-hosted massive sulfide-style mineralization by synorogenic, shear-hosted lode gold mineralization. On the basis of model ages based on sulfide Pb isotope ratios, these two events are separated by approximately 300 m.y.Subvolcanic intrusive rocks from within the host sequence to mineralization at the Orion Two and Hornet deposits have SHRIMP II zircon U-Pb ages of crystallization of 2929 + or - 3 and 2934 + or - 6 Ma, respectively. These ages are within error and provide a minimum age for the host sequence, and consequently, the volcanic-hosted massive sulfide mineralizing event, at Mount Gibson. On the basis of geochemical evidence, the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude, and consequently ca. 2930 Ma is probably the approximate age of the host rocks to mineralization. The regional granitoid, which intrudes the base of the greenstone belt, has a crystallization age of 2935 + or - 3 Ma.A post-tectonic granite porphyry, which crops out approximately 25 km north-northeast of the Mount Gibson mine site, has a SHRIMP II zircon U-Pb crystallization age of 2623 + or - 7 Ma. This granitoid has identical field relationships, similar whole-rock geochemistry, and similar Pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the Deep South pit and is consequently interpreted to be cogenetic and contemporaneous. The 2623 + or - 7 Ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at Mount Gibson.A morphologically and chemically distinctive population of zircons extracted from the sample of 2934 Ma altered felsic schist from the Hornet deposit gives a SHRIMP II zircon U-Pb age of 2627 + or - 13 Ma. These zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at Mount Gibson. The interpreted mineralization age is within error of the inferred age of the postmineralization, post-tectonic granitoid. The age is also within error of the majority of absolute ages for gold mineralization from across the Yilgarn craton, which generally fall in the range 2630 + or - 10 Ma.SHRIMP zircon U-Pb geochronology thus provides strong support for the two-stage model proposed for mineralization at Mount Gibson. Synvolcanic volcanic-hosted massive sulfide-style mineralization occurred at ca. 2930 Ma and was overprinted by lode gold mineralization some 300 m.y. later, at ca. 2630 Ma.

Constraints on the age of granitoid emplacement, metamorphism, gold mineralization, and subsequent cooling of the Archean greenstone terrane at Big Bell, Western Australia

The Big Bell gold deposit (40 t Au produced) is located in a narrow terrane of the Meekatharra-Wydgee greenstone belt in the Murchison province of the Yilgarn craton, Western Australia. At the mine site, the greenstones consist of amphibolites and metakomatiites of the 3.0 Ga Gabanintha Formation, rotated into a subvertical position and foliated parallel to stratigraphic contacts. The amphibolite-metakomatiite sequence is intruded parallel to foliation by swarms of granodiorite-tonalite dikes. The largest dike, exposed in the Big Bell open pit and dated at 2737 + or - 4 Ma (U-Pb zircon), is surrounded by a zoned contact-metamorphic aureole (> 1 km wide) of amphibolite facies grade. The dikes represent the oldest and outermost intrusive phase of the batholith to the southeast, where the border granodiorite pluton has been dated at 2700 + or - 7 Ma (U-Pb zircon).The gold-bearing metasomatic rocks at Big Bell are subdivided into central muscovite-microcline and biotite-plagioclase gneisses, and into outer calcic skarns. Both gneisses and skarns form steeply dipping zones bound to the contacts of altered granodiorite dikes. The timing of main gold-sulfide-scheelite mineralization is constrained by a concordant U-Pb almandine age of 2662 + or - 5 Ma from a low-grade cummingtonite-hornblende contact skarn. Main mineralization in the mine area thus postdates amphibolite facies metamorphism by about 80 m.y. Muscovite from the Main lode microcline gneiss yielded an Ar-Ar plateau age of 2639 + or - 16 Ma, interpreted as reset by the thermal effect of the granite pluton at the northwestern margin of the greenstone terrane. The granite U-Pb zircon age (2627 + or - 8 Ma) agrees within error with the Ar-Ar muscovite age. Metamorphic recrystallization related to granite intrusion was of very limited extent (

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.

Petrophysical Characteristics of BIF – Hosted Gold Deposits and the Application of Downhole EM to Their Exploration, With Examples From Hill 50 Gold Mine, Mt Magnet, Western Australia

Hill 50 Gold Mine is located at Mt Magnet, 560 km NNE of Perth, in the Murchison Province of the Yilgarn Craton. The Mt Magnet greenstone belt consists of a sequence of mafic, ultramafic and felsic volcanic rocks, with interbedded volcanogenic sediments, predominantly banded iron formations (BIFs). The belt has been deformed into a principal structure, known as the Boogardie Synform. Major NNE ? NE striking faults, superimposed on the synform, are referred to as Boogardie Breaks and exhibit important control over many of the ore deposits. Historically, most of the gold production in the Mt Magnet district has come from BIF ? hosted deposits (e.g. Hill 50 and Boomer ? styles). Hill 50 ? style deposits are hosted by BIF bars of the Sirdar Formation, having a width generally greater than 20 m, and being structurally controlled by the Boogardie Breaks. Gold mineralisation is closely associated with pyrrhotite and to a lesser degree, pyrite. Hillcrest South and Brown Hill West prospects are currently being explored using a Hill 50 model. Petrophysical measurements carried out on samples from these areas demonstrate that pyrrhotite ? rich BlFs are characterised by lower magnetic susceptibilities and higher Koenigsberger Ratios, conductivities and densities. Use of downhole EM in locating the pyrrhotite ? rich BIF has been highly successful.Boomer ? style deposits are hosted by multiple NW ? trending BIF units, and breccias, and are distinguished by the close association of gold mineralisation with pyrite. Petrophysical testing on samples from the Boomer pit showed that the pyrite ? rich rocks are characterised by higher densities and magnetic susceptibilities than their pyrite ? poor counterparts. Although the conductivities are also slightly higher, Boomer ? style deposits are not detectable using downhole EM. The differing conductivities observed in unmineralised BlFs and BlFs with significant pyrite andlor pyrrhotite can be explained by varying textural and mineralogical characteristics of the metallic lustre minerals involved.

An overview of the relationship between granitoid intrusions and gold mineralisation in the Archaean Murchison Province, Western Australia

In the Archaean Murchison Province of Western Australia, granitoid batholiths and plutons that intruded into the ca. 2.7–2.8 Ga and ca. 3.0 Ga greenstone belts can be divided into three major suites. Suite I is a ca. 2.69 Ga monzogranite-granodiorite suite, which was derived from anatexis of old continental crust and occurs as syn-tectonic composite batholiths over the entire province. Suite II is a trondhjemite-tonalite suite (termed I-type) derived from partial melting of subducted basaltic crust, which intruded as syn- to late-tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated. One of the Suite II trondhjemite plutons has a Pb−Pb isochron age of 2641±36 Ma, and one of the structurally youngest tonalite plutons has a minimum Pb−Pb isochron age of 2630.1±4.3 Ma. Suite III is a ca. 2.65–2.62 Ga A-type monzogranite-syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province. Gold deposits in the province are mostly hosted in brittle-ductile shear zones, and were formed at a late stage in the history of metamorphism, deformation and granitoid emplacement. At one locality, mineralisation has been dated at 2636.8±4.2 Ma through a pyritetitanite Pb−Pb isochron. Lead and Sr isotope studies of granitoids and gold deposits indicate that, although most gold deposits have initial Pb isotope compositions most closely similar to those of Suite II intrusions, both Suite I and Suite II intrusions or their source regions could have contributed solutes to the ore fluids. These preliminary data suggest that gold mineralisation in the Murchison Province was temporally and spatially associated with Suite II I-type granitoids in the northeastern part of the province. This association is consistent with the concept that Archaean gold mineralisation was related to convergent-style tectonic settings, as generation of both Suite II I-type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust, and old sialic crust or its anatectic products may also contribute solutes to the ore fluids. Integration of data from this study with other geological and radiogenic isotope constraints in the Yilgarn Block argue against direct derivation of gold ore fluids from specific I-type granitoid plutons, but favour a broad association with convergent tectonics and granitoid magmatism in the late Archaean.

A time scale for granitoid emplacement in the Archean Murchison Province, Western Australia, by single zircon geochronology

Abstract In the Late Archean Murchison Province of Western Australia four major granitoid suites intruded into two stratigraphically distinct supracrustal sequences: the Luke Creek and Mount Farmer Groups. Single zircon UPb ages from five granitoid samples, representing three of the four suites, were measured by ion microprobe. The earliest of the suites, the pegmatite-banded gneiss, intruded Luke Creek Group greenstones at ∼2919 ± 12 Ma (1σ). The Luke Creek Group and the pegmatite-banded gneiss protolith were subsequently deformed and then overlain by Mount Farmer Group greenstones. Large volumes of monzogranite then intruded greenstones throughout the province; two samples from this suite gave crystallization ages of 2704 ± 51 and 2681 ± 6 Ma. This package of greenstones, gneiss and monzogranite was folded and metamorphosed and then intruded by two suites of post-folding granitoids. Two samples from the more southern of these suites gave ages of 2641 ± 5 and 2602 ± 14 Ma. Two additional samples were collected from a tonalite pluton previously assigned to the northern post-folding granitoid suite; they gave crystallization ages of 2760 ± 8 and 2784 ± 22 Ma (1σ). This tonalite is now interpreted as a subvolcanic pluton associated with the andesite and dacite volcanism which produced the nearby Mount Farmer Group greenstones. No xenocrystic zircons with ages significantly in excess of 3.0 Ga were found in any of the seven samples, implying that little or no mid-Archean or older rocks are present in the region. Lower concordia intercept ages and 207 Pb/ 206 Pb ages from individual analyses indicate the presence of Mid- to Late-Proterozoic activity throughout the Murchison Province. Concordia intercept ages were calculated using a least-squares fit of a plane in three dimensions. This approach obviates both the need to assign a common lead isotopic composition and the assumption that no pre-Recent events have disturbed the population's UPb system. A further refinement of the data assessment process involved the use of the internal scan-by-scan scatter for defining the precision of the individual Pb / Pb isotopic ratios. Compared with the observed scatter, ion counting error estimates were found to frequently underestimate the uncertainties in the 207 Pb/ 206 Pb and 204 Pb/ 206 Pb ratios.

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.

Crustal evolution of Archaean granitoids in the Murchison Province, Western Australia

Four suites of granitoids intruded the supracrustal greenstone sequence in the Murchison Province of the Archaean Yilgarn Craton during a 300 million year period. The earliest granitoid suite intruded the base of the developing greenstone sequence as a series of thin subhorizontal tabular plutons of monzogranite and granodiorite at ∼2.9Ga. This suite has been deformed and metamorphosed, and is now a pegmatite-banded gneiss. At about 2.7 Ga, thick, subhorizontal, tabular plutons of monzogranite intruded the base of the greenstone sequence. This suite, which now forms much of the regions between greenstone belts, was folded and recrystallized during regional deformation and metamorphism. Two distinct but contemporaneous suites of post-folding granitoids intruded the greenstone belts at ∼2.6 Ga, largely post-dating regional metamorphism. One suite of post-folding granitoids comprises tonalite, trondhjemite, granodiorite and monzogranite plutons, confined mainly to the north of the Province. The other suite comprises quartz-rich monzogranite and syenogranite plutons, confined mainly to the south of the Province. Pegmatite-banded gneiss, recrystallized monzogranite, and the northern suite of post-folding granitoids were all derived by partial-melting of mafic crustal rocks. Most post-folding granitoids from the southern suite were derived by partial-melting of siliceous crustal material at least as old as basal greenstones. The modes and sites of intrusion of all granitoid plutons were controlled by active tectonic processes or by structural features of the crust. Widespread ∼2.6 Ga RbSr ages of pegmatite-banded gneiss and recrystallized monzogranite reflect post-metamorphic cooling which was contemporaneous with intrusion of post-folding granitoids.