Greenstone Pile Research Articles

Structural features and age of gold mineralization in the Troia Massif, Borborema Province, NE Brazil: A Paleoproterozoic (∼2029 Ma) hypozonal orogenic gold deposit overprinted by the late Neoproterozoic Brasiliano/Pan–African orogeny

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn+1 is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn+2 represents a strike–slip NE–trending shear zones, and (4) the late Dn+3 event is characterized by ductile–brittle deformation. Evolution from Dn+1 to Dn+3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW–ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high–temperature calc–silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (±Te, Ag) occurs along late stage brittle structures (Dn+3). The U–Pb titanite age of 2029 ± 28 Ma for the calc–silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Eburnean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). This is in agreement with the U–Pb zircon age of 575 ± 3 Ma obtained for the syn–tectonic Guaribas dikes, which bracketed the age of Dn+3. Therefore, as Dn+1, Dn+2 and Dn+3 evolved by progressive deformation; they are all of Neoproterozoic age. This two–stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan–African orogeny.

Two stages of gold mineralization at Hutti mine, India

The Hutti gold mine is located in a high-angle, NNW–SSE-trending shear zone system, which hosts nine discrete auriferous shear zones (reefs). On a clockwise, retrograde PT path two separate stages of deformation/metamorphism (D2/M2 and D3/M3) occurred synchronous with two distinct stages of gold mineralization, both of which were associated with different fluid types. Stage 1 mineralization developed during D2/M2, where the amphibolite host rocks were altered by a metamorphic fluid with a \( {{\delta }^{{18}}}{{O}_{{{{H}_2}O}}} \)of 7.5–10.1 ‰, rich in K, S, As, and Au at pressure and temperature conditions of around 3 kbar and 530 + 20/−30°C, respectively. The stage 1 auriferous shear zones are enveloped by a zoned alteration consisting of a distal biotite–chlorite and proximal biotite–plagioclase assemblage. Subsequently, D2/M2 was overprinted by D3/M3 deformation and metamorphism at 300–400°C and <2 kbar that formed the stage 2 mineralization. The stage 2 mineralizing fluid which originated from outside the greenstone belt (δ18Ofluid of 3.2–6.8 ‰) was rich in Si, Au, and W. This mineralization stage is distinct by the emplacement of laminated quartz veins central to the shear zone, containing locally visible gold at concentrations of up to 1 kg Au/t. The laminated quartz veins are surrounded by a millimeter-scale chlorite2–K-feldspar alteration halo, which replaced the stage 1 biotite–plagioclase assemblage. The oxygen isotopic composition of the stage 2 fluid suggests a mixture of a magmatic fluid with an oxygen isotopic composition in the range of 6 to 10 ‰ and an isotopically light formation fluid that resulted from fluid–rock interaction in the greenstone pile. The two fluid fluxes at stages 1 and 2 both contributed to the overall gold mineralization; however, it was the second fluid pulse, which gave the Hutti mine its status as the largest gold mine in India. The metamorphic evolution was thereby important for the first stage, whereas the second stage was controlled by tectonism and intrusion of the high-heat production Yellagatti granite that re-established the fluid plumbing and mineralizing system.

Application of mineral equilibria modeling to constrain T and X CO2 conditions during the evolution of the Magdala gold deposit, Stawell, Victoria, Australia

Mineral equilibria modeling involving solid solution calculations has been combined with mineral assemblage information from the alteration zones associated with gold mineralization to determine the T and XCO2 conditions for the formation of the Magdala gold deposit at Stawell, Victoria, Australia. Economic gold mineralization is primarily hosted within the stilpnomelane alteration zone of the Stawell Facies that is adjacent to the Magdala Basalt. Evolution of the Magdala gold deposit involved at least three fluid infiltration events: (1) a CO2-bearing fluid during the D2 deformation event produced carbonate spots throughout the chlorite zone; (2) a CO2–S–K-bearing fluid, accompanied the D3–4ab deformation and produced a muscovite zone and siderite rims on ankerite; and (3) a CO2–K–S–Au-bearing fluid during the D4c deformation event produced the stilpnomelane zone of the Stawell Facies, the proximal and distal alteration zones within the Magdala Basalt, and the main economic gold mineralization. Mineral equilibria modeling constrains the temperature of formation of the Magdala deposit to T = 345–390°C at 3kbar, substantially lower than indicated by other previous classical thermobarometry methods. Furthermore, this method has allowed the characterization of the mineralizing fluid and constrained its composition to XCO2 < 0.08 at 3kbar. The timing and composition of the mineralizing fluids are similar to that of metamorphic fluid generated from devolatilization of a greenstone pile with peak of metamorphism occurring earlier and at deeper levels in the crust.

Conductive incubation and the origin of dome‐and‐keel structure in Archean granite‐greenstone terrains: A model based on the eastern Pilbara Craton, Western Australia

The Archean East Pilbara Granite‐Greenstone Terrane (EPGGT) of the Pilbara Craton in Western Australia preserves a prolonged record of voluminous mafic‐dominated volcanism and felsic plutonism, commencing at circa 3515 Ma and ultimately resulting in the development of spectacular “dome‐and‐keel” structures. Early crustal growth was dominated by basaltic magmatism and the regional development of a 12–18 km thick greenstone sequence by circa 3335 Ma, overlying widespread 3470–3430 Ma tonalite‐trondhjemite‐granodiorite (TTG) suites emplaced into the middle crust. Stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal‐scale dome‐and‐keel formation in the southeastern EPGGT was not initiated until circa 3325 Ma but proceeded rapidly over the interval 3325–3308 Ma. This scenario is consistent with a “conductive incubation” period of 10–100 Myr duration, following the burial of radiogenic granitic crust beneath the accumulated greenstone pile. For heat production parameters appropriate to the 3515–3308 Ma EPGGT, we show that the burial of 3470–3430 Ma granitoid‐rich crust beneath the ≥3335 Ma Euro Basalt and its precursors led to long‐term crustal temperature increases of 170–234°C at 35 km depth, with a corresponding reduction in effective viscosity of 2–5 orders of magnitude. In combination with the greenstone‐over‐granitoid density inversion, the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome‐and‐keel formation. Similarly, cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large‐scale dome‐and‐keel amplification.

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.

Fluid inclusion studies of gold-bearing quartz veins from the Yirisen deposit, Sula Mountains greenstone belt, Masumbiri, Sierra Leone

The auriferous veins at Yirisen, Masumbiri, Sierra Leone, occurring mainly in the form of sericitic quartz-sulphide lodes and stringers, are hosted in metamorphosed volcano-sedimentary assemblages invaded by at least two generations of granitic intrusions. Detailed microthermometric studies of fluid inclusions from the veins coupled with laser Raman spectroscopic analysis show that the inclusions contain aqueous fluids of variable salinity (5 to 60 wt.% NaCl equivalent) and dense carbonic fluids (pure CO 2: 1.08> d>0.88 g/cm 3). Optical observations and analysis on opened inclusions by scanning electron microscopy (SEM) reveal that some of the aqueous inclusions contain a number of daughter minerals: halite, sylvite, Ca-, Fe-, Mg- and possibly Li-bearing chlorides, and anhydrite; nahcolite occurs also in some of the CO 2 inclusions. The SEM runs also detected a small amount of electrum, suggesting that silver might be a bi-product of the mineralisation. The aqueous and carbonic fluids remained immiscible throughout the formation and evolution of the hydrothermal veins. A few mixed (H 2O+CO 2) inclusions apparently resulted from accidental trapping of both fluids in the same cavity. The wide range of salinities observed in the aqueous inclusions is attributed to the mixing of relatively hot, low-salinity aqueous fluids and colder, high-salinity brines. The CO 2-rich and low-salinity H 2O inclusions are considered to be derived from the metamorphic decarbonation/dehydration of the greenstone pile whilst the high-salinity brines are believed to be basinal in origin. Pressure–temperature ( P– T) conditions of entrapment, inferred from the intersection of representative isochores of the immiscible fluids, indicate that the formation of the veins started at T=400°C and P about 4 kbar, in the presence of the high-density CO 2 and low-salinity H 2O fluids. At about 200°C, pressure fluctuations (incremental opening of the vein) correspond to the trapping of the lower-density CO 2 inclusions and high-salinity brines. It is proposed that the decarbonation/dehydration processes (possibly aided by later magmatic processes) expelled and mobilised the gold from the greenstone pile and concentrated it in the CO 2-bearing hydrothermal fluid in the form of Au–chloride complexes. High thermal gradients are believed to have caused the upward migration of this fluid from the bottom of the greenstone pile through structurally controlled conduits. We contend that phase separation of the H 2O–CO 2 metamorphic fluid, aided possibly by some wall–rock alteration, most probably triggered a decrease in ligand activity and thus, precipitation of the gold into lodes. Percolation of the basinal brines is thought to have remobilised some of the gold together with some silver.

Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications

Strain varies systematically from weakly-developed, outward-dipping, S-tectonites in the ∼3320–3310±10 Ma Mount Edgar Batholith to intensely deformed, subvertical, L-tectonites in greenstones of the Warrawoona syncline. A consistent ‘greenstone-down/batholith-up’ sense of shear is recorded in batholithic domal margins and adjacent high-grade supracrustal rims: lineations converge to a central zone of subvertical extension (zone of sinking) along the synclinal axis. At domal margins, early kinematic granitoid sheets and ‘intrusive diatexites’ are subconcordant to a well-developed, dome-parallel schistosity, but late- to post-kinematic intrusives are discordant, high-level plutons. All granitoids are the same age, within analytical error. These syn-doming features conform with structural tests for diapirs, and differ from those expected during metamorphic core complex formation and cross-folding. Diapirism is part of a larger process involving partial convective overturn of the crust. Based on strain patterns and kinematic criteria, we argue that deformation was initiated by sinking of greenstones, which rapidly subsided; the domes then rose passively as accommodation structures. Ongoing doming ( D 4) partitioned strain to the southern rim of the batholith and uplifted a wedge of the Warrawoona Syncline keel. We consider that crustal overturn occurred in response to mantle plume activity: Eruption of a 5–10 km thick, ∼3325 Ma, mafic–ultramafic greenstone pile onto an older granite–greenstone terrain created a negatively buoyant crust, but convective overturn occurred some 5–20 Ma later, triggered by widespread anatexis (thermal softening). Convective overturn may have been a common Archaean process, occurring in response to a hotter mantle, and represents an end-member deformation mechanism that includes thrust-accretion of lithotectonic assemblages in other Archaean terranes, such as the Superior Province of Canada.

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.

The age of the Kambalda greenstones resolved by ion-microprobe: implications for Archaean dating methods

A “chert” horizon within the Archaean basalt pile at Kambalda, Western Australia, contains zircons from air-fall tuffs erupted as the sediment was laid down. Ion-microprobe analyses of these magmatic zircons constrain the long-sought age of the Kambalda lavas to 2692 ± 4Ma(2 σ), and show that regional metamorphism and deformation of the greenstone pile occurred within 30 Ma of deposition. Rare zircon xenocrysts are evidence of pre-greenstone crust as old as 3450 Ma, exposed remnants of which are yet to be identified. Subsets of both the magmatic and xenocryst zircons show evidence of hitherto-unsuspected low-temperature U and/or Pb mobility in the Proterozoic, long after stabilisation of the craton. The ion-microprobe data unravel complexities of isotopic inheritance and alteration, illustrating the hazards of using bulk or whole-rock samples in Archaean isotopic investigations. Identification of interflow “cherts” as a precisely dateable rock type opens the way to direct ion-probe dating of mafic-ultramafic piles, and close numerical age control of the complex, but ill-exposed, stratigraphic and structural relations in this richly-mineralised greenstone belt.

The genesis and tectonic control on archaean gold deposits of the Western Australian Shield — A metamorphic replacement model

Gold deposits occur in greenstone belts world wide, and contribute to anomalously high gold production from Archaean terranes. As in other cratons, Archaean gold mineralization of Western Australia represents a complex array of deposit styles. Despite this, most deposits are clearly epigenetic, and large deposits have a number of features in common, including their strong structural controls, distinctive wallrock alteration (Fe-sulphide, K-mica±albite, CaMgFe carbonates), consistent metal associations (AuAgAsSbWB; low base metals), commonly Fe-rich host rocks, great depth extension and lack of appreciable vertical zonation. These shared characteristics, combined with their ubiquitous occurrence, indicate that Archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts. Auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic (porphyry) intrusions. Although these gold systems involved low-salinity, lowdensity, reduced, near-neutral H 2OCO 2 fluids carrying gold as reduced sulphur complexes, the origin of the fluids is equivocal. Most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic (granitoid or felsic porphyry) orggins, but the lack of consistent spatial relationships between specific, volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids. The metamorphic-replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile, with high geothermal gradients inhibiting significant melting. CO 2 possibly formed by the decarbonation of early alteration, related to mantle degassing along crustal-scale, synbasinal fault zones. Auriferous fluids were channelled along greenstone-scale faults, in part developed during reactivation of crustal-scale faults in a strike-slip regime. Gold deposition occurred largely under greenschist facies conditions (about 300–400°C, 1–2 kb) in response to decreasing gold solubility with declining temperature. However, a major control on gold deposition was fluid/wallrock interaction. Many large deposits formed by sulphidation of Fe-rich host rocks, with synchronous deposition of Fe-sulphides and gold. However, the variable nature of gold-depositing reactions, including lowering of fO 2 and pH, allowed a multitude of small, and some large, deposits to form wherever that fluid circulation occurred. In consequence, several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks. There are few constraints on the source of components (Au, S, K, CO 2) added to gold deposits, but even giant deposits such as the Golden Mile, Kalgoorlie could have formed from a realistic greenstone source volume (ca. 8×8×5 km). Convective circulation of fluids could have contributed to the generation of high fluid-rock ratios. On the regional scale, the markedly heterogeneous distribution of large gold deposits, gold productivity and host rocks to deposits can be accommodated by the metamorphic-replacement model. The most favourable conditions for development of auriferous hydrothermal systems operated in younger (ca. 2.7±0.1 Ga) rift-phase greenstones where greatest extension and crustal thinning produced high geothermal gradients, crustal-scale synbasinal faults, and rapid extrusion and burial of volcanics, including abundant komatiites. Iron-rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits. The least favourable conditions existed in older (ca. 3.5-3.4 Ga) platformphase greenstones, where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites, commonly in very shallow-water environments. This allowed intense synvolcanic alteration of both gold source rocks and potential host rocks. The generally smaller gold deposits formed mainly in ultramafic or greywacke hosts. Younger (ca. 3.0 Ga) platform-phase greenstones appear intermediate in nature but, unlike other greenstones, have significant epigenetic gold deposits in originally oxide-facies BIF, which were deposited on relatively deep-water platforms. Similar controls appear to exist on a world scale, with gold mineralization peaking at ca. 2.7±0.1 Ga in response to development of major rift zones in thickened, relatively mature continental crust. Interestingly, the giant Witwatersrand goldfield formed at about the same time.

Lead isotope constraints on the age and source of gold mineralization in the Archean Yilgarn Block, Western Australia

Model mineralization ages based on whole earth lead isotope growth curves are used to date sulfides (mainly galena with minor Fe sulfides and Pb tellurides) related on one hand to stratiform base metal and Ni-Cu mineralization and on the other to predominantly lode-type gold mineralization in greenstone belts of the Archean Yilgarn block, Western Australia. Galenas related to undoubted syngenetic, volcanogenic base metal mineralization and Fe sulfides to komatiite-associated Ni-Cu deposits yield Pb model ages that are broadly consistent with Sm-Nd, Pb-Pb, and U-Pb zircon greenstone belt geochronology; such deposits from the Murchison province give model ages of ca. 3.0 b.y. or greater, whereas those of the Eastern Goldfields province give model ages of ca. 2.85 to 2.80 b.y. for the northeastern part of the province and ca. 2.75 to 2.70 b.y. for the highly mineralized Norseman-Wiluna belt. The majority of galenas related to gold mineralization record model ages of ca. 2.85 to 2.75 b.y. for the Murchison province and ca. 2.75 to 2.70 b.y. for the Southern Cross province, with most deposits from the Norseman-Wiluna belt having model ages around or below ca. 2.7 b.y. A few examples of gold mineralization, all but one economically insignificant, have galenas with more complex multistage lead isotope histories with mineralization ages lying in the Proterozoic.Lead model ages of galenas from gold deposits in the Murchison province are consistently younger then those related to syngenetic mineralization; those from gold deposits in the Southern Cross province broadly coincide with the best estimates of the age of peak metamorphism; and those from the Norseman-Wiluna belt are indistinguishable or slightly younger than the ages of syngenetic base metal and Ni-Cu mineralization in the same belt. The lead isotope compositions of most gold-related galenas yield 238 U/ 204 Pb (mu ) values that are consistent with a previously proposed epigenetic model for economically important gold mineralization; in this model, lead (and by inference, gold) was derived from basalts and komatiites by major hydrothermal systems which developed due to devolatilization of the greenstone pile during regional metamorphism from between 250 m.y. to less than 50 m.y. after eruption.The small deposits that give younger mineralization ages may be related to, or affected by, minor thermal events which occurred later in the history of the Yilgarn block and which were also responsible for resetting rock and mineral Rb-Sr systems. The contrast in isotopic histories between galenas (and possibly Fe sulfides) from major and minor gold deposits may have application as an exploration tool.Significantly higher 238 U/ 204 Pb values for sulfides from the Norseman gold deposits provide support for the suggestion that crust older than 3.0 b.y. existed prior to formation of the greenstone belts of the Yilgarn block.

Dynamothermal transition zone between Archaean greenstone and granitoid gneiss at Lake Dundas, Western Australia

A 500 m wide shear zone occurs between the base of an Archaean greenstone sequence and adjacent granitoid gneiss complex on the shores of Lake Dundas, Western Australia. The dynamothermal margin remains distinguishable due to the preservation of upper amphibolite facies assemblages, related granitoid anatectites and mylonitic, schistose and gneissose fabrics developed parallel to the contact, which itself lies subparallel to the greenstone bedding surface. The margin contrasts with less deformed greenschist to low amphibolite facies assemblages which characterize lithologies within the greenstone belt, many of which retain igneous textures and relict primary phases. Structural, petrological and textural evidence indicates that the dynamothermal contact originally evolved as a subhorizontal ductile shear zone during juxtaposition of the greenstone pile with granitoid gneiss and that its formation preceded regional folding, greenschist facies overprinting and granitoid intrusion which occurred at about 2700 Ma. The amount of heat generated within the transition zone during thrusting was limited to maximum temperatures of c. 650°C due to the buffering effect of granitoid anatexis.

Geochemical and stable isotopic studies of the Crown and Mararoa reefs, Norseman, Western Australia

The quartz reefs of Archean age at Norseman, Western Australia are restricted to the flatter-dipping sections of reverse faults and the gold mineralization is mainly stratabound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events. Bucky' quartz crosscuts laminated quartz and was deposited during an episode of hydraulic fracturing. Gold and sulfides are concentrated on fractures in the laminated quartz reefs. Wallrock assemblages of the Crown and Mararoa reefs require two temporally distinct phases of alteration synchronous with the emplacement of laminated and bucky quartz reef. The first involved the deposition of hydrothermal actinolite and biotite, whereas chloritic overprinting of these assemblages characterizes the second phase of alterarion. Both potassic and chloritic assemblages were modified by the crystallization of metamorphic actinolite and tremolite. In general, single stage alteration is characterized by the addition of volatiles, K 2 0 and CaO, and the loss of Na 2 O. Changes in whole-rock geochemistry during chloritic overprinting of the potassic assemblages include losses of CaO, Al 2 0 3 , Na 2 0 and K 2 0, and a gain of MgO. The δ 18 O values of the Norseman wallrooks range from 5.8 to 9.1 ‰ with a mean value of 7.2 ±0.9 ‰ (16 sampes ). Reef quartzes also exhibit a narrow range of δ 18 O values (11.7 ± 0.4°‰, 26 samples). The wallrock assemblages are cornpatible with a mineralization temperature ≥ 400°C. The calculated δ 18 O value of the water in equilibrium with the quartzes at these temperatures is 7‰. Waters involved in the subsequent metamorphism have identical δ 18 O values which suggest they have a common source, connate fluids and hydrous minerals in the greenstone pile. The observed timing relations and isotope systcmatics suggest that the ore fluids were connate brincs which were mobilized during tectonic dewatering of the volcano-sedimentary pile and channelled up early faults.