Steffen Hagemann

Lode-gold deposits of the Yilgarn block: products of Late Archaean crustal-scale overpressured hydrothermal systems

Abstract Although the lode-gold deposits of the Yilgarn block are hosted by a variety of rocks, and their structural style, associated alteration and ore mineralogy are also variable, common parameters suggest that they represent a coherent group of epigenetic deposits, most of which formed during a widespread (500 000 km2) and broadly synchronous (2635 ± 10 Ma) hydrothermal event in the closing stages of the Late Archaean tectonothermal evolution of the host granitoid-greenstone terrains. Progressive variations in deposit parameters can be correlated with the metamorphic grade of the enclosing greenstone successions. These systematic variations, combined with evidence for their timing and the P-T conditions of their formation, indicate that the deposits form a continuum in which gold deposition took place from < 5 km depth (1 kbar, 180°C) to > 15 km depth (> 5 kbar, 700°C), marking hydrothermal fluid flow and fluid evolution through the middle and upper crust. The primary ore fluid appears to have been an overpressured, low salinity H2O-CO2-CH4 fluid originating from a deep source. Upward fluid advection was strongly channelized along vertically extensive conduits. Although there is a gross regional association between clusters of gold deposits and craton- or greenstone-scale deformation zones, these do not appear to have been the primary fluid conduits, at least during their major phase of structural and magmatic activity. Further, all kinematic types of lower-order structure are mineralized, and the fossil fluid conduits exposed at the mine scale are extremely variable, some with no obvious fault or shear control. A potentially unifying hypothesis that can explain this extreme variability in the nature of the conduits is that fluid flow was focussed into zones of low mean stress in the granitoid-greenstone terrains. This can explain the selective occurrence of gold deposits adjacent to irregular or fault-bounded granitoid contacts in some goldfields, the selective mineralization of competent units (e.g. dolerites) in elongate greenstone belts that contain such units and are oriented sub-perpendicular to the far-field compressive stress, and the lack of mineralization in major, largely planar, shear zones undergoing simple shear. The orientation of the greenstone belts with respect to the far-field compressive stress appears to be a crucial factor in defining the potential for strike-extensive zones of low mean stress. This potentially can explain why granitoid-greenstone belts with a high density of sub-parallel craton- and greenstone-scale deformation zones are most highly mineralized and commonly contain the giant gold deposits which, themselves, are located in geometrically anomalous zones within these greenstone belts.

The Bronzewing lode-gold deposit, Western Australia: P–T–X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins

Abstract The Bronzewing lode-gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton of Western Australia. Gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies. Individual ore bodies are controlled by a complex, gold-bearing quartz vein system that comprises shear and extension veins formed during a progressive D2 deformation event. Gold is localized in quartz along fractures and deformed grain boundaries, and is interpreted to have formed late in the formation of the veins. Detailed petrography and microthermometry on primary, pseudosecondary and secondary fluid inclusions trapped in gold-bearing shear and extension veins revealed five types of fluid inclusions: (1) CO2±CH4–H2O–NaCl inclusions of variable salinity (0.1 to 17.5 eq. wt.% NaCl) containing between 10 and 99 mol% CO2 and molar volumes that range from 22 to 76 cm3; (2) H2O–NaCl inclusions of variable salinity (0.4 to 22.1 eq. wt.% NaCl); (3) CO2±CH4 inclusions with up to 58 mol% CH4 and molar volumes between 54 and 73 cm3; (4) CH4–H2O inclusions with CH4 ranging from 80 to 90 mol%; and (5) CH4 inclusions with low molar volumes of 19 to 23 cm3. Types 1, 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary, pseudosecondary and secondary trails and clusters within the gold-bearing quartz vein system. These co-existing fluids are interpreted to have formed by fluid immiscibility of a low-salinity, homogeneous parent fluid at about 300°C and 1500 bars. Locally, Type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100°C less than the co-genetically trapped Type 1 aqueous-carbonic inclusions. This discrepancy is interpreted to have involved CO2 effervescence in response to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are rare, and are only present locally in secondary trails and clusters in extension veins. Significant pressure fluctuations, but relatively constant homogenization temperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic decompression of the hydrothermal fluids, due to seismic activity along the shear zones that host the gold-bearing veins, triggered fluid immiscibility. The process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the D2 shear zone hosted vein system at Bronzewing.

Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia

Abstract Fluid inclusions that are interpreted to be related to vein filling of quartz veins at the Three Mile Hill and Marvel Loch gold deposits in amphibolite-facies rocks, and the Griffins Find deposits in granulite-facies rocks in the Yilgarn Craton of Western Australia are dominantly low-salinity H 2 O–CO 2 ±CH 4 mixtures or CO 2 –CH 4 fluids. At each deposit, inclusions have a wide range of compositions with respect to both the aqueous:carbonic ratio and the CO 2 :CH 4 ratio of the carbonic phase, and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock. Inclusion densities suggest a range of P–T conditions of entrapment, and are in general not consistent with the conditions of vein formation indicated by vein assemblages. Diffusional addition of H 2 into inclusions, diffusional loss of H 2 O, and reduction of inclusion volume are possible during cooling and uplift along the inferred P–T path. The inclusion populations could have been derived from an originally uniform population of low-salinity, aqueous dominated H 2 O–CO 2 inclusions by a combination of these processes. Inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition, and, to an extent, by variations in fluid salinity. If inclusion re-equilibration is the cause of inclusion variability, it was of variable intensity within a single vein system, and within individual clusters of inclusions in some samples, and is suggested to have been a function of the local petrological and textural environment.

Constraints on the timing of lode‐gold mineralisation in the Wiluna greenstone belt, Yilgarn Craton, Western Australia

40Ar‐39Ar ages of hydrothermal muscovites constrain the age of gold mineralisation in the Wiluna greenstone belt, Western Australia. A single muscovite sample from the Matilda M1 deposit, hosted in the greenschist‐amphibolite facies Matilda domain, has a 40Ar‐39Ar plateau‐like segment corresponding to an age of 2623 ± 12 Ma (2 σ). This is interpreted as the probable age of mineralisation and is similar to radiometric ages of mineralisation from other parts of the craton. Two muscovite samples from an altered microdiorite dyke from the East Lode deposit, located within the prehnite‐pumpellyite facies Wiluna domain have 40Ar‐39Ar plateau‐like segment ages of 2565 ± 12 Ma and 2563 ± 12 Ma. A third sample, consisting of muscovite from an altered high‐MgO basalt, has an enigmatic spectrum with no obvious plateau‐like segment and the highest temperature steps have apparent ages of ca 2600 Ma. Two alternative interpretations are considered for the data from the East Lode deposit: (i) mineralisation in the Wiluna...

Banded iron formation to iron ore: A record of the evolution of Earth environments?

Banded iron formations (BIF) are the protolith to most of the world’s largest iron ore deposits. Previous hypogene genetic models for Paleoproterozoic “Lake Superior” BIF-hosted deposits invoke upwards, down-temperature fl ow of basinal brines via complex silica and carbonate precipitation/dissolution processes. Such models are challenged by the necessary SiO 2 removal. Thermodynamic and mass balance constraints are used to refi ne conceptual models of the formation of BIF-hosted iron ore. These constraints, plus existing isotope and halogen ratio evidence, are consistent with removal of silica by down- or up-directed infi ltration of high-pH hypersaline brines, with or without a contribution from basinal brines. The proposed link to surface environments suggest that Paleoproterozoic BIF-ore upgrade may provide a record of a critical time in the evolution of the Earth’s biosphere and hydrosphere.

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

Abstract 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.

A mineral system approach to iron ore in Archaean and Palaeoproterozoic BIF of Western Australia

Abstract This review paper examines banded iron formation-hosted higher-grade (>58 wt% Fe) iron ore types present in the two main metallogenic districts of Western Australia, the Yilgarn Craton and the Hamersley Province. The principal iron ore deposits from both districts exhibit variation in ore properties and genesis within and across districts, but also striking similarities. There are five critical elements involved in iron ore formation and preservation: (a) BIF iron fertility defined by stratigraphic and geodynamic setting; (b) Si-dissolving fluid flow; (c) high permeability at a range of scales; (d) exhumation and supergene modification; and (e) preservation of BIF-hosted iron ore bodies by surficial modification, cover or structures (downdrop, overthrust). Several subsidiary or constituent processes are important for the formation of distinct iron ore types and have expressions as (mappable) targeting elements. Deposits in the Hamersley Province record the presence of basinal brines and meteoric fluids, whereas deposits in the Yilgarn Craton, while less well constrained, suggest the influence of metamorphic/magmatic and meteoric fluids. A scheme for BIF alteration related to ore formation in a crustal depth continuum is presented, which integrates pressure-/temperature-dependency of assemblages, fluid–rock ratios and Si-dissolution capability and is a conceptual guide to prospective zones for iron ore.

Mapping of hydrothermal alteration and geochemical gradients as a tool for conceptual targeting: St Ives Gold Camp, Western Australia

Camp- to deposit-scale alteration halos at the kilometrescale are documented in the St. Ives gold camp, the Yilgarn Craton, Western Australia. St. Ives has sulphide-oxide mineral footprints, which are interpreted to represent different hydrothermal fluids, a more reduced and a more oxidized fluid. Boundaries where reduced and oxidized fluid domains border each other are particularly suitable for gold precipitation, suggesting a redox control on gold mineralization. Oxidized zones can be identified using detailed gravity and aeromagnetic images as well as camp-scale, first-fresh-rock, multielement whole-rock geochemistry and PIMA data. Stable isotope variations also match well spatially with reduced and oxidized zones.

Geochemistry and geology of spatially and temporally associated calc-alkaline (I-type) and K-rich (A-type) magmatism in a Carboniferous continental arc setting, Pataz gold-mining district, northern Peru

The Pataz–Parcoy gold-mining area, in the Eastern Andean Cordillera of northern Peru, is located on the western margin of the Amazonia craton, in a transtensional jog on the Cordillera Blanca Fault. Episodic subduction, accretion and rifting have taken place in the Eastern Andean Cordillera since the Mesoproterozoic. In the Pataz district, the Cambrian–Ordovician Vijus and Atahualpa formations comprise dacitic and rhyodacitic volcaniclastic rocks that formed by fractionation and/or crustal assimilation of a regionally more abundant andesitic parent melt. Mississippian magmatism began with enriched tholeiitic magmas that lack a Ti anomaly and formed by melting undepleted metasomatised asthenosphere. These magmas assimilated variable amounts of continental crust and were emplaced as Vista Florida Group volcaniclastic rocks, and dioritic plutons of the Pataz batholith. Ponding of these mantle-derived magmas in andesitic lower crust caused partial melting, which generated the volumetrically dominant, granodioritic component of the calc-alkaline (I-type) Pataz batholith. Within a few million years but following rifting, and uplift of the Pataz batholith, relatively K-rich (A-type) magmas formed by melting of a mid-crustal tonalitic source, and were emplaced in the upper crust as the latitic to rhyolitic Lavasen Volcanics and Esperanza subvolcanic complex. Anatexis in the mid crust was promoted by a second batch of magma derived from the undepleted metasomatised asthenosphere, evidence of which is preserved as mafic components of the Esperanza subvolcanic complex and post-Esperanza dolerite dykes. A K-rich magma chamber, the source of the Lavasen Volcanics and the Esperanza subvolcanic complex, is proposed as a possible source of the ore fluid, which deposited gold in veins hosted by the Pataz batholith.

Structural and hydrothermal alteration evidence for two gold mineralisation events at the New Celebration gold deposits in Western Australia

The New Celebration gold deposits (∼2 Moz Au) are located within the Boulder segment of the first-order, transcrustal Boulder-Lefroy fault zone about 35 km southeast of Kalgoorlie, in the Kalgoorlie Terrane of the Eastern Goldfields Province, Western Australia. The gold deposits are hosted by ultramafic rocks (komatiite), differentiated gabbro/dolerite and variably thick (0.5 to 5 m, locally up to 80 m) felsic porphyry dykes that have intruded the ultramafic–mafic rock contact. Host rocks have undergone regional metamorphism to upper greenschist facies. At least four deformation events are recorded at New Celebration: (1) D1NC is represented by vertical stratigraphic contacts, the expression of regional scale upright folds. (2) D2NC deformation is expressed as a well-developed, NNW-trending, steeply WSW-dipping penetrative shear foliation (S2NC) and foliation-parallel, boudinaged quartz–carbonate veins (V2NC), which are representative of the regional D2 Boulder fault zone; movement on the fault is sinistral oblique slip. (3) D3NC deformation resulted in NNE-trending, WNW-dipping, short strike length (<50 m) faults, quartz–carbonate–epidote–chlorite fault fill veins and widely spaced S3NC foliation. Subhorizontal L3sNC slickenline lineations on D3NC fault planes indicate strike-slip movement during D3NC (no kinematic indicators are observed), whereas the geometry of S–C fabrics in the S3NC foliation suggests sinistral movement. (4) D4NC deformation is poorly developed by west-dipping, curviplanar faults that cross cut the S2NC foliation. Lineations formed during D2NC deformation include: (a) moderate to steep, SSE to SSW-plunging L2mNC mineral elongation lineations, (b) moderate, NW-plunging L2iNC intersection lineations between S- and C-foliation planes, and (c) moderate to steep, SSE to SSW-plunging L2sNC slickenline lineations. Moderate to steep SSW-plunging quartz L2mNC elongation lineations, in conjunction with the orientation of S–C fabrics, constrain the sense of movement on the shear zone as sinistral oblique-slip. The S2NC foliation is also developed in thin (1–5 m width) M1 plagioclase-rich porphyry dykes, thus indicating their emplacement pre- to syn-D2NC deformation. The mylonitic fabric in the first-generation Magmatic 1 (M1) porphyry dykes contrasts with the ‘fresh,’ undeformed igneous textures of second generation Magmatic 2 (M2) quartz–feldspar porphyry dyke, which preferentially intruded along the mafic–ultramafic rock contacts. The lack of internal ductile deformation fabrics and a predominance of brittle structures (e.g. fracture network at the margins) in the M2 porphyry dyke indicate its emplacement into a brittle deformation environment. Two gold events are interpreted at the New Celebration deposit. (1) An ‘early’ Mylonite-style gold event in high-strain, quartz–ankerite–biotite–sericite mylonite in mafic rocks and M1 porphyry dykes, which is interpreted to be synchronous with the D2NC deformation. Minute inclusions of gold (<100 μm) are hosted within pyrite that is aligned parallel to the S2NC foliation planes. Gold in mafic mylonite is in equilibrium with ankerite–sericite ± biotite ± pyrite and is also spatially and temporarily associated with gold and non-gold-bearing tellurides such as calerverite, petzite, hessite, altaite, melonite and a bismuth telluride. (2) A ‘late’ gold event that consists of Contact and Fracture-infill styles. The Contact-style gold mineralisation is located in high-Mg basalt within S2NC foliation planes that wrap around the M2 porphyry dyke during late D2NC deformation. Gold is in equilibrium with pyrite–sericite–ankerite. TheFracture-infill-style gold mineralisation is located in brittle fracture networks formed at the margin of the M2 porphyry dyke. ‘Free’ gold and inclusions of gold in pyrite are in equilibrium with pyrite–sericite2 ± ankerite. No telluride species are observed in either Contact or Fracture-infill-style mineralisation. The exact timing of the Fracture-infill-style gold mineralisation is equivocal; it may have formed late-D2NC or during D3NC or even during a later deformation event. The New Celebration gold deposits are a rare example of an orogenic gold system located in a first-order, transcrustal fault system, the Boulder-Lefroy fault. Its complex fault zone architecture and long-lived nature of fault movement is interpreted to be at least in part responsible for the high gold endowment when compared with the typically barren first-order fault zones. Magnetite and biotite in the outer alteration zones are spatially related to M1 dykes, and the spatial and temporal restriction of tellurides to the M1 dyke suggest that the orogenic New Celebration gold deposits were sourced from magmatic fluids, at least during the early stages of the formation of the deposits.