T. C. McCuaig

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.

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earths continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Spatio-temporal constraints on lithospheric development in the southwest–central Yilgarn Craton, Western Australia

The Archean western Yilgarn Craton contains an extensive record of supracrustal formation from ca 3730 to ca 2675 Ma, as well as evidence of an ensialic crustal component as old as ca 4400 Ma. These features make the western Yilgarn Craton one of the oldest crustal provinces on Earth and ideal for the study of Archean crustal evolution. Spatial analysis of new and collated U–Pb age data define three broad pulses of granite emplacement at ca 3000–2820, ca 2805–2720 and ca 2720–2600 Ma, with a period of regional quiescence at 2820–2805 Ma. Within these pulses, major peaks in granite production are defined at ca 2920, ca 2890, ca 2845, ca 2790, ca 2750, ca 2690, ca 2665, ca 2655, ca 2630, and ca 2615 Ma; with lesser inherited material as old as 3670 Ma. In the western Yilgarn Craton, all terranes show evidence of granite activity at ca 3000–2820 Ma. The South West Terrane and Southern Cross Domain share granite pulses at ca 2950–2920, 2880–2820 and 2800–2720 Ma, although during these intervals granite magmatism tends to dominate in one terrane, i.e. ca 2805–2780 Ma granite activity predominantly occurs in the South West Terrane, while 2780–2720 Ma activity is focused in the Southern Cross Domain. Including the period of quiescence, granite production is relatively minor between ca 2820 and ca 2720 Ma relative to the 3000–2820 Ma and 2720–2600 Ma intervals, suggesting limited crustal development at this time. This period corresponds with widespread greenstone formation throughout the western Yilgarn Craton. The major pulse of granite emplacement and crustal evolution occurs at ca 2700–2600 Ma, with the main phases of activity at ca 2680–2650 Ma in the Southern Cross Domain and ca 2640–2620 Ma in the South West Terrane. These pulses coincide with a craton-wide transition in granite geochemistry from high-Ca to low-Ca at ca 2650 Ma and suggest significant variations in the method and timing of melt generation. Results from this study provide new constraints on the spatio-temporal evolution of the lithosphere in the western Yilgarn Craton. The spatial distribution of these age data suggest that existing terrane boundaries should be revised with the South West Terrane separated into at least two distinct domains, and the boundary between the Youanmi and South West Terranes moved westward to correspond with the eastern extent of charnockite granites.

Contrasting komatiite belts, associated Ni-Cu-(PGE) deposit styles and assimilation histories

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew–Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) MgO contents of 25–30% in inferred parental magmas; and (ii) Al/Ti ratios of ∼20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e.g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni–Cu–(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.

A geophysically constrained multi-scale litho-structural analysis of the Trans-Tanami Fault, Granites-Tanami Orogen, Western Australia

The Trans-Tanami Fault in the poorly exposed Paleoproterozoic Granites-Tanami Orogen of Western Australia is an ∼100 km long curvilinear structure with ∼6 km right lateral displacement. Multi-scale integration and analysis of aeromagnetic, gravimetric, reflection seismic and remote sensing data have constrained the relative timing and architectural relationship of this structure. Interpretation of regional scale long-wavelength potential field (gravity and magnetic) anomalies, which are commonly used to define first-order structures, show that the fault is not a terrane boundary. Structural interpretation of short-wavelength potential field data illustrates that the structural domains on either side of the fault represent the products of a non-homogeneous stress regime developed between rigid granitic plutons. Additionally, 2D joint forward modelling of gravity and magnetic data and interpretation of reflection seismic data confirms the vertical displacement across this fault to be negligible indicating a predominant lateral displacement. The lateral displacement along a portion of this structure has exploited a pre-existing plane of a north-dipping thrust fault. Where this early thrust fault terminates, the Trans-Tanami Fault displaces previously unfaulted rock as a wrench fault step-over. These observations differ from previous findings in the area by constraining the absolute displacement of this structure and through the recognition of a wrench fault system that includes lateral step-overs between re-activated early thrust fault planes.