John L. Walshe

Geodynamic modelling of the Century deposit, Mt Isa Province, Queensland

This paper is concerned with an example of quantitative modelling of orebody formation as a guide to reducing the risk for future mineral exploration. Specifically, the paper presents a detailed 3–D numerical model for the formation of the Century zinc deposit in northern Queensland. The model couples fluid flow with deformation, thermal transport and chemical reactions. The emphasis of the study is a systems approach where the holistic mineralising system is considered rather than concentrating solely on the mineral deposit. In so doing the complete plumbing system for mineralisation is considered with a view to specifying the critical conditions responsible for the ore deposit occurring where it does and having the size and metal grades that are observed. The numerical model is based on detailed geological, tectonic, isotopic and mineralogical data collected over the past 20 years. The conclusions are that the Century zinc deposit is located where it is because of the following factors: (i) a thermal anomaly is associated with the Termite Range Fault due to advection of heat from depth by fluid flow up the Termite Range Fault; (ii) bedding‐plane fissility in the shale rocks hosting the Century zinc deposit has controlled the wavelength and nature of D1 folding in the vicinity of the deposit and has also controlled increases in permeability due to hydrofracture of the shales; such hydrofracture is also associated with the production of hydrocarbons as these shales passed through the ‘oil‐window’; (iii) Pb–Zn leached from crustal rocks in the stratigraphic column migrated up along faults normal to the Termite Range Fault driven by topographic relief associated with inversion at the end of the Isan Orogeny; these fluids mixed with H2S derived at depth moving up the Termite Range Fault to mix with the crustal fluids to precipitate Pb–Zn in a plume downstream from the point of mixing. Critical factors to be used as exploration guides are high temperatures, carbonaceous fissile shales now folded into relatively tight D1 folds, fault‐controlled plumbing systems that enable fluid mixing, depletion of metals upstream of the deposit and,in particular,a very wide Fe‐depletion halo upstream of the deposit.

A detrital model for the origin of gold and sulfides in the Witwatersrand basin based on Re-Os isotopes

Abstract The Re-Os systematics of gold and sulfides from the Witwatersrand basin were utilized to determine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin. Gold from a gravity concentrate from the Western Areas Gold Plant and gold from the Vaal Reef have very high Os concentrations of approximately 73 to 10000 ppb and 3 to 32 ppb Re, resulting in 187Re/188Os ratios of 0.010 to 0.185. The gold has subchondritic 187Os/188Os ratios between 0.1056 to 0.1099 and an average value of 0.1067. Rhenium depletion ages (TRD) range from 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga. Pyrite from the Vaal Reef have Os concentrations ranging from 0.26 to 0.68 ppb, Re concentrations of 1.7 to 2.8 ppb and187Re/188Os ratios of approximately 14 to 87. The pyrite samples have measured 187Os/188Os ratios of 0.84 to 4.7 and define an isochron with an age of 2.99 ± 0.11 Ga (MSWD = 0.77). The Os isotopic data from the direct measurement of gold preclude introduction of gold to the Witwatersrand basin from crustally derived metamorphic or hydrothermal fluids between 2.7 to 2.0 Ga. The unradiogenic 187Os/188Os ratios, old TRD ages of the Western Areas and Vaal Reef gold samples, as well as the contemporaneously old age of the Vaal Reef pyrite are consistent with detrital deposition of gold during the formation of the Witswatersrand basin. The Os data will allow for minor hydrothermal remobilization and/or overprinting of hydrothermal gold on preexisting detrital gold grains but does not support the introduction of gold solely by hydrothermal fluids.

Alkalic porphyry Au – Cu and associated mineral deposits of the Ordovician to Early Silurian Macquarie Arc, New South Wales

Twenty-one alkalic porphyry deposits of Late Ordovician to Early Silurian age occur in two mineral districts in New South Wales. The Cadia and Northparkes districts formed in shoshonitic volcanic centres where a major basement structure (the Lachlan Transverse Zone) cut the Ordovician Macquarie Arc obliquely. Processes of mineralisation in both districts were centred in and around quartz monzonite porphyry complexes that intruded the volcanic centres. These composite intrusive complexes comprise pipes, dykes and stocks. Hydrothermal alteration in and around the intrusions produced a complex sequence of potassic, calc-potassic, sodic, propylitic and late stage, typically fault- and fracture-controlled phyllic assemblages. Hematite dusting was a common alteration product giving the intrusions and the altered volcano-sedimentary host sequences a distinctive pink-orange coloration. Several of the deposits have bornite-rich cores, chalcopyrite-dominant annuli and pyritic outer haloes. Gold is well correlated with bornite in most of the deposits, and with chalcopyrite at Cadia Hill. The mineralising intrusions have Sr and Nd isotopic compositions consistent with derivation from a depleted mantle source regime, although the Nd data are permissive of limited crustal contamination. Epidote peripheral to the porphyry deposits has Sr isotopic compositions indistinguishable from the host intrusions, precluding the involvement of external seawater in the mineralising processes. In contrast, slightly elevated initial Sr values have been detected in epidote from nearby skarn deposits, indicative of incorporation of a minor component of Sr from limestone dissolution or seawater mixing. The alkalic porphyry deposits are difficult exploration targets because intensely developed hydrothermal alteration zones are restricted to within a few hundred metres of the monzonite complexes.

Characterising the hydrothermal alteration of the Broadlands–Ohaaki geothermal system, New Zealand, using short-wave infrared spectroscopy

Abstract Hydrothermal clay minerals present in the Broadlands–Ohaaki geothermal field were characterised by field portable short-wave infrared spectroscopy. Three major alteration zones, an upper smectite, a middle illite and a lower illite–chlorite, are spectrally separable. The zoning pattern is generally consistent with the thermal structure of the geothermal field, although occasionally zone boundaries cut present-day isotherms. The data indicate that temperature is the major control on clay zoning and permeability plays a subordinate role. Both beidellite and montmorillonite are common in the upper, low-temperature smectite zone. Kaolinite, mainly of low crystallinity, marks the margin of the field where cool acidic ground waters inflow. In the middle alteration zone, illite, dominantly K-rich, shows a narrow compositional variability. Some highly permeable zones are characterised by illite with low octahedral Al contents. Ammonium-bearing illite and buddingtonite are present locally in permeable horizons within the illite zone, where temperatures are above 200°C. Chlorite is most abundant in the lower alteration zone (temperature >250°C), although it also occurs unevenly in the upper and middle alteration zones. Chlorite varies from Mg- to Fe-rich varieties (but mostly with Mg# values

Mineral systems of Australia: an overview of resources, settings and processes

Australia produces more than 22 mineral commodities from a wide variety of deposit types, but >90% of the value of mine production is obtained from just 10 commodities. Both production and resources are underpinned by a few major deposit classes and a small number of world‐class deposits. Most major deposits lie in highly endowed metallogenic provinces and formed at discrete intervals during major metallogenic epochs, notably: 2.7–2.6 Ga, 2.4–2.2 Ga, 1.7–1.6 Ga, 1.2 Ga, 500–480 Ma, 440–420 Ma, 380–350 Ma, 320–280 Ma, 240–220 Ma and 100–0 Ma. These lie within the global metallogenic epochs that have previously been shown to coincide with the accretion and breakup of the major continents. Many of the major metallogenic provinces formed by accretion during convergent tectonic regimes and can be linked to particular orogenic events and/or changes in the apparent polar wander path. Giant deposits appear to have formed from large systems commonly located at major crustal structures. The interplay of source, transport and trap is critical. Formation of ore bodies occurs when hydrothermal fluids are released in a focused manner, migrate along ‘open’ transport zones, and interact with suitable traps to deposit metals. Typically this occurs late in the tectonic evolution of the province or basin and may be associated with uplift and/or basin inversion. The efficiency of the trap site is a key factor in determining ore‐deposit size, and is enhanced by major physical and chemical contrasts between trap and fluid. Emplacement of mineralised intrusions and migration of mineralising fluids occurs in response to geodynamic triggers involving changes to the stress regime that may include far‐field stresses propagated across the Australian continent. With the exception of supergene deposits, mineralisation typically appears to have occurred within short time frames (<1 million years) in response to the geodynamic triggers. Formulation of the next generation of predictive, geologically credible models requires an integrated, holistic mineral systems approach with a strong focus on crustal architecture and the relative timing of critical events.

Finite element modeling of fluid–rock interaction problems in pore-fluid saturated hydrothermal/sedimentary basins

In order to use the finite element method for solving fluid-rock interaction problems in pore-fluid saturated hydrothermal/sedimentary basins effectively and efficiently, we have presented, in this paper, the new concept and numerical algorithms to deal with the fundamental issues associated with the fluid-rock interaction problems. These fundamental issues are often overlooked by some purely numerical modelers. (1) Since the fluid-rock interaction problem involves heterogeneous chemical reactions between reactive aqueous chemical species in the pore-fluid and solid minerals in the rock masses, it is necessary to develop the new concept of the generalized concentration of a solid mineral, so that two types of reactive mass transport equations, namely, the conventional mass transport equation for the aqueous chemical species in the pore-fluid and the degenerated mass transport equation for the solid minerals in the rock mass, can be solved simultaneously in computation. (2) Since the reaction area between the pore-fluid and mineral surfaces is basically a function of the generalized concentration of the solid mineral, there is a definite need to appropriately consider the dependence of the dissolution rate of a dissolving mineral on its generalized concentration in the numerical analysis. (3) Considering the direct consequence of the porosity evolution with time in the transient analysis of fluid-rock interaction problems; we have proposed the term splitting algorithm and the concept of the equivalent source/sink terms in mass transport equations so that the problem of variable mesh Peclet number and Courant number has been successfully converted into the problem of constant mesh Peclet and Courant numbers. The numerical results from an application example have demonstrated the usefulness of the proposed concepts and the robustness of the proposed numerical algorithms in dealing with fluid-rock interaction problems in pore-fluid saturated hydrothermal/sedimentary basins

Epidote–clinozoisite as a hyperspectral tool in exploration for Archean gold

Hyperspectral analysis at seven gold deposits within the eastern Yilgarn Craton of Western Australia has revealed significant occurrences of previously unrecognised clinozoisite, and spatial relationships between the distribution of clinozoisite, epidote and gold deposits. Here we report the development of an index to allow the systematic spectral mapping of the epidote–clinozoisite solid solution. The combination of the wavelength position and depth of the 1550 nm absorption was used to characterise the solid- solution series spectrally. The spectral responses from CSIRO HyChips™, fitted with an Analytical Spectral Devices (ASD) FieldSpec-3 spectrometer, and a SisuCHEMA™ spectral-imaging camera were calibrated against electron microbe analyses of epidote–clinozoisite. The spectral-imaging camera helped resolve correlations for samples with complex paragenetic histories. Textural studies found genetic links between epidote and Mg-chlorite, and between clinozoisite and Fe-chlorite, with each mineral combination part of separate, diagnostic hydrothermal assemblages. Spectra from epidote–clinozoisite-dominated veins showed that shifts in the 2250 nm absorption correlate with epidote–clinozoisite composition and not with chlorite composition, and that coexisting amphibole phases have a closer compositional tie than chlorite in the given samples. The genetic affiliation, yet compositional discordance, between coexisting epidote–clinozoisite and chlorite suggests that the compositional spectral index associated with each are wholly independent, but in combination are diagnostic for the mapping of separate hydrothermal assemblages. Of the newly defined compositional relationships, vein-hosted clinozoisite was found to be a proxy for pre-existing structurally-controlled hydrothermal tschermakite. A comparison of spectral and stable isotopic characteristics from diamond drill hole CD5026, St Ives mining camp, shows correlations between the epidote–clinozoisite spectral index and δ13C of carbonate and δ34S of sulfide. Such correlations imply a redox control on the distribution of clinozoisite and epidote, and mean that the spectral logging of epidote–clinozoisite transitions can serve as a proxy for mapping paleoredox gradients.

Five questions for fun and profit: A mineral system perspective on metallogenic epochs, provinces and magmatic hydrothermal Cu and Au deposits

We argue that the empirical basis of deposit classification schemes is an inhibitor of the adventurous conceptual thinking required to aid efficient mineral exploration. It ought to be possible to construct a limited number of process — based, scale-integrated, system models to describe mineral and hydrocarbon deposits, provinces and epochs. We suggest that the geodynamic controls on reservoir construction and focused fluid release may be a unifying theme across many classes of deposits, effecting the formation of mineral provinces and epochs. The common architectural, geodynamic and fluid reservoir characteristic of porphyry Cu-Au and orogenic Au classes of deposits serve to illustrate the concept. The “Five Question” description of mineral systems is utilized.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

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.