Final Reports (Stage One Projects)

Project  Project Y3 - Camp to Deposit-Scale Alteration Footprints in the Kalgoorlie-Kambalda Area

Introduction

The project initially focused on the St. Ives gold camp with additional studies on the Golden Mile and selected areas in the Kalgoorlie-Kambalda corridor. Additional resources were provided by St. Ives Gold Company Pty Ltd and Placer Dome to support researchers on-site at St. Ives, Kanowna Belle and Wallaby. These resources are managed through MERIWA project M358 and selected results of this project, as well as information from the pmd *CRC Y2 project, are integrated into the Final Report.

Research Summary

The project team was tasked to determine camp- to deposit-scale hydrothermal alteration footprints to identify possible vectors which can assist exploration within a set of five strategic parameters/questions:

High Impact Outcomes

pmd *CRC Project Y3 has concluded that an assessment for exploration targeting should follow these steps:

For exploration purposes, it is critical to collect data sets which allow the mapping of flow pathways and the chemistry of the hydrothermal fluids and also determine the location of zones where fluid chemistry changed dramatically, i.e. the location of chemical gradients. The best data sets to track chemical domains and zones of drastic change in the fluid chemistry (gradients) are:

Summary Research Outcomes

Most of the study was conducted in the Kalgoorlie-Kambalda corridor which forms the southern part of the "Golden Corridor."

Three dimensional seismic interpretations reveal that a significant N-S trending subsurface anticlinorium, with an undulation about an E-W axis, underlies the Kalgoorlie-Kambalda region. Significant faults dip both east and west away from the antiform. Consequently, gold deposits occur in the footwall of the master faults.

Three main fluid reservoirs (ambient fluids, magmatic fluids, mantle/deep crustal fluids) are identified based on stable isotope and fluid inclusion evidence. Near surface environments potentially show the influx of surface-derived, meteoric fluids, but their relationship to gold mineralisation is unclear. Importantly, fluid types are likely to be identified in specific trends in the stable isotope plots rather than by a unique stable isotope composition.

In order to form large-tonnage, high-grade gold deposits it is postulated that both fluid-flow and contrasting thermochemical conditions (gradients) have to be sustained at the site of ore deposition. Zones of thermochemical gradients (i.e. sites best suited for gold precipitation) can be interpreted from alteration patterns.

St Ives

Importantly, if multiple fluids of contrasting chemistry were present in the system, evidence should be found at all scales from camp- to micro-scale. Fluid inclusion studies at St. Ives give evidence of rapidly changing fluid conditions with fluctuations from saline H2O-rich fluids prior to gold precipitation to CO2-rich fluids during gold precipitation to extremely saline H2O fluids post gold. Distinct fluctuations from CH4-rich, to H2O-CO2 fluids back to CH4-rich fluids have also been identified at New Celebration, further emphasizing the importance of different hydrothermal fluids in the system.

The Golden Mile

At the Golden Mile, Fimiston style gold mineralisation is bracketed by feldspar porphyry dykes, which overprint both limbs of the Kalgoorlie fold pair. These feldspar porphyry dykes both pre-date and cross cut mineralisation. Importantly, dykes and mineralisation are overprinted by the regional NNW-trending foliation.

New Celebration

At New Celebration, two gold events are recognised. An early event hosted in porphyry dykes is interpreted to be synchronous with D3NC deformation and contains gold-bearing and gold-absent tellurides. The late gold event formed within a brittle fracture network at the margins of M2 porphyries and consists of free gold and gold in pyrite.

Hydrothermal Alteration Distribution Maps

The most critical data sets for exploration are camp-scale hydrothermal alteration distribution maps. These maps have been developed for the St. Ives camp, Kanowna Belle and Wallaby deposits.

In the St. Ives camp, oxidized magnetite-pyrite±hematite assemblages are spatially associated with major gold deposits such as Victory-Defiance and Revenge. The domains of oxidised assemblages are focused on gravity lows which indicate abundant porphyry intrusions at depth. During gold precipitation, the oxidation state of the hydrothermal fluid increased from magnetite stable to hematite stable. Domains of reduced assemblages (pyrrhotite-pyrite) occur flanking the oxidized domains. Most importantly, high grade gold intersections (>100 ppm) and also most of the known ore bodies occur at the domain boundary, but within the oxidised domain.

The Kanowna Belle gold deposit and surrounds also exhibit distinct alteration patterns in whole-rock multielement geochemistry and PIMA data sets. A V-bearing phengitic mica domain (oxidised) is characterised by high AlOH SWIR wavelengths and occurs at the Kanowna Belle deposit. It borders a Ba-rich muscovite domain (reduced) characterised by low AlOH SWIR wavelengths. Gold occurs in the oxidised domain in close proximity to the domain boundary with the reduced domain. In addition, a regional scale reduced-acidic domain characterised by paragonite, pyrite and carbonate can be distinguished which could potentially indicate pH gradients. Back calculation of multielement whole-rock geochemical data sets into modal mineralogy allow identification of reduced and oxidised domains and mapping of these domain in three dimensions.

In the district surrounding the Wallaby deposit, the broad areas of low AlOH wavelength mica imply widespread acidic conditions. The coexistence of chloritoid with short wavelength mica in these areas is consistent with reduced-acidic conditions. The Wallaby deposit occurs within a domain of longer AlOH wavelength mica, at the boundary between the reduced-acidic domain and the more oxidized-neutral domain.

The spatial distribution of sulphur isotopes in all three locations indicate that negative S-isotopes correlate well with the oxidised domains, whereas zero to positive S-isotopes correlate well with the reduced domains. Zones where S-isotopes switch from negative to positive host the gold deposits, further emphasizing the importance of chemical gradients. Based on the S-isotope data sets, it appears that the absolute state of oxidation is not critical for gold precipitation. However, it is critical to have evidence of both oxidised and reduced fluids in the system-i.e. zones of maximum geochemical gradient.

For exploration it is essential to determine the structural architecture and the spatial distribution of fluid chemistry. The combination of this information in 3D provides the best guide to mineralisation. Whilst traditional structural geology and geophysical data sets provide information on the architecture of the system, new data sets have to be collected to understand the chemical architecture.

At St.Ives, traditional mineral mapping from drill core provides information on the chemical zonation, but at Kanowna Belle and Wallaby, multielement whole-rock geochemical data sets and PIMA data sets provide the best guide. The key to localising mineralisation is to find suitable structural positions which show evidence of hydrothermal fluids of strongly different chemical composition. Zones with strong redox, pH and S gradients are best suited to host mineralisation.

Figure

Fig. 27 (see Final Report Diagram numbering)  Screenshot of 3D model of the geology of the Victory-Defiance area in the St. Ives cold camp (view to NW).  The model shows that magnetite (oxidised assemblage) is focused in the Victory-Defiance area while pyrrhotite (reduced assemblage) flanks the oxidised domain. Gold orebodies occur preferentially at the interface between oxidised and reduced domains.

List of Partners

* Please note that references and appendices are not published in this Final Report Summary. These can be found in the Final Report and available via the links below.

Full Report PDF31.5Mb

Appendices

For Further Information Contact

Dr Peter Neumayr
University of Western Australia
Tel: 08 6488 3423
Email: pneumayr@cyllene.uwa.edu.au

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Logos of core participants
Monash University University of Western Australia University of Melbourne James Cook University Geoscience Australia Victorian Institute of Earth and Planetary Sciences CSIRO Centre for Exploration Targeting AMIRA