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Final Reports (Stage One Projects)

Project F3: Micrometallogeny of Hydrothermal Fluids

Introduction

This report is a synopsis of research compiled and carried out within the Predictive Mineral Discovery CRC F3 project "Micrometallogeny of hydrothermal fluids". The F3 project's original objectives were:

• Geology-driven terrain and ore fluids investigations to evaluate the chemistry and fluid processes within mineral systems in order to extend the focus of fluid studies beyond direct ore deposit analysis.
• LAICPMS and PIXE technique and methodology development were techniques utilized throughout the project and methodologies were developed for their combined application.
• Diamond-cell autoclave experiments component of the project was a scoping-collaboration in year one of F3 with the Museum of South Australia. 
• Database development and fusion with numerical modelling resulted in development of a web-based database for fluid inclusion research has been successful and is currently accessed at http://www.ga.gov.au/minerals/research/methodology/geofluids/flincs_about.jsp

High Impact Outcomes

• Terrain-scale - regions that contain evidence for both evaporitic and magmatic fluid reservoirs are most prospective.
• District to deposit-scale - the most prospective systems will contain evidence for ultrasaline magmatic fluids and also fluid inclusions with a wide range of salinities and temperatures.
• The most Cu-rich fluid inclusions occur in magmatic settings and have Br/Cl ratios consistent with a magmatic source.

Research Results Summarised 

Distinct differences between hydrothermal fluids that formed barren to weakly mineralised regional alteration systems and iron oxide-copper-gold (IOCG) deposits have been recognised in Proterozoic IOCG districts. These differences include:

• Ultrasaline, multisolid inclusions are abundant in IOCG deposits but are mostly absent in regional alteration systems.
• Br/Cl ratios from PIXE analysis of individual fluid inclusions suggest that fluid salinity in the IOCG deposits was derived from halite dissolution and magmatic sources whereas regional fluids are characterized by higher Br/Cl ratios consistent with a bittern brine origin. These results are consistent with step-heating measurements of halogens using bulk inclusion techniques.
• Noble gas analyses also identify a magmatic/mantle fluid component in the IOCG deposits in addition to surface derived fluids that experienced crustal residence. There is no evidence for a magmatic/mantle fluid in the regional alteration systems.

Fluid mixing is interpreted to be a critical process in IOCG ore deposition. A combination of data suggests that major IOCG deposits formed by the mixing of the evaporite-related brines with a pulse of Cu-rich ultrasaline magmatic fluid. Evidence supporting this mixing model includes:

• Fluid inclusions in IOCG deposits display very wide ranges in salinity and temperatures. Different fluid inclusion types occur in the same paragenetic stages but there is an overall decrease in salinity through pre-, to syn- and post-ore stages.
• The most Cu-rich fluid inclusions occur in magmatic settings and have Br/Cl ratios consistent with a magmatic source. The most Cu-rich fluids in IOCG deposits also have Br/Cl ratios consistent with magmatic fluids.
• Br/Cl and Cu PIXE analyses of fluid inclusions in all settings suggest mixing between Cu-rich magmatic fluids and Cu-poor evaporite derived fluids.
• The largest IOCG deposit, Ernest Henry, has the strongest magmatic/mantle-like noble gas signature further suggesting a key role for magmatic fluids.

List of Partners

• Geoscience Australia
• CSIRO Exploration and Mining
• James Cook University
• La Trobe University
• Monash University
• University of Western Australia

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

Full Report 24.7Mb

For Further Information Contact

Project Leader
Tim Baker
Economic Geology Research Unit
School of Earth Sciences
James Cook University
Tel: +61 7 4781 4756
Email: Timothy.Baker@jcu.edu.au

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