Final Reports (Stage One Projects)

Project I2+3 - Total Systems Analysis of the Mt Isa Eastern Succession

Introduction

Most of the rocks of the Eastern Succession of the Mt Isa inlier described in this report were formed between 1760 and 1500 Ma, post-dating the Archaean and early Palaeoproterozoic history. The geodynamic settings of both the depositional and orogenic parts of this history have been extensively debated in the past and are still unresolved at present.

Progress has been made in this report through an examination of the isotopic signatures of crustal blocks comprising the basement to the cover sequences, by analysing the geodynamic implications of mafic rocks and the voluminous late-orogenic felsic magmatism, and by a more detailed examination of the tectonothermal evolution.

High Impact Outcomes
The Research Program

The project was organised into six modules, results of which can be found in the Final Report as follows:

Summary Research Outcomes (a new view of IOCG genesis)

The lower crustal basement of the Eastern Succession of the Mt Isa inlier is divided into a western block and an eastern block, the latter with a distinctly younger crustal residence age. The overlying metasedimentary and meta-volcanic rocks were deposited in extensional basins and comprise two major Cover Sequences (CS2 and CS3). CS2, consisting of mafic and felsic volcanic rocks overlain by carbonates, was deposited from ca 1780 to 1690 Ma diachronously from west to east, possibly in a back-arc environment. CS3 (quartzites, pelites, carbonates and volcanic rocks), was deposited from ca 1655 to 1600 Ma, with important lateral facies changes from west to east.  Major thickness changes in both sequences are localised over NS trending structures, which were presumably basin margins. Low pressure metamorphism and albitisation occurred at the time of EW extensional basin formation.

The Isan Orogeny from ca 1600 Ma to 1500 Ma was dominated by EW compression that lasted at least episodically until ca 1500 Ma.  From ca 1550 to 1500 Ma, voluminous mafic and felsic potassic magmatism was emplaced at mid-crustal levels. Despite having A-type geochemical signatures, the granites are syn-tectonic. P-T-t paths were complex, showing an initial anticlockwise segment, followed by a clockwise segment and then heating events corresponding to granite intrusion. Metamorphism was caused by advective heating in the lower-middle crust, due to melt migration, and conductive heating in the upper crust. The major crustal structures produced during the orogeny were km scale upright folds and steep faults. The coincidence of these major structures with thickness changes in the cover sequences suggests that positive inversion was a dominant process during the orogeny. A preliminary 3D geological model has been constructed which identifies some key relationships of the basement and cover sequences, and provides important pointers for further research in resolving the architecture of the region.

Iron Oxide Copper Gold (IOCG) deposits in the Eastern Succession have strong spatial associations with the tectonic CS2/CS3 contact, with faults and fault bends and with mafic intrusions. The role of mafic rocks in IOCG genesis appears to be important in many cases: metals may have been transported and deposited from magmatic-hydrothermal fluids on crystallisation of the mafic rocks, or from metamorphic fluids that subsequently leached them. In the largest IOCG deposit, Ernst Henry, fluids from highly fractionated Williams Batholith rocks may have mixed with primitive mantle or gabbro-derived fluids. In all cases strong structural controls in the form of NS and NE-SW faults, bends and intersections are apparent. The importance of structural controls on fluid pathways is demonstrated by numerical modelling, which predicts known deposits and corresponds closely with prospectivity results. The prospectivity studies highlight a number of targets under cover, based on geophysical data.

The base metal endowment of the Eastern Succession can be seen as a product of several geological processes in a volume of crust that has experienced high fluid fluxes and prolonged deformation and thermal anomalism. Reactivation of structures and lithological diversity are also ingredients of a highly distinctive geological history that has lead to extensive mid-Proterozoic mineralisation.

Development Program - Recommendations arising from Project I2+3

The role of mafic rocks in IOCG genesis needs to be investigated in detail by ore deposit studies including detailed geochemical, petrological, fluid and isotopic research designed to test the hypothesis that mafic rocks are a critical ingredient in IOCG genesis.

The most important further work on architecture is the completion of the 3D model by creation of volumes for all rock units, and by incorporation of infill cross-sections between the existing skeleton. Geophysical forward modelling and inversion of the resultant model are urgent tasks to verify the basement architecture.

As a result of building the 3D model it has become apparent that several major structures need detailed field-based, modern structural investigation in the Eastern Succession.

The creation of the Tectonostratigraphy model also emphasised the need for a better stratigraphic understanding of the Eastern Succession based on high calibre new geochronological work.

It will not be possible to choose between several viable alternatives for the basic architecture of the Eastern Succession unless the stratigraphy is better understood. Map patterns cannot be correctly interpreted at present, even on a 1: 500 000 scale, because of the fundamental uncertainties in the stratigraphic relations of the major cover sequences.

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 available via the links below.

Full Report PDF75.9Mb

Project I2 Appendices

 

For Further Information Contact

Project Leader
Dr Tom Blenkinsop
School of Earth Sciences
James Cook University
Tel: +61 7 47814318
Email: thomas.blenkinsop@jcu.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