Kinematic and structural controls of Au metallogenic systems in the Early Earth: Examples from the eastern Pilbara Craton, NW Australia

Abstract

This thesis is divided into eight chapters excluding the Introduction (Chapter 1):- Literature Review: Chapter 2 provides a detailed overview of the historical mining activities within the East Pilbara Superterrain. Details such gold production, grades and mining techniques are described, together with size and orientation of the orebodies. The alteration zone is described in detaiL At the end of Chapter 2, a comparison between the different gold deposits is made, and according to their importance, specific gold deposits are selected for detailed studies. Structural and Kinematic Setting of Greenstone-Hosted Gold Deposits and Structural and Kinematic Setting of Turbidite-Hosted Gold Deposits: Chapters 3 and 4 describe the kinematic and tectonic history of the main research areas in the Marble Bar and Nullagine Domains. The reason for classifying the gold deposits in two separate domains is the fact that gold deposits in the Marble Bar Domain are mainly hosted by greenstones wrapping the Mount Edgar Batholith, while in the Nullagine Domain the gold mineralisation is hosted by turbidites. Furthermore, the area around each gold deposit has been studied in terms of kinematic and structural setting. Later, the kinematics of structures at the gold deposits is correlated with the regional tectonic studies of previous works (e.g. Kloppenberg et aL, 2001; Huston et aL, 2002; Zegers et al., 2002). The timing of the gold deposition, in relation to the tectonothermal events, is constructed. Mineral Associations. Paragenetic Sequences and Alte~ation Associated with the Greenstone-Hosted Gold Deposits and Alteration. Paragenetic Sequences, Mineral AssociationsTurbidite- Hosted Gold Deposits: - Chapters 5 and 6 describe the petrogenetic characteristics of several deposits in each domain, as well as alteration zones and mineral assemblages. The paragenetic sequence of each mineral phase in relation to other mineral phases is constructed, and tables containing paragenetic relations between the mineral phases are presented. This work was undertaken using reflected light microscopy and qualitative analysis using Jeol JXA8600 Superprobe. The evolution of the gold bearing fluid is discussed for some deposits. Microstructural and Fabric Studies of Chert Leaders. and Au Mineralisation within the Warrawoona Greenstone Belt: Microstructural Fabric and Geochemical Studies:- Chapter 7 Illite Crystallinity: - Chapter 8: Geochronology: - Chapter 9 deals with the geochronological dating of some gold deposits, to establish the relationship between the gold deposition event and the tectonothermal events that occurred in the Pilbara Craton. This chapter presents a different dating technique from the Pb isotope model ages extensively used for the gold deposits of the Pilbara Craton. The Pb isotope technique offers many disadvantages as even within a single deposit, an age variation due to mixing of Pb isotopes is observed (Huston eta!., 2002). A compilation of the Pb isotope ages using galena has been completed by Huston et a!. (2002) and is presented at the beginning of Chapter 9. In addition to the compilation of the ages of the gold deposits in the Pilbara Craton, a summary of geochronological dating of tectonic events is also provided. This summary has been presented by Hickman (1984), Collins et a!. (1998), Zegers et a!. (1998) and Van Kranendonk et a!. (2002). In addition to the compilations of the age dating, as well the compilation of the tectonothermal events that took place in the Pilbara Craton, the relation between both compilations is made. In order to precisely date the gold deposits of the Warrawoona Mining Centre and later compare those data with Pb isotope age dating using galena samples from the same area, two felsic dykes with well-constrained kinematics, were collected (Samples KL00/04 and KL00/05). The felsic dyke KL00/05 display similar kinematics and has stretching lineation with similar orientation as the stretched gold-bearing pyrites from Klondyke Queen Mine. This sample is interpreted as syn-tectonic to the gold deposition event. Sample KL00/04 on the other hand, displays no notable deformation and is interpreted to be younger than the felsic dyke sample (KL00/05). This sample is interpreted as post-tectonic to the gold deposition event. Dating of these two felsic dykes gave a best constrain for the gold deposition event in the Warrawoona Mining Centre. The gold deposits of the Sharks Well Mining Centre were also selected for dating. Within this mining centre, the Sample SG00/01 had been collected. Sample SG00/01 consists of felsic volcanic rock that crosscuts the gold-bearing quartz vein at Mount Ada Mine. Dating of this sample provided a minimum age constraint for the age of the gold depositionat Sharks Well Mining Centre. Both the gold deposits from the Warrawoona Mining Centre and Sharks Well Mining Centre were dated using Pb-Pb and U/Pb simultaneous solution nebulisation and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the University of Utrecht, Netherlands. The LA-ICP-MS uses a short wavelength _Lambda Physik Complex Argon Fluoride 193 nm Excimer Laser, associated with a Micromass Platform ICP single collector quadrupole mass spectrometer. In addition to the two mining centres, one attempt to date the gold deposits from the Mosquito Creek 'Block was conducted. In order to date the deposits from the Mosquito Creek Block, chemical Th-U-Pb dating of monazite was undertaken using the Jeol JXA8600 Superprobe at the University of Utrecht, Netherlands. However, the technique could not be applied to monazite crystals from the Mosquito Creek Mining Centre as they have small sizes and contain .little Th. Conclusions: - Chapter 9 summarises the strata-tectonic history of the Marble Bar and Nullagine domains with respect to the genesis of gold mineralisation.