Tripp, Gerard Ignatius (2013) Stratigraphy and structure in the Neoarchaean of the Kalgoorlie district, Australia: critical controls on greenstone-hosted gold deposits. PhD thesis, James Cook University.

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Abstract

The remarkably prodigious gold endowment of the Neoarchaean was controlled by specific characters of the stratigraphy, structural geology, and geochemistry of greenstone belts. This study attempts to synthesise stratigraphic, structural and geochronological data to understand the geology of the Kalgoorlie greenstones, and by representing those data on maps to assess the controls on major gold deposits. Voluminous, previously unavailable regional - scale data sets from exploration and mining provide new insights into the controls on gold deposits in the north Kalgoorlie district of the Eastern Goldfields Province (EGP) in Western Australia.

The end of mafic volcanism is constrained by a new U-Pb SHRIMP age determination for the Golden Mile Dolerite at 2685±4 Ma (host to the largest Archaean greenstone gold deposit). This new age determination indicates a possible three million year time gap between the intrusion of the Golden Mile Dolerite (>2681 Ma) and overlying Lakewood dacite volcanics (<2678 Ma), allowing for the possibility that the Golden Mile dolerite was a high-level intrusion at the Upper Basalt / Black Flag Group interface.

The stratigraphy of post Upper Basalt rocks, including three newly defined formations, is separated on the basis of this work into four units from oldest to youngest: 'Talbot formation'; White Flag Formation; 'Gibson-Honman formation' (~2675 Ma); and 'Gidji Lake formation' (~2660 Ma). Gibson-Honman formation includes felsic volcanic rocks at Gibson-Honman Rock, Lakewood dacitic volcanics, and Perkolilli rhyolitic volcanics. Gidji Lake formation includes felsic volcaniclastic rocks at Gidji, Binduli Porphyry Conglomerate and Grave Dam Grit at Kanowna. The stratigraphy includes unconformities between White Flag Formation and Talbot formation (locally), and between Gibson-Honman and Gidji Lake formations. The Panglo member in the Kanowna district is interpreted as a correlative of the Kurrawang Formation, and represents a major change to the district stratigraphy that identifies late, unconformable clastic sequences in the vicinity of the Kanowna Belle gold deposit.

Five major deformation events are separated into pre-Kurrawang and post-Kurrawang deformation episodes, which record a change from early extension to bulk contraction, with the latter including syn-orogenic clastic sedimentation. Major map scale F1 folds are located at Kanowna and the Golden Mile in a restricted distribution that may reflect typically poor preservation of D1 fabrics. Localised syn-orogenic extension resulted in the deposition of linear fault-controlled, late clastic sedimentary sequences at <2650 Ma (Kurrawang, Panglo), unconformably overlying F2 folded rocks. The deposition of those sequences marks the onset of penetrative deformation over a relatively short time interval (~2650 – 2639 Ma). A regionally pervasive S3 foliation transects F1 and F2 fold axial planes. The S3 foliation trends uniformly over the district, and is axial planar to F3 folds with inclined axial planes in the late clastic sequences. The maximum age of the regional S3 foliation is constrained by the <2650 Ma rocks that are folded and foliated by it, and a minimum age at 2639±3 Ma from Lode-Au veins that were synchronous with and overprinting the regional foliation.

The Zuleika Shear Zone marks a domain boundary that juxtaposes crustal blocks with minor stratigraphic differences and thickness of sequences, yet the blocks do not have significantly different deformation intensity or metamorphic grade. Fault bounded domains in the Kalgoorlie Terrane were therefore not diverse crustal blocks amalgamated by strike slip or accretion, but were more likely subjacent depocentres, possibly half-grabens, bounded by early extensional faults such as the Zuleika Shear Zone and Bardoc Tectonic Zone.

Geological sequences of the Yindarlgooda Dome show no major differences in lithotectonic assemblages, stratigraphic sequence, or age, from sequences in the adjacent Boorara Domain. On this basis, the allocation of 'terrane boundary' status to the Mount Monger Fault as separating a western back-arc (Kalgoorlie Terrane) from an eastern accreted volcanic arc (Gindalbie Terrane) is suspect, and casts doubt on the application of subduction / arc-accretion models to greenstones of the southern EGP. Models that propose rifting of a pre- ~2700 Ma greenstone basement may be better analogues for the tectonic setting of the Kalgoorlie greenstones.

At a regional scale, major gold districts in the EGP have spatial and temporal relationships with unconformable, late clastic sedimentary sequences that mark areas of thick greenstone preservation. There is diversity of mineralisation styles and settings in the largest gold deposits, including early syn-volcanic hydrothermal alteration and mineralisation systems, and late-tectonic, vein hosted lode-Au deposits. High-level mineralisation styles were developed early in the formation of the greenstone belts with spatially coincident, late-tectonic mineralisation, indicating persistent mineral systems at fixed locations throughout several uplift and deformation cycles. That persistence suggests fundamental structural controls.

Structural controls on the different mineralisation styles include early-formed disseminated sulphide replacement mineralisation (e.g. Binduli) deformed and cut by late, syn-tectonic lode-Au vein style mineralisation (e.g. Binduli; Kundana). The Binduli district is spatially associated with the ~2664 Ma Binduli Porphyry Conglomerate unconformity that is interpreted to separate the Gibson-Honman and Gidji Lake formations. Kanowna Belle gold deposit is hosted in hypabyssal porphyritic rocks that have a spatial association with a ~2660 Ma unconformity at the base of the Grave Dam sequence (Gidji Lake formation). Unconformable sedimentary and volcaniclastic rocks of the Gidji Lake formation (~2660 Ma) are present in the vicinity of the eastern margin of the world class Mount Charlotte / Fimiston gold camp. This spatial association of major gold deposits with rocks located above an unconformity internal to the Black Flag Formation is a new understanding of the critical controls on Neoarchaean gold deposits and is a key criterion for area selection in exploration.

Item ID:	43748
Item Type:	Thesis (PhD)
Keywords:	Archaean; deformation; Eastern Goldfields Province; EGP; geochemistry; geochronology; geology; gold deposits; gold mineralisation; gold mineralization; greenstone belts; Kalgoorlie; Neoarchaean; ore deposits; stratigraphy; WA; Western Australia
Copyright Information:	Copyright © 2013 Gerard Ignatius Tripp
Additional Information:	Appendices 1, 4 & 5 (data) and appendix 6 (Geoscience report) are not available through this repository.
Date Deposited:	16 Jun 2016 04:47
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34% 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 33% 04 EARTH SCIENCES > 0403 Geology > 040311 Stratigraphy (incl Biostratigraphy and Sequence Stratigraphy) @ 33%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 100%
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