University of Tasmania
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Genetic and chemical characterisation of the host succession to the Archean Jaguar VHMS deposit

posted on 2023-05-26, 23:30 authored by Belford, Susan Margaret
Jaguar, an Archean Cu-Zn-rich volcanic-hosted massive sulfide deposit, is situated in the Teutonic Bore volcanic complex in the Eastern Goldfields of the Yilgarn Craton, Western Australia. The Jaguar deposit is one of only three VHMS deposits that have been mined in the Eastern Goldfields. The deposit is hosted in a succession dominated by coherent facies and their associated volcaniclastic facies, with minor non-volcanic facies. The rocks (c. 2.69 Ga) have been affected by regional greenschist facies metamorphism and deformation, and hydrothermal alteration is intense around the massive sulfide orebody. Using only drillcore across a 1 km x 1.6 km area, twenty-five principal lithofacies have been recognised in the study area and are organised into five groups: 1) coherent rhyolite, dacite, andesite, basalt and dolerite facies; 2) monomictic volcanic breccia and conglomerate; 3) polymictic volcanic breccia and conglomerate; 4) volcanic sandstone and mudstone; and 5) non-volcanic mudstone and chemical facies. The environment of deposition was submarine and below-storm-wave-base. The stratigraphy at Jaguar has been reconstructed using observations and interpretations based on facies analyses and relationships, rock fabric and microstructure that are supported by the application of immobile element geochemistry. The succession is split into Footwall (FW), Mineralised Package (MP)and Hangingwall (HW) units. The FW is divided into four volcanic lithofacies having distinctive composition. It is marked by an absence of sedimentary units between the volcanic facies. The deepest footwall is andesite lava (DFA) which was succeeded by a rhyolite dome (FR). These units are overlain by coherent andesite (FA) and then pillow basalt (FB). The FW (DFA, FR, FA and FB) and the MP dacite magmas were closely related. The MP is a complex assemblage of intercalated coherent, autoclastic, resedimented and non-volcanic lithofacies divided into six sub-units: 1) the Dacite MP (MPD) comprising coherent dacite, monomictic dacite breccia, and monomictic pumice-rich breccia facies; 2) the Conglomerate MP (MPC) comprising polymictic dacite breccia, polymictic conglomerate and pillow-fragment basalt breccia facies; 3) the Pumice-rich MP (MPP) comprising polymictic pumice-rich breccia and pumice granule sandstone facies; 4) the Laminated MP (MPS) comprising laminated volcanic mudstone, non-volcanic mudstone and black shale, polymictic conglomerate, volcanic sandstone and chert fades; 5) the Laminated Chert MP (MPH) comprising chert fades; and 6) the Sulfide ore (MPO). The basal unit of much of the MP is laminated chert (MPH) (30-100 cm thick), which directly overlies the FB. Chemical deposition of the chert occurred on the sea floor as an exhalative precipitate immediately prior the eruption of the dacite (MPD). In places, plastic deformation and brecciation occurred possibly during contemporaneous local seismic events (related to growth faulting) which locally exposed the FB. The dacite (MPD) was erupted as a series of small domes or flows on to the seafloor in an unstable environment. The coherent centres of these domes pass through autoclastic margins into pumiceous carapace. Pumice formed by non-explosive, quench fragmentation of pumiceous carapace on the vesiculating dome. Spalling and debris flows of unstable primary breccias deposited the associated polymictic breccias (MPG) and incorporated plastically deformed clasts of seafloor chert. The polymetallic sulfide orebody was formed in this environment. Within the MPG, primary sulfide clasts indicate that the sulfide body was forming contemporaneously with the MP. Tectonic instability and growth faulting exposed the growing massive sulfide, creating local debris flows containing sulfide clasts. This instability caused elutriation of finer-grained particles into the water column where they were moved about by water and weak gravity currents, before settling out of suspension (MPS). The HW comprises coherent and associated autoclastic lithofacies interbedded with laminated volcanic and non-volcanic mudstone facies. It is divided into four major volcanic units, defined by packages of associated volcanic lithofacies having distinctive composition. From the base of the hangingwall, these units are informally named: the Hangingwall Andesite (HA), the Hangingwall Basalt (HB), the Upper Porphyritic Andesite (HUA) and the Upper Quartz Rhyolite (HUR). The HA and the HB have been further subdivided into single mappable units, assisted in the case of the HA, by distinct geochemical characteristics. There was a profound change in composition from the FW to the HW magmas. Volcanism resumed after the MP, more highly fractionated andesites (HA) interfingering with volcaniclastic and pelagic sediments. These units are overlain by a thick succession of compositionally monotonous basalts (HB), that includes fades dominated by fountain deposits (fluidal-clast breccia) to pillow lava. Transient growth faults caused episodic to gradual subsidence that formed wedge shaped thickening in the HA and lower HB units. Subsidence had ceased by the time the upper HB was erupted. Dolerite sills with the same composition as the HB were likely feeders to the HB lavas. They intruded the succession up to mid-level HB. At this time, evidence points to a change in the stress regime from extension to compression. The composition of the HB/D magmas was different from earlier magmas. The aggregate thickening of the succession from sill inflation was between 150 and 200 m. The inflation was probably not uniform; intrusion of more magma in the south of the area likely caused tilting to the north. The remainder of the hangingwall was deposited in an apparently seismically stable environment and each major volcanic event was followed by deposition of significant mudstone (plus sandstone and carbonaceous mudstone). The lateral continuity of almost all units, and the lack of repetition of the sequence, does not support the presence of subtle thrust ramp repetitions, despite substantial evidence of shear related deformation in some sedimentary interbeds. All the sedimentary younging evidence unequivocally indicates younging to the west, implying no obvious major folds. The deformation of the sequence was not significant enough to influence stratigraphic reconstruction. The dominant sulfide minerals are pyrite, pyrrhotite, sphalerite and chalcopyrite (and locally, magnetite). Galena and arsenopyrite are present as minor phases and trace amounts of tetrahedrite-tennantite and geochronite were identified. The majority of the ore minerals have been subject to varied amounts of strain. A low-strain window is the primary source of evidence that the deposit was syn-volcanic and formed predominantly beneath the seafloor. The evidence of the infill of open space textures, the colloform intergrowths of sulfide and chert, the sulfide replacement of spherulites plus replacement fronts within the dacite all support this conclusion. Where pyrite has been deformed, it has commonly failed in a brittle manner. Where there is interconnectivity of sulfide grains, most sulfides (excluding pyrite) show evidence of ductile flow deformation and durchbewegung textured bands. Relicts of undeformed colloform pyrite may remain within these ductile bands, and where pyrite has boudinaged or failed cataclastically, fine fractures perpendicular to the band have been filled with other sulfides. Where clasts of gangue have been dragged into the bands, typical durchbewegung texture is developed. Most bands appear to have been annealed post-deformation. Multi-element and REE spidergrams suggest that the coherent rocks are similar to BABB, with Nb-Ta depletion indicative of a subduction-related arc signature. Discrimination diagrams (developed for Phanerozoic rocks) suggested a complex, early back-arc setting for Jaguar. This conclusion is consistent with an ensimatic rift environment where the rift was an early back-arc basin probably over a subducting slab, and coincided with a period of modest extension at local scales. Although only a small part of the whole succession was examined, ore formation appears to have been localised at a volcanic centre, during a transition in magma composition and productivity.


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Copyright 2010 the author No access or viewing until 18 October 2011. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references

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