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

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Belford, Susan Margaret (2010) Genetic and chemical characterisation of the host succession to the Archean Jaguar VHMS deposit. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 2010 the author

Additional Information:

No access or viewing until 18 October 2011. After that date, available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:58
Last Modified: 11 Mar 2016 05:53
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