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Geology, host rock succession, and hydrothermal alteration of the Waterloo volcanic-hosted massive sulphide deposit (Northern Queensland, Australia)

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Monecke, T (2002) Geology, host rock succession, and hydrothermal alteration of the Waterloo volcanic-hosted massive sulphide deposit (Northern Queensland, Australia). PhD thesis, Technischen Universitat Bergakademie Freiberg.

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Abstract

The Waterloo volcanic-hosted massive sulphide (VHMS) deposit is located in the Charters
Towers Region in northern Queensland, Australia. The deposit forms part of the CambroOrdovician
Seventy Mile Range Group that represents a major belt of E-W striking and subvertical
dipping volcanic-sedimentary rocks. The volcanic host rocks and the base metal mineralisation
of the Waterloo deposit are not exposed in surface outcrops because of a thick
cover by Pliocene fluvial sedimentary rocks. Exploration diamond drilling (total -10 km of
core) led to the delineation of a relatively small high-grade base metal resource of 243,500
tonnes ore grading 3.8 % Cu, 13.8 % Zn, 3.0 % Pb, 74 glt Ag, and 1.2 glt Au. The mineralisation
comprises several small semiconnected stratiform, blanket-like, pyrite-chalcopyritesphalerite-
galena massive sulphide lenses.
The structural style of the Early Ordovician Waterloo sequence has been constrained using
macroscopic structural techniques. The massive sulphides at Waterloo are interpreted to be
syn-volcanic in origin because they have been overprinted by the same generations of tectonic
structures as the host stratigraphy. The Waterloo sequence was tilted into a subvertical position
during north-south compression that is possibly Mid- to Late Ordovician in age. This
regional folding event also resulted in the development of an axial plane cleavage that is particularly
well developed in a high strain zone surrounding the massive sulphides. The spatial
relationship between the folded bedding plane and the axial plane cleavage as well as the consistent
south facing of the bedding of volcaniclastic sediments indicate that the deposit is located
at the southern limb of a major east-west trending antiform. This antiform has a shallow
plunge to the west. The Waterloo sequence was affected by two faulting events that are
younger than the regional folding. Early steeply dipping ENE striking faults interpreted to be
Silurian or Devonian were accompanied by significant dip-slip normal movement, whereas
younger strike-slip faults have no affect to the geometry of the Waterloo sequence.
Based on the improved understanding of the structural style of the Waterloo sequence, the
volcanic facies architecture of the host sequence was investigated to unravel the temporal and
spatial relationships between volcanism and massive sulphide formation. The massive sulphides
formed in a below storm wave base depositional enviromnent on top of a nonexplosive,
near-vent, andesite-dominated facies association containing coherent volcanic units
and related juvenile volcaniclastic rocks. The massive sulphide lenses are overlain, and partially
hosted in, a coarse quartz-feldspar crystal-rich sandstone and breccia facies. These rocks
are interpreted to be mass flows that record contemporaneous probably explosive dacitic to
rhyolitic volcanism outside the Waterloo area. The still wet and unconsolidated coarse sediment
in the immediate hanging wall of the massive sulphides was intruded by a feldspar porphyritic
dacite cryptodome that was partly emergent at the ancient seafloor. The emplacement
of the cryptodome indicates that the magmatic source feeding the volcanism within the Waterloo
area shifted towards an acidic composition at the time of massive sulphide formation.
Dacite cryptodome volcanism at Waterloo was followed by the waning of the hydrothermal
activities. The subsequent period of relatively quite sedimentation was occasionally interrupted
by the emplacement of syn-sedimentary basaltic to andesitic sills and was followed by
the mass flow deposition of a coarse feldspar-quartz sandstone and breccia facies. Finally
there was a period of intense non-explosive, near-vent basalt to andesite-dominated volcanism.
Petrochemical investigations demonstrated that the coherent volcanic rocks of the Waterloo
sequence belong to a subalkaline volcanic suite. The basalt, andesite, and dacite of the Waterloo
sequence are cogenetic. The petrographic and petrochemical characteristics of the reworked
volcaniclastic facies suggest that the material was derived from a petrogenetically
similar volcanic source of dacitic to rhyolitic composition. The geochemical signatures of the
most primitive volcanic rocks from the Waterloo sequence are similar to modern subductionrelated
volcanics, such as back-arc basin basalts forming during the early stages of back-arc
basin evolution. Based on these findings and the results of previous regional studies, it is suggested
that volcanism in the Waterloo area occurred in a bac - arc basin that developed on
thinned Precambrian continental lithosphere flanking a continental margin volcanic arc.
Mineralogical investigations on the volcanic rocks hosting the massive sulphides revealed that
two types of alteration can be distinguished. Least altered rocks were affected by weak regional
alteration that was caused by the combined effects of devitrification, hydration, burial
diagenesis, seawater interaction, regional metamorphism of the lower greenschist facies, and
defOlmation. In contrast, volcanic rocks located in the footwall and the immediate hanging
wall of the massive sulphides were subject to a combination of hydrothermal and regional
alteration.
The spatial distribution of alteration mineral associations as well as the mineralogical and
geochemical attributes of the hydrothermal altered rocks constrain the environment of hydrothermal
alteration. The massive sulphide lenses at Waterloo are underlain by an extensive
footwall alteration halo that is typified by a semiconformable zonation defined by an inner
zone of silicic-altered volcanics (pyrite-quartz-muscovite) that laterally passes into a zone of
phyllic alteration (pyrite-muscovite-chlorite-quartz, pyrite-paragonite-muscovite-chloriteintermediate
NaIK mica-quartz, and pyrite-muscovite-albite-chlorite-paragonite-intermediate
NaIK mica-quartz-calcite) and a zone consisting of propylitic-altered volcanics (albitechlorite-
epidote-muscovite-paragonite-quartz-calcite-pyrite and albite-chlorite-epidote-quartzcalcite).
It is demonstrated that the development of the zonation of the alteration halo can be
directly linked to the nature and evolution of the fluids interacting with the volcanic rocks in
the different parts of the hydrothermal alteration halo. Hydrothermal alteration in the upflow
zones of the mineralising fluids resulted in the formation of large amounts of muscovite on
the expense of primary rock-forming silicates by the combined effects of potassium and hydrogen
metasomatism. This type of alteration was principally linked to the acidity of the mineralising
hydrothermal fluids. The alteration in the upflow zones also involved a sulphidisation
of the rocks due to the reaction of ferrous iron contained in rock-forming silicates and the
volcanic glass matrix with HzS supplied by the hydrothermal fluids. Silicification was pronounced
in the upflow zones because the mineralising fluids cooled by moving down a temperature
gradient. Outward percolation of the hydrothermal fluids into zones surrounding the
thermal upflow was accompanied by a rapid neutralisation of the strong acids and, therefore,
an increasing reactivity of CO2 with respect to hydrogen metasomatism. The percolation of
seawater into the zones surrounding the high temperature upflow zones was intrinsically involved
in the development of the alteration zonation. Heating of seawater, by moving up a
temperature gradient, resulted in a pronounced sodium metasomatism in the outer parts of the
alteration halo that caused the formation of sodium silicates (albite, intermediate NaIK mica,
and paragonite) at the expense of primary rock-forming silicates such as feldspars and earlier
formed products of hydrothermal alteration, such as muscovite. In contrast to the footwall
alteration halo, alteration of the volcanic facies overlying the ore horizon is limited in extent
and rapidly fades in intensity with increasing distance from the sulphides. The zonation of the
hanging wall alteration is defined by an inner zone of phyllic alteration (pyrite-muscovitequartz,
pyrite-muscovite-paragonite-intermediate NaIK mica-chlorite-quartz, muscovitechlorite-
quartz, and muscovite-paragonite-intermediate NaIK mica-chlorite-quartz) and an
outer zone comprising propylitic-altered volcanics (albite-muscovite-chlorite-paragoniteintermediate
NaIK mica-quartz-calcite and albite-muscovite-chlorite-epidote-quartz-calcite).
Phyllic alteration in the immediate hanging wall of the massive sulphides can be accounted
for by the ongoing intense alteration following the burial of the ores by the mass flow derived
coarse quartz-feldspar sandstone and breccia facies and the emplacement of the dacitic cryptodome,
whereas the outer zone of propylitic alteration records the waning of the hydrothermal
activities where alteration occurred at successively decreasing temperatures in a more
oxidising environment.
Based on the results of this study it is suggested that the genetic relationship between volcanism
and massive sulphide formation can be constrained by integrating volcanological studies
on the host rock sequence with detailed mineralogical and geochemical investigations of the
hydrothermally altered rocks. The volcanological investigations demonstrate that the mineralisation
event occurred in close temporal and spatial relationship to felsic volcanism culminating
in the emplacement of a dacite cryptodome in the immediate hanging wall to the massive
sulphides. The findings of the alteration halo study are consistent with this observation
because the mantle-derived volcanism in the Waterloo area may not only have provided the
heat to drive the hydrothermal system, but may also have acted as a source of chemical components,
such as volatile species that controlled the acidity of the mineralising hydrothermal
fluids.

Item Type: Thesis (PhD)
Date Deposited: 21 Jul 2009 00:30
Last Modified: 27 Jun 2016 03:19
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