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The geology, timing of mineralisation, and genesis of the Menninnie Dam Zn-Pb-Ag deposit, Eyre Peninsula, South Australia


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Roache, M 1996 , 'The geology, timing of mineralisation, and genesis of the Menninnie Dam Zn-Pb-Ag deposit, Eyre Peninsula, South Australia', PhD thesis, University of Tasmania.

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The Menninnie Dam Pb - Zn - Ag deposit is located on the northern Eyre Peninsula of South
Australia, approximately 1-2 km south of the main mass of the Gawler Range Volcanics. The
deposit has no surface expression and was located by drilling of aeromagnetic anomalies in
deeply weathered, flat lying terrain. Mineralisation occurs over= 3 km strike and is hosted by
upper amphibolite facies (= 700°C and 7 kbars) marbles and diopside-rich calc-silicate
lithologies, correlated with the Katunga Dolomite and lower units of the Cook Gap Schist of
the Middleback Subgroup, a subdivision of the Hutchison Group (1964- 1845 Ma.). The
Katunga Dolomite is bound to the west by a shear zone(> 50 m wide) formed during the latter
stages of the Kimban Orogeny (1740- 1710 Ma.). Syn-deformation granite dykes intruded
into the shear zone, and, to a lesser extent, into the Katunga Dolomite and Cook Gap Schist.
Late tectonic pegmatite dykes intruded into the Katunga Dolomite and lower units of the Cook
Gap Schist, where they produced zones of K-feldspar- calc-silicate metasomatism.
Previous researchers have incorrectly suggested that the Menninnie Dam Pb - Zn - Ag deposit
is a Broken Hill-type. However, Pb - Zn - Ag mineralisation post-dates metamorphism,
deformation and metasomatism and has replaced the host marble, and to a lesser extent,
diopside-rich calc-silicate lithologies. On the basis of lithologic relationships, most
mineralisation formed within 100 - 600m of the palaeosurface, and resulted in a central zone
of stockwork and matrix to marble and calc-silicate breccias, flanked by veins. Sulphide and
gangue minerals consist of a simple assemblage of pyrite, quartz, sphalerite, galena and CaMn-
Mg-Fe carbonate, with accessory chalcopyrite, chlorite, adularia, sericite, fluorite,
rhodonite, talc, phlogopite, dolomite, hematite and matildite. Three paragenetic stages of
mineralisation are present with an early pyrite stage, followed by a sphalerite galena, and late
pyrite stages. Metal abundances have a bell-shaped distribution along the length of the deposit
and the highest values are associated with the zone of stockwork and breccia mineralisation.
Post metamorphic, porphyritic rhyolite intruded the Hutchison Group, and interacted with
(heated?) groundwater resulting in explosive fragmentation of the host rocks and formation of
polymictic breccia pipes. Some of these erupted onto the palaeosurface and formed layered
polymictic breccias. Rhyolite continued to intrude through polymictic breccia pipes, and
resulted in formation of peperite at the margins. Some rhyolite intrusions erupted onto the
palaeosurface and formed volcanic breccias and rhyolite lavas. Polymictic breccias contain
clasts of paragenetically early sulphide and gangue minerals, and have an altered matrix that
includes paragenetically late sulphide minerals, indicative of syn-mineralisation emplacement.
U-Pb zircon dating of the rhyolite intrusions constrains the timing of mineralisation to I 594 ±
7 Ma. which is indistinguishable from that determined for the Hiltaba Suite granitoids and comagmatic
Gawler Range Volcanics. Modelling of regional gravity and aeromagnetic data indicates the Menninnie Dam deposit lies
near the north-western margin of a::::: 20 km diameter Hiltaba Suite granite that intruded to
within I - 3 km of the palaeo-surface. Lead was derived from the underlying Hiltaba Suite
granite and leached from Cook Gap Schist. Lead isotope ratios have a spatial distribution on a
prospect scale, and the least radiogenic ratios correspond with the highest metal values and the
central zone of stockwork and breccia style mineralisation, consistent with a single zone of
fluid up-flow. Carbonate gangue is interpreted to have precipitated via interaction of the
mineralising fluid (o18Q = -2.0%o; o13C = -6.9%o) in equilibrium with H2C03(aq) (> 0.01
molal) and the host marbles (o18Q = 15.5 to 21.09%o; o13C = -1.1 to 1.6%o) between 200°
and 125°C. Hydrous phyllosilicates associated with mineralisation have calculated fluid
values of o18Q = -0.7 to -2.0%o and oD = -43 to -48%o, indicative of a mixed meteoric - .
magmatic origin for the mineralising fluids. Sulphide o34S values range from -3.0 to 8.2%o,
with most between 4 to 6%o. The lack of evidence for sulphur isotope fractionation between
different sulphide minerals is consistent with non-equilibrium precipitation of sulphides from a
reduced fluid, low temperature kinetic effects and l or a H2S : metal ratio ~ 1. Sulphur was
sourced from either the magma, the country rocks, or a combination of both.
Primary fluid inclusions hosted by sphalerite and quartz have a range of trapping temperatures
and salinities interpreted to have resulted from mixing of=140°C and =27 wt.% NaCI
equivalent Na-Ca-K-Cl brine with a = 180°C dilute chloride water. Thermodynamic
modelling has shown that sufficient concentrations of Pb and Zn (> 1 ppm) can be transported
together with reduced sulphur CIS = 0.002 molal) in a low temperature (150°C) saline brine
(=6 molal) to form the Menninnie Dam deposit. The physiochemical attributes of the
mineralising fluid at 150°C are estimated to have been log f02 = -46 and pH= 4.6. Dilution
through mixing with heated groundwater was a possible base metal depositional mechanism
but is predicted to have been less effective than the pH increase that resulted from dissolution
of the host marbles.
Soon after cessation of the mineralising event, the stratigraphy was mantled by a single
cooling unit> 260m thick of lithic-rich(45% and up to 20m across) welded ignimbrite
(MD ignimbrite). The thickness, abundance and size of lithic clasts in the MD ignimbrite, and
shallow intrusion of granite are consistent with an intracaldera setting. Following welding and
cooling of the MD ignimbrite, the lower part of the MD ignimbrite and the Hutchison Group
near the southern end of the Menninnie Dam deposit were partially altered to a texturally
destructive quartz - chlorite - carbonate - calc-silicate assemblage by a hot, low salinity water
(190- 356°C and 0- 3 wt. % NaCI equiv.). Carbon, hydrogen, and oxygen isotopes are
consistent with a meteoric water that had undergone partial isotopic exchange with igneous
rocks. Mineral textures, whole rock geochemistry, lead and sulphur isotope data are
consistent with the Menninnie Dam Pb- Zn- Ag mineralisation being partially dissolved and
reprecipitated by this event, with no addition of metals or sulphur.

Item Type: Thesis - PhD
Authors/Creators:Roache, M
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