The geology, timing of mineralisation, and genesis of theudMenninnie Dam Zn-Pb-Ag deposit, Eyre Peninsula, SouthudAustralia

摘要

The Menninnie Dam Pb - Zn - Ag deposit is located on the northern Eyre Peninsula of SouthudAustralia, approximately 1-2 km south of the main mass of the Gawler Range Volcanics. Theuddeposit has no surface expression and was located by drilling of aeromagnetic anomalies inuddeeply weathered, flat lying terrain. Mineralisation occurs over= 3 km strike and is hosted byudupper amphibolite facies (= 700°C and 7 kbars) marbles and diopside-rich calc-silicateudlithologies, correlated with the Katunga Dolomite and lower units of the Cook Gap Schist ofudthe Middleback Subgroup, a subdivision of the Hutchison Group (1964- 1845 Ma.). TheudKatunga Dolomite is bound to the west by a shear zone(> 50 m wide) formed during the latterudstages of the Kimban Orogeny (1740- 1710 Ma.). Syn-deformation granite dykes intrudedudinto the shear zone, and, to a lesser extent, into the Katunga Dolomite and Cook Gap Schist.udLate tectonic pegmatite dykes intruded into the Katunga Dolomite and lower units of the CookudGap Schist, where they produced zones of K-feldspar- calc-silicate metasomatism.udPrevious researchers have incorrectly suggested that the Menninnie Dam Pb - Zn - Ag depositudis a Broken Hill-type. However, Pb - Zn - Ag mineralisation post-dates metamorphism,uddeformation and metasomatism and has replaced the host marble, and to a lesser extent,uddiopside-rich calc-silicate lithologies. On the basis of lithologic relationships, mostudmineralisation formed within 100 - 600m of the palaeosurface, and resulted in a central zoneudof stockwork and matrix to marble and calc-silicate breccias, flanked by veins. Sulphide andudgangue minerals consist of a simple assemblage of pyrite, quartz, sphalerite, galena and CaMn-udMg-Fe carbonate, with accessory chalcopyrite, chlorite, adularia, sericite, fluorite,udrhodonite, talc, phlogopite, dolomite, hematite and matildite. Three paragenetic stages ofudmineralisation are present with an early pyrite stage, followed by a sphalerite galena, and lateudpyrite stages. Metal abundances have a bell-shaped distribution along the length of the depositudand the highest values are associated with the zone of stockwork and breccia mineralisation.udPost metamorphic, porphyritic rhyolite intruded the Hutchison Group, and interacted withud(heated?) groundwater resulting in explosive fragmentation of the host rocks and formation ofudpolymictic breccia pipes. Some of these erupted onto the palaeosurface and formed layeredudpolymictic breccias. Rhyolite continued to intrude through polymictic breccia pipes, andudresulted in formation of peperite at the margins. Some rhyolite intrusions erupted onto theudpalaeosurface and formed volcanic breccias and rhyolite lavas. Polymictic breccias containudclasts of paragenetically early sulphide and gangue minerals, and have an altered matrix thatudincludes paragenetically late sulphide minerals, indicative of syn-mineralisation emplacement.udU-Pb zircon dating of the rhyolite intrusions constrains the timing of mineralisation to I 594 ±ud7 Ma. which is indistinguishable from that determined for the Hiltaba Suite granitoids and comagmaticudGawler Range Volcanics. Modelling of regional gravity and aeromagnetic data indicates the Menninnie Dam deposit liesudnear the north-western margin of a::::: 20 km diameter Hiltaba Suite granite that intruded toudwithin I - 3 km of the palaeo-surface. Lead was derived from the underlying Hiltaba Suiteudgranite and leached from Cook Gap Schist. Lead isotope ratios have a spatial distribution on audprospect scale, and the least radiogenic ratios correspond with the highest metal values and theudcentral zone of stockwork and breccia style mineralisation, consistent with a single zone ofudfluid up-flow. Carbonate gangue is interpreted to have precipitated via interaction of theudmineralising fluid (o18Q = -2.0%o; o13C = -6.9%o) in equilibrium with H2C03(aq) (> 0.01udmolal) and the host marbles (o18Q = 15.5 to 21.09%o; o13C = -1.1 to 1.6%o) between 200°udand 125°C. Hydrous phyllosilicates associated with mineralisation have calculated fluidudvalues of o18Q = -0.7 to -2.0%o and oD = -43 to -48%o, indicative of a mixed meteoric - .udmagmatic origin for the mineralising fluids. Sulphide o34S values range from -3.0 to 8.2%o,udwith most between 4 to 6%o. The lack of evidence for sulphur isotope fractionation betweenuddifferent sulphide minerals is consistent with non-equilibrium precipitation of sulphides from audreduced fluid, low temperature kinetic effects and l or a H2S : metal ratio ~ 1. Sulphur wasudsourced from either the magma, the country rocks, or a combination of both.udPrimary fluid inclusions hosted by sphalerite and quartz have a range of trapping temperaturesudand salinities interpreted to have resulted from mixing of=140°C and =27 wt.% NaCIudequivalent Na-Ca-K-Cl brine with a = 180°C dilute chloride water. Thermodynamicudmodelling has shown that sufficient concentrations of Pb and Zn (> 1 ppm) can be transportedudtogether with reduced sulphur CIS = 0.002 molal) in a low temperature (150°C) saline brineud(=6 molal) to form the Menninnie Dam deposit. The physiochemical attributes of theudmineralising fluid at 150°C are estimated to have been log f02 = -46 and pH= 4.6. Dilutionudthrough mixing with heated groundwater was a possible base metal depositional mechanismudbut is predicted to have been less effective than the pH increase that resulted from dissolutionudof the host marbles.udSoon after cessation of the mineralising event, the stratigraphy was mantled by a singleudcooling unit> 260m thick of lithic-rich(45% and up to 20m across) welded ignimbriteud(MD ignimbrite). The thickness, abundance and size of lithic clasts in the MD ignimbrite, andudshallow intrusion of granite are consistent with an intracaldera setting. Following welding andudcooling of the MD ignimbrite, the lower part of the MD ignimbrite and the Hutchison Groupudnear the southern end of the Menninnie Dam deposit were partially altered to a texturallyuddestructive quartz - chlorite - carbonate - calc-silicate assemblage by a hot, low salinity waterud(190- 356°C and 0- 3 wt. % NaCI equiv.). Carbon, hydrogen, and oxygen isotopes areudconsistent with a meteoric water that had undergone partial isotopic exchange with igneousudrocks. Mineral textures, whole rock geochemistry, lead and sulphur isotope data areudconsistent with the Menninnie Dam Pb- Zn- Ag mineralisation being partially dissolved andudreprecipitated by this event, with no addition of metals or sulphur.