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• W29713941 abstract "Mount Dore-style breccia-hosted copper-gold deposits define a 70 kilometre-long, north-trendinglineament from Kuridala (65 kilometres south of Cloncurry), southwards. The type deposit lies 130kilometres south of Cloncurry, and a detailed study of it was undertaken to produce a metallogenic modelapplicable (with suitable modifications) to all deposits having this style.Regional geology results from a combination of (i) at least two cycles of ensialic riftsedimentation, (ii) later compressional tectonics and associated metamorphism to a maximum middleamphibolite grade, and (iii) intrusion of late-tectonic granitoids (Beardsmore et al., 1988 and Newbery etal., in prep.). Mount Dore-style deposits are largely restricted to rocks of the upper part of the MiddleProterozoic Maronan Supergroup, a newly-recognized package of rift-basin sediments. The precise age ofthis unit is presently unknown; it could belong to either rift episode, or be older or younger.The Mount Dore deposit occurs within steeply east-dipping quartz-muscovite schists andcarbonaceous slates of the uppermost Maronan Supergroup structurally overlying meta-calcarenites,calcilutites, marbles and metabasalts of the Staveley Formation. The structural history includes early,subhorizontal (D1) detachment of the Staveley Formation from older units, followed by upright, northtrending,tight to isoclinal folding (D2), accompanied by peak metamorphism in the lower to middleamphibolite facies (Jaques et al., 1982). The events are tentatively dated at 1545 Ma, by analogy with D2and metamorphic history derived for the western part of the Mount Isa Inlier (Page and Bell, 1986).Northwest-trending of open, upright folds belonging to the D3 deformation are scatteredacross the region, and one of these passes through the Mount Dore orebody. Latest tectonism produced theMount Dore Fault Zone, a moderately- to steeply east-dipping reverse fault-zone about 250 metres wide,which passes through Mount Dore and reactivates the D1 structure. The fault zone contains a thin sliver ofuppermost Maronan Supergroup, sandwiched between footwall Staveley Formation and hangingwall(truncated) Mount Dore Granite. The granite is dated at 1510 Ma (Nisbet et al., 1983).Mount Dore displays a complex history of brecciation and alteration. Both are related tomovement along the Mount Dore Fault Zone and to associated hydrothermal activity. Brecciation was acontinuum process, with any particular event first producing angular, commonly tabular, crenulatedschistose fragments. The crenulation is identified with S3, but is randomly orientated from clast to clast,arguing for post-D3 brecciation. Subsequent reworking of the early fragments involved tectonic andhydrothermal milling. Replacement and infill in the breccias are extensive. Early alteration produced Kfeldspar(or biotite), tourmaline, sericite and quartz. Later alteration produced carbonate (dolomite andcalcite), apatite and chlorite. All phases are associated with all brecciation styles, but the most pervasivealteration is associated with the intensively milled breccias.Sulphide mineralization is associated temporally with carbonate alteration, and occurs late in thehistory of development of the Mount Dore deposit. Primary sulphide mineralization comprises pyrite andchalcopyrite, with minor sphalerite and galena. Pyrite is early, and is replaced by the other phases.Chalcocite also clearly replaces earlier pyrite, but is restricted to shallow depths, and probably formed bydeep leaching of the deposit during Recent weathering. Alteration, fluid inclusion and stable isotope geochemistry identify a primary deep-seated, hot(>500oC?), oxidized, CO2-bearing, highly-saline (65-70 wt% salt) metamorphic or magmatic fluidcontaining K+, Na+, Fe2+, Ca2+, B, SiO2, H+, Cl- and possibly SO2. After initial separation and loss of animmiscible CO2-rich phase, the residual aqueous fluid became more dilute with time, probably by mixingwith cooler, lower salinity (<20 wt% salt), low-CO2 fluid, possibly also of metamorphic origin. A modelaccounting for mineralization at Mount Dore invokes dilation and hydraulic brecciation during movementalong the Mount Dore Fault Zone, where the fault intersects D3 corridors of shallowly-dipping beddingand S2 foliation. Early potassic and silicic alteration released ore metals (Cu, Pb, Zn, Ag, Co, U, Au) to thefluid from the host rocks at this time. Sulphide precipitation was controlled by sulphate reduction withcarbon released from host. Pyrite scavenged most of this, and later Cu-, Pb- and Zn-sulphides formed byscavenging of S from pyrite.Data concerning other Mount Dore-style deposits (Mount Elliott, S.W.A.N., Hampden) arelimited, but suggest they may have formed by similar processes, with superficial differences arising fromvariations in geological setting. These deposits apparently all formed during a single metallogenic eventrelated to late tectonism in the eastern part of the Mount Isa Inlier. A speculative regional model proposesemplacement of at least one large allochthonous slab of Maronan Supergroup over the carbonate-evaporitesuccessions of the Mary Kathleen Group. The latter passed highly saline, CO2-bearing connate andprograde metamorphic fluids upwards into and along the decollement. Subsequent upright to inclined F2antiforms may have ponded these fluids, allowing them to stew for some time in contact with relativelymetal-rich rocks in the overriding plate. Alternatively, or additionally, the fluid may have migrateddissolved in Williams Batholith magmas, which were produced by partial melting of deep crustal materialprobably at the peak of regional metamorphism. Eventual release of hydrothermal fluid to higher crustallevels occurred only when vapour separation occurred in the rising plutons, and when permeable, latetectonicreverse faults, which also controlled the solid-state emplacement of at least some of the plutons,breached F2 structures. Passing rapidly upwards along the faults, the fluids encountered local dilatantzones, where high fluid fluxes and rapidly changing physical and chemical conditions instigated extensivealteration and sulphide precipitation. Low salinity fluids of meteoric, or more likely upper-platemetamorphic derivation also migrated into the dilatant zones when the deeply penetrating fault structuresbecame available, and subsequently mixed with the saline fluids, perhaps initiating some styles ofmineralization in the process.Epigenetic mineralization across the Cloncurry Fold Belt (and perhaps the entire Mount Isa Inlier)appears to be the result of large-scale devolatilization of the crust during the waning stages of regionaldeformation and metamorphism. The characteristics of individual deposits depends on the combination oflocal factors such as structure and rock types available adjacent to these structures for leaching of metals." @default.
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• W29713941 date "1992-12-01" @default.
• W29713941 modified "2023-09-27" @default.
• W29713941 title "Petrogenesis of Mount Dore-style breccia-hosted copper ± gold mineralization in the Kuridala-Selwyn region of northwestern Queensland" @default.
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