Beardsmore, Trevor John. "Petrogenesis of Mount Dore-style breccia-hosted copper ± gold mineralization in the Kuridala-Selwyn region of northwestern Queensland." Thesis, 1992. https://researchonline.jcu.edu.au/1344/1/01front.pdf.
Анотація:
Mount Dore-style breccia-hosted copper-gold deposits define a 70 kilometre-long, north-trending
lineament from Kuridala (65 kilometres south of Cloncurry), southwards. The type deposit lies 130
kilometres south of Cloncurry, and a detailed study of it was undertaken to produce a metallogenic model
applicable (with suitable modifications) to all deposits having this style.
Regional geology results from a combination of (i) at least two cycles of ensialic rift
sedimentation, (ii) later compressional tectonics and associated metamorphism to a maximum middle
amphibolite grade, and (iii) intrusion of late-tectonic granitoids (Beardsmore et al., 1988 and Newbery et
al., in prep.). Mount Dore-style deposits are largely restricted to rocks of the upper part of the Middle
Proterozoic Maronan Supergroup, a newly-recognized package of rift-basin sediments. The precise age of
this 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 and
carbonaceous 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 middle
amphibolite facies (Jaques et al., 1982). The events are tentatively dated at 1545 Ma, by analogy with D2
and metamorphic history derived for the western part of the Mount Isa Inlier (Page and Bell, 1986).
Northwest-trending corridors of open, upright folds belonging to the D3 deformation event are scattered
across the region, and one of these passes through the Mount Dore orebody. Latest tectonism produced the
Mount 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 of
uppermost 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 to
movement along the Mount Dore Fault Zone and to associated hydrothermal activity. Brecciation was a
continuum process, with any particular "event" first producing angular, commonly tabular, crenulated
schistose 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 and
hydrothermal milling. Replacement and infill in the breccias are extensive. Early alteration produced Kfeldspar
(or biotite), tourmaline, sericite and quartz. Later alteration produced carbonate (dolomite and
calcite), apatite and chlorite. All phases are associated with all brecciation styles, but the most pervasive
alteration is associated with the intensively milled breccias.
Sulphide mineralization is associated temporally with carbonate alteration, and occurs late in the
history of development of the Mount Dore deposit. Primary sulphide mineralization comprises pyrite and
chalcopyrite, 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 by
deep 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 fluid
containing K+, Na+, Fe2+, Ca2+, B, SiO2, H+, Cl- and possibly SO2. After initial separation and loss of an
immiscible CO2-rich phase, the residual aqueous fluid became more dilute with time, probably by mixing
with cooler, lower salinity (<20 wt% salt), low-CO2 fluid, possibly also of metamorphic origin. A model
accounting for mineralization at Mount Dore invokes dilation and hydraulic brecciation during movement
along the Mount Dore Fault Zone, where the fault intersects D3 "corridors" of shallowly-dipping bedding
and S2 foliation. Early potassic and silicic alteration released ore metals (Cu, Pb, Zn, Ag, Co, U, Au) to the
fluid from the host rocks at this time. Sulphide precipitation was controlled by sulphate reduction with
carbon released from host. Pyrite scavenged most of this, and later Cu-, Pb- and Zn-sulphides formed by
scavenging of S from pyrite.
Data concerning other Mount Dore-style deposits (Mount Elliott, S.W.A.N., Hampden) are
limited, but suggest they may have formed by similar processes, with superficial differences arising from
variations in geological setting. These deposits apparently all formed during a single metallogenic event
related to late tectonism in the eastern part of the Mount Isa Inlier. A speculative regional model proposes
emplacement of at least one large allochthonous slab of Maronan Supergroup over the carbonate-evaporite
successions of the Mary Kathleen Group. The latter passed highly saline, CO2-bearing connate and
prograde metamorphic fluids upwards into and along the decollement. Subsequent upright to inclined F2
antiforms may have ponded these fluids, allowing them to "stew" for some time in contact with relatively
metal-rich rocks in the overriding plate. Alternatively, or additionally, the fluid may have migrated
dissolved in Williams Batholith magmas, which were produced by partial melting of deep crustal material
probably at the peak of regional metamorphism. Eventual release of hydrothermal fluid to higher crustal
levels occurred only when vapour separation occurred in the rising plutons, and when permeable, latetectonic
reverse 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 dilatant
zones, where high fluid fluxes and rapidly changing physical and chemical conditions instigated extensive
alteration and sulphide precipitation. Low salinity fluids of meteoric, or more likely upper-plate
metamorphic derivation also migrated into the dilatant zones when the deeply penetrating fault structures
became available, and subsequently mixed with the saline fluids, perhaps initiating some styles of
mineralization 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 regional
deformation and metamorphism. The characteristics of individual deposits depends on the combination of
local factors such as structure and rock types available adjacent to these structures for leaching of metals.