Brecciation within the Mary Kathleen Group of the Eastern Succession, Mt Isa Block, Australia: Implications of district-scale structural and metasomatic processes for Fe-oxide-Cu-Au mineralisation

The Eastern Succession of the Proterozoic Mt Isa Block, including the Cloncurry District and the Mary Kathleen Fold Belt (MKFB), contains numerous examples of Fe-oxide-Cu-Au mineralisation. Most deposits, including Ernest Henry, Eloise, Starra and Mt Elliott formed after the peak of ca. 1600-1575 Ma upper greenschist to amphibolite facies metamorphism, and during the waning phases of the Isan Orogeny. Mineralisation was broadly synchronous with emplacement of voluminous phases of the Williams and Naraku batholiths (ca. 1550 - 1500 Ma) and widespread brecciation and accompanying metasomatism. Brecciation and metasomatism were best developed within Cover Sequence 2 stratigraphy, and in particular within calc-silicate rock and meta-siltstone stratigraphy of the Corella Formation, the predominant rocks of the Mary Kathleen Group. The geometry and distribution of brecciation in the Corella Formation was in part controlled by retrograde buckle folding imposed on a heterogeneous rock sequence that was fractured and boudinaged both pre- and syn-buckle folding. Brecciation is far more widespread in the Cloncurry District relative to the MKFB, reflecting in part a larger proportion of stratigraphy in the Cloncurry District that was at low angles to the shortening direction during the waning phases of the Isan Orogeny, favoring refolding and consequent fracturing. Variations in regional structural trends reflect strain partitioning around competent intrusive bodies, fault reactivation and refolding. Other contributing factors for brecciation include low temperature conditions during late deformation, and the proximity to voluminous intrusions, the emplacement of which likely resulted in transient elevated fluid pressure and strain rates, favoring fracturing and brecciation. The relative paucity of brecciation in most stratigraphic units outside of the Corella Formation reflects a high proportion of incompetent stratigraphy in these sequences (e.g. voluminous micaceous schists within the Soldiers Cap Group), which were able to• accommodate strain by plastic flow. The broad-scale geometry of the Cloncurry District reflects Cover Sequence 3 rocks overlying Cover Sequence 2 rocks, the two sequences being separated by early faults. Marbles within the Corella Formation, and schists in other stratigraphic sequences were not prone to brittle failure, and acted as low permeability barriers to fluid flow. These horizons allowed for the attainment of elevated fluid pressures within large volumes of brecciated rock. During the final stages of brecciation, these low competence marbles and schists were fractured and brecciated, predominantly within discrete fault zones. This shift from widespread brittle-ductile to purely brittle deformation likely reflects progressive cooling, as well as locally elevated fluid pressure and/or strain rate associated with pluton emplacement and degassing. A synchronous district-scale shift from compression to transtension facilitated the development of vertically continuous zones of dilation within faults, resulting in very large fluid pressure gradients and catastrophic fault valving. Brecciation was accompanied by widespread metasomatism that ranges from high temperature (400° - 600°C) Na-(Ca)-rich assemblages (e.g. albite ± actinolite, clinopyroxene, scapolite, magnetite, titanite, etc.) to retrograde (<400°C) chloritic assemblages. Interpretation of stable (0 and C) and radiogenic (Sr) isotopes and mineral chemistry is consistent with this spectrum of alteration assemblages reflecting metasomatic fluids of two predominant origins. The oxygen and carbon isotopic signature of carbonates from Na-(Ca) assemblages indicates that fluids responsible for this style of alteration were not simply equilibrated with magmatic rocks, but were exsolved from crystallizing plutons. Low temperature, low salinity fluids of inferred meteoric origin were introduced late in the paragenesis, and do not appear to have contributed significantly to the mass budgets of eu-Au ore systems in the district. Extensive fluid-wallrock interaction prior to mineralisation appears to have been important in the genesis of some deposits that record K- and Fe-rich alteration haloes, including for example the Ernest Henry deposit. However, the occurrence of skarn-like, intrusion-proximal mineralisation that lacks significant K- and Fe enrichment at for example Mt Elliott, indicates that fluid-wallrock interaction was not a necessary precursor for all styles of Cu-Au mineralisation in the Cloncurry District.

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