Abstract

The Eastern fold belt of the Proterozoic Mount Isa inlier has undergone numerous episodes of deformation, hydrothermal alteration, and iron oxide copper-gold mineralization. The structural and lithological controls on the location and geometry of five iron oxide copper-gold deposits (Starra, Mount Elliott, SWAN, Mount Dore, and Lady Ella) in the Selwyn-Mount Dore corridor were studied through geologic mapping, core logging, structural analysis, and examination of 3-D grade distribution. All the deposits in this area occur in close proximity to the Selwyn shear, a long-lived, regional structure which developed at the contact between carbonate-rich and siliciclastic-dominated rock packages. This fault and a series of subparallel structures have been the focus of multiple reactivation events and have controlled the distribution of alteration and mineralization, which occurred in this area at ~1595, 1570, 1530, 1515, 1500, and 1485 Ma.

Ironstone-hosted mineralization at Starra developed in a preexisting zone of sodic and iron oxide alteration along the N-striking Starra shear. Kinematic indicators along the Starra shear reveal that it has been reused numerous times; early D2 thrusting, post-D2 sinistral shearing, and pre-D4 (syn-D3?) dextral brittle faulting were all associated with sodic, sodic-calcic, and iron oxide alteration. Either the post-D2 or pre-D4 strike-slip faulting events can explain the steeply plunging nature of the orebodies at Starra. However, the dextral pre-D4 event produces dilational structures in ironstones and is associated with regionally important sodic and iron oxide alteration along NE- to ENE-trending faults, which intersect the Starra shear close to the main orebodies; we consider this the more likely main mineralizing event at Starra. Both strike-slip faulting events correspond with a change from E-W–oriented compression during the main phase of the Isan orogeny to transpression that may have been driven by crustal overthickening during orogenesis.

At Mount Elliott and SWAN, mineralization is associated with post-D4 dextral and reverse movement across a sinistral fault jog along the NNW-striking Mount Elliott fault during transpression. Mineralization, alteration, and veins at these deposits clearly overprint all ductile structural features. Overall, the orebody geometry is controlled by the orientation of the Mount Elliott fault, but high-grade ore shoots are controlled by breccia formation in elongate pods which are parallel to preexisting F2 fold axes. In contrast, gently plunging grade shells at the slightly younger Mount Dore and Lady Ella deposits formed in response to renewed E-W compression and reverse displacement along N- to NE-striking fault segments of the Mount Elliott fault and newly formed subparallel faults. Examination of the structural setting of Mount Dore and Lady Ella also demonstrates that strain partitioning between mechanically heterogeneous rock sequences (metasedimentary rocks, metabasalts, granite, and strongly silicified rock types) exerted strong controls on the location of later mineralized systems.

The important episodes of iron oxide copper-gold mineralization in the Eastern fold belt between 1595 and 1485 Ma appear to be associated temporally with compressive or transpressive structural regimes. These events developed in response to far-field tectonic stresses that evolved in response to the collision of Laurentia with the proto-Australian continent during the Isan orogeny. Late in the orogenic history, the emplacement of voluminous A-type batholiths associated with hot-spot plume activity likely generated localized transient shortening events, which are also associated with iron oxide copper-gold mineralization. This unique geodynamic evolution was responsible for the large-scale metal enrichment and remobilization into suitable chemical and structural traps over significant time periods in the Eastern fold belt iron oxide copper-gold district.

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