ABSTRACT

   The Selwyn Line is the name adopted for an open arcuate zone of iron-copper-gold mineralisation, located ~150 km southeast of Mount Isa, in the Mount Isa Proterozoic inlier, northwest Queensland, Australia. Since the discovery of gold mineralisation in the Selwyn Hills in the 1970's, 5 small mines have been operated. These exploited high grade ironstone hosted gold-copper mineralisation, located within a zone of poly-aged shearing and complex alteration. The mines are located within the Starra Shear which has a history of ductile-brittle, brittle-ductile and brittle deformation. The structure is altered over widths of between 100 m and 500 m, and for over 10 km along strike. The alteration involved early albite +quartz +calcite +scapolite +actinolite replacement of the host sheared meta-sediments of the Staveley Formation. This was followed by a magnetite/hematite+biotite+quartz(+pyrite) overprint, and finally post tectonic oxidation of the magnetite, and chlorite-calcite alteration with associated chalcopyrite and gold mineralisation, and limited pyrite formation (Rotherham 1997). The high grade zones exhibit indications of strong structural controls. They typically plunge steeply to the north along magnetite bearing extensional duplex structures and shear planes which include portions of the hanging wall bounding shears and link structures to the footwall bounding shears. The dimensions of the mined and mineable resource (<8 Mt) have always been regarded as small for typical Fe-Cu-Au systems. However resource work done on lower cut-off grades indicates that the system is more typical of the large systems of this sort found elsewhere in the district, with a pre-mining global resource of at least 29 Mt at 2.5g/t Au and 1.4% Cu (at a 1.5% Cu Equiv. cutoff using a factor of ~ 0.8 depending on where the ore is from), 49 Mt at 1.76 g/t Au and 1.05% Cu using a 1.0% cut off, or 95 Mt at 1.1g/t Au and 0.74% Cu at a 0.5% cutoff. These are embraced within a larger mineralised system with a global resource of 253 Mt @ 0.48g/t Au and 0.34% Cu at measured, indicated and inferred categories, to a depth of ~300m below surface (using a 0.2% Cu Equiv. cut off ).
   The main observations made in this paper are: 1). The Selwyn Line is fairly typical of iron-copper-gold systems in NW Queensland in that it is part of a very large alteration system containing large tonnes of mineralised magnetite ironstones with associated low tenor Cu and Au grades; 2). In the Selwyn Line (excluding the 222 Mine) the ore grade zones discovered to date have been confined to duplexes within crudely tabular 'tramway' shear systems, in which multiple localised sites for ore mineral deposition were generated. The cumulative tonnage of these sites is substantial but still remains constrained by the overall tightness of the late (brittle-ductile) strain zones which were dominated by flattening (D4). The dimension of ore grade zones in these systems is mostly a function of the volume of the zones in which late extensional deformation was developed (D4 to D6, Adshead-Bell, 2000); 3). At 222 Mine to the south, the structural location is more complex with multiple 'north' trending S2/S4 shears intersecting the S1 'hinge zone' shear and its associated intra-folial ironstones at higher angles. This has provided a site for greater volumes of late stage alteration and mineralisation. The resultant lower grade tonnages drilled to date have already intersected larger orebodies here (>5 Mt), and suggest there is further potential for the delineation of a major deposit in this system.

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