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Two massive to banded strata-bound magnetite-rich ironstones (Fe2O3total + SiO2 = 96%, Fe2O3total > 60%) host Au-Cu mineralization in an intracratonic rift setting within the Mount Isa eastern succession. The deposits are of international interest because of the present divergent views on exhalative versus epigenetic genesis; these ores have features which support both origins.

Starra is the main orebody cluster, consisting of four geographically separate lodes, totaling 5.3 million metric tons at 5.0 g/metric tons Au, and 1.98 percent Cu. The origin of the Starra ores is complicated by an intense deformational history. The lodes lie on the margin of a major D1 decollement, which was subsequently reactivated during D2 and D4. Starra ores are deformed by all recognizable stages of deformation, occurring both in folded and unfolded segments. The footwall is extensively altered to albite-magnetite-pyrite-bearing assemblages, whereas the hanging wall shows only sporadic albite alteration overprinted by D4 calcite gash veining not spatially related to ore.

Ores are massive to banded, characterized by Fe, Si, Au, Cu, W, and Sn enrichment and by Pb, Zn, Ag, and Ba depletion. Au shows good correlations with Si, W, and Cu but is inversely correlated to Fe in the only lode studied in detail. Trough Tank, 40 km southeast of Starra, a similar but less highly strained deposit, is characterized by Co, Mo, and P enrichment in addition to the above elements.

A syngenetic exhalative origin with transport of Au as Au chloride complexes at 280° to 380°C into a low S oxidizing environment is invoked. This best explains local and regional features such as high background Au levels in banded iron-formation, a lack of replacement textures in massive ore, zoning of geochemistry and mineralogy, high Cu/Au ratios, and location at the boundary between a basic-acid sequence and calcareous metasediments.

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