Soldiers Cap Group iron-formations, Mt Isa Inlier, Australia, as windows into the hydrothermal evolution of a base-metal-bearing Proterozoic rift basin

The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill-type deposit) and barren iron oxiderich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite-garnet-rich, laterally extensive (1.9 km) example, the Weatherly Creek iron-formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron-formations associated with Broken Hill-type Pb-Zn deposit iron-formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre-deformational alteration all indicate that the Soldiers Cap Group iron-formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, are consistent with components of both high-temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn-oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron-formation deposition. Most chemical sedimentation pre-dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional-scale aquifer by deep-seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron-formations requires a depositional setting such as a sea-floor metalliferous sediment blanket or series of brine pools, with iron-formation deposition likely confined to much smaller fault-fed areas surrounded by Fe-Mn-P-anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep-seated heating, extension and magmatism.