Carbonate alteration of the Upper Mount McRae Shale at Mount Whaleback, Western Australia – implications for iron ore genesis

Abstract

The genesis of giant hematite ore deposits in northwest Australia remains a contentious topic in part because key evidence supporting a unifying genetic model has not been found at all deposits. In particular, several papers have proposed that carbonate replacement of silica layers in banded iron formation (BIF) by basinal brines is a requisite first step toward ore formation. This initial hypogene stage is inferred primarily from the presence of silica-poor, carbonate-rich rocks found below ore and along normal faults at the Mount Tom Price and Giles Mini deposits. However, until recently such rocks have not been identified at the largest martite-microplaty hematite deposit, Mount Whaleback. In the present study, samples of the upper Mount McRae Shale are examined for their chemistry, mineralogy and petrography. These samples were collected from several key locations, including within drill core that penetrates an area immediately beneath ore along the Mount Whaleback fault at Mount Whaleback. Compared with unaltered black shale from Wittenoom Gorge in the north and altered black and red shale at Mount Whaleback, reddish-green shale below the Mount Whaleback deposit is significantly enriched in MgO and CaO and depleted in SiO 2 . Samples of this shale consist predominantly of fine-to medium-grained dolomite and ankerite cut by chlorite and carbonate veins, contrasting with unaltered equivalents that contain mostly stilpnomelane, K-feldspar and relatively minor carbonates. This alteration is distinct from assemblages developed during regional metamorphism, and possibly represents hydrothermal alteration after metamorphism. Although several questions remain unanswered, these results support models that invoke an early hypogene stage during the formation of the martite-microplaty hematite deposits in the Hamersley Province.

Barren ground depressions, natural H2 and orogenic gold deposits: Spatial link and geochemical model

“…The Mount Whaleback iron deposit is located at ∼ 25 km to the east of the fairy circles. Shales predominantly containing dolomite and ankerite are found below this deposit (Webb et al, 2006). We also observed other geomorphological features in the Hamersley province recalling fairy circles at ∼ 50 km to the north of the discovery of Getzin et al (2016) (see kmz file for exact location).…”

Petrology of banded iron formation-related country rocks at the northern limit of the Mbalam iron ore district, southeastern Cameroon

“…Wittenoom and Mt. Whaleback, Australia (Webb et al, 2006). Major BIF deposits around the world have recorded gold contents as high as 5 ppm and reserves up to 44.6 Mt (Pereira et al, 2007;Morales et al, 2016;Vitorino et al, 2020).…”