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Mineralium Deposita

, Volume 39, Issue 5–6, pp 632–645 | Cite as

Carbonate alteration of the Upper Mount McRae Shale beneath the martite-microplaty hematite ore deposit at Mount Whaleback, Western Australia

  • Adam D. Webb
  • Gerald R. Dickens
  • Nicholas H. S. Oliver
Article

Abstract

The formation of large martite-microplaty hematite ore deposits in northwest Australia remains a contentious topic in part because important evidence supporting a unifying genetic model has not been observed at all deposits. Carbonate replacement of silica has been found along normal faults below ore at the Mount Tom Price and Giles Mini deposits, which suggests an early hypogene process during ore formation. However, such rocks have not been identified at the largest martite-microplaty hematite deposit, Mount Whaleback. In this study, samples of the Mount McRae Shale are examined for their chemistry, mineralogy and petrography. These samples were collected from several key locations, including an area that immediately underlies ore along the Mount Whaleback fault at Mount Whaleback. Compared to unaltered black Mount McRae Shale from Wittenoom Gorge in the north and altered black and red Mount McRae Shale at Mount Whaleback, reddish-green Mount McRae Shale along the Mount Whaleback fault is greatly enriched in MgO and CaO and depleted in SiO2. This chemistry arises from significant amounts of fine- to medium-grained ferroan-dolomite and ankerite and cross-cutting chlorite and carbonate veins. The composition is distinct from that produced during regional metamorphism, and most likely represents hydrothermal alteration after metamorphism. The lack of carbonate-rich, silica-poor rocks in the overlying Dales Gorge Member at Mount Whaleback is consistent with pervasive oxidation of most rocks in the region during or after ore genesis, a process that removed carbonates. 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.

Keywords

Martite-microplaty hematite ore deposits Northwest Australia Carbonate replacement of silica Chemistry, mineralogy and petrography Hypogene stage 

Notes

Acknowledgements

This research was funded through an ARC Large Grant to N.H.S. Oliver and G.R. Dickens. We gratefully acknowledge BHP Billiton Iron Ore and its geologists, particularly Simon Hewson, Peter Croft, Peter Waters, Mal Kneeshaw and Mawson Croaker, for providing access to samples and data at Mount Whaleback and other logistical support. However, our interpretation of the data does not necessarily reflect the opinions of BHPBIO. We also thank Elvy Grigolato, Kevin Blake and Sharon Ness of the JCU-AAC for help with the XRF, XRD and electron-microprobe analyses. Steffen Hagemann, Alexandre Raphael Cabral and an anonymous reviewer are thanked for their detailed and insightful review comments.

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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  • Adam D. Webb
    • 1
  • Gerald R. Dickens
    • 1
    • 2
  • Nicholas H. S. Oliver
    • 1
  1. 1.School of Earth SciencesJames Cook UniversityTownsvilleAustralia
  2. 2.Department of Earth SciencesRice UniversityHoustonUSA

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