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Mineralium Deposita

, Volume 39, Issue 2, pp 143–158 | Cite as

Mass flow sedimentology within the HYC Zn–Pb–Ag deposit, Northern Territory, Australia: evidence for syn-sedimentary ore genesis

  • Tim Ireland
  • Stuart W. Bull
  • Ross R. Large
Article

Abstract

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.

Keywords

Base metal mineralisation HYC Mass flow Sedimentology 

Notes

Acknowledgements

This paper is the result of ongoing studies of north Australian sediment-hosted stratiform base metal deposits at the University of Tasmania, and would not have been possible without the financial and logistical support offered by the CODES Centre for Ore Deposit Research. Damien Nihill, Steve Peverly and Matt Furness of McArthur River Mining (MRM) provided invaluable support on-site through providing access to core, underground exposures and electronic resources. MRM also contributed financially to the project.

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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  1. 1.Centre for Ore Deposit Research (CODES)University of TasmaniaHobartAustralia

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