The mineralogical and lithogeochemical footprint of the George Fisher Zn-Pb-Ag massive sulphide deposit in the Proterozoic Urquhart Shale Formation, Queensland, Australia

Abstract

The Proterozoic Carpentaria Province (McArthur basin and Mount Isa Inlier) in northern Australia comprises a number of world class clastic dominated (CD-type) Zn-Pb massive sulphide deposits, formally known as SEDEX deposits. In order to identify the geochemical footprint of any mineralizing system it is necessary to characterize compositional variability of the host rock to mineralization. In the southern Carpentaria, establishing the baseline composition of the host rock is complicated by varying degrees of tectonic overprint, a lack of metamorphic indicator minerals, and the overall size of the ore forming systems. In this study, samples from drill-holes intersecting the main ore bodies at the world class George Fisher CD-type massive sulphide deposit have been compared to samples from a drill-hole intersecting barren, correlative lithologies of the Urquhart Shale Formation (ca. 1654 Ma). Bulk rock lithogeochemical (X-ray fluorescence, inductively coupled plasma mass spectrometry and LECO) and mineralogical (X-ray diffraction) analyses have been combined with petrographic observations to (1) establish the baseline composition of the Urquhart Shale Formation and (2) determine the geochemical and mineralogical footprint of the CD-type system at George Fisher. The absence of metamorphic indicator minerals, combined with the preservation of illite in un-mineralized Urquhart Shale, suggests that in this part of the Mount Isa area, the host rocks did not reach greenschist facies conditions (>300 degrees C). Chlorite in the un-mineralized Urquhart Shale is very fine grained (<= 10 mu m) within interstitial pore spaces with other phyllosilicates (e.g., illite), and is interpreted to be diagenetic in origin. Relative to the un-mineralized Urquhart Shale, the first stage of sulphide mineralization (Zn-dominated, stratabound) at George Fisher is associated with decreased abundances of albite, chlorite, and calcite, and higher abundances of dolomite and phyllosilicates (muscovite and phlogopite). These mineralogical transformations are associated with strong minor and trace element depletion (Sr and Na) and enrichment (Tl and Mn). An element index based on this suite of elements (GF index = 10(400Tl+Mn/10Sr+Na)) is highly effective in differentiating between the background Urquhart Shale Formation and the alteration footprint at George Fisher and may provide an additional tool for geochemical exploration programmes in the Mount Isa area. This study affirms the benefit of combining lithogeochemical, mineralogical, and petrographic data in order to understand the host rock baseline composition and the alteration footprint of Carpentaria CD-type massive sulphide systems.