Published:January 01, 1995
J.R. Vearncombe, A.W. Chisnall, J.N. Christensen, M.C. Dentith, S.L. Dörling, A.R. Reed, 1995. "Discussion", Zinc-Lead Mineralization on the Southeast Lennard Shelf, Canning Basin, Western Australia, J.R. Veamcombe, S.L. Dörling, M.C. Dentith, A.W. Chisnall, J.N. Christensen, N.J. McNaughton, P.E. Playford, M.J. Rayner, A.R Reed
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The movement of MVT-forming brines has been tied to the histories of adjacent sedimentary basins, but the precise causal link is still not well established because the timing of MVT mineralization is usually poorly constrained relative to basin evolution (see Chapter 1). Thus, the precise dating of MVT mineralization would allow discrimination among the various models for the transport of the fluids responsible for ore deposition.
Geological relationships which may give a time constraint on mineralization are often ambiguous. Until the successful development of Rb-Sr dating of sphalerite (Nakai et al., 1993), some deposits, such as Pine Point (Canada), were constrained only by the age of the host rocks representing the maximum age of the deposit, and a knowledge of when it was first found by man (see Garven and Sverjensky, 1 994; Lyons, 1994). On the Lennard Shelf we can be more precise because the deposits are hosted in Devonian limestones, and Zn-Pb mineralization in faults and veins is truncated by the unconformity at the base of the late Carboniferous and Permian Grant Group. Locally, as at Blendevale, detrital clasts of mineralized limestone are preserved in erosional channels filled with Grant Group sandstone, thus providing a minimum age for mineralization as late Carboniferous. However, as with MVT deposits worldwide, the precise date on mineralization is controversial.
Figures & Tables
Zinc-Lead Mineralization on the Southeast Lennard Shelf, Canning Basin, Western Australia
Mississippi Valley-type (MVT) deposits are carbonate-hosted, Zn-Pb deposits with their type examples in the Mississippi Valley area of the USA. MVT deposits are: (i) stratabound but epigenetic in the nature of their mineralization; (ii) occur on the periphery of sedimentary basins in platform facies above basement highs; (iii) lack any association with igneous activity; (iv) have a simple mineralogy of marcasite, pyrite, galena and sphalerite. These deposits are the result of the passage of large volumes of metal-bearing brines (Sverjensky, 1986; Anderson and Macqueen, 1988), and thus provide a record of the large-scale movement of crustal hydrothermal fluids (Bethke and Marshak, 1990; Halliday et al., 1991).
MVT deposits are controversial, with current scientific debate about: (i) the causes of large-scale movement of crustal hydrothermal fluids and the focussing mechanism necessary to form the deposits; (ii) the timing of mineralization; (iii) the nature of the deposition site; (iv) the source of the sulfur required for base-metal precipitation and whether the sulfur was acquired at the deposition site or was carried with the ore solution; (v) the nature and characteristics of any alteration halo to the mineralization. All of these controversies are discussed in this field guide, but two specific aspects are singled out for special attention. These are the nature of the deposition site and the timing of mineralization.