Implications for mineralisation

The Isa Superbasin contains one of the greatest concentrations of base metal (Pb-Zn-Ag) mineralisation in the world. The major sediment hosted Pb-Zn deposits include the Mount Isa, Hilton, Century, Lady Loretta, and Walford Creek, and George Fisher (Figure 1). It has been a long held view that Pb-Zn mineralisation has been intimately associated with basin forming processes and in particular an intracontinental rift settings and their subsequent sagphase (see Goodfellow et al., 1993; McGoldrick and Keays, 1990; Large et al., 1988). All deposits post-date major epochs of intracontinental extension (Betts et al., 1999; O'Dea et al., 1997b). One of the distinct characteristics of the sediment hosted Pb-Zn deposits is that they are not hosted within a single stratigraphic horizon within the sag-phase of the Isa Superbasin (Goodfellow et al., 1990; Betts et al., 1999), and thus no single mineralising event is responsible for their formation. For example, the Mount Isa and Hilton deposits (Figure 1) are hosted within stratigraphy deposited at 1562±7 Ma (Page and Sweet, 1998), the Walford Creek deposits (Figure 1) are hosted within stratigraphy that is aged at ~1640 Ma (Page and Sweet, 1998; Rorhlach et al., 1998), and the Century deposit is hosted within stratigraphy aged ~1595 Ma (Broadbent et al., 1998; Page and Sweet, 1998). Genetic models for mineralisation are also variable. Traditionally, Pb-Zn deposits are considered to be SEDEX-types (Goodfellow et al., 1993), detailed studies are suggesting different processes of mineralisation. Some deposits have multiple genetic models for the formation (cf. McGoldrick and Keays, 1990; Perkins, 1996).

Models include early diagenesis beneath the watersediment interface (Mount Isa, Hilton: McGoldrick and Keays, 1990). Similar ore depositional processes have been proposed for the HYC deposit in the McArthur Basin and the Walford Creek Deposit (Rorhlach et al., 1998). The Walford Creek Deposit has characteristics associated with several deposit types (Rorhlach et al., 1998). These include mineralisation is similar to those formed with SEDEX deposits, cavity fill and stratiform Pb-Zn deposition during early diagenesis followed by cavity fill, replacement, and MVT-style veins. The variation of mineral style within the Walford Creek Deposit reflects the continuum of the sediment hosted mineralisatiuon processes during basin evolution and subsequent inversion (Rorhlach et al., 1998). The syn-sedimentary and early diagenetic concepts for mineralisation have been challenged. Several recent studies have suggested that mineralisation occurred during basin inversion (e.g., Century: Broadbent et al., 1996) and regional shortening of the Isan Orogeny (e.g., Mount Isa and Hilton: Perkins, 1996; 1998). The Century deposit is hosted within turbiditic siliclastic black shale and siderite-rich siltstone of the upper McNamara Group (Andrews, 1998; Broadbent et al., 1998). This differs from the deposits hosted lower in the stratigraphy that are hosted within shallow marine carbonates and evaporitic-rich sequences. Century is a shale hosted mineralization style formed during tectonically driven migration of basinal fluids during initial N-S-directed basin inversion, sharing many similarities with Mississippi Valley-type mineralisation (Broadbent et al., 1998).

An epigenetic model for Mount Isa has been applied by Perkins (1996; 1998) explaining several paragenetic and overprinting relations of alteration and sulphides. This model differs significantly from the SEDEX or diagenetic models commonly supposed for the deposit (see Large et al., 1988; McGoldrick and Keays, 1990) because sulphide deposition involves fluid-wall rock reactions rather than mixing of sea water and hydrothermal fluids. Perkins (1996; 1998) utilised overprinting relations to suggest that mineralisation occurred after the main phase of E-W shortening of the Isan Orogeny.

Despite the various genetic models for the sediment hosted Pb-Zn ores of the Western Fold Belt, a common feature of the deposits is their strong structural control, or a spatial association, with the large extensional faults (Lister and Betts, in review). Mineralising events may be related to periods of extensional faults reactivation. Post-rift seismic activity along normal and transverse faults may have localised stresses, providing pathways for the ascent of mineralising fluids. Reactivation of normal faults are documented by Andrews (1998) and Rohrlach et al. (1998). The size of the rift faults may also be significant in determining the location of mineralisation. Larg e volumes of fluid, sourced from the deep crust, are likely to be channelled along larger basin bounding faults. Smaller rift faults may not be as integral to the fluid plumbing system.

At the Mount Isa deposit, the Paroo Fault is the fundamental control of the zonal pattern of mineral distribution in the orebody whereas smaller scale dilational dolomite veins locally control the distribution of sulphides (Perkins, 1996). Other examples of Pb-Zn deposits forming along major extensional structures includes Century which is spatially associated with the Termite Range Fault (Broadbent et al., 1998) and the Walford Creek Deposit which occurs along the basin-bounding Fish River Fault (Rohrlach et al., 1998).

An asymmetric extension model for the Mount Isa Rift Event has significant implications for the formation of these large Pb-Zn-Ag orebodies. Pb-Zn- Ag deposits are located to the west of the Mount Isa Rift (Figure 1), coincident with the predicted location of maximum asthenospheric upwelling, magmatism, and the highest geothermal gradients. Most models associated with the development of sediment hosted Pb-Zn deposits, particularly diagenetic and SEDEX models, invoke a fluid convection system driving fluids throughout the basin (Russell et al., 1981; Goodfellow et al., 1993). Models commonly require a local magmatic source (e.g., shallow level pluton) as a driving mechanism for fluid convection (Goodfellow et al., 1993; Goodfellow et al., 1990). Such models also require a prolonged transit time for hydrothermal fluids to reach the sea floor (Goodfellow et al., 1990). In the Isa Superbasin, there is little evidence from exposed supracrustal sequences or from the geophysical data, with the exception of the Mount Isa and Hilton deposits, that such plutons are temporally or spatially associated with the large Pb-Zn deposits. Other convection models (eg. Russell et al., 1981) do not require a local source. Rather these models call upon a downward propagating convection cell along extensional faults that tap into elevated heat sources, caused by an elevated geotherm associated with an rifted continental crust (Russell et al., 1981). The proposed asymmetric extension model predicts the location of maximum asthenosphere upwelling and presumably highest geothermal gradients to be located to the north and west of the rift axis. The model conveniently explains the absence of Pb-Zn deposits within the rift zone. There remains a temporal problem between timing of the mineralisation (post-rift) and the timing of elevated geothermal gradients during rifting. An elevated crustal geotherm probably continued on the rift flanks after the cessation of rifting, although it gradually dissipated as the asthenosphere downwelled and the basin subsided. Thermal subsidence also promotes burial and increasing the depth extent of high geothermal gradients, effectively allowing the mid to upper crust to heat up while the Moho cools (McLaren et al., 1997), allowing favourable thermal conditions to drive convecting fluids in the basin.

Recognition of favourable or common sedimentary environments for the sequences hosting the mineralisation is significant for ground selection for Pb-Zn mineralisation. Particular sedimentary environments produce chemically favourable site for the deposition of Pb-Zn mineralisation (Goodfellow et al., 1993) regardless if mineralisation occurred at the sea floor during deposition or during later diagenesis. Favourable stratigraphic packages associated with the deposition of Pb-Zn mineralisation include carbonaceous chert, shale, siltstone, and coarser clastic lithologies deposited in a anoxic marine environment (Goodfellow et al., 1993). Lacustrine depositional environments have also been suggested (Muir, 1983). Stratigraphic studies of the northern Mount Isa terrane have shown that fluid migration may have been controlled by unconformities beneath sequence boundaries (McConachie and Dunster, 1996). Metal precipitation was due to the reduction of organic matter in shale near the top of organic-rich high-stand system tracts (McConachie and Dunster, 1996). Pb- Zn-Ag mineralisation is often hosted within condensed stratigraphic sections defined by black shale sequences (McConachie and Dunster, 1996). Although, the interpreted tectonic setting of the Isa Superbasin differs from that of McConachie and Dunster (1996) the same principles are applicable to a sag-basin. Condensed sections are likely to form where subsidence and accommodation space is greatest, throughout the northern Mount Isa terrane. Muchez et al. (1994) proposed that gravity driven fluid flow plays a significant role in the development of diagenetic Pb-Zn-Ag orebodies. During sag-phase evolution of the basin, fluid migration would be directed towards the locations of greatest thermal subsidence (i.e. the northern Mount Isa terrane).