Abstract. As host to several world-class sediment-hosted Pb–Zn deposits and unknown
quantities of conventional and unconventional gas, the variably inverted
1730–1640 Ma Calvert and 1640–1575 Ma Isa superbasins of northern
Australia have been the subject of numerous seismic reflection studies with
a view to better understanding basin architecture and fluid migration
pathways. These studies reveal a structural architecture common to inverted
sedimentary basins the world over, including much younger examples known to
be prospective for oil and gas in the North Sea and elsewhere, with
which they might be usefully compared. Such comparisons lend themselves to
suggestions that the mineral and petroleum systems in Paleo–Mesoproterozoic
northern Australia may have spatially, if not temporally overlapped and
shared a common tectonic driver, consistent with the observation that
basinal sequences hosting Pb–Zn mineralization in northern Australia are
bituminous or abnormally enriched in hydrocarbons. Sediment-hosted Pb–Zn
mineralization coeval with basin inversion first occurred during the
1650–1640 Ma Riversleigh Tectonic Event towards the close of the Calvert
Superbasin with further pulses taking place during and subsequent to the onset of the 1620–1580 Ma Isa Orogeny and final closure of the Isa Superbasin.
Mineralization is typically hosted by the post-rift or syn-inversion
fraction of basin fill, contrary to existing interpretations of Pb–Zn ore
genesis where the ore-forming fluids are introduced during the rifting or
syn-extensional phase of basin development. Mineralizing fluids were instead
expelled upwards during times of crustal shortening into structural and/or
chemical traps developing in the hangingwalls of inverted normal faults.
Inverted normal faults predominantly strike NNW and ENE, giving rise to a
complex architecture of compartmentalized sub-basins whose individual
uplifted basement blocks and doubly plunging periclinal folds exerted a
strong control not only on the distribution and preservation of potential
trap rocks but the direction of fluid flow, culminating in the co-location
and trapping of mineralizing and hydrocarbon fluids in the same carbonaceous
rocks. An important case study is the 1575 Ma Century Pb–Zn deposit where
the carbonaceous host rocks served as both a reductant and basin seal during
the influx of more oxidized mineralizing fluids, forcing the latter to give
up their Pb and Zn metal. A transpressive tectonic regime in which basin
inversion and mineralization were paired to folding, uplift, and erosion
during arc–continent or continent–continent collision, and accompanied by
orogen-parallel extensional collapse and strike-slip faulting best accounts
for the observed relationships.
Stratiform and Strata-Bound Zn-Pb-Ag Deposits in Proterozoic Sedimentary Basins, Northern Australia