Litho-chemo-stratigraphic, structural and mineral prospectivity aspects of the Rosebery Group, an enigmatic Cambrian volcano-sedimentary succession in western Tasmania

The Middle to Late Cambrian Rosebery Group is a 2-5 km wide, continuous belt of highly deformed, marine siliciclastic and volcaniclastic strata, situated on the western flank of the Mt Read Volcanics (MRV), in western Tasmania. It occupies a transitional position between volcanogenic sequences to the east, known for their world class volcanic hosted massive sulphide deposits (VHMS), and broadly coeval sediment-dominated packages to the west. The transitional character is manifested by rapid lateral facies and provenance changes, which have historically led to confusing and contradictory stratigraphic relationships. Also contributing to the complexity is an unusual structural style characterised by abrupt change in younging direction across anastomosing faults zones, in some cases containing slivers of basement strata, and disappearance and reappearance of units along strike. This study examines these problems through a combination of litho- and chemo-stratigraphic and structural techniques. The belt is bounded to the west by a N-S trending domain of basement inliers that define a major sub-basin boundary. To the east is the Rosebery Fault, a moderately E-dipping thrust born during the principal phase of basin inversion in the mid-Devonian. The Rosebery VHMS deposit (28.3 Mt at 14.3% Zn, 4.5% Pb, 0.6% Cu, 145 g/t Ag and 2.4 g/t Au) occurs within the hangingwall block of the Rosebery Fault, hosted largely by a narrow interval of siltstone and sandstone. The host interval records a brief episode of relative volcanic quiescence that separates an older feldspar-phyric dacitic phase of magmatism (Hercules Pumice Formation), from a younger heterogeneous magmatic phase comprising quartz + feldspar-phyric rhyodacite and basaltic-andesite components (White Spur Formation). In the eastern part of the Rosebery Group, within the immediate footwall of the Rosebery Fault, VHMS style mineralization occurs in strongly sericite altered, quartz-phyric volcaniclastic sandstone. The mineralisation is characterised by banded and disseminated concentrations of mainly sphalerite, galena and pyrite, and includes a drill hole interval of 9.8 m at 8.3% Zn, 4.5% Pb, 0.4% Cu, 514 g/t Ag and 5.5 g/t Au. The host package is dominated by volcanogenic facies that have affinities with the Rosebery ore body hangingwall sequence. Defined herein as the Marianoak Formation, the package is considered chrono-stratigraphically equivalent to western intervals dominated by siltstone, sandstone, and conglomerate, but containing a distinctive pumice breccia unit known as the Natone Volcanics. Lower levels of the Marianoak Formation (MRF 1) comprise subaqueously erupted and deposited, juvenile fragment-rich, quartz-feldspar-phyric pumice breccia, and volumetrically minor peperitic rhyolite intrusions and mudstone interbeds. VHMS-style occurrences are towards the top of this interval. The pumice breccia shows minor cross-strata mineralogical variation, mainly in the abundances of quartz and feldspar crystal components. MRF 1 is overlain by a pumice-poor sequence of feldspathic volcaniclastic breccia and volcaniclastic sandstone, rich in intermediate volcanic fragments, interpreted to represent minor epiclastic reworking (VBX I/VSST, MRF 2). Narrow intervals of rhyolite pumice breccia and monomict, jigsaw fit breccia facies (VBX II/VSST, MRF 2) punctuate the facies association, recording episodic felsic explosive and less significant effusive volcanism that typified MRF 1. The rhyolite monomict, jigsaw fit breccias and peperite, suggests a local rhyolitic source for the deposit rather than input from adjacent sub-basins. The fundamental two-fold lithostratigraphic framework of the volcanogenic Marianoak Formation corresponds with marked differences in whole rock geochemical signatures, which in turn reflect changes in magma composition. MRF 1 pumice breccia facies are characterised by incompatible element-rich signatures indicative of rhyodacitic magmatism. Parts of the sequence possess remarkably coherent chemical signatures, with notably elevated Th/Sc and Th/Ti values, indicating derivation from a homogenous magmatic source, most likely from a single volcanic centre. MRF 2 is characterised by a more heterogeneous signature, with an abrupt increase in Ti and Sc abundances relative to various incompatible elements at its base. Although episodic rhyodacitic input persists throughout the unit, the dominance of more mafic compositions records a progressive shut-down of explosive felsic volcanism. Chemical analysis of the White Spur Formation reveals a very similar upsection profile to that of the Marianoak Formation, providing the basis of a robust chemo-stratigraphic correlation of packages across the surface of the Rosebery Fault. The Hercules Pumice Formation, or stratigraphic footwall to the Rosebery orebody, has a distinct chemistry, with no known equivalents in the Marianoak Formation (although equivalents at depth, beyond the present limits of drilling and sampling remain possible). The VHMS-style occurrences within MRF 1 of the Marianoak Formation are stratigraphically positioned above the Rosebery ore body host, and that the latter is yet to be penetrated by drilling within the Rosebery Fault footwall block. Pumice breccias of the Natone Volcanics (i.e. western Rosebery Group) are shown to be direct chemo-stratigraphic equivalents of MRF 1 of the Marianoak Formation and lower levels of the White Spur Formation. In this case, however, the pumice breccias are entirely enclosed within basement-derived non-volcanogenic strata. The facies relationships are interpreted to indicate deposition in a basin marginal position, distal to major volcanic centres, with volcanic input only from the most violent and volumetrically significant eruptive events. Situated at the equivalent stratigraphic position of the Hercules Pumice Formation is the basement-clast-rich Salisbury Conglomerate. It is likely that the tectonic phase heralded by the influx of coarse basement-sourced debris at the basin margin is linked to the coeval change in magmatic composition and deposition of epiclastic sediments of the Rosebery Host Rock Member towards the basin's interior. Lateral facies variants of the Rosebery Group accumulated in a series of broadly N-S trending sub-basin compartments. Devonian inversion of these sub-basins led to a domainal structural style characterised by dismembered upright folds. Sheared out fold hinges and consequential facing flips correspond locally with fault-bounded basement inliers that were emplaced along short-cuts transecting the footwall blocks of inverted grabens. The eastern sub-basin is considered to remain prospective for VHMS-style mineralisation as it is yet untested and is inferred to be a stratigraphicequivalent of the Rosebery orebody. However, cross section construction indicates that this level is relatively deep (> 500 m below surface), at least in the vicinity of the Rosebery Mine.