Structural controls on mineralisation in the Rendeep area, Renison Tin Mine, Tasmania

Mineralisation in the Rendeep area occurs below 1700m RL (>550m from surface) at the northern end of the Renison Tin Mine, Renison Bell, western Tasmania. Probable reserves of the Rendeep area are .3.3Mt at 1.96% Sn (Thomas, 1994), compared to remaining proven and probable reserves of 6.4Mt at 1.41% Sn for the upper mine (Thomas & Roberts, 1994). The Renison deposit is hosted by supratidal to intertidal dolomitic and siliciclastic sediments of the Renison Mine Sequence which comprises the upper ‚ÄövÑvÆ80m of the late Precambrian Success Creek Group and the lower ‚ÄövÑvÆ60m of the Cambrian(?) Crimson Creek Formation. Minor facies variations occur in the Rendeep area but gross thickness variations are the result of structural processes. The strongly layered Renison Mine Sequence tends to deform by interstratal slip in contrast to the rigidity of the relatively massive underlying and overlying formations. Devonian D3 deformation, which dominates the Rendeep area, resulted from the radial stress field produced by the forceful emplacement of the Pine Hill Granite into the D2 Renison Bell Anticline axis (Ktto, 1994). The granite surface plunges northeast under the Rendeep area from its local high beneath the central part of the Renison deposit. The Federal-Bassett Fault developed over the local granite high and is transitional to a monoclinal fold in the Rendeep and North Bassett areas. Syn-intrusive, sub-vertical extension produced 770m vertical displacement of the Renison Mine Sequence in the North Bassett/Rendeep area by combined ductile and brittle processes, with ductile processes favoured by elevated temperatures and pressures(?) closer to the granite. The North Bassett Fault formed by layer-parallel extension of the Renison Mine Sequence close to its contact with overlying Crimson Creek Formation. Conjugate subvertical brittle-ductile extensional structures, Faults A and Z, formed subparallel to the principle stress direction. Strain transferral from the North Bassett Fault onto Faults A and Z produced a broad, concave-east open fold in the North Bassett Fault and Renison Mine Sequence north of 66400m N. The fold axial region, which plunges ‚ÄövÑvÆ700 S, became a major upflow zone for hydrothermal fluids during later brittle reactivations. Draping of the Renison Mine Sequence along Fault Z and normal movement on the antithetic Csar Fault produced the Rendeep Graben/Syncline. Dilation of the North Bassett Fault above the Rendeep Graben/Syncline focussed hydrothermal fluid flow. Brittle dip-slip reactivation of the Federal-Bassett Fault occurred late in the D3 deformational event, faulting the crystalline carapace of the cooling Pine Hill Granite and releasing hydrothermal fluids. D3 dextral wrench deformation accompanied the decay of the radial stress field associated with the Pine Hill Granite intrusion and a return to the regional Tabberabberan stress field (Ktto, 1994). Brittle strike-slip reactivations occurred on all major faults and a weak dextral kink fold formed in the Renison Mine Sequence interstitial to Fault A and the North Bassett Fault, the hinge zone of which became a local focus for hydrothermal fluid flow. Rendeep mineralisation is predominantly \stratabound\" consisting of dolomite horizons of the Renison Mine Sequence replaced by an assemblage of pyrrhotite ¬¨¬± talc ¬¨¬± cassiterrte and minor arsenopyrfte. Stratabound mineralisation becomes more localised about feeder structures on approaching the Pine Hill Granite due to the effect of increasing temperatures and pressures(?) on the brittle/ductile transition in dolomite. Ductile behaviour in dolomite may inhibit fluid/rock interaction by restricting fracture propagation. A radius of 400m from the Pine Hill Granite is proposed as the minimum distance to economic stratabound tin mineralisation based on sparse drilling information in the Rendeep area. Hydrothermal fluid flow paths are identified by integrating the modelled tin grade distribution sulphur and oxygen isotope data and fluid inclusion data with the structural interpretation. Possible extensions to Rendeep mineralisation are predicted by extrapolating the hydrothermal fluid flow paths along controlling structural features."