REMOBILIZATION, SYNTECTONIC PROCESSES AND MASSIVE SULFIDE DEPOSITS

Ductile remobilization by mechanical and chemical processes is most effective in the approximate temperature range of 350-degrees-500-degrees-C under prograde and retrograde metamorphism. The relative behaviour of individual phases in mixed assemblages is expressed in terms of the viscosity contrast. A layer comprising a two-phase mixture has an effective viscosity between the limiting viscosities of the discrete phases. Where adjacent layers have different effective viscosities, the composite behaviour reflects the effective-viscosity contrast and obeys the principles of folding. Despite behavioural convergence in mixtures, substantial effective-viscosity contrasts exist between massive sulphide bodies, some carbonate rocks, and most silicate rocks; intense folding and remobilization manifest this. Internal and external ductile remobilization have the capacity to form oreshoots comprising enriched and/or thickened portions of mineralization within or external to other parts of the mineralized system. Internal shoots comprise hinge-zone concentrations, macroboudins and partitions, and elongation geometries. External oreshoots, including those arising from ''transposition'' and thrusting, fuel the debate on remobilization versus syntectonic emplacement of ore. A conceptual approach demonstrates that remobilization geometry will reflect the geometric relationships of the precursor mineralization and hostrocks, and the degree, extent and nature of the remobilization mechanisms. Dynamic interplay between ductile deformation and advective transfer can produce complex parageneses and microstructures which are indistinguishable from those described for syn-tectonic emplacement. The scope of this problem is illustrated with particular reference to deposits of the Cobar region, Australia. Guidelines are provided for discriminating between remobilized pre-tectonic and syn-tectonic deposits, but doing so is difficult, because a better understanding of the processes which give rise to many traditional pieces of evidence renders the evidence inconclusive. The best available technique requires thorough geometric and kinematic analysis on all scales of observation, coupled with a preparedness to accept polymodal genesis.