An introduction to remobilization: Information from ore-body geometry and experimental considerations

Abstract

Mobilization involves the translocation and concentration of originally dispersed metallic constituents; remobilization is the translocation resulting in modified concentration and distribution of pre-existing metalliferous concentrations. Mobilization and remobilization proceed by “chemical” (liquid-state), “mechanical” (solid-state), and “mixed” processes. The paradox of remobilization is that the more complete the process, the more difficult becomes substantiation. Regional metamorphic remobilization is commonly invoked to explain the morphology of supposedly once-stratiform massive sulphides and their feeder and alteration systems. Analysis of the principal dimensions of stratiform massive sulphides in non-metamorphic and regionally metamorphosed deposits shows that, whereas comparative morphology may be a guide to remobilization, it does not constitute proof. Similarly, feeder and alteration morphology suggests that geometric analysis may be of value in establishing mechanical remobilization; but the more complex alteration distributions which exist could well lead to misinterpretation in highly deformed terranes. Ore composition, internal structure and zonation influence the ore's capacity to be remobilized. Mechanical remobilization requires rheological divergence between sulphides and their host rocks. Experimental deformation of dry sulphides shows that most common sulphides pass through the brittle-ductile transition before entering the field of regional metamorphism, but that the strength-order of sulphides, silicates and carbonates remains dynamic until all are undergoing steady-state flow. Within this context, experimental strength-orders support galena (weakest) < pyrrhotite < chalcopyrite < sphalerite < pyrite (strongest), with some variation in the relative positions of the middle three. They also suggest that the middle three are of similar strength to carbonate rocks and that some silicate rocks are weaker than pyrite. In mixed sulphide and sulphide-silicate assemblages, behavioural convergence results from strength-extremes being diluted by weaker/stronger phases; in effect, an aggregate-strength exists. The presence of an aqueous phase further modifies mechanical behaviour; massive sulphides probably deform by dominant dislocation flow, whereas intercalated sulphides and silicates express a complex interplay of solution-transfer and other flow mechanisms. Experimental considerations indicate that, during diagenesis and early prograde metamorphism, the brittle-ductile transition will result in widely distributed micro- and mesoscale mechanical remobilization. Nevertheless, there is usually insufficient gross behavioural contrast to engender discordant macroscale remobilization. Possible exceptions, particularly in relation to thrusting, are outlined, but we conclude that macroscale mechanical remobilization during the brittle-ductile transition is an uncommon occurrence. Experimental data were also examined relative to subsequent prograde (to peak) metamorphism, and strength-contrasts during decaying and retrogressive metamorphism. Because the largest strength-difference between sulphides and the host-sequence is in the lower part of the 350°–650°C metamorphic range, we contend that macroscale mechanical remobilization is more probable during prograde metamorphism in the 350°–500°C interval. We similarly believe that macroscale mechanical remobilization is more likely during deformation accompanying retrogressive cooling in the range 500°–350°C; and we note that it may also result from deformation exploiting the brittle/ductile contrasts between silicate and sulphide rocks during the water-deficient, non-retrogressive, metamorphic-decay cycle. Chemical mobilization and remobilization require the dissolution of base metals by metamorphic fluids. The compositions of hydrothermal (low-grade metamorphic) fluids and their precipitates in active geothermal systems, show that the fluids can dissolve, transport, and form concentrations of base and precious metals. Similarly, fluid-inclusion compositions from low- to medium-grade metamorphic rocks show that such fluids can dissolve a few ppm of base metals. In both cases, low salinities and low dissolved base-metal contents raise doubts over whether such metamorphic fluids can form ore-grade base-metal concentrations; we submit that they can. Metamorphogenesis is favoured by high initial base-metal concentrations, preferential dissolution of sulphides over silicates, and fluid-dominated metamorphism as found in a multiple-pass fluid system. Preferential dissolution of sulphides and the need for high water/rock ratios are supported by theoretical considerations, experimental work on mafic, felsic and sedimentary source-rocks, and field-based studies. In the context of mobilization, the work strongly implies that prograde metamorphic devolatilization, and a mobile fluid phase have substantial ore-forming capability; retrograde revolatilization does not. Remobilization is more complex. Although favoured by high initial metal concentrations, dissolution is inhibited by the low permeability of homogeneous, low-silicate, massive sulphides, and the consequent peripheral fluid-channeling. It is conversely enhanced by fine grainsize in discordant, fluid-focussing (retro-grade) shear zones. In all cases, for remobilization to form a new ore body, both fluid-flow and the precipitation process must be focussed and efficient.