Cyclicity in the lead-zinc-silver-bearing sediments at Mount Isa mine, Queensland, Australia, and rates of sulfide accumulation

Abstract

Markov probability matrices have been constructed showing the degree of upward and downward ordering in the lead-zinc-silver sulfide ores at Mount Isa mine, Queensland. Application of the difference matrix approach (Gingerich, 1969) and the concept of entropy (Hattari, 1976) revealed the sediments to be strongly ordered and cyclic. Vertical repetitions of the cyclic sequences dolomite --> sphalerite --> galena --> sphalerite --> dolomite; dolomite --> sphalerite --> dolomite; dolomite --> sphalerite with pyrite --> dolomite; and dolomite --> pyrite --> dolomite were found to be a function of lateral zonation of sulfide-rich layers. It is suggested that the metals and sulfur were supplied in pulses, possibly pumped up a fault by earth movements. Progressive mixing of this fluid with the sea water led to sequential precipitation of galena, sphalerite, and pyrite across the deposit. Transgression and regression of the system is responsible for the low-probability upward and downward transitions between the sulfide layers of lower probability. A generalized cross section has been drawn of a typical sulfide bed which, if repeated in the sequences, would give probability matrices approximating those found.A major conclusion of this model is that sulfides in submarine exhalative systems are probably rapidly deposited and a few centimeters of sediment may represent only a few weeks of deposition. Consideration of some general features of other submarine exhalative deposits also indicates high deposition rates.