Syngenetic Dolomitization and Sulfide Mineralization

Abstract

Excellent stratigraphic control and diagenetic changes make it possible to demonstrate the temporal relationship as well as the process of Pb-Zn mineralization in well exposed Precambrian rocks of Helikian age on northern Baffin Island, N. W. T., Canada. Initial deposition of shallow-marine, cratonal sediments was interrupted by regional tilting. Emerging areas were extensively karsted and brecciated, affecting in particular laminated algal carbonates. Subsequent subsidence led to the development of a topographically restricted marine environment and in turn to evaporative conditions. The brecciated carbonate and the extensive subsurface channel system which was formed during the preceding episode of karsting, provided perfect passageways for a refluxing dolomitizing brine. Steady-flow conditions and a continuous supply of seawater resulted in the complete dolomitization of the brecciated and channelled hostrock. Seawater must have contributed most if not all the Mg during dolomitization. Continuous circulation of a seawater brine must have introduced other metal ions as well, which were most likely carried as chloride complexes. These combined with a possibly seasonal supply of sulfide ions generated by sulfate-reducing bacteria. Precipitation of Pb and Zn sulfides took place in open fractures (disseminated occurrences) and in channelways (massive occurrences). Karsting, dolomitization and sulfide mineralization were terminated prior to the deposition of another Helikian sequence of clastics and carbonates which are only partially dolomitized and carry no Pb and Zn sulfides.

Excellent permeability, a proper drainage system for a refluxing brine and a continuous supply of evaporating seawater are the essential criteria that account for this Arctic Pb-Zn deposit. The simplicity of this pattern suggests that syngenetic dolomitization and sulfide mineralization are a common geological event and may be responsible for other Pb-Zn occurrences.