Breccia-Hosted Lead—Zinc Deposits in Carbonate Rocks

Abstract

Although several deposit types have either been shown or suggested to be associated with paleokarst features in carbonate rocks, lead-zinc sulfide deposits of the "Mississippi Valley-type" (MVT) have received the most research and exploration. MVT deposits are not simply sulfide-filled open caves; rather, they are emplaced in carbonate brec-cias and consist of sulfides and carbonates, as either matrix or cement, filling spaces between angular carbonate fragments. Evidence of replacement is characteristically minimal, except in high-grade portions of deposits. Two main hypotheses have been proposed to account for the origin of the breccias. One school advocates solution collapse brought about by the incursion of meteoric water into the carbonate strata (i.e., meteoric karst) during a period of emergence. Emplacement of ore into the breccias is envisaged as a separate and later event. The other school proposes solution collapse induced by a slightly earlier phase of the ore solutions themselves (hydrothermal karst) with no meteoric involvement. Circumstantial evidence, such as the nearly universal presence of an overlying unconformity, the morphological similarity of the ore-bearing breccia zones to known paleokarst solution collapse breccias, the presence of solution-thinned strata below the breccia zones, and the relative paucity of wallrock alteration, has led to a majority opinion favoring the meteoric karst hypothesis.