STEFANI, ANN B., and ISABEL P. MONTANEZ, Department of Earth Sciences, University of California, Riverside, CA

ABSTRACT: Association of Secondary Porosity Evolution and Dolomitization in Mineralized Cambro-Ordovician Knox Group, TN

Carbonates of the Cambro-Ordovician Knox Group host Mississippi Valley-type Pb-Zn ores in the southern Appalachian mining districts. Mineralization is associated with porous zones, which commonly occur as porous coarse crystalline dolomites that may be brecciated or fractured. It is postulated that mineralization was associated with extensive dissolution of limestone host, and that early dolomite host rocks were relatively unaltered by late diagenetic fluids. These diagenetic fluids presumably were undersaturated with respect to calcite, and became dolomitizing via fluid-rock interaction processes. Porous dolomite zones associated with limestone intervals are the result of limestone (dissolution followed by occlusion of porosity by burial dolomites.

This integrated study suggests that secondary porosity development in Knox carbonates is genetically related to the earliest stage of burial dolomitization. The spatial distribution of porous dolomite zones, however, is ultimately related to the distribution of early diagenetic facies. Intervals of nonporous dolomites formed in mud-rich regressive facies during third-order sea level falls. These syndepositional dolomites underwent little porosity modification with burial. In contrast, more heterogeneous facies of transgressive cycles were partially dolomitized or remained limestone early in their burial history. Partly dolomitized facies apparently had high initial permeabilities and acted as fluid conduits for later diagenetic fluids. Extensive fluid-rock interaction of dolomites in ransgressive cycles produced considerable porosity. Porous dolomites exhibit a complex paragenetic history including multiple generations of dolomitization, dissolution, and silicification. Limestones proximal to `dolomite conduits' are partially dolomitized, and have minor to moderate amounts of porosity closely associated with burial dolomites. Limestones, separated from porous dolomites, however, are tight and contain well preserved allochems and marine to shallow burial nonferroan calcite cements. Minor amounts of porosity are associated with fractures developed in these limestones.

Secondary porosity development in Knox carbonates is closely associated with migration of deep burial dolomitizing fluids through conduits developed during early diagenesis, rather than associated with wholesale dissolution of limestone host. Subsequent stages of burial dolomites progressively occluded secondary porosity; present-day burial dolomites range from nonporous to vuggy dolomites.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.