The timing, composition, and source of subsurface diagenetic waters responsible for sulfide and carbonate mineralization in solution cavities of the ordovician galena group limestone, North Aurora, Illinois, USA

Geologists have long recognized the important role of continental-scale fluid flow as a mechanism for diagenesis, including the dissolution of rock and the deposition of Mississippi Valley-Type (MVT) minerals in sedimentary basins. Tens-of-centimeter scale calcite crystals concentrically zoned with sulfide minerals and growing within meter-scale solution cavities were recently discovered on the flank of the Michigan Basin in the North Aurora mine (Kane County, NE Illinois). These mineral deposits are a reflection of the temporal changes in chemical composition and the source of paleo-fluids migrating through the midcontinent. Meter-scale solution cavities were created by dissolution along northwest-southeast trending fractures in the Wise Lake Formation limestone of the Ordovician Galena Group. On the basis of plane-light and cathodoluminescent petrography and scanning electron microscopy, a series of repeating diagenetic events are identified within the cavities consisting of dissolution followed by marcasite, pyrite, and calcite precipitation. The repetition of these diagenetic events and the paragenetic ordering of mineralization, in conjunction with changes in isotopic (87Sr/86Sr) and elemental concentrations (Ca, Na, Sr, Fe, Mg, and Mn) in calcite growth zones, suggest changes in the fluid chemistry and the source of the subsurface diagenetic waters that were responsible for dissolution and mineralization. The oldest, innermost calcite cement zone within the cavity has a chemical composition (87Sr/86Sr 0.71015±0.00002 standard SRM976, high Mg and high Fe) that best reflects deep basinal brine that may have previously interacted with K-feldspar rich rocks such a those near the Cambrian-Precambrian unconformity. Later, and younger calcite cements have successively lower isotopic and elemental values (87Sr/86Sr 0.70980-0.70898±0.00002, low Mg, low Fe), suggesting a weakening of deep basinal brine upwelling and mixing with formation waters. Comparison of the paragenesis of North Aurora with those from other regional MVT deposits indicates a related succession of diagenetic events, thus supporting possible involvement of a similar diagenetic fluid during dissolution and mineralization. However, the paragenesis at North Aurora most resembles the paragenesis of the Upper Mississippi Valley Zn-Pb district deposits, which are thought to be sourced from fluids expelled out of the Illinois Basin. Conversely, the geographic location, structural boundaries, and similar paragenesis to MVT deposits in the Michigan basin cannot rule out a Michigan or Appalachian Basin fluid source. The results of this study have important implications with respect to MVT mineral deposits and factors controlling cavernous porosity development. This study provides a means for predicting regional subsurface paleo-fluid flow and thus developing a more accurate model for economic mineral and hydrocarbon exploration