Michigan Basin derived mississippi valley-type CaCO (sub 3) and sulfide mineralization within fractured Ordovician bedrock in Kane County, NE Illinois USA

Tens-of-centimeter-scale euhedral calcite crystals growing within bedrock fractures exhibit concentric zoning of sulfide mineralization and cathodoluminescence (CL) that reflect temporal changes in the timing and chemical composition of fluids that were flowing through the fractures. On the southwest flank of the Michigan Basin in the North Aurora Mine (Kane County, NE Illinois), dissolution along vertical north-west trending fractures has created meter-scale cavities in the extensively dolomitized Wise Lake Formation limestone of the Ordovician Galena Group. Calcite crystals precipitated within the cavities exhibit a complex paragenetic sequence composed of multiple events of calcite and sulfide mineral precipitation. The calcite exhibits at least three distinct CL defined cm-scale layers of cementation interlayered with the sulfides. These CL and sulfide zonations, in conjunction with changes in preliminary 87Sr/86Sr, suggest changes in fluid chemistry and source as the subsurface waters migrated through the fracture system. Preliminary strontium isotope data indicates that the youngest, outermost thin zone of calcite precipitation has 87Sr/86Sr ratios (0.70865-.070914±0.00002, standard SRM976) are consistent with those of Lower to Middle Ordovician seawater. The other zones of calcite cementation contain significantly more radiogenic 87Sr/86Sr ratios (0.709884-0.710236±0.00002). The geographic location and orientation of the fractures suggest that the fluids were sourced from the Michigan Basin and may represent an Albion Scipio type of diagenetic history. However, close proximity to the Upper Mississippi Valley mining district does not preclude an Illinois Basin fluid source. The change in radiogenic 87Sr/86Sr ratios in successive layers of calcite cementation suggests a change in the subsurface flow path and possibly source of the diagenetic waters.