MINERALOGY AND STABLE ISOTOPE CHEMISTRY OF THE CARBONATE ZONE AT THE FLAMBEAU COPPER MINE, LADYSMITH, RUSK COUNTY, WISCONSIN

The Flambeau copper deposit is a 1.8-1.9 Ga exhalative massive sulfide deposit in central Wisconsin. It was subjected to metamorphism of greenschist to amphibolite grade during the Penokean Orogeny, and later extensive oxidation and supergene enrichment before the deposition of Cambrian sandstone. During the mining operation from 1993 to 1997, a unique stratiform carbonate zone was found interlayered amid the metavolcanic rocks and supergene enriched massive sulfides. The carbonate mainly consists of vuggy dolomite, siderite and ankerite with numerous amounts of hematite, and microcrystals of a variety of sulfide minerals such as bornite, chalcopyrite, siderite, galena, marcasite, and gypsum. Its textural features include stalactitic and dendritic habits with abundant cavities, suggestive of formation by open space filling at low temperature and pressure. This implies that the carbonate zone must have formed after the Penokean metamorphic event, but before the incursion of the Cambrian sea. Some hypotheses for formation include a possible Precambrian hydrothermal event or a bacterial redeposition of a primary carbonate or sulfate deposit. Samples from the carbonate zone have been submitted for oxygen, sulfur and carbon stable isotope analysis, in an attempt to understand its conditions of formation. δ¹³C _VPDB standards on stalactitic, matrix and vein dolomite range from -4.53 to -6.84. δ¹⁸O _VPDB on the same material range from -5.83 to -6.54. Sulfide isotope data was not yet available at the time of abstract submission.