REVISITING THE SUFIDE PIT BRECCIA AT DECATURVILLE IMPACT STRUCTURE, MISSOURI: IMPLICATIONS FOR THE AGE OF MINERALIZATION

Re-examination of exposures in the central uplift of the Decaturville impact structure provides evidence for two stages of mineralization, indicating both pre-impact and post-impact ages. Decaturville structure is widely accepted as having an impact origin; shatter cones and planar deformational features (PDFs) have been described by previous investigators. The structure is approximately 6 km in diameter. The central uplift area and adjacent overturned strata are 3.5 km across. Middle Cambrian through Lower Ordovician megabreccia blocks comprise the bulk of material in the central uplift area. A block of pegmatitic granite was emplaced near the center. Regionally, the depth to crystalline basement is approximately 300 m, providing a minimum constraint on the amount of uplift. The sulfide pit constitutes a small area of approximately 20 x 120 m in the western part of the central uplift.

Isolated Precambrian, Cambrian, Middle and Upper Ordovician, and Middle Silurian blocks have been reported in the moat as well, but the country rock of this area is primarily folded and faulted parautochthonous Lower Ordovician strata. Although the structure preserves remarkable crater morphology, the paucity of crater-fill material suggests that the structure was deeply eroded after impact. No overlying undeformed strata have been identified, and as a consequence, the age of impact is poorly constrained stratigraphically.

The argument for a late Pennsylvanian or early Permian age rests on the mineralization of the sulfide pit and paleomagnetic studies. Some discrete pyrite crystals appear to have been brecciated, indicating a phase of mineralization prior to impact. Most of the sulfides, however, including galena and secondary emplacement of pyrite, post-dated the impact. Pyrite coats on clasts and mineralized fronts in the sulfide breccia suggests the second phase of mineralization was more pervasive. This poses the problem: which phase of breccia is associated with MVT-mineralization? Based on the inclusion of Middle Ordovician through Middle Silurian strata, which largely were removed by erosion in this region during early Mississippian time, the impact is likely older than recognized previously.