Mafic and ultramafic plutons associated with the New Madrid rift complex; a possible major source of the copper-cobalt-nickel mineralization of Southeast Missouri

Abstract

The main rift structure of the New Madrid rift complex, the Reelfoot rift, strikes northeast-ward, extending from northeastern Arkansas through the Missouri and western Kentucky into southern Illinois. The recently discovered Bloomfield lineament zone transects the Reelfoot rift in the boot heel area and strikes north-northwest, passing through the Mine La Motte-Fredericktown subdistrict of the Southeast Missouri lead-zinc-copper mining district.Studies suggest a strong relationship between the proximity of the New Madrid rift complex to the Southeast Missouri district and the anomalously high amounts of Cu, Co, and Ni in the region--particularly in the Mine La Motte-Fredericktown subdistrict. Supporting evidence comes from three sources: (1) mineral zoning and paragenetic sequence, (2) geotectonics of the New Madrid rift complex, and (3) regional variations of 87 Sr/ 86 Sr and 18 O/ 16 O isotope ratios.A detailed microscopic and SEM-EDS study of the Pb-Zn-Cu ores from the Viburnum No. 28, Brushy Creek, and Goose Creek mines in the Southeast Missouri district indicates a complicated paragenetic sequence characterized by repeated periods of mineral deposition. The sequence is composed of 19 minerals including abundant galena, sphalerite, chalcopyritc, pyrite, marcasite, dolomite, calcite, and quartz as well as minor siegenite, bornite, bravoite, blaubleibender covellite, covellite, gersdorffite, digenite, enargite, tennantite, dickite, and malachite. In addition to the mineralogy and paragenetic sequence, a number of significant findings resulted from a comprehensive microscopic study of ore textures from the three mines.The paragenetic sequence of the main colloform stage of deposition indicates that the ore solutions evolved in the following general order: (1) Cu rich, (2) Co-Ni rich, (3) Pb-Zn rich, and (4) Fe rich. The general depositional sequence corresponds with mineral zoning and is similar throughout the entire lead district. Most importantly, the paragenetically early Cu-Co-Ni sulfides may represent a regional early development of the Cu-Co-Ni-rich mineralization similar to that in the Mine La Motte-Fredericktown subdistrict and the southern extension of the Viburnum Trend. This early Cu-Co-Ni-rich zone always lies at the lowest stratigraphic level and, in the Mine La Motte-Fredericktown subdistrict, indicates a southerly source for the ore fluids.It is proposed that the source of at least part of the Cu, Co, Ni, and some siderophile elements of the Southeast Missouri lead deposits may be the result of basinal brine leaching of a series of probably multiaged, mafic and ultramafic intrusions or their sediments controlled by the New Madrid rift complex. One such branch of the complex, observed on Landsat-2 imagery, is expressed as a fracture zone that extends north-northwest from the Reelfoot rift toward Fredericktown, Missouri, namely the Bloomfield lineament zone. Strong 87 Sr/ 86 Sr and 18 O/ 16 O isotope anomalies show, respectively, a consistent decrease and a consistent increase from south to north in southeast Missouri with the contour lines strikingly tracking the Bloomfield fracture zone. This suggests that the lineament zone was a channel for ore fluids entering southeast Missouri via the Mississippi embayment. The fracture zone may represent a series of transform faults or a portion of a failed continental rifting event. The zone is an epicenter trend and a distinct first-order lineament which appears to form part of the northeast boundary of the northwest-trending Missouri gravity low.