REDISCOVERING SOUTHEAST MISSOURI MISSISSIPPI VALLEY-TYPE PB-ZN DEPOSITS: THE COBALT-NICKEL ENRICHED HIGDON DEPOSIT, MADISON AND PERRY COUNTIES

The Higdon Mississippi Valley-type (MVT) Pb-Zn-Ni-Co deposit is located ~11.5 km NE of Fredericktown, MO in the Mine LaMotte-Fredericktown sub-district, one of several clusters of MVT deposits flanking the St. Francois basement uplift including the currently producing Viburnum Trend. The Higdon deposit was discovered in the 1950s and an attempt to mine the deposit was made in the 1960s. In the 1990s, extensive exploration by The Doe Run Co. revealed a larger deposit and greater Ni-Co content than initially estimated.

The Higdon deposit resembles other SEMO MVT deposits in most features such as the host stratigraphic section overlying Precambrian igneous basement, localization of Pb-Zn mineralization in grainstone beds and solution collapse breccias of the Bonneterre Fm. above pinch outs of Lamotte sandstone against basement highs, and general paragenetic sequence.

The Higdon deposit is unusual in that sulfides occur in the lower one-third of the Bonneterre Fm. and upper 50 ft of the Lamotte sandstone; a lower stratigraphic position than ore in the Viburnum Trend. Several faults aligned parallel to the NW-striking Simms Mountain and Mine Lamotte fault systems appear to have exerted unusual control on dissolution, collapse brecciation and mineralization at the base of the Bonneterre Fm. Faults of this trend also appear to bound the deposit, especially the high Ni-Co portion.

Petrographic and microprobe studies at Higdon determined a sulfide sequence of pyrite, marcasite, chalcopyrite, bravoite, siegenite, gerdorsffite, sphalerite and galena. The sulfides are fine-grained and disseminated in contrast to the more common open space filling and replacement textures of the Viburnum Trend. Finally, the Higdon deposit is anomalous in the tonnage and grade (0.17 % Ni, 0.14 % Co) of its Ni-Co resource with significant values contained in siegenite (30.9 % Ni, 23.9 % Co), bravoite (0.04 % Ni, 6.3 % Co), gersdorffite (26.1 % Ni, 8.0 % Co) and marcasite-pyrite (0.3 % Ni, 0.2 % Co).

Data for the Higdon deposit suggest Ni-Co fluids migrating through basement rocks along NW-trending faults may have mixed with connate Pb-Zn brines moving laterally through the Lamotte sandstone and other permeable units. Mafic intrusions located SE of the deposit in the New Madrid rift zone represent a possible source of Ni and Co.