Abstract
The basement-Lamotte Formation unconformity in southeastern Missouri shows evidence of several distinctive alteration events, some of which may be related to migration of ore-forming fluids or K-rich basinal brines. The alterations are spatially related to the unconformity, suggesting that the unconformity was a preferred pathway for fluid migration. Unconformity-related rocks in five cores penetrating both the Lamotte Formation and the basement have been characterized by bulk chemistry, mineralogy, textures, and paleotopographic position. The alteration history along the unconformity includes: (1) Pre-Lamotte hydrothermal alteration affecting basement, (2) Pre-Lamotte weathering, and (3) Post-Lamotte hydrothermal and diagenetic events. The third group is particularly complex, including early diagenetic hematite and calcite cementation, later quartz cementation, secondary biotite growth, K-metasomatism that generated adulaira, and K-leaching that left a narrow dickite-rich zone along the unconformity. Sev...
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