LITHOLOGY AND ALTERATION ASSEMBLAGES OF THE BOSS IRON-COPPER DEPOSIT, IRON AND DENT COUNTIES, SOUTHEAST MISSOURI

Igneous host rocks for the West Dome of the Boss iron-copper deposit are dominantly trachytes that intersect rhyolites. There are also minor diorites, chlorite schists, and undifferentiated intermediate rocks. All exhibit alteration to varying degrees.

Rhyolites are the earliest lithology, and are associated with a proposed collapsed caldera complex. Trachytes represent a shallow intrusive that post-dates the rhyolites. Both lithologies exhibit potassium enrichments and secondary quartz emplacement related to a potassic alteration assemblage. A Rb-Sr isotope date of 1.136 Ga „b 0.170 Ga may represent the potassic alteration event. Granitic dikes post-date rhyolites and trachytes. The dikes lack potassium enrichment, suggesting that they were in equilibrium with, or post-dated, potassium alteration.

Propylitic, phyllic and hematitic assemblages altered rhyolites, trachytes and granites. Assay values for propylitized core suggest that sulfur associated with this alteration assemblage resulted in additional chalcopyrite mineralization. The phyllic alteration assemblage is one of the most extensive present at West Dome. The extent is unusual in the province. Hematitic alteration may be related to the previously noted assemblages. The assemblages suggest that the Boss deposit formed at shallow depths.

Comparisons of West Dome to other deposits in the Southeast Missouri Iron Metallogenic Province and in the Gawler Province, South Australia, as well as to criteria set forth for Olympic Dam-type deposits, clearly places Boss West Dome in this family of iron-copper-gold deposits.