GEOCHEMICAL SIGNATURES OF SULFIDES FROM IOA-IOCG DEPOSITS IN SE MISSOURI, USA: RELATION TO MAFIC TO INTERMEDIATE INTRUSIONS, FELSIC INTRUSIONS, AND CALDERA LAKES

To distinguish hydrothermal events and clarify the source of metals in four IOA deposits and the Boss IOCG deposit, the paragenesis, texture, and chemical composition of pyrite and chalcopyrite were determined by EMP and LAICPMS. Trace element abundances were compared to previous sulfur isotope results.

Pyrite from the IOA deposits and the Boss IOCG deposit is enriched in Co, Ni, Se, and As ± Te as well as Bi-Pb-Cu-Ag-bearing mineral inclusions. Pyrite intergrown with primary magnetite in IOA deposits is enriched in Ni and locally has rims that are enriched in Co and As. Pyrite intergrown with secondary hematite-chlorite ± carbonate in veins and breccia cements is also enriched in Co and As. At Boss, pyrite in West Dome magnetite-sulfide ores, composed of chalcopyrite, carrollite ± sphalerite and galena, are highly enriched in Co and Se. In contrast, pyrite in magnetite-sulfide ores in Central Dome, which has lesser chalcopyrite ± hematite, has trace element enrichments that are similar to IOA pyrite associated with secondary hematite.

Chalcopyrite in IOA deposits and the Boss IOCG deposit is enriched in Se, In, Zn, and Sn ± Ag, Bi, Pb, Te, and Sb. In IOA deposits, chalcopyrite in magnetite ores is more enriched in Zn ± Sn and Ag than chalcopyrite in secondary hematite-chlorite in veins and breccia cements. At Boss, West Dome chalcopyrite with pyrite or bornite is highly enriched in Se whereas Central Dome chalcopyrite is enriched in Sn and Ag.

The sulfur isotope composition of pyrite ± chalcopyrite from IOA deposits (excluding REE-breccia pipes from Pea Ridge) and the Boss IOCG deposit has two distinct modes at 3 and 13 ‰. The isotopically lighter sulfur occurs in pyrite and chalcopyrite from primary magnetite ores in IOA deposits and West Dome magnetite-sulfide ores, whereas the isotopically heavier sulfur occurs in pyrite and chalcopyrite from secondary hematite-chlorite veins and breccia cements and Central Dome magnetite-sulfide ores.

Together, the trace element variations and sulfur isotope values indicate that sulfide minerals in magnetite iron ore in IOA deposits and magnetite-copper ore at West Dome are from mafic to intermediate composition intrusions. Sulfides associated with secondary hematite in IOA deposits and magnetite-copper ore at Central Dome were likely derived from caldera lakes and/or felsic intrusions.