Stromatolitic carbonate in moat-fill of a 1.48 Ga caldera was converted to mineralized calc-silicate skarn by interaction with magma-derived brine at Ketcherside Gap in southeastern Missouri. The skarn records early low fO2 conditions similar to those of reduced W and Au skarns. Initial skarn-forming conditions were: PL ⩽ 221 bar (22.1 MPa), XCO2 ⩽ 0.10, T = 450–400°C, log fO2 ≈ −30, and log fS2 ⩽ −13. Late skarn records combined effects of T↓, fO2↑, fS2↑, XCO2↓, and appearance of an immiscible CO2-rich vapour. The absence of a contact aureole indicates that skarn reactions were driven by advective heat transfer from an infiltrating fluid. Elsewhere in the region, hypersaline, synvolcanic fluids produced oxidized endoskarn in rhyolite. Development of carbonate-hosted, reduced skarn in caldera moat sediments is attributed to: (1) the nature of the host rock and reaction along a lengthy flow path; (2) early, but temporary, dilution of brine influx by CO2-producing reactions; and (3) cooling, possibly accompanied by increased brine influx with time.