The ores at Darwin occur as massive replacement bodies in silicated limestones of Pennsylvanian and Permian age adjacent to a Jurassic quartz monzonite stock. Three types of ore have a definite spatial relationship to the quartz monzonite: (1) pyrite-sphalerite-galena ores, (2) pyrite-pyrrhotite-magnetite-sphalerite-galena ores, and (3) galena-Ag-Bi-Se ores.The delta ³⁴ S values of all sulfide minerals range from +4.4 to --5.7 permil. The delta ³⁴ S values for individual minerals tend to decrease with respect to both space and paragenetic time. The delta (sub sl-gn) values range from 1.5 to 2.5 permil and correspond to a temperature range of 325 degrees + or - 55 degrees C. The sphalerite-galena sulfur isotope temperatures at a given locality are reproducible to + or -30 degrees C and are consistent with temperatures determined by other means.Analyses of water in three samples of fluid inclusions in sphalerite indicate that the ore fluids had delta D values of --66 + or - permil, a total salinity that reached at least 20 percent, and K/Na atomic ratios of 0.23 + or - 0.03. Isotopic and thermochemical data indicate that the ore fluids had average values of delta ³⁴ S (sub Sigma S) = 3 permil, delta ¹³ C (sub Sigma O) = -- 3.5 permil, Sigma S = 0.01 mole/Kg H ₂O, f (sub CO ₂ ) = 12 + or - 8 bars, and Sigma C = 0.15 + or - 0.06 mole/Kg H ₂ O.The delta ¹⁸ O values of postsulfide calcites have a range of 12.8 + or - 1.5 permil in the pyrite ores and 17 + or - 2 permil in the pyrrhotite ores. Similar values for relict limestone host rock in these areas indicate that oxygen isotopic equilibrium was established between the calcite precipitating fluids and the wall rock. The delta ¹³ C values of the calcites, however, range from --5.8 to --3.6 permil and are considerably more negative than the limestone in the host rock, indicating that carbon isotopic equilibrium was not established between the fluids and the wall rock.Thermochemical data indicate that the pH of the ore fluids was about 4.8 as they traveled through the quartz monzonite at T nearly equal 350 degrees C. When the fluids completely equilibrated with the relict limestone and calc-silicate host rock, the pH increased to 6.7. This increase in pH as the fluids traveled upward and away from the quartz monzonite was responsible for the spatial distribution of iron sulfide assemblages, for variations in delta ³⁴ S values of sulfides, and probably caused precipitation of the ore.