The Sunnyside mine, in the Eureka mining district, San Juan County, Colorado, produces 700 tons/day of gold-silver-copper-lead-zinc-cadmium ore and is currently the State's leading gold producer (28,500 ounces in 1974). Production is from veins filling faults and fractures along the northern side of the Eureka graben, which formed during resurgent doming of the San Juan-Uncompahgre calderas about 28 m.y. ago. Mineralization in the Sunnyside system occurred between 13.0 and 16.6 m.y. ago. The Sunnyside mine workings extend through a vertical range of 610 meters, from 3,230 to 3,840 meters elevation, and extend laterally for approximately 2,100 meters.Six periods of mineralization have been recognized. The products of these, and their approximate volumetric abundances, are: (I) pyrite-quartz ores (5%); (II) banded quartz-sulfide ores (30%); (III) massive galena-sphalerite-chalcopyrite-bornite-hematite ores (40%); (IV) gold-telluride-quartz ores (1-2%); (V) manganese ores (20%); and (VI) quartz-fluorite-carbonate-sulfate ores (5%). Heating studies of fluid inclusions in quartz, fluorite, and rhodochrosite indicate that temperatures ranged between 250 degrees and 320 degrees C during mineralization for periods I-V and between 170 degrees and 245 degrees C for period VI. Samples collected at 3,800 meters elevation on the Spur vein show evidence that period VI fluids boiled at approximately 240 degrees C. These data together with the P-V-T data of dilute salt solutions indicate that P (sub H 2 O) was between 110 and 220 bars during periods I-V and about 35 bars for period VI. Freezing temperatures of fluid inclusions indicate a range from approximately 0 to 3.6 equiv. weight percent NaCl for the ore-forming fluids. The ore-forming fluids were largely K-Na-Ca solutions whose concentrations ranged from 0.03 to 0.27 weight percent K; 0.23 to 0.65 weight percent Na; and 0.03 to 0.26 weight percent Ca. Metal concentrations in inclusion fluids range from less than 10 to approximately 1,000 ppm for Cu, Fe, Zn, and Mn; these values agree well with values calculated from thermodynamic data. The dominant species in the fluid was H 2 O, which accounted for more than 0.99 mole fraction of fluids excluding dissolved salts. CO 2 was the only other gas species present in significant proportions. SO 2 2 as well as H 2 S was detected in period IV fluids. Period VI fluids contain numerous odd-mass hydrocarbons. The pH of periods I-V fluids ranges from 4.3 to 5.9 at a (sub K (super +) ) = 0.02 and appears to have been in equilibrium with the assemblage sericite + quartz. Total sulfur concentration is estimated to be 0.05 + or - 0.02 molal; f (sub O 2 ) and f (sub S 2 ) conditions estimated from the stability relationships of minerals together with gas data from inclusion fluids indicate that the dominant sulfur species in the ore-forming fluid were in the oxidized form (HSO 4 (super -) , KSO 4 (super -) , and NaSO 4 (super -) ) rather than in the reduced form. The delta D values of inclusion fluids from quartz, fluorite, sphalerite, and galena range from -96 to -135 per mil during periods I-VI. The delta 18 O values of water calculated from oxygen isotopic composition of quartz and carbonates ranges from -5.7 to -8.0 per mil for periods I-V and from -7.2 to -14.0 per mil for period VI. The delta 18 O values of whole rocks for wall rocks adjacent to a major vein range from +1.9 to -3.9 per mil. These delta D and delta 18 O data suggest that the hydrothermal fluids were predominately meteoric water which underwent moderate isotopic exchange with country rocks during periods I-V, but only slight exchange during period VI. The delta 13 C values of period VI carbonate minerals are -2.8 and -3.8 per mil for calcite and -6.3 to -7.9 per mil for rhodochrosite, indicating a limestone source for carbon in calcite while suggesting either a meteoric or magmatic source for carbon in rhodochrosite. Sulfur isotope values are 6.0 to +2.7 per mil for sulfides and +15.5 to +22.0 per mil for sulfates. These values together with consideration of the physicochemical conditions of the ore-forming fluids suggest that upper Paleozoic marine evaporites with delta 34 S = +12 per mil may have been the source for Sunnyside sulfur. Isotopic studies of lead and rubidium-strontium suggest that metals in the Sunnyside ore fluids were scavenged from both Tertiary volcanic rocks and from the 1.4 to 1.8 b.y. Precambrian basement. Geological and geochemical studies suggest a model for Sunnyside ore deposition in which a probable recharge area for the ore-forming fluids was located west and southwest of the San Juan-Silverton calderas. Fluids were channeled through and out of the caldera along fractures radiating to the northwest and the northeast. In-flowing fluids picked up carbon, salts, and sulfur by solution of Mesozoic and/or Paleozoic sedimentary rocks. As these dilute fluids became heated, metal-carrying capacity increased and metals were leached from wall rocks. At this same time, some SO 4 (super 2-) was reduced to H 2 S by reaction with the volcanic rocks. Fluids carrying both metals and reduced sulfur continued their northerly flow until they encountered the structures of the Eureka graben. Here the primary mechanism of precipitation was decreasing temperature.