Ok. The site is north-east to the sea volcano visible on the world map (in the center), in the (accessible) mountain valley that holds another volcano. Use the elevation map and you will spot it easily.
I used modded entity file, but it shouldn't affect terrain generation i think...
Created in DF v0.34.11.
[WORLD_GEN]
[TITLE:LARGE MOD]
[SEED:dvoGkuy2Pwc7sDpcuy3J]
[HISTORY_SEED:Ltd6m4jOd8z1Gk6JfHtW]
[NAME_SEED:6m5EdLH894Ei3YKi3TrB]
[CREATURE_SEED:BpX7lnZArCyISexQ8zs9]
[DIM:257:257]
[EMBARK_POINTS:1324]
[END_YEAR:300]
[BEAST_END_YEAR:100:50]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:1600:1600]
[RAINFALL:0:100:800:800]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:0:100:800:800]
[VOLCANISM:0:100:800:800]
[SAVAGERY:0:100:1600:1600]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:4:2:2:1:1:1]
[DRAINAGE_FREQUENCY:1:2:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:4:1:1:1:1:3]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[MINERAL_SCARCITY:1000]
[MEGABEAST_CAP:150]
[SEMIMEGABEAST_CAP:150]
[TITAN_NUMBER:40]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:52]
[NIGHT_TROLL_NUMBER:26]
[BOGEYMAN_NUMBER:26]
[VAMPIRE_NUMBER:26]
[WEREBEAST_NUMBER:26]
[SECRET_NUMBER:52]
[REGIONAL_INTERACTION_NUMBER:52]
[DISTURBANCE_INTERACTION_NUMBER:52]
[EVIL_CLOUD_NUMBER:26]
[EVIL_RAIN_NUMBER:26]
[GOOD_SQ_COUNTS:100:1000:2000]
[EVIL_SQ_COUNTS:100:1000:2000]
[PEAK_NUMBER_MIN:50]
[PARTIAL_OCEAN_EDGE_MIN:2]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:15]
[REGION_COUNTS:SWAMP:1032:7:6]
[REGION_COUNTS:DESERT:1032:7:6]
[REGION_COUNTS:FOREST:4128:13:12]
[REGION_COUNTS:MOUNTAINS:8256:9:9]
[REGION_COUNTS:OCEAN:8256:7:6]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:8256:13:12]
[REGION_COUNTS:HILLS:8256:13:12]
[EROSION_CYCLE_COUNT:200]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:15]
[LEVELS_ABOVE_LAYER_2:10]
[LEVELS_ABOVE_LAYER_3:10]
[LEVELS_ABOVE_LAYER_4:10]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:18]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:1500]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:8256:16512:8256]
[RAIN_RANGES:8256:16512:8256]
[DRAINAGE_RANGES:8256:16512:8256]
[SAVAGERY_RANGES:8256:16512:8256]
[VOLCANISM_RANGES:8256:16512:8256]