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Geology and structural control of ore deposition in the Creede district, San Juan Mountains, Colorado

1965; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp487

2330-7102

Thomas A. Steven, James Clifford Ratte,

Geological and Geochemical Analysis

Sixty million dollars worth of silver, lead, zinc, and gold has been extracted from mines in the Creede district of Colorado.The principal source of this production has been from veins along faults that formed during long-continued subsidence con• current with late Tertiary volcanic activity in the central San Juan Mountains.Most subsidence was in a large volcanic caldera, 10-12 miles in diameter, which we have called the Creede caldera.The known mineralization was confined largely to a complex graben that extends north-northwest from the caldera.The rock units in the Creede area record a complex sequence of volcanic events, which can be divided with respect to recurrent subsidence.The oldest unit is the Outlet Tunnel Member of the La Garita Quartz Latite; the ash-flow eruptions responsible for this unit culminated in collapse along a series of faults roughly parallel to upper East Willow Creek, and along several northwest-trending normal faults.The rhyolite of Miners Creek, the Bachelor Mountain Rhyolite, the Shallow Creek Quartz Latite, and the Phoenix Park Member of the La Garita Quartz Latite were erupted, at least in part, onto the irregular surface formed by the earlier collapse.The rhyolite of Miners Creek is a local steep-sided accumulation of rhyolite flow and pyroclastic rocks.This is overlapped by the Bachelor Mountain Rhyolite-a thick mass of pumiceous pyroclastic material that grades from a densely welded rock having a fluidal texture in the lower part (Willow Creek Member), through a compact rock that is obviously a welded tuff (Campbell Mountain Member), to a non welded to slightly welded tuff at the top (Windy Gulch Member).The upper two members intertongue to the west with quartz latitic lava flows and breccias of Shallow Creek Quartz Latite.To the east, the Bachelor Mountain rocks intertongue with and are overlain by crystal-rich welded tuffs of the Phoenix Park Member of the La Garita Quartz Latite.Eruptions of the Bachelor Mountain Rhyolite culminated in collapse of the probable vent area and in displacement along a series of northwest-trending step faults.These step faults were the first to be active in the area of the present Creede graben.Succeeding eruptions deposited first a local accumulation of pumiceous tuffs and welded tuffs that constitutes the Farmers Creek Rhyolite.This activity was followed in turn by deposition of thick units of welded tuff comprising the Mammoth Mountain Rhyolite and Wason Park Rhyolite.Concurrent eruptions of dacitic lava and pyroclastic rocks to the southwest deposited the Huerto Formation; tongues of these rocks extend into the Creede district where they underlie, are intertongued with, and overlie the Mammoth Mountain and Wason Park Rhyolites.Indirect evidence suggests that the Creede caldera may have subsided concurrently with at least some of these volcanic eruptions.Crystal-rich quartz latitic ash was erupted later, during the time of caldera subsidence, to form the tuffs and welded tuffs of the Rat Creek Quartz Latite and the densely welded tuffs of the Nelson Mountain Quartz Latite.The last major eruptions during that time resulted in the accumulation of more •than 6,000 feet of crystal-rich welded tuff in the core of the Creede caldera to form the Snowshoe Mountain Quartz Latite.The caldera sank concurrently with accumulation of the Snowshoe Mountain Quartz Latite, and talus and landslide debris from the exposed walls intertongue with and overlie the pyroclastic caldera fill.Minor eruptions during caldera subsidence resulted in local volcanic necks and flows that cut and are intertongued with the more widespread units deposited then.Caldera subsidence was followed by pronounced doming of the caldera core due, presumably, to magmatic pressure.Local flows, domes, and pyroclastic breccias of Fisher Quartz Latite were then erupted along the margin of the caldera or along fissure zones extending outward from the caldera.Concurrently, lake and stream deposits, volcanic ash, and travertine from many mineral springs were deposited in the structural moat around the margin of the caldera to form the Creede Formation.The last major period of faulting followed deposition of the Creede Formation.Several of the faults in the Creede graben that formed during earlier periods of subsidence were active again at this time.The known veins in the Creede district are localized along faults that were active during the last major period of movement.The Alpha-Corsair fault, the Amethyst fault and related hanging-wall fractures, and the Solomon-Holy Moses fault were widely mineralized and have provided most of the ore mined in the Creede district.Some disseminated ore in the Creede Formation has been mined in the Monon Hill area.