2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 10
Presentation Time: 4:00 PM


THORSON, Jon P., Consulting Geologist, 5515 Nuthatch Road, Parker, CO 80134, jonthorson@rmi.net

In the Paradox Basin, epigenetic, post-lithification, deposition of copper sulfides in stratiform sediment-hosted deposits is the result of two sequential episodes of basinal fluid expulsion. The earlier fluid was a sulfur-bearing, reducing, aqueous fluid that bleached red sandstones as hematite was reduced to pyrite. This fluid rose buoyantly through the section along marginal breccia zones adjacent to salt diapirs, or along faults, and flowed laterally in permeable strata. Regional geology and burial history analyses suggest a late Jurassic age for the early fluid event as the black shale interbedded with the Paradox Fm. evaporites entered the oil window. The second episode of fluid expulsion was oxidized warm saline brines carrying copper, silver, and other base metals. At Lisbon Valley and the Cashin Mine minerals deposited by the second fluid are zoned from hematite ± native copper, through chalcocite, bornite, and chalcopyrite zones, to peripheral pyrite + lead and zinc. Copper sulfides are disseminated, partially or completely filling intergrannular spaces in sandstone bleached during alteration by the first fluid. Gangue minerals in feeder fault structures are dolomite and barite. Homogenization temperatures from fluid inclusions in these gangue minerals are between 70 and 110° C; salinities are variable but as high as 21% NaCl equivalent Fluid inclusions from bleached Jurassic sandstones and from the Cashin fault contain hydrocarbon droplets from a Pennsylvanian Paradox black shale source rocks. Paleomagnetic data and burial history analyses suggest a Late Cretaceous or early Tertiary age for the second episode of fluid expulsion as Cutler Formation arkosic red beds subsided into the oil window. The copper deposits at Lisbon Valley and the Cashin Mine are the result of both fluid expulsion episodes. The earlier fluid prepared otherwise unfavorable host rocks by reducing hematite to pyrite and depositing hydrocarbons. The second fluid deposited copper as sulfides that replaced pyrite, calcite, bituminous hydrocarbon and coaly plant material.