Alteration history of volcaniclastic sediments in the upper Oligocene Creede Formation, southwestern Colorado

The mineralogy, chemistry, and mineral textures of tuffs and tuffaceous sediments from two drill cores, CCM-1 and CCM-2, were studied to better understand depositional and diagenetic conditions of the Creede Formation of Colorado. Similarities between phenocrystic crystal assemblages and relict shard shapes and sizes in correlated tuffs indicate minimal offset between CCM-1 and CCM-2. Vertically zoned sequences of authigenic silicates in tuffs reflect alteration of silicic-intermediate vitric tuffs within an elevated geothermal gradient (100-110 °C/km). The abundance of clinoptilolite and Al-smectite in drill core CCM-2 is consistent with diagenetic waters having low to moderate salinity and a pH of 7-9. The higher-grade mineral assemblage, including analcime, quartz, minor heulandite, and albite, in CCM-1 reflects hotter fluids migrating through more permeable strata. The presence of kaolinite and mixedlayer clays in nontuff lithologies in CCM-1 indicates intermittently low cation/H+ ratios and SiO_2(aq) activities. Alteration occurred in a relatively open chemical system based on calculated gains and losses from silicic to intermediate end-member compositions of Fisher Quartz Latite compared to Creede Formation tuffs. On the basis of comparisons with studies of zeolite formation in other burial diagenetic systems, the observed authigenic silicate assemblage (opal-CT, quartz, clinoptilolite, and analcime) in tuffs of the Creede Formation probably formed within ∼3-4 m.y. of deposition. This assemblage was later overprinted by a hydrothermal event (K-feldspar, I/S, C/S, chlorite, and quartz) in CCM-1 at 17.5 Ma. Authigenic silicate assemblages in tuffs poorly constrain the salinity of diagenetic pore waters because they could form in waters ranging from fresh to moderately saline with a pH ≤ 9. The inferred low to moderate salinities for silicate alteration and relatively open chemical system suggest that salinities were progressively lowered from moderate to high during deposition of ancient Lake Creede to lower salinities during silicate reactions associated with later burial diagenesis and hydrothermal alteration.