INSIGHT INTO THE COMPOSITION, ORIGIN, AND AGE OF CLASTIC DIKES IN OURAY COUNTY, COLORADO

Clastic dikes cut Paleozoic to Cenozoic strata between Placerville and Ouray. Trends and dimensions, clast populations, and U-Pb detrital zircon age analyses were employed to test hypotheses on the mechanisms and timing of emplacement of these clastic dikes.

Within the study area, clastic dikes trend 250˚ to 320˚ with vertical to near vertical dips. The dikes are up to 5 m thick and have strike lengths up to a kilometer. The dikes are clast to matrix supported and contain angular to rounded fragments up to 35 cm in dimension. Proterozoic basement rocks dominate clast populations in dikes near Ouray; fragments of Mesozoic sedimentary rocks and ~66 Ma granodiorite make up lesser proportions. Clastic dikes at Stony Mountain contain pieces of Proterozoic basement, underlying sedimentary units, and gabbro from the 27 Ma Stony Mountain stock. All the dikes preserve different degrees of chlorite + epidote alteration, and most contain secondary calcite and minor Cu mineralization.

Zircon populations from dike samples reveal the highest proportion of ages from 1800 to 1300 Ma, smaller and dispersed populations from 1200 to 25 Ma, and a small number of ~2500 Ma zircons. A dike exposed near Ouray yielded a minimum age of ~64 Ma similar to ages of adjacent granodiorite plutons. Field relations show that these dikes are older than the ~30 Ma San Juan Formation constraining emplacement from 64 to 30 Ma. The dike exposed at Stony Mountain contains ~25 Ma zircons, and is cut by a 12 Ma rhyolite pluton, which constrains timing of emplacement from the Oligocene to Miocene.

The results of this study reveal that the clastic dikes in the Ouray area formed thousands of meters below the paleosurface from 64 to 25 Ma where Proterozoic basement fragments were entrained. Clast populations and features in the clastic dikes are consistent with explosive sub-surface gaseous eruptions that were emplaced along pre-existing fractures. Zircon population ages along with the close proximity with igneous masses, and field evidence for the transition from magmatic dikes to fragmented magmatic and clastic dikes, adds support for a heritage involving Late Mesozoic to Cenozoic magmas.