EXPLORING THE TIMING AND ORIGIN OF THE MINERALIZED BRECCIA SYSTEM IN SILVER GULCH, SOUTHWESTERN COLORADO

There is a close spatial relationship between mineralized (Cu, W) breccia pipes, breccia dikes, and altered diorite dikes within the Dunmore fissure-vein system, ~8 km south of Ouray in Silver Gulch. The relationships, origin, and timing of these features were previously not well constrained. In this investigation, field analyses combined with petrographic and U-Pb zircon analyses were applied to constrain the geologic history of the western part of the Dunmore system. The information is critical to understanding the origin and evolution of features in this system, and for making correlations with other regional Cenozoic events.

Field studies establish the breccia features and related igneous dikes in the west Dunmore system formed after the ~30 Ma San Juan Formation. A 1:1800 scale field survey delineated a large mass of breccia with distinct zones of Cu-Fe or W mineralization. Several breccia dikes and sills extend from this zone into the Proterozoic Uncompahgre Group. The breccias are matrix to clast supported with angular to rounded, granule- to boulder-sized fragments composed mostly of quartz, quartzite, and fine-grained rock with quartz phenocrysts; the clasts are cemented with quartz + sericite ± pyrite.

One breccia sample yielded a population of zircon ages from 1400 to 25 Ma, but the youngest zircons are discordant. A deformed diorite dike, which cuts all other features, constrains magma emplacement from 27.5 to 28.5 Ma. These age constraints indicate the western Dunmore system formed between 27.5 and 30 Ma. The timing is similar to the emplacement of breccia dikes at Stony Mountain at ~27 Ma, but is not correlative with the formation of mineralized breccia pipes on Red Mountain at ~24 Ma.

The breccia dikes and chimneys in the west Dunmore system are interpreted as products of gas-charged explosive eruptions possibly related to magmas. The magmatic-hydrothermal volatiles followed fractures into high levels of the crust, where they expanded to create breccia zones. Rounded fragments and pieces of reworked breccia indicate multiple eruptive phases. Spatial proximity of the breccia and igneous features to the Dunmore system, along with age constraints, support the genetic relationship to Oligocene magmatism at ~27 Ma close in time to the formation of calderas in the western San Juan Mountains.