THE NATURE AND ORIGIN OF PEBBLE DIKES AND ASSOCIATED ALTERATION: TINTIC MINING DISTRICT, UTAH
The pebble dikes cut an Eocene rhyolite lava flow as well as underlying folded Paleozoic sedimentary rocks. The dikes have a strong northeast trend and appear related to a monzonitic swarm of narrow dikes of the same orientation which are centered on a shallow intrusion of porphyritic biotite-hornblende-pyroxene monzonite. The main swarm is roughly 1 km by 2.5 km. Locally, pebble dikes grade into these igneous dikes and at least one igneous dike contains rounded quartzite pebbles similar to those seen in pebble dikes. Dikes are often enveloped by a halo of low-grade argillic and silicic alteration accompanied by the introduction of minor sulfides.
Our preliminary observations of the pebble dikes suggest they were emplaced during the violent expansion (boiling) of a superheated fluid as it escaped from depth to the surface, carrying brecciated rock debris, and in some cases magma, with it. Clasts that traveled the longest distance then exhibit the highest degree of rounding. Heat to drive pebble dike emplacement was likely provided by the introduction of the porphyritic monzonite to the subsurface. Future stable isotopic and fluid inclusion work will constrain the origin of the fluid, whether meteoric (shallow groundwater) or magmatic, as well as the association of pebble dikes to Tintic district mineralization.