THE NATURE AND ORIGIN OF PEBBLE DIKES AND ASSOCIATED ALTERATION: TINTIC MINING DISTRICT, UTAH

Pebble dikes, which are tabular bodies of breccia dominated by rounded fragments of multiple rock types enclosed in a fine-grained clastic matrix, are common in many productive mining districts that are closely associated with igneous systems. In the Tintic mining district (Pb-Zn-Ag-Cu-Au), Utah, pebble dikes consist of subangular to rounded fragments of quartzite, shale, carbonate, and two types of igneous rock. All fragments appear to have been derived from underlying units. Dikes are thin, typically less than 0.3 m wide to as much as 1 m. Some dike segments may exceed 100 m in length. The average of the 10 largest clasts is typically less than 4 cm and correlates positively with dike width. Contacts are sharp and an envelope of fine breccia surrounds roughly half of the dikes. Fragments from immediate wall rock that have been incorporated into the dikes are angular. Fracture systems parallel and splay out above and below ‘blades’ of pebble dike.

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.