In many ore deposits throughout the world, brecciation often accompanies or occurs in association with mineralization (Sillitoe, 1985). Such is the case in the Tintic Mining District (Ag-Pb-Zn) of north-central Utah, where unique breccia features called pebble dikes occur alongside significant mineralization. Pebble dikes are tabular bodies of breccia, which consist of angular to rounded clasts of quartzite, shale, carbonate, and minor igneous rock cemented in a fine-grained clastic matrix. All clasts now lie above or adjacent to corresponding source rocks. Dikes are thin, typically less than 0.3 m wide to as much as 1 m, and can exceed 100 m in length. The average of the largest clast sizes is less than 3 cm but correlates positively with pebble dike width. Contacts are sharp and an envelope of fine breccia surrounds roughly half of the dikes. Pebble dikes are mostly hosted in an Eocene rhyolite lava flow, which displays argillic to silicic alteration when in contact with a pebble dike, but are also hosted in an assortment of folded Paleozoic sedimentary rocks. The dikes show a strong northeast trend in orientation, following a regional fabric of northeast-trending strike-slip and oblique-slip faults.The formation of pebble dikes has been historically attributed to the intrusion of the Silver City Stock, the Tintic District's main productive intrusion (Morris and Lovering, 1979; Hildreth and Hannah, 1996; Kim, 1997; Krahulec and Briggs, 2006). However, pebble dikes are spatially associated with a previously unrecognized porphyritic unit, informally named the porphyry of North Lily, which is texturally, mineralogically, and chemically distinct from the Silver City Stock, and like pebble dikes, is emplaced in northeast-trending plugs and dikes. Pebble dikes show a strong spatial correlation to outcrops of the porphyry of North Lily. Additionally, clasts of the porphyry of North Lily have been found in pebble dikes, while pebble dike quartzite clasts have been found as xenoliths in the porphyry of North Lily. These similarities and interactions suggest simultaneous formation. Low-grade alteration associated with pebble dikes indicates that they formed at elevated temperatures (<150°C). Stable isotope characteristics of rhyolite altered during the emplacement of pebble dikes suggests that the dikes formed in the presence of heated groundwater, with little to no magmatic water association. The overall physical, spatial, and chemical characteristics of pebble dikes of the Tintic Mining District suggest that they formed by the mobilization of breccia in the explosive escape of groundwater that had been heated by the porphyry of North Lily. This escape occurred along pre-existing northeast-trending faults and fractures. Pebble dikes then became pathways for later ore fluids, easing the creation of the district's abundant mineral resources.

The mineral resources of the Tintic Mining District are influenced by three major events in its geologic history; the Mesozoic Sevier Orogeny, Paleogene volcanism and Late Neogene Basin and Range extension. In this paper a detailed analysis of each these geologic events is presented to help us understand the structural host, mineralization and exhumation of the Tintic Mining District ore. A kinematic analysis of the faults was completed to determine the origin of NE-trending faults, Sevier Orogeny or Basin and Range extension, in the northern part of the East Tintic Mountains in Allens Ranch 7.5' quadrangle, near the eastern margin of the Great Basin of central Utah. The structural history of the NE-trending faults found in the quadrangle was reconstructed to determine stress directions and fault kinematics. Maximum paleostress direction for the East Tintic fold and thrust system is between 80º–100º with fold axes oriented at ~350º. For example, the Gardison Ridge and Tintic Prince faults are NE-trending right-lateral transverse faults that formed at ~30º to paleostress directions similar to those of the Sevier Orogeny. The dominant NE-trending faults in the region are likely due to (1) differential shortening during progressive orocline development, (2) the pre-deformational Pennsylvanian-Permian Oquirrh basin geometry, and (3) the influence of the Leamington transverse zones of the Provo salient. Conversely, mixed paleostress directions for the north-trending Tintic Davis Canyon fault show it is a Basin and Range extension-related normal fault that may have originated as a Sevier related fault. Other N-trending faults within the quadrangle are only related to Basin and Range extension. However, large offset, range-bounding faults are buried by valley fill throughout the quadrangle and no young fault scarps are identified cutting Lake Bonneville deposits. An Oligocene to Miocene suite of extrusive volcanic units in the quadrangle correlates well with those of the East Tintic and Soldiers Pass volcanic fields. The Paleogene volcanic section is dominated by a suite of high-K calc-alkaline extrusive rocks (35 to 32 Ma). This intermediate to silicic sequence was followed by eruption of the mildly alkaline Mosida Basalt during the Miocene (19.5 Ma) marking the transition from subduction-related intermediate and silicic volcanism to extension-related mafic volcanism in the eastern Great Basin.