Abstract

Polyphase deformation and magmatism in the East Tintic mining district of central Utah formed highly permeable damage-zone conduits for water circulation and mineralization, yet clay-rich and re-cemented fault core zones form impermeable barriers that compartmentalize and isolate local ground-water systems at scales of a few kilometers. The effects of the Eureka Lilly fault, a major structure in the area, are most pronounced. It separates two distinct ground waters with a vertical separation of 50 m that have different temperatures and compositions, indicating little to no communication across the fault. Damage zones parallel to the fault form very permeable networks of interconnected and open fractures that discharge 11,000 to 34,000 L/min in otherwise impermeable units. Fracture density measurements show that the damage zone is discontinuous with localized fracture networks separated by mostly coherent blocks.

Three distinct types of ground water are found near the Eureka Lilly fault: (1) strongly thermal (>54°C), Na⁺- and Cl⁻-rich ground water with high total dissolved solids (TDS) at <1385 m elevation in the footwall of the fault; (2) modestly thermal (<27°C), Mg²⁺- and SO^2-₄-rich ground water with intermediate TDS at >1434 m elevation in the hanging wall of the fault; and (3) cold, low-TDS, shallow ground water that extends only 100 m below the surface, leaving a >243 m dry zone between it and waters 1 and 2. These observations bring into question simple basin-flow models involving ground water moving easily through bedrock and across faults between basins.