Tops and Bottoms of Porphyry Copper Deposits<subtitle>The Bingham and Southwest Tintic Districts, Utah</subtitle>

Abstract

A comprehensive look at the closely spaced blast hole samples, exploration drill data, and a resource model interpolated from diamond exploration drilling data show that the distinct molar tooth shape of the Bingham Canyon orebody has strong lithologic controls at depth. At and below the level of current mining Cu occurs in the Bingham stock as a ring approximately 4000 feet in diameter, centered on a barren core located at the eastern end of the quartz monzonite porphyry (QMP). A smaller ring of Cu (about 2500 feet in diameter) is centered on the southwest end of the QMP. The mineralized cylinders are broken where chemical and physical differences between the mineralized intrusive rocks and the barren quartzites on the south and west sides of the Bingham stock cause gaps in the regular Cu distribution, leading to the distinctive root zones of the Bingham porphyry system. These root zones are artifacts of favorable host rock and not products of multiple mineralization centers or structural control. Blasthole and diamond drill hole assay data show broadly continuous grade through monzonite (MZ), QMP and latite porphyry (LP) dikes, though locally grade changes occur across LP contacts. A drop in grade within quartz latite porphyry (QLP) dikes is common. The overall consistency of grade across intrusive contacts including the LP suggests that the main pulse of mineralization, associated with the QMP, spanned the period of subsequent LP intrusion. Mineralization had notably waned by the time of QLP emplacement, leading to lower Cu grades in the QLP throughout the system.