CONSTRAINTS ON THE AGE AND DEPTH OF CARLIN-TYPE GOLD MINERALIZATION IN THE FOOTWALL OF A METAMORPHIC CORE COMPLEX: PEQUOP MOUNTAINS, NEVADA

The Pequop Mountains of NE Nevada contain the Long Canyon and West Pequop Carlin-type Au deposits (CTDs). Gold is finely disseminated in Paleozoic silty carbonates and dissolution breccias associated with faults, folds, and dikes. Pequop CTDs exist ~120 km east of well-known Au belts in platform carbonates considered less prospective for large CTDs compared to the slope-facies setting that hosts most deposits. Well-studied CTDs along the Carlin trend formed during the Eocene (42-36 Ma) at shallow depths of 1-3 km in rocks unaffected by regional metamorphism. In contrast, CTDs of the Pequops are of uncertain age and are hosted in variably metamorphosed rocks of the Ruby Mountains-East Humboldt metamorphic core complex (RMEH), which was exhumed from greater depths than host rocks of the Carlin trend. The timing of RMEH exhumation is debated and bears on the depth that CTDs formed.

U/Pb zircon ages from altered and mineralized intrusions in the Pequops are Jurassic and Eocene, consistent with regional magmatism. These ages support an Eocene or younger age for Au deposition in the Pequops similar to CTDs in the major Au belts. Apatite (closure T ~450°C) from Jurassic and Eocene intrusions yield U/Pb intercept ages of 41.0 to 23.9 Ma. Apatite U/Pb ages are youngest to the west where dikes intrude the oldest and metamorphosed part of the Proterozoic to Paleozoic passive margin sequence; ages are oldest to the east where intrusions cut the youngest, non-metamorphosed sedimentary rocks. Apatite fission-track ages (AFT) of igneous samples along a range-wide NE-SW transect record cooling below ~110°C and also show westward-younging from 163.1 to 16.5 Ma.

U/Pb apatite and AFT data are consistent with exhumation in part through eastward tilting of the Pequops from the Eocene through the mid-Miocene, but clustering of several apatite U/Pb ages from 41-32 Ma may alternately reflect cooling of an Eocene intrusion at depth. A 163 Ma AFT age on a mineralized Jurassic sill at Long Canyon indicates that mineralization was at least locally insufficient to anneal apatite. Eastward variation in AFT from ~22-163 Ma, respectively, in the eastern Pequops suggests the area straddled the Oligocene 110°C isotherm required to anneal apatite, thus was relatively shallow. Thus, Eocene or younger mineralization at Long Canyon was also shallow, possibly <4 km.