Apex Stock, Gilpin County, Colorado, USAi

Regional Level Types
Apex Stock	Intrusion
Gilpin County	County
Colorado	State
USA	Country

The Apex stock cuts Precambrian rocks, is elongate north-south, and is about 5 km long by 1 km wide. It is composed of hornblende-biotite quartz monzonite and is cut by dikes of similar composition. Rice and others (1982) report K-Ar hornblende dates of 79.3+1.9 Ma and 76.7±3.1 Ma. The main body of the stock gives fission-track dates of 71.3±18.0 Ma (AFT) and 61.7±6.3 Ma (ZFT) (sample A-l, tables 1 and 2). Dates from both minerals are concordant at about 60±5 Ma. Barker (1979) reports dates of 60.1±5.5 Ma and 57.3±4.4 Ma. Sample A-2 (tables 1 and 2), from the same location as A-l, is from one of a series of younger, crosscutting dikes and yields an AFT date of 66.8±7.5 Ma and a ZFT date of 58.5±5.2 Ma. Dates from minerals in A-2 are slightly younger than dates from corresponding minerals in sample A-l. Apparently there are problems either with excess radiogenic argon in the hornblende and the stock formed about 60 Ma, or fission tracks in the zircons and apatites have been reset to about 60 Ma by a thermal event apparently associated with a larger intrusion probably related to the dikes sampled by A-2. The apatite ages from both samples A-l and A-2 show the same ages as from the zircon in the same sample, indicating there has been no thermal resetting younger than about 60 Ma, in contrast to the thermal history of the nearby Empire stock.

Pyritic gold formed the principal ore in the vicinity of the Apex stock. Most of the veins trend northeast and are located just east of the stock (Levering and Goddard, 1950); however, two veins cut the stock. Low-grade molybdenite deposits are present in the stock and in associated veins and breccias (Barker, 1979).