ASBOG FG Practice Test 13

Field problem: Corvid Hills mining district

A junior exploration company has hired you as project geologist for the Corvid Hills mining district. Before you plan a drilling program, review the regional abstract below and the geologic map in the figure, then answer the questions in this set.

The Corvid Hills are a semi-arid upland of rounded ridges and dry washes. Mean annual precipitation is 12 in., mostly from summer thunderstorms, and potential evapotranspiration exceeds 55 in. The water table lies 150 to 250 ft below most hilltops. Soils are thin, calcareous gravelly loams, and the land is used for cattle grazing and mineral exploration.

The oldest exposed rock is unit Q, a vitreous quartzite at least 1,000 ft thick. It is overlain conformably by unit L, a gray fossiliferous limestone 300 ft thick; unit S, a black pyritic shale 300 ft thick; and unit C, a red conglomerate and sandstone more than 800 ft thick. Graded beds and cross-beds show that all four units are right side up. They are folded into the north-trending Corvid anticline, whose limbs dip 40° and whose axial surface is vertical. The east-trending Hollins fault displaces the fold.

Unit G, a hornblende–biotite granodiorite stock, cuts the folded strata and the Hollins fault; the stock itself is not displaced. Where G is in contact with L, the limestone has been converted to garnet–pyroxene rock containing magnetite and chalcopyrite. Unit V, flat-lying andesite flows and tuffs, covers the eastern part of the district; its basal conglomerate contains cobbles of the granodiorite.

Copper was discovered here in 1879. Early miners worked a rust-colored outcrop of limonite, hematite and jarosite above pyritic shale near the stock, and their shafts reached soft, sooty black copper ore close to the water table. The old Raven shaft closed in 1931. The company now proposes vertical drill hole DH-2 to test the contact zone south of the Hollins fault.