Abstract

The Lisbon Valley area has a history of copper prospecting and mining dating from the 1890s. Early mining produced small amounts of high-grade oxide copper or chalcocite ores. Since the 1960s, several companies have evaluated lower grade disseminated deposits and made unsuccessful attempts at production. Constellation Copper Corporation has evaluated, engineered, and permitted an open pit mining project designed to treat 48 million tons grading 0.467 percent Cu by heap leach and the solvent extraction and electrowinning process.

The Lisbon Valley Copper Project includes three deposits (Centennial, Sentinel, and GTO) in the Cretaceous Burro Canyon and Dakota Formations. Copper concentrations occur mostly as intergranular disseminations in sandstones and pebble conglomerates with subordinate amounts in veinlets along structures. The copper deposits have oxide zones, characterized mostly by malachite and azurite to a depth of about 150 feet below the surface; below that, ore minerals are predominately chalcocite. Minor amounts of bornite and chalcopyrite occur in the fringes of the deposits and are surrounded by distal zones with lead and zinc geochemical anomalies. Petrography and fluid inclusion studies suggest that the ore minerals were deposited from warm, very saline brines by replacement of calcite, pyrite, organic material, and hydrocarbons. Alteration is subtle but includes clays, silica overgrowths, calcite and dolomite.

The mineralized areas are located adjacent to strands of the Lisbon Valley fault where it splays out at the plunging southeast end of the Lisbon Valley salt anticline. On the scale of individual deposits, the permeability of the host rocks, proximity to apparent feeder structures, and the amount of available reductant control the thickness and grade of the copper deposits.