Abstract The Lisbon Valley area has a history of copper prospecting and mining dating from the 1890's. Early mining produced small amounts of high-grade oxide copper or chalcocite ores. Since the 1960's 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.

Upper Pleistocene rhyolitic ash-flow and air-fall tuffs, erupted from several centers, were sampled in 23 pumice-filled basins over an area of 16,000 km 2 . Fifty pumice-matrix samples were analyzed for as many as 20 trace elements. Ba, Fe, Hf, Rb, Sm, Sr, Th, Ti, and Zr were particularly useful in “fingerprinting” correlations between basins and in corroborating the stratigraphy previously established within individual basins. On the basis of similar trace elements, a tephra and an overlying ash-flow sheet (together, a unit here named the Los Chocoyos Ash) appear to have formed from a multiphase eruption. The tephra, whose volume exceeded 100 km 3 , blanketed an area greater than 1 × 10 6 km 2 . The second phase of the eruption produced an ash flow of greater than 200 km 3 . Areal geochemical patterns within the ash-flow sheet are probably related to sequentially less explosive eruptions of progressively more mafic ash flows. Changes in chemical composition, size of pumice and lithic fragments, thickness, and elevation all suggest a source for the Los Chocoyos Ash in the Lake Atitlán cauldron. Chemical data suggest correlation of the H-tephra member of the Los Chocoyos Ash with the most prominent D layer of the Worzel ash of the equatorial Pacific.