VOLCANIC STRATIGRAPHY AND HOST ROCKS OF GOLD AND COPPER ORE IN THE YANACOCHA MINING DISTRICT, PERU

VOLCANIC STRATIGRAPHY AND HOST ROCKS OF GOLD AND COPPER ORE IN THE YANACOCHA MINING DISTRICT, PERU LONGO, Anthony A., Dept. Geosciences, Oregon State Univ., Corvallis, OR, longot@geo.orst.edu , and SADERHOLM, Eric, Newmont Mining Corp., Denver, CO, esad4070@yanacocha.newmont.com Yanacocha is a high-sulfidation mineral system in northern Peru with more than 50 million ounces of gold in oxide ore and an unknown resource of gold and copper in sulfide ore. Gold production to date exceeds 9 million ounces from four mines in the core of the district. Regional field studies over an area of more than 1000 km2 and detailed core logging of rocks from the Yanacocha ore deposits have resulted in major revisions to the volcanic stratigraphy. It consists of six rock packages that include the following in stratigraphic order: (1) early hornblende-bearing medium-K andesite, with debris flows, volcaniclastic sediment, and basaltic-andesite lava flows that overlie the Cretaceous basement; (2) biotite-rich high-K andesite lithic tuff, ignimbrite, and debris flows; (3) hornblende- and biotite-bearing high-K trachyandesite to dacite ignimbrite that may represent the initial phase of local volcanism, (4) quartz-biotite rhyodacite ignimbrite with related dike rocks that intrude sequences 1 to 3; (5) hornblende-bearing low-Mg medium- to high-K andesite rocks that represent a late phase of dome-related volcanism; and (6) late medium-K andesite dikes and lava flows that represent the last phase of magmatism at Yanacocha. Hydrothermal processes that led to the deposition of gold and copper ore favor specific pyroclastic flows. Rocks found to most commonly host ore include the biotite-rich high-K andesite tuff of sequence 2, the hornblende-biotite trachyandesite tuff of sequence 3, and the hornblende-bearing high-K andesite tuff, most notably the San Jose Ignimbrite, of sequence 5. Acidic-altered accidental fragments found in the pyroclastic rocks of sequences 3 to 5 suggest that acid-sulfate alteration may have preceded the eruption of sequence 3. The age of Au mineralization is bracketed between sequences 5 and 6. Ongoing studies will integrate field geology with radiometric 40Ar/39Ar ages to more clearly define the stratigraphy, the volcanic history, and the timing of volcanism and Au-Cu mineralization at Yanacocha.