Presented at the Geological Society of America meeting, Denver, October 26-30, 1996

EVOLUTION OF PORPHYRY COPPER-GOLD TO LOW SULFIDATION EPITHERMAL GOLD MINERALIZATION: MARICUNGA BELT, NORTHERN CHILE

MUNTEAN, John L., and EINAUDI, Marco T.,
Dept. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305, munt@pangea.stanford.edu

Maricunga porphyry gold deposits are located in Miocene subvolcanic andesitic to dacitic intrusive centers. Gold is closely associated with quartz veinlets that are mostly <0.5mm in width. Porphyry copper-type veinlets associated with pervasive potassic alteration include discontinuous A veinlets of magnetite-quartz-biotite-chalcopyrite to more continuous, quartz-rich, B veinlets with variable amounts of magnetite, chalcopyrite, pyrite, and specular hematite. A and B veinlets are cut by banded veinlets which on the basis of physical characteristics and mineralogy can be classified as "low-sulfidation epithermal."

The banded veinlets contain over 90% granular to locally toothy quartz with slightly wavy walls. Dark gray bands, whose color is due to increased density of fluid inclusions and m-scale grains of magnetite, commonly occur as symmetric pairs near the vein walls. The bands are commonly botryoidal and are continuous through quartz grains, suggesting the quartz recrystallized from chalcedony. The veinlets also contain common pyrite, local K-feldspar, calcite, and chlorite, and local trace chalcopyrite, bornite (without pyrite), and sphalerite. Alteration envelopes are lacking. Fluid inclusion data indicate temperatures and salinities lower than for A and B veinlets. Banded veinlets do not form true stockworks and exhibit structural patterns that include steeply-dipping radial and shallowly-dipping concentric patterns. Zones of abundant banded veinlets generally contain 0.5-2g/t Au and <0.1% Cu (hypogene), whereas zones of abundant A and B veinlets contain mostly <1g/t Au and 0.1-0.4% Cu (hypogene).

Banded veinlets and pervasive propylitic alteration predominate at the Verde deposit at Refugio. Along a 300m vertical exposure at the Pancho deposit at Refugio, A and B veinlets and pervasive potassic alteration predominate at the lowest levels and are cut by banded veinlets that predominate at higher levels. The banded veinlets are cut by quartz-alunite ledges at the highest levels. At the Cavancha deposit at La Pepa, banded quartz veinlets cut A and B veinlets and pervasive potassic alteration and are cut by gold-bearing pyrite-quartz veins with sericitic envelopes which may be feeders to high-grade quartz-alunite-enargite-barite ledges (locally >100g/t Au) that are within 100m of Cavancha. At all of the deposits, A, B, and banded veinlets cut and are cut by intrusions suggesting a genetic relationship between quartz vein formation and intrusive activity. Magmatic-hydrothermal brines apppear to have evolved in space and time to cooler, rock-dominated, dilute, low-sulfidation fluids responsible for copper-poor, gold mineralization associated with banded veinlets, whereas acidic, high-sulfidation fluids that locally formed high-grade Au-quartz-alunite ledges may have evolved later along different structural and chemical paths.