Geology and geochemistry of the Copper Canyon porphyry copper deposit and surrounding area, Lander County, Nevada

Abstract

The Copper Canyon deposit, largest producer of copper in the Battle Mountain mining district, is about 19 km southwest of the town of Battle Mountain, in north-central Nevada. The district is at the north end of the Eureka-Battle Mountain mineral belt a northwest-southeast zone of metal-mining districts. Du~al Corp. has mined copper, gold, and silver from the Copper Canyon deposit by open-pit methods since 1967. The deposit formed in chemically favorable wallrock adjacent to a small potassic-altered granodiorite intrusion. The granodiorite was emplaced under about 1,500 m of cover into a complex thrust sequence of lower and upper Paleozoic rock in late Eocene or early Oligocene time (about 38 million years ago). Emplacement is inferred to have been controlled partly by the Virgin fault, a major north-striking normal fault in this part of the district, along which movements continued after crystallization of the intrusion. Experimentally determined minimum melting curves, together with univariant breakdown curves for primary biotites from this intrusive, suggest that biotite may have crystallized at about 800°C and suggest high PH 2 0: Ptotal equilibrium ratios during the crystallization. The deposit occurs predominantly as hypogene replacement sulfides in altered rocks belonging to the Middle Pennsylvanian Battle Formation. Smaller amounts of ore occur in the Upper Cambrian Harmony Formation and in the Pennsylvanian(?) Pumpernickel Formation. Most ore was deposited in beds that originally contained hematite and calcite. The hypogene minerals include pyrite, chalcopyrite, pyrrhotite, marcasite, arsenopyrite, sphalerite, molybdenite, galena, and gold. Gold is the second most economically important metal in the deposit. The most common gangue minerals are quartz, K-feldspar, biotite, and white mica, typical of the potassium silicate alteration facies. Early-stage sulfide deposition, beginning at temperatures of about 300°C and continuing to at least 360°C, may have occurred when metal-bearing :fluids reacted with hematite-rich beds of the Battle Formation. Oxygen fugacities in the environment of ore deposition appear to have been very low during this stage. Later, during the intermediate stages, siderite-and marcasite-bearing assemblages developed as the :fluide became enriched in C02 and became possibly more acidic because of the dissociati<* of hydrogen gas. Most gold may have been 1 Duval Corp., Battle Mountain, Nev. 89820. deposited very late in the metallization sequence, along with quartz that fills vugs. Ore deposits peripheral to the intrusion at Copper Canyon are zoned successively from copper-gold-silver deposits near the center to gold-silver to lead-zinc-silver deposits in the northernmost parts of the area. Over 2,900 rocks across all three zones were analyzed for 31 elements in an area of about 16 sq km, up to about 3.2 km north of the main intrusion at Copper Canyon. The chemical data indicate many very high local concentrations of metals near the lead-zinc-silver and the gold-silver peripheral vein d...

Geology and geochemistry of the West Ore Body and associated skarns, Copper Canyon porphyry copper deposits, Lander County, Nevada, with a section on electron microprobe analyses of andradite and diopside

“…The West Ridge fault locally has abundant silicified breccia along its trace, and it is a major splay off the Copper Canyon fault (plate 1). The uneroded part of the upper thrust plate is about 60-90 m thick east of the West Ridge fault but thickens rapidly to the west (Roberts, 1964;Theodore and Blake, 1975). Although most of the ore in this plate is in a tectonic block that belongs structurally higher than the block in which the east ore body occurs, the elevation of the west ore body is now about 60 m lower than some of the lowest ore in the east ore body (Theodore and Blake, 1975).…”

“…These two deposits are within 300m of a small altered granodiorite body (Nash and Theodore, 1971;Theodore and Blake, 1975), dated by the potassium-argon method as late Eocene or early Oligocene (38 m.y.) (Theodore and others, 1973).…”

“…Our data will show that eventual emplacement of the granodiorite at its present level followed development of skarn and preceded the bulk of the mineralization in the east ore body. In the east ore body, most ore occurs as hypogene sulfides that replace former hematite-and calcite-bearing beds in the lower member of the Battle Formation of Middle Pennsylvanian age (Theodore and Blake, 1975). In the west ore body, a copper-gold-silver skarn, concentrations of sulfides occur as replacements and fracture fillings of skarn which formed previously in favorable calcareous argillite and limy shale beds, mostly of the Pennsylvanian and Permian (Erickson and Marsh, 1974) Pumpernickel Formation (fig.…”

“…The uneroded part of the upper thrust plate is about 60-90 m thick east of the West Ridge fault but thickens rapidly to the west (Roberts, 1964;Theodore and Blake, 1975). Although most of the ore in this plate is in a tectonic block that belongs structurally higher than the block in which the east ore body occurs, the elevation of the west ore body is now about 60 m lower than some of the lowest ore in the east ore body (Theodore and Blake, 1975). The west ore body occupies an irregular ovoid zone about 370m long in an east-west direction along the northern contact of the granodiorite.…”

“…Theodore, Silberman, and Blake (1973) reported the potassium-argon ages of plutonism in the district, and Theodore and Blake (1975) described the geology of the east ore body, one of the major ore bodies at Copper Canyon in production in 1975. Three other reports (Nash and Theodore, 1971;Theodore and Nash, 1973;Batchelder and others, 1976) discuss the chemistry and distribution of the ore-forming fluids.…”