TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS REVISITED: VERTICAL AND LATERAL EXTENTS FROM YERINGTON, NV, AND BUTTE, MT (Invited Presentation)

Porphyry ore deposits form at a depth range of 1 to10 km. Magmatic fluids are directed by both magmatic-hydrothermal hydrofractures that dominate in the ductile zone near magma and faults related to far-field tectonic forces in the shallow brittle regime.

At Yerington, a >1300 km³ batholith has been offset by normal faults and tilted ~90°W to expose a magmatic “floor” at 8 km depth, and a volcanic roof at ca. 1 km depth. Four porphyry copper systems (at 1-2.5, >2, 2-3, and 2.5-4 km depth) are coincident with voluminous (~20-30% of ore) granite porphyry dikes that were emplaced vertically and extend from the deep source Luhr Hill granite at 4-8 km depth to <0.5 km depth. Magmatic hydrothermal alteration follows dike swarms from below ore (weak K-sil & musc) to ore (K-sil>Ser: biot, Ksp, musc) to the subvolcanic environment (AA pyroph, alun, topaz ± musc), and is flanked by sodic-calcic and propylitic alteration likely produced by sedimentary brines. Lateral magmatic fluid flow only occurred in the shallow environment (<1 km). The short duration of magmatic-hydrothermal activity (~100,000 yr; Schöpa et al., 2017) is consistent with only minor concurrent telescoping and 300 - >500 m erosion of AA altered volcanic rocks.

In contrast, the Butte district is slightly tilted (10-30°), and current mine and drilling information extends from 6 to 9 km original inferred depths. Granite porphyry dikes are sparse (<0.1 vol.%) in ores. Magmatic-hydrothermal alteration produced relatively wide Cu-Mo ore shells with Cu sulfide-rich EDM (biot-Ksp-musc) and younger quartz-molybdenite veins that overlie deep K-sil (biot) and underlie shallow weak sericitic and propylitic zones. Magmatic-hydrothermal activity spanned 4-5 m.y., and allowed for extreme exhumation of several km of rock between early porphyry Cu-Mo centers (6-8 km depth) and late Main Stage lodes (2-5 km). The lodes extend along conjugate sets of strike-slip faults with minor normal movement that allowed ~300-400°C fluids to move laterally for >5 km and produce polymetallic veins with sericitic alteration.

Both Yerington and Butte ores are derived from arc magmas emplaced during crustal shortening and thickening, associated rapid exhumation (0.2-1.5 mm/yr), and inferred elevated topography that drove lateral fluid flow of both near-surface meteoric and magmatic fluids.