Electrical studies conducted on the Lake City Caldera, Colorado, July 1984

Abstract

A second steeply dipping resistivity gradient between stations 5 and 9 east of Burrows Park suggests a contact between the intracaldera quartz syenite intrusive and the Sunshine Peak tuff/megabreccia. The Sunshine Peak tuff/megabreccia deposit is shown on the simplified cross-section ( fig. 4). Gravity and magnetic modeling also suggests a boundary (Grauch, personal commun., 1984) at this location.The high resistivity values beneath station, 9, 10, and 11 suggests the intracaldera quartz syenite intrusive extends down to the maximum depth of exploration.Stations 12 and 8 show a gentle east-dipping resistivity gradient (400 and 630 ohm meter contours). The gradient appears to follow the contact between the intracaldera quartz syenite intrusive and the Sunshine Peak tuff/meggabreccia.The near surface (<_ 1000 m) low resistivities «400 ohm-m) of stations 8, 13, and 15 are related to the widespread alunization of Red Mountain, local alteration, and mineralization near station 15. Station 13 ( fig. 4) is approximately located over the conduit for the quartz latite intrusion (Lipman, 1976). Because station 13 has low near-surface resistivities, the depth of exploration is limited by electrical channeling in the conductive surface. The resistivity values do suggest a lower limit for the alteration, because the resistivities of stations 8, 13, and 15 show increasing resistivties with depth.

Mineral resources of the Redcloud Peak and Handies Peak Wilderness Study Areas, Hinsdale County, Colorado

“…2). The low at L4 suggests a local area of low-density rock, and coincides with an area of hydrothermally altered rock that was inferred from electrical studies (Pierce and Hoover, 1985). Hydrothermal solutions at shallow depth may have altered the minerals of the rocks in this area to lower density minerals.…”

“…Electrical Survey and Results Pierce and Hoover (1985) conducted an audiomagnetotelluric traverse across both of the study areas ( fig. 7) and a telluric traverse through Burrows Park just north of the Handles Peak Wilderness Study Area.…”

“…Three areas of anomalously low resistivity were defined within the study areas. The ring fault of the Lake City caldera marks a deep zone of very low resistivity, which Pierce and Hoover (1985) interpreted as a zone of alteration and possible mineralization. This area generally coincides with gravity and aeromagnetic lows, L4 on figure 5 and L2 on figure 6.…”

“…In addition, a steeply dipping resistivity gradient occurs near Silver Creek, and a gently east-dipping resistivity gradient occurs between Middle Fork Alpine Gulch and East Fork Alpine Gulch. Pierce and Hoover (1985) interpreted both of these anomalies as manifestations of contact zones between the resurgent intrusion of the Lake City caldera and the Sunshine Peak Tuff. Epithermal ore deposits are those formed by precipitation of metals from hydrothermal solutions at moderate temperatures (200-300 °C) and relatively shallow depth (a few hundred to several thousand feet).…”

“…Vertical exaggeration 1.6:1. (Modified from Pierce and Hoover, 1985. ) veins at the surface may be rich in ore at depth, and, conversely, rich veins at the surface may be barren at depth.…”