Initial results from VC‐1, First Continental Scientific Drilling Program Core Hole in Valles Caldera, New Mexico

Abstract

Valles Caldera 1 (VC‐1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC‐1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856‐m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC‐1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480‐, 540‐, and 845‐m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.

Evolution of a mineralized geothermal system, Valles Caldera, New Mexico

“…Sparse occurrences of molybdenite, sphalerite, chalcopyritc, galena, and arsenopyrite were identified in hole VC-1 (Fig. 4B; Goff et al, 1986;Keith, 1988). Argentiferous pyrite is documented beneath the Redondo Creek graben (Hulen and Nielson, 1986) A comparison between Au, As, and K20 contents in the upper part of the Sulphur Springs system is shown in Figure 9B.…”

Light lithophile element (boron, lithium) abundances in the Valles Caldera, New Mexico, VC‐1 core hole

“…The most likely host for boron and lithium is smecti1c-illi1c, which is prescnt throughout the scdimcncary section. Samplcs low in boron and lithium at 546 m and 307.8 mare adjacent to an aquifcr at 539 m [Goff et al, 1986] and fault zone ab318 m [Goff et al, 1986]. Circulaùng high tempcraturc lluids (275 -300°C) may have parùally broken down the clay mincrals and lcachcd boron and lithium from the system.…”

“…and core holc VC-1 is the first bote complclCd undcr this program (Figun: 1). One of the major objectives of VC-1 was lO pencuaic a high-icmperature hydrothennal outflow plume ncar its source [Goff et al, 1986]. The core hole thus providcs a unique opportunity ro examine the relationship of boron and lithium in diffcrent rock types in an active hydrothcrmal environmcnL…”

The DOE Thermal Regimes Drilling Program through 1987

“…The Inyo chain was chosen for a drilling investigation because (1) it is the YOlmgest of the rhyolitic volcanoes in the contiguous United States and hence the youngest of easily accessible rhyolitic systems, (2) its surface-accessible portions had been well characterized, and (3) a well-defmed concept had been developed concerning its subsurface configuration. That concept, which was based on evidence that the linear array of vents had been active contemporaneously 600 years ago, was that the eruptions and accompanying surface formation were the surface manifestation of the emplacement at shallow depth of a rhyolite dike.…”

“…Objectives were to (1) intersect the hydrothermal outflow plume of Valles Caldera at a point midway between the source reservoir in the caldera and the terminus at hot springs outside the caldera; (2) investigate the structure, stratigraphy, and hydrothermal alterations in QuaternaryPrecambrian rocks in the ring-fracture zone; and (3) investigate the petrogenesis of the youngest postcaldera volcanic rocks.…”