Eos, Vol. 68, No. 30, July 28, 1987  Scientific Drilling  at Sulphur Springs,,  Valles Caldera,  New Mexico:  Core Hole VC 2A  PAGES 649, 661-662 Fraser Goff, 1 Dennis L. Hulen, 2 Peter Lysne, 3 Lisa Nielson, 2 Jamie N. Shevenell, 1 and John C. Gardner, 1 Jeffrey B. Rowley 1 A scientific core hole has been drilled into the western ring fracture zone of the Valles Caldera, N.Mex. Hole VC-2A, the second scientific core hole in the calde- ra, was cored through a faulted and brecciated sequence of intracaldera tuffs and volcaniclastic rocks to a depth of 528 m at Sulphur Springs. As of November 1, 1986, the unequilibrated bottom hole temperature was 212°C. The rocks that have been penetrated are intensely altered and contain molybdenite mineraliza- tion (MnC>2) that is less than or equal to 1.1 m.y. in age. The active hydrothermal system at Sulphur Springs consists of a thin (5-m) acid condensation zone overly- ing vapor- and water-dominated zones. The latter two zones are apparently sepa- rated by a region of tightly sealed rock. Objectives  Valles Caldera scientific drillhole 2A (VC- 2A) is the second scientific core hole to be drilled into the Valles Caldera as a part of the Continental Scientific Drilling Program (Fig- ure 1). The primary objective of VC-2A was to penetrate the vapor zone beneath the acid sulfate hot spring system of Sulphur Springs, which is located on the western edge of the resurgent dome inside the l.l-m.y.-old calde- ra. Secondary objectives were to core through the interface between the vapor zone and hot water-dominated zone, to obtain structural and stratigraphic data on the caldera fill rocks along the boundary between the ring fracture and the resurgent dome, and to de- 'Los Alamos National Laboratory, Los Ala- mos, N.Mex. 2 University of Utah Research Institute, Salt Lake City. 3 Sandia National Laboratories, Albuquer- que, N.Mex. Cover. Core hole VC-2A at the Valles Caldera, N.Mex., on May 1, 1987, after perforation at 490 m, 210°C, during stim- ulation. Steam and flashed water shoot about 35 m skyward. For more informa- tion about the drilling of VC-2A and the scientific results obtained afterward, see the article "Scientific Drilling at Sulphur Springs, Valles Caldera, New Mexico: Core Hole VC-2A" by F. Goff et al., p. 649. (Photograph by R. Myers, Sandia Na- tional Laboratories, Albuquerque, N.Mex.) 0096-3941/87/6830-649$ 1.00 Copyright 1987 by the American Geophysical Union termine mechanisms of ore deposition in an active caldera hydrothermal system. Technology and Safety  Surface conditions at Sulphur Springs sug- gested that VC-2A would provide a design and drilling challenge for a coring operation using a diamond drill bit. Steel fixtures that have been used in the area, such as culverts, are quickly corroded by surface acid condi- tions, a fact that did not bode well for the usual drilling rods and hole casings. Further- more, the presence of hydrogen sulfide and high-temperature fluids at the surface sug- gested that hydrogen embrittlement of down- hole components could be a problem. Thus a number of unique design and operational features were used in the drilling of VC-2A to ensure that it would be usable for a period of at least 5 years. Details of the core hole de- sign and coring operations can be found in the work of Lysne et al. [1987]. Three particu- larly interesting aspects of the operation are discussed below. Hydrogen sulfide is a component (up to 1500 ppm) of the gases emanating from fu- maroles in the Sulphur Springs area. This gas is dangerous to life in concentrations above 300 ppm. Because of the very likely occur- rence of hydrogen sulfide in well gasses and drilling fluids, a monitoring system was de- signed, and personnel were trained in emer- gency procedures, such as the use of air packs and cardiopulmonary resuscitation. The monitoring system included seven detec- tors, which were located near the drill rig, at the operations trailers, at living trailers, and in a drainage area provided for flow testing of the well. Information from these detectors was routed to a central panel that automati- cally activated low (10 ppm) and high (20 ppm) alarms (the Occupational Health and Safety Administration (OSHA) ceiling stan- dard is 20 ppm). Data from all of the sensors were recorded to computer disk at 5-minute intervals throughout the drilling operation. A core tube loading chamber was designed especially for VC-2A. It consists of a 10-ft (~3-m)-long section of HQ rod (3.5 in drill pipe) with a packer for the wireline at the top and a 4-inch (~10-cm) ball valve at the bot- tom. Other components were a pressure gage, attachment for the mud line, and the overshot assembly. In operation, the water swivel was removed, and the valve end of the chamber was screwed to the top of the rod string. The valve was opened, allowing the overshot to be lowered into the hole, and the core tube was then withdrawn into the cham- ber. The valve was closed, and the chamber was removed. Thus the rods were open for a minimum time, and mud could be pumped downhole while core was being recovered. 5 KILOMETERS Fig. 1. Location map showing Valles-Toledo calderas, existing CSDP borehole sites (VC- 1 and VC-2A), and previous geothermal wells drilled in the Sulphur Springs area and Sul- phur Springs fault map [adapted from Goff et al, 1985; Hulen and Nielson, 19866]. Stippled pattern shows area of relatively intense argillic alteration. A large landslide complex occurs east of Sulphur Springs.