Dating Sinter Deposits in Northern Dixie Valley, Nevada- the Paleoseismic Record and Implications for the Dixie Valley Geothermal System

A series of fossil spring deposits are exposed along the Dixie Valley fault just south of the producing geothermal field, in an area now characterized by active fumaroles and steaming ground. These deposits are composed of both travertine and sinter that have trapped pollen and other plant material during their formation. Radiocarbon dates on the organic material indicate that the youngest hot spring sinters range in age from about 3.4 ka to essentially modern. Banded travertine composed of calcite, dolomite, hematite, and barite may represent deposition from a warm spring at about 5040 ka. Older quartz-rich sinters are between 3.4 ka and Lake Dixie (11-12 ka) in age. The mineralogy and texture of the opaline sinters are consistent with their young ages. One sinter (with a modern C age) consists of botryoidal heads of vitreous, opaline silica or “geyserite” that likely formed from actively spouting eruptions of boiling fluids along the fault zone. X-ray diffraction analyses indicate that the sinter is composed of original opal-A that has not undergone the transition to the more crystalline opal-CT or cristobalite (opalC). Slightly older (2.2 to 3.4 ka) sinters appear to be admixtures of opal-C or opal-CT, microcrystalline quartz, and calcite. These sinters are predominantly thinly laminated to porous opal-CT, and contain abundant plant remains and clasts of even older microcrystalline quartz sinter. The radiocarbon dates reveal the episodic nature of spring activity along the Dixie Valley fault. Paleoseismic studies identify several large magnitude earthquakes in the central Dixie Valley area that may be related to the spring activity. Two of these seismic events are historic, the 1915 M7.7 Pleasant Valley and the 1954 M6.8 Dixie Valley earthquakes. Other large magnitude Pleistocene and Holocene earthquakes identified in the area include The Gap event (3.72.2 ka), an older Holocene event (age not constrained), the West Stillwater event (<5.6 ka), and another late Pleistocene event (~12-34 ka). Based on the age relationships between the spring deposits and local paleoseismic events, we suggest that: 1) the 2.0 or 3.4 ka sinter may have initiated with The Gap event; 2) the “modern” geyserite may be associated with either the 1915, Ms =7.7 Pleasant Valley, or the 1954, Ms =6.8 Dixie Valley earthquakes; and 3) that warm spring activity at about 5 ka may be associated with the M7 earthquake on the west side of the Stillwater Range. The maximum possible age for geothermal waters of the Dixie Valley system is estimated to be 12-14 ka, hence these warm and hot spring deposits represent surficial discharges of the modern geothermal system. This portion of the Dixie Valley fault, just a few kms southwest of the producing geothermal field, has actively discharged geothermal fluids in the past few thousand years, up into modern times. INTRODUCTION AND GEOLOGIC SETTING The Dixie Valley geothermal reservoir is associated with an active normal fault, the Dixie Valley (or Stillwater) fault that occurs along the eastern margin of the Stillwater Range. In the geothermal field, the high-temperature (240o C) fluids produced from the fault zone at depths of 2-3 km power a 62 MW electrical plant operated by the Caithness Energy Corp. Active fumaroles are present along the surface trace of the Dixie Valley fault at the northern and southern ends of the geothermal field (Fig. 1). The Senator Fumaroles occur near the northern margin of the production area and smaller fumaroles (informally called the Section 10/15 fumaroles; Blackwell et al., 2000a) are active along the rangefront south of the geothermal field where fossil travertine and sinter deposits are also present. These travertine and sinter deposits occur at the northern endpoint of surface ruptures along the Dixie Valley fault that are Holocene in age (Fig. 2; Caskey et al., 1996; 2000). In this paper we describe the mineralogy, occurrence, age and paleoseismic significance of these spring deposits. Future work will include the study of smaller sinter deposits in this area that have yet to be mapped or dated, including fossil sinter deposits at the nearby Dixie Comstock gold mine that Vikre (1994) suggested were related to the Dixie Valley geothermal system. The Section 10/15 fumarole and sinter area occurs as part of the DVPP (Dixie Valley Power Partners, including ESI and Caithness Corp.) geothermal lease where two deep exploration wells (62A-23 and 3614) were drilled in 1993 and 1994, south of the (then) Oxbow field (see Fig. 1). The sinter area is important because: 1) well 36-14 is the hottest well drilled in the entire Great Basin (with temperatures of 280 oC at about 3050 m; Benoit, 1994; Blackwell et al., 2000a; 2000b); 2) compared to a single, major fault in the current production area, the fault zone in the DVPP lease block appears to be composed of multiple faults, both inboard and outboard of the main rangefront fault (Smith et al., 2001); and 3) wells along this portion of the fault zone have variable permeabilities, well 62-23 appears to be tight (Blackwell et al., 2000a) while fluid production from well 36-14 is capable of supporting 3.5 MW (S. Petty, pers. comm.). Although the fault structure complicates development drilling, there is significant potential for further geothermal exploration and development (Robertson-Tait et al., 2000). The local stress regime based on borehole studies in well 66-21 (Hickman et al., 1997; Barton et al., 1998), and paleoseismic studies of the Dixie Valley fault (Bell and Katzer, 1990; Caskey et al., 1996; 2000a; Caskey and Wesnousky, 2000b) has been well documented. The Hickman and Barton studies indicate that present-day fluid pressures in well 66-21 (Fig. 1) are artesian compared with subhydrostatic pressures in the main geothermal reservoir, and that faults near this well are not critically stressed for frictional failure. Even though the faults and fractures in well 66-21 were found to be optimally oriented for normal faulting, a high ratio of Shmin to Sv appears to have a great effect on the fracture permeability in this nonproductive well. The Dixie Valley geothermal field occurs in an area known as the “Stillwater seismic gap”, a 45 km-long section of the Dixie Valley fault that lies between the 1915 (Ms 7.7) Pleasant Valley and 1954 (Ms 6.8) Dixie Valley fault rupture zones to the north and south, respectively (Fig. 2). Recent paleoseismic studies have provided new constraints on the timing and distributions of large magnitude late Pleistocene and Holocence earthquakes in area including: 1) The Gap event (3.7-2.2 ka) (Caskey, unpub. data) which broke along the section of the Stillwater seismic gap lying south of the geothermal field and overlapped with the 1954 rupture zone by about 20 km; 2) an older (?) Holocene event (age not constrained) that broke along the section of the Stillwater gap lying north of the geothermal field (Caskey and Wesnousky, 2000); 3) the West Stillwater event (<5.6 ka but older than The Gap event) which broke along a ~40 km section of the West Stillwater fault on the west side of the Stillwater Range (Caskey, unpub. data); and 4) a paleoseismic event (~12-34 ka) that broke in the area of the 1954 earthquake (Bell and Katzer, 1990). The Gap event produced vertical displacements of up to 5 m along the Dixie Valley fault. Surface ruptures of this event can be traced to about 10 km south of the geothermal field. A total rupture length of 45 km together with an average slip for the event of 2 to 4 m (Caskey, unpub. data) equate to an estimated moment magnitude of 7.1 to 7.3 for The Gap event. Similarly, the measured length of rupture associated with the West Stillwater event and vertical surface displacements on the order of 1-3 m indicate that these ruptures are associated with an M~7 earthquake. Ruptures for this event essentially overlap with the 1954 rupture trace on the east side of the range.