GRC Transactions, Vol. 38, 2014 Gravity Modeling of the Rye Patch Known Geothermal Resource Area, Rye Patch, Nevada Mark R. McDonald1 and William D. Gosnold2 1North Dakota Geological Survey, Bismarck ND 2Department of Geology and Geological Engineering, University of North Dakota, Grand Forks ND Keywords A gravity study was performed at the Rye Patch KGRA in an Rye Patch, gravity, modeling, geophysics, exploration attempt to further define the geothermal reservoir and/or to identify potential drilling targets in the area by delineating approximate fault locations, lengths, trends and dip angles. The gravity survey ABSTRACT was augmented with existing seismic, aeromagnetic, remote sens- ing, and geochemical data. A gravity survey was conducted at the Rye Patch Known Geo- thermal Resource Area located at Rye Patch, Nevada. The purpose Geology of the study was to attempt to further delineate the geothermal reservoir and/or to identify potential drilling targets. The survey The Rye Patch geothermal area is located in the Humboldt Riv- consisted of collecting data at 264 new stations to augment data er Valley in Pershing County, Nevada. As previously mentioned, from 203 stations collected in 2008. Information from previous the study area is bounded by the Humboldt Range on the east seismic, aeromagnetic and geochemical investigations was also examined and incorporated. After filtering the data to remove the strong regional overprint caused by the large density contrast between the low density alluvium within the valley versus the near-surface higher density rock in the higher elevations, 2-D models were created across three profiles. Based on estimates arrived at by using 2nd derivative methods, fault dip angles are on the order of 80o and are consistent with previous conceptual models of the site. The modeling indicates that there are a series of parallel normal faults within the valley and that the fault blocks appear to be rotated back to the east creating a series of sub-basins within the valley. Introduction The Rye Patch Known Geothermal Resource Area (KGRA) is located along U.S. Interstate 80 approximately 190 km northeast of Reno, Nevada between the cities of Winnemucca and Lovelock, Nevada (Figure 1). It includes what has been referred to as the Humboldt House geothermal anomaly to the north and the more southerly Rye Patch geothermal anomaly. It is bounded on the east by the Humboldt Range and on the west by the Rye Patch Reservoir. The northern boundary of the KGRA is demarcated by the northeast-trending Midas Lineament and the southern boundary by a northwest-trending fault zone that offsets the Humboldt Range from the West Humboldt Range located to the south (Waibel et al., 2003). Figure 1. Map showing location of Rye Patch, Pershing County, Nevada. 533 McDonald and Gosnold and by the Rye Patch Reservoir on the west (Figure 2). Johnson silts, sands and gravels with interbedded sinter deposits (Johnson, (1977) presents a detailed discussion of the Geology of Pershing 1977; Michels, 2002). The deposits capping the valley floors and County. In the study area, the Humboldt Range is described as the along the mountain flanks consist of alluvium, flood-plain and being a north-trending anticlinal structure with a complex network playa lake deposits, and sand dunes. A generalized stratigraphic of faulting formed by the Nevadan orogeny during the Jurassic column is shown in Figure 2. and Cretaceous periods (Johnson, 1977; Hastings, 1993). The oldest exposed rocks consist of the Lower Triassic Koipata Structure Group which consists of the Limerick Greenstone, the Rochester Rhyolite and the Weaver Formation. The Koipata Group is over- Hastings et al. (1993) and Sanyal et al. (2006) report that the lain by the Star Peak Group, also Triassic in age. The Star Peak Mesozoic rocks dip to the west-northwest at an angle of between Group consists of the principally carbonate rocks of the Prida and 20 to 40 degrees. The overlying sedimentary and volcanic rocks Natchez Pass Formations. Pelitic and fine-grained clastic rocks of of Paleogene, Neogene and Quaternary age are nearly horizontal the Upper Triassic Grass Valley Formation, part of the Auld Lang and the contact between the Mesozoic and Cenozoic rocks is Syne Group, overlie the Star Peak Group. Burke and Silberling unconformable with the dip of the unconformity approximately (1973) report that the Dun Glen Formation also of the Auld Lang 30 degrees to the northwest (Sanyal et al., 2006). Syne Group, is present at the southern end of the Humboldt Range. The major Basin and Range (Cenozoic) faulting generally On the western side of the Humboldt Range, the upper member trends northward and bounds the mountain ranges with parallel of the Natchez Pass and the Grass Valley Formation are thrust piedmont faults occurring in the alluvial valleys and possible over the Prida Formation along the Humboldt City thrust fault antithetic faults on the opposite side of the valley and this general and the Grass Valley Formation has been locally metamorphosed pattern is observed in the study area. These north-trending fault to argillite, phyllite, slate and quartzite (Silberling and Wallace, systems cut and are cut by shorter east-west trending transfer faults 1969; Johnson, 1977). or accommodation zones (Faulds and Varga, 1998). The Midas Within the valley, the Grass Valley Formation is overlain by Lineament, a left lateral strike-slip zone, truncates the Humboldt Paleogene and Neogene clays, shales, and volcanics. The Paleo- Range on the north (Rowan and Wetlauger, 1981; Hastings et al., gene and Neogene deposits are followed by Quaternary clays, 1993) and another east-west trending transfer fault or accommoda- tion zone offsets the West Humboldt Range and the Humboldt Age Group Formation Description Range to the south (Waibel et al., 2003). Clays, silts, sands and gravels with inter- The normal faulting has been intermittent from about 16 Quaternary -- Unnamed bedded sinter deposits Ma to the present with maximum displacement that may be on the order of several thousand meters (Johnson, 1977). Ehni Paleogene and Volcanics, sands, gravels, clays, and shales -- Unnamed (2001) states that at Rye Patch, geothermal fluids may be fed Neogene with minor amounts of limestone to cavernous limestone beds (presumably at the Mesozoic/ Unconformity Cenozoic contact noted above) by high angle normal faults. It Limestone and dolomite with interbedded should also be noted that evidence suggests that the geothermal Dun Glen sandstone and argillite fluid circulation may be enhanced in areas where fault systems Auld Lang merge (Smith, 2003). Mudstone and fine-grained sandstone that Upper Syne Grass Thrust faults within the Humboldt Range include the is variably recrystallized to argillite, slates Triassic Valley Golden Stairs Thrust and the Humboldt City Thrust along and quartzite which the upper member of the Natchez Pass and the Grass Limestone with volcanics and terrigenous Valley Formation are thrust over the Prida Formation. These Natchez clastics grading into massive carbonates thrust faults are believed to have been formed during the Pass Massive limestone with interfingers of Nevadan orogeny (Johnson, 1977). Known normal faults mafic volcanics include the Range Front Fault along the base of the Humboldt Upper Member - limestone and dolomite Range and the Rye Patch Fault, a piedmont fault located Star Peak to the west of the Range Front Fault and near the existing Middle with thin interbeds of chert plant. Seismic sections, LiDAR, and aeromagnetics suggest Triassic Middle Member - silt, shale and siltstone Prida that there may be other parallel faults to the west of the Rye with interbedded carbonates Patch Fault and dip reversals identified in the most recent Lower member - siltstone, sandstone, and seismic survey may indicate antithetic faults further to the carbonates west (Ellis, 2011). A hypothetical structural cross-section is Unconformity presented as Figure 3. Weaver Rhyolitic tuffs, flows and volcaniclastics Lower Altered felsite with tuffaceous sedimen- Geothermal Setting Koipato Rochester Triassic tary rocks The general conceptual model that has been developed Limerick Altered andesitic flows and volcaniclastics for Basin and Range geothermal systems is that water flows Figure 2. Generalized stratigraphic column of rocks exposed in the Humboldt downward along faults and returns to the surface along simi- Range. lar pathways. Richards and Blackwell (2002) and Blackwell 534 McDonald and Gosnold et al. (2003) present three possible scenarios to account for the existing plant. Based on the numerous studies, explorations, wells different geothermal and groundwater systems and features that and other borings, and observations at the site, it appears that all are commonly found in the Basin and Range including fumaroles three scenarios of Richards and Blackwell (2002) as presented near range front faults, hot springs located further down in the diagrammatically in Figure 3 may be operating in the KRGA. valleys, and areas with shallow warm outflow dependent on the Specifically, the sinter deposits located in the Humboldt House groundwater piezometric surface and flow. These scenarios are area are indications of hot springs and fumaroles (MacKnight et illustrated in Figure 3. al., 2005) indicative of cases a) and possibly c) of that figure. MacKnight et al. (2005) also speculate that as the sinter deposits are younger than the Pleistocene Lake Lahontan sediments, that they must be fairly recent (< 10,000 y) and the linear nature observed by LiDAR suggest that they were structurally con- trolled by piedmont or antithetic faults in that area. The shallow, laterally-flowing outflow discharge plumes have been identified by temperature measurements in wells and exploration borings, shallow temperature measurements and drill holes and mining operations associated with the Florida Canyon mine (Waibel, et al., 2003) and are indicative of scenario b) of Figure 3. In ad- dition, the fumarole reported by Croffutt (1872), if it was close to the Humboldt Range as speculated, is also an indication of scenario c).