Text And Ref For The Geology Of The Huntsman Ranch, Elko .
Text to accompany Nevada Bureau of Mines and Geology Map 163GEOLOGIC MAP OF THEHUNTSMAN RANCH 7.5’ QUADRANGLE,ELKO COUNTY, NEVADAbyAlan R. Wallace1, Michael E. Perkins2, and Robert J. Fleck31U.S. Geological Survey, Reno, NVUniversity of Utah, Salt Lake City, UT3U.S. Geological Survey, Menlo Park, CA2INTRODUCTIONThe Huntsman Ranch quadrangle is 3 kilometers northeast ofthe town of Carlin in western Elko County (Fig. 1). Thequadrangle is in the middle of a topographic lowland that isflanked by the Adobe Range to the east, Swales Mountain tothe north, Marys Mountain to the west, and the Piñon Rangeto the south. Perennial Susie Creek and ephemeral Dry SusieCreek and Dry Gulch flow southward through thequadrangle, and all drainages feed into the Humboldt Riverjust south of the quadrangle border; perennial Maggie Creekflows southeast to the river through the southwest corner ofthe quadrangle.The Overland Trail, used by tens of thousands ofpioneers in the mid 1800s, followed the Humboldt River. Aspur of the trail over the southern Adobe Range—theGreenhorn Cutoff—was used by some wagon trains to avoidthe narrow Carlin Canyon along the river just southeast ofthe quadrangle. This spur traversed the southeastern part ofthe quadrangle, passed the hot springs shown in section 8between Susie and Dry Susie Creeks, and descended alongSusie Creek back to the river.The quadrangle also lies along the eastern edge of thenorthwest-trending Carlin gold trend, from which more than50 million ounces of gold have been produced (Thompsonand others, 2002). The Gold Quarry mine, which exploitsone of the largest gold deposits along the trend, is 8 km westof the quadrangle (Fig. 1).The middle Miocene sedimentary rocks that are exposedthroughout much of the quadrangle are the remnants of abroader Neogene sedimentary basin—here called the Carlinbasin—that once covered the Gold Quarry deposit and otherparts of the Carlin trend. Miocene sediments deposited inadjacent Miocene basins also host significant Mioceneepithermal gold-mercury deposits in northern Nevada, theclosest of which are in the Ivanhoe district, 50 km northwestof the quadrangle (Wallace, 2003).Figure 1. Location of the Huntsman Ranch 7.5-minute quadrangle (solidrectangle), surrounding 7.5-minute quadrangles, and geographic featuresmentioned in the text. Stippled areas are the approximate outlines ofinterpreted middle Miocene highlands that shed debris into the Carlinbasin, which is shown as white across much of the map. SchroederMountain is a topographic high, cored by Paleozoic sedimentary rocks,that formed after Miocene sedimentation. V pattern shows theapproximate location of the Carlin gold trend through the Carlin basinarea. Geologic maps of the surrounding quadrangles: 1, Emigrant Pass(Henry and Faulds, 1999); 2, Welches Canyon and 3, Rodeo Creek NE(Evans, 1980); 4, Swales Mountain SW (unmapped); 5, Swales Mountainand 6, Adobe Summit (Evans and Ketner, 1971); 7, Hunter (Ketner,1973); 8, Grindstone Mtn., 9, Carlin East, and 10, Carlin West (Smith andKetner, 1978); 11, Schroeder Mtn. (Evans and Cress, 1972).1
Studies of the Carlin basin and the Huntsman Ranchquadrangle were initiated to expand regional studies of thelate Cenozoic paleogeographic evolution of the region(Wallace and others, 2004, 2008; Wallace, 2005), includingthe roles of the basins in the formation of epithermal Au-AgHg deposits and the processes that took place after theformation of the late Eocene gold deposits along the Carlintrend. In addition, the Miocene sedimentary rocks form oneof the aquifers affected by dewatering at the GoldQuarry open-pit mine, and evaluation of the effectsof the sedimentary facies and faults on ground-watercompartmentalization in that aquifer became an outgrowth ofthe basin studies (Wallace, 2005).The Huntsman Ranch quadrangle and the Miocenesedimentary rocks of the Carlin basin have received littleattention. This was due in part to the general absence ofexposures of the Eocene and Paleozoic rocks that wererelated to or host major gold deposits in adjacent areas, andin part to the generally non-economic nature of the Mioceneunits. Regnier (1960) provided the first geologic descriptionsof the area in his study of the Cenozoic geology of the Carlinarea. Subsequent to Regnier’s studies, the geology of thesurrounding quadrangles was published in a series of 7.5minute quadrangle maps; these included, clockwise from thesouthwest: Emigrant Pass (Henry and Faulds, 1999),Schroeder Mtn. (Evans and Cress, 1972), Swales Mountainand part of the Adobe Summit quadrangle (Evans andKetner, 1971), and Hunter (Ketner, 1973) (Fig. 1). Inaddition, Smith and Ketner (1976, 1978) mapped the geologyof the Piñon Range area, the northern part of which includesthe south end of the Carlin basin. These mapping effortsfocused principally on the Eocene and older geologic unitsand features, and stratigraphic and structural details of thepost-Eocene units generally were not provided. Plume (1995)studied the hydrology of the Carlin basin as it related todewatering at the Gold Quarry mine, and Harlan and others(2002) provided some local detail on the late Cenozoicgeology in the vicinity of the Gold Quarry deposit. Overall,however, little was known about the Miocene and youngergeologic history of the Carlin basin before the present study.As a supplement to the map-unit descriptions, this textinsert discusses the Miocene sedimentology in the HuntsmanRanch quadrangle and adjacent areas, as well as thepaleogeographic evolution of the basin. All data andobservations from this and adjacent quadrangles are based onthis study except as cited otherwise.sedimentary rocks (PPsu), which depositionally overlie theallochthon (―overlap‖ rocks). Additional post-allochthonunits, including the Mississippian Tonka and MelandcoFormations and the Pennsylvanian Moleen and StrathearnFormations, are exposed a few kilometers southeast of thequadrangle (Ketner, 1973; Smith and Ketner, 1976; Trexlerand others, 2003), and they may be present beneath thesoutheastern part of the quadrangle. In the Gold Quarry area8 km west of the quadrangle, rocks of the Roberts Mountainsallochthon structurally overlie lower Paleozoic carbonaterocks of the lower plate (Evans and Cress, 1972; Evans,1980). These lower-plate rocks likely are present beneath theupper-plate and overlap rocks in the quadrangle, although atan unknown depth.Very limited exposures of early Tertiary rocks in thequadrangle include late Eocene rhyodacite to rhyolite flowsand tuffs. These units are similar to and locally continuouswith units exposed in the Swales Mountain area to the north(Evans and Ketner, 1971) and the southern Adobe Range tothe east (Ketner, 1973; this study). However, they are absentin several locations both within and near the quadranglewhere Miocene sediments were deposited directly onPaleozoic rocks. In the northeastern part of the quadrangle, afeldspathic sandstone underlies Eocene tuffs in a small area.The sandstone contains sparse, small, altered pumice or ashgrains and thus is not a pre-Tertiary unit, such as the SilurianElder Formation. The unit may be in part related to theEocene Elko Formation, a largely pre-volcanic clasticsedimentary unit that is exposed in nearby areas. Haynes(2003) described very sparse (less than 1 percent) volcanicmaterial in the Elko Formation, similar to this unit.The Elko County geologic map (Coats, 1987) showsmiddle Tertiary andesite rocks exposed in a small area westof Susie Creek and north of the Huntsman Ranch site in thisquadrangle and in extensive areas to the northeast and east.The state geologic map (Stewart and Carlson, 1978) showsthe same units but designates them as Miocene. The presentstudy shows that those areas previously mapped as andesitein this quadrangle actually are gravel deposits (QTg) that capa terrace surface. The Eocene rhyodacite flow units that areexposed in the northeastern corner of the quadrangle are notshown on the other geologic maps, and they are continuouswith the Eocene ―andesite‖ flows in the Adobe Summitquadrangle (Coats, 1987).The area of the Carlin basin was part of a V-shaped(plan view), late Eocene paleotopographic high that extendednorthwest from the northern Piñon Range through thesouthern Tuscarora Mountains and bifurcated to the northeastalong the Adobe Range (Haynes, 2003). Although Haynes’paleogeographic reconstructions are generalized at a smallscale, it appears that the area of this quadrangle wastopographically high in the late Eocene. This would explainthe apparent absence of the Elko Formation, which wasdeposited on the flanks of and in basins between the Eocenehighlands (Haynes, 2003).In the few areas where early Tertiary rocks are exposedbeneath the Miocene sedimentary rocks in and near theHuntsman Ranch quadrangle, the early Tertiary rocks dipmore steeply than the Miocene sedimentary rocks. In thenortheastern part of the quadrangle, the difference in dipPRE-MIOCENE GEOLOGYThe extensive exposures of Miocene rocks conceal most ofthe pre-Miocene geology of the quadrangle. On the basis ofexposures in this and surrounding quadrangles, Paleozoicsedimentary rocks, with a discontinuous cover of late Eocenerocks, underlie the Miocene sedimentary rocks. Paleozoicunits exposed in this quadrangle include the OrdovicianVinini Formation (Ov), which is part of the upper plate of theRoberts Mountains allochthon (Roberts and others, 1958),and the Mississippian Webb Formation (Mw) and undividedPermian and Pennsylvanian carbonate and clastic2
ranges from 5º to 17º, and similar to slightly greaterdifferences were measured in the southern Swales Mountainand western Hunter quadrangles. To the west in the EmigrantPass quadrangle (Henry and Faulds, 1999), dip differencesaverage about 18º. There, pre-Miocene north-striking normalfaults tilted the Eocene rocks and one overlying 25-Ma tuffto the east prior to Miocene sedimentation, with some postsedimentation fault reactivation. Similar pre-Miocene faultshave not been found in or near the Huntsman Ranchquadrangle. However, the extensive Miocene rocks couldconceal any older faults, and some of the many faults that cutMiocene sediments could be reactivated older faults.Additional data indicate that at least some of the tiltingappears to have occurred in the late Oligocene to earlyMiocene. Thermochronologic data along the Carlin trend(Fig. 1) suggest Oligocene to early Miocene uplift anddenudation (Tosdal and others, 2003), and alunite dates fromthe trend indicate supergene weathering during the sameperiod of time (summarized in Wallace and others, 2008).Local red-colored regoliths in Paleozoic rocks beneathMiocene sedimentary rocks in this quadrangle may haveformed then as well. Some clasts in the basal HumboldtFormation sediments were derived from late Eocene plutonsin the Swales Mountain and southern Tuscarora Mountainsareas. Although the depth of emplacement of these plutons isunknown, they had been uplifted and exposed by thebeginning of sedimentation in the Carlin basin shortly before16.3 Ma.Carlin formation to describe post-mineralization sedimentaryrocks that were laterally continuous with or similar toRegnier’s Carlin formation. However, Smith and Ketner(1976), in their study of Cenozoic sedimentary rocks on theeast and west sides of the Piñon Range south of Carlin,determined that the Regnier’s Carlin formation wasequivalent to the Miocene Humboldt Formation in the typearea east of the Piñon Range, and that the Raine Ranchformation was late Eocene to early Oligocene in age and thusunrelated to the Humboldt Formation.Mapping for the present study within and beyond theCarlin basin (Wallace and others, 2004, 2008; Wallace,2005) shows that the Miocene Carlin depositional basinextended beyond the limits of the modern basin andconnected with the depositional system that produced theHumboldt Formation in its type area (Smith and Ketner,1976). Therefore, the name Humboldt Formation is appliedto the middle Miocene sedimentary rocks in this quadrangle,in keeping with the rationale described by Smith and Ketner(1976) and, in part, Sharp (1939).GeochronologyTephra correlations were obtained from ash-rich samples ofthe Humboldt Formation in the quadrangle and nearby areas,and 40Ar/39Ar dates were obtained on some of those samples.Two additional tephra samples were dated using 40Ar/39Armethods but do not have tephra correlations. Dating of theandesite (Ta) and rhyolite (Trf) flow units in this quadranglewas attempted but not successful. The Palisade Canyonrhyolite, which is widely exposed in the western andsouthern parts of the basin and is exposed in a small area inthis quadrangle, was dated by Armstrong (1970) and Henryand Faulds (1999). The very limited Eocene volcanic rocksin the quadrangle were not dated because of their directcorrelations with dated units in the Swales Mountain area tothe north (Henry and Ressel, 2000).Samples for tephrochronology and isotopic dating werecollected from three general areas in and immediatelyadjacent to the quadrangle: (1) the tephra-bearing upper twothirds of the Humboldt Formation north of Dry Gulch in thenorth-northwest part of the quadrangle (sections 1 and 2, Fig.2); (2) along Susie Creek in the northeastern part of thequadrangle (section 3); and (3) from the Great White Hopediatomite mine a few hundred meters south of the quadrangle(section 4). The samples in the first area were collected fromoutcrops at the base of the epiclastic and ash-rich member(Thm) up through the top of the ash-rich member (Tha).Samples for the second area were collected from outcrops ofthe epiclastic and ash-rich member on both sides of SusieCreek near the Huntsman Ranch site, as well as from roadcuts in the ash-rich member at the north edge of thequadrangle just north of Susie Creek. The third suite ofsamples was collected from the ash-rich member in open cutsand outcrops at and near the diatomite mines in the CarlinEast quadrangle. The units sampled extend into theHuntsman Ranch quadrangle. M. Perkins collected severalsamples at the third area prior to this study, and those tephracorrelations are shown in Figure 2 and Table 1 along with thenew data.MIOCENE GEOLOGYMiocene sedimentary units are exposed throughout most ofthe quadrangle. Epiclastic sediments and ash were depositedin fluvial and lacustrine environments from before 16.3 Mauntil after 15.1 Ma, and minor andesite and rhyolite flowswere erupted during sedimentation. Post-sedimentationnormal faults have cut and gently tilted all of the sedimentaryand volcanic units. The following text provides detaileddescriptions of the units and interpretations of thesedimentary environments. Nearly complete stratigraphicsections are exposed just east of Susie Creek southeast of theHuntsman Ranch site and in section 11, and in areas to thenorth and west in the northwestern part of the quadrangle.Nomenclature of Miocene Sedimentary UnitsSharp (1939) assigned the name Humboldt Formation tomany of the Cenozoic rocks in the region, especially thosewest of the Ruby Mountains and East Humboldt Range nearElko. Subsequent work by others removed some units, suchas the Eocene Elko Formation, from that formation (VanHouten, 1956), but the name Humboldt Formation wasretained for the Miocene units.Regnier (1960), in his study of the sedimentary rocks inthe Carlin area, informally divided the sedimentary rocks inthe Carlin basin into the near-source pyroclastic andvolcaniclastic Raine Ranch formation and overlying fluvialand ash-rich Carlin formation. As the major Eocene golddeposits of the Carlin trend were discovered and mined sincethe early 1960s, numerous authors informally used the name3
40Ar/39Ar dates in the Carlin basin. Inset map of quadrangle shows general locations ofsampling areas; those areas and specific locations are cited in Table 1 and shown on the geologic map. Tephra composition types are based on regionaltephra sources and types discussed in Perkins and others (1998) and Perkins and Nash (2002). Stratigraphic symbols are those used on the geologicmap. Thicknesses of units are schematic only.Figure 2. Schematic diagram showing tephra correlations andTable 1. Tephra correlations and 40Ar/39Ar dates, Huntsman Ranch quadrangle.[--, no sample or not determined; all 40Ar/39Ar dates calculated using 28.02 Ma FCT sanidine standard (Renne and others, 1998).]Area(fig. eLongitude40Ar/39Ar*date (Ma)Tephraage 283CLN04-1284CLN04-1287CLN04-1288, mTha(Trf)**(Trf)**(Trf)**40 50.77840 50.77840 50.61640 50.60440 50.57240 51.80840 51.80840 51.70640 51.61240 51.61240 51.39240 51.39240 51.35340 44.88740 44.88840 44.89540 44.97840 44.97840 52.38240 52.38240 51.27840 51.3340 49.82440 49.85740 37.3440 42.740 36.92-116 1.096-116 1.096-116 5.713-116 5.724-116 5.767-116 5.657-116 5.657-116 5.679-116 5.66-116 5.66-116 5.329-116 5.309-116 5.236-116 3.548-116 3.547-116 03.475-116 03.659-116 03.659-116 0.52-116 0.52-116 5.135-116 5.127-116 02.439-116 00.926-116 11.20-116 16.1-116 11.8316.28 0.04-15.75 0.0316.03 0.05-15.76 0.02--15.92 0.08--16.31 0.25-15.17 0.0315.2 0.03-----15.73 0.0615.56 0.0516.38 0.116.16 0.0315.26 0.0315.32 0.0415.4 1.0*** 16.0 16.0 15.9-- 15.9 15.915.9415.7-16.015.9215.94 15.3-15.28 16.0 15.8 15.5 15.516.1-15.5-------This studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyThis studyPerkins, unpubl. data, 2002Perkins, unpubl. data, 2002Perkins, unpubl. data, 2002This studyThis studyThis studyThis studyThis studyThis studyWallace and others (2008)Henry and Faulds (1999)Armstrong (1970)* Sample 049-21E dated using incremental step heating on plagioclase. All other 40Ar/39Ar dates were single-crystal analyses of multiple sanidinecrystals.** Palisade Canyon rhyolite (rhyolite of Marys Mountain in Henry and Faulds, 1999).*** K/Ar whole-rock date; recalculated from published 15.0 1.0 Ma date.4
Sample locations within the quadrangle are shown onthe geologic map and in Figure 2. Table 1 provides samplelocation and geochronologic data. 40Ar/39Ar dates wereobtained on single-crystal analyses of multiple sanidinecrystals from the same sample, and dates and uncertaintiesare the weighted mean of multiple analyses. Sample 049-21Ewas dated using incremental step heating on plagioclase.Tephra correlations were determined using the chemical andpetrologic methods described in Perkins and others (1998).All published and new 40Ar/39Ar dates reported here werecalculated using the 28.02 Ma standard for Fish Canyon Tuffsanidine (Renne and others, 1998).In the northwestern part of the quadrangle, the memberis composed of pebble conglomerate, pebble- to cobblebearing sandstones, and sandstone; boulders up to a meter indiameter are abundant in some beds. Lithologies of the clastsare identical to various Paleozoic sedimentary and Tertiaryvolcanic rocks exposed in the southern Tuscarora Mountainsto the west and Swales Mountain to the north. The sandcomponent is dominantly epiclastic but contains variableamounts of reworked ash and small pumice.The beds and sedimentary structures in this area are notwell exposed. As exposed, the dominant bedform isstructureless sand with minor to abundant amounts of matrixsupported (―floating‖) small pebbles, cobbles, and, to a lesserextent, boulders up to a meter in diameter. The sandy matrixweathers readily, resulting in a residual surface lag of claststhat commonly gives the impression that the source rockshad m
the north, Marys Mountain to the west, and the Piñon Range to the south. Perennial Susie Creek and ephemeral Dry Susie Creek and Dry Gulch flow southward through the quadrangle, and all drainages feed into the Humboldt River just south of the quadrangle border; perennial Maggie Creek
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