OVERVIEW GEOLOGIC OF THE RAIN SUBDISTRICT
Anthony A. Longo1, Tommy B. Thompson2, and J. Bruce Harlan1
ABSTRACT
Gold ore in the Rain subdistrict developed along the unconformity between the Mississippian Webb Formation and the underlying Devonian Devils Gate Limestone. Hydrothermal breccia developed in thin-bedded to laminated mudstone of the middle to lower Webb Formation and collapse breccia in medium- to thick-bedded limestone of the upper Devils Gate Limestone that served as channelways for gold-bearing hydrothermal solutions. These breccias were exposed at the Rain open-pit deposit and extend underground for more than 3 miles (5 km) northwest of the open pit. Pipe-like breccia bodies, some containing higher-grade gold mineralization, developed within the Devils Gate Limestone below the collapse breccia zones. Sandstone of the Devonian Oxyoke Canyon Formation also hosts weak gold mineralization 1,400 feet (430 m) below the Rain open-pit deposit. Deep gold mineralization in the Devils Gate Limestone and Oxyoke Canyon Formation is poorly understood, and the economic potential has not been established. The stratigraphic framework in the Rain subdistrict is discussed as a time-stratigraphic depositional sequence in relation to the Antler orogeny. Paleozoic rocks in the Rain subdistrict are divided into four sequences of tectonofacies: (1) autochthonous pre-Antler orogeny lower-plate carbonates of the eastern assemblage miogeocline, (2) autochthonous proto-Antler flysch deposits that lie disconformably on the underlying carbonates, (3) allochthonous syn-Antler upperplate eugeoclinal to flysch siliciclastics, and (4) post-Antler orogenic molasse siliciclastics (overlap assemblage). The autochthonous (lower-plate), allochthonous (upper-plate), and overlap assemblages are sedimentary rocks whose designations pertain to whether the rocks lie below or above the Roberts Mountains thrust, or overlap the upper- and lower-plate rocks. Lower-plate rocks include tectonofacies sequences 1 and 2. Pre-Antler lower-plate carbonate rocks are micritic to sandy dolostone to sandstone of the lower-plate Devonian Nevada Group, and micritic limestone of the lower-plate Devonian Devils Gate Limestone. Proto-Antler lower-plate flysch deposits are mudstone to arkosic sandstone and conglomerate of the Mississippian Webb and Chainman Formation. The Devils Gate Limestone and Webb Formation mudstone are exposed in the area from BJ Hill to the Rain open-pit deposit, and the coarse-grained sandstone of the Chainman Formation forms the dominant outcrop pattern from the Rain open-pit deposit to the Saddle deposit. Syn-Antler upper-plate eugeoclinal to flysch siliciclastics at Rain display a variety of lithologic types that belong to the Devonian Woodruff Formation. These consist of shaley and cherty mudstone to rhythmically-banded chert and cherty siltstone adjacent the Rain open-pit deposit, and phosphatic chert,
1 2
Newmont Mining Corporation Ralph J. Roberts Center for Research in Economic Geology
mudstone, shale, cherty siltstone and sandstone west of the Saddle deposit. Imbricated thrust slices of allochthonous Woodruff Formation may thicken the section and place a cherty clastic sequence in contact with phosphatic nodular chert in Woodruff Creek. Tectonic windows through the upper-plate Woodruff Formation into the lower-plate Chainman and Webb Formations are also found west of the Saddle deposit in Woodruff Creek. Post-Antler orogeny molasse siliciclastics, or the overlap assemblage, include clastic rocks of the Late Mississippian-Early Pennsylvanian Diamond Peak Formation that overlap the Woodruff Formation. These rocks consist of coarse conglomerate, sandstone, siltstone, shale, and minor limestone. The sandstone and siltstone of the Diamond Peak Formation are similar to the clastic rocks of the Mississippian Chainman Formation, and without careful observation, these rocks can be misinterpreted as either rock formation. For example, conglomerates of the Diamond Peak Formation are characterized by chert and white quartzite cobbles that range in size from pebbles to boulders. Whereas, green chert pebbles are most characteristic of clastic rocks in the Chainman Formation. Three major fault zones controlled gold mineralization in the Rain subdistrict. The Rain fault zone is a N60W-trending structural corridor that defines the Rain horst and includes the Rain fault, the Dike fault, and the SB fault. The N30Etrending Northeast fault zone truncated the Rain fault zone southeast of the Rain open pit, and the north-trending Emigrant fault zone hosts the Emigrant Springs deposit in mudstone of the Webb Formation along the contact of the Devils Gate Limestone. These fault zones juxtapose the stratigraphic relationships of the Rain lithologies in a normal sense of vertical displacement by up to 3,000 feet (900 m). Both the Rain and the Emigrant fault zones are described as having associated horst and graben blocks based on the apparent vertical displacement of stratigraphy across their boundaries. Their subsequent horst blocks are interlaced with a complex network of splays and crosscutting faults, some of which controlled gold mineralization. These complex structural relationships support a flower structure interpretation for the Rain fault zone that can be attributed to wrench faulting. Hydrothermal activity at Rain began with an early passive stage of silicification along the contact to the Devils Gate Limestone and Webb Formation. During this stage, calcite dissolution and concurrent dolomite replacement of the Devils Gate Limestone developed beneath a portion of the silicified rock. Early iron sulfides, arsenian pyrite, native gold, and rutile developed with quartz during silicification. At least two stages of hydrothermal brecciation and one event of collapse breccia preceded the main ore stage. These breccias were accompanied by the precipitation of barite. Later main-stage ore filled the remaining voids in the breccias with pyrite encapsulated by arsenian overgrowths containing gold. Breccias were flooded with quartz, rutile, and apatite. Late vug filling and veinlets
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included minerals of barite, orpiment, and cinnabar. Supergene phosphates, chalcedony, iron oxides-hydroxides-sulfates, alunite, and kaolinite developed from the oxidation of the iron sulfide-bearing breccia. The age of gold mineralization at Rain is approximately 31.710.3 Ma based on fission track dating of hydrothermal apatite, predating the oldest supergene alunite age of 22 Ma. Gold mineralization at Emigrant Springs postdates the age for an altered monzonite dike of 37.50.8 Ma. Pennsylvanian in the northern Pion Range. The misidentification of a lithology, such as confusing siliceous mudstone from the allochthonous Devonian Woodruff Formation for siliciclastic mudstone from the autochthonous Mississippian Webb Formation, has resulted in a misinterpretation as to the location of the Webb-Devils Gate contact horizon. This in turn has led to drilling many hundreds of feet in upper-plate siliceous assemblage rocks that are barren of gold. Successfully pinpointing this horizon within the Rain subdistrict requires an understanding of the rock formations within the Devonian and Mississippian systems, and the ability to distinguish rocks of the autochthonous Mississippian Chainman and Webb Formations from rocks of the allochthonous Devonian Woodruff Formation and overlap rocks of the Late Mississippian-Early Pennsylvanian Diamond Peak Formation.
INTRODUCTION
A combination of stratigraphic and structural relationships was responsible for the localization of gold ore in the Rain subdistrict. Distinct lithologic types from three separate Devonian and Mississippian formations have been juxtaposed along major faults (the Rain and Emigrant faults) with stratigraphic throw of up to 3,000 feet (914 m) (Mathewson, 1993). The sedimentary rocks include the upper-plate cherty mudstone of the Devonian Woodruff Formation in fault contact with lower-plate limestone of the Devonian Devils Gate Limestone and siliciclastic mudstone of the Mississippian Webb Formation. Gold ore is found in the Webb Formation and the Devils Gate Limestone spatially associated with altered igneous dikes and tuffisites that intrude the major fault zones. Three characteristics of a major ore-controlling fault at Rain are as follows: (1) distinct surface geochemical anomalies, (2) the presence of altered igneous intrusions, and (3) a distinct gravity signature (gravity surveys delineate the margin of the structurally uplifted block that defines the Rain horst). Therefore, the elements that lead to the discovery of gold orebodies within the Rain subdistrict combine a thorough understanding of stratigraphic relationships with the recognition in the field of the major ore-controlling fault. At Rain, medium- to thick-bedded, fine-grained limestone of the Devils Gate Limestone lies below thin-bedded to finelylaminated mudstone and claystone of the Webb Formation. The importance that this contact has on the localization of gold mineralization cannot be overstated for the deposits within the district. After numerous deep core holes were drilled into the Devils Gate Limestone along the Rain fault, geologists at Rain interpreted that early hydrothermal activity resulted in carbonate dissolution and dolomitization of the limestone that underlies platy mudstone (Mathewson, 1993). Evidence suggests that alteration induced a volume loss in the limestone section that caused subsequent collapse and brecciation within both rock formations (Devils Gate Limestone and Webb Formation). Collapse breccia formed as a result of reactive fluid channeling along the Rain fault and the limestone-mudstone contact. Multiple episodes of hydrothermal brecciation in Webb Formation mudstone and claystone eventually developed a permeable breccia through which ore fluids passed. This process gradually allowed stoping upward along the Rain fault to significant levels above the upper contact of the Devils Gate Limestone (Williams, 1992; Williams and others, 2000). A challenge presented to geologists in the Rain subdistrict is that many lithologic types are similar in appearance throughout the various rock formations from Middle Devonian to Middle
PREVIOUS WORK
Early work in the northern Pion Range (Smith and Ketner, 1975a,b) established the geologic framework for the Rain subdistrict. Over the years, the work of numerous geologists and geophysicists from Newmont Mining Corporation, the University of Nevada, Reno, Colorado State University, and the United States Geologic Survey provided many contributions to the understanding of the stratigraphy, the igneous rocks, and the structure as discussed in this report. This work includes that of Knutson and West (1984), Thoreson (1990a), Jory (1992a), Williams (1992), Mathewson (1993), Mathewson and others (1994), Lane and Heitt (1994), Jones and others (1995), Heitt (1996), Longo (1996), Longo and others (1996, 1997), Read and others (1998), Shallow (1999), and Williams and others (2000).
THE RAIN SUBDISTRICT AREA
The area designated the Rain subdistrict lies within the northern Pion Range along the southern part of the Carlin trend in Elko County, Nevada (fig. K-1). The center of the subdistrict is located 9 miles (14.5 km) southeast of the city of Carlin at the Rain open-pit deposit, and it encompasses an area of over 30 square miles (77 km2). To date nine gold deposits have been discovered (fig. K-2). Rain is considered a subdistrict to the larger Carlin and Maggie Creek districts in the northwest and therefore part of the Carlin trend; however, it is separated from the central portion of the Carlin trend by a distance of nearly 15 miles (24 km) between Rain and Maggie Creek (fig. K-1).
Tectonic Setting of the Rain Subdistrict
The earliest and potentially the most significant orogenesis began in Early Mississippian time with the onset of the Antler orogeny. Deposition of mudstone of the Early Mississippian Webb Formation is evidence for an initial phase of the westward prograding Antler highland in the Rain subdistrict. Westward progradation continued during Mississippian time as evidenced by a coarsening-upward sequence of sandstone to pebble conglomerate that defines a transitional contact between the Webb Formation and the overlying Chainman Formation (Mathewson, 1993; Longo and others, 1996). By Late
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Tusc Mac Gold Quarry
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Figure K-1. Map of the Carlin trend showing the major gold deposits.
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Rain Subdistrict
Mississippian time, eastward-directed thrusting overrode the synorogenic flysch sequence of the Webb and Chainman Formations. During this deformation, siliciclastic rocks of the allochthonous late Devonian Woodruff Formation were thrust eastward over rocks of the Chainman Formation, thus forming the upper plate of the Roberts Mountains thrust (Mathewson, 1993; Longo and others, 1996). As a result, lower-plate autochthonous rocks are relatively undeformed and characterized by broad and open folds, whereas the allochthonous rocks are highly deformed into tight, isoclinal to crenulated inclined folds. Post-Antler orogenic overlap siliciclastic rocks of the Late Mississippian Diamond Peak Formation were then deposited eastward over highly deformed siliciclastic rocks of the upperplate Woodruff Formation. The overlap contact in the Rain subdistrict is characterized by limestone, shale, and marl of the Diamond Peak Formation in angular unconformity with chert and chert-cemented clastic rocks of the Woodruff Formation. In the northern Pion Range, a post-Antler deformational event (the Late Jurassic Humboldt orogeny) is evident within the Diamond Peak Formation rocks as gentle to tight, north to northwesttrending folds (Ketner, 1977). In the Mesozoic or Tertiary, paleoshearing along the Rain fault zone (fig. K-2), named after the prominent Rain fault, formed a structural corridor for the emplacement of lamprophrye and tuffisite dikes, and served as a conduit for gold-bearing hydrothermal fluids. Geologists are still debating time constraints on the gold mineralization and emplacement of the intrusions. Recurrent movement on the Rain fault zone is interpreted to have occurred during middle Tertiary Basin and Range deformation. The Rain fault zone forms a structural discontinuity over a strike length similar in magnitude to the Post fault zone in the northern Carlin trend.
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Figure K-2. Generalized structural map of the Rain subdistrict showing gold footprints, principal faults, and geographic locales discussed in text.
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Geophysics
Geophysics plays an integral role in gold exploration along the major structural features in the Rain subdistrict. Gravity surveys have helped to delineate fault boundaries by measuring density contrasts along the major structural discontinuities. Gravity anomalies define the boundaries and lateral extent of the Rain fault zone and Rain horst northwest of the Rain openpit deposit. A gravity anomaly also defines the north-trending Emigrant Springs horst block east of the Rain deposit.
Exploration and Production History
The original Rain claims were staked by Price (Turk) Montrose, a local barite prospector, over a barite-bearing jasperoid outcrop. Based on similarities between this exposure and goldbearing outcrops along the Carlin trend, Mr. Montrose submitted his claims to Newmont Mining Corporation in 1979 (Knutson and West, 1984). Newmont acquired the property on the basis of samples with up to 0.48 opt (16.5 g/t) gold from the Rain jasperoid discovery outcrop. Detailed exploration began with systematic rock chip and soil geochemistry in the Rain subdistrict followed by drilling. Soil samples with anomalous arsenic, locally exceeding 1,000 ppm, helped to delineate the northwest-striking Rain jasperoid. Initial reverse circulation exploration drilling in 1982 and 1983 defined an initial reserve of more than 680,000 ounces (21 t) of gold. Subsequent infill and step-out drilling in the area of the Rain open pit eventually increased this to over one million ounces (31 t) in 1994. Satellite deposits at Emigrant, Gnome, Snow Peak, and the Southern Mineralized Zone (SMZ) were also discovered during this time (fig. K-2).
140,000
Construction of the Rain access road began in July 1987, and mining began in October of the same year. The first gold bar was poured in June 1988 (fig. K-3). Open-pit mining continued from 1988 through 1994 resulting in the recovery of 707,949 ounces (22 t) of gold (table K-1). Production peaked in 1990 and 1991 when 135,500 ounces (4.2 t) of gold were produced each year (fig. K-3). In 1992, a reevaluation of exploration potential in the Rain subdistrict began with a program of detailed geologic mapping, close-spaced gravity surveys, rock chip geochemistry, comprehensive data compilation, and deep drilling. This work greatly enhanced the understanding of the geologic setting and controls on gold mineralization. During a period of nearly 5 years (19921997), exploration was rewarded with continued discovery outward along the northwest extension of the Rain fault and southward along the Emigrant fault. The discovery of the Rain Extension in 1992 by Mathewson (1993) initiated the momentum, and exploration continued to push the envelope outward. This work led to the discovery of the Tess deposit in 1993 and 1994 (Mathewson and others, 1994; Jones and others, 1995), and the discoveries of the NW Tess and Saddle deposits (fig. K-2) in 1995 and 1996 (Longo and others, 1996, 1997). Underground mining began at Rain in early 1994 with the development of Stope 1 and Stope 2 in the immediate hanging wall of the Rain fault. Rain Underground geologists split the Rain Extension into three segments called Stope 1, Stope 2, and Zone 3, and the Tess deposit into three segments called Zone 4, Zone 5, and Zone 6 (fig. K-2). These deposits were high-grade extensions of the Rain open-pit deposit (fig. K2). Stope 1 was the first underground mine developed on the Carlin trend, and the mine was accessed by a portal and decline
Open pit mill production 120,000 Underground mill production Open pit leach production Underground leach production
Ounces of Gold Produced
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Figure K-3. Annual gold production from the Rain Mine, 1988 to 1999.
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Table K-1.MRain reserves and resources (Crouse, 1990; Jory, 1990; Thoreson, 1990a; Lane and Heitt, 1994; Heitt, 1996; Williams, 1997; Harlan, 1999; Odell, personal commun., 1999; Roman, personal commun., 2000; Mallete, 2001)
Ore Type and Cutoff Grade Date Drilled Date Mined Oxide 19821987: 100-foot grid 1987: 100-foot. grid 19821983: 400-foot grid 1990: 200-foot grid 19932000 1994 19881994 Gold Produced
Deposit
Gold Reserve and Resource
Rain open-pit
1,017,300 oz (15.5 million tons@0.066 opt) 31.6 t (14 Mt@2.3 g/t) Oxide
707,949 oz 22.0 t 19,000 oz 591 kg
SMZ
30,000 oz (1.5 million tons@0.019 opt) 0.9 t (1.4 Mt@0.65 g/t) Oxide Cutoff: 0.01 opt
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2/3 oxide Cutoff: 0.150 opt oxide 0.250 opt refractory Sulfide Cutoff: 0.30 opt 10 g/t Cutoff: 0.20 opt 7 g/t 19962000
Emigrant Springs
360,200 oz (14.6 million tons@0.025 opt) 11.2 t (13.1 Mt@0.86 g/t)
Planned for 2002@$325 Au
On standby with $270/oz. Au
Rain Underground
265,047 oz (1.154 million tons@0.230 opt) 8.2 t (1.04 Mt@7.9 g/t)
19941998: Stope 1, Stope 2, and Zone 3 19992000: Zone 4
Total until 2000: 114,815 oz 3.57 t Zone 4 in 1999: 30,024 oz 933 kg No mine plan None
Saddle/NW Tess
782,000 oz (1.37 million tons@0.572 opt) 24.3 t (1.23 t@19.6 g/t)
1,475,000 oz (3.99 million tons@0.370 opt) 45.9 t (3.59 Mt@12.7 g/t)
Rain Subdistrict
along the northwest wall of the open-pit. The decision to conduct underground test mining was based on a resource of only 4,375 ounces (136 kg) of gold (A. London, personal commun., 1999). Since mining of Stopes 1 and 2, drifting and underground diamond drilling led to a steady increase in the underground reserves and resources. Stope 1, Stope 2, and Zone 3 were mined out between 1994 and 1998. Production from Zone 4 began in late 1998, and by 1999 it totaled 97,166 tons (88,170 t) grading 0.309 opt (10.6 g/t) for 30,024 ounces (936 kg) of gold (table K-1 and fig. K-3). Zone 4 became the largest and highest-grade underground reserve identified to date in the Rain subdistrict. Underground exploration pushed the development drift out to Zone 6 of the Tess deposit approximately 4,000 feet (1,220 m) to the northwest along strike with the Rain fault. By year 2000, underground mining at Rain had produced 114,815 ounces (3.56 t) of recovered gold (table K-1). Widespread surface drilling indicates mineralization continues along the Rain fault for at least 3,000 feet (900 m) beyond Zone 6 of the Tess deposit (Mathewson and others, 1994; Jones and others, 1995; Longo and others, 1996; 1997; Read and others, 1998; Mallete, 2001). This area contains the underground sulfide resource of the Saddle/NW Tess deposit (fig. K-2). Manually constructed, resource polygons for the Saddle/NW Tess deposit indicate 782,000 ounces (24.2 t) averaging 0.572 opt (19.6 g/t) using a 0.3 opt (10.3 g/t) cutoff (table K-1) (Mallete, 2001).
Oxyoke Canyon Formation The Devonian Oxyoke Canyon Formation lies below and is transitional into the overlying Telegraph Canyon Formation (fig. K-5). The transitional sequence into the Telegraph Canyon Formation consists of 240 feet (73 m) of sandy dolostone that grades into cream-colored quartzite. Total thickness beneath the Rain open-pit deposit, including the transitional sequence, is at least 360 feet (110 m). The Oxyoke Canyon Formation is 244 feet (74 m) thick in a Mobil Oil drill hole completed within the Rain graben north of Woodruff Creek (SWc, NWc, Sec. 19, T32N, R53E) (fig. K-2). Gold mineralization is known to exist within the clastic units of Oxyoke Canyon Formation below the Rain open-pit deposit where grades up to 0.351 opt (12 g/t) gold are associated with black silica stockwork veins and oxidation, and lower grade disseminated gold is found scattered within the transitional dolomitic sandstone sequence. Present data suggest potential is low for underground mineable gold ore. Telegraph Canyon Formation Rocks of the Devonian Telegraph Canyon Formation range from a basal silty limestone and dolomite to medium and dark colored micritic dolomite near the top. Thicknesses range from 480 feet (146 m) below the Rain open-pit deposit to 1,235 feet (376 m) north of Woodruff Creek in the Mobil Oil drill hole (fig. K-2). This unit appears to grade into the overlying Devils Gate Limestone. DEVONIAN DEVILS GATE LIMESTONE The Devils Gate Limestone is exposed only in the area of the Rain open-pit deposit and the Emigrant Springs deposit (fig. K-4). Numerous drill holes penetrate the unit elsewhere in the district. Rocks of the Devils Gate Limestone are thin- to thickbedded, medium to dark gray, micritic limestone that is a moderate ledge former and weathers into variably-sized blocks. Locally, the limestone is fossiliferous with stromatoporoid colonies and horn coral. Thicknesses are 485 to 860 feet (148 to 262 m) at the Rain open-pit deposit and 735 feet (224 m) in the Mobil Oil drill hole north of Woodruff Creek (fig. K-2). Gold mineralization was discovered at depth below the Webb Formation in the Devils Gate Limestone at the Saddle (Longo and others, 1997) and BJ Hill deposits (Mathewson and others, 1994) (fig. K-2). High-grade gold ore in these deposits occurs in breccias of silicified, dolomitized, and oxidized limestone that are interpreted as near vertical and irregular, pipe-like, collapse breccias. Gold grades in these breccias average 0.22 opt (7.5 g/t) at the BJ Hill deposit and 0.15 opt (5.1 g/t) in the Saddle deposit. Grades as high as 0.63 opt (22 g/t) gold in the Saddle deposit occur in matrix-supported, calcite-healed breccia with sooty sulfide.
STRATIGRAPHY
Rock types observed for 5 miles (8 km) along the Rain fault, and those to the east along the Emigrant fault, are discussed below and shown on the geologic map (fig. K-4) and the stratigraphic section (fig. K-5). The stratigraphy is discussed as a time-stratigraphic depositional sequence in relation to the evolution of the Antler orogeny. Stratigraphic relationships, the relative positions of tectonostratigraphic sequences, and the major structural features are displayed in figure K-5 for both the Rain horst and the Rain graben. The discussion addresses the autochthonous miogeoclinal pre-Antler orogeny lower-plate carbonates, the autochthonous proto-Antler lowerplate flysch deposits, the allochthonous syn-Antler upper-plate siliciclastics, and the post-Antler overlap molasse sequence. Descriptions of the Tertiary Elko Formation and a discussion on the igneous dikes and sills that intruded the fore mentioned Paleozoic rock units are included in the synopsis.
Pre-Antler Orogeny Lower-Plate Carbonates
DEVONIAN NEVADA GROUP Rocks of the Devonian Nevada Group include the Oxyoke Canyon and Telegraph Canyon Formations. Neither unit crops out in the Rain subdistrict, and their lithologies are known only from drill cuttings of two deep reverse circulation drill holes that were collared within the Rain open-pit.
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Rain Subdistrict
Early Antler Orogeny Flysch Deposits of the Lower Plate
WEBB FORMATION Exposures of the Mississippian Webb Formation occur in the area of the Rain open-pit and Emigrant Springs deposits (fig. K-4) as recessively weathered outcrops of platy, black to dark gray or light brown to yellow brown mudstone that is typically found as float or talus. The lower mudstone sequence in the Webb Formation is the dominant host for gold mineralization
in the Rain deposits. Discovery outcrops for the Rain open-pit deposit were large blocky ribs of pervasively silicified mudstone of the Webb Formation with barite along the Rain fault. Exposures along the Emigrant fault are in blocky, silicified, and locally liesegang banded, reddish-gray to greenish-gray outcrops. The Webb Formation consists of a sequence of noncalcareous, platy, impermeable, carbonaceous mudstone and siltstone with <10% coarse clastics as discontinuous lenses of sandstone and conglomerate (Mathewson, 1993; Mathewson and others, 1994). Rocks are well bedded with thicknesses of
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Quaternary alluvium/colluvium Tertiary andesite lava flows Tertiary Elko Formation limestone, siltstone, mudstone, tuff Lamprophyre intrusive rock Diamond Peak Formation conglomerate, sandstone, shale
UPPER PLATE ROCKS Dw
Woodruff Formation cherty mudstone, siltstone, limestone Chainman Formation sandstone, conglomerate, siltstone Webb Formation mudstone, siltstone, sandstone Devils Gate Limestone limestone
Fault, ball on downthrown side, dashed where inferred, dotted where hidden Thrust fault 0 0 6000 feet 2000 meters
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OVERLAP ROCKS DMdp
Figure K-4. Generalized geologic map of the Rain subdistrict.
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Mississippian/Pennsylvanian Diamond Peak Formation
NF AU L T
(Exists as scabs within the Rain horst only observed in West Woodruff Creek) angular unconformity Lamprophyre and tuffisite dikes
Devonian Woodruff Formation
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R AI
Mississippian/ Pennsylvanian Diamond Peak Formation
Coarse comglomerate sandstone, siltstone with marl, shales and limestone at base Thin rhymically bedded blank cherty mudstone and shales with phosphate nodules Thickness = 0 5000 feet
Thickness = 0 450 feet
angular unconformity
Roberts Mountains thrust
Devonian Woodruff Formation
Thin-bedded limestones Micritic with pyrite framboids+ radiolaria
Mississippian Chainman Formation
Clastic Facies Conglomerates + arkosic sandstone Thickness = 1100 1470 feet Transitional Facies Mudstone, arteosic sandstone and conglomerate Thickness = 50 250 feet
Increased silt and sand content in upper portions
Webb Formation
Coarsing upward sequence of carbonaceous mudstone with <10% coarse clastics Thickness = <400 800 feet unconformity Thickness > 1650 feet Roberts Mountains thrust Predominately black cherty mudstone
Devils Gate Limestone
Locally fossiliferous micritic limestone Thickness = 485 860 feet
?
Mississippian Chainman Formation
Telegraph Canyon Formation
Devonian Nevada Group
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Thickness up to 1750 feet
Dolomites + various dolostones
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Thickness = 400 1235 feet
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Oxyoke Canyon Formation
Dolostone + Quartzite Thickness = 240 400 feet
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Rain horst
Rain graben
Figure K-5. Stratigraphic column of the Rain subdistrict.
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Rain Subdistrict
individual beds ranging from less than 1/2 inch to 3 feet (1 to 90 cm). Silty limestones of the basal Tripon Pass Member of the Webb Formation have been observed only in the Emigrant Springs area. Secondary calcification and carbonization developed as an outer halo to the mineralized system and could be misinterpreted as calcareous and carbonaceous facies. Mudstone of the Webb Formation is transitional and grades into an upward coarsening sequence of arkosic rocks in the Mississippian Chainman Formation. Thicknesses of the Webb Formation range from less than 400 feet (122 m) near the Rain open-pit deposit to more than 800 feet (244 m) west of the Saddle deposit. Drill core data from the Saddle deposit indicate a structural thickening of the Webb Formation in proximity to the Rain fault, parallel faults, and crosscutting northeast faults. Additionally, drill-hole information suggests that lateral facies changes have caused a variation in the thickness of the Webb Formation. A red clay horizon has been recognized along the contact of the Devils Gate Limestone to the Webb Formation from the Rain open-pit deposit to the Tess deposit. Red clay, however, is not present along the contact from the Tess deposit through the Saddle deposit. Instead, sulfidic clays with remnant igneous textures have been observed along the unconformity. These textures resemble rocks termed lamprophyres by some geologists that work the Carlin trend. A controversy exists as to what constitutes Webb Formation. Rocks belonging to the type section of the Webb Formation of Smith and Ketner (1975a) are highly contorted, siliciceous mudstone and claystone, with tiny chert nodules, that lie in contact with siliciclastic rocks of the upper-plate Devonian Woodruff Formation. Some geologists disagree with the Webb designation of Smith and Ketner (1975a). Field evidence indicates that rocks composing the Webb type section are allochthonous siliciclastic rocks of the upper-plate. Mathewson (1993) described these rocks as allochthonous Webb Formation, where as Longo and others (1997) interpreted them as allochthonous upper Woodruff Formation. Nonetheless, siliceous lithologies in the type section of the Webb Formation are distinctly different from the mudstone and siltstone that host gold mineralization in the Rain deposits. Mudstone of the Webb Formation at Rain is transitional into the overlying Chainman Formation and only mildly deformed with broad open folds. Moreover, the Webb Formation at Rain is autochthonous and belongs to the lower-plate of the Roberts Mountains thrust. CHAINMAN FORMATION The Mississippian Chainman Formation is exposed as small ledges along South Ridge, Rain Hill, and Saddle Hill (figs.K2 and K-4). It also forms low blocky outcrops in drainages and float throughout the northern part of the Rain horst and in a window through the allochthon in Woodruff Creek. Chainman Formation rocks typically weather into tan to brown sharpedged plates and blocks. The Chainman Formation is a package of autochthonous siliciclastic lithologies that are conformable with mudstone of the Webb Formation. These lithologies may be divided into two facies: (1) a fine clastic facies that is transitional with the underlying Webb Formation, and (2) an upper facies of coarse clastic rocks that grade downward or laterally into the fine clastic facies (fig. K-5). These facies change sharply both laterally and vertically along the Rain fault zone and are referred to as turbidites by Tosdal (personal commun., 2002). Drill-hole data indicate thickness of the Chainman Formation ranges from 1,100 to 1,470 feet (335 to 448 m) in the Saddle area where the upper-plate Woodruff Formation is present, and is less than 850 feet (260 m) at the Rain open-pit deposit where significant erosion has exposed lithologies lower in the section. Transitional Facies. Lithologies of the transitional facies are part of a coarsening upward sequence and include mudstone, siltstone, arkosic sandstone, and pebble conglomerate. These lithologies are always observed as transitional with the lower mudstone of the Webb Formation and the clastic lithologies of the overlying coarse clastic facies. Thicknesses range from 50 to 250 feet (15 to 76 m) and average 220 feet (67 m) thick. Lateral facies changes and structural disruption are responsible for the variation in thickness observed along the Rain fault zone. Coarse Clastic Facies. The coarse clastic facies rocks of the Chainman Formation crop out from Rain Hill to the Saddle area and in windows through the Devonian Woodruff Formation along the eastern end of Woodruff Creek (fig. K-4). These rocks are coarse, poorly sorted conglomerates and sandstones that contain cobbles and pebbles of chert and other clastic rocks that are interpreted to have come from the paleo-Antler highlands to the west. Green chert pebbles are most characteristic of this unit. Furthermore, the cobbles and pebbles in the Chainman coarse clastic facies are predominately pure quartz sandstones and quartzites and less arkosic than the surrounding matrix.
Syn-Antler Orogeny Upper Plate Siliciclastics
Timing of the emplacement of the Roberts Mountains thrust (RMT) in the Rain subdistrict is constrained to the upper Osagean and possibly lower Meramecian series between the deposition of the late Early Mississippian Chainman Formation clastic facies suite and the late Mississippian Diamond Peak Formation. Upper plate Woodruff Formation is in thrust fault contact above the Chainman Formation. In turn, the Diamond Peak Formation lies in a depositional unconformity above the allochthonous Woodruff Formation (fig. K-5). The sole of the RMT (Woodruff Formation above Chainman Formation) is best observed in trenches and road cuts within the Saddle Hill area, 2 miles (3.3 km) west of the Rain open-pit deposit (figs. K-2 and K-4). DEVONIAN WOODRUFF FORMATION Rocks of the Woodruff Formation are allochthonous in the upper plate of the RMT above autochthonous Chainman Formation rocks. The RMT has been identified in both the Rain
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horst and Rain graben. Thicknesses of the Woodruff Formation in the Rain horst are up to 450 feet (137 m), more than 1,650 feet (500 m) in the Rain graben (Santa Fe Gold Company report, 1996), and 2,740 feet (835 m) in the Mobil Oil drill hole (fig. K-2). Thicknesses of the Woodruff Formation in the Rain graben immediately northeast of the Rain open-pit deposit range from 490 to 800 feet (137244 m). The lower Woodruff Formation is composed of rocks that range from black cherty mudstone, similar in appearance to silicified Webb Formation mudstone, to thin, rhythmically bedded, cherty mudstone, shale, and siltstone. Phosphate nodules in the mudstone and shale are diagnostic. Rare, thin-bedded, micritic limestone with radiolaria and pyritic framboids are characteristic of middle to upper Woodruff Formation rocks; however, the unit is generally decalcified in proximity to the Rain fault zone (Longo and others, 1997). Geologic mapping and core logging along the western extension of the Rain fault zone indicate that the Woodruff Formation typically coarsens upward. This may be a true lithologic change or a tectonostratigraphic variation due to imbricate thrust sheets within the Roberts Mountains allochthon. A sequence of cherty sandstone and siltstone with characteristics similar to the Quarry Siltstone of Rota (1993) has been recognized as part of the Woodruff Formation toward the west in Woodruff Creek, and is interpreted to be high in the section (fig. K-5). Cherty siltstone, sandstone, and even pebbly sandstone have been observed as rhythmically bedded and conformable with classic cherty Woodruff Formation mudstone along the north high wall in the Rain open-pit. In west Woodruff Creek, the unit contains siltstone and finegrained sandstone with nodules and concretions that are interbedded with chert and nodular chert. Past workers (Smith and Ketner, 1975a; Mathewson, 1993) have assigned both autochthonous and allochthonous Webb and Chainman Formation designations to the clastic-dominant lithologies defined in this report as part of the RMT allochthon. As discussed above, a controversy exists with the Webb Formation as defined by Smith and Ketner (1975a), which is interpreted here as part of the RMT allochthon. Further radiolarian and other fossil dating are necessary to constrain the ages of the imbricated thrust slices within the upper-plate of the RMT.
reclassified all of these rock units as Mississippian to Pennsylvanian Diamond Peak Formation (figs. K-4 and K-5). The Diamond Peak Formation is interpreted as a postAntler orogeny overlap sequence of molasse clastic rock that was eroded from the Antler highlands during Late Mississippian to Early Pennsylvanian age. The basal contact occurs regionally as an angular unconformity on highlydeformed, allochthonous Woodruff Formation rocks. Thicknesses range from 1,000 to 5,000 feet (3001,500 m) (Smith and Ketner, 1975a). The Diamond Peak Formation is composed of coarse conglomerate, sandstone, siltstone, lesser marl and shale, and minor limestone. Marl, shale, and limestone are generally basal units that are locally observed in direct contact with the angular unconformity above the allochthonous Woodruff Formation. Conglomerates of the Diamond Peak Formation are characterized by chert and white quartzite cobbles that range in size from pebbles to boulders.
Post-Paleozoic Rocks
TERTIARY ELKO FORMATION The Elko Formation includes interbedded tuff, calcareous and tuffaceous siltstone, marl, and limestone. Outcrops are rare but are present east of the Northeast fault zone and south of the Emigrant Springs deposit where they unconformably overlie the Webb Formation (fig. K-4). The dominant rock types are gray bioclastic to silty limestones interbedded with thinly laminated tuffaceous siltstone. The siltstone commonly has a fetid odor, and fossil ostracods have been identified in the limestone (Mathewson and others, 1994). Locally, fine-grained tuffs, some rich in biotite, are interbedded with the sedimentary rocks. IGNEOUS INTRUSIONS AND TUFFISITE DIKES Igneous dikes in the Rain subdistrict were first recognized within the Rain open-pit deposit by McFarlane (1987), the Northwest Rain Extension by Mathewson and others (1994), and the Emigrant Springs deposit by Mathewson and others (1994). Argillized igneous dikes have been mapped on the surface continuously for over 3 miles (5 km) from the Tess deposit into west Woodruff Creek (Longo and others, 1996). These dikes intrude along the Rain fault, the Dike fault, and the northeast faults. They are spatially associated with gold mineralization in the Rain Underground, Tess, and Saddle deposits. Similar igneous dikes and sills intrude a complex structural zone that includes north-, north-northeast-, and northwest-trending faults in west Woodruff Creek (fig. K-4). An altered monzonite porphyry intrusion intruded along a fault within the Emigrant Springs deposit (McComb, 1994; Mathewson and others, 1994), and an altered feldspar-biotite porphyry intrusion was found at Redridge (figs. K-2 and K-4) near the intersection of the Northeast and Emigrant faults (Jones and others, 1995; Longo and others, 1997; Read and others, 1998). Five textural types have been recognized from igneous intrusions in the Rain subdistrict: (1) felty with abundant altered mafic laths in an argillized aphanitic matrix; (2) porphyritic felty with remnant mafic phenocrysts in an argillized matrix of needle-like laths of relict plagioclase or biotite; (3)
Post-Antler Orogeny Molasse Overlap Sequence
MISSISSIPPIAN-PENNSYLVANIAN DIAMOND PEAK FORMATION Overlapping molasse sequences in the Rain subdistrict have been designated Diamond Peak Formation (Smith and Ketner, 1975a), Tonka Formation (Iverson, 1991; Poole, personal commun., 1995), Tess Formation (Mathewson, 1993), and Rain Formation (Mathewson, 1993). Mathewson (1993) and Poole (personal commun., 1993) identified one fossil of siphonodendron Amplexis zamphentis classified as Meramecian (Late Mississippian) age. No other fossil data exist to support the separate designations; therefore, Longo and others (1996)
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Rain Subdistrict
amygdaloidal with large amygdules of kaolinite, barite, and amythestine quartz a in fine-grained, felty matrix; (4) aphanitic; and (5) a feldspar and biotite porphyry described by McComb (1994) at Emigrant Springs. Felty textures appear to be networks of plagioclase or biotite microlites that locally display a preferred orientation with a pilotaxitic-like texture. All intrusive types are positively anomalous in numerous trace elements, including As, Sb, Hg, Mo and V. Alteration is typically intense, and the alteration minerals are kaolinite, alunite, sericite, quartz, K-feldspar, jarosite, and iron oxide minerals. Kaolinite and alunite appear to overprint sericite and K-feldspar. Limonitic oxide minerals are common. Dikes in deposits along the Rain fault zone are termed lamprophyre. The term lamprophyre, as used here, is a field term for late igneous dikes with a relict porphyritic texture of feldspar, biotite, and olivine in an altered groundmass (Rock, 1977). Petrographic data collected by McComb (1994 and 1996) and Baker (1997c) support the use of the term. McComb (1996) reported that the dike rocks all have a relict felty texture and appear to have had a significant mafic content. Some relict mafics appear to have been olivine and others biotite. Semiquantitative XRF analysis shows that SiO2 contents range from 43.50% to 65.89% (average = 53.96%) and implies an intermediate to mafic composition. However, the dikes are altered and the SiO2 content may not be reliable. For instance, the higher SiO2 content is due to secondary quartz introduced as an alteration product and quartz veinlets not related to the original composition The intrusive rocks at Rain are similar to other dike rocks termed lamprophyres throughout the Carlin trend (Teal and Jackson, 1997b) in that they are late and intrude breccias, they contain relict olivine and biotite phenocrysts, and they display a high degree of alteration especially of the groundmass. Samples from drill core and the Rain Underground Mine constrain the timing of the lamprophyre intrusions (Longo and others, 1997). Petrographic examination (Baker, 1997c) has shown the previously silicified mudstone was microbrecciated along dike contacts, and micro-xenoliths of silicified siltstone occur within the dikes along contacts. Also bleached, recrystallized, secondary quartz occurs along margins of sulfidic igneous rocks that intruded oxidized silicified breccia (Longo and others, 1996, 1997). These observations suggest that the emplacement of dikes at Rain postdate silicification, brecciation, and possibly oxidation. A sample of dike rock from the Emigrant Springs deposit (RCR-9; 676677 feet), which was described as an altered monzonite porphyry (McComb, 1994), was dated using the UPb SHRIMP method (Mallete, 2001). The age of this dike is 37.50.8 Ma on magmatic zircon thus placing an upper constraint on the age of mineralization at Emigrant Springs (Garwin, 2001). The age is similar to the 36.81.1 Ma age of the quartz monzonite Bullion stock in the Railroad mining district (Smith and Ketner, 1975b) that is located 8 miles (13 km) south of the Emigrant Springs deposit. Furthermore, Ressel and Henry (personal commun., 2002) dated biotite from a dike in the Saddle deposit (RCR-61; 2,092 feet) by the 40Ar/39Ar method and found that its age is 38.890.20 Ma. The date is similar to the age of the monzonite porphyry dike at Emigrant Springs. Williams and others (2000) have shown that some dikes at the Rain Underground Mine are not igneous intrusions, but instead are fragmental rocks termed tuffisites. They are interpreted to have developed during intense hydrothermal brecciation and milling and consist of 5 to 80% lapilli ranging from 0.04 to 0.5 inches (112 mm) in diameter in a rock flour matrix. The rock flour is finely milled illite, quartz, barite, pyrite, and traces of clays, phosphates, and alunite (Williams and others, 2000). Flow banding along the tuffisite margins is common, whereas the interiors exhibit no flow structure. Tuffisites have not been recognized west of the Northwest Rain Extension.
STRUCTURAL GEOLOGY
Three major fault zones controlled gold mineralization in the Rain subdistrict (fig. K-2): (1) The Rain fault zone is a N60W-trending structural corridor that defines the Rain horst and includes the Rain fault, the Dike fault and the SB fault. (2) The N30E-trending Northeast fault zone truncated the Rain fault zone east of BJ Hill and Snow Peak, and the Tertiary Elko Formation was juxtaposed against the Mississippian Chainman Formation (figs. K-2 and K-4). (3) The north-trending Emigrant fault zone hosts the Emigrant Springs and South Emigrant Springs deposits in mudstone of the Webb Formation along the contact of the Devils Gate Limestone. These fault zones (fig. K-2) juxtapose the stratigraphic relationships of the Rain sedimentary rocks (fig. K-4) in a normal sense of vertical displacement by up to 3,000 feet (900 m) (Mathewson, 1993; Longo and others, 1997; Read and others, 1998). Both the Rain and the Emigrant faults zones are described as having associated horst and graben blocks based on the apparent vertical displacement of stratigraphy across their major horst bounding faults. The horst blocks are interlaced with a complex network of splays and crosscutting faults, some of which controlled gold mineralization. An unusual stratigraphic relationship that has been recognized along strike of the Northeast fault indicates that the sediments in the north are down-dropped to the northwest of the fault and those to the south are down-dropped to the southeast (fig. K-4). Crosscutting fault relationships suggest that the Rain fault is the oldest fault in the Rain subdistrict (Mathewson and others, 1994; Longo and others, 1996). The Northeast fault truncates the Rain fault to the east of BJ Hill (fig. K-2) and bounds the northwest margin of the Emigrant Springs horst block (fig. K4). The Emigrant fault forms a major structural boundary along the west side of the Emigrant Springs deposit and crosses the Northeast fault in Section 26 (fig. K-2) where it maintains its normal fault geometry. Both faults form an unusual intersection in that neither fault displays significant displacement by the other (fig. K-4). Northeast-trending faults regularly crosscut the Rain fault zone and drop the section down to the northwest into Woodruff Creek (figs. K-2 and K-4).
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Rain Fault Zone
The Rain fault zone is a west-northwest-striking network of Subparallel faults with an anastomosing width of 1.1 to 2.7 miles (1.84.3 km). It is interpreted as a major, long-lived structural zone or deep crustal suture similar in magnitude to the Post fault zone (Teal and Jackson, 1997). It is traceable on the surface for over 5 miles ((8 km) from BJ Hill to Woodruff Creek (figs. K-2 and K-4). Three prominent high-angle faults have been mapped that define the primary structural fabric and major structural boundaries of the Rain fault zone. These include: (1) the Rain fault, the northern fault that bounds the Rain horst; (2) the Dike fault, a dike-filled fault splay within the Rain horst Subparallel to the Rain fault; and (3) the SB fault, the southern fault that bounds the Rain horst. Other faults include splays and crosscutting features (northeast, oblique northwest, and north-striking crosscutting faults) that displace the structural fabric. Structural complexities have been well documented within the Rain fault zone. Longo (1996) and Longo and others (1997) observed a complex swarm of faults within the Rain horst that are locally dike-filled and subparallel to the Rain fault and include the Dike fault. These subparallel faults, and associated stratigraphic displacement, define a collapse feature along the Rain fault that overlies the Tess and Saddle deposits (fig. K6). Jory (1992b) and Williams and others (2000) recognized right lateral offset along the Rain fault through detailed mapping of fault striations and drag folds in the Rain open-pit deposit and Rain Underground Mine. Mathewson (1993) and Longo and others (1997) observed complex structural relationships at the intersection of the Rain and Northeast faults. These include minor thrust faults, high-angle reverse and normal faults, and possible sigmoidal bends in the Rain fault near the intersection to the Northeast fault (due to scale these complexities are not shown on figs. K-2 and K-4). These relationships resemble the structural kinematics as discussed by Bohannon and Howell (1982) at the junction of the San Andreas, Garlock, and Big Pine wrench fault systems in California. Williams (1997b) recognized reverse movement along faults that are subparallel to the Rain fault in Zone 4 of the Rain Underground Mine. Detailed mapping of these underground exposures led Williams and others (2000) to identify a positive flower structure, as defined by Wilcox and others (1973), to describe the structural complexities observed in Zone 4 (fig. K-7). RAIN FAULT The Rain fault strikes west-northwest, dips southwest, and displays apparent reverse displacement (Lane and Heitt, 1994; Heitt, 1996; Longo and others, 1996; Teal and Jackson, 1997b; Williams and others, 2000). Drilling across the Rain fault indicates high-angle reverse-fault geometry to depths of more than 2,000 feet (660 m) With the aid of gravity surveys, rocks southwest of the Rain fault have clearly been defined as part of an uplifted block referred to as the Rain horst. The fault strikes N4050W in the Rain open-pit deposit and has a strike of N6585W in the Rain Underground Mine.
Dips range between 68 and 80 to the southwest in the open pit but flatten to as low as 38 to the southwest in the Rain Underground Mine. The fault acted as a conduit and, in places, a structural boundary to gold mineralization. Sedimentary rocks have been displaced by as much as 3,000 feet (990 m) in a vertical sense along the Rain fault (Mathewson, 1993). Upper-plate Woodruff Formation in the footwall is juxtaposed against lower-plate rocks of Devils Gate Limestone, Webb Formation, and Chainman Formation in the hanging wall (fig. K-5). Evidence for this displacement has been observed in the Rain Underground Mine where a sharp contrast in styles and intensity of folding is apparent across the Rain fault. Footwall rocks consist of intensely folded upper-plate Woodruff Formation, whereas hanging wall rocks exhibit broad southwest-plunging open folds within mudstone of the Webb Formation. Mineralized Webb Formation and Devils Gate Limestone in the hanging wall are in contact with barren, nonreactive siliciclastics of the Woodruff Formation in the footwall. Gravity surveys across the Rain horst indicate that a carbonate sequence at depth along the Rain fault (Nevada Group Dolomites) extends to the north of the carbonate rocks (Devils Gate Limestone) higher in the section (Beetler, 1993). This has been interpreted to mean that the Devonian Nevada Group dolomite at depth in the Rain horst is northeast of its upper contact with rocks of lower density. Therefore, the Rain fault changes dip from southwest to northeast and must have the geometry of a normal fault adjacent the Nevada Group dolomite (fig. K-5). Extensive drilling along the Rain fault verifies the reverse or southwest dip of the Rain fault above the contact to the Nevada Group dolomite, and deep drilling confirms the presence of Nevada Group dolomite north of the surface projection for the Rain fault (Read and others, 1998). DIKE FAULT The Dike fault is a prominent fault found within the Rain horst, and at N60W, the overall surface strike of the fault is parallel to that of the Rain fault. It displays both apparent normal and reverse displacement, and generally dips to the northeast (Longo and others, 1997). The fault is host to a type of dike rock (figs. K-4 and K-5) termed lamprophyre by McComb (1996g). Surface geology indicates the dike crops out continuously along the Dike fault for a strike length of 2.7 miles (4.4 km) from the Tess deposit to Woodruff Creek (figs. K-2 and K-4). Geochemical anomalies from surface sampling and deep core drilling along the fault suggest it acted as a conduit for gold mineralization. SB FAULT The overall surface strike of the SB fault is also subparallel to the Rain fault, and it bounds the southern margin of the Rain horst (figs. K-2 and K-4). Results from geophysical surveys of gravity and IP support the inferred southern margin of the Rain horst.
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Figure K-6. Generalized cross section of the Rain underground deposit along N1000 exploration grid with drill holes projected into plane of section.
Additional Structural Features of the Rain Subdistrict
Additional structural features in the Rain subdistrict are the result of cross faults that intersect the Rain fault zone. These produced zones of discontinuity where gold ore was truncated or displaced. Within these zones, gold grades are generally lower and unpredictable to absent. Furthermore, crosscutting faults were intruded by igneous and tuffisite dikes (Longo and others, 1997; Williams and others, 2000). They include the following fault sets (figs. K-2 and K-4): (1) Faults that strike from N20E to N40E and generally dip northwest have been mapped on the surface and in the Rain Underground workings. The most important include, from northwest to southeast, the Bama, the Petan, the Three, the Tess, the Pyrite, the EOS 2, and the Sharpstone faults (figs. K-2 and K-8). (2) Northwest-striking faults that trend from N40W to N75W are interpreted as splays to the Rain fault zone and still poorly understood. These faults have been recognized on the surface at the Saddle deposit and west into Woodruff Creek (figs. K-2 and K-9) and appear crosscut by the northeast-striking faults. The 2 Bad fault in the Saddle area of the longitudinal section (fig. K-6) is the best example. (3) North-striking faults crosscut the Rain horst in Zone 3, Zone 4, Zone 6, the Saddle deposit, and along Woodruff Creek. The most important include the Spring, the QNE 1, and the Quandary faults in Zone 4 (fig. K-8), and an unnamed fault between the Tess deposit and the Saddle deposit (figs. K-2 and K-4). Rain Mine geologists have
SW
interpreted that some gold mineralization in Zone 4 was controlled by north-striking faults (Harlan, 1999; Williams and others, 2000). Surface and underground mapping provide evidence that these faults displace the Rain fault and lithology. They crosscut the Rain horst at regular intervals, and down drop Paleozoic rocks progressively to the northwest (fig. K-6). For example, the depth to the Devils Gate Limestone increases progressively to the northwest toward Woodruff Creek with a vertical displacement of 1,800 feet (550 m) over a lateral distance of 1,400 feet (510 m) from the Rain open-pit to the Saddle deposit. Devils Gate Limestone crops out at an elevation of 6,600 feet (2,000 m) adjacent the Rain open-pit deposit. It is displaced to an elevation of 6,350 feet (1,935 m) in Stope 1, to 5,860 feet (1,790 m) in Zone 4, to 5,300 feet (1,615 m) in the Tess deposit, and to 4,800 feet (1,463 m) west of the Saddle deposit. Oblique northwest-striking faults crosscut the Rain horst at Saddle Hill and Snow Peak (figs. K-2 and K-4). Both the oblique northwest- and north-trending faults host igneous dikes (McComb, 1996g; Longo, 1996). A series of west northweststriking and northeast-dipping faults have been mapped in board along the margin of the Rain horst subparallel to the Rain fault. Some of these, such as the Dike fault, are intruded by igneous and tuffisite dikes, whereas others, such as the SB fault (fig. K-2), are interpreted to bound the southern margin of the Rain horst (Longo and others, 1997). Secondary north- to northeast-striking fault sets observed in the Rain Underground Mine (Williams and others, 2000) and in the Saddle deposit (Longo and others, 1997), controlled the igneous and tuffisite dikes, as well as the ore-bearing breccias. Williams and others (2000) interpreted them as a
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Figure K-7. Cross section of a positive flower structure from the Rain Underground Mine (from Williams and others, 2000).
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Figure K-8. Average grade contour map for the Rain ore system.
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Figure K-9. Grade-thickness map of a portion of the Rain open-pit ore zone (on the southeast) and in the underground workings.
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conjugate fault system within the Rain fault zone. Other northeast-striking faults are not mineralized and display postmineral offset. NORTHEAST FAULT The Northeast fault is a major structural discontinuity that truncates the Rain fault zone to the east (figs. K-2 and K-4). It places Tertiary Elko Formation in contact with Mississippian Chainman Formation along its southern extent, and places Woodruff, Chainman, and Diamond Peak Formations in contact with Webb Formation along its northern extent (fig. K-4). The Northeast fault is defined by a gravity anomaly to the southsouthwest of the junction between it and the Rain fault zone, as well as by geochemical anomalies of mercury and arsenic. It is a complex structure that is poorly understood, but it has been interpreted as a scissors fault (Mathewson and others, 1994) and a tear fault with left-lateral displacement (Longo and others, 1997). Lamprophyre and monzonite dikes (McComb, 1994) have been observed along its northern extension (Mathewson and others, 1994). EMIGRANT FAULT The Emigrant fault is a north-striking, normal fault that bounds the western margin of a prominent gravity anomaly east of the Rain open-pit deposit and the Northeast fault. Outcrops of mineralized baritic and silicified Webb Formation occur in the footwall south of the intersection with the Northeast fault in the SEc of Section 26 (fig. K-4). The Emigrant fault hosts lamprophyre and monzonite dikes (McComb, 1994) to the north and controlled gold mineralization in the Emigrant Springs deposit (fig. K-2) for nearly 2 miles (3 km) south of the intersection with the Northeast fault (figs. K-2 and K-4). The Emigrant fault appears to crosscut the Northeast fault and cause only minimal displacement; however, the intersection between the two faults is poorly understood. Across the Emigrant fault south of the intersection, the Woodruff and Chainman Formations are juxtaposed against Webb Formation (Sa Sec. 26; fig. K-4), and north of the intersection (Na of Sec. 26; fig. K-4), the Diamond Peak Formation in the west is juxtaposed against Woodruff and Chainman Formations to the east (McMillin personal commun., 1997). The fault breaks into splays to the north in Section 23 (fig. K-4). and Bettles (1992) described high- and low-angle structures in the Betze-Post and Meikle deposits that were purportedly caused by wrench faulting, including strike-slip, thrust, highangle reverse, and normal faults. Recent work (Mathewson, 1993; Mathewson and others, 1994; Jones and others, 1995; Heitt, 1996; Longo, 1996; Longo and others, 1997; Williams, 1997b; Williams and others, 2000) has shown that the Rain fault zone is of similar magnitude and degree of complexity as the mega-shears (i.e., the Post and Good Hope faults) found within the Richmond Spur, BetzePost, and Meikle areas. Many of the complexities observed along the Rain fault zone have been attributed to wrench faulting. Field mapping and deep core drilling in the Rain, Tess, and Saddle deposits support the interpretation that displacement along the Rain fault was mainly reverse-slip that changed to normal-slip in the Nevada Group dolomite sequence. Transpressive deformation as discussed by Sylvester and Smith (1976) could explain the high-angle and low-angle apparent reverse faults observed at both the Rain and Emigrant Springs deposits. The complex structural patterns observed at the Rain Mine resemble the geometry of a positive flower structure that commonly form within contractional duplexes akin to wrench fault tectonics (Bartlett and others, 1981), whereas at Saddle Hill a collapse feature has been interpreted within a set of subparallel faults that include the Dike fault. Structural complexities along the Rain fault zone are not fully understood, and although this paper advocates wrench fault tectonics, the interpretation is based on locally derived datasets from windows into the Rain fault zone. Other ideas (Mathewson and others, 1994; and Tosdal, personal commun., 2002) that favor the structural kinematics of intracontinental compressional tectonics and inversion tectonics (Coward, 1994) dispute the hypothesis for wrench fault tectonics.
DESCRIPTION OF THE RAIN UNDERGROUND ZONE 4 DEPOSIT
Zone 4 is an orebody in the hanging wall of the Rain fault between the Pyrite fault on the east and the Quandary-QNE1 fault zone on the west (fig. K-9). Production from Zone 4 began in late 1998 utilizing underground cut and fill and long-hole open-stope mining techniques. Ore production began with an initial reserve of 101,059 ounces (3.13 t) of gold in 373,959 tons (339,000 t) of ore averaging 0.270 opt (9.3 g/t). Nearly all of this material is moderately to strongly oxidized, and gold is recovered by conventional cyanidation. During 1999, a total of 97,166 tons (88,172 t) of ore was mined from Zone 4 at an average grade of 0.309 opt (10.6 g/t) gold. New drill data and deposit modeling increased the year-end 1999 reserve to 411,409 tons (374,000 t) grading 0.316 opt (10.8 g/t) for a total of 129,903 ounces (4.04 t). Zone 4 is the largest and most intensively drilled underground orebody at Rain. In plan view, the orebody measures approximately 1,050 by 600 feet (320 by 183 m) with the long dimension roughly parallel to the Rain fault (fig. K-9). The deposit is dominated by four structural components: (1) the northwest-striking Rain fault, (2) a set of north-striking
Discussion
Wrench-fault tectonics has been suggested by other workers along the Carlin trend to explain the evolution of complex structures that hosted the gold mineralization (Putnam and McFarlane, 1990). Moore (1995b) proposed transpressive wrench faulting to explain regional-scale tectonics in the Richmond area of the southern Tuscarora Range. He interpreted reverse faults at Richmond to be compressive in nature, and discussed contractional duplexes that formed by shortening along a lateral-slip transcurrent fault zone as a possible explanation. This style of tectonism would form a series of up-thrust blocks bounded by high-angle reverse faults whose cross-sectional c...