geologic overview of the rain subdistrict

7/27/2019 Geologic Overview of the Rain Subdistrict

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GEOLOGIC OVERVIEW OF THE RAIN SUBDISTRICTAnthony 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 andthe 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 upper-

plate eugeoclinal to flysch siliciclastics, and (4) post-Antler

orogenic molasse siliciclastics (overlap assemblage). Theautochthonous (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,

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 WoodruffCreek. 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 N30E-

trending 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 stratigraphicrelationships 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, calcitedissolution 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 veinlets1Newmont Mining Corporation2Ralph J. Roberts Center for Research in Economic Geology


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Rain Subdistrict

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.

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 WebbFormation. 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 finely-

laminated 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

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 anunderstanding 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.

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 Universityof 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 SubdistrictThe 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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Winnemucca

Elko

Carlin

Ely

Reno

Lovelock

Las Vegas

80

80

H U M B O L D T E L K O

N Y EM I N E R A L

ESMERALDA

LYON

L I N C O L N

C L A R K

P E R S H I N G

C H U R C H I L L W H I T E

P I N EE

U

R

E

K

A

L

A

N

D

E

R

WA

SHOE

80

80

80

EmigrantPass

GoldQuarry

M ac

Tusc

PeteCarlinUniversalGas Pit

Lantern

BeastBlue Star

Bobcat

North Star

W est Leevil le

Four CornersTurf

Genesis

Deep Star

Betze-Post

Rodeo ( Goldbug)

Meikle

Dee

Capstone

Bootstrap

Tara

Rain

Emigrant

Carlin

E u

re ka

C o u

n ty

E lk

o C

o u n

ty

Mike

0 2 4 6 miles

0 42 6 8 10 kilometers

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 arehighly 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 upper-

plate 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 northwest-

trending 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 forgold-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.

1513 18 17 16

24 19 2021

25

30

29 28

36

3132

33

2

11

1

12

6

7

5

8

4

9

14 13

23 2422

2625

27

363534

2 13

121110

Saddledeposit

Raindeposit

Tessdeposit

SM Z

Gnome

Snow Peakdeposit

Emigrantdeposit

RainHill

BJHill

SouthRidge

SaddleHill

Topographic landmark

Major fault

Intercept grade (G) = opt Au

Intercept thickness (T) = feet

Minimum 10-foot intercept

Gold deposit footprint

BAMA

FAUL

T

PETAN

FAULT

SB

FAULT

SHARPSTO

NEFAULT

NORTH

EAST

FAULT

FRIDAY

FAULT

EMIGRANT

FAULT

TESS

FAULT

RAIN

FAULT

RAIN

FAULT

PARALLEL

WoodruffCreek

0 6000 feet

0 2000 meters

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 open-

pit deposit. A gravity anomaly also defines the north-trendingEmigrant 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 gold-

bearing 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 theRain 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).

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. K-

2). Stope 1 was the first underground mine developed on the

Carlin trend, and the mine was accessed by a portal and decline

20,000

40,000

60,000

80,000

100,000

120,000

140,000

Ounce

sofGoldProduced

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Years

Open pit mill production

Underground mill production

Open pit leach production

Underground leach production

Figure K-3. Annual gold production from the Rain Mine, 1988 to 1999.


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Rain Subdistrict

TableK-1.MRainreservesandresources(Crouse,1990;Jory,1990;Thoreson,1990a;LaneandHeitt,1994;H

eitt,

1996;William

s,1997;Harlan,1999;Odell,personalcommun.,1999;Roman,pe

rsonalcommun.,2000;Mallete,2

001)

Deposit

GoldReserveandResource

OreTypeand

DateDrilled

DateMined

Gold

Produced

CutoffGrade

Rainopen-pit

1,017,300

oz([email protected])

Oxide

19821987:

19881994

707,949oz

31.6t(14

[email protected]/t)

100-footgrid

22.0t

SMZ

30,000oz

([email protected])

Oxide

1987:100-foot.

1994

19,000oz

0.9t([email protected]/t)

grid

591kg

Emigrant

360,200oz([email protected])

Oxide

19821983:

Plannedfor

Onstandbywith

Springs

11.2t([email protected]/t)

Cutoff:0.01opt

400-footgrid

2002@$325Au

$270/oz.Au

1990:200-footgrid

Rain

265,047oz([email protected])

2/3oxide

19932000

19941998:Stope1,

Totaluntil2000:

Underground

8.2t(1.04


Cutoff:0.150opt

Stope2,andZone3

114,815oz

3.57t

oxide0.250opt

19992000:Zone4

Zone4in1999:

refractory

30,024oz

933kg

Saddle/NW

782,000o

z([email protected])

Sulfide

19962000

Nomineplan

None

Tess

24.3t(1.2


Cutoff:0.30opt

10g/t

1,475,000

oz([email protected])

Cutoff:0.20opt

45.9t(3.5


7g/t


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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 (88,170t) 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 theunderground 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).

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 discussedas 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 lower-

plate 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-PlateCarbonates

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.

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 OxyokeCanyon Formation is 244 feet (74 m) thick in a Mobil Oil

drill hole completed within the Rain graben north of Woodruff

Creek (SW c

41

c

41 c

41

c

41 c

41 , NW

c

41

c

41 c

41

c

41 c

41 , 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 thick-

bedded, 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.


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Rain Subdistrict

Early Antler Orogeny Flysch Depositsof 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 typicallyfound 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 non-

calcareous, platy, impermeable, carbonaceous mudstone and

siltstone with


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?

?

?

?

?

?

?

Transitional Facies

Dolostone + Quartzite

Thickness = 240 400 feet

Telegraph CanyonFormation

Oxyoke CanyonFormation

Dolomites + various dolostones


Locally fossiliferous micritic limestone


Coarsing upward sequence ofcarbonaceous mudstone with


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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 igneoustextures 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 FORMATIONThe Mississippian Chainman Formation is exposed as small

ledges along South Ridge, Rain Hill, and Saddle Hill (figs.K-

2 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 sharp-

edged 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 faultzone.

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 OrogenyUpper 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 Formationlies 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 Formationmudstone, 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 withcharacteristics 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 fine-

grained 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-dominantlithologies 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.

Post-Antler Orogeny MolasseOverlap Sequence

MISSISSIPPIAN-PENNSYLVANIANDIAMOND 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

siphonodendronAmplexis zamphentisclassified as Meramecian

(Late Mississippian) age. No other fossil data exist to support

the separate designations; therefore, Longo and others (1996)

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 post-

Antler 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 highly-

deformed, 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)


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179

Rain Subdistrict

amygdaloidal with large amygdules of kaolinite, barite, and

amythestine quartz in a 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% to65.89% (average = 53.96%) and implies an intermediate to mafic

composition. However, the dikes are altered and the SiO2content

may not be reliable. For instance, the higher SiO2content 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 andothers, 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 U-

Pb SHRIMP method (Mallete, 2001). The age of this dike is37.50.8 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 aredescribed 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 truncatesthe Rain fault to the east of BJ Hill (fig. K-2) and bounds the

northwest margin of the Emigrant Springs horst block (fig. K-

4). 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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180

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 faultshave 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 andinclude 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. K-

6). 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). Theserelationships 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 thehanging 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 (NevadaGroup 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 samplingand 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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Rain Subdistrict

N106400E8

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E-5000

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Main

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ZONE5

(mostly

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ZONE4

(approx.

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ZONE3

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STOPE2

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Intrusiverockand/ortuffisite

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Fault

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withdrillholesprojectedintoplaneofsection.


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182

Additional Structural Featuresof 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 andothers, 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

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 northwest-

striking and northeast-dipping faults have been mapped inboard 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

Figure K-7. Cross section of a positive flower structure from the Rain Underground Mine

(from Williams and others, 2000).

HIDD

ENFAULT

FLOW

ERFAULT

RAIN

FAULT

REVELATIONFAULT

PHANTOMFAULT

Ddg

Ddg

Mw

Mw

Dw

Mw

AWAY

AWAY

TOWARD

TOWARD

0 200 feet

0 60 meters

0.15 opt (5.1 g/t) gold

Breccia

Tuffisite

Mississippian Webb Formation

Devonian Woodruff Formation

Devonian Devils Gate Limestone

Mw

Dw

Ddg

NESW


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183

Rain Subdistrict

N104

,000

Averagegrade(optAu)

Majorfaults,ballondownthrown

side,arrow

indicatingdipdirection

andrelativedisplacement

0.10-0.149

E96,500

E97,000

E94,000

E94,500

E95,000

E95,500

E96,000

N103

,500

N103

,000

N102

,500

N102

,000

RAIN

FAULT

BAND

ITFA

ULT

TESSFAUL

T

QNE1FAULT

SPRINGFAULT

DIKEF

AULT

EOS2

FAULT

EOS1

FAUL

T

MYSTE

RYFAU

LT

BARI

TEFA

ULT

PYRITE

FAULT

QUANDAR

YFAULT

DS13FAULT

SHOP

FAUL

T-UG

2FAULT

SHOP

FAU

LT-P

IT

RAIN

FAULT

R

AIN

FAULT

GALENFA

ULT

0

200feet

0

60meters

0.15-0.199

0.20-0.249

0.25-0.299

0.30-0.349

>0.35

FigureK-8.AveragegradecontourmapfortheRainoresystem.


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184

N104

,000

GradeX

TrueT

hickness(opt-feet)

Majorfaults,ballondownthrown

side,arrow

indicatingdipdirection

andrelativedisplacement

E96,500

E97,000

E94,000

E94,500

E95,000

E95,500

E96,000

N103

,500

N103

,000

N102

,500

N102

,000

RAIN

FAULT

BAND

ITFA

ULT

TESSFAUL

T

QNE1FAULT

SPRINGFAULT

DIKEF

AULT

EOS2

FAULT

EOS1

FAUL

T

MYSTE

RYFAU

LT

BARI

TEFA

ULT

PYRITE

FAULT

QUANDARY

FAULT

DS13FAULT

SHOP

FAUL

T-UG

2FAULT

SHOP

FAU

LT-P

IT

RAIN

FAULT

RAIN

FAULT

GALENFA

ULT

0

200feet

0

60meters

>35

30-34

25-29

20-24

15-19

10-14

5-9

FigureK-9.Grade-thicknessma

pofaportionoftheRainopen-pito

rezone(onthesoutheast)andinth

eundergroundworkings.


18/22

185

Rain Subdistrict

conjugate fault system within the Rain fault zone. Other

northeast-striking faults are not mineralized and display post-

mineral 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 south-

southwest 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 SE c

41

c

41

c

41

c

41 c

41 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 causeonly 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 (S a

21

a

21

a

21

a

21

a

21 Sec.

26; fig. K-4), and north of the intersection (N a

21

a

21

a

21

a

21

a 21 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).

Discussion

Wrench-fault tectonics has been suggested by other workersalong 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 character is that of a flower structure. Lauha

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, high-

angle 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 andGood Hope faults) found within the Richmond Spur, Betze-

Post, 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 RAINUNDERGROUND ZONE 4DEPOSIT

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


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186

faults, (3) a set of west-northwest striking faults subparallel to

the Rain fault, and (4) a set of northeast-striking post-mineral

faults. The structural fabric described here was identified by

detailed mapping of underground exposures, close-spaced core

drilling, and computer modeling and can be used to model

other ore zones elsewhere along the Rain fault.

In Zone 4, the Rain fault strikes N55W to N75W and

dips 40 southwest. It is a sharp planar feature with striations

and broad corrugations that rake at low to moderate angles.Williams and others (2000) proposed a component of right-

lateral offset along the Rain fault based on the orientation of

small-scale drag folds and fault striae. Lamprophryre dikes

and tuffisites are distinctly absent, and in many places, the fault

plane is healed by hydrothermal barite and quartz, and lacks

evidence of postmineral offset. The Rain fault is crosscut by

numerous late structures that strike north and northeast.

A subset of the Rain fault zone includes faults subparallel

to the Rain fault. In Zone 4, these faults occur as en-echelon

breaks with strike lengths that rarely exceed 800 feet (244 m).

They are found in sets of less-than-1-inch-wide fractures that

form disrupted zones several feet wide, and include four faults,

the Flower and Hidden faults, that dip to the southwest similarto the Rain Fault, and the Phantom and Revelation faults, that

dip northeast toward the Rain fault These faults display evidence

for reverse movement, and the resultant geometry has been

identified as a positive flower structure (fig. K-7) (Williams

and others, 2000). Like the Rain fault, they contain hydrothermal

quartz and barite, and show no apparent postmineral

displacement. Tuffisite and lamprophyre dikes intruded the

subparallel faults and are characteristically unmineralized. The

lamprophryre dikes are intensely clay-altered and in some places

misinterpreted as fault gouge. Gold ore commonly occurs along

these faults with the thicker and higher-grade ore in the footwall.

The Devils Gate Limestone-Webb Formation contact is usually

displaced by less than 30 feet (9 m) across structural zonesdominated by the subparallel faults.

Subparallel faults have been recognized within the other

Rain underground deposits, but were recognized more strongly

developed in Zone 4. The Zone 4 orebody is six times the

width of the Stope 1, Stope 2, and Zone 3 orebodies Therefore,

mine geologists at Rain consider these faults an important

control to ore.

Based on grade-thickness plots (fig. K-9) and the

localization of breccia, it is estimated that the gold ore in Zone

4 was controlled in part by a series of north-striking cross faults.

These structures include, from east to west, the Pyrite, the Spring,

the QNE1, and the Quandary faults (fig. K-9). The QNE1 and

Quandary faults form a structurally complicated zone that

separates high-grade oxide ore in the west part of Zone 4 from

low-grade refractory ore in the east part of Zone 5 (fig. K-9).

Mine geologists at Rain believe these faults acted as barriers to

gold mineralization and oxidation. The intersection of the Rain

fault with the QNE1 and Quandary faults is characterized by

intense decalcification and dolomitization in Devils Gate

Limestone. Large pods (up to 50 feet [15 m] thick) of mineralized

and silicified hydrothermal breccia float in a clay matrix-

supported breccia that is interpreted to have formed as a result

of post-mineral collapse. The north-striking faults are interpreted

as ore-controlling, conjugate structures that formed during right-

lateral strike-slip movement along the Rain fault (Williams and

others, 2000). However, these structures display evidence for

significant post-mineral reactivation and offset.

The fourth major structural component observed within

Zone 4 is a group of west-dipping, northeast-striking faults

with post-mineral offset that displace the Paleozoic section

downward to the west (fig. K-9). Grade-thickness and geologic

maps indicate that these faults cut off the orebodies and relatedbreccia. They do not seem to have controlled gold

mineralization or lamprophyre and tuffisite intrusions, and have

been interpreted as faults caused by Basin and Range

extensional tectonics.

OREBODY CHARACTERISTICSALONG THE RAIN FAULT

Three breccia types are present in the orebodies at Rain and

include the following: (1) an early collapse breccia has been

recognized in the Devils Gate Limestone at depth in the BJ

Hill and Saddle deposits, (2) a multiple-stage silicified

hydrothermal breccia within the Webb Formation hosts the gold

in the Rain open-pit and underground orebodies, and (3) a late

postmineral collapse breccia with open-space caverns in the

Devils Gate Limestone and fragments of the multistage

silicified breccia below the Devils Gate-Webb Formation

contact horizon.

Collapse breccia preceded gold mineralization in the area

northwest of the Rain open pit and served as channelways for

gold-bearing hydrothermal fluids. An early collapse of the

section formed as a result of the early dolomitization and

dissolution of the underlying Devils Gate Limestone, and

resultant collapse of overlying rock into the open spaces.

Mineralized pipes of collapsed breccia with fragments of theWebb Formation occur in the Devils Gate Limestone at the BJ

Hill (Mathewson and others, 1994) and the Saddle (Longo and

others, 1996) deposits. Gold grades in these breccias average

0.22 opt (145 g/t) at the BJ Hill deposit and 0.15 opt (5.1 g/t)

in the Saddle deposit. The breccias are healed with dolomite

(BJ Hill) and calcite (Saddle) cement, and gold ore is associated

with the sooty black sulfides that are disseminated in the

carbonate cement.

The Rain open-pit and underground orebodies are in a

multiple-stage hydrothermal breccia mass in the hanging wall

of the Rain fault along the contact between the Webb Formation

and the underlying Devils Gate Limestone. Anomalous gold

values occur throughout the breccia mass; however, ore gradesare generally confined to its upper portion, a 17- to 40-foot (5-

12 m) thick zone of crackle breccia that locally extends as

discordant masses into the siltstones and mudstones of the

overlying Webb Formation. Fragments of Webb Formation are

shattered but not significantly displaced within the crackle

breccia, and original bedding attitudes are preserved. Crackle

breccia fragments have been cemented by a matrix of quartz,

barite, pyrite, and local sphalerite. In the upper part of the

crackle breccia the matrix occurs in narrow veinlets that thicken

downward to the lower contact, which is a sharp boundary

with underlying heterolithic breccia.


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187

Rain Subdistrict

The heterolithic breccia is generally matrix supported and

was interpreted to have formed in three stages (Williams, 1992):

(1) an early crackle breccia, (2) a later heterolithic breccia,

and (3) the latest milled, hydrothermal breccias observed in

the tuffisite dikes. The matrix in these breccias exhibits flow

orientation. Fragments consist of jasperoid and dolostone from

the underlying Devils Gate Limestone, and siltstone of Webb

Formation with variable amounts of alteration, oxidation

minerals, and barite.Collapse brecciation clearly postdates the hydrothermal

breccias. A late collapse breccia event has been recognized in

the Devils Gate Limestone that contains fragments of the

silicified heterolithic breccia. Locally, some caverns in the

Devils Gate Limestone remained open due to bridging over

the top of a cavity. The collapse breccia is commonly

superimposed on an erosional unconformity at the top of the

Devils Gate Limestone in which terra rossa cemented limestone

fragments.

Mineralogy of Ore and Wall-Rock

Alteration AssemblagesThe paragenetic sequence for ore and gangue minerals in the

Rain deposits is shown in figure K-10. Development of the

Rain hydrothermal system began with a passive stage of

silicification along the contact to the Devils Gate Limestone

and Webb Formation. This consisted of early iron sulfides,

arsenian pyrite, native gold, quartz, and rutile. During this stage,

calcite dissolution and concurrent dolomite replacement of the

Devils Gate Limestone occurred beneath the silicification.

Mathewson and others (1994), Longo and others (1996), and

Heitt (1996) recognized a spatially associated carbon depletion

and outward enrichment (very fine, non-sulfidic black soot in

fractures and along laminae of the mudstone), sulfidation (earlydisseminated pyrite that precipitated within pore spaces and

fractures in the mudstone and developed an outer halo to the

silicified ore zone), and carbonate enrichment above the

silicified ore zone. From drill holes in the Tess and Saddle

deposits, Longo and others (1996) identified a carbonate halo

consisting of calcite veinlets, rare dolomite veinlets, and

carbonate replacement above silicified crackle breccia in the

Webb Formation. The halo is most intense 5 to 30 feet (1.59

m) above the ore horizon where calcite fills fractures and

replaces siltstones depleted in carbon. The carbon was pushed

outward and enriched in fractures and along laminae in

mudstone of the Webb Formation. A pyrite-rich halo with

veinlets of calcite and dolomite extends more than 300 feet(90 m) above the orebodies. The sulfidation, along with

carbonization and carbonate mobilization and enrichment, are

interpreted to have been synchronous with early ore stage

development.

The early stage of passive silicification at Rain was

followed by the main ore stage, and subsequently by late

veining and vug filling. The main ore stage was preceded by at

least two events of hydrothermal brecciation and one event of

collapse breccia (Williams, 1992; Mathewson, 1993) all with

intervening barite precipitation (fig. K-10). Breccia and open

spaces were filled by quartz with encapsulated elemental gold

and pyrite with local arsenian overgrowths containing

submicron gold (fig. K-11), and locally, breccias were flooded

with quartz-apatite-rutile. Late veinlet and vug minerals include

barite, minor amounts of sphalerite, orpiment, and cinnabar.

Supergene phosphates, chalcedony, iron oxides-hydroxides-

sulfates, alunite, and kaolinite developed due to oxidation of

iron sulfide-bearing breccia (Williams, 1992; Shallow, 1999).

The age of gold mineralization at Rain has been establishedby fission track dating of hydrothermal apatite in the silicified

breccias; that date is 31.710.3 Ma (T. Thompson, unpub. data).

It predates the oldest supergene alunite age of 22 Ma (Odekirk,

written commun., 1989). Gold mineralization at Emigrant

Springs postdates the age for an altered monzonite dike of

37.50.8 Ma on magmatic zircon (Garwin, 2001). Some

evidence suggests that the early passive stage of silicification,

and possible early brecciation, predates the lamprophyre dikes.

At Rain the lamprophyre dikes are intensely clay-altered;

however, biotite from a dike in the Saddle deposit was dated at

38.890.20 Ma (Ressel and Henry, personal commun., 2002).

CONCLUSIONS

Gold orebodies in the Rain subdistrict are found consistently

in a zone along the Rain fault at the contact of the Devonian

Devils Gate Limestone and the Mississippian Webb Formation.

The ore zone contains hydrothermal and collapse breccias in

mudstone of the middle to lower Mississippian Webb

Formation and micritic limestone of the upper Devonian Devils

Gate Limestone. Mineralized pipe-like bodies of dolomite- and

calcite-cemented breccia developed within the Devils Gate

Limestone below the main ore zone and contain high-grade

gold ore (Mathewson and others, 1994; Longo and others,

1997). Similar breccias described by Williams and others(2000) in the Rain open pit have supergene alunite-cement and

much lower grade. Sandstone of the Devonian Oxyoke Canyon

Formation also hosted weak gold mineralization 1,400 feet (420

m) below the Rain open-pit deposit (Read and others, 1998).

Gold occurs as elemental gold encapsulated in quartz as

well as in submicron substitutions in arsenian rims over pyrite

(Shallow, 1999). The hydrothermal breccia that contains the

gold orebody is silicified and contains iron sulfides, significant

barite, apatite, and minor amounts of sphalerite, orpiment, and

cinnabar.

While regional stratigraphic relationships remain a subject

of debate, the miogeoclinal Devils Gate Limestone, the WebbFormation, and the Mississippian Chainman Format

geologic overview of the rain subdistrict

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