AGE AND POSSIBLE SOURCE OF AIR-FALL TUFFS OF THE …

 

 

CONTRIBUTIONS TO THE GOLD METALLOGENY OF NORTHERN NEVADA OPEN-FILE REPORT 98-338

AGE AND POSSIBLE SOURCE OF AIR-FALL TUFFS
OF THE MIOCENE CARLIN FORMATION,
NORTHERN NEVADA

By Robert J. Fleck, Ted G. Theodore, Andrei Sarna-Wojcicki, and Charles E. Meyer

ABSTRACT

40Ar/39Ar laser-fusion ages have been obtained from
alkali-feldspar-bearing air-fall tuffs of the Carlin Formation,
a Miocene clastic and volcaniclastic sedimentary unit of north-
central Nevada. Ages of these tuffs are grouped tightly, ranging
from 14.4 to 15.1 Ma, but even this limited range may be
exaggerated by contamination during deposition with feldspar
from underlying rhyolitic volcanic rocks. These tuffs and much
of the Carlin Formation post-date at least some of the gold
mineralization in the Midas and Ivanhoe mining districts,
where ages of 15.3 and 15.1 Ma, respectively, are reported for
vein adularia. Younger quartz-adularia veins in the Santa Renia
Fields quadrangle cut detrital units of the Carlin Formation
that overlie the air-fall tuff horizons northwest of the dated
locations, suggesting that circulation of hydrothermal fluids
probably continued during deposition of the formation.

The age, location, and major-element chemistry of the
glass shards in the tuffs of the Carlin Formation are consistent
with derivation from silicic volcanic centers associated with
the Yellowstone hotspot and/or the northern Nevada rift. These
parameters are most consistent with a subalkaline source in
the Owyhee Plateau of Idaho, such as the Juniper Mountain
volcanic center, but tuffs also could have been derived from
local sources in the Snowstorm or Sheep Creek Mountains or
from more distant calderas of the McDermitt or Lake Owyhee
volcanic fields 50-60 km to the west or northwest. Additional
studies of minor- and trace-element compositions of the tuffs
and better age and chemical characterization of middle
Miocene volcanic centers of the region are necessary for more
precise tephrochronology of 14- to 16-Ma volcanism in
northern Nevada and adjacent areas of Oregon and Idaho.

GEOLOGIC SETTING

The Carlin Formation was defined by Regnier (1960)
for a sequence of largely unconsolidated silts, sands,
fanglomerate, and pyroclastic rocks that overlie Ordovician
to Miocene(?) strata in the vicinity of Carlin, Nev.,
approximately 50 km south-southeast of the Santa Renia Fields

quadrangle (fig. 1). In that area the Carlin Formation is
approximately 130-200 m thick (Regnier, 1960). Originally
thought to be entirely Pliocene in age (Regnier, 1960), based
on the presence of vertebrate fossils approximately 5 km
northeast of Carlin, the strata were assigned a Miocene and
Pliocene age by Evans and Cress (1972) and Evans (1972,
1974). As mapped in the western part of the Santa Renia Fields
quadrangle by T.G. Theodore, J. Kelly Cluer, and Stanley C.
Finney (unpub. data, 1997), four units are identified, but these
were not found to be mappable throughout the area (fig. 2). In
the quadrangle, the 30 m-to-50 m-thick basal tuff unit includes
thin beds of partly welded tuff and lesser amounts of siltstone,
mudstone, and gravel. This unit rests on Miocene rhyolite
porphyry informally named the Craig Rhyolite of Bartlett and
others (1991). W.E. Brooks (written communication, 1997)
reports an 40Ar/39Ar incremental-heating plateau age of
15.03– 0.05 Ma for similar rhyolite porphyry approximately 5
km southwest of location 4 (fig. 2) in the adjacent Willow
Creek Reservoir SE quadrangle. The basal tuff unit is overlain
by a unit, as much as 400 m thick, including mostly
unconsolidated silts, sands, and sedimentary breccia, as well
as abundant volcanic ash and air-fall tuff. Because the volcanic
debris is present variously as unconsolidated and consolidated
deposits, the terms, ash, tuff, and tephra are all correct and are
used here depending upon the context. Bedded air-fall tuff
crops out prominently throughout the quadrangle as nearly
white bands across slopes (fig. 3). Beds in the various units of
the Carlin Formation are either horizontal or gently dipping;
most exposed sequences are horizontal and ample evidence is
present in the form of pisolites and abundant worm burrows to
indicate subaqueous deposition, probably lacustrine. The
organisms commonly used brown mud or silt to fill their
burrows through the air-fall tuff. Roadcuts through the
formation reveal that many well-layered sequences of air-fall
tuff are broken by high-angle normal faults that have
separations of as much as 1 to 1.5 m. Where the air-fall-tuff-
rich unit laps unconformably onto lower Paleozoic rocks, an
underlying, thin, yellow-brown, sandy silt or massive olive-
brown mudstone is present. In places the underlying units
form mud lumps in beds of gray-white, air-fall tuff. Thin beds
of granitic debris, possibly derived from the Jurassic Goldstrike

176

CARLIN-TYPE GOLD DEPOSITS

120°

118°

116°

114°

112°

45°

44°

43°

41°

Idaho

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Montana

YP
2.1–0.6
Ma

Snake River Plain
Volcanic Province

H
6.5–4.3
Ma

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McDermitt Volcanic
Field

16.1–15
Ma

Oregon

42°

13.8
Ma

O–H

_
>13.9–12.8
Ma

B–J
12.5–10
Ma

P
~10 Ma

TW
10–8.6
Ma

Nevada

Utah

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Idaho

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Northern
Nevada
Rift

Santa Renia Fields
Quadrangle

0

200 KM

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Figure 1. Index map showing location of the Santa Renia Fields quadrangle, Nev., the Snake River volcanic province (modified
from Perkins and others, 1995), the McDermitt volcanic field (modified from Rytuba and McKee, 1984), and the northern
Nevada rift (Zoback and Thompson, 1978; Zoback and others, 1994). Dashed lines within Snake River volcanic province
approximate the major volcanic fields proposed by Perkins and others (1995): O–H, Owyhee-Humboldt volcanic field; B–J,
Bruneau-Jarbidge volcanic field; TW, Twin Falls volcanic field; P, Picabo volcanic field; H, Heise volcanic field; YP, Yellowstone
Plateau volcanic field. Age range of major explosive volcanic activity in Snake River volcanic province modified from Pierce
and Morgan (1992) and Perkins and others (1995). Dashed lines within McDermitt volcanic field are locations of major
calderas with age range modified from Rytuba and McKee (1984).

pluton approximately 10 km to the southeast, also are present
locally in the air-fall-rich sequence. The tuff-bearing unit is
overlain, in turn, by poorly exposed silts and sands, mostly
unconsolidated, that are conspicuously free of air-fall tuff.
All of these units are overlain by unconsolidated fanglomerate
deposits — typically as much as 130 m thick and locally as
much as 600 m thick — that appear to have been derived
principally from Paleozoic rocks in the area of Beaver Peak,
approximately 10 km to the east.

The spatial and temporal relationship of epithermal and
hot-springs-generated ore deposits in the northern Great Basin
to magmatic and hydrothermal activity has been documented
by field and geochronologic studies of these deposits (McKee
and others, 1974; Silberman and others, 1976; Noble and
others, 1988; Wallace and others, 1990). Noble and others
(1988) summarize evidence for mineralization throughout the
17- to 14-Ma period in this region. West of the Santa Renia
Fields quadrangle, sedimentary strata correlated with the
Carlin Formation overlie volcanic rocks thought to post-date
mineralization in the Hollister mine in the Ivanhoe mining

district (McKee and others, 1974; Alan R. Wallace, written
commun., 1997). Locally within the quadrangle, however,
some sequences of the Carlin Formation are cut by low-
temperature quartz-adularia veins. Roughly 4.8 km west of
the Dee gold mine, near the west edge of the quadrangle, these
veins are present as fracture and fragment coatings within
fanglomerate of the formation (fig. 4). Rhyolite flows and
hypabyssal domes that appear to postdate some of the Carlin
sedimentary rocks show evidence of weak alteration (Alan R.
Wallace, written commun., 1997), and support the continuation
of mineralization during Carlin sedimentation.

SAMPLING AND ANALYTICAL
TECHNIQUES

Sample Collection of Air-Fall Tuffs

Samples of bedded air-fall tuff were obtained from the
Carlin Formation in the central part of the Santa Renia Fields

177

CONTRIBUTIONS TO THE GOLD METALLOGENY OF NORTHERN NEVADA OPEN-FILE REPORT 98-338

116°30′

41°07’30”

116°22’30”

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SQUAW CREE K THRUST

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COYOTE T H R U S T

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4

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1

5

Creek

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R O S

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FA U LT

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Rossi Ba
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7

6

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(cid:0)(cid:0)

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Rodeo Creek Au
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Queen Ba

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A n t e l o p e

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F

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N

A

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Rossi Storm Au
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Dee Au

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Hill

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Ren Au

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K F

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L C

L

E

B

(cid:0)(cid:0)

Tara Au
(cid:0)
(cid:0)
(cid:0)

(cid:0)

(cid:0)

(cid:0)

(cid:0)

(cid:0)

(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)

(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)

(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)
(cid:0)

(cid:0)(cid:0)

(cid:0)(cid:0)

1 KM

41°00′

0

(cid:2)

(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)

Figure 2. Geologic sketch map of the Santa Renia Fields quadrangle, Nev., showing locations of samples
dated from Miocene Carlin Formation. Modified from T.G. Theodore, J.K. Cluer, and S.C. Finney (unpub.
data, 1997). Location numbers same as in tables 1–3.

(cid:0)(cid:0)

(cid:0)(cid:0)

(cid:0)(cid:0)

(cid:0)(cid:0)

(cid:0)(cid:0)

(cid:0)

(cid:0)
(cid:0)(cid:0)
(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)

178

(cid:0)(cid:0)

(cid:3)

(cid:0)
(cid:3)
(cid:2)
(cid:1)

(cid:1)
(cid:0)

N

CARLIN-TYPE GOLD DEPOSITS

Figure 2—Explanation

CORRELATION OF MAP UNITS

QUATERNARY

UNCONFORMITY

(cid:0)(cid:1)
(cid:1)
(cid:0)

UNCONFORMITY

(cid:3)

(cid:0)(cid:2)
(cid:3)

(cid:3)
(cid:2)
(cid:0)
(cid:1)

UNCONFORMITY

(cid:0)(cid:0)(cid:0)

MIOCENE

TERTIARY

(cid:3)
(cid:2)
(cid:2)(cid:3)

(cid:0)(cid:0)(cid:0)

UNCONFORMITY

North of Coyote thrust

South of Coyote thrust

T
S
U
R
H
T
E
T
O
Y
O
C

UNCONFORMITY

S
N
I
A
T
N
U
O
S M

T
S
U
R
H

T
R
E
B
O
R

T

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

(cid:0)

PENNSYLVANIAN

SILURIAN AND
DEVONIAN

SILURIAN

ORDOVICIAN, SILURIAN,
AND DEVONIAN

ORDOVICIAN

(cid:0)(cid:1)(cid:0)(cid:1)

(cid:0)(cid:0)

DESCRIPTION OF MAP UNITS

Alluvium and other unconsolidated deposits (Quaternary)

(cid:2)(cid:3)

(cid:2)(cid:3)(cid:0)(cid:1)(cid:2)(cid:3)

Carlin Formation—Fanglomerate deposits, unconsolidated (Miocene)

Carlin Formation—Silts and sands, mostly unconsolidated (Miocene)

Carlin Formation—Silts and sands, mostly unconsolidated, sedimentary
breccias, and abundant air-fall tuff (15.1 to 14.4 Ma, Miocene)

Carlin Formation—Tuff, partly welded, and minor silts and sands,
mostly unconsolidated (Miocene)
Carlin Formation—Undivided (Miocene)

(cid:0)(cid:1)(cid:2)(cid:3)(cid:0)(cid:1)

(cid:0)(cid:0)

(cid:0)(cid:0)

Porphyritic rhyolite vitrophyre (north of Antelope Creek) and peralkaline
rhyolite (west of Boulder Creek). Same as informally named Craig rhyolite
of Bartlett and others (1991) (Miocene)

Strathearn Formation—Chert-pebble conglomerate (Pennsylvanian and Permian)
(See page 180 for remainder of map units and symbols)

quadrangle after repeated visual examination to assure
collection of the freshest and least contaminated material.
Twelve of these samples from seven different localities (fig.
2) were selected for initial 40Ar/39Ar dating and several
additional samples were used for tephrochronology. Many
additional localities were examined but rejected because of
inadequate exposures or alteration of the tuffs. Most samples
represent material obtained from approximately 10-cm-thick,
well-bedded stratigraphic intervals with minimal evidence of

reworking. Some outcrops of the tuff, however, are severely
bioturbated such that only traces of bedding surfaces remain.

40Ar/39Ar Laser-Fusion Dating

The 40Ar/39Ar dating technique was first utilized in its
common form by Merrihue and Turner (1966) and has since
become utilized more commonly than conventional K-Ar

179

CONTRIBUTIONS TO THE GOLD METALLOGENY OF NORTHERN NEVADA OPEN-FILE REPORT 98-338

Figure 2—Explanation cont’d.

(cid:0)(cid:0)

(cid:0)(cid:0)

Undivided autochthonous carbonate rocks below Roberts Mountains thrust—Includes
laminated and shaly limestone of Silurian and Devonian Roberts Mountains
Formation, ooid packstone of the informally named Devonian Bootstrap limestone,
and laminated limey mudstone of the Devonian Popovich Formation

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)

Elder Sandstone in upper plate Roberts Mountains thrust—Mostly siltstone and minor
chert (Silurian)

Undivided siliceous sedimentary rocks in upper plate of Roberts Mountains thrust
south of Coyote thrust—Mostly chert and argillite of Ordovician Vinini Formation,
chert and argillite of probable Devonian Slaven Chert, and minor siltstone of
Silurian Elder Sandstone. May include small areas of outcrop of cherty rocks belonging
to Devonian Rodeo Creek unit (see text) (Ordovician, Silurian, and Devonian)

(cid:0)(cid:1)(cid:2)(cid:3)

Undivided siliceous sedimentary rocks in upper plate of Coyote thrust—Siltstone,
mostly, and some chert, probably belonging to Ordovician Vinini Formation
(Ordovician)

(cid:0)(cid:1)

(cid:0)(cid:0)

Vinini Formation—Quartzarenite, mostly (Ordovician)

Contact

Fault—Dashed where approximately located; dotted where concealed; bar
and ball on down-dropped side

Thrust fault—Sawteeth on upper plate. Dashed where approximately located;
dotted where concealed

Mine dump

1

BX–1

Fossil locality described in text; locality 4 same as table 1

Diamond drill hole Baxter no.1 (see text)

dating because of its increased precision and utility
(Dalrymple, 1989). Although the early advances with 40Ar/
39Ar involved step-wise heating of samples and evolving the
argon gas in multiple increments, total fusion of extremely
small amounts of material through the use of a continuous
laser (laser-fusion 40Ar/39Ar) has become the technique of
choice for mineral separates from thermally undisturbed
samples (York and others, 1981; Dalrymple and Duffield,
1988). Laser-fusion 40Ar/39Ar dating has evolved to dating
of single or small numbers of mineral grains and is especially
useful to detect and overcome the effects of xenocrystic
contamination (LoBello and others, 1987; Fleck and Carr,
1990) and incomplete extraction of radiogenic Ar from alkali

feldspar (McDowell, 1983).

The laser-fusion technique used in this study follows
procedures described by Dalrymple and Duffield (1988),
Dalrymple (1989), and Fleck and Carr (1990). These analyses
were performed on the same mass spectrometer and using the
same argon extraction-system described by Dalrymple (1989).
Samples used in 40Ar/39Ar dating were irradiated for 16 hours
in the U.S. Geological Survey TRIGA Reactor Facility in
Denver, Colorado. An intralaboratory standard sanidine,
85G003, (Taylor Creek Rhyolite, 27.92 Ma) was used for
calculation of the neutron flux in all irradiations. The age of
this monitor mineral is as reported by Duffield and Dalrymple
(1990), standardized to an age of 513.9 Ma for interlaboratory

180

CARLIN-TYPE GOLD DEPOSITS

Figure 3. Photographs showing field relations of air-fall tuff-rich unit of Miocene Carlin
Formation, Santa Renia Fields quadrangle, Nev. A, Distribution of air-fall tuff west of Santa
Renia Springs near upper reaches of Antelope Creek, Santa Renia Fields quadrangle, Nevada;
B, character of well exposed air fall tuff at map location 3 (fig. 2).

standard hornblende, MMhb-1 (Samson and Alexander, 1987)
as measured in the Menlo Park laboratory and the
intralaboratory standard biotite, SB-3 (Lanphere and others,
1990). Decay and abundance constants for all ages reported
are those recommended by Steiger and Jager (1977).

Best-estimate ages for each sample are represented by
the weighted means of replicate laser-fusion analyses, with

the inverse variance of propagated, within-run (i.e., “internal”)
errors used as the weighting factors (Taylor, 1982). The
goodness of fit parameter, MSWD or Mean-Square-of-
Weighted-Deviates (McIntyre and others, 1966), is calculated
for these means and is used to determine the presence of any
error component in excess of estimated analytical error (an
“external” error). Where the MSWD of a mean is greater than

181

CONTRIBUTIONS TO THE GOLD METALLOGENY OF NORTHERN NEVADA OPEN-FILE REPORT 98-338

C

D

Figure 3. (cont’d) Photographs showing field relations of air-fall tuff-rich unit of Miocene
Carlin Formation, Santa Renia Fields quadrangle, Nev. C, gently dipping beds of air-fall tuff
near upper contact of unit in west-central part of quadrangle, Nev. D, close-up of thin layers
of air-fall tuff and pisolites approximately 2-5 cm above sample 96TT134 (loc. 3, fig. 2)

1.0, analytical errors are multiplied by the square root of
MSWD, as discussed by Ludwig (1988) and Fleck and others
(1996), to incorporate this error.

Tephrochronologic Methods

Volcanic glass shards from samples of air-fall tuff (tephra

182

layers) of the Carlin Formation were separated and analyzed
by electron microprobe using methods described by Sarna-
Wojcicki and others (1984). Briefly, samples were wet-sieved
with water in plastic sieves fitted with nylon screens, retaining
the 200 to 100 mesh size fraction (about 80 to 150 microns,
respectively) for separation of glass shards. This fraction was
placed in water in an ultrasonic vibrator, treated with a 10
percent solution of HCl for a few minutes to remove authigenic

CARLIN-TYPE GOLD DEPOSITS

Figure 4. Photographs of quartz-adularia veins (white) in air-fall tuff-
rich unit of Miocene Carlin Formation approximately 4.8 km west of Dee
gold mine. A, Veins cementing largely unconsolidated fanglomerate
deposits; B, veins along fracture surface in fanglomerate.

carbonate adhering to the glass particles, and leached with an
8 percent solution of HF acid for about 30 seconds to one
minute to remove other coatings or altered rinds that may have
been present on the glass shards. The glass shards were then
separated from other components of each tephra sample using
(1) a magnetic separator and (2) heavy liquids of variable
density made from mixtures of methylene iodide and neothene
or acetone. Most glass samples analyzed in the present study
were already partly processed during separation of alkali
feldspar (sized and most crystalline material was separated),
and the volcanic glass was suitable for analysis.

Each glass separate was mounted in epoxy resin in
shallow holes drilled into Plexiglas slides, and the slides were
ground-down and polished with diamond paste to expose the
shards and prepare a smooth, uniform surface for analysis.
The polished sample was coated with carbon, and 15 to 25

individual glass shards were analyzed for Si, Al, Fe, Mg, Mn,
Ca, Ti, Na, and K using a JEOL 8900 electron-microprobe.
Details of the analytical techniques are reported by Sarna-
Wojcicki and others (1984).

ANALYTICAL RESULTS

Results of individual laser-fusion 40Ar/39Ar experiments
on samples from air-fall tuff or ash layers collected within the
Santa Renia Fields quadrangle are shown in table 1 and
summarized in table 2. Apparent ages range from 15.1 Ma to
14.4 Ma, but internal precision varies as shown by the MSWD.
Values of MSWD greater than about 2.5 are commonly
considered to reflect scatter beyond reasonable expectations
of analytical error and are interpreted as indicating geologic

183