Download Time-Space Development of an External Brine

Time-Space Development of an External Brine-Dominated, Igneous-Driven
Hydrothermal System: Humboldt Mafic Complex, Western Nevada
DAVID A. JOHNSON† AND MARK D. BARTON
Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721
Abstract
The Humboldt mafic complex, west-central Nevada, is a large composite Middle Jurassic basaltic-composition volcano-plutonic center that has exceptionally extensive (>900 km3), intense (nearly complete leaching of
many elements) sodium-rich hydrothermal alteration. Mapping of exposures at multiple structural levels allows
assessment of the time-space development of hydrothermal alteration and cogenetic magnetite and hematite
± copper sulfide mineralization.
Alteration varies from early, deep and proximal marialitic scapolite-hornblende to shallow and distal albiteactinolite-chlorite and chlorite-carbonate assemblages. These associations reflect large compositional changes
in host rocks (mass-transfer), whereas distal and deep propylitc assemblages are less intensely modified. Substantial quantities of iron are present in massive, breccia-form, and stratabound magnetite and hematite bodies at intermediate and shallow depths. Lesser amounts of copper, cobalt, and other metals are sporadically enriched at shallow levels.
Field, petrological, and geochemical constraints require that the fluids were dominantly or entirely nonmagmatic, external brines that circulated in response to the heat and permeability increases associated with repeated basaltic intrusion. The Humboldt system represents a mafic end-member among iron oxide-rich copper-bearing hydrothermal systems (Barton and Johnson, 1996) and, in the larger context, and an end-member
in the spectrum of igneous-related hydrothermal systems.
Introduction
IGNEOUS-RELATED iron oxide-rich hydrothermal systems are
common throughout western North America in Jurassic rocks
(Barton et al., 1988; Barton, 1996). This kind of system has attracted attention globally because some occurrences host
large concentrations of Cu, Au, and other elements (e.g.,
Hitzman et al., 1992). The origin of these hydrothermal systems is controversial and revolves around the source of the
hydrothermal fluids, magmatic versus externally derived
(Barton and Johnson, 1996; Hitzman et al., 1992; Williams,
1994). Fe oxide-rich deposits occur with igneous rocks that
range from mafic to felsic (Barton and Johnson, 1996). In this
and other respects, they contrast to many other kinds of igneous-related mineral deposits which occur with narrow
ranges of igneous compositions. Alkali-dominated hydrothermal alteration is voluminous and large volumes of magnetiteor hematite-rich rock (replacement and open space) typify
generally sulfide-poor mineralization.
The hydrothermal system associated with the Humboldt
mafic complex (Fig. 1) represents an exceptionally large,
compositional end-member within the Fe oxide group. In aggregate, the system covers parts of three mountain ranges in
the western Great Basin over an area approximately 40 by 70
km in present exposure. This basalt-related hydrothermal system provides an important contrast with other Great Basin
Jurassic Fe oxide-rich districts that are associated with intermediate- to felsic-composition igneous rocks. The comparison of Fe oxide-rich systems associated with different kinds of
igneous rocks and with other styles of mineralization, as at
Yerington, allows better assessment of the roles that igneous
rocks play in the hydrothermal system.
† [email protected]
This study grew out of recognition that regionally extensive
sodium-rich hydrothermal alteration in the western United
States typically had associated Fe oxide-rich mineralization
(Barton et al., 1988; Battles and Barton, 1995). Work in the
Humboldt mafic complex was undertaken as part of a comparison of compositionally diverse systems in the Jurassic of
the Great Basin (Johnson, 2000). The Humboldt mafic complex is favorable for study due to good exposure of multiple
structural levels and a good pre-existing geologic framework.
This was critical to building an understanding of the characteristics and origin of the widespread iron oxide ± copper
mineralization in this complex and its implications for similar
systems elsewhere.
This study draws upon earlier work on the complex and
provides part of the framework for geochemical studies in
progress. Early studies concentrated on descriptive information on the iron oxide occurrences in the Mineral basin
district (Reeves and Kral, 1955; Nickle, 1968). These early
studies were complemented by regional geologic and petrographic work of Speed (1962, 1976), who covered the entire
complex. Geologic maps of the region were published by
Page (1965) and Speed (1976). Vanko (1982) and Vanko and
Bishop (1982) studied aspects of the petrology of the igneous
and scapolitized rocks.
Mining history and Fe-Cu-Co-Ni resources
Copper and cobalt-nickel-copper mineralization within the
Humboldt mafic complex was discovered early in Nevada’s
mining history. Most copper occurrences are high in the volcanic section and found in the central part of the complex in
the Stillwater Range. The Boyer Copper mine was found prior
to 1860, and in 1861 shipped several wagon trains of rich copper sulfide ore to Sacramento for treatment (Lincoln, 1923).
Early observers suggested that these copper occurrences
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JOHNSON AND BARTON
Cortez Mtns
(mid J, dacite-rhyolite)
Humboldt
mafic complex
(mid J, basalt)
Yerington District
(mid J, dacite-rhyolite)
Eagle-Palen Mtns
(mid J, dacite-rhyolite)
Fe-oxide-rich
hydrothermal system
N
W
E
Igneous Rocks
S
Intrusive \ Volcanic
mainly
mid-Mesozoic
KILOMETERS
250
0
250
500
FIG. 1. Map of the western United States and Mexico showing the location of Humboldt mafic complex, Mesozoic igneous rocks, and iron oxide occurrences. Igneous rocks from King and Beikman (1974). Iron oxide occurrences from MRDS
database.
were similar in grade and extent to porphyry copper deposits
such as Ely, Bingham, and Yerington (Carpenter, 1911).
Nickel-cobalt-copper occurrences in Cottonwood Canyon
were located in 1882 and shipped ores to Swansea, Wales
(Lincoln, 1923; Ferguson, 1939).
Igneous and sedimentary rocks related to the Humboldt
mafic complex contain more than 50 occurrences of iron
oxide mineralization (Johnson, 1977; Moore, 1971; Willden
and Speed, 1974), ranging from small prospects to deposits
exceeding 100 Mt of resource. Production of iron began in
the late 1880s and continued intermittently through World
War II (Reeves and Kral, 1955; Willden and Speed, 1974).
After World War II, Japanese steel companies contracted
with Nevada mines to supply iron ore. In addition to shipments made to Japan, small quantities of ore were shipped
to the Ford Motor Company in Dearborn, Michigan. Production continued until 1968, when contracts expired. Small
shipments, mainly from stockpiled ore, have continued on
and off until the present mainly for use in the cement industry. Total production to 1971 is at least 4 Mt of iron ore.
Published resource estimates for 15 of the 31 largest mines
and prospects is greater than 170 Mt of measured and indicated reserves at an average grade of 33 wt percent iron
(Moore, 1971). Total resource estimates for all evaluated occurrences are probably greater 500 Mt (unpub. reports,
United States Steel). In addition to metals, small amounts of
gypsum were produced from Jurassic sedimentary rocks in
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the West Humboldt Range and Mopung Hills (Willden and
Speed, 1974).
Geologic Framework
The Humboldt mafic complex and related host rocks are
exposed in the West Humboldt, Stillwater, and Clan Alpine
ranges and in the Buena Vista Hills covering an area of approximately 1800 km2. The outcrop pattern forms a crude ellipse elongated east-west with a two to one axis ratio (Fig. 2;
Speed, 1976). Stratigraphic relations and major structural relationships are summarized in Figure 3.
The Humboldt mafic complex was emplaced at ~170 Ma
into an extending back-arc trough (Saleeby et al., 1992; Johnson, 2000) in the mildly alkaline Jurassic arc (Miller, 1978).
The mafic rocks intrude and overlie Late Triassic to Early
Jurassic pelitic assemblages of the Lovelock assemblage
(Speed, 1976). The lowest portion of the complex consists of
numerous intrusive bodies which intrude and feed the overlying volcanic pile. Plutonic rocks are dominantly gabbroic.
West-northwest-oriented mafic dikes are abundant and
locally form sheeted dike swarms. These are best exposed in
the central portions of the complex. The overlying volcanic
pile is mainly composed of basaltic lavas and tuffs with interbedded volcaniclastic and other sedimentary rocks (Johnson, 2000). Volcanic rocks overlie or interfinger with sandstones and evaporites of the Middle Jurassic Boyer Ranch
Formation (central areas) and the carbonates and evaporites
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DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
Generalized Facies of Mid-Jurassic
Lovelock
Sediments
km
0
Boyer Ranch Fm
Upper qtz arenites
Basal conglomerates, carbonates,
evaporites
Lovelock & Muttlebury Fm
Carbonates, evaporites, &qtz arenites
Fe n
cem
aker
Thru
20
st
N
83
Fencem
aker T
hrust
Lithology
Qal
Alluvium & sediments of Lake Lahontan
Ji
QTb
Pleistocene to Miocene basalt
Jv
Tv
Felsic tuffs, lavas, and intercalated
sedimentary rocks
Kg
Granites, granodiorites, & qtz monzonites
Js
Mafic to intermediate intrusive
rocks of the Humboldt mafic complex
Trs
Undivided basinal
pelitic sequence
Mafic to intermediate volcanics of
Undivided
the Humboldt mafic complex
Trs carbonate
Undivided Jurassic sediments,
sequence
carbonates, evaporites, coarse clastics
Lovelock, Muttlebury, & Boyer Ranch Fm
FIG. 2. Geologic map of the Humboldt mafic complex. Modified from Speed (1976) and Page (1965). Also shown is the
reconstructed sedimentary facies for the Middle Jurassic sedimentary rocks (modified from Oldow et al., 1990).
of the Lovelock and Muttlebury Formations (western areas;
Speed and Jones, 1969; Johnson, 2000).
Structural relationships within the Humboldt mafic complex are complex. These include (1) compressional, preHumboldt mafic complex structures (Elison and Speed, 1988;
Oldow et al., 1990; Wyld and Wright, 2000), (2) extensional
and possible compressional structures, syn-Humboldt mafic
complex (Johnson, 2000; Speed, 1966, 1976; Speed and Page,
1964), (3) compressional, post-Humboldt mafic complex
thrusting along the Lunning-Fencemaker thrust belt (Oldow
et al., 1990, Wyld and Wright, 2000), and (4) mid-Tertiary and
Basin and Range extension (Hudson and Geissman, 1991;
John, 1995; Johnson, 2000). These relationships have led previous workers to a variety of interpretations for the origin of
the Humboldt mafic complex (Speed,1966, 1976, Dilek et al.,
1988, Dilek and Moores, 1995). Heretofore largely undescribed, Tertiary extension within the Humboldt mafic complex complicates the structural and lithologic story in the preTertiary rocks (Johnson, 2000).
Pre-Humboldt complex folding and thrusting are exposed
in Triassic and lower Jurassic rocks throughout the complex
(Oldow et al., 1990; Wyld and Wright, 2000). These compressional structures die out up section and are missing in the
Middle Jurassic sediments (Oldow et al., 1990). Igneous rocks
were probably initially emplaced into multiple centers as
demonstrated by the distribution of mafic dike swarms.
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Volcanic rocks were erupted into the shallow basin, eventually
building a topographic high, as shown by (1) interfingering of
the lower volcanic flows with the basin margin Boyer Ranch
Formation, (2) the presence of rare pillow structures in the
lower portions of the volcanic pile and the absence of these
features in higher stratigraphic sections, and (3) the high
abundance of debris flows sourced from topographic highs
preserved throughout the volcanic section. Dike swarms
feeding the overlying volcanic pile developed in a number of
locations, such as the Buena Vista Hills and the White Rock
Canyon area, commonly with a west-northwest orientation
(Fig. 4). Orientations of dike swarms and Jurassic faults are
roughly parallel to each other. Other faults composing the
contacts between the mafic complex and host rocks as well as
contacts within the Middle Jurassic sedimentary units place
younger rocks on top of older rocks (Speed, 1976) and likely
have normal displacement. These observations are consistent
with emplacement of the complex into a northeast-southwest
extending Jurassic basin. Similar Jurassic extension is documented for the Dunlap Formation in western Nevada, a
roughly correlative unit to the Boyer Ranch Formation
(Oldow and Bartel, 1987). This interpretation contrasts with
earlier interpretation of emplacement of the complex as a single lopolithic mass which displaced surrounding mid-Jurassic
sedimentary rocks outward along a number of thrust faults
(see Speed, 1966; and Speed and Page, 1964).
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JOHNSON AND BARTON
mafic complex region are summarized in Figure 5. Other evidence is also compatible with considerable extension including northwest-southeast elongation of the outcrop pattern,
the bend in the trace of the Fencemaker thrust, and paleomagnetic data.
Lovelock Assemblage
Tr pelites
JTr pelites-sandstones
J Lovelock and
Muttlebury Fm
Lovelock
Boyer Ranch Fm
Assemblage basal cgl
upper sandstone
East
West
Tectono-stratigraphic column,
Humboldt mafic complex
Humboldt
Assemblage
Tr carbonates
Clan Alpine
Range
Stillwater
Range
Buena Vista
Hills
West Humboldt
Range
Fencemaker
Thrust
Igneous rocks
QT basalts
T young felsic volcanics
T old felsic volcanics
J mafic volcanics &
volcaniclastics
J gabbroic rocks
FIG. 3. Schematic tectono-stratigraphic column for the Humboldt mafic
complex.
Large degrees of mid-Tertiary extension in west central
Nevada is documented in a number of areas in the western
Great Basin (Proffett, 1977; Seedorff, 1991; Hudson and
Geissman, 1991; John, 1995). At least two episodes of extension are recorded in the region of the Humboldt mafic complex. Young tilting is represented by Basin and Range (<13
Ma?) normal faulting which controls range fronts and tilts
Miocene and younger basalts and sediments (John, 1995).
Older is an earliest Miocene (~24 Ma) extensional event documented in the southern Stillwater Range (south of the Humboldt mafic complex; Hudson et al., 1993; Hudson and Geissman, 1991), which tilts volcanic units of the Job Canyon
caldera up to 90° (John and Silberling, 1994, John, 1995).
Hudson et al. (1993) estimate that Cenozoic extension in this
region is between 70 and 100 percent.
Within the complex, the maximum possible Cenozoic extension is not well constrained because the older units of the
Tertiary volcanic sequence which occur to the south in the
Stillwater Range are missing. It is likely at least 50 percent,
compatible with 45° dips on many of the Tertiary volcanic
rocks, but it could be 100 percent or more (Johnson, 2000).
Tertiary volcanic rocks dip up to 75°; Jurassic volcanic and
sedimentary rocks also have variable dips and in a few places,
such as in the Buena Vista Hills, they are overturned (Fig. 4).
Tertiary tilting of various fault blocks within the Humboldt
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Igneous Rocks
Basaltic volcanic rocks and gabbroic intrusions constitute
most of the complex (Fig. 2). Gabbroic rocks are exposed in
all three ranges and compose the largest portion of complex
at intermediate and deep structural levels. Volcanic and volcaniclastic rocks are exposed in the Stillwater Range, Buena
Vista Hills, and in drill core at the western edge of the Carson
Sink. A third type, composed of mafic dike swarms, is volumetrically subordinate to other types, but are important at intermediate structural levels in many locations throughout the
complex.
Hornblende gabbro, olivine gabbro, and microgabbro constitute the majority of intrusive rocks. A wide range of textures are found within this group ranging from fine- to
coarse-grained, from equigranular to strongly porphyritic,
and from nonfoliated to strongly foliated varieties. The largest
portion of the plutonic rocks consists of hornblende- and
olivine-bearing equigranular gabbros with subordinate coarsegrained picrites and anorthosites, and variably porphyritic
fine-grained gabbros and dikes. Equigranular gabbros are
subophitic in texture with 0.5–2 cm plagioclase and augite
with variable amounts (10–20%) of amphibole (magnesiohastingsite to hornblende) and olivine (<1–5%; Fo75–82).
Minor minerals include magnetite and ilmenite (0.5–1.5%),
minor chromite in the olivine-rich rocks, and accessory apatite and titanite. The latter is magmatic and rims ilmenite.
Orthopyroxene is commonly present only in olivine-bearing
gabbros.
Equigranular gabbros commonly grade outward into finergrained (0.1–0.3 cm) diabasic microgabbro, which is commonly exposed near the periphery of the complex, possibly as
a border phase or as small bodies intruding the sedimentary
host rocks, and near the plutonic-volcanic contact throughout
the complex (Speed, 1976). Microgabbros typically contain
hornblende, augite, and minor olivine. Accessory minerals include magnetite (~1%), apatite intergrown with the hornblende, and igneous biotite (Phl75–90). Contacts between
equigranular and the finer-grained gabbros are commonly
gradational. Where crosscutting relationships are exposed,
the microgabbro is typically older.
Strongly porphyritic rocks occur at intermediate to shallow
levels and intrude the volcanic pile. These rocks are commonly trachytic textured and are composed of 5–20 percent
plagioclase phenocrysts 1–>2 cm in side 5–20 percent and
groundmass (0.2–0.5 cm) plagioclase, augite and hornblende.
Rarely, these bodies can be shown to feed overlying porphyritic volcanic flows. Due to their relatively shallow distribution, these rocks are commonly strongly altered (Fig. 6b).
Where contacts are exposed, strongly porphyritic rocks cut
equigranular varieties.
Minor picritic gabbros are distributed throughout the periphery of the complex at deeper levels and are commonly
overlain by anorthositic rocks. Picritic gabbros commonly
have magmatic foliations defined by plagioclase (1–4 cm)
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DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
25
Lovelock
16
21 10
Structural Data
Orientation of Jurassic mafic dikes
Fen
20
60
15
70
70
20 Strike & dip of foliation
ce m
aker
Thrust
25
20
50
Strike & dip of beds
70
30
Horizontal beds
60
40
50
68
5
10
5
30
Normal fault
45
80
15
15
Young Dikes
Thrust fault
36
72
Oldest Dikes
46
60
43
TCzv
Jv
5
72
aker T
hrust
35
34
35
Fencem
25
25
30
52
55 20
10
14
25
27
20
67
12
16
28
15
20
km
0
20
15
25
55
25
60 20
30
60
Alluvium & sediments of Lake Lahontan
Post-Humboldt rocks
30
55
12
10
20
50
Mafic to intermediate intrusive rocks of the
Humboldt mafic complex
Mafic to intermediate volcanics of the
Humboldt mafic complex
Ji
18
60
30
40
Lithology
Qal
37
32
33
16
20
23
N
42
20 50
5
48
8080
83
30
45 60
45
33
23
Js
Undivided Jurassic sediments, carbonates, evaporites, coarse
clastics Lovelock, Muttlebury, & Boyer Ranch Fm
Trs
Pre-Humboldt pelitic and carbonate rocks
FIG. 4. Structural orientations of Jurassic mafic dikes, iron oxide bodies, and igneous foliations in the Humboldt mafic
complex. Data from Johnson (2000), Reeves and Kral (1955), and Wilden and Speed (1974).
Lovelock
Tertiary & Quaternary Structural Domains
25
16
40
25
Fe n
20
60
-Representative orientation of
Tertiary units
-Oldest and youngest units shown
where exposed
-Oldest (large symbol)
-Youngest (small symbol)
-Oldest unit used where undivided
cem
aker T
hrust
20
10
30
5
40
45
15
83
20
5
15
32
Fencem
aker T
hrust
25
18
10
5
52
27
20
23
30
40
20
N
20
km
0
17
15
20
20
15
25
30
Qal
Alluvium & sediments of Lake Lahontan
QTb
Pleistocene to Miocene basalt
Tv
Felsic tuffs, lavas, and intercalated sedimentary rocks
55
30
20
20
50
FIG. 5. Tertiary tilting domains within the Humboldt mafic complex. Data from Johnson (1977), Johnson (2000), Page
(1965), Speed (1976), and Wilden and Speed (1974).
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JOHNSON AND BARTON
FIG. 6. Outcrops and hand specimens from variably altered and mineralized rocks. a. Scapolitized dike cutting earlier
scapolite veins, Buena Vista Hills (lens cap for scale). b. Pervasively scapolitized porphyritic gabbro. Remnant plagioclase remains in approximately 10 percent of the feldspar phenocrysts. Mafic minerals are replaced by magnesio-hornblende with
rare magnetite. c. Albite veins cutting pervasive albite-hematite altered volcanic flow from the Bradshaw mine. d. Albitehematite replaced vesiculated flow top exposed in White Rock Canyon. e. Crustiform massive magnetite-apatite replacement, from White Rock Canyon (magnet for scale). f. Magnetite-apatite ± amphibole veins cutting scapolite-amphibole altered clasts in a magnetite-amphibole cemented breccia. g. Chalcopyrite-hematite veins cutting weakly albitized and
chloritized volcanic lavas from the upper portion of White Rock Canyon. h. Stratabound, laminated possibly syngenetic(?)
hematite ± silicas interbedded with volcaniclastic rocks from Cottonwood Canyon (magnet for scale).
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DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
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active tectonic environment includes the incorporation of
high-temperature-alteration clasts within the debris flows
that are in turn altered to other assemblages or overlain by
later syngenetic hematite.
Fresh igneous rocks within the Humboldt mafic complex
are rare. The least-altered analyzed igneous rocks range from
calc-alkaline to slightlly alkaline basalts and trachybasalts. Igneous rocks range in SiO2 from 45.8 to 53.7 wt percent. with
the majority of the samples between 48.6 and 51.9 wt percent
SiO2 excluding picritic rocks and moderately chloritized samples. The content of sodium and potassium varies between
2.0–5.5 wt percent Na2O and 0.3–2.5 wt percent K2O. Rocks
demonstrating higher alkali contents are weakly altered (Fig.
7). Major and trace element contents and neodymium isotopic compositions of these rocks are consistent with magmatism in an evolving arc to back-arc environment (Gleason et
al., 2000; Johnson, 2000).
Hydrothermal Alteration and
Fe oxide (-Cu) Mineralization
Sodium-rich hydrothermal alteration and Fe oxide and Cu
sulfide mineralization are distributed widely and systematically throughout the Humboldt mafic complex. The complex
is characterized by >900 km3 of sodium-rich alteration that
consists of deep and proximal scapolite-rich mineral assemblages that are overlain and/or overprinted by shallow or distal albite/oligoclase-dominated assemblages (Fig. 8). Magnetite and hematite mineralization occurs at intermediate and
shallow levels. Cu sulfide ± hematite assemblages are rare
within the deeper, mainly magnetite-rich, mineralization but
14
Fresh rocks.
Altered rocks.
12
10
Na2O+K2O (wt%)
which is intergrown with large kaersutitic amphiboles (1–6
cm) and shows nice orthocumulate textures with olivines (<1
cm, with chromite inclusions) and large poikolitic orthopyroxenes. Anorthositic gabbros are also coarse grained (>2 cm)
and equigranular with a magmatic foliation defined by tabular plagioclase (An40–60) and hornblende.
At higher levels, small stocks, individual dikes, and sheeted
dike swarms intrude fine-grained gabbroic and overlying volcanic rocks. Dikes within the swarms have both single- and
double-sided chilled margins. The majority of these rocks are
fine-grained basalts with chilled margins and sparse, small
(0.2–0.4 cm) plagioclase ± augite phenocrysts in a finegrained (<0.1 cm) groundmass. Up section, the dikes and hybabyssal intrusions become less abundant and, in a few
places, can be demonstrated to feed lava flows (White Rock
Canyon). Dike swarms occur within a number of separate
centers, commonly at least several kilometers long. These
areas tend to localize the most intense hydrothermal alteration and provide some of the best evidence for synmagmatic
alteration within the complex (e.g., in the Anderson RanchBuena Vista mine area; see Johnson and Barton, 2000).
The volcanic pile comprises predominantly mafic lavas, pyroclastic, and volcaniclastic rocks. Lower and upper sequences in the pile differ based upon the proportion of massive lavas and well-bedded volcaniclastic rocks. The best
preserved volcanic section is exposed on the eastern side of
the Stillwater Range. There, the lower sequence consists of
massive basaltic lava flows, volcanic-derived debris flows,
minor lapilli tuffs, and reworked volcaniclastic rocks. These
rocks are commonly composed plagioclase and augite phenocrysts. Olivine and magnetite are minor and not always present. Individual flows commonly have vesiculated flow tops
which may be strongly altered (Fig. 6d). In Cottonwood
Canyon these rocks are interpreted to conformably overlie
and interfinger with the Boyer Ranch Formation (Speed and
Jones, 1969, Riehle, 1969). Overlying the lower sequence is a
dominantly stratified sequence composed of basalt and
basaltic andesite lavas and tuffs, reworked tuffs, and bedded
volcaniclastic rocks. The upper sequence typically contains a
smaller volume of dikes. The presence of lapilli tuffs and
other pyroclastic units and the lack of subaqueous sediments
indicate that most of the volcanic rocks were erupted subaerially or in very shallow bodies of water. Pillow structures
are rare; they have been found only in lower sequence rocks
in a few areas within the Stillwater Range (Dilek and Moores,
1995; Johnson, 2000).
Igneous development began with intrusion of basaltic
magma into a shallow, evaporite-bearing basin as recorded by
the Boyer Ranch and correlative Lovelock and Muttlebury
Formations. Basaltic volcanic rocks interfinger with parts of
the Boyer Ranch Formation. Continued eruption in an active
tectonic environment led to a volcanic pile 1–2 km thick
which periodically shed debris flows incorporated within the
volcanic section. Synmagmatic alteration, described later,
began shortly after the first magmatic input into the system.
The most strongly altered areas centered upon zones of
sheeted dikes and resulting structural zones. These areas provide the best evidence for the evolving system. In addition to
multiple crosscutting magmatic and hydrothermal relationships, other evidence for a coeval hydrothermal system in an
8
6
4
2
0
40
45
50
55
60
SiO2 (wt%)
65
70
FIG. 7. Total alkali-silica igneous rock classification grid after Le Bas et al.
(1986). Composition of fresh and altered rocks is shown.
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JOHNSON AND BARTON
Lovelock
Fe
nce
m ak
er
N
km
0
20
83
Fence
m
a ke r T
hrust
Generalized Alteration
Albitization
Ab-Chl-Dol-Anal±Hm±Py±Cpy
(weak / pervasive)
Scapolitization
Scap-Hbld-Spn±Ep±Mt±Act
(weak / pervasive)
Lithology
Q-Tr
Pre- and post Humboldt mafic complex rocks
Mineralization
Mafic to intermediate intrusive and volcanic rocks of the
Ji, Jv, Humboldt mafic complex. Includes undivided Jurassic
Js sediments, carbonates, evaporites, coarse clastics Lovelock,
Muttlebury, & Boyer Ranch Fm.
Fe mine / occurrence
Cu occurrence
Ni-Cu-Co occurrence
FIG. 8. Generalized alteration map of the Humboldt mafic complex showing distribution of sodium-rich alteration and
location of iron oxide, copper sulfide, and nickel-cobalt-silver-copper-uranium occurrences (compiled from unpublished
mapping of D.A. Johnson and M.D. Barton).
are widespread at shallow levels. In addition to copper-sulfide
mineralization, rare “five-element-suite” (Co-Ni-Ag-As-U)
sulfide and arsenide assemblages with accessory Cu occur in
two areas near the edge of the complex.
Alteration associations
Hydrothermal alteration assemblages are conveniently divided into five major groups reflecting mineralogy and bulk
composition changes. These groups are (1) scapolite-hornblende sodic or sodic-calcic alteration, (2) albite/oligoclaseactinolite sodic-calcic alteration, (3) albite-chlorite sodic alteration, (4) carbonate-chlorite alteration, and (5) propylitic
alteration (Table 1, Fig. 9). The terminology of sodic-calcic alteration follows that used in the Yerington district (Carten,
1986; Dilles and Einaudi, 1992; Dilles et al., 2000) and by
Battles (Battles, 1990; Battles and Barton, 1995) for other occurrences in the western United States.
Early, virtually ubiquitous proximal alteration is characterized by scapolite-dominated sodic-calcic alteration composed
of scapolite-hornblende ± clinopyroxene-titanite ± magnetite
± apatite assemblages exposed in deeper parts of the complex
(Fig. 8). These assemblages replace both intrusive and volcanic rocks (Fig. 6a, b and Table 1) (Vanko and Bishop, 1982;
Johnson, 2000). In most cases, marialitic scapolite (ca. Na4Al3
Si9O24Cl) replaces igneous plagioclase, volcanic groundmass,
and mafic minerals whereas pargasitic hornblende ± diopside
pyroxene replaces igneous augite and hornblende. Minor titanite ± magnetite forms in the original Fe-Ti-oxide sites.
Based upon petrography and textures, scapolitic alteration is
subdivided into several assemblages for mapping (Table 1,
Fig. 9). Strong scapolitization (Sp-2, SP-2a assemblages) is
pervasive and commonly texture destructive, transforming
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fine-grained rocks into coarse-grained equigranular rocks (cf.
Fig. 6a). Strong scapolitic alteration is spatially concentrated
near the plutonic-volcanic contact in the central portions of
the complex (e.g. Mineral basin district) along structurally
complex zones, and/or associated with dike swarms (e.g.
Buena Vista mine area, Fig. 8). Magnetite is commonly destroyed in deep exposures containing strong, pervasive
scapolitic alteration (Fig. 6b; see map in field guide). Strong
scapolitic alteration consists of greater than 80 vol percent of
hydrothermal minerals in mapped exposures. In contrast, incipient to moderate scapolitic alteration typically preserves
igneous textures. Igneous feldspars and groundmass minerals
are partially replaced and/or mantled by scapolite and mafic
minerals are converted to hornblende, actinolite, and chlorite. Exposures mapped as moderate scapolitic alteration contain between 15 to 80 vol percent of the above assemblages.
Distal to SP-2 and SP-2a assemblages are SP-1 scapolitic assemblages. These assemblages consist of a lower volume of
scapolite in conjunction with higher abundance of epidote
and are commonly less texture destructive than SP-2 assemblages. Other scapolitic assemblages are volumetrically minor
in the mapped portions of the complex and occur near iron
oxide mineralization (Fig. 9; Table 1; see Johnson, 2000).
Sodic plagioclase-actinolite (sodic-calcic) assemblages are
restricted in distribution. These assemblages are characterized by the replacement of igneous plagioclase or hydrothermal scapolite by combinations of albite, oligoclase and
epidote, mafic minerals by actinolite ± pyroxene, and irontitanium oxides by titanite. Plagioclase-rich sodic ± calcic
alteration is divided into two assemblages based upon the modal
mineralogy, mode of occurrence, and the spatial distribution of the assemblage. Albite (without oligoclase)-actinolitic
134
Alteration assemblages
Pervasive replacement
Vein/open space
iron oxide/sulfide
mineralization
Relict Minerals
P=igneous,
H=hydrothermal
Spatial/
host
Comments
SP-1
Scap+Ep+Hbld+Ti±Mt±Ap
Ep, Scap-Hbld±Mt
P/Plag cores, Hbld, Cpx
Deep-Int/I±V
Pervasive to weak; distal to SP-2
SP-2
Scap+Hbld+Ti±Mt±Ap±Cal±All
Mt±Ap±Hbld±Scap.
ms Mt+Ap±Scap±Hbld
P/Plag cores, Hbld, Cpx
Deep-Int/I-V
SP-2a
Scap±Hbld±Ti±Ap±Cal±Ep±Mt(r)
Scap+Hbld+Ti+Ap
P/Plag cores in bx fragments Deep-Int/I-V
Pervasive, texture destructive, commonly
bx fill, Scap >65% producing white
outcrops, (A) at BVH
SP-3
Scap+Bt+Hbld±Ep±Mt
Mt±Ap
P/Plag, Hbld
Int/I-V
Locally developed in WRC, NBVH
SP-4
Scap±Ab+Act+Cal+Ti±Ep±Mt±Ap
Mt+Act+Ap+Ti-Ab±
Cpy(r)
vugs
H/Scap
Deep-Int/I-V
Widespread with Ep, occurs as vug filling
and replacements w/in ms repl bodies
SC-1
Ab+Act+Mt+Ap+Ti+Rut±Ep
Mt/Hm+Act±Ap
H/Scap, Hbld, textures
Int/I-V
Overprints scapolitic alteration, coarsening
grain size, rare Ep at BVH
SC-2
Ab/Olig+Ep+Act+Ti±Chl±Mt±Py
Ab+Act+Ep±py
P/Plag cores, Hbld, Cpx
Int/I-V
Widespread
S-1
Ab+Chl±Mt/Hm±Ti/Rut
Mt+Ab+Chl; Hm+Cpy±Bn H/Mt redistributed
Shal-Int/V-I-B
Overprints scapolitic alteration, Ap stable
S-2
Ab+Chl+Ank±Hm±Py±Cpy±Rut±Trml
stratabound Hm±Qtz,
Hm+Cpy±Bn+Chl,
Qtz±Ab+Co+Ni+Cu+
As+U
P/Plag, (Hbld, pyx)
H/Scap, Hbld, Mt, Ti
Shal-Int/V-I-B
texture and mineral sites preserved, Ank
after mafic sites, Hm+Cpy±Bn veins;.
5 element suite hosted by Boyer Ranch
sediments, Trml in B
C-1
Ank+Ab+Chl±Hm±Rut
Ank+Hm+Py
P/Plag, (mafics)
H/Ab, Chl,
Shal-Int/V-I-B
Distal; texture and mineral sites preserved
C-2
Ank±Chl±Ab±Hm
Ank+Brt±Qtz±Py
Ank±Hm±Cpy±Py
P/Plag, (mafics)
H/Ab, Chl,
Shal-Int/V-I-B
Distal; texture and mineral sites preserved
C-3
Cal+Chl+Ep±Ab±Act+Hm±Mt
Cal±Hm
P/Plag, Hbld, Cpx
Deep-Int/I
Locally well developed in SWR and WHR
Propylitic
P-1
Act+Chl+Ab+Ep+Cal+Ser±Preh±Py±Mt Ep±Ab, Act±Ab, Chl
P/Plag, (mafics)
Deep-Shal/I-V
Widespread at all structural levels
Propylitic(?)
P-2
Ab+Anal+Cal+Ser±Preh±Rut
P/Plag
H/Scap-Hbld
Int/I-V
Proximal; hydrothermal texture preserved/
replaces scapolitized rocks
Alteration
type
Scapolitic
(Sodic±calcic)
Albite-actinolite
(Sodic±calcic)
135
Albitic
(Sodic)
Carbonate-chlorite
Symbol
used in
figures
Cal, Anal
135
Alteration assemblages include widespread replacement and vein envelopes; alteration assemblages are divided by the respective mineralogy, mineral abundance, timing, and associated textures;
mineralization assemblages include both open-space filling and replacement
Abbreviations: (host rock): I = intrusive rock, V = volcanic sequence, B = Boyer Ranch Fm; (spatial): Int = intermediate levels, Shal = shallow; (occurrence): ms = massive, repl = replacement, bx
= breccia; (minerals): Ank = ferroan dolomite, All = allanite; (locality abbreviations in addition to abbreviations used in text): WHR=West Humboldt Range, SWR=Stillwater Range, BVH=Buena
Vista Hills, NBVH=northern BVH, WRC=White Rock Canyon
DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
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TABLE 1. Table of Hydrothermal Mineral Assemblages and Associated Vein Types
136
Mt
Ti Ap
Ti
Cpx
Hbld
Cpx
Mt Ap
Ti
Hbld
Mt
Ap
Ti
Hbld
Cal
fics
Hbld
ma
Mt
Ap
Ti
60
40
Ol
Ep-All
Cal
Plag
Plag
(remnant)
Ig: An45-55
rimmed by
Ab: An3-10
Plag
*
Cal
Scap
Ep
Ep
Chl
mafics
Ank
°
Plag
Plag
Chl
Hbld
Chl?
Anal-Cal
Hbld
Cpx
Cpx
Ser
Cal
Preh
Ep
An
al
Plag
Plag
Ab
Plag
Ab
Ab
Ab
Scap
Ab
weak
SP-1
SP-2
Ab
Ab
Ab
C-3
P-1
P-2
C-1 C-2
strong
SP-2a
SP-3
SP-4
SC-1
SC-2
S-1
S-2
C-1,2
Occurrence
Fresh
Time
Act
Chl
Ank
Scap
0
Space
Mt
Hm Rut
Act
l
Scap
Fracture/open
space
Replacement
Ep
Cal
Mt
Py
Ca
Ab
Scap
20
Cal-Hm
Brt
Qtz Mt
Hm
Hm
py Ru Hm
Py-C
Rut t
Py
-C Ac Chl
Qtz
py t
Chl
Chl
Ep-Ab
Chl±act±Mt
Cal-Preh
Cal
Ank-Brt
Ank-Qtz±Py±Cpy
Hm-Qtz
Ank-Chl-Ab
Ab-Cal-Hm
Ab-Ank-Chl±Py
Ab
Mt
Chl-Ab±Mt
Ab-Hm±Chl
Hm
Rut
Mt
Ti
Ti
Chl
Ap
Ru Cp Act
x
t
Hbld
Act
Cpx
Hbld-Act
Ep-Ab±Act±Py
Ab
Ap
Bt
Plag
Mt
Plag
Volume %
Opx
Cpx
Act-Ab±Mt
Mt-Ab
Mt
Ti
Ep
Cpx
Act-Ab±Mt±Ti±Ap
Ap±Act
Ab
Hbld-Ap-Ab
Hbld-Ap±Mt±Ti
Scap±Bt±Mt
Ap
Mt
Scap-Ti-Hbld-Ap±Mt
Scap±Hbld
Scap-Hbld-Ti-Ap±Mt
Mt-Ap±Hbld±Ti±Chl
Ap±Hbld±Mt
Ep-Mt
Mt
Hbld
ma
fic
s
Hbld
80
Mt-Scap-Ap-Ti
Scap-Hbld-Ti±Ap±Mt
Ap Ti
Cr Ilm Mt Bt Ap
Plag
Early
Mineralogy in order of decreasing abundance
Silicate-rich assemblages
dominately veins
Late
100
Silicate-rich assemblages
dominately replacement
Pervasive replacement, vein, and breccia mineral assemblages
JOHNSON AND BARTON
Late
Early
Distal
Proximal
= rare py
= rare cpy
First appearance of sulfide minerals in the BVHM area.
Time-space relationships.
= time-space relationships demonstrated by mapping and petrography
= variable relationships
= timing and spacial distributions not known
Fracture/Open space
Replacement
= breccia fill and open space fill
= replacement of breccia clast and host rock
= fracture fill
= replacement of host rock
FIG. 9. Typical modes, styles, and distribution of various alteration assemblages including iron-oxide mineralization. Ank
= ferroan dolomite. Modal mineralogy may be more variable than expressed here. Alteration assemblages are generally organized from early to late assemblages from left to right. Likewise, alteration intensity within each assemblage increases from
left to right. Local reversals occur in the timing of scapolitic and albitic assemblages. Volumes of secondary sodium-rich minerals, iron-oxides, and sulfides are highlighted. Volumes of albite and scapolite are highly variable with respect to volcanic or
intrusive rock protolith. Remnant igneous feldspar and mafic minerals are in italics. Textural information concerning replacement versus open space and degree of brecciation is summarized above each diagram. Remnant igneous minerals do
not occur in open-space filling. Generalized time-space distributions are indicated at the bottom of each diagram. Mineral
modes exclude overprinting assemblages. Fracture/open-space line is not indicative of time, but of modal mineralogy.
hornblende-titanite ± epidote ± magnetite (SC-1) assemblages sporadically overprint magnetite-rich scapolitic alteration in the Buena Vista Hills and eastern Stillwater Range
and are commonly associated with iron oxide mineralization
(Fig. 9). The second assemblage consists of albite/oligoclaseepidote-actinolite ± pyroxene (SC-2) assemblages and is similar to plagioclase-rich sodic-calcic assemblages described
elsewhere in the Great Basin (Carten, 1986; Dilles and Einuadi, 1992; Battles and Barton, 1995). These assemblages are
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exposed throughout deeper plutonic rocks, especially near
the margin of the complex and at shallower levels within hybabyssal intrusive rocks. These sodic-calcic assemblages
grade from pervasive replacements in deep portions of the
system along epidote-actinolite-albite veins to weakly developed assemblages along epidote-albite veins and lithologic
contacts in hybabyssal rocks.
Distal and/or later albite-chlorite (sodic) alteration types constitute the second most common hydrothermal assemblages
136
DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
(Fig. 8). Sodic assemblages are defined by the presence of
abundant albite and chlorite without actinolite (Table 1, Fig.
9). Feldspathic minerals are totally replaced by albite (An<10)
and mafic minerals are replaced by chlorite ± iron oxides.
Other common minerals include the replacement of titanite
by rutile and the introduction of ferroan dolomite giving altered rocks an orange color in outcrop. Strong albitic alteration was mapped where mineral assemblages replaced both
groundmass and phenocryst sites and the abundance of secondary hydrothermal minerals is greater than 70 vol percent.
Weak albitic assemblages are widespread, affecting nearly all
volcanic rocks. Weak assemblages were mapped where the
abundance of hydrothermal minerals was between 10 to 70
vol percent of the outcrop.
Sodic alteration in the mapped areas is divided into an albite-chlorite-magnetite-stable assemblage (S-1) and an albitechlorite-hematite ± ankeritic dolomite assemblage (S-2).
Albite-chlorite assemblages overprint all sodic-calcic assemblages. Sodic assemblages mostly occur at shallow to intermediate levels within the complex where they replace volcanic, volcaniclastic, and shallow level intrusive rocks. Both
pervasive and fracture-controlled types are common; the
character depends upon the rock type and structural level
(Fig. 6c). Where pervasive, igneous and earlier hydrothermal
textures are typically preserved, except near structures and
other highly permeable zones (Fig. 6d). At deeper levels,
sodic assemblages occur along the margins of the complex
and are dominantly structurally controlled, overprinting hightemperature assemblages.
Carbonate-chlorite alteration grades outward from albitic
assemblages and consists of ferroan dolomite-chlorite ± albite
± hematite ± rutile ± sulfides. It is locally divided into two assemblages based upon the volume of associated albite (C-1,
C-2 assemblages; Table 1, Fig. 9). Both varieties are distal to
and overprint albitic and sodic (-calcic) alteration in volcanic
rocks and gabbroic rocks. At these shallow levels carbonatechlorite alteration is widespread and pervasive, especially
near structures and along favorable lithologic contacts. At
deeper levels the assemblages are commonly fracture-controlled and occupy small volumes (see alteration map in field
guide for the Buena Vista mine area). In the most intensely
altered rocks, ferroan dolomite replaces plagioclase phenocrysts, groundmass, and matrix minerals; chlorite ± ferroan
dolomite ± hematite replace mafic minerals, and rutile replaces igneous and hydrothermal titanite (Table 1, Fig. 9).
Early ankerite-carbonate veins have albitic envelopes. Assemblages lacking albite commonly are associated with a variety
of carbonate ± quartz ± sulfide ± barite veins. These veins are
best developed at shallow levels within the volcanic pile
(White Rock Canyon area).
Propylitic alteration assemblages are common throughout
the complex at all stratigraphic levels, affecting both intrusive
and volcanic rocks. Propylitic alteration is divided into two
distinct assemblages: P-1 and P-2 (Table 1, Fig. 9). P-1 assemblages consist of minerals typical of propylitic alteration as defined by Meyer and Hemley (1967): plagioclase is variably replaced by epidote, carbonate, prehnite, chlorite ± sericite and
is commonly mantled by albitic plagioclase. Mafic minerals
are replaced by actinolite-chlorite ± pyrite. Magnetite is typically stable although other oxides may be replaced by pyrite.
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137
Veins of chlorite ± actinolite and epidote with albite ± actinolite ± pyrite selvages, and calcite-prehnite are common within
propylitized rocks. At shallow structural levels, greater than
75 percent of nonalbitically altered volcanic rocks are pervasively altered to these assemblages. At deeper levels, the
freshest rocks are also propylitized. There, propylitic P-1 assemblages are interpreted to grade into weak sodic ± calcic
assemblages in deeper settings based upon spatial relationships. The other assemblage, P-2, consists of calcite-analcite
± sericite ± chlorite assemblages replacing scapolitized rocks
and is included here with propylitic due to the probable lack
of chemical exchange. P-2 assemblages occur at intermediate
to deep levels of the complex replacing scapolitized gabbroic
and volcanic rocks. These assemblages occur as selvages to
analcite-calcite veins. In these rocks scapolite is replaced by
analcite, calcite, and sericite and hydrothermal amphibole is
commonly weakly replaced by chlorite ± actinolite (Fig. 9).
Propylitic assemblages represent formation of new hydrous
and carbonate minerals by volatile addition from higher temperature igneous and hydrothermal assemblages and results
in, at most, minor changes in the major elements and variable
changes in trace elements such as copper (Johnson, 2000).
Fe oxide (-Cu-Co) mineralization
Hydrothermal iron oxides and sulfides are widespread in
the complex and have systematic vertical and lateral distribution patterns (Fig. 8). They consist of four main types: (1)
massive to breccia-hosted bodies of magnetite-rich mineralization; (2) replacement and stratabound bodies of hematiterich mineralization; (3) minor but widespread copper sulfide
mineralization; and (4) rare nickel-cobalt sulfide and arsenide
mineralization.
At intermediate structural levels, iron oxide mineralization
consists of discordant massive to disseminated magnetite-apatite and various silicate minerals replacing scapolitized shallow level intrusions and volcanic rocks and as open-space filling in veins and breccia bodies (Fig. 6e, f). Magnetite-rich
mineralization follows the transition between scapolitic and
albitic alteration. Early magnetite mineralization commonly
consists of magnetite-scapolite-hornblende ± titantite ± apatite assemblages. These assemblages generally replace pervasively scapolitized rocks including scapolitized breccias
forming disseminated to massive bodies.
Early magnetite mineralization is commonly cut by magnetite-apatite ± hornblende-rich assemblages where scapolite
is volumetrically less significant (Fig. 9) forming massive replacement bodies (Fig. 6e) and open-space veins and breccias
(Fig. 6f). The bodies are composed of both fine-grained (<
0.1 cm) and coarse-grained (0.5–3 cm) magnetite and common dendritic, hopper-shaped apatite that displays crustiform
textures in both veins and replacement bodies. Multiple generations of these mineralized assemblages are demonstrated
by crosscutting mineralized and nonmineralized dikes and
the incorporation of early magnetite-rich mineralization into
later breccia bodies containing mineralized clasts cemented
by magnetite.
Late magnetite-rich assemblages consist of magnetitehornblende-chlorite ± apatite that are interpreted to be part
of the transitional (scapolite-albite) SP-4 assemblages and
later SC-1 assemblages. Early albitic alteration is magnetite
137
138
JOHNSON AND BARTON
stable and commonly redistributes magnetite locally, coarsening the grain size. It is not known if there is significant newly
added magnetite during these stages as these assemblages are
only identified overprinting magnetite-rich rocks. Later albite-chlorite assemblages largely overprints silicate gangue
and oxidizes magnetite to hematite. Magnetite-rich bodies
commonly lie up section from strong, magnetite poor,
scapolitic (SP-2, 2a) assemblages and down section from
strong albitic alteration (Johnson and Barton, 2000).
Up section from the magnetite-rich mineralization in the
Stillwater range are large (> 50 m × 100 m) hematite-rich replacement bodies in volcanic and volcaniclastic rocks and
small, laminated, stratabound bodies composed of hematite ±
fine-grained quartz (Fig. 6h). Hematite replacement bodies
are best developed in volcaniclastic rocks and along flow tops
within the mafic lavas. These assemblages are composed of
hematite ± chlorite ± albite. Magnetite is sometimes found as
euhedral grains within the volcaniclastic units but is largely
oxidized to hematite. These bodies are spatially associated
with albitic alteration, typically lying above pervasive alteration. Good cross cutting relationships have not been seen
within these bodies so that exact timing and relations to other
assemblages is not well established. Small, laminated, stratiform bodies of hematite crop out discontinuously for less than
150 m and are commonly less than 1 m thick, interbedded
with volcaniclastic rocks (Fig. 6h). These bodies are composed of fine-grained, finely laminated hematite intergrown
with variable amounts of quartz. Fine layering within these
bodies is conformable with layering seen in the clastic rocks
both above and below exposures and often shows draping
around clasts and soft sediment deformational features. Many
of these occurrences lack hematite mineralization in the surrounding rocks. It is our belief that some of these bodies may
be syngenetic in origin having formed on the paleosurface
from exiting hydrothermal fluids due to their stratigraphic location, unaltered nature of surrounding units, and the finely
laminated sedimentary textures preserved within the
hematite and quartz.
Sulfide mineralization is minor but relatively widespread
within albitic and carbonate-chlorite altered portions of the
volcanic sequence. Mineralization occurs as discontinuous
veinlets and cement within local breccia bodies and consists
of iron-copper sulfides, dominantly pyrite, chalcopyrite, and
bornite, along with hematite, ferroan dolomite, and rare
quartz and barite (Fig. 6g). Copper sulfide mineralization is
best developed at the Boyer Copper mine and the Bradshaw,
Anaconda, and unnamed prospects in the Stillwater Range
and at small prospects in the Copper Kettle area. In these
areas, chalcopyrite and bornite occur in abundant large veins,
in breccia cement, and as vein and vesicle filling in flow tops.
Grades and tonnages are not known, but copper mineralization at the Anaconda and Boyer Copper mines is exposed
over areas of 0.5 and 1 km2, respectively.
In addition to the dominantly copper-rich mineralization,
nickel-cobalt-copper sulfide and arsenide mineralization associated with high silver contents and reported uranium mineralization occurs in Cottonwood Canyon at the Lovelock and
Nickel mines (Ransome, 1909; Elevatorski, 1978). These occurrences are typical of five-element-suite deposits described
elsewhere (Ruzicka and Thorpe, 1996). The Ni-Co-Cu
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mineralization occurs as quartz-carbonate-sulfide-arsenide
veins in strongly albitized and carbonated quartz arenites of
the Boyer Ranch Formation and basaltic volcanic rocks
(Willden and Speed, 1974; Johnson, 2000).
Mass Transfer and Sources of Components
in the Hydrothermal System
Geologic, geochemical, and mass balance evidence require
external derivation of the Na- and Ca-bearing brines from the
surrounding evaporite-bearing wall rocks and/or coeval surface fluids. Hydrothermal circulation of these brines led to
large metasomatic changes, primarily gain of sodium with
some calcium, chlorine and water but with the loss of most
other constituents.
Nature of fluids and sources of components
The fluids involved in the hydrothermal system are moderately saline Na-Ca brines as inidcated by fluid inclusions,
many of which are halite-bearing, and by the stability relationships of marialitic scapolite and other minerals in the hydrothermal system (M.D. Barton, D.A. Johnson, J.D. Gleason, unpub. data). Three lines of evidence indicate much or
all of the water and dissolved salts came from external
sources: (1) mass balance—the amount of sodium and chlorine added during alteration exceeds by a factor of at least 10
the maximum amount that could plausibly have been available from the original basaltic magma; (2) hydrogen, oxygen,
and strontium isotope data point to large scale-exchange with
nonmagmatic sources; and (3) evaporites (presumably with
coeval surface or connate brines) constitute a significant part
of the Jurassic stratigraphy, complete with solution-collapse
features in sections near the intrusion (Speed and Clayton,
1974; Speed, 1975). Most salinities are compatible with a lowtemperature, halite-saturated source. It is easy to envision
how other more dilute and locally highly saline inclusions can
form by fluid flow paths that intersect the vapor-brine solvus
or, for dilute cases only, mix with local, relatively fresh, nearsurface waters.
Other constituents in the hydrothermal system, notably
first-row transition metals and the rare earth elements, are
mobilized in large quantities during alteration of the igneous
rocks and need not be sourced elsewhere by other mechanisms (see below). Neodymium isotopic data supports multistage remobilization and concentration of the rare earth elements from a primary igneous source (Gleason et al., 2000;
Johnson et al., 1995). Sulfide and sulfate sulfur is enriched in
34
S (commonly >8 ‰ for sulfides) and is consistent either
with reduction of an external sulfate or with the pronounced
fractionation of sulfur that could take place in a sulfur-limited
system such as this.
Mass transfer
Components are strongly redistributed in both shallow and
deep parts of the hydrothermal system (Figs. 10 and 11).
Gains and losses correlate with changes in mineralogy and
give insight into the chemical environment during hydrothermal alteration. The most obvious changes are the large additions of sodium to altered rocks at all levels of the complex resulting from the conversion of igneous feldspars to scapolite
or sodic plagioclase. Concurrent with the addition of sodium
138
139
DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
is the loss of iron from deep levels and the local redistribution
of iron at shallow levels. The loss of iron in deep levels results
from the replacement of mafic minerals by calcium- and magnesium-rich amphibole ± pyroxene, the loss of magnetite, and
the conversion of iron-titanium oxides into titanite and rutile.
Besides sodium and iron, other components such as potassium, magnesium, phosphorous, manganese, barium, and the
base metals are strongly removed during sodic-calcic alteration at deeper levels (Fig. 10).
Reconstructions based on the mapped distribution of alteration allow estimates of the total mass transfer in the system
(Johnson, 2000). Approximately 50 billion tonnes of sodium
was added to the complex during sodic ± calcic and sodic alteration. Approximately 20 billion tonnes of calcium and 15
billion tonnes of magnesium were mobilized from some of
the deep rocks but largely redistributed within the system at
local to complex-wide scales. Iron and phosphorous are
strongly depleted in deep and in many intermediate level
rocks.
Metals such as Cu, Zn, Pb, Co and Ni are removed during
sodic ± calcic alteration at deeper levels (Fig. 10; Johnson,
2000). The quantities of metals redistributed within or lost
from the hydrothermal system are comparable to the
amounts contained in world-class deposits (Fig. 12). Copper
and zinc removed during sodic-calcic alteration total approximately 20 to 40 and 30 to 45 million tonnes, respectively.
These changes have implications for the associated iron oxide,
iron-copper sulfide, and five-element-suite mineralization.
Rare earth and metal concentrations can be directly attributed to trapping of only a small fraction of the masses moved.
In a sulfur-poor environment only the siderophile elements
Sodic-calcic alteration
B
100
Ñ
H
J
H
F
B
B
H
F
H F Ñ
FF
B
B H
Ñ
B F
B
F
Ñ
B F
B
F
J B B
B
F
H
B
Ñ
J
J
Ñ
F
B
B
J
Ñ
Ñ
J
H
H H F Ñ Ñ
Ñ B
H F
H J
J J B
B
Ñ
Ñ
H
J
B
J
B J B
B
B
Ñ
H
B B
B
H
Ñ
J
B
Scapolitized porphyritc gabbro (9423a/9414o)
F
Scapolitized gabbro (9415c/9414o)
Scapolitized mafic dike (9419c/8611)
H
Scapolitized volcanic rock (9110j/949e)
Ñ
Scapolitized-weakly mineralized dike (9419c/949e)
J
Albitized mafic lava flow (941p/949e)
FIG. 10. Characteristic chemical gains and losses spider diagram for sodiccalcic alteration in gabbroic, volcanic, and dike rocks in the Humboldt mafic
complex. Samples are normalized to constant of Al2O3. Numbers in parenthesis are sample numbers of altered and fresh pairs (altered/fresh). See
Johnson (2000) for details on normalization and major element data.
Lithology
Areal extent
Mt
Oxide
Gains and Losses
Hm
Carbonate-chlorite-albite
Albitic alteration
(Ab-act-chl-mt-hm)
Scapolitic alteration
(Scap-hbld-spn-cal-mt-ap)
Oxide
Lithology
Mt-Hm
Sulfide
Py
Cp-Bn
Other (5 element &
Pb-Zn)
Hm-(Cp-Bn-Py)
Distal 5 element suite
Possible Pb-Zn-Ag
Mt-Hm-Ap-Act
Volcanic and
sedimentary rocks
Intrusive rocks
(gabbro & mafic dikes)
REE
Ba
Pb
Zn
Cu
Co
Mineralization
K2 O
Rare sulfide Common
Alteration
CaO
Mt-Hm-Ap-Act
Ore
Na2O
High (contacts, flow tops, structures)
Alteration Mineralization
Hm-Cu
Depth
Low (contacts and structures)
Permiability
Generalized
MnO
-100
Ñ Ñ
B
J
FeO*
-50
Ñ
B
J
H
F
J
FF
J
J
Ñ
B FÑ Ñ
F
H
F B B
B
P2O5
0
SiO2
50
Na2O
CaO
K2O
SiO2
TiO2
P2O5
FeO*
MnO
MgO
LOI
Co
Ni
Cu
Zn
Pb
Ba
Rb
Sr
La
Ce
Yb
% Change from Fresh Protolith
150
Gains and Losses
Leaching
Addition
Not trapped/lost
Variable
FIG. 11. Summary diagram for mass-transfer of various components within the Humboldt mafic complex with respect to
structural level and dominant alteration type. Components may be added to the system, redistributed within the system, or
lost to the system.
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139
140
JOHNSON AND BARTON
Time-Space Synthesis of Magmatic and
Hydrothermal Events
Volume rock (km3)
10
a)
0.1
1.0
10
100
Deposit Types
Candelaria
Olympic Dam
Porphyry copper
1
10,000
Fe-oxide
0.1
0
00
0,
00
1,
0.01
t
at
Cu
d
d 0t
te
ne 00
ra
ai 0,
nt
nt ,00
ce
co 100
on
C
Copper (%)
1,000
Humboldt 100
107
108
109
1010
?
Lost?
106
1,000
Copper (ppm)
0.01
1011
10
1013
1012
Mass rock (tonnes)
Volume rock (km3)
0.01
b)
0.1
1.0
10
100
1,000
Fe retained in
volcanics
Fe lost
Fe mobilized
nc
Co
100
ted
tra
at
?
00 t
d Fe
00,0
aine
Humboldt (HM) resource
Humboldt mafic complex
occurrences
Global occurrences
en
10
00,0
cont
10,0
Iron (%)
HM-calculated
resource
Humboldt
Fe resource Olympic Dam
estimated
1
05
106
107
108
109
1010
1011
1012
1013
Mass rock (tonnes)
FIG. 12. Grade-tonnage diagrams for metals trapped and masses mobilized during hydrothermal alteration in the Humboldt mafic complex compared to other types of deposit types. Ellipses illustrate the amounts of metals mobilized at deep structural levels. a. Copper grade-tonnage diagram
showing various global iron-oxide ± (Cu-Au) systems (large diamonds) and
copper porphyry deposits (grey field and small open diamonds). b. Iron
grade-tonnage diagram showing various global iron-oxide systems compared
to various deposits in the Humboldt mafic complex and the masses of materials mobilized. Masses of metals removed from deep levels are comparable
to some of the world’s largest ore deposits. Metals trapped and associated
grades are dependant upon system efficiency, trapping mechanisms, and the
ability to focus hydrothermal fluids. Given favorable conditions some metals
such as copper appear to be partially trapped within the hydrothermal system. Modified from Johnson (2000).
and the most chalcophile elements are precipitated. Considerable iron and phosphorous are reprecipitated at intermediate to shallow levels in upflow zones as focused and dispersed
magnetite/hematite-apatite-rich mineralization. Copper with
minor cobalt and nickel are present mainly in sulfide-bearing
mineralization in shallow, distal settings. Less chalcophile elements such as zinc and lead appear to be lost from the hydrothermal system (Fig. 11).
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Figure 13 shows a geologically consistent interpretation of
the development of the igneous and hydrothermal system
near one of the dike swarms as a function of time and depth.
The number of intrusive and crosscutting hydrothermal
events is largely schematic, although all of the relationships
and events shown can be demonstrated in a number of places
throughout the complex and actually probably represent a
minimum number of events for the complex. Temporal relationships between the magmatic and hydrothermal system
are largely evidenced by the numerous diking events which
can be used as time lines to document timing of various assemblages, especially where mid-Tertiary tilting reveals contiguous sections demonstrating a large paleovertical component (see the field Guide, Buena Vista mine area). Separate
intrusive bodies show similar overall relations, but are generally limited in the number of magmatic pulses seen. Timespace relationships on the margin of the complex and, perhaps, on the margin of individual intrusive centers would be
different (cf. diagrams for Yerington in Dilles et al., 2000). Although generalized, this figure highlights the demonstrable
intimate link between magma emplacement, formation of voluminous hydrothermal alteration, and the more restricted
development of iron oxide and sulfide mineralization.
The mid-Jurassic record begins with the accumulation of
evaporitic sedimentary rocks, likely in an extending, restricted
basin. These sediments compose the reworked eolian sandstones of the Boyer Ranch Formation to the east, and the carbonate-rich Lovelock and Muttlebury Formations to the
west. These features are part of the broader Jurassic evaporite-eolian sand package that was a response to monsoon-driven aridity on the western margins of the Americas following
the breakup of Pangea (Parrish, 1993).
Mafic magmas intruded and were erupted on these sediments, probably from a number of distinct centers. These
centers are best evidenced by the presence of well-exposed,
spatially distinct basaltic dike swarms. The number and size
of these centers are not well established, but the best documented one is in the southern Buena Vista Hills, and
stretches at least 8 km east-west (Johnson and Barton, 2000).
This length is similar to volcanically active segments in modern back-arc basins. Sedimentation continued with volcanism
but became dominated by volcaniclastic rocks and rare syngenetic hematite-rich chemical sediments.
A volcanic pile, ultimately about 2 km thick, was constructed by repeated eruption, surface reworking and, in
deeper sections, intrusion by basaltic magmas. Thus, the top
surface in Figure 13 moves upward with time. This is important to help understand the complex hydrothermal overprinting that obscures the earliest alteration record. Rare clasts of
intensely scapolitized rocks in the oldest recognizable intrusions indicate that brines circulated soon after initiation of
magmatism, yet initial hydrothermal alteration formed from a
heating brine would have likely have contained sodic albite
formed as the fluids and host rocks heated. If originally present, such assemblages were destroyed by higher temperature scapolitic alteration as the oldest igneous rocks were
buried and progressively heated.
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DEVELOPMENT OF A HYDROTHERMAL SYSTEM: HUMBOLDT MAFIC COMPLEX, W. NV
Alteration
Fe-oxide Mineralization
Carbonate-chlorite
Hm replacement and possible
syngenetic mineralization
Massive and disseminated
Mt±Ap±silicate gangue
Mt-Hm stability,
Hm above
Fe carb-Chl-Hm-Rut±Ab±Py±Cpy
C1
S2
?
?
ost
bro/h
S1 SC1
S2
Gab
?
sed. & vol. rocksl
intrusive rocks
2a
Seds & debris flows
SC2
SC2
S1
ct
conta
Seds & debris flows
C1
S1
SP1
rock
Seds & debris flows
S1
SP1
Mid-J clastic and evaporite-carbonate rocks
overlain by Tr-J pelitic sequence.
C2
C1
S2
Paleosurface
Intrusive rocks
C2
Scapolitic
Scap-Hbld-Ti±Ep±Mt±Ap±Cc
SC1
(km)
Ab-Act-Ep-Ti±Chl±Ap±Mt±Py
2a
SP
Depth
Albite-actinolite
Magma
1
SP
4
Ab-Chl-Rut±Mt/Hm±Fe carb±Py±Cpy
1
SP
2
Volcanic and volcaniclastic rocks
Sodic
0
141
SP
SP2
SP2
SP2
SP2
Magma
Mt/Hm
line
Decreasing T or mixing
Magma
Time
Permeability
High
Jurassic sediments and volcanic rocks
Low
High
Intrusive rocks
Low
High
Structural preparation
Low
High
Topography
Low
Moderate
Extension
Low
FIG. 13. Time-space diagram showing the evolution of the Humboldt mafic complex and summarizing magmatic and hydrothermal events.
Regardless of the precise time of its initiation, hydrothermal activity continued steadily throughout recognizable magmatic history, waxing and waning with individual intrusive
pulses (Fig. 13). Even the youngest dikes have, at minimum,
a propylitic overprint. Many examples of mutually crosscutting relations between dikes and sodic-calcic alteration
demonstrate this at deeper levels (Buena Vista mine area;
Johnson and Barton, 2000). At shallower levels in the complex, albite-bearing assemblages formed contemporaneously
with the deep scapolite as demonstrated by the mafic dikes
that are scapolitized at depth and albitized at shallower levels
(for example in White Rock Canyon, eastern Stillwater
Range). Intermittently (between intrusive events) retrograde
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albitization (lower-temperature assemblages) collapse to
deeper levels and overprints scapolitic alteration only to be
cut in turn by younger scapolitic veins and associated dikes.
In the central portions of the complex, only in the waning
stages of the system (the last few percent of intrusive rocks)
are dikes altered incompletely to albitic or chloritic assemblages (Fig. 13).
Fluid flow lines can not be fully illustrated on Figure 13
given the evidence for external derivation of the Na- and Cabearing brines. External fluids must flow in from outside the
diagram. During inflow, fluids would heat and create some
of the distal and shallow sodic plagioclase-stable alteration
assemblages.
141
142
JOHNSON AND BARTON
Magnetite (-hematite-sulfide) mineralization formed at the
same time as the sodic alteration. It too shows repetitive formation associated with diking and brecciation events. Magnetite-rich veins, replacements, and breccias are concentrated in and over the major dike complexes, where evidence
for repeated brecciation, remineralization and intrusion is
common (e.g., Buena Vista mine area). Shallower types—
syngenetic hematite-silica, bedding-parallel replacement
hematite-magnetite, and Cu-Fe sulfide-hematite veins and
replacements—are less clearly associated with dike centers,
but likely formed concurrently with the magnetite-rich types
through much of the history. This is demonstrated by the location of syngenetic bodies within the volcanic pile, the association of some hematite-sulfide mineralization with albitedominated alteration in veins and breccia bodies, and the
general overlap with albite and chlorite-carbonate styles of alteration. In addition to crosscutting relationships demonstrated by the dikes, high-temperature magnetite and lowertemperature, syngenetic hematite-silica mineralization occur
as clasts in debris flows. These flows are in turn covered by
later flows and altered to albitic assemblages. This demonstrates an active tectonic environment where higher temperature parts of the systems become exposed at the surface and
are subsequently buried and are overprinted by later alteration assemblages. (Johnson, 2000).
Concluding Remarks
The Jurassic Humboldt mafic complex represents an endmember in the spectrum of intrusion-related hydrothermal
systems. It is part of a broader pattern in the western United
States that includes multiple styles of iron oxide-rich hydrothermal systems, including districts such as Yerington,
Nevada, Eagle Mountains, California, and the Cortez Mountains, Nevada (Fig.1). Excellent exposures in tilted fault
blocks of the otherwise undeformed and unmetamorphosed
complex allow study of deep and shallow, proximal and distal
parts of the Humboldt magmatic and hydrothermal systems,
and interpretation of its temporal and spatial development.
The Humboldt system reflects protracted fluid convection
driven by heat of repeated basaltic intrusions. Externally derived brines circulated to depths of at least 4 km and achieved
temperatures of at least 500°C. Accompanying sodic-calcic
hydrothermal alteration and iron oxide-dominated, locally
Cu-bearing mineralization resulted from the interaction between these brines and the hot mafic rocks. Profound leaching of many constituents from large volumes of this complex
is only partly reflected in the metals and other constituents
that precipitated in identified upwelling zones or at the paleosurface. Some elements, such as zinc, were likely lost due to
the lack of suitable traps. Others, like iron, may mass balance
over larger volumes.
The Humboldt complex shares characteristics with many
Fe oxide (-Cu-Au) systems. These features include voluminous sodic (-calcic) alteration, iron oxide-rich sulfide-poor
mineralization and copper as the principal base metal. These
systems may have multiple origins; they certainly have many
differences. In this particular example, as with some other
Phanerozoic examples (Salton Sea, California; Danakil Depression, Ethiopia; Siberian traps; Newark Supergroup,
Pennsylvania), basaltic magmatism in evaporitic basins results
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in a common and geochemically sensible type and style of
mineralization (Barton and Johnson, 1996). The links to the
intermediate and felsic variations on this theme deserve further exploration.
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