SOCIETY OF ECONOMIC GEOLOGISTS
INTERNATIONAL EXCHANGE LECTURE - JUNE
6-km Vertical Cross Section Through
Porphyry Copper Deposits,
Yerington District, Nevada:
Multiple Intrusions, Fluids, and Metal Sources
Marco T. Einaudi, Stanford University
Stanford, California, U.S.A.
Gaps in understanding of individual
porphyry copper deposits result from limited exposures of
the tops, sides, and bottoms of ore zones. In order to build
models of porphyry systems, investigators have relied on
observations from numerous localities that are thought to
represent different paleolevels. This approach does not
account for variability between deposits and for important
features located several kilometers below and to the side of
ore zones. Missing are direct observation and analysis of:
potential source regions for fluids and their dissolved
components; deep pathways for inward-convecting and
thermally prograding non-magmatic fluids whose presence
is predicted by numerical models of mass and heat
transport; the effects of prograding fluids on cycling of
metals and other components; and time-space relations of
intrusions and hydrothermal evolution on a system-scale.
These and other questions can be addressed in the
Yerington mining district, Nevada, where post-ore tilting
during extreme custal extension (150%) in Late Tertiary
time rotated the Jurassic porphyry copper deposits 90_ on
their sides (Proffett, 1977). Present exposures are Jurassic-
age cross-sections, with up to the west. The geological
data base for the district was acquired through some 40
man-years of effort, initially (1967-1975) by geologists of
the Anaconda Company, and later (1975-1990) at Stanford
University. Studies have been published on district
geology and structure (Proffett and Proffett, 1976; Proffett,
1977; Proffett and Dilles, 1984), district copper skarns
(Einaudi, 1977, 1982; Harris and Einaudi, 1982), geology
and alteration-mineralization of the porphyry copper
deposit at the Yerington mine (Carten, 1986), petrology
and U-Pb ages of Jurassic plutonic and cogenetic volcanic
rocks (Dilles, 1987; Dilles and Wright, 1988; Proffett and
Dilles, 1991), and geology, alteration, fluid inclusions, and
stable isotope characteristics of the porphyry copper
deposit at Ann Mason (Dilles et al., 1992; Dilles and
Einaudi, 1992). The focus of this contribution is on the
time-space evolution of hydrothermal fluids of diverse
origin that generated the porphyry copper deposits in the
district. The presentation is a composite from detailed
studies at the Yerington mine and Ann Mason prospect and
reconnaissance study of the paleosurface in the Buckskin
Range west of Yerington. The composite view extends
from the paleosurface (2.5 km above the top of copper ore,
where copper ore is defined as rock containing > 0.3 %
Cu) downward to depths of 6 km (2 km below the bottom
or copper ore), and laterally 2.5 km from the edge of ore.
The Yerington district is located in the western edge
of the present-day Basin and Range Province in the former
site of a subduction-related magmatic arc that developed
along the western margin of North America in Jurassic
time (Dilles and Wright, 1988). Porphyry copper deposits
in the district, exposed in the Singatse Range, are
associated with granite prophyry dike swarms emanating
from a deep granite cupola emplaced into an earlier quartz
monzodiorite batholith. Based on U-Pb dates, the batholith
and its related granite dike swarms and hydrothermal
systems had a total life span of less than 1 m.y. at 169-168
Ma (Dilles, 1987; Dilles and Wright, 1988). The batholith
is overlain by a 2 to 3 km-thick cogenetic andesite/quartz-
latite flow-dome complex with interbedded volcaniclastics
(Proffett and Dilles, 1991); there is no evidence to suggest
the presence of stratavolcanoes. The batholith intruded a 3
km thick sequence of Middle to Upper Triassic volcanic
arc andesite-rhyolite and Upper Triassic to Middle Jurassic
shelf sequence of volcaniclastic, argillaceous, and
carbonate sedimentary rocks, including evaporites and
eolian sandstone at the top (Einaudi, 1977). Carbonate
units of Late Triassic age contain small copper skarns at
distances of 3 to 5 km from porphyry ore (Einaudi, 1977;
Harris and Einaudi, 1982). Basin and Range extension on
normal faults dipping east has rotated the district 90_ to the
west (Proffett, 1977).
The description of attitudes, directions, and the
interpretation presented below is given in the context of
Jurassic time, rather than present day.
The batholith, genetically linked to all
porphyry deposits in the district, is a composite calc-
alkaline pluton, 15 X 15 km in plan view, with a floor at
~7-8 km depth and a roof at ~ 1-2 km depth where it
intrudes its cogenetic volcanics. The petrologic summary
that follows largely is taken from Dilles (1997). The
batholith ranges in composition from early hornblende
quartz monzodiorite (60 % SiO2), through hornblende-
biotite quartz monzonite (66 % SiO2), to late hornblende-
biotite granite (68 % SiO2). Sr isotope (87Sr/86Srinitial =
0.7040) and whole-rock O isotope (*18O = 6.8) values and
major and trace element compositions indicate parent
magmas were high-K andesites, probably derived by
fractionation of basalt with little crustal contamination.
Similar data also are consistent with granite being derived
from quartz monzodiorite by crystal fractionation. All
intrusive phases have an essential mineralogy of
plagioclase, microperthitic K-feldspar, quartz, hornblende,
biotite, magnetite, sphene, apatite, ilmenite, and zircon. In
contrast with the coarse- to medium-grained equigranular
textures and equidimensional shapes of the earlier
intrusions, the youngest intrusions, closely associated in
space and time with porphyry copper mineralization, are
strongly porphyritic granite dikes with 50% aplitic (0.02-
0.05 mm) groundmass of mostly quartz and alkali feldspar.
The porphyry dikes occur in three separate swarms within
the batholith, each localized over individual cupolas on a
deeper granite pluton whose top is at 5-6 km paleodepths.
The dike swarms strike NW and have steep dips, controlled
by a regional fracture pattern.
Porphyry copper deposits - Grades,
Six million tons of copper are contained in
three porphyry copper deposits: 162 million tons (mt) of
0.6 % Cu in the Yerington mine; 495 mt of 0.4 wt % Cu
and ~0.01 wt % Mo in the Ann Mason deposit; and >
500 mt of 0.4 wt % Cu in the Bear-Lagomarsino deposit.
Gold and silver assays of drill core from the copper ore
zones are typically < 0.03 and 0.17 ppm, respectively.
Lead and zinc are both < 50 ppm. Sulfide mineral
assemblages and percentages vary as a function of
veinlet/vein types and wall-rock alteration. Below, styles
of alteration-mineralization are identified by the alteration
Early potassic (phlogopite-Kspar)
alteration at 2-4 km depth.
Most of the
copper ore was deposited during potassic alteration,
consisting of phlogopite-(magnetite-rutile) after hornblende
and biotite, Kspar after plagioclase, and very abundant
quartz-sulfide veinlets. Sulfide assemblages are bornite-
magnetite-(digenite) (at Yerington mine only, where it is
earliest and central with hypogene grades of 2-3 wt % Cu),
bornite-chalcopyrite-magnetite, chalcopyrite, and fringing
chalcopyrite > pyrite. These low-sulfur sulfide
assemblages are disseminated and in quartz veinlets and
there is a direct correlation between volume percent quartz
veinlets and grade of copper: highest grades of copper at
the Yerington mine occur in zones with 10-25 vol% quartz
veinlets. The majority of veinlets occur as semi-sheeted
sets parallel to the regional fracture pattern, rather than as
stockworks linked to localized hydrofracting. Shreddy
phlogopite after hornblende characterizes a fringing
chalcopyrite > pyrite zone, where quartz veinlets are
sparse, hydrothermal Kspar is absent, rock magnetite
reappears, and grades drop below 0.2% Cu. At the
Yerington mine, three successive, partly superimposed,
potassic ore-forming events were associated with each of
three early dike emplacement events. Based on direct
observation, at least one of these early dikes did not vent to
the surface, but terminated in an upward-flaring, igneous-
matrix breccia that fingered and died-out upwards.
Early sodic-calcic (actinolite-oligoclase)
alteration at 4-6 km depth.
alteration occurred at depth and laterally along the margins
of granite cupolas, but overlapped upward and centrally
with potassic alteration. Sodic calcic assemblages are
characterized by actinolite after hornblende and biotite,
oligoclase after Kspar, abundant sphene, local epidote,
local tourmaline, and absence of biotite, magnetite, and
sulfides. Veinlets contain oligoclase-quartz-(actinolite)
locally with patchy chalcopyrite-pyrite where
superimposed on potassic ore. The most intense sodic-
calcic alteration yielded an oligoclase-quartz-sphene rock
with halos of oligoclase-quartz-sphene-actinolite-(epidote).
Weak sodic-calcic assemblages are transitional to propylitic
assemblages. Detailed mapping in the Yerington mine
indicates that sodic-calcic alteration at depth developed
contemporaneously with potassic alteration higher in the
system during each of several, separate intrusive events.
Superposition of sodic-calcic onto potassic resulted in
strong depletion of copper. Some late dikes post-date the
termination of potassic and sodic-calcic alteration; these
late dikes pre-date the late stages of hydrothermal alteration
Late sodic (chlorite-albite) alteration at 2-
4 km depth.
Sodic alteration was
superimposed on all porphyry dikes and on potassic and
propylitic zones. At deeper levels, it consists of albite after
feldspars, chlorite-(sericite) after ferromagnesian minerals,
rutile, sphene, and sparse pyrite > chalcopyrite. Sodic
assemblages occur along fractures and as halos on sparse
quartz-albite veinlets and are flanked by a zone of
chloritized biotite. At higher structural levels (near 2 km
depth), sodic assembalges consists of albite-sericite-
(chlorite) accompanied by tourmaline, 1-2 % pyrite, minor
rutile, and rare quartz-pyrite-(tourmaline) veinlets. The
sericite:chlorite ratio increases upward in a probable
transition to sericite-quartz-pyrite assemblages (see
Very late sericitic alteration and
tourmaline breccias at 0-2 km depth.
Youngest sericitic alteration, consisting of sericite-quartz
with 2 to 10 vol % pyrite, formed as well-defined halos on
through-going pyritic fractures and pebble breccias with
steep Jurassic dips. At the Yerington mine, all sericitc
alteration postdated all porphyry intrusions within the 2.5
km of vertical exposure available. The volume of sericitic
alteration expands upward from 1-2% of the rock in
potassic ore zones at 2-3 km depths to ~50% of exposures
at 1 km paleodepth. Near the paleosurface in the Buckskin
Range, sericite-quartz-pyrite related to the three underlying
porphyry copper centers coalesces into a regional zone of
pervasive sericite. Veins are filled with pyrite and lesser
chalcopyrite, hematite, quartz, and sericite. The
pyrite:chalcopyrite ratios in veins are dependent on local
background sulfide assemblage and content. In potassic
ore zones with original bornite-magnetite at the Yerington
mine, sericitic veins contain chalcopyrite-(magnetite) or
chalcopyrite>pyrite and constitute ore. In contrast, in
previously low-grade upper portions of the pattern, sericitic
veins typically have pyrite-chalcopyrite ratios that exceed
50:1. Pyrite veins are bordered by sericite-pyrite selvages
with minor quartz and rutile; some veins display an
outermost chloritic (intermediate argillic) halo. In most
cases, intermediate argillic alteration is not present as an
outermost selvage on sericictic veins. At higher structural
levels, innermost silica-pyrite halos are present bordered by
outer sericite-pyrite halos.
Tourmaline breccias root at about 2.5 km
paleodepth and extend upward at least 2 km. They are up
to several meters wide, and contain matrix-supported clasts
averaging 1 to 5 cm in largest dimension in a matrix of
quartz, tourmaline, 1 % pyrite, and trace rutile. Clasts are
angular to subrounded and consist of sericitic and sodic
assemblages similar to near-by wall rocks, indicating lack
of large-scale upward transport (e.g., these are not pebble-
dikes). Tourmaline breccias cut and/or re-open pyrite-
sericite veins, identifying them as the youngest event.
Advanced argillic alteration at the
breccias, tuff, and sediment, comprising a section up to 1.5
km thick in the Buskskin Range, are the extrusive
equivalents of quartz monzodiorite. These are cut by flow-
banded porphyry dikes that may correlate with late granite
porphyry dikes of Singatse Range exposures. Sericitic
alteration is pervasive in the upper part of the section in
many areas of the Buckskin. Surface outcrops are intensely
leached by weathering, but limonite mapping indicates that
original sulfide content increased upward, averaging 4 %
py:cp > 50:1. Fine-grained specular hematite, likely
hydrothermal, is abundant locally. Within zones of
pervasive sericitic alteration are stratiform silica ledges, up
to 75 m thick and with strike lengths up to 400 m, that
replaced mostly tuff or sedimentary units. These ledges are
composed largely of fine-grained quartz; textures and
character of limonite suggests they contained abundant
fine-grained pyrite. Advanced argillic minerals, including
alunite and pyrophyllite, occur within the silica ledges and
on their borders. Also present are andalusite, diaspore,
corundum, and zunyite. The mineral assemblages and
paragenetic sequence among these minerals has not been
worked out, and the origin of alunite (supergene, hotspring,
or magma-hydrothermal) is unknown.
Character and sources of hydrothermal
yield whole-rock *18O values of 6.7 to 7.0 and biotite *D
values of -85 to -88, typical magmatic values, which are
calculated to be in equilibrium with 700C water with *18O
values of 7.4 and *D values of -65. Igneous quartz in the
early equigranular phases of the batholith contains liquid-
rich L-V fluid inclusions with halite, sylvite and hematite
daughters, interpreted to have been trapped during
crystallization of quartz monzodiorite and quartz
monzonite. These fluids did not generate ore.
Fluids associated with potassic alteration.
Primary L-V fluid inclusions (+daughters)
in quartz, without coexisting V-rich fluid inclusions,
homogenize by halite dissolution at 150 to 550 C and yield
calculated salinities of 32 to 62 wt % NaCl equiv. Whole-
rock and/or Kspar have *18O = 6.5 to 8.4 and biotite has *
D = -68 to -69. Values from deeper samples are calculated
to be in equilibrium with magmatic water at 500 to 600 C.
These data, in conjunction with the geologic and petrologic
data presented above, are consistent with copper deposition
and exchange of K for Na from upward flowing and
cooling fluids released episodically during dike
Fluids associated with sodic-calcic
Application of phase equilibria
suggests temperatures of 360 to 480 C for oligoclase-
actinolite assemblages, and peristerite solvus relations
suggest temperatures greater than 375 to 400 C. Primary
L-V fluid inclusions (+daughters) in quartz and epidote
homogenize by halite dissolution at 200 to 375 C and yield
salinities of 31 to 41 wt % NaCl equiv. Whole-rock and/or
oligoclase have *18O = 5.7 to 8.4 and actinolite has *D = -
68 to -69. Therefore, sodic-calcic fluids were dominantly
nonmagmatic (*18O = < 3-6 and *D = -50 at ~ 400C)
and likely originated as isotopically evolved, 18O-enriched
formation waters, which circulated convectively into the
deep root zone of the granite stocks. These fluids,
exchanging Na for K on heating, were capable of leaching
and transporting precisely those components that were
added to the ore zone (e.g., K, Fe, Cu, S, etc.). High
salinities of these fluids may have been achieved by their
interaction with evaporite, shale, and carbonate in the
Triassic-Jurassic sedimentary wall rocks.
Fluids associated with sodic and sericitic
Whole-rock and/or albite have
*18O = 6.7 to 9.6 and chlorite has *D = -79 to -85.
Primary fluid inclusions have not been identified in vein
minerals associated with sodic alteration. The youngest,
sericitic, alteration displays a distinct enrichment in 18O
and D relative to other alteration types: whole-rock and/or
quartz have *18O = 9.8 to 10.4 and sericite has *D = -61.
Possibly primary L-V fluid inclusions in quartz
homogenize to liquid at 100 to 250 C and yield salinities of
2 to 13 (mostly 2 to 5) wt % NaCl equiv. Pressure
corrections result in a mode for late-stage fluids of 220 ±
20 C. Tourmaline breccias contain the only vapor-rich
inclusions noted in hydrothermal minerals at Yerington;
their potential coexistence with L-rich fluid inclusions
present in the samples, which would suggest boiling, is
dicounted because hydrostatic pressure estimates are
considerably less than the geologic depth estimate.
Sericitic alteration was caused at high water:rock ratios by
fluids with calculated (225 C) *18O = 0 and *D = -20 to -
55, whose origin most likely was 18O-shifted (heavy)
coastal meteoric water or seawater.
At least three
hydrothermal fluids of different origin were involved in
generating the alteration-mineralization patterns at
Yerington. Earliest stages, cyclically repeated with each of
several porphyry emplacement events, involved high-
temperature flow and interaction of two fluids: (1) a
cooling (thermally retrograding), saline, hydrothermal fluid
of magmatic origin linked to potassic alteration and metal
deposition, and (2) a heating (thermally prograding), saline,
hydrothermal fluid of nonmagmatic origin linked to sodic-
calcic alteration and metal leaching. These early, multiple
patterns of sodic-calcic and potasssic alteration were
broadly overprinted in their upper portions by (3) lower
tempertaure, dilute, and convecting, nonmagmatic
hydrothermal fluids of surface water origin that caused
sodic and sericitic alteration.
Application to further understanding
There are many implications of the Yerington study
that require further work and application to other districts.
Among the more important are the sources of metals in
ores, recognition of prograde fluid paths, and sources of
Sources of metals and other components.
In spite of the overwhelming evidence
presented in numerous studies over the past decades for
direct magmatic sources of many of the components in
porphyry copper ore zones, the evidence from Yerington
indicates that potential contributions from external sources
should continue to be sought. For example, the point of
transition from heating to cooling of nonmagmatic fluids is
the point of transition from sodic-calcic (metal leaching) to
potassic (metal deposition). Comparison of multiple
analyses of fresh and altered rocks at Yerington indicates
that up to 50 ppm Cu was leached during sodic-calcic
alteration. This leaching could be the source of up to 30%
of the copper deposited in the Ann Mason ore zone.
Ultimately, though, this copper had its source in the magma
of the Yerington batholith.
Prograde fluid paths.
spite of the fact that most porphyry copper deposits are
associated with multiple intrusions and therefore are likely
to undergoe cyclical heating and cooling, most models of
porphyry copper deposits deal only with unidirectional
cooling. The prograde cycle rarely has been recognized, in
part because it is most likely to be found at depths below
ore zones. At El Salvador, the andalusite-corundum
assemblages on the margin of the late porphyry intrusion
have been interpreted as the result of prograding solutions
(Gustafson and Hunt, 1975; Hemley et al., 1980).
Prograding fluids are capable of destroying ore zones and
likely contribute to the variability of metal-distribution
patterns observed in porphyry deposits.
Sources of hydrothermal fluids.
It has been generally accepted that
hydrothermal fluids in porphyry copper deposits have two
sources: magmatic and meteoric. The Yerington study and
recent studies of Bingham and Tintic, Utah (Bowman et al.,
1987; Norman et al., 1991) indicate that this two-fluid view
Carten, R.B. (1986) Sodium-calcium metasomatism:
chemical, temporal, and spatial relationships at the
Yerington, Nevada, porphyry copper deposit: Econ.
Geol., v. 81, p. 1495-1519.
Dilles, J.H. (1987) The petrology of the Yerington
batholith, Nevada: Evidence for the evolution of
porphyry copper ore fluids: Econ. Geol., v. 82, p.
Dilles, J.H., and Wright, J.E. (1988) The chronology of
early Mesozoic arc magmatism in the Yerington
district, Nevada, and its regional implications: Geol.
Soc. America Bull., v. 100, p. 644-652.
Dilles, J.H., and Einaudi, M.T. (1992) Wall-rock
alteration and hydrothermal flow paths about the
Ann-Mason porphyry copper deposit, Nevada--A 6-
km vertical reconstruction: Econ. Geol., v. 87, p.
Dilles, J.H., Solomon, G.C., Taylor, H.P., Jr., and
Einaudi, M.T. (1992) Oxygen and hydrogen
isotopes characteristics of hydrothermal alteration at
the Ann-Mason porphyry copper deposit,
Yerington, Nevada: Econ. Geol., v. 87, p. 44-63.
Einaudi, M.T. (1977) Petrogenesis of the copper-
bearing skarn at the Mason Valley mine, Yerington
district, Nevada: Econ. Geol., v. 72, p. 769-795.
Einaudi, M.T. (1982) Description of skarns associated
with porphyry copper plutons, southwestern North
America, in Titley, S.R., ed., Advances in the
geology of porphyry copper deposits, southwestern
North America: Tucson, Univ. Arizona Press, p.
Harris, N.B., and Einaudi, M.T. (1982) Skarn deposits
in the Yerington district, Nevada: metasomatic
skarn evolution near Ludwig: Econ. Geol., v. 70, p.
Proffett, J.M. (1977) Cenozoic geology of the
Yerington district, Nevada, and implications for the
nature and origin of basin and range faulting: Geol.
Soc. America Bull., v. 88, p. 247-266.
Proffett, J.M., and Dilles, J.H. (1984) Geologic map of
the Yerington district, Nevada: Nevada Bur. Mines
Geology, Map 77.
Proffett, J.M., and Dilles, J.H. (1991) Middle Jurassic
volcanic rocks of the Artesia Lake and Fulstone Spring
sequences, Buckskin Range: Geol. Soc. Nevada, Field trip
16 guidebook compendium, v. 2, p. 1031-1036.
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