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55

Introduction

THE YERINGTON DISTRICT, Nevada, contains porphyry Cu(Mo),Cu skarn, Fe oxide, and Cu sulfide ores in metasedimentary rocks, and shallow batholith-hosted Cu-Au-Fe oxide lodes. Inaggregate, the geologic resource and production of the dis-trict includes 6 Mt Cu in sulfide ores and >100 Mt of Fe inoxide ores (Dilles and Proffett, 1995). All these deposits aredirectly associated with the Jurassic Yerington batholith, which serves as either host rock or as source for heat and ma-terials that produced contact metasomatism in the aureole tothe batholith. Hydrothermal alteration has affected morethan 100 km3 of rock and was produced by a similar volumeof hydrothermal fluids. Jurassic rocks are superbly exposed incross section from the subvolcanic environment to ca. 7 kmpaleodepth, due to Cenozoic normal faulting and westward

tilting (Proffett, 1977), so that the three-dimensional geome-try of a large upper-crustal paleohydrothermal system can bedescribed in detail.

The purpose of this paper is to summarize the current stateof knowledge and viable theories for the sources of these hy-drothermal fluids, their flow paths through the Yeringtonbatholith and its wall rocks, and the evolution of the fluids in

both space and time. These features are summarized by a se-ries of maps and cross sections illustrating the temporal evo-lution and three-dimensional geometry of a batholith-relatedhydrothermal system. This is necessarily a brief progress re-port drawn from 35 years of geologic research sponsored by the Anaconda Company, research universities, and the Na-tional Science Foundation.

The focus below is on two significantly different sources of fluids that have both different point sources and differenttime-temperature-position flow paths. Magmatic fluids areclosely related to the youngest and central intrusion of the

Overview of the Yerington Porphyry Copper District:Magmatic to Nonmagmatic Sources of Hydrothermal Fluids,

Their Flow Paths, Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores

JOHN H. DILLES,†

Department of Geosciences, Wilkinson Hall 104, Oregon State University, Corvallis, Oregon 97331

MARCO T. EINAUDI,

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305

JOHN PROFFETT,

Geological Consultant, P.O. Box 772066, Eagle River, Alaska 99577

AND MARK D. BARTON

Department of Geosciences, University of Arizona, Tucson, Arizona 85721

Abstract

The Yerington district, Nevada, hosts at least four porphyry copper deposits and several small Fe oxide-cop-per-gold lodes within a middle Jurassic batholith and its volcanic cover. The contact aureole of the batholithcontains early garnet-pyroxene hornfels and endoskarn, later copper-bearing andradite skarn deposits, and lat-est-stage large Fe oxide-copper-gold replacement deposits. The Jurassic host rocks have been faulted and tilted60° to 90° W by Cenozoic normal faulting (Proffett, 1977) so that the modern exposures represent cross sec-tions of a complex paleohydrothermal system from the volcanic environment to about 7 km depth.

This paper summarizes field, petrologic, and geochemical data that support the origin of hydrothermal wall-rock alteration and ore deposition due to two different types of fluids. Magmatic brines were derived from thecrystallization of the youngest equigranular intrusion of the Yerington batholith, the Luhr Hill granite. Brinesseparated from the granite and were emplaced upward together with granite porphyry dikes to produce cop-per-iron sulfdes and associated K silicate alteration in the porphyry copper deposits and copper skarns. In theupper part of the hydrothermal system, magmatic fluids are an important source of acids and sulfur that pro-duced sericitic and advanced argillic alteration.

A second type of ore fluid is brine derived from formation waters trapped in the Triassic-Jurassic sedimen-tary section intruded by the batholith. These fluids were heated by the batholith and circulated through its crys-talline parts. Hornfels and endoskarn were produced along the contact of an early intrusion. Following intru-sion of the porphyry dikes, sedimentary brines circulated up to 3 km into the batholith and upon heatingproduced sodic-calcic alteration there. Ascent of these brines, particularly after the waning of magmatic fluidinput, may have caused shallow-level chlorite-dominated alteration in igneous host rocks and Fe oxide-Cu-Aulodes and replacement deposits in the batholith and its contact aureole, respectively.

† Corresponding author: e-mail, [email protected]



Yerington batholith, the Luhr Hill granite, and associatedgranite porphyry dikes (Proffett, 1979; Dilles, 1987; Dillesand Proffett, 1995). The flow paths of these fluids were gen-erally upward and outward from porphyry centers.

The second major type of fluids includes sedimentary brines or formation waters genetically related to watertrapped within the evaporite-bearing Jurassic-Triassic sedi-

mentary sequence at Yerington. These brines were heated by the Yerington batholith and convectively circulated throughthe contact aureole and the crystalline parts of batholith. Theflow paths were generally inward toward the batholith, andthen upward and locally outward.

Geology

Early Mesozoic rocks

The rocks of the Yerington district include Mesozoic crys-talline rocks, Cenozoic volcanic rocks, and Quaternary alluvialdeposits (see Proffett and Dilles, 1984). The oldest rocks con-sist a sequence of late Triassic or older intermediate and sili-cic volcanic rocks intruded by middle to late Triassic age plu-

tons (Dilles and Wright, 1988). Unconformably overlyingthese rocks are a series of marine sedimentary and volcani-clastic rocks, from bottom to top as follows: limestone andblack argillite; tuffaceous and limy beds; a dacitic tuff; a thickupper Triassic (Norian) limestone; tuffaceous siltstones andargillites; a thin marble; an evaporite gypsum; and an aeoliansandstone (Fig. 1). These rocks are overlain by the ArtesiaLake Volcanics, a ca. 1-km-thick sequence of intermediate tosilicic lavas, tuffs, and volcaniclastic rocks, which representsthe first of a voluminous series of Middle Jurassic igneousrocks.

Yerington batholith

The Artesia Lake Volcanics are thought to represent the

early, extrusive part of the magmatic system that developedinto the Yerington batholith. Thus, the Yerington batholith isemplaced into the base of the Artesia Lake Volcanics. Thebatholith consists of three major equigranular intrusions,each progressively smaller in volume, more deeply emplaced,and more silicic in composition (Dilles, 1987). The batholith was emplaced and crystallized in about 1 m.y. based on theU/Pb zircon ages of 169.4 Ma for early McLeod Hill quartzmonzodiorite and of 168.5 Ma for a late, mineralized graniteporphyry dike (Dilles and Wright, 1988).

McLeod Hill quartz monzodiorite contains hornblende andbiotite and was emplaced as a series of dikelike bodies intothe overlying volcanics and into the adjacent hornfels. Early endoskarn and metasedimentary hornfels in the contact aure-

ole are closely associated with this intrusion. There are nu-merous internal, steeply dipping contacts within the unit.The second is the Bear intrusion, which generally was em-

placed into the McLeod Hill body, but locally was emplacedinto volcanics and possibly into older hornfels (Fig. 1). It is acompositionally zoned body, ranging from a fine-grained,graphic-textured granite roof and border a few hundred me-ters thick downward into a medium-grained, relatively homo-geneous body of hornblende quartz monzonite. Hydrother-mal alteration of the granite and its roof is widespread andconsists of weak K silicate and sericitic alteration associated

with minor pyrite mineralization; it is unclear if there is any associated Cu or Au mineralization.

The youngest equigranular unit of the batholith is the LuhrHill intrusion, which consists of a deeply emplaced, medium-to coarse-grained K feldspar megacryst-bearing hornblende-biotite granite. A series of granite porphyry dikes are coge-netic with the Luhr Hill. The dikes contain about 50 vol per-

cent phenocrysts of plagioclase, biotite, hornblende, quartz,and 1-cm-long K feldspar in an aplitic groundmass of ~0.02-mm-diameter K feldspar, quartz, and minor plagioclase. Theporphyries were derived from the Luhr Hill granite, becausethey are compositionally and mineralogically very similar andbecause they grade downward into granite via an increase ingrain size of the groundmass. Most porphyries intrude upwardthrough cupolas of the granite from source areas within a few hundred to a thousand meters of the upper contact (Fig. 1).Porphyry copper deposits at Ann-Mason and the Yeringtonmine are associated with dike swarms that consist of a seriesof separate dikes emplaced through a granite cupola. At theYerington mine at least five separate dike intrusions (desig-nated QMP-N, 1, 2, 2.5, and 3; Proffett, 1979; Einaudi et al.,

pers. commun., 2000) have been mapped and identified. Atleast three temporally separate intrusions are mapped at Ann-Mason (Dilles, 1987). The Cu skarns near the Mason Valley,Bluestone, and Douglas Hill-Casting Copper-Ludwig mineareas are all associated with porphyry dikes. In addition, nu-merous aplite and pegmatite dikes cut the upper contact of theLuhr Hill granite and are in turn cut by younger porphyry dikes.

Younger Jurassic igneous rocks

Following the emplacement of the Yerington batholith, aseries of subareal intermediate- to silicic-composition lavas,domes, ignimbrites, and volcaniclastic sedimentary rocks thatform the Fulstone Spring Volcanics were deposited. Theseare preserved in the western part of the Yerington district in

the Buckskin Range (Proffett and Dilles, 1991). A U/Pb zir-con age of 166.5 Ma was obtained from a dome in the upperpart of the Fulstone (Dilles and Wright, 1988). However, it ispossible that the base of the Fulstone section may be age-equivalent to youngest granite porphyry dikes related to theLuhr Hill Granite because the Fulstone contains quartz latitelava flows, breccias, and ignimbrites that somewhat resemblethe porphyries (20 to 40 vol % phenocrysts of plagioclase, horn-blende, biotite, quartz, and local 1- to 2 cm-long K feldspar).

A series of quartz monzodiorite porphyry dikes traverse theYerington district, strike east-west, dip steeply, and have aU/Pb zircon age of ca. 165 Ma. They intruded the Fulstone volcanics and commonly were emplaced along faults thatbound and downdrop the Yerington batholith ca. 1 km relative

to the surrounding rocks (Proffett and Dilles, 1984). The dikesare inferred to be older than the Shamrock batholith (U/Pbzircon age of 165 Ma), which is a large body of hornblende-biotite granite that lies south of the Yerington batholith.

Cenozoic rocks and structure

Following a long period of erosion or nondeposition, a seriesof Oligocene and early Miocene ignimbrites and some lavaflows blanketed the Yerington district (0.5–2 km thick: Prof-fett and Proffett, 1976). These rocks are overlain by middleMiocene-age andesitic lavas. During andesitic magmatism,

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OVERVIEW OF THE YERINGTON PORPHYRY COPPER DISTRICT 57

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120%

Print,

Fig. 1

F I G . 1 . N o r t h - s o u t h g e o l o g i c c r o s s s e c t i o n t h r o u g h t h e Y e r i n g t

o n b a t h o l i t h b a s e d o n t h e g e o l o g y e x p o s e d i n t

h e A n n - M a s o n - L u d w i g , B l u e H i l l , a n d M a c A r t h u r f a u l t

b l o c k s i n t h e S i n g a t s e R a n g e a n d e x p o s u r e s i n t h e B u c k s k i n R a n g

e ( P r o f f e t t a n d D i l l e s , 1 9 8 4 , 1 9 9 1 ) . F r o m D i l

l e s ( 1 9 8 7 ) .



rapid extension via closely spaced, east-dipping normal faultsbegan and terminated about a million years later (Proffett,1977; Dilles and Gans, 1995). These oldest faults are cut andoffset by two sets of east-dipping normal faults, the youngerof which include the active fault system bounding the modernbasins and ranges in the area. As a consequence of the down-to-the-east normal faulting, the pre-Miocene rocks, including

the mineralized Mesozoic rocks, were tilted approximately 60°to 90° W (Proffett, 1977; Geissman et al., 1981) so that thecurrent exposures represents cross sections of the Jurassic pa-leohydrothermal systems at paleodepths ranging from the volcanic environment (Buckskin Range) to about 7 km depth.

Magmatic Fluids

Genesis of fluids

The Yerington batholith represents a relatively typical, shal-lowly emplaced differentiated granitoid suite with an alkali-calcic geochemistry. The magmas were water-rich (2.5–4 wt.%) on the basis of hornblende crystallization early during so-lidification; strongly oxidized (NNO buffer + 2 to 3 log units)

on the basis of Fe oxide compositions of the assembage mag-netite + sphene + quartz; and contained abundant sulfate(>1000 ppm) based on S-content of apatite (Dilles, 1987;Dilles and Proffett, 1995; Streck and Dilles, 1998). Thus,crystallization of each major intrusion of the batholith gener-ated magmatic-derived aqueous fluids.

Magmatic-hydrothermal fluids, attendant hydrothermal al-teration, and porphyry Cu-Fe ± Mo sulfide mineralization areclosely associated temporally and spatially with emplacementof granite porphyry dikes (these were previously called quartzmonzonite porphyry in Proffett and Dilles, 1984). Porphyry copper deposits at the Yerington mine (Proffett, 1979) and atthe Ann-Mason prospect (Dilles and Einaudi, 1992) are well-studied, and Bear and MacArthur deposits represent separate

centers that are similar. Petrologic evidence indicates theLuhr Hill granite reached water-saturation during cooling atabout 50 percent crystallinity, and that the resultant magmaticaqueous fluids that separated were rich in dissolved Cl, Na,K, Fe, S, and Cu (Dilles, 1987; Dilles and Proffett, 1995).These fluids accumulated in the cupolas of the Luhr Hillgranite (Dilles, 2000) and ultimately were released via fluidoverpressuring or tectonic fracturing. The released fluids hy-drofractured overlying rock and allowed upward movementof both the hydrothermal fluids and porphyry magmas. Mostporphyry dikes were emplaced upward from the cupola re-gions of the Luhr Hill granite and these dikes served to focusmagmatic-hydrothermal fluid-flow. During emplacement of most dikes, the least principal stress was apparently oriented

in a southwest-northeast direction, and thus fracturing andmost dike emplacement occurred along a prominent joint setstriking northwest-southeast and dipping steeply. In the mod-ern, west-tilted exposures, these dikes and fractures now strike N 70° W and generally dip an average of 45° N.

Magmatic-hydrothermal alteration associated with the McLeod Hill and Bear intrusions

Magmatic-hydrothermal fluids were apparently releasedduring crystallization of the McLeod Hill quartz monzodior-ite and Bear quartz monzonite, but the role and importance

of these fluids in hydrothermal alteration and mineralizationis not clear. The advanced argillic and sericitic alteration inthe overlying Artesia Lake Volcanics could be in part due tofluids released during crystallization of the McLeod Hill andBear intrusions, and these alteration zones in part predatesome shallowly emplaced porphyry dikes. Certainly, mag-matic fluids containing sulfur, acids, and alkalis were released

from the granitic roof of the Bear intrusion and produced rel-atively low temperature, widespread hydrothermal mineralassemblages that include K feldspar, sericite, chlorite, andpyrite in the granite and overlying McLeod Hill quartz mon-zodiorite and Artesia Volcanics in the central Singatse Range.

The emplacement and crystallization of McLeod Hill andpossibly the Bear are closely related to formation of garnet-pyroxene hornfels in the contact aureole (Einaudi, 1977,2000; Harris and Einaudi, 1982) and pyroxene-plagioclase en-doskarn within the batholith near its contact (Harris and Ein-audi, 1982; Dilles and Einaudi, 1992). The role of the mag-mas, as a material source compared to a heat source, isunclear. The hydrothermal fluids were apparently dominatedby saline sedimentary brines heated by the McLeod Hill and

possibly Bear intrusions (see below). However, these intru-sions may have contributed considerable amounts of mag-matic water to the metasomatic fluids, which dominantly flowed upward along the contact and within the sedimentary- volcanic sequence. The nearly isochemical nature of themetasomatic alteration that produced the garnet-pyroxenehornfels (Einaudi, 2000) suggests that magmatic fluids that would have contributed alkalis, sulfur, and iron were subordi-nate to sedimentary brines.

Magmatic-hydrothermal alteration associated with the Luhr Hill intrusion

The magmatic-hydrothermal fluids related to the Luhr Hillgranite produced a series of porphyry dike-centered alter-

ation and mineralization zones. Along the porphyry dikeswarms immediately above the Luhr Hill granite cupolas, thehydrothermal alteration and Cu(-Mo) sulfide mineralizaton ismost intense and produced at least four porphyry copper de-posits: Yerington, Ann-Mason, Bear, and MacArthur (cupolasare not identified for the latter; Figs. 2, 3). Fluids of magmaticorigin are responsible for K silicate alteration and in mixtures with local ground waters for both sericitic and advancedargillic alteration. Oxygen and hydrogen isotope data indicate waters for K silicate alteration were entirely magmatic whereas sericitic waters were about half magmatic (Dilles etal., 1992). Sulfur isotope compositions in K silicate andsericitic alteration are similar and indicate a magmatic originfor sulfur (Dilles et al., 1995).

The magmatic fluids were high salinity and produced thehigh-temperature K silicate alteration (biotite ± K feldspar)and bornite ± chalcopyrite ± magnetite mineralization char-acteristic of both the central parts of the Yerington and Ann-Mason porphyry deposits. Apparently, magmatic aqueous flu-ids were focused along the dike swarms and resulted in thehighest water:rock ratios, and where fluids cooled they pro-duced the most intense metasomatism, highest quartz veindensities, and highest percentages of Cu-bearing sulfides.Granite porphyries dikes emplaced off-axis of the major dikeswarms or in the upper part of the dike swarms typically were

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, B l u e H i l l , a n d M a c A r t h u r a r e a s .



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FaultMine or Prospect

& replacementsFe-oxide lode

VolcanicsArtesia Lake

Hill

Luhr

RangeWassukWestern

AreaPumpkin Hollow

AreaLudwig

Range

Wassuk

alterationK-silicate

Endoskarnalteration

Sodic-calcic

monzoniteBear quartzmonzodiorite &

McLeod Hill quartzOuter contact of

VolcanicsFulstone Spring

EXPLANATION

intrusions-contactMcLeod Hill & Bear

-outer contactLuhr Hill granite

dikesGranite porphyry

hornfelsSkarnoid &

Skarn

ton Mine1.Yering-

18.

8.Douglas Hill

4.Ann-Mason

Prospect19.Anaconda

9.McConnell

7.Casting Cu

5.Bluestone Lights Cu-Fe14.Northern

F a u l t

View Au17.Mountain

12.Lyon Fe-Cu

Valley6.Mason

11.Ludwig

~4 km depthHill GraniteTop of Luhr

Mine15.Bluejay Hill

Fe Mine13.Minnesota

Prospect3.MacArthur

Prospect2.Bear

Singatse

Range

HillBlue

16.Buckskin

BuckskinRang

e Easter Prospect

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FIG. 3. Generalized plan map of selected hydrothermal alteration zones projected to the Lower Tertiary erosion surface

from about 1–4 km below the Jurassic paleo-surface (or 0–3 km below the LTS). The geologic contacts are from John Prof-fett (fig. 1 of Dilles and Proffett, 1995), and are shown in plan view at the elevation of the LTS (ca. 1 km Jurassic paleodepth).Because Jurassic rocks were tilted ca. 10° W prior to formation of the LTS, this is a slightly enclined plan view. Zones showninclude K-silicate alteration (and Cu-Fe sulfide) located near porphyry Cu centers, sodic-calcic alteration, and shallow Fe oxide Cu-Au lodes in the batholith, and endoskarn, hornfels, Cu skarn, and Fe oxide Cu-Au replacements in the contactaureole. Widespread propylitic-actinolite, shallow sericitic and chloritic alteration are not illustrated. Mine numbers corre-spond to deposits whose class and resources are listed in table 1 of Dilles and Proffett (1995).



accompanied by lesser amounts of magmatic fluids. Nonethe-less, all dikes in the 1 to 6 km depth range in the Ann-Masonand other Yerington district exposures contain some hy-drothermal biotite and minor Cu-Fe sulfide, attesting to thepassage of magmatic hydrothermal solutions. Distal dikes lo-cally intrude the contact aureole of the Yerington batholith(e.g., the Ludwig area) where they are closely associated with

andradite, hedenbergite, and actinolite-rich skarn that con-tains chalcopyrite and pyrite (Figs. 1, 2; Einaudi, 2000). Theiron, copper and sulfur-rich ore fluids were apparently de-rived from these porphyry dikes.

The magmatic fluids were released as a series of pulses ac-companying dike emplacement. Within the main Yeringtonmine orebody, at least five granite porphyries (designatedQMP) have been identified. The oldest, QMP-1 accompaniesthe strongest K silicate alteration and Cu-Fe sulfides. Por-phyry QMP-N post-dates QMP-1, and is itself poorly miner-alized. Porphyry QMP-2 is younger than QMP-N and accom-panies moderate K silicate alteration and Cu-Fe sulfides.Porphyries QMP-2.5 and QMP- 3 record progressively less Ksilicate alteration and Cu-Fe sulfide mineralization appar-

ently because they contained less magmatic fluid (J.M. Prof-fett and M.T. Einaudi, unpub. data). Sericitic alteration ac-companies pyrite-quartz “D” veins (Gustafson and Hunt,1975) and postdates porphyry QMP-2.5 in the Yeringtonmine (Proffett, 1979). Throughout the Yerington district, within each porphyry copper deposit there is both a time evo-lution at a single point from early K silicate with Cu-Fe sul-fides to late sericitic alteration with pyrite; similarly, although we must infer age relations, it is quite possible that at any given time hydrothermal alteration evolved from deep K sili-cate to shallow sericitic or advanced argillic alteration (Fig. 4).

Sericitic alteration is widespread both above the Ann-Mason and MacArthur porphyry deposits (Fig. 2) and in theoverlying Artesia Lake Volcanics in the Buckskin Range. In

the latter (oxidized) exposures, the most intense alteration in-cludes advanced argillic assemblages with pyrophyllite,quartz, alunite (hypogene?), pyrite, and sericite (Hudson,1983; Dilles and Proffett, 1991, Lipske and Dilles, 2000). Al-though age relations are not clear in most exposures, locally,D veins with sericitic envelopes postdate and crosscut pyro-phyllite-bearing advanced argillic alteration. Pyrophyllite-bearing assemblages are cut by porphyry dikes (that may belate dikes related to the Luhr Hill granite, or possibly relatedto the Fulstone Spring Volcanics) that are in turn cut by younger D veinlets with selvages of hematite-sericite-pyritealteration (Lipske and Dilles, 2000).

One interpretation of the temporal-spatial relationships of K silicate, sericitic, and advanced argillic assemblages follows.

Two assumptions are made. All these alteration types are as-sumed to have been produced by fluids with large magmaticcomponents, based on the stable isotopic data and the ability of magmatic fluids alone as potential ore fluids to provideabundant acids and sulfur. The second assumption is thateach pulse of magmatic fluid is directly related to porphyry dike emplacement. The Ann-Mason, Yerington mine, andBuckskin data suggest that early magmatic-hydrothermal flu-ids produced a high-temperature plume of brine that roseand produced K silicate and Cu-Fe sulfide mineralization(with local, later molybdenite) in the individual porphyry

centers. Due to depressurization at high temperature, on as-cent these fluids would have intersected the brine-vapor im-miscibility field in the system water-NaCl (Sourirajan andKennedy, 1962). A buoyant, low-density vapor likely sepa-rated from brine (cf. Henley and McNabb, 1978) may haverisen to the subvolcanic environment where it condensed andmixed with local ground water to produce the acid fluids re-

sponsible for the pyrophyllite-sericite assemblages. Alterna-tively, these two environments may not have been linked, ormay have been linked by sericitic alteration lying in an inter-mediate position between deep K silicate and shallow advanced argillic zones. This process of hydrofracturing, ascentof fluids, immiscibility, and coupled deep K silicate and shal-low advanced-argillic alteration may have occurred numeroustimes accompanied by successive granite porphyry dike em-placements that represent the main dike swarms in individualporphyry centers. As the Luhr Hill magma crystallized anddegassed the late porphyry dikes were emplaced from adeeper source into cooler wall rocks. These fluids were ap-parently sulfur-rich but copper-poor and they cooled and de-pressurized along a path such that they did not intersect the

water-NaCl immiscibility field. Instead, at the level of theporphyry copper orebody, they produced sericitic alterationand pyrite mineralization. As fluids ascended to the subvol-canic environment they produced some sericite-pyrite alter-ation, but may have lost most of their acids and sulfur and sodid not produce strong alteration in the late porphyry dikesemplaced at this level. Rather, they may have mixed with localground waters to produce the weak K metasomatism and thechlorite + hematite ± sericite ± K feldspar assemblages ob-served in the base of the Fulstone Spring Volcanics (Fig. 2;Dilles and Proffett, 1991; Lipske and Dilles, 2000).

Sedimentary Brines

The Triassic-Jurassic sedimentary section at Yerington rep-

resents a late Mesozoic marine sequence that ended with de-position of an evaporite and an aeolian sandstone, which indi-cates that marine conditions gave way to subareal conditionsthrough time. The lower Jurassic evaporite – sandstone asso-ciation is widespread in western Nevada, and represents atime of marine regression, formation of closed marine basins,drying of these marine basins, and subareal dune sand andfluvial sand deposition (Speed, 1974, 1978). The Triassic-Jurassic section at Yerington thus would have contained alarge volume of marine and possibly meteoric pore fluids re-stricted by impermeable sedimentary units (shales, etc.). Al-though evaporite in the upper part of the section now con-tains only gypsum, it is likely that originally the gypsum beds would have contained saline brines and very likely evaporitic

halite and other salts. Thus, the sedimentary section intrudedby the Yerington batholith contained a vast volume of trappedpore fluids that likely ranged from halite-saturated brines tolow-salinity sea water. When the batholith was emplacedthese fluids were heated and convectively circulated, leadingto extensive sodic-calcic alteration and associated Fe oxide-bearing mineralization.

Early hornfels and endoskarn

A contact metasomatic aureole surrounds the Yeringtonbatholith and is well-developed in sedimentary lithologies


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QMD QM PG GP1 GP-LGP2 FULSTONE

QMD QM PG GP1 GP-LGP2 FULSTONE

0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

D e p t h ( k m )

D e p t h ( k m )

A

B

Distal to Central Porphyry Dikes

Proximal to Central Porphyry Dikes

Time (early to late intrusions)

??

McLeod

Hill Bear Luhr Hill & Porphyries

Weak K-silicate & sericitic (weak K)

Igneous Alteration

Equigranular intrusions of Yeringtonbatholith

Igneous Rocks

Sodic-Calcic (NaCa) with sodic plagioclase+ actinolite + epidote + sphene

Porphyry dike (related to Luhr Hilgranite or Fulstone)

Fulstone Spring Volcanics

Propylitic (PA, withactinolite+epidote)

K-silicate (K) with biotite-CuFe-sulfide – K-feldspar

Chlorite-hematite-sericite–magnetite (chl-hm-ser) withfeldpar stable

Sericite + quartz + pyrite (S)

Advanced argillic (AA) withpyrophyllite – alunite + pyrite

Magnetite-chalcopyrite

-chlorite lode & replacement

NaCa

mt-cp-chl

chl-

hem-

ser

P

chl-

hem-

ser

chl-

hem-

sermt-cp-chl

PA

PAPA

????

Artesia Lake Volcanics

S S

AA SAA

NaCa

NaCa

PAPA

PA

PA

K

chl-

hem-

ser

chl-

hem-

ser

weak K

NaCa

Flow Paths of Fluids

Upward flow (magmatic fluids,

sedimentary brines, etc.)

Inward flow (into diagram,

mainly sedimentary brines)

chl-

hem-

ser

FIG. 4. Time-space evolutionary model for hydrothermal alteration of the Yerington batholith. A. Proximal to porphyry centers. B. Distal to porphyry centers. Note that alteration in the distal batholith environment is linking to alteration in thecontact aureole, where protolith composition and resultant assemblages differ (see text and Fig. 5). Abbreviations are: quartzmonzodiorite (QMD), quartz monzonite (QM), porphyritic granite (PG), and granite porphyry (GP). Granite porphyry is di-

vided into early dikes (GP-1 equivalent to QMP-1 in Yerington mine), intermediate age dikes (GP-2, equivalent to QMP-2of Yerington mine), late porphyries (GP-L equivalent to QMP-2.5 and 3 of Yerington mine, and porphyries that may be feed-ers to the base of the Fulstone Spring Volcanics in the Buckskin Range. See text for details.



within about 4 km of the Jurassic paleosurface. The contactmetasomatic aureole south of the Yerington batholith con-tains a huge volume of garnet-pyroxene hornfels and smaller volumes of garnet-pyroxene and plagioclase-pyroxene en-doskarn within sills and dikes of the McLeod Hill quartzmonzodiorite (Harris and Einaudi, 1982; Einaudi, 2000; Fig.2). As noted above, large volumes of thin-bedded mixed clas-

tic, carbonate, and tuffaceous lithologies have been broadly chemically homogenized. Such a process must have accom-panied metasomatic fluid flow, which as documented by Ein-audi (2000) must have been broadly up along the McLeodHill quartz monzodiorite contact and outward away from thebatholith along permeable sedimentary beds. These fluids ac-complished broad-scale chemical homogenization and extrac-tion of large amounts of carbonate and water from the sedi-mentary rocks at temperatures in excess of 400°C. Thepaucity of alkali, iron, and acid metasomatism and hydrother-mal sulfides suggests the principal metasomatic fluid was aCa-rich brine derived from the sedimentary section. En-doskarn in the McLeod Hill quartz monzodiorite has seenlarge additions of calcium and loss of iron and potassium dur-

ing metasomatism. Strontium isotopic analyses of endoskarnin the McLeod Hill intrusion (Dilles et al., 1995) suggest thattwo-thirds of the strontium in these samples is derived fromthe Triassic-Jurassic sedimentary section as transferred intothe batholith via the sedimentary brines.

Sodium-calcium metasomatism

Sodium-calcium alteration is widespread within the Yering-ton batholith and is closely associated in time with emplace-ment of the Luhr Hill granite and the related granite por-phyry dikes. This alteration represents strong sodium and orcalcium metasomatism as characterized by addition of sodicplagioclase, actinolite, epidote, and sphene to rocks. A variety of arguments suggests that this alteration represents influx of

sedimentary brines into the Yerington batholith following hy-drofracturing and porphyry dike emplacement produced by rise of magmatic fluids. These observations include presenceof radiogenic strontium derived from the Triassic-Jurassicsedimentary rocks within altered batholithic rocks (Dilles etal., 1995); alkali exchange phase equilibria that indicatesodium metasomatism was produced by a heating (or pro-grade) fluid (Carten, 1987; Dilles and Einaudi, 1992); and nu-merous porphyry dike age relations with sodic-calcic alter-ation veins, which indicate that dikes cut veins but are cut inturn by younger veins (Carten, 1987; Dilles and Einaudi,1992). The scenario of sodic-calcic metasomatism is that sed-imentary brines moved laterally through the contact aureoleinto the batholith at 2 to 6 km depth, traversed into the

batholith for 1 to 3 km along joint sets and granite porphyry dikes, and then flowed upward along the dikes and joints. Themain upward flow occurred along porphyry dikes immedi-ately outside of the main upward flow path of the magmatic-hydrothermal fluids (Figs. 1–3). Alteration effects are maxi-mum where sedimentary fluids reached their highest inferredtemperatures (~400°–450°C) along the deep parts of the por-phyry dikes near the apices of the Luhr Hill granite cupola(Dilles and Einaudi, 1992). In these areas, sodic-calcic alter-ation is superimposed on earlier K silicate alteration within aporphyry representing a single dike emplacement, but

younger dikes are observed to cut off the older alterationtypes and be affected by a younger paired set of K silicate andsodic-calcic alteration (Carten, 1987). In the center of theAnn-Mason and Yerington orebodies, relatively late fluiddominated by sedimentary brine produced assemblages withsodic plagioclase, hydrobiotite or chlorite, local sericite, andrutile. These assemblages cut and replace earlier K silicate as-

semblages. Causative fluids were likely sedimentary brinesthat acquired a moderately low pH, (necessary to produce ru-tile, sericite, and chlorite), as a result of either fluid coolingand resultant dissociation of HCl or by mixing with a smallproportion magmatic fluid (Dilles et al., 1995).

“Propylitic”-actinolite alteration

At Ann-Mason and elsewhere in the Yerington district,there is widespread “propylitic”-actinolite alteration that is in-termediate in depth (~2.5–4.5 km depth) between deep-levelsodic-calcic alteration and the high-level, chlorite-rich alter-ation described below. Such alteration types are proposedhere (see Dilles and Einaudi, 1992) to be caused by sedi-mentary brines but at a lower temperature (~300°C) than the

deeper sodic-calcic alteration. Propylitic-actinolite alterationis characterized by addition of actinolite, epidote, chlorite,minor calcite, hematite, sulfide, and magnetite in mafic min-eral sites, and by weak alteration of plagioclase to epidote,fine-grained white micas and clays, and minor K feldspar. Veins are dominated by epidote along fractures similar todeeper sodic-calcic assemblages. Locally, at the Easterprospect in the MacArthur area, magnetite-apatite-chalcopy-rite veinlets form in the upper parts of the propylitic-actino-lite zone. In the interpretation here, the propylitic-actinolitezones represent alteration by sedimentary brines followingcomplex flow paths, for example into the batholith laterally ordownward as input into the sodic-calcic alteration zone, orpossibly upward parallel to the magmatic fluids as the outflow

zones for sodic-calcic alteration fluids. Temporal constraintson propylitic-actinolite alteration are minimal, but the alter-ation affects all Yerington batholith units as well as the Arte-sia Lake Volcanics and hence could be lengthy. An alternativeinterpretation is that propylitic alteration represents the out-flow zone for rising and outward-spreading magmatic fluids. We do not support this hypothesis because the sodic-calcic al-teration zone commonly lies in an intermediate position be-tween K silicate and propylitic-actinolite alteration; flow paths of magmatic fluids are mapped as near vertical; and themagmatic fluid signature of sulfide and acids is weak in thepropylitic-actinolitic zone.

Magnetite-hematite lodes and chlorite-rich alteration

In the upper part of the Yerington batholith hydrothermalsystem and its contact aureole, there is abundant silicate al-teration and iron-oxide mineralization that is not clearly linked to magmatic-hydrothermal fluids. In many cases, thesezones contain few porphyry dikes and those that are presentcontain little sulfide or hydrothermal biotite or sericite.

In the Artesia Lake Volcanics, uppermost exposures of theMcLeod Hill quartz monzodiorite, and base of the FulstoneSpring Volcanics, several Fe oxide-Cu-Au vein or lode sys-tems are associated with broadly distributed wall rock alter-ation to chlorite, hematite, and local sericite, K feldspar, and


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magnetite (and relict feldspar). Lodes exposed in the ArtesiaLake Volcanics at the Buckskin mine, southern BuckskinRange (Gibson, 1987), are similar to those in the McLeodHill Quartz Monzodiorite at Bluejay Hill, 5 km east of Yer-ington. In both cases, lodes are up to 15 m wide and consistof wall rock replaced by mixtures of quartz, hematite, chlo-rite, and minor sericite, pyrite, chalcopyrite, tourmaline, and

traces of sphalerite, galena, and gold. At Bluejay Hill the lodesystems can be traced downward about 1 km into zones of chlorite and quartz veins associated with epidote, magnetite,and sulfide veins with propylitic-actinolite and local sodic-cal-cic alteration. In the southwest part of the Ann-Mason area, veinlets with quartz, biotite, chlorite, magnetite, and hematitealso pass downward over ~1 km into epidote vein zones withpropylitic-actinolite alteration. Veinlets at the Buckskin mine(Gibson, 1987) and southwest Ann-Mason both contain hy-persaline (halite-bearing) fluid inclusions similar to those foundin deeper sodic-calcic veinlets (Dilles and Einaudi, 1992).

In the uppermost part of the Artesia Lake Volcanics, baseof the Fulstone Spring Volcanics, and in porphyry dikes cut-ting the upper part of the Artesia in the central Buckskin, wall

rock is widely altered to chlorite, sericite, quartz, andhematite with relict feldspar. Veinlets of specular hematiteand quartz with minor pyrite and chalcopyrite have distinctinner sericitic alteration envelopes and are enclosed in rockaltered to chlorite-sericite-hematite ± albite ± K feldspar(Lipske and Dilles, 2000). These veinlets postdate advancedargillic alteration in the area.

At the Minnesota Iron mine in northern Buckskin Rangeand in Lyon Fe-Cu-Au deposit in the Pumpkin Hollow area10 km southeast of Yerington (ca. 2 and 100 Mt Fe containedin ores, respectively; Dilles and Proffett, 1995) iron oxides re-placed earlier calc-silicate hornfels, minor Cu skarn, and mar-ble. The ores consist of massive and vein-filling magnetite ±hematite ± chalcopyrite ± pyrite with associated silicate alter-

ation minerals such as chlorite, actinolite, talc, serpentine,quartz (and calcite). These latter minerals locally replacedand destroyed early, high-temperature anhydrous calc-silicateminerals that include garnet and pyroxene. Similarly, in theLudwig area, small volumes of quartz-calcite-hematite-mag-netite with local actinolite, chlorite, talc and minor apatite re-place early andradite- and salite pyroxene-skarn (Harris andEinaudi, 1982; Einaudi, 2000).

The Fe oxide Cu-Au association in both the lodes in thebatholith and the replacement deposits in the contact aureolehave similar associations and likely a common parentage.These deposits are dominated by addition of Fe oxide miner-als and low-temperature silicate alteration without importantsulfur addition or hydrolytic alteration (clay, sericite). The de-

posits formed at least in part later than skarn and advancedargillic porphyry alteration (Figs. 4, 5). The deposits also lieabove or outboard from zones of strong sodic-calcic alteration where Fe, K, and Cu were removed from the Yeringtonbatholith (Figs. 2, 4; Carten, 1987; Dilles et al., 1995), andform along northwest-southeast striking structural pathwaysfor fluid flow. In the model proposed here, these Fe oxide de-posits are formed where deep sodic-calcic fluids moved upward and outward from the center of the Yerington batholithalong permeable fracture zones, cooled, reacted with wallrocks, and saturated in hematite and magnetite (Fig. 4).

Surficial-Derived Fluids

The role of dilute surficial ground water or meteoric waterat Yerington is not clear, but isotopic evidence and hy-drothermal alteration patterns (Dilles et al., 1992; Dilles andEinaudi, 1992) suggest a minimal role. Dilute ground watermay have been important in the subvolcanic environmentdominated by sericite, chlorite, clay and local K feldspar-

bearing assemblages. However, this weak acid and alkalimetasomatism may have been caused by ground water withmoderate salinity that had formed in the Jurassic subaerial toevaporitic environment (Barton, 1998). The young event em-placement of the Shamrock was also accompanied by sodic-calcic alteration, which argues that sedimentary brines con-tinued to dominate at depth (Battles and Barton, 1995).

Conclusions: Time-Space Evolutionof Hydrothermal Flow

Figure 5 is a simplified cross section through the Yeringtonbatholith that illustrates time-space evolution of the hy-drothermal fluid flow, alteration, and ore deposition. In frame

A, early equigranular intrusions created hornfels and en-doskarn in the contact aureole. These fluids were dominatedby sedimentary brines with lesser magmatic fluids.

In frame B, following intrusion of the Luhr Hill granite,crystallization led to aqueous fluid saturation, hydrofractur-ing, early porphyry intrusion, and upward movement of mag-matic hydrothermal fluids to form deep K silicate alteration with Cu-Fe sulfide mineralization and shallow sericitic andadvanced argillic alteration in the subvolcanic environment.Fracturing allowed sedimentary brines to move into thebatholith, be heated, and cause sodic-calcic and propylitic-actinolite alteration flanking the K silicate alteration zones.

In frame C, the process in B was repeated multiple times insuccession accompanying emplacement of a successive series

of granite porphyry dikes (shown here schematically as oneevent). Most Cu-Fe sulfide and K silicate alteration, as well aslate molybdenite, formed in this time interval along the por-phyry dikes, as did andradite garnet skarn with Cu sulfide inthe contact aureole. The subvolcanic environment was domi-nated by sericitic and advanced argillic alteration waning intime to sericitic alteration only. Sodic-calcic and more distalpropylitic-actinolite alteration increased with time as moresedimentary brine circulated through the batholith. As thisfluid circulated upward through the top of the batholith andoutward into the upper part of the contact aureole, early Feoxide-Cu-Au lode deposits began to form associated withchlorite-rich hydrothermal alteration.

In frame D, young porphyries related to the Luhr Hill

granite were emplaced together with hydrothermal fluidsthat produced pyrite-rich and Cu sulfide-poor mineralizationand abundant sericite in the porphyry Cu(Mo) center. Con-tinued circulation of sedimentary brines produced flankingsodic-calcic and more proximal albite-rich alteration. Dis-charge of these brines led to a peak in the amount of Feoxide Cu-Au mineralization and associated hydrous silicatealteration (chlorite, etc.) in the contact aureole and subvol-canic zone, where it is superimposed on the earlier skarnzones and both sericitic and advanced argillic alterationzones, respectively.

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In frame E, porphyries possibly related either to the LuhrHill granite or to a new magma at depth were emplaced to-gether with extrusive lavas and pyroclastic rocks that form thebase of the Fulstone Spring Volcanics. Weak chlorite-hematite-calcite (with relict feldspar) alteration dominatesthe shallow environment and may be age-equivalent to late,deep propylitic-actinolite or possibly sodic-calcic alteration.

Hematite-magnetite-copper-iron sulfide-gold veins are sparsein the shallow environment. Hydrothermal fluids are domi-nated by low-temperature, sedimentary brines with little mag-matic input.

Acknowledgments

This paper represents a synthesis based our own observa-tions and data, and the excellent field data collected by a gen-eration of Anaconda Company geologists and the university researchers they supported. Geologist who have contributedinclude John Proffett, Rick Carten, Nick Harris, CharlesMeyer, G. Hunter Ware, Ken Howard, John Geissman, Bill Wright, Don Gustafson, Jay Raney, Geoff Wilson, Bart Stone,Guillermo Salas, David Johnson, Kurt Friehauf, and many

others.

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overview of the yerington porphyry copper district - magmatic to nonmagmatic sources of hydrothermal...

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