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EconomicGeology

Vol. 80, 1985, pp. 1467-1514

Ore-RelatedBreccias n Volcanoplutonic rcs

RICHARD H. SILLITOE

8 WestHill Park, HighgateVillage, LondonN6 6ND, England

Abstract

Anoverviewfbrecciaselatedoa vriety fbasemetal, reciousetal, ndithophile

element depositsn volcanoplutonic rcspermitsdefinitionof six possiblemechanismsor

subsurface brecciation.

1. Release fmagmatic-hydrothermalluidsromhigh-level ydrous agma hambersuring

second oilingandsubsequentecompressionenerates spectrum f breccia ypes n which

fragments aysuffer ollapsend/or scent. ingle r multiplentrusion-relatedreccia ipes

andpre- and ntermineral recciasn porphyry opperdepositsrovidewidespreadxamples.

2. Magmaticheatingand expansion f meteoricpore fluidsmay ead to brecciation, om-

monlyof late or postmineral geand ncluding ebbledikes, n porphyry-type ndrelated

deposits. agmatic eating f rocks aturated ith seawater aycause ubmarineydrothermal

eruptionsate n the emplacementistories f Kuroko-typemassiveulfide eposits; anyof

the resultantbreccias nderwent imited sedimentaryransport.Overpressuringf heated

fluids eneath emipermeable,artlyself-sealedap ocksmay ead o brecciationndsubaerial

hydrothermal ruptionsn shallow pithermal reciousmetalsettings;magmatic eatingor

tectonicdisturbancemay have riggeredbrecciation.

3. Interaction f coolgroundwaterswithsubsurface agma angenerate hreatomagmatic

explosions. ostmineral hreatomagmaticiatremes ssociated ith porphyrysystems nd

premineraldiatremeswith epithermalprecious 4- base)metal depositswere generated n

thismanner;heseattainedhe palcosurfaceo produce yroclasticase urge nd all deposits

that accumulated s uff ringsaroundmaarcraters.

4. Magmatic-hydrothermalrecciationmay ead o disruption f rocks hrough o the pa-

lcosurface,ecompression,nd ragmentationnderuption f the top part of an underlying

magma hamber.Pre- andpostmineralmagmatic iatremes f this sortare nferred o accom-

panya few porphyry-type ndotherbaseandpreciousmetalsystems;hey were manifested

at the palcosurfacey accumulationsf pyroclasticall and low deposits.

5. Brecciasmay esult rommechanical isruption f wallrocksduringsubsurface ovement

of magma. ny ntrusion-relatedepositmay nclude uch ntrusion reccias.

6. Tectonicbreccias esulting rom fault displacementmay accompany ny type of ore

deposit.

A continuum xists etweenmanyof thesebreccia ypes nd t is difficult o dentifyunique

criteria for their unambiguous istinction.

Introduction

BRECCIASith an enormous arietyof characteristics

are common, erhapsubiquitous, ccompanimentso

a wide spectrumof hydrothermalore deposits. hey

have ascinated ndperplexedminersandgeologists

for at least 200 years. Ore-related brecciaswere

identified orrectly uring he ate 19thcentury e.g.,

in Cornwall,England;Hunt, 1887, p. 421-422), and

in 1896, Emmonsprovidedan explicitdescription f

the Bassick nd Bull-Domingo recciapipes n Col-

orado. The common occurrence of breccias as hosts

for, or associatesf, hydrothermal re depositswas

generallyappreciatedby the early 20th century, as

evidenced y perceptive eviewsof their character-

isticsandproposalsor their originby Locke (1926),

Walker (1928), andEmroohs1938). Notwithstanding

their early recognition,however, it has only been

during the last decadeor so that someof the more

subtlevarietiesand expressionsf brecciation ave

beenappreciated. ven oday, argematrix-rich od-

ies of breccia are often confused with volcanosedi-

mentary ormationsndelongatematrix-poor reccias

are incorrectlyassigned tectonicorigin.Worsestill,

ore-related reccias ot uncommonlyass nnoticed.

Ore-related reccias ere ast eviewed y Bryner

(1961). Mayo (1976) presented n historical verview

of subsurfacereccias f igneous ffiliation, ut only

a fewof hisexamplesreassociatedithoredeposits

This paper beginswith a brief discussion f classifi-

cationproblems ndproceedso a description f the

characteristics, lterationand mineralization eatures,

and possible riginsof six categories f ore-related

breccias. The treatment is based on the writer's field

studiescombinedwith a perusalof the voluminous

literature on ore-related breccias.

Attention s restricted o volcanoplutonic rcsbe-

cause hey containa greater number and variety of

ore-relatedbreccias han any other metallogenic et-

0361-0128/85/439/1467-4852.50 1467

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1468 RICHARD H. SILLITOE

ting and have provided most of the examplesde-

scribed in the literature. Discussion is focused on

breccias hat were generated n subsurface nviron-

mentsby hypogene rocessesn associationith eco-

nomically ignificant asemetal,preciousmetal,and

lithophileelementdeposits. ubaerial olcanicbrec-

cias are not dealt with, except for those that accu-

mulatedn close roximityo theirsubsurfaceeeders.

Although this paper is restricted to ore-related

breccias,t shouldbe emphasizedhat numerous x-

amplesof apparently imilarbreccias evoidof even

subeconomic amounts of mineralization are known

from arc terranes n manypartsof the world (e.g.,

Gates, 1959; Morris and Kopf, 1967; Busselland

McCourt, 1977).

Classification

A comprehensive enetic classification f ore-re-

lated breccias emainselusive.The proliferationof

genetic ermsused o describebreccias ends o ob-

scurerather than illuminate he subject: ntrusion,

intrusive, explosion,eruption, collapse, phreatic,

phreatomagmatic, ydrothermal, fiuidization, gas

fluxion,steamblast, hydraulic racture (hydrofrac),

and uffisitic re ust someof the qualifters sed,com-

monly ooselyor evenerroneously,n the literature.

The difficultquestion f originhasbeen urther com-

pounded by attempts to explain the formation of

brecciasn general y a singlemechanism.n common

with Bryner (1961) and Richard (1969), the writer

prefers he notionof multiple origins or ore-related

breccias nd s n sympathy ith Joralemon1952, p.

256) when he stated: "It is inconceivable hat all

brecciachimneys ere formedby the sameprocess,"

and "Nature evidently ovesa breccia, and if no vi-

olent phenomenons available, he breccia s formed

just the same"

In principle, ore-relatedbrecciasare amenable o

classificationn he basis f either geneticor descrip-

tive criteria, n the sameway asRecentvolcanic ocks

(e.g., Wright et al., 1980). Ideally, the descriptive

criteriawouldbe diagnostic f a breccia'sgenesis.n

the case of ore-related breccias, however, it has

proved mpossibleo infer the processeliably rom

observed eometric, ithologic,and texturalcharac-

teristics. xisting lassificationchemes, uchas hose

by Wright andBowes 1963), Kents 1964), andBry-

ner( 1968), are nadequateecause f the subjectivity

of manyof the descriptive arameters mployed, s

well asbecause f the lackof support or manyof the

resultinggeneticassumptions.

In this paper, ore-relatedbreccias re discussedn

the context f a broadgenetic ramework,which akes

into account he overlapnow widely recognizedbe-

tween intrusive, volcanic, and hydrothermalpro-

cesses.With the exceptionof tectonicbreccias, he

primary division is based on the inferred role of

magmaand/or aqueous luids n breccia ormation,

and further subdivisions on the basisof ore deposit

type. The resultingscheme,which dictates he or-

ganizationof this paper, is summarizedn Table 1.

Assignment f a breccia o the appropriate ategory

doesnot rely solelyonbrecciacharacteristicsut also

takescognizance f the overallenvironment f brec-

ciation, n particular he relationship o, and condi-

tionsof, accompanyingre deposition. he recogni-

tionof modern nalogsor severalypesof ore-related

breccias lsoprovesuseful.

Magmatic-hydrothermalreccias re products f

the release f hydrothermalluids rommagma ham-

bers, rrespective f the originalsourceof the fluids

concerned magmatic,meteoric, connate, or ocean

waters). Hydromagmaticincludinghydrovolcanic

breccias, sdefinedby Macdonald 1972) and Sher-

idan and Wohletz (1981), are generatedby the in-

teractionof magmaand an externalsourceof water,

suchas groundor surface ocean, ake) waters.The

hydromagmaticategorys subdividednto phreato-

magmatic reccias,where both water and magmadi-

rectlycontributedo formation f the observed rod-

ucts,and phreaticbreccias,n which only magmatic

heat had access o the external water source.Mag-

matic (including olcanic)breccias esult from frag-

mentationand eruption of magma rom subsurface

TABLE . Subdivision f Ore-RelatedBreccias mployed n this Paper

Magmatic-hydrothermalreccias

Hydromagmatic

(hydrovolcanic)

breccias

Magmatic volcanic)breccias

Intrusion breccias

Tectonic breccias

Phreatic breccias

Phreatomagmatic reccias

Pipes related to intrusions

Porphyry-typedeposits

Epithermalprecious 4- base)metal deposits

Porphyry-type nd other intrusion-related eposits

Kuroko-typemassive ulfidedeposits

Porphyry-type nd epithermalprecious 4-base)metal deposits

Porphyry-type ndotherbaseandpreciousmetaldeposits

Any intrusion-related eposits

Any type of ore deposit

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ORE-RELATED BRECCIAS IN VOLCANOPLUTONIC ARCS 1469

chambers.The remaining categoriesof subsurface

breccia--intrusion and tectonic--are only briefly

considered or the sake of completeness.ntrusion

breccias re a direct productof the passive ubsurface

movement f magma. ectonicbreccias re primarily

the productsof tectonic processes,n which water

may or may not have participated.The widely em-

ployed term hydrothermal breccia describes he

productsof magmatic-hydrothermalnd hydromag-

matic processes nd therefore providesa valuable

designation or many ore-relatedbreccias.

An additionalcategory,amagmatic-hydrothermal,

maybe introduced o includebreccias enerated y

hydrothermal luidsof, say, meteoricor cormateor-

igin, uninfiuenced y magmatism. he breccias ec-

ognized rom Mississippi alley-type ead-zincde-

posits,sediment-hostedmassivesulfide ead-zinc de-

posits, unconformity-typeuranium deposits, and

sediment-hostedipesand bodiesare all assignable

to this category. However, since these ore deposit

typesare generallyabsent rom arc terranes,amag-

matic-hydrothermal recciasare not considered ur-

ther.

Magmatic-HydrothermalBreccias

Pipes elated to intrusions

General remarks: This section describesbreccias,

confined o singleor multiple pipes, that possess

closegenetic connectionwith unaltered and unmin-

eralized intrusive rocks, either batholiths or stocks.

There seems o be a gradation rom districtscharac-

terized by one or morebrecciapipesassociated ith

fresh intrusiverocks o districts n which the pipes

constituteonly parts of larger volumesof pervasive

alteration-mineralization f porphyry type (see be-

low). Although most of the brecciassummarized n

Table '2are demonstrably ot partsof porphyrysys-

tems,and thereforeare not underlainby porphyry-

type mineralization,CopperCreek (Grimour,1977)

andKidston R. H. Sillitoe,unpub. ept., 1980) could

be the high-levelmanifestationsf largelyconcealed

bodiesof porphyrycopper-molybdenumndClimax-

type porphyry molybdenummineralization,respec-

tively.

It is clear from Table 2 that there is no agerestric-

tion for mineralizedbrecciapipes.Known examples

range from Archcan hroughProterozoicand Paleo-

zoic to Meso-Cenozoic. Most of the western American

brecciapipes are Mesozoicor Cenozoic n age, al-

though he absence f examplesn Table 2 younger

than Eocene s noteworthy. This observation s inter-

preted o reflecteraplacement f the breccias t hyp-

abyssal epths 1-3.6 kin; Soand Shelton,1983) and

the time required for their subsequent nroofing.

Characteristics: The intrusion-related breccias un-

der considerationere are restricted o pipes hat may

occur ndividuallyor in closelyspaced lustersof up

to 200 or more (Table2). Pipes also ermedchimneys

or columns)are generallyroughly circular to ovoid

in cross ectionandpossesserticaldimensions hich

are observedor inferred to be several imes greater

than their maximum horizontal dimensions. Horizon-

tal dimensions re commonly n the range of 50 to

300 m but are as great as 1,300 X 900 m at Kidston

(PlacerExplorationLtd., 1981) or as ittle as 3 m in

the Cabeza de Vaca district (Sillitoe and Sawkins,

1971). The full vertical extent of a pipe is nowhere

observable, lthoughminimumverticaldimensionsf

725 to 860 m are known for four districts Table 2).

Unless ilted subsequento emplacement, ipes are

only uncommonlynclinedat more than 15 from the

vertical.

Severalexamples f partly bifid pipes have been

recorded. The San Antonio de La Huerta pipe in

Sonora,Mexico, divides downward into two prongs

(R. H. Sillitoe, unpub. rept., 1975), whereas the

Childs-Aldwinklepipe in the Copper Creek district

(Kuhn,1941), the Ilkwangpipe (Fletcher,1977), and

the A-B pipe at Inguar/tn Sawkins, 979) all bifurcate

upward.

The contacts etweenbrecciapipesand their wall

rocks are commonly abrupt, and in many cases,

markedby a zone of closelyspaced ertical fractures

(or sheeting) rom 1 to 5 m wide (Fig. 1). Fractures

may be mineralizedor lined with fault gouge.Sheet-

ing is not presentas a singleuninterruptedannulus

but is made up of severalstraight o gently curved

bandsof fractures,whichcommonlyend to be more

markedlycurvedat oneof their ends.Overlapof these

several engthsof sheeting ends o give a polygonal

outline to pipes. Alternatively, breccia and unfrac-

tured wall rocksmay grade nto eachother over dis-

tances of several meters.

The upward and downward erminations f pipes

are not commonlyobserved.Locally, as in the San

Pedro de Cachiyuyodistrict (Sillitoe and Sawkins,

1971), pipes are seen o be cappedby dome-shaped

roofsoverlainby columns f alteredbut unbrecciated

rock,and t seems nlikely hat manyof thesebreccia

pipes approached he palcosurface.Where the bot-

tomsof pipeshavebeen observed, s n the A-B pipe

at Inguarm Sawkins, 979) and he SanAntoniode

La Huerta pipe (R. H. Sillitoe, unpub.rept., 1975),

they are irregular but grossly iat, and breccia ter-

minates abruptly against ess altered intrusive or

country ocks.The CopperPrincepipe n the Copper

Creek district s underlain y a mineralized pen is-

sure (Kuhn, 1941; Joralemon,1952), whereas the

lensoidExtensi6nSanLuis pipe at Inguarm s tran-

sitional ownwardo a shear one V. F. J. Escand0n

unpub. alk 1974).

The breccias re normally haracterized y angular

to subroundedragments anging n size from a few

centimeterso severalmeters nd, ocally,severalens

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TABLE 2.

SelectedExamplesof Mineralized

Locality Host rocks Age (m.y.)

No. of pipes Surface Vertical

(total/ dimensions dimension

mineralized) (m) (m)

Fragment

form Rock flour

Tribag, On- Granite, mafic vol-

tario, Canada canics, elsite

1,055

4/3 up to 700 X 300 >860 Angular Absent except

East breccia)

Chadbourne, Andesitic + rhyo-

Ontario, Can- litic volcanics

ada

Golden Sun- Calcareous sedi-

light, Mon- ments, atite por-

tana phyry

Victoria, Limestone, sand-

Nevada stone

Copper Creek, Granodiorite,an-

Arizona desitic volcanics

Ortiz, New Quartzite, pyroclas-

Mexico tics

Los Pilares, Latitic q- andesitic

Sonora, volcanics

Mexico

Washington Andesitic, atitic

dist., Sonora, q- trachytic

Mexico volcanics

La Colorada, Trachytic q- rhyoli-

Zacatecas, tic pyroclastics

Mexico

Inguarfm,Mi- Granite,granodio-

choactn, rite, granodiorite

Mexico porphyry

Tu'rmalina,

Peru

Granodiorite

Archean

Early Ter-

tiary

135(?)

68

Oligocene

-55(?)

45.7

53.6 t

35.6

Tertiary

1/1 300 x 120 >750

1/1 200 x 200 , >550

>4/1 >200 x 75 >800

>200/8 up to 180 >270

3/1 970 X up to 600 >150

1/1 600 X 300 >725

13/2 up to 100 >400

9/6 up to 100 X 40 >300 (600

inferred)

10/3 up to 600 X 300 225

1/1 150 X 150 >600

Angular Absent

Angular to sub- Absent

rounded

Angular, ocally Present n

rounded parts

Angular o Absent

rounded

Angular o Locally pres-

rounded ent

Angular Absent

Angular or Present n

rounded somepipes

Mainly Abundant

rounded

Angular to 10 to >50%

rounded

Angular o sub- Absent

rounded

San Pedro de

Cachiyuyo,

Chile

Cabeza de

Vaca, Chile

El Bolsico,

Chile

San Francisco

de Los Andes,

Argentina

Granodiorite

Granodiorite, an-

desitic volcanics

Quartz diorite,

quartz diorite

porphyry

Sandstone, shale,

siltstone

Paleocene

24/10 up to 250 X 130 216

62 >100/5 up to 70 > 100

Paleocene

Late Carbon-

iferous-

Early

Permian

4/1 180 x 95 >170

3/1 70 x 15-30 >35

Angular o sub- Absent

rounded

Angular o

locally

rounded

Angular o

rounded

Angular

Absent

Abundant

Absent

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ORE-RELATED RECCIASN VOLCANOPLUTONICRCS 1471

Breccia Pipes Related to Intrusive Rocks

Hydro-

thermal

alteration Principal hypogene

(t = tour- metallicminerals

maline) (in order of abundance)

Principalgangue

minerals

Structural

control

Related

intrusive rock

Ore reserve

and/ormined

(M = million, t

= metric tons)

Reerence

Sericitic, Pyrite, chalcopyrite, Quartz, calcite,

chloritic, pyrrhotite,magnetite, ankerite, au-

argillic molybdenite montite

Sericite-cal- Pyrite

cite

Silicifica-

tion, seri-

citic

Calc-silicate

Sericitic (t),

K silicate

Sericitic

Sericitic,

chloritic

Sericitic, K

silicate,

chloritic

Sericitic

Propylitic

(t)

Sericitic,

chloritic

(t)

Sericitic (t)

Sericitic (t)

Sericitic (t)

Silicifica-

tion (t)

Pyrite, chalcopyrite,

bornitc, galena,sphal-

erite

Pyrite, chalcopyrite


molybdenite, ornitc

Pyrite, magnetite,hema-

tite, scheelite

Specularitc, yrite, chalo

copyrite,scheelite


molybdenite, chee-

lite

Pyrite, sphalerite,ga-

lena, tetrahedrite,

chalcopyrite

Chalcopyrite,pyrite,

scheelite


molybdenite, rseno-

pyrite, wolframite,

scheelite



specularitc,scheelite

Chalcopyrite,molybde-

nite, pyrite, specular-

itc

Pyrite, arsenopyrite,

bismuthinite, chalco-

pyrite

Quartz, albite,

calcite, anker-

itc, dolomite

Quartz, barite,

sericite, fiuora-

patire

Calcite, diopside,

garnet, quartz

Quartz, sericite,

chlorite, tour-

maline

Calcite

Quartz, calcite,

chlorite

Quartz, tourma-

line

Quartz

Quartz, epidote,

tourmaline,

chlorite, cal-

cite

Quartz, tourma-

line

Quartz, tourma-

line

Quartz, tourma-

line, K-feld-

spar,calcite

Quartz, tourma-

line, sericite,

calcite

Tourmaline,

quartz

Faults, oints,

contacts

Fault related

Not recognized

Absent

Probablyabsent

Not recognized

Not recognized

At leastpartly

fault related

Not recognized

N 20 W + N

70 E faults(?)

Not recognized

Absent

Absent

Not recognized

Jointing

Felsite stock(?)

Syenite(?)

body

Latite por-

phyry

stock(?)

Quartz fatire

porphyry

stock(?)

Latite

Quartz latite

porphyry(?)

Unknown

Granodiorite

pluton(?)

Quartz monzo-

nite(?)

Granodiorite

q- granodio-

rite por-

phyry stock

Granodiorite

pluton

Granodiorite

pluton

Granodiorite

pluton

Granodiorite

pluton

Granodiorite

pluton

i Mr, 1.6% Cu;

40 Mt, 0.2%

Cu (Breton

pipe)

1.8 Mr, 4.5

ppm Au

31 Mr, 1.9 ppm

Au

2.2 Mr, 2.4%

Cu, 0.05% Bi

3,714 t Cu,

3,151 t Mo

7 Mt, 1.7 ppm

Au, 0.05%

WOa

19 Mr, 2.6%

Cu; 44 Mt,

0.8% Cu

1.2 Mr, 1.7%

Cu, 0.14%

W, O.O6%

Mo

2 Mr, 4% Pb

q- Zn, 120

ppm Ag

6 Mt, 1.2 to

1.5% Cu,

0.04% WOa

13,600 t Cu,

1,360 t Mo

>0.6 Mt, 3.7%

Cu

High-gradeCu,

minor W

2.7 Mr, 1.27%

Cu, 0.12%

Mo

38 t Bi

Armbrust (1969),

Blecha 1974),

Norman and

Sawkins 1985)

Walker and Cregh-

cur (1982)

Porter and RipIcy

(1985)

Atkinson et al.

(1982)

Kuhn (1941), Jora-

lemon (1952),

Simons 1964)

Lindquist 1980),

Wright (1983)

Wade and Wandtke

(1920), Locke

(1926), Thorns

(1978)

Sillitoe (1976),

Simmons and

Sawkins 1983)

Albinson 1973)

Escand6n unpub.

talk, 1974), Silli-

toe (1976),

Sawkins 1979)

Carlson and Sawk-

ins (1980)

Sillitoe and Sawkins

(1971)

Parker et al.

(1963), Sillitoe

and Sawkins

(1971)

Pimentel (1979), C.

Llaumett (unpub.

rept., 1981)

Llambias and Mal-

vicini (1969)

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14 7 2 RICHARD H. SILLITOE

TABLE2--(Continued)

Locality Host rocks Age (m.y.)

No. of pipes Surface Vertical

(total/ dimensions dimension Fragment

mineralized) (m) (m) form

Rock flour

Y16j'firvi, in- Intermediate volca- 1,800 to

land nics 1,900

Ilkwang, Quartz monzonite 69

S. Korea

2/1 700 X 5-80 380 Angular Absent

1/1 80 X 50 >100 Angular to Absent

rounded

Khao Soon, Argiilaceous edi-

Thailand ments

Redbank, Trachytic volcanics,

Northern dolomite, sand-

Territory, stone,shale

Australia

Triassic(?)

1,575(?)

Kidston, Gneiss,granodio- Middle Car-

Queensland, rite boniferous

Australia

1/1 800 X 400 >300 Angular to sub- Absent

rounded

50/9 up to 135 >330 Angular Generally ab-

sent

1/1 1,300 X 900 >250 Angular o sub-

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Hydro-

thermal

alteration

(t = tour-

maline)

Principalhypogene

metallic minerals

(in order of abundance)

Principal gangue Structural Related

minerals control intrusive rock

Ore reserve

and/or mined

(M = million, t

= metric tons)

Reference

Silicifica-

tion,

chloritic

Sericitic

Sericitic, si-

lieifiea-

tion

Arsenopyrite,chalcopy- Tourmaline Not recognized Granodiorite 4 Mt, 1.4% Cu,

rite, pyrrhotite, pluton 0.04%

scheelite WOa

Pyrrhotite, chalcopy- Quartz, tourma- Absent Quartz monzo- 3,500 t Cu, 40

rite, arsenopyrite, line nite stock t W

wolframite

Ferberite, pyrite Quartz Nearby fault Unknown W

K-feldspar- Chalcopyrite

chlorite

Sericitic, Pyrite, sphalerite, ga-

carbonate lena

Dolomite, quartz, E to NE linea- Trachyte

chlorite ments plugs(?)

Quartz, calcite, Not recognized Rhyolite dikes

sericite + stock(?)

Himmi et al. (1979)

Fletcher (1977)

Ishihara et al.

(1980)

3.5 Mt, 1.8% Orridge and Mason

Cu (1975), Knutson

et al. (1979)

39 Mt, 1.76 Bain et al. (1978),

ppm Au Placer Explora-

tion Ltd. (1981)

partially detachedand, in places,disaggregatedo

produce abular fragments.

Intrusion-related reccias arely reveal evidence

to suggest ppreciable erticaldisplacementf frag-

mentsduringpipe emplacement.n fact, n partsof

somepipes, ragments ppearmerely o havebeen

pulledapartandcanbe fittedback nto heiroriginal

positions s n a jigsaw Fig. 4). Normally he lithol-

ogiesof fragments loselymatch hoseof their wall

rocks,hereby ommonlyroducing onolithologic

breccias. hereseveralock ypes djoin pipe, ittle

mixingof fragments f different ithologies as aken

placeandcontacts eyond he pipemaybe extended

through he breccia Fig. 5). There s, however,nor-

mallya relativelysmalldownward isplacementf all

fragmentst most evelswithina pipe.Thishasbeen

quantifiedby comparison ith distinctivewall-rock

lithologies t several ocalitiesand amounts o 25 m

at WashingtonSimmonsndSawkins, 983), 100 m

at RedbankOrridgeand Mason,1975) andTribag

(Normanand Sawkins,1985), >125 m at Panuco,

Mexico (Buchanan, 983), and a maximumof 160 m

at Los Pilares Wade and Wandtke, 1920; Fig. 5).

Locally,however, here s evidenceor somemixing

andupward ransport f fragments,sat La Colorada

and Kidston.

Breccias re commonly ocated n the upper parts

of, or immediatelyabove, plutonsor stocks,or are

distributed around their sloping margins. n some

districts,pipesmay be interpreted o have extended

from the upper parts of a pluton into its roof rocks.

In severaldistricts,ncludingsomeconfined o sizable

plutons,smallvolumes f fine-grained orphyritic n-

trusive rock are temporally,spatially,and probably

geneticallyassociated ith the brecciationprocess.

The intrusive ock mayoccurasdikesandsmallbod-

ies, angularbreccia fragments,and irregular, partly

disaggregatedmasseswithin the pipe. The last type

of occurrence rovides vidence hat the magmawas

plastic during brecciation. These minor intrusions

have been emphasized rom the Chilean districts

(Parker et al., 1963; Sillitoe and Sawkins, 1971),

CopperCreek (Simons, 964), Tribag (Blecha,1974),

Victoria (Atkinsonet al., 1982), and Kidston (Placer

ExplorationLtd., 1981), and suggesthe presence n

depthof largerbodiesof the same ntrusive ockwith

which pipe formationwas inked. Sucha body was

encountered y drillingsome 00 m beneath he out-

crop of the Breton pipe at Tribag (Blecha, 1974).

Table 2 suggestshat there s no generalagreement

on he role of structuren localization fbrecciapipes.

The impressions gained rom the literature that the

importanceassigned o structuralcontrol saysmore

about he proclivityof the observer han t doesabout

the localizationof breccia pipes This statement s

borne out by comparing he interpretations f Kuhn

(1941) and Simons 1964) for the Copper Creek dis-

trict. On the basis of available evidence, it is tenta-

tively concluded hat major regionalstructures lay

little part in brecciapipe formationand, f structural

control s significant,t is likely to be by minor aults,

fracturesand oints. One of the most detailed struc-

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FIG. 1. A typicalsheeted onebordering brecciapipe. lk-

wang, southernKorea.

tural studies f a brecciapipe and ts environswas

undertakent Chacritas,hile,byReyes ndCharrier

(1976),whoconcludedhatneither he position or

the shape f the pipewasstructurallyetermined.

-?

FIG. 3. Spheroidal ragmentand ts mould.

Alteration and mineralization: Most intrusion-re-

latedbreccias arrycoppermineralization, lthough

molybdenum,ungsten nd/orgoldarecommonlylso

economicallymportantcommoditiesTable 2), and

a minor onnage f bismuth re wasexploited t San

Franciscode Los Andes (Llamblasand Malvicini,

1969). Breccias t Chadbourne, oldenSunlight,Or-

tiz, and Kidston Table2) are exploitable olely or

their gold (and subordinate ilver) contents.A few

breccias are different and contain silver-lead-zinc or

tungstenmineralization Table 2).

All breccias f this ypeunderwento some egree

thehydrothermaleplacementndopen-space-filli

stageseferred o below,a factwhichstrongly ug-

gests hat alterationand mineralizationwere neces-

saryconsequencesf the brecciation rocess. ow-

ever, (50 percentof brecciasn anydusterof pipes

are ore bearing (Table 2), a characteristichat has

often rustratedhe explorationistJoralemon,952).

Sericitizations the mostcommon lteration ype

FIG. 2. Shingle reccia emented y massiveourmalinerom

a breccia ipe.Yabricoya istrict,Chile.Geology ickhandle s

FIG. 4. Typical igsawbrecciacementedby tourmalineand

sericitized long ragmentmargins nd ractures. pproximately

one-third natural size.

5/21/2018 015 Sillitoe

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ORE-RELATED BRECCIAS 1N VOLCANOPLUTONIC ARCS 1475

E W

SHEEEDI

' '"'Kv ,1 VOLCANIC

1001, , 2.5-3.0uORE=

0 melers ]00

FIO. 5. Crosssection hrough the Los Pilaresbreccia pipe,

Sonora,Mexico. It shows he distributionof copper orebodiesas

an annulus n the mginal pt of the breccia well as smaller

bodieswithin it, and he depression f the latite-andesite ontact

within the brecciapipe. Taken from Wade and Wandtke (190),

with lithologicnomenclature rom Thorns 1978).

in the brecciapipesdiscussedn this sectionand is

commonly ccompaniedy tourmalineTable2; Figs.

2 and 4). Chloritizationand silicificationwere also

commonlydeveloped,propylitic and K silicate as-

semblagesrerecordedn a few pipesor parts hereof,

and calc-silicate lteration s presentat Victoria (At-

kinson et al., 1982). Alteration generally ends

abruptlyaround he marginsof pipes, especially t

sheeted ones,but in someexamples e.g., Ilkwang;

Fletcher,1977) mayextenda few metersor even ens

of meters nto the wall rocks.Marked changesn al-

teration ype are observedn somepipes:sericitiza-

tion changes ownward o propylitization t Los Pi-

lares WadeandWandtke, 1920) and ransitionsrom

sericitic o K silicateassemblagesavebeen noted n

the lowermostportionsof pipesat Washington Sim-

mons ndSawkins, 983), Childs-Aldwinkle, opper

Creek district (Kuhn, 1941), and Los Verdes, Buena

Esperanzadistrict, Mexico (R. H. Sillitoe, unpub.

rept., 1975).

The alteration replacement) tagen brecciapipes

tookplace mmediately fter,andperhaps lsoduring,

fragmentation.t was ollowed y anepisode f open-

space illing, during whichboth gangueand metallic

mineralswere precipitated Table 2). Both are com-

monlycoarse rainedandwell crystallized, ndpeg-

matitic extures re common.n copper-bearingipes,

the open-space-filling tage commencedwith the

outward growth from fragmentsof tourmaline and/

or quartz, followedby any scheelite,wolframite,or

arsenopyritend inallyby pyrite (and/orpyrrhotite),

chalcopyrite,ndmolybdenite. phalerite ndgalena

followedby carbonates nd/or ate quartz may con-

stitutea final illing.Ore minerals t Inguartn, 1Bol-

sico,andLa Colorada re dispersedn interfragment

rock lour nstead f present sopen-spaceillings. n

contrasto manybrecciavarieties seebelow),most

of the intrusion-related breccias considered here un-

derwentonly singlemineralization ventsand gen-

erally ackevidenceor rebreeciation f earlymin-

eralization;Golden Sunlightand Kidstonare, how-

ever, exceptions.

Instead f beinghomogeneouslyineralized,many

breccias ontainonly restrictedvolumes f ore-grade

material.This is commonlypresentalong part of a

pipemargin,mmediatelydjoininghe sheeted one,

asat Victoria,LosPilares Fig. 5), Turmalina,E1Bol-

sico Fig.6), lkwang, ndSan ranciscoeLosAndes.

At Los Pilares, he marginalannulus f ore thickens

substantiallyt both endsof the ovoidpipe. At Y18-

jSrvi, the four steepore shoots re locatedclose o

the northeastern nd of the extremelyelongatepipe

(Himmiet al., 1979).Enhanced ermeabilityesulting

from more originalopen spacebetween ragments

andproximity o the sheeted one, s believed o ac-

count or the higher-grademineralizationn the mar-

ginalpartsof pipes.The highest radeof goldore at

Kidstonoccursat the southwestern nd of the pipe

in an exceptionally ide (up to 300 m), inward-dip-

ping,quartz-filled heeted one,whichcutsPrecam-

briangranitewall rocks, he breccia, ndpostbrecci

rhyolitedikes Bainet al., 1978; Fig. 7).

Ore maybe restricted o portions f pipe nteriors.

The goldorebody t Ortiz coincides ith the part of

the star-shapedrecciahat carrieshe east ock lour

(Lindquist,1980). Orebodies n the Breton breceia

at Tribag are confined o domal ractures,which are

oval to circular n plan, extend nto the wall rocksof

the breccia Blecha,1974), and probably esulted

from late subsidenceNormanand Sawkins, 985).

Total Cu O.30

7 .... Mo '0.25

,,

,,,. .,

'"' ' "

','i

' ' ^ I:j 's

o " ' 'g--- ..... ' "--- o

IN SlTU BRECCIATION ' ,

CSTS:L^STS '?STCSTS

SHEED o 2 5oo m SHE.D

I I i i

ZE ZE

. 6. Relationship etwee. copper a.d molybde.um co.-

re.rs a.d brecciachactedstics acrosshe 1 Bolsico recciapipe,

Chile. Mappi a.d sampli. carried o.t alo the SV] adit o

the 3,030-m level. Compiled rom PimeteJ ]gTg) ad C. Jau-

mett (u.pub. rept., ]gS]).

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Sheeted

uartz veins

Contact

LateRhyolitemicrogranite'.-'.':

aleozoic Breccia pipe

PrecambrianGranite

etamorphic rocks

FIG.7. Surface apof hebreccia ipeatKidston, ueensland,

Australia, o showdistribution f gold-bearing nnular ractures

andpostbrecciaikes. aken romBainet al. (1978).

At Chadbourne,gold is concentratedn cylindrical

shoots fbreccia, up to 40 m wide, that have he same

plungeas he pipe (Walker and Cregheur, 1982).

Metals are commonly onedat the scaleof a pipe.

For example, t Turmalina he molybdenum ontent

exceedshat of copper n the upperpartsof the pipe

but decreases teadilydownward (Carlsonand Saw-

kins, 1980), whereas n the Childs-Aldwinklepipe at

Copper Creek the molybdenum ontent emainsun-

changed0.6-1.2%), but the coppercontent ncreases

from i percent at the top to 6 to 8 percent on the

800-ft level (Kuhn, 1941). In contrast,molybdenum

increasesn gradedownward n the Washingtonpipe

(Simmons nd Sawkins,1983). Horizontal metal zon-

ing may alsobe present,as at E1 Bolsico,where Pi-

mentel (1979) reporteda zonation rom copper-mo-

lybdenum hroughmolybdenum o a low-gradecore

inward from the sheetedcontact Fig. 6).

Studies of fluid inclusions n open-space-filling

minerals from intrusion-related breccias reveal that

the mineralizing luidsranged n temperature rom

310 to 470C and n salinity rom 1 to 50 equiv.wt

percentNaC1 seeSoandShelton,1983). The higher

temperatureand higher salinity luidsare similar o

those nvolved n early (K silicate) tages f porphyry

deposit ormation (Sheppardet al., 1971) and like

them maybe reasonablynferredasat leastpartly of

magmatic-hydrothermal rigin.

Origin: All the principal mechanismsor breccia

pipe formationwere proposed, t least n basic orm,

many years ago and recent studiesof breccia pipe

formation have all utilized one of these mechanisms

with at most minor modification or embellishment

(Table 3). Bearing n mind the downwardmovement

of fragmentsand the existenceof up to 20 percent

openspacen manypipes,anybrecciationmechanism

must be capableof generatingan appreciablevoid.

Five hypotheses ave been entertained or the pro-

ductionof a void (Table 3): (1) localizeddissolution

and upward removalof rock material by fluids re-

leased rom coolingmagma Locke,1926), (2) release,

perhapsexplosively,of volatiles rom magmawith

material carried physicallyupward (Walker, 1928;

Emmons,1938), (3) downwardmovementof magma

by either shrinkage or withdrawal (Hulin, 1948;

Perry, 1961), (4) development f a bubbleon the roof

of a stockor plutonby accumulation f exsolvedluids

(Norton and Cathies,1973), and (5) productionof

dilatent zoneson major faults during displacemen

(Mitcham, 1974).

The first four hypothesesall account for the

ubiquitous associationobserved between breccia

pipes, ntrusive ocks,and alteration-mineralization

whereas the fifth does not and therefore is discounted

asa generalbrecciationmechanism.

The four proposedmechanismsor breccia pipe

formationmay not necessarily e considered s mu-

tually exclusive nd might all contribute n varying

degrees o brecciationf consideredn the contextof

Burnham's 1979, 1985) model for energy release

during eraplacement nd solidification f hydrous

magmas t high crustal evels.As quantified y Burn-

ham 1985), energy s dissipatedromhydrousmagma

during exsolution f an aqueous luid phaseby the

second boiling reaction (water-saturatedmelt--

crystals aqueousluid), and hen by decompression

of both the exsolved ow-densityaqueous luid and

the water-saturatedresidual melt. Decompression

causes xpansion f previouslyexsolved luid, exso-

lution of additional luid, and the expenditureof a

greateramountof energy handuringsecond oiling.

Asdiscussedy Allman-Wardet al. (1982) andBurn-

ham (1985), processesriggered by and accompa-

nyingdecompressionppear o account atisfactorily

for the formationof brecciapipes, especiallywhere

fluid is released rom the top of a restrictedcupola

(givinga singlepipe) or is preferentiallychanneled

by inhomogeneoustructurallypreparedwall rocks

above a more extensivepluton (giving a swarm of

pipes).

Violent and rapid expulsionof fluid from magma

wouldbe capable f generating teep ensile ractures,

or reopening xisting aultsor fractures, nd further

widening hem by hydraulic ractureof their walls.

Decompression ausedby propagationof fractures

into higher evel, owerpressureperhaps ydrostatic)

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TABLE3. Some SuggestedMechanisms or Formation of Breccia Pipes

Principal mechanism Modification

Violent releaseof fluid from magma Emmons,1938; Llambias

and Malvicini, 1969; Knutsonet al., 1979; Allman-Ward et

al., 1982; Burnham,1985; PorterandRipley, 1985 )

Subsurface hockmetamorphismGodwin, 1973)

Collapsedue to excavation f exsolved apor bubble (Norton

and Cathies, 1973)

Collapse nto void formed by rock dissolutionmineralization

stoping;Locke, 1926; McKinstry, 1955; Sillitoe and Sawkins,

1971; Mills, 1972)

Rock dissolution longminor aultswith only subsidiary ol-

lapse Kuhn,1941 ; JohnstonndLowell, 1961 )

Readjustment pon coolingof underlyingmagmawith only

subsidiary ollapseButler, 1913 )

Collapse nto void formedby magmawithdrawal Perry, 1961;

Blecha, 974;Atkinson t al., 1982 )

Collapsento void formedby shrinkage ue to coolingof

magma Hulin, 1948)

Collapse nto dilatent zone formed on major fault (Mitcham,

1974)

Chemicalbrecciation n situ followingpipe formationby an-

other mechanismSawkins,1969)

Combinedwith decreasen magma ressureArmbrust, 969 )

Due to magmaadvance hydraulic amming;Kents, 1964)

Due to magma dvance nd followedby solution-inducedol-

lapse Fletcher,1977 )

With ventingof rock flour to give void for collapse Scherken-

bach,1982; SimmonsndSawkins, 983 )

Following racturingdue to magmatic ulsationsReyesand

Charrier, 1976 )

Due to releaseof fluid (WalkerandCregheur,1982 )

Mechanismroposedor single ipeor groupof pipes

regimeswould result in increased luid release from

themagma, ndan ncreasedateof fluid"streaming"

(Burnham, 985),bothofwhichcould esult n mixing

and milling of fragments,productionof rock flour

matrix, and varying degreesof upward transportof

material. Such conditions would also facilitate intru-

sionof smallvolumes f magmanto and aroundde-

velopingbrecciapipes.

If fluid pressures ropped o valuesbelow those

necessaryo maintain he channel penat depth,cav-

ing and spalling f the wallsof the partly evacuated

conduitmight be induced.Open-space nd shingle

breccias, heeted ones,arching oof fractures, nd

exfoliated ragmentsmight all be produced n this

way. The close association f rock flour and open

space recciasn the samepipe swarmand, ocally,

even n a singlepipe accordswell with such luctua-

tions n fluid pressure uringdecompression.

It is uncertain f the fracturing nd fragmentation

involved n the generationof sheetingand shingle

breccia anbe attributed olely o the effects f de-

compressionr whether he preexistence f an array

of concentric nd radial ractures roduced y up-

ward-directedfluid)pressuresReyes ndChattier,

1976) is also equired.As a cause or hypogene x-

foliation n theseand other breccias seebelow), an

instantaneousrop in confiningpressureduring de-

compression Godwin, 1973; Sillitoe, 1976; Allman-

Ward et al., 1982) is preferred to other proposed

mechanisms,uchas nterclast ttrition e.g.,Gavasci

and Kerr, 1968), mechanicaldetachmentof altered

clast ims (e.g., Simons,1964; Sillitoe and Sawkins,

1971), and hermalspalling f fluid-heated lastse.g.,

McBirney, 1959; Warnaars, 1983).

Featuressuchas fragment oundingand mixing,

rock flour generation,and differential vertical dis-

placementof fragmentshave been considered y

many workers e.g., Mayo, 1976; Woolseyet al.,

1975; McCallurn, 1985) to be compatiblewith the

operation of fiuidization as a transportmechanism

during he formation f subsurfacereccias,ncluding

some of those under consideration in this section.

However, n view of the great disparity n particle

sizesn rock lourbreccias,t seems nlikely hatmore

than a small ractionof a brecciawasever truly flu-

idized (cf. Wolfe, 1980). If particlesof a givensize

were fluidized, hen finer grainedmaterialwouldun-

dergoelutriation o accumulate t the top of the pipe

above ines-depleted reccia cf. Wilson, 1980); this

vertical zoning s never observed. t is more likely

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that brecciaswhich underwent significantupward

movementdid so as slurries, n much the sameway

as the chaotic ragmentassemblagesn debris flows

(P. T. Delaney, writ. commun.,1984).

A discretevoid filled by fluid could alsobe pro-

ducedon a pluton's oof asa resultof either localized

liftingof the roof ocks uring luid eleaseBurnham,

1985; Fig. 8a) or, perhaps essprobably,by with-

drawal of magma Perry, 1961; Fig. 8c). Burnham

(1985) calculated hat energy released nstanta-

neouslyduring decompression y a unit massof

magmawould be sufficient o lift an equivalentmass

of rock for a height of 990 m, given no frictionalre-

sistance, nd therefore confirmed he feasibilityof

generatinga void in this way. The reality of fluid-

filled voidsat the topsof magmachamberss con-

firmedby the existence t Panasqueira,ortugal,of

a lensoidmassof quartz that was precipitated n a

cavityat the apexof a granitecupola Kelly andRye,

979). However,brecciapipe formationwas nhibited

at Panasqueira ither because luid pressures ere

insufficient o instigatehorizontalextensionailure or

because he 14-m height of the cavity was too little

to induce appreciablecaving.

Fluid corrosion f quartz-rich ocksmight alsobe

effectiven producing r enlarging oidsnear he tops

of plutonsor in their immediate oof rocks Locke,

1926; Fig. 8b). The mechanisms viable duringcool-

ing of a fluid from 520 to 340C at a constant res-

surenot exceeding 00 bars the regionof retrograde

solubility or quartz; Fournier, 1983). Sericitization

of feldsparslso esultsn the production f significant

void space 15-20% of the feldsparvolume; W. C.

Burnham,writ. commun.,1984). Evidence or partial

dissolution f igneous ocks s providedboth by the

corrodedand porous ragments ound in somebrec-

ciasandby the existence f unbrecciatedeplacement

pipes.These are particularlycommonnear the roofs

a b c

_ ___

.....

...............................................................

G. 8. Schematic epresentationofbreccia pipesabovea plu-

ton roof that were formed with three different types of transitory

void development: a) domingof roof rocksby accumulation f

exsolved luid, (b) dissolution f roof rocksby exsolved luid, and

(c) magmawithdrawal.

WHIPSTICKMINE Extrapolatedormerposition f contact

:::++::':{.REPLACEMENTiPE

0 meters 100

FIG. 9. The bismuth-and molybdenum-bearing hipstick e-

placementpipes, New SouthWales, Australia.Taken from Weber

et al. (1978).

of felsic plutons n easternAustralia and comprise

steep,narrow 1-10 m), branching odies,of roughly

circular o ellipticalcross ection, illedwith remnants

of sericitizedntrusive ockandpegmatitic ggregates

of quartz,molybdenite, ismuthinite, olframite, nd

other minerals Blanchard,1947; Fig. 9). The evi-

dence favorsproductionof premineralizationopen-

ingsby rock solution,with the pipesperhaps ot being

wide enough to have permitted caving and breccia

formation McKinstry,1955).

Geometric elationships ear he bottomsof pipes,

asschematizedn Figure 8, may prove useful or dis-

tinguishing etweenvoids ormedby fluid overpres-

sures, ock dissolution, nd magmawithdrawal.

In most ntrusion-related reccias,only one brec-

ciation event occurred and was probably accom-

plishedby low-densityaqueous luids (W. C. Burn-

ham, writ. commun.,1984). It was followedby the

open-space-filling tage of mineralization, n which

high-salinity luidsplayed an importantrole (see So

andShelton, 983). Fluid flow hroughmanybreccias

seemso havebeensluggishf the coarse,ocallypeg-

matitic texture of ore and gangueminerals s attrib-

uted o slowcrystallizationather han o a low degree

of fluid supersaturation.

Porphyry-typedeposits

General remarks:Most porphyry systems, e they

dominated by copper, molybdenum,gold, tin, or

tungsten, ontainone or morevarietiesofbreccia cf.

Richard, 1969). Breccias re reported rom 50 to 60

percent of porphyry systems, s in westernCanada

(Seraphim and Hollister, 1976) or the Philippines

(Sillitoe ndGappe,1984). More arecertainly resent

but either are not exposedor have not been recog-

nized.The breccias ange rom minoradjuncts o de-

posits o the economicallydominantparts of some

porphyrysystems, sat BossMountain,Copper Flat,

Cumobabi,Los Bronces Disputada),and Ardlethan

(Table 4). Even porphyry-typemineralization s old

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asearly Archcan s well endowedwith breccias Bar-

ley, 1982).

Characteristics:hemost bundantndwidespread

brecciasn porphyry ystemsre grouped nder his

category.They exhibit a broad spectrumof charac-

teristics Table 4), many of them sharedwith the in-

trusion-related recciapipesdealt with above.

The breccias ommonly ccuras ensoid,ovoid,or

circularpipelikebodieswith steep o verticaldips

(Table 4). Pipesmay.occur inglyor in groupsof as

many as 25 at Copper Basin (Johnstonand Lowell,

1961) and 35 at Cumobabi (Scherkenbach t al.,

1985). Additionalgeometriesncludedikes, rregular

bodies,carapaceso dikesor plugs e.g., IslandCop-

per, Cargill et al., 1976; andE1Abra, Ambrus,1977),

and annularconfigurationse.g., aroundan unbrec-

ciatedcore at Duluth, Cananea,Perry, 1935).

The brecciabodies ange n horizontaldimensions

from a few meters to a maximum of 2 X 0.7 km for

the composite ipe at LosBroncesWarnaars, 983).

Known vertical dimensions are likewise considerable

and commonly ange from 500 to 1,000 m at Red

Mountain Quinlan,1981; Fig. 10), Cananea Perry,

1935, 1961), and Ardlethan Paterson, 976) to at

least1;100 m at LosBroncesWarnaars t al., 1985).

An upward ncreasen the rockvolumeoccupied y

breccia s recorded romsome ocalities, .g., Sierrita-

Esperanza West and Aiken, 1982) and Toquepala,

Peru (Zwengand Clark, 1984).

The formof pipelike recciasn porphyry ystems

is, in general,ess egular han hat of brecciapipes

divorced rom porphyry ystems.rregularembay-

ments and offshoots from the main breccia bodies are

commonplacendcontacts ith the enclosing arts

of the porphyry ystem re commonly radational,

althoughheycanbe sheeted ndabrupt e.g,Whim

Hill breccia t Santa ita;Norton ndCathies, 973).

A numberof examples f both he topsandbottoms

of porphyry-relatedbrecciashave been described.

Examples f bottoming, haracterizedy a rapid

transition from breccia to stockworked or fractured

rock, nclude he Transvaal reccia t Cumobabiat

350 m; Scherkenbach, 982) and the Whim Hill

breccia t Santa ita atabout100 m as wo separate

lobes;NortonandCathies,1973). Upward ermina-

tions fbrecciasave eendescribedrom heCapote

pipe at Cananea, which fades out into a mineralized

limestoneorizon 00m beneathhesurfacePerry,

1935; Meinert, 1982), and the 148-155 pipe at Red

Mountain,which tops out about 1,200 m below the

surfaceQuinlan, 981;Fig. 10). Given hisevidence

from Cananeaand Red Mountain,and observations

elsewhere e.g., CopperFlat, Dunn, 1982; andSanto

Nifio, Philippines, illitoeand Gappe,1984) sug-

gestingmarkedupwarddecreasen the sizeofbreccia

bodies,t is nferredhatmost orphyry-relatedrec-

ciaswere originally"blind."

It is clear romTable 4 that breccia ragmentsange

from angular o roundedand that comminuted ock

flour mayor maynot contribute o their matrices.t

wouldappear hat heterolithologic recciaswith sub-

rounded r rounded ragments nda rock lourmatrix

(rock flour breccias; ig. 11) are more widespread

than intrusion-related reccia pipes (Table 4). The

rock flour matrix ocallyexhibits rregularbut gen-

erally steep alignmentof its constituentparticles,a

fabric attributed to upward fluid streaming e.g.,

Central breccia at Los Bronces,Warnaarset al., 1985;

Llallagua,Fig. 12; andOk Tedi, Arnoldand Fitzger-

ald, 1977). Tabular ragments re uncommon. em-

nantopenspace etween ragmentss frequentlyob-

servedbut in manycases mountso only a few vol-

ume percent of the brecciaand comprisessolated,

roughly riangular peningsn tightly itting ragment

arrays.Clast-supported reccias re the norm (Fig.

11) although very gradationo bodiescomposed n-

tirely of rock lour sknown.Only a smallpercentage

of breccias ossessesn igneousmatrix in the sense

that it is composed f an intrusiverock). Examples

include a smallpart of the brecciasat BossMountain

(Soregaroli,975), BethlehemBriskey ndBellamy,

1976), Granisle Kirkham, 1971), and Ok Tedi (Ar-

nold and Fitzgerald, 1977).

Individualporphyry-related reccias lsoseem o

exhibit a greater variety of textures han isolated

brecciapipes.This feature attains ts extreme devel-

opment at Los Bronces,where a sequenceof seven

principalbreccias achdistinguished n the basisof

the size and form of clasts, he nature and amount of

matrix, and the degree and type of alteration-min-

eralization onstitutes single omposite ipe (War-

naars, 1983; Warnaarset al., 1985).

Thedegreeof fragment isplacementn porphyry-

relatedbrecciass variedbut, in general, s greater

than n intrusion-relatedreccia ipes,an observation

supported y the frequency f heterolithologicrec-

cia.Particularly oteworthys the ncreased vidence

for the ascent of clasts--intrusive clasts were dis-

placedupwardby 200 m in the Infiernillo brecciaat

Los Bronces Warnaars,1983) and K silicate-altered

clastswere carriedupwardat least 100 m at Mocoa

(Sillitoeet al., 1984a).Descentof fragmentss also

documented,however, and amounts o 250 to 300 m

at Los Bronces (Warnaars, 1983; Warnaars et al.,

1985) and >330 m in the Capote pipe at Cananea

(Perry,1961). Elsewhere, owever, sat CopperFlat

(Dunn, 1982), fragmentdisplacements considered

to be minimal.

Thebreccias escribedn thissection enerally re

closely elated o oneor moreporphyry tocks. ost

brecciasre rooted n porphyryntrusions,lthough

in somecases, s at Cananea Perry, 1935), Questa

(Leonardsont al., 1984), Red Mountain Quinlan,

1981), and Ardlethan Paterson, 976), muchof the

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TABLE . SelectedExamples f Magmatic-Hydrotherma

Hydrothermal

Principal ost Formof breccia alterationt --

Locality rocks Age m.y.) body Fragmentorm Rock lour tourmaline)

Bethlehem, Granodiorite 200 Steepelongate Angular o

B.C., Can- anastomosing rounded

ada bodies

BossMountain, Granodiorite 105 Irregular enslike Angular o

B.C., Can- vertical body rounded

ada

Galore Creek, Alkalic volcanics, 174 to 198 Steep pipelike Angular o

B.C., Can- syenitepor- bodies rounded

ada phyry

Island Copper, Quartz-feldspar 154 Carapace o Rounded

B.C., Can- porphyry, an- steep dike

ada desitic volca-

nics

Mt. Pleasant, Granitepor- 330 to 340 Pipelikebody Angularand

N. B., Can- phyry rounded

ada

Sacaton,Ari- Quartz monzo- 64.5 Large rregular Mainly subangular

zona nite porphyry, body to subrounded

monzonite

porphyry,

granite

Sierrita-Esper- Quartz monzo- 57 Irregular up- Angular o

anza,Ari- nite porphyry, ward-flared rounded

zona quartz monzo- bodies

nite, quartz

diorite, andesi-

tic volcanics

Copper Basin, Quartz diorite, 64 25 vertical pipes Angular o

Arizona quartz monzo- rounded

nite, quartz

monzonite

porphyry

Red Mountain, Latitic and an- 60 Steep pipe Angular

Arizona desitic volca-

nics

Copper Flat, Quartz monzo- 73.4 Steepelongate Angular, ittle dis-

New Mexico nite pipe placed

SantaRita, Granodiorite 63 Elongatepipe Angular,sub-

New Mexico porphyry (Whim Hill rounded

breccia)

Questa,New Andesiticvolca- 23

Mexico nics

Cananea,Son- Granite, lime- 59.9

ora, Mexico stone, quartz-

ite, rhyolitic to

andesitic vol-

canics

Cumobabi, Quartz monzo- 40.0

Sonora, Mex- nite porphyry

ico or andesitic

volcanics

Bodyabovecu- Subangular(?)

pola of aplite

porphyry

Eight principal Angular o sub-

pipes rounded

35 irregular Angularbut

pipes and rounded at La

bodies Verde pipe

Abundant Biotitic

0 to 70% Biotitic

Present lo- K silicate

cally (+ garnet)

Abundant Pyrophyllite-

sericite

Abundant Quartz-topaz

5 to 20% K silicate

Abundant in K silicate

upper

parts

Absent Quartz-K-feld-

spar

Absent K silicate

+ sericitic

Absent K silicate

Present K silicate

Absent

Absent

Absent,

present at

La Verde

pipe

K silicate

Sericitic, K sil-

icate, skarn

destruction

K silicate or

sericitic (t)

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Breccias ssociated ith Porphyry-type eposits

Principalmetallic Principalgangue Age relative o Economic

minerals minerals porphyrydeposit significance Reference

Chalcopyrite, bornite,

pyrite, molybdenite

Molybdenite,pyrite


Pyrite, chalcopyrite,mo-

lybdenite

Biotite,chlorite, Largelypremineral High-grade arts Briskey ndBellamy

tourmaline, of orebodies (1976)

quartz

Quartz

Biotite, garnet, an-

hydrite

Early ntermineral Ore largely e- Soregaroli1975),

stricted o Soregaroli nd

breccias Nelson (1976)

Premineral Part of orebody Allen et al. (1976)

Quartz, pyrophyllite Premineral

Part of orebody Cargill et al. (1976)

Wolframite,molybdenite,

arsenopyrite, native

bismuth, bismuthinite


lybdenite, specularite

Quartz, fluorite Premineral

Quartz Premineral

Main part of W- Kooiman et al.

Mo orebody (1984)

Hostsmuch of Cummings 1982)

West orebody


lybdenite

Quartz, biotite Early mineral

High-gradeore West and Aiken

(19S2)


lybdenite

Quartz Largely premineral

Three pipescarry Johnston nd Lowell

high-gradeCu- (1961)

Mo ore

Chalcopyrite,pyrite, mo- Quartz, K-feldspar, Premineral

lybdenite anhydrite,calcite

Pyrite, chalcopyrite, Quartz,biotite, K- Early mineral

magnetite,molybde- feldspar, luorite,

nite calcite, apatite

Pyrite, chalcopyrite, Quartz, K-feldspar, Early mineral

magnetite, molybde- biotite

nite

Molybdenite

Quartz, K-feldspar, Premineral

biotite

High-gradeore,

especiallyon

contacts

High-gradecen-

tral part of ore-

body

Part of supergene

orebody

Main orebody

Quinlan (1981)

Dunn (1982)

Kerr et al. (1950),

Rose and Baltosser

(1966), Norton

and Cathies 1973)

Leonardson et al.

(1984)

Chalcopyrite,bornite,

pyrite, sphalerite, mo-

lybdenite, galena

Pyrite, molybdenite,

chalcopyrite, etrahe-

drite

Quartz, carbonate,

phlogopite (La

Colorada), chlo-

rite

Quartz, biotite, K-

feldspar,anhy-

drite, apatite, sid-

erite or quartz,

tourmaline

Intermineral

Premineral

High-gradeore

Four bodiescarry

Mo ore

Perry (1935, 1961),

Meinert (1982)

Sillitoe (1976),

Scherkenbach et al.

(1985)

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TABLE . (Continued)

Hydrothermal

Principalhost Form of breccia alteration t --

Locality rocks Age (m.y.) body Fragment orm Rock lour tourmaline)

La Caridad, Quartz monzo- 54.5

Sonora,Mex- nite porphyry,

ico diorite, grano-

diorite

Mocoa,Colom- Dacite porphyry, 166

bia andesitic-daci-

tic volcanics

Quebrada Quartz monzo- 38

Bianca, nite, quartz

Chile and feldspar

porphyries

E1 Abra, Chile Diorite 33 to 35

Los Bronees,

Chile

Llallagua,

Bolivia

Panguna,

Papua New

Guinea

Quartz monzo- 7.4 to 4.9

nite, andesitic

volcanics

Quartz atite 20

porphyry, ar-

gillite

Andesitc, diorite, 3 to 5

granodiorite

Ok Tedi, Quartz monzo-

PapuaNew nite porphyry

Guinea

Ardlethan, Adamellite,

N. S.W., quartz-feldspar

Australia porphyry

Irregular to Rounded o sub-

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Principalmetallic Principalgangue Age relative to Economic

minerals minerals porphyry deposit significance Reference


Quartz, tourmaline Intermineral Part of chalcocite

ore zone

Chalcopyrite, yrite, mo- Quartz, K-feldspar, Intermineral Partlyhigh-grade

|ybdenite sericite, chlorite ore

Pyrite, chalcopyrite, or- Quartz, biotite, K- Intermineral Lens-shaped

nite, molybdenite feldspar, ericite, bodycontains

tourmaline Cu-Mo ore

Chalcopyrite,bornite

Biotite Intermineral Part of ore zone

Pyrite, chalcopyrite, Tourmaline,quartz, Intermineral Partsof four

specularite, mo- ohiorite, sericite, breccias consti-

lybdenite anhydrite tute hypogene

ore

Cassiterite, pyrite

Chalcopyrite, bornite


lybdenite

Tourmaline,quartz Pre- and inter- Partly ore

mineral

Quartz, biotite, K- Intermineral High-gradeore

feldspar

Quartz, biotite Intermineral Part of orebody

Saegartet al. (1974),

R. H. Sillitoe (un-

pub. rept., 1975)

Sillitoe et al. (1984a)

Hunt et al. (1983)

R. H. Sillitoe and H.

Neumann (unpub.

rept., 1970), Am-

brus (1977)

Warnaars (1983),

Warnaars et al.

(1085)

$illitoe et al. (1975),

Grant et al. (1980)

Baldwin et al. (1978)

Arnold and Fitzger-

ald (1977)

Pyrite, arsenopyrite,

sphalerite,galena,

chalcopyrite,cassiterq

itc

Quartz, tourmaline, Early and inter- Comprisesmost

sericite, chlorite, mineral of the nine ore-

siderite, fluorite bodies

Paterson 1976), P. J.

Eadingtonand

R. G. Paterson un-

pub. rept., 1984)

propylitizationt LosBroncesWarnaars t al., 1985),

quartz-topaz lterationat Mt. Pleasant Kooimanet

al., 1984), andskarn-destructiveuartz-chlorite-car-

bonate-hematite lteration at Cananea (Meinert,

1982). K silicate lteration s notablymoreabundant

than in intrusion-related recciapipes.At some o-

calities,both K silicate-alteredand sericitizedbreccias

are present n closeproximity e.g., Mocoa;Sillitoe

et al., 1984a); elsewhere sericitic alteration over-

printedearlyK silicate ssemblagese.g.,Cumobabi;

Scherkenbach,982) or characterizeshe apexand

flanksof a largelyK silicate-alteredipe (e.g.,Red

Mountain;Quinlan,1981). At Cumobabi, recciaso-

catednear he centerof thehydrothermalystem re

K silicatealtered and constitutemolybdenum re

whereasmore peripheralbreccias re propylitized

and/orsericitized ndare devoidof ore to explored

depths Sillitoe,1976; Scherkenbach,982).

Quartz s the mostwidespread ementingmineral,

although t is absentor minor at E1 Abra and Galore

Creek. n K silicate-alteredrecciast is accompanied

by K-feldspar nd/orbiotite, to which one or moreof

chlorite, luorite, apatite,siderite, ourmaline,mag-

netite, and specularitcmay be added.The K silicate

assemblageresent sa matrix o breccias t Questa

(Leonardsont al., 1984), CopperFlat (Dunn, 1982),

and he Colorada ipeat CananeaPerry,1935, 1961)

is pegmatitic in texture. Tourmaline tends to be a

morecommon onstituent f sericitizedbreccias Ta-

ble 4). Garnetoccurs sboth an alterationandmatrix

mineralat Galore Creek (Allen et al., 1976). One or

moreof chalcopyrite, yrite, andmolybdenites also

present as a matrix component,even in rock flour

breccias. assiterites the economically ost mpor-

tant cementingmineral at Llallaguaand Ardlethan,

as is wolframite at Mt. Pleasant.

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148-155

BECCIA riPE

1

/I I

o m.. '

FIG. 10. Diagrammatic ross ectionhrough he Red Mountain

porphyry copper system,Arizona, to show he central position

of the 148-155 breccia pipe. Taken from Quinlan (1981).

FIG. 12. Swirly flow texture n rock flour matrix o hetero-

lithologic reccia.Oruro in deposit,Bolivia.Approximatelyalf

natural size.

Breciasnporphyryystemsrecommonlyhar-

acterizedby higher contentsof exploitablemetals

than the surroundingstockworks.The situation

reachesan extreme at BossMountain (Soregaroli,

1975), CopperFlat (Dunn, 1982), Cumobabi Scher-

kenbach,1982), and Los Bronces Warnaarset al.,

1985), where the porphyry copper stockworks e-

yond he breccias o not attainore grades. lsewhere,

however, ncluding slandCopper,Cananea,Mocoa,

QuebradaBianca,Questa, lallagua,Mt. Pleasant, nd

Ardlethan, reccias onstitutehe highest radeparts

of the orebodies. ocally,asat LosBroncesWarnaars,

1983) andMocoa Sillitoeet al., 1984a),metalgrades

are appreciably nhanced y the presenceof previ-

ouslymineralized lastsn the breccias.n some rec-

cias, he metalbudget s distinctlydifferent rom hat

characteristic f the porphyrydeposit sa whole.As

examples, t QuebradaBianca, dikelikebrecciacar-

,, -- _. ,,' - , *. ; . '

. -.. ... q . . - , '- *

.- .. **":-:'..../.,.;:.:..,d - .%" ..'.- ;-.. ,[ ,,

O. 11. Tie rock flour breccia.EI Abra porphyrycopper

depo,it, hile.

ries more than 15 timesthe averagemolybdenum

gradeof the restof the deposit Hunt et al., 1983)

and at SantoTomas I, Philippines, mall pipelike

breccias avemarkedly igherMo to Cu andMo to

Au ratios han the rest of the deposit Sillit.oe nd

Gappe, 1984).

In commonwith intrusion-related reccia pipes,

some recciasn porphyry ystemsrecharacterize

by a preferreddistribution f ore minerals. xamples

maybe cited rom he 148-155pipeat RedMountain

wherecopper,molybdenum,ndsilver rades round

the margins re several imesgreater han those n

its nterior Quinlan, 981),and rom heDonoso ipe

at Los Bronces,where copper s concentratedn a

seriesof downward-closinghells Warnaars,1983).

Based n he exampleselectedor Table4, breccia

emplacementn porphyry ystemsangesn age rom

premineral o intermineral.n premineral xamples

there is no evidenceof any earlier stages f miner-

alization, and at some ocalities, such as Bethlehem

(Briskey ndBellamy,1976), the mainmineralized

stockwork crosscuts the breccia bodies. Where brec-

ciasare designated searly mineral Table4), there

is only minor evidence rom constituentragments

for prebrecciationlteration ndmineralization.his

is exemplified y low-grade yrite-chalcopyrite in-

eralizationrelated to pervasivesericitization hat

predated he brecciation-K ilicatealterationevent

at CopperFlat (Dunn, 1982), a barren prebreccia

stageof quartz-K-feldspareiningat Sierrita-Esper

anza WestandAiken,1982), andprebreccia uartz-

topaz alterationat Ardlethan P. J. Eadington nd

R. G. Paterson, npub.rept., 1984). In contrast, n-

termineral recciaswere emplacedater thanoneor

more main stages f alterationand mineralization

Evidence or this conclusions commonly rovided

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by the restriction f ore-bearing einlets o individual

fragmentsn a breccia Fig. 13), as emphasizedor

Granisle ndelsewhere y Kirkham 1971), or by the

truncation f alteration ndstockwork einlets y an

entire brecciabody. Intermineralbrecciasmay also

contain clasts of mineralized breccia derived from

preexisting odies, relativelycommoneatureat Los

Bronces Warnaars,1983) and elsewhere.

In somecases, opperand molybdenumntroduc-

tion to intermineralbreeeias ccompaniedenewed

(or continued)K silicate lteration,whereas lsewhere

it was associatedwith localized serieitie, or in most

Philippineexamples, hiotitlealteration Si litoeand

Gappe, 1984).

Stable sotopestudies o determine he sourceof

fluids responsible or alteration-mineralization f

breeeias n porphyrysystems ave not been carried

out. However, the coincidence of brecciation and K

silicate lteration uring he earlydevelopmenttages

of manyporphyry ystemsTable4) suggestshatme-

teoric-hydrothermalluids generallywere subordi-

nate to fluidsof direct magmatic-hydrothermalar-

entageSheppardt al., 1971).

Origin: Most workers n the last two decadeshave

attributed he principal reeeiasn porphyrysystems

to theviolent elease f magmatic-hydrothermalluids

from coolingstocks e.g., Phillips,1973; Seraphim

and Hollister, 1976). It is clear that the model of

Burnham 1979, 1985) andothers or brecciation y

fluid liberationduringsecond oiling, ollowedby

decompressionf the released luids, s as effective

in explaininghe widevarietyof breeeiasn porphyry

systems s t is the isolated ntrusion-relatedbreeeia

pipesdescribed bove.Furthermore,he widespread

stockworkracturesn porphyrysystems ayalsobe

FIG. 13. Intermineralbreeeiawith quartzveinlet confined o

clastnear he middleof photograph. hlorite-bearingock lour

matrix.

attributed to the same late magmatic processes

(Burnham,1979).

The spectrumof texturesand relationships um-

marizedabove or breccias n porphyrysystemsmay

be attributed o the samemechanismssed o explain

comparable eatures n isolatedbreccia pipes. It is

thereforeno longernecessaryo invokeseparateor-

igins or texturallyand geometrically ifferentbrec-

cias hat occur n closeproximity n many porphyry

systems;hey may all be related to the sameOverall

mechanism.

Rock lourbreccias howing vidence f mixing nd

upward transportof fragments re apparentlymore

widespread n porphyry systems han in isolated

brecciapipesandmaybe due to the efficient elease

of argervolumes f fluids romsubvolcanicorphyry

stocks han from the roofsof deeper seatedplutons

(seeBurnham,1985). A more protracted eleaseof

fluids,or severalstages f releaseas a resultof mul-

tiple intrusion,effectivelyexplains he intermineral

positionof manybreccias n porphyrysystems.n-

termineralbrecciationmayalsobe favoredby the re-

duction of rock permeability resulting from early

stages f K silicate lteration particularly uartzpre-

cipitation)and mineralizationseeFournier, 1983).

Phreatic Hydromagmatic)Breccias

Epithetrealprecious _ base)metal deposits

General remarks:Epithermal preciousmetal de-

positsmay be subdividedconveniently nto three

principalcategoriesBonham nd Giles, 1983): vol-

canic-hostedeposits, ot spring-relatedeposits, nd

carbonate-hostedCarlin-type) deposits.A shallow

(< 1,000 m) level of emplacements nferred or most

epithermaldeposits. n associationf epithermalde-

positswith volcanic tructures r landforms,ncluding

flow-domecomplexes,maar-diatreme ystems, nd

caldera ing fractures Table5), emphasizeshe shal-

low depthsof emplacement.n fact,severalof the hot

spring-relateddepositsattained the contemporary

surface sshown y their associationith sintersTa-

ble 5). As a consequence f their shallowsettings,

mostdeposits ange from Miocene to Pleistocenen

age and lack large volumesof associatedntrusive

rocks Table 5).

It is widely accepted hat brecciasare a common

accompanimento volcanic-hostedndhot spring-re-

lated epithermaldeposits ndare considered y Ber-

gerandEimon 1983)andBonham ndGiles 1983)

as an integralpart of the latter category. heir im-

portance n many Carlin-typedeposits asalsobeen

emphasizedecently Sillitoe,1983a).

Characteristics: broadrangeof breccia ypes s

found n epithermalsystemsTable 5). Their geom-

etriesrange rom smallveinsandveinlets Fig. 14) to

largepipes, abularmasses,nd rregularanastomos

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TABLE . SelectedExamplesof Phreatic Breccias

Locality

Host rocks

Age (m.y.) Volcanicsetting

Form Fragmentcharacteristics

Equity Silver,

B.C., Canada

Dacitic tuffs

59 None known

Irregular abular Angular o rounded, t

body least wo generations

Cinola, B.C.,

Canada

Delamar, Idaho

Cripple Creek

(Globe Hill),

Colorado

Summitville,

Colorado

Red Mountain,

Colorado

Round Mountain,

Nevada

Buckhorn,

Nevada

Buckskin,

Nevada

Hasbrouck

Mountain,

Nevada

Conglomerates,

siltstones

Rhyolite domes,

plugs, lows

Latite-phonolite

intrusions

Quartz latite por-

phyry dome

Rhyolitic to

quartz latitic

volcanics,

quartz latite

porphyry plugs

Metasediments,

ignimbrite

Basaltic andesitic

volcanics,argil-

lite

Rhyolitic pyro-

clastics

Volcaniclastic

sediments, g-

nimbrite

Late Ceno- None known

zoic

15 Rhyolite low-

dome com-

plex

27 to 28 Interior of dia-

treme

22 to 23

22.5

25

Late Tertiary

15.5

16.3

Dome on older

calderaring

fracture

Ring fracture of

older caldera

On caldera ing

fracture

Graben

Rhyolite low-

dome com-

plex

Rhyolite low-

dome com-

plex

Extensive, oorly Angular o rounded(?)

defined bodies

Irregularvein Angular,monolithologic

andpipelike to subrounded,et-

bodies erolithologic

Irregular bodies Angular,monolithologic

and pipes o to rounded,hetero-

>330 m lithologic; hree

generations

Pods, ipes,and Angular o subrounded,

tabular bodies mono- or heterolitho-

logic, three

generations

Pipes o >370 m Angularo rounded, et-

erolithologic

Upward-flared

pipelikebody

to >350 m

Pipelikebody

+ subaerial(?)

patches

Pipelikebody

Extensive irregu-

lar bodies

Angular o subangular,

heterolithologic,

movedupward

Angular

Angular o rounded,

sortedparallel o con-

tact

Angular o rounded,het-

erolithologic,moved

upward

Northumberland,

Nevada

AlligatorRidge,

Nevada

Limestone, dolo-

mite, shale, silt-

stone

Limestones,

shales

84.6(?)

Tertiary(?)

None known

None known

Structurallyand

stratigraphi-

cally con-

trolled bodies

Irregular(?)

bodies

Angular

Angular

La Coipa, Chile

Rosia Montana,

Romania

Chinkuashih,

Taiwan

Wau, Papua New

Guinea

Siltstone, dacitic

ignimbrite

4- tuff

Dacite porphyry

Sandstone,shale

Phyllites,explo-

sion breccia

Miocene(?)

Late Mio-

cene

Pleistocene

2.4

Dacite domes

Probable flow-

dome com-

plex

Dacite porphyry

flow-dome

complex

Tuff ring around

maar

Irregular pipes

and bodies

Breccia pipes to

500 m

Smallpipes

and dikes to

>200 m

Anastomosing

veins and

pods,subaerial

apron

Angular o subrounded

Angular o rounded(?)

Angularor rounded,

movedupward,het-

erolithologic

Angular o rounded,het-

erolithologic

1486

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Associatedwith PreciousMetal Deposits

Hydrothermal Principalhypogene

alteration minerals

Ore deposit type

and ore reserve

(M = million,

t: metric tons)

Relation to

palcosurface

Relation to

orebody

Reference

Advanced Quartz,pyrite,arseno- BulkAg-Cu-Au-Sb; Subsurface Constitutes re

argillic pyrite, tetrahedrite, 28 Mt, 106 ppm (1,000 m?)

chalcopyrite, phaler- Ag, 0.38% Cu,

itc, galena 0.96 ppm Au

Silicification Quartz,pyrite,marcasite BulkAu; 41 Mt, Subsurface,roba- Constitutes re

1.85 ppm Au bly shallow

Silicification, Quartz,pyrite, nauman- Bulk Ag-Au;9 Mt, Shallow ubsurface Partlyore

argillic nite, argentitc 86 ppm Ag, i and at paleosur-

ppm Au face (sinter)

Quartz, sericite, Quartz, fluorite,carbon- Bulk Au, 2 orebod- Subsurface

chlorite, ate, celestite,anhy- ies; 4 Mt, 1.3

montmoril- drite, pyrite, galena, to 1.8 ppm Au

1onite sphalerite,chalcopy-

rite, pyrrhotite

Silicification, Quartz, alunite,pyrite, Au-Ag-Cu odes Shallow ubsurface

advanced enargite,covellite, na- and pipes

argillic rive sulfur

Silicification, Quartz, clays,pyrite, en-

advanced argite, chalcocite,co-

argillic veilitc, bornitc,sphal-

erite, galena

Silicification Pyrite

Silicification, Quartz, pyrite, marcasite

kaolinitc, ad-

ularia, seri-

cite

Silicification, Quartz, pyrite, stibnite,

alunite sulfosalts, innabar

Silicification, Quartz, pyrite, acan-

adularia, llite thite, stibnite,pyrar-

gyrite, chalcopyrite

Silicification Quartz, barite, pyrite

(jasperold)

Silicification Quartz, calcite, barite,

(jasperoid) pyrite, stibnite

Silicification, Quartz, pyrite, sphaler-

advanced itc, galena,chalcopy-

argillic rite, sulfosalts

Silicification, Quartz, rhodochrosite,

adularia, pyrite, sphalerite, ga-

argillic lena, chalcopyrite

Silicification Pyrite, enargite,quartz,

alunite

Minor

Quartz, calcite, manga-

nocalcite,pyrite, galena,

sphalerite

Cu-Au-Ag pods

and pipes

Bulk Au; 204 Mt,

1.2 ppm Au

Bulk Au; 4.6 Mt,

1.54 ppm Au

Vein and stock-

work Au-Ag

Bulk Au-Ag

Carlin-type Au;

40 Mt, 2.4

ppm Au

Carlin-type Au;

4.5 Mr, 4.1 ppm

Au

Bulk Ag-Au pros-

pect

Au

Cu-Au veins

+ breccias

Bulk Au-Ag

Subsurface

Shallow subsurface

Shallow subsurface

and paleosur-

face(?) sinter

fragments)

Shallow subsurface

and palcosur-

face (sinter)

Shallow subsurface

(

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FIG. 14. Typicalbreccia einlet esulting romhydraulic rac-

ture. Matrix comprises ilicifiedrock flour. Epithermal precious

metal prospect,Chile.

ingbodies. everal pithermal reccias ossessnown

verticalextentsof 200 to 500 m (Table 5). The reg-

ularlyshaped ipesat RedMountain Burbank, 941)

and Chinkuashih Chu, 1975) are reminiscentof the

intrusion-relatedipesdescribed bove.t is herefore

significanthat marginal heeted onesand a close

relation of breccias o quartz latite porphyry plugs

are characteristic f severalpipesat Red Mountain

(Burbank, 941; FisherandLeedy, 1973) and arge

isolatedspheroidal ragmentswere reported from

Chinkuashih Chu, 1975).

At severalocalities, uch sRoundMountain Mills,

1972), there s a marked pward lare o pipe-shaped

bodies,which is interpreted o be due to their ap-

proach o the contemporaryand surface. n fact, at

Buckhorn (Monroe and Plahuta, 1984), Buckskin

(Vikre, 1983), HasbrouckMountain Graney,1984),

Delamar R. H. Sillitoe ndH. F. Bonham, r.,unpub.

observations,981), La Coipa (R. H. Sillitoe,unpub.

rept., 1980), and Wau (Sillitoe et al., 1984b), brec-

ciationbreached he palcosurface. emnants f sub-

aerialbrecciaapronsare still preserved t Wau. Es-

sentially ubaerial reccias t the Milestone rospect

(Delamar),Buckhorn,and HasbrouckMountaincon-

tain fragments f sinteraswell asa varietyof under-

lying rocks,whereas t La Coipa (and n placesat

McLaughlin,California)surfacehot springsinters

underwent brecciation more or less in situ. Silicified

logsaccompany inter ragments t Milestone.

The texturesof epithermal reccias re extremely

varied.Rock lourandopen-spacereccias re both

widespreadBerger ndEimon,1983) andbothmay

occur in individual breccia bodies. Rock flour is com-

monly maskedby silicification Fig. 14). There is

commonly vidence or relatively estrictedupward

displacementf fragments,ut this sclaimedo attain

200 m in rock flour breccias t ChinkuashihChu,

1975).Appreciable penspaceswidespreadn some

breccias, speciallyhose hat underwenthypogene

leaching uringadvanced rgillicalteration, sat Red

Mountain Burbank, 941). Some pithermal reccias

display cleargradationo stockworkracturinge.g.,

Delamar,HasbrouckMountain,GlobeHill at Cripple

Creek, AlligatorRidge,and Equity Silver).

Many epithermalbrecciasprovide evidenceof

multiple stages f silicification,mineralization, nd

brecciation,and at some ocalitiesa temporal se-

quence,with eachbrecciaexhibiting ts own distinc-

tive characteristics,aybe determined. or example,

Thompson t al. (1985) proposedour stages fbrec-

ciation,each accompanied y mineralization,n the

GlobeHill area at Cripple Creek. The intermineral

(and, ocally,evenpostmineral)imingof brecciation

is emphasized t many ocalities y the restrictionof

distinctive ypesof silicification r sulfideveining o

isolated ragments. or example, smanyas our va-

rieties of silicified limestone occur .in breccia in the

Taylor district,Nevada Loveringand Heyl, 1974).

A structuralcontrolof epithermalbrecciass em-

phasizedmore frequently than for deeper seated

brecciasassociatedmore closelywith plutonsand

stocks. Minor faults are considered to have localized

the Red Mountainpipes Burbank, 941; Fisherand

Leedy, 1973) and he Chinkuashih recciadikesand

pipes Chu, 1975), whereas majoroblique-slipault

abuts and probably localized the Cinola breccias

(Cruson t al., 1983). n the GlobeHill areaat Cripple

Creek, faulting took place during brecciationand

actedas an importantspatialcontrol Thompson t

al., 1985). High-angle aultsand stratigraphic ori-

zons, especially imestone-shale ontacts, ocalized

much of the silicification and brecciation in carbonate-

hostedepithermal deposits,as at Northumberland

(Motter and Chapman,1984) and Alligator Ridge

(Klessig,1984). Structuresof volcanicorigin also

controlled brecciation and mineralization in several

epithermaldistricts, sat Wau whereshort ow-angle

extensional tructures etween a diatremering fault

(seebelow)anda regional ault ocalizedbrecciation

(Sillitoeet al., 1984b).

Alteration and mineralization: The dominant fea-

ture that distinguishespithermalbrecciasrommost

magmatic-hydrothermalreccias s the widespread

occurrence f quartzasbotha pervasiveeplacement

of, and a cement o, fragments.t is generally ine

grained ndcommonly halcedonic,ndcharacterizes

all but threeof the examplesited n Table5. Silicified

carbonateocksaregenerally eferred o as asperoid.

In epithermalpreciousmetal depositswhere silic-

ification s widespread,here is a close elationship

between he development f pervasive halcedonic

silicaand brecciation, sseenat Summitville Steven

andRatt6,1960), n the carbonate-hostedpithermal

depositsTable5), andelsewhere. he brecciapipes

at RedMountain Burbank, 941) are capped y mas-

5/21/2018 015 Sillitoe

23/48

ORE-RELATED BRECCIAS IN VOLCANOPLUTONIC ARCS

1489

sivesilicification. ilicifications accompaniedy, or

gradesnto,advancedrgillic lterationich n alunite

at Summitville,Red Mountain,La Coipa,and Chink-

uashih ut is surrounded y less cidalteration ypes

at the other localities listed in Table 5. Patches of

silicificationndassociatedrecciation realso ypical

of the similar ones f advanced rgillic lterationhat

characterizehe upper (volcanic) artsof porphyry

coppersystemsSillitoe,1983b).

The presence r absence f advanced rgillical-

teration is the dominant control on the sulfide and

ganguemineralogyf the breccias. ulfur-richUl-

fides,especially yrite, enargite, uzonite,and cov-

ellitc, generallycement silicifiedbrecciaswithin

zonesof advanced rgillicalteration,whereasmuch

smaller mounts f pyrite, either aloneor accompa-

niedby sphalerite, alena, halcopyrite,ennantite-

tetrahedriteand/orargentitcoccurwhere advanced

argillicalteration s absent.Breccias ssociated ith

Carlin-typedepositsend to be cemented y a re-

strictednumberof minerals, f whichquartz,calcite,

pyrite,barite,andstibnite re the mostwidespread.

Epithermal recciasommonlyonstituteoldand/

or silverore. Breccias ayprovide he main ocifor

ore, asat Red Mountain Burbank, 941), or maysim-

ply host ome f the highest radeportions f anore

body, asat HasbrouckMountain Graney,1984) or

Northumberland Motter and Chapman, 1984). In

theseand mostof the other examplesn Table 5 the

preciousmetalmineralizations presentmainly n the

brecciacement.Locally,however,as at Buckskin

(Vikre, 1983), preciousmetalsare presentonly in

veins and stockworks hat cut breccia. At Wan, much

of the gold n the brecciass presentn clasts f vein

material $illitoeet al., 1984b). n contrast,he brec-

cia pipe at RoundMountain s barren,althought is

surroundedy ore (Mills, 1982). Manyof the brec-

ciated asperoids ssociated ith carbonate-hosted

epithermal eposits ontainonly trace amounts f

preciousmetals, lthough t NorthumberlandndAl-

ligatorRidge Table5) they are integralpartsof the

orebodies.

Onlysparsenformations available oncerninghe

fluids nvolved n the formation f the epithermalde-

positsisted n Table 5 (e.g., CrippleCreek,Thomp-

sonet al, 1985; EquitySilver,Wodjakand Sinclair,

1984). In commonwith most epithermal precious

metal deposits, owever, he ore fluidsare assumed

to have been dominatedby meteoricwater (e.g.,

O'Neil andSilberman,974;Radtke t al., 1

015 sillitoe

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