Geology

1. DISCUSSIONS 533
andMcDougal(1971,p. 768,Fig. 6) for Tennessee
was alsoreportedby Mullens (1964, p. 525-526,
Fig. 66) for the Upper Mississippi
Valley. The
solutionfeaturesare also similar as can be seenby
comparing
Fulweilerand McDougal'sFigures2-5
with Figures37-39 of Heyl andothers(1959). In
generaloutlinethe mine workingsin the two areas
alsoare similar. (CompareFig. 6 of Fulweilerand
otherswith P1.5 of Heyl andothersand P1.25 of
Mullens.)
The postulated
methodsof formationof the ore
structuresare very similar (compareMcCormick
andothers,1971,with Mullens,1964,p. 513-521).
The effects of solution of limestone and dilation of
thicker bedded dolomite on the volume of ore in
certain types of depositsare similar. (Compare
Wedow, 1971,with Reynolds,1958,and Mullens,
1964,p. 519-520.)
Given all the similarities, how can we establish
which interpretationis more nearly correct?A
possible
methodinvolves
determining
the origin of
the conjugate
joint systems
that preferentially
con-
trolledore deposition
in bothdistricts. The Upper
Mississippi
Valley districtdeposits
alinealongtwo
differentsetsof joints in the CubaCity-Shullsburg
area. One conjugate
set is relatedto the Meekers
Groveanticline(Mullens,1964,p. 476-479,Figs.
54, 55, and66), whichis youngerthanSilurian. If
the conjugatesets in easternTennessee
can be
provedto be olderthanearlyMiddleOrdovician,
thenthepaleoaquifer
theoryis supported.If, how-
ever,the jointscan be provedto be relatedto de-
formationof Permianor later age,then the paleoa-
quifertheorywouldseem
to beuntenable.
ALLEN V. HEYL
THOMAS E. MULLENS
U.S. GEOLOgiCAL
SURVEY,
DENVERFEDERALCENTER,
DENVER,
COLORAr•0
80225
REFERENCES
Fulweiler, R. E., and McDougal, S. E., 1971, Bedded-ore
structures,
JeffersonCity Mine, JeffersonCity, Tennessee:
EcoN. GEOmG¾,
V. 66, no. 5, p. 763-769.
Heyl, A. V., Jr., Agnew,A. F., Lyons,E. J., and Behre,C.
H., Jr., 1959, The geology of the Upper Mississippi
Valley zinc-lead district [Ill.-Iowa-Wis.]: U.S. Geol.
Survey Prof. Paper 309, 310 p.
McCormick,J. E., Evans,L. L., Palmer, R. A., and Rasnick,
F. D., 1971, Environment of the zinc diposits of the
Mascot-Jefferson City District, Tennessee: EcoN.
GEoI.
oG¾,
v. 66, no. 5, p. 757-762.
Mullens, T. E., 1964, Geology of the Cuba City, New
Diggings, and Shullsburg quadrangles,Wisconsin and
Illinois: U.S. Geol. Survey Bull. 1123-H, p. 437-531.
Reynolds,R. R., 1958,Factors controlling the localizationof
ore depositsin the Shullsburg area, Wisconsin-Illinois
zinc-leaddistrict: EcON.G•o•.oa¾,
v. 53, no. 2, p. 141-163.
Wedow, Helmuth, Jr., 1971,Modelsof mineralizedsolution-
collapse structures from drilling statistics--An aid to
exploration:EcoN. GEoma¾,
v. 66, no. 5, p. 770-776.
Publication authorized by the Director, U.S. Geological
Survey.
ORIGIN OF COPPER-BEARING BRECCIA PIPES
In theirrecent
paperSillitoeandSawkins(1971)
haveprovided
a wealthof information
on thenature
of the Chileancopper-bearing
breccia
pipesandhave
presented
a strongcasefor an originby collapse
accompanying
and followingintense
hydrothermal
fluidactivity. By way of reinforcing
their sugges-
tions as to the mechanism of formation of this kind
of brecciapipe, I wish to comment
on a couple
of aspects
of breccia
pipesin Chileand elsewhere.
The aspects
to whichI refer are the breccia
pipe
terminations
andthebreccia
pipematrix.
In their Figure 11 the authorsshowa typical
Chileanbrecciapipe terminatingupward beneath
a sheetedbut unbrecciate
dome-shaped
roof. Min-
eralizedbrecciaalso terminatesupward beneatha
dome-shaped
roof in the Capoteand East Breccia
pipesat Cananea,
Mexico (Perry, 1961). The
typicalChilean
pipe(Fig. 11) is shown
asterminat-
ing downward
in a funnel-shaped
neck,although
Sillitoeand Sawkinspoint out that this is hypo-
thetical,
for thelowerlimit of nopipewasaccessible
during their study. Presumably
their proposed
bottomconfiguration
is based
uponobservations
by
Locke (1926) andJoralemon(1952) to whomthey
refer. It may be worthwhile to point out that
similar conditionshave been describedby Kuhn
(1941) and by Kents (1964). The latter actually
observed
the bottomingof a rupture brecciawithin
lessthan half a meter; the maze of tight fractures
of the rupture breccianarrowed down to a few
vertical feeder fractures.
Theseadditionalillustrations
of upwardanddown-
ward restrictions
of brecciapipesprovideno support
for an originby explosion
or by the withdrawalof
underlyingmagma. Similarly,there is no evidence
to supporta brecciaorigin by faultingor intrusion.
Nor would an appealto fluidizationby hot gases
haveanymerit in viewof therelativetightness
of the
do/nedroof; sucha roof could not have provided
channels
sufficiently
opento allowthephysical
trans-
port of the enormousvolumesof rock fines that
would have had to have been removed in order to
accommodate
the hydrothermal
mineralmatrix and
the numerousvoids. Hence, an origin by solution
and collapse
seems
to be the only onethat is com-
patiblewiththedata.

2. 534 DISCUSSIONS
Anotherinteresting
aspect
of pipesof thiskind is
the scarcityor lack of rock fines in the matrix.
Either the fines have somehow been removed or
nonewere generated. Of the various mechanisms
of
rock brecciation,
collapse
with a minimumof later
settlingwouldcreatethe leastamountof rock fines.
Certainlyany finesthat may havebeengenerated
wouldbe moresusceptible
thancoarse
fragments
to
solution
by corrosive
fluids. The largeamountof
gangue (mostly quartz) and sulfides (especially
chalcopyrite,
lesspyrite, molybdenite
and other sul-
fides) cementing
the brecciafragments,
the many
vugswith crystalsprojectinginto them, and other
texturesindicativeof spacefilling are testimony
to
the great volumeof spacethat must have existed
prior to the introductionof the principal gangue
and ore minerals. This, in turn, is a measureof
the great volume of rock that had to be removed
prior to and during rock brecciation. The scarcity
or lack of rock fines and the presence
of quartz-
sulfidematrix are not uniqueto the Chileanpipes
but are typicalof many brecciapipeselsewhere
in
the world, for example, Copper Basin, Arizona
(Johnstonand Lowell, 1961); Bagdad, Arizona
(Anderson, Scholz, and Strobell, 1955); Copper
Creek,Arizona (Kuhn, 1941): SanJuan,Argentina
(Llambias and Malvicini, 1969); ore brecciasat
Toquepala,Peru (Richard and Courtright,1958);
and the Tribag mine, Ontario (Armbrust, 1969).
The two aspectsof the breccia pipes reviewed
above,the needfor large volumesof through-going
corrosivefluids,and the top and bottomrestrictions
on pipesmay seemat first to be contradictory,
but
they are not. Even thoughrestrictionsof the kind
referred to led Perry (1961) to conclude:"Since
the tops of many pipesare coveredand sealedby
unbroken roofs, the evidenceis conclusivethat re-
moval of material must have occurred from below,"
he describes
brecciacolumnsas having peripheral
zonesof fracturing around and over them and he
attributesthe shapeand depthof the Duluth pipe
to regionalfractureplanes. That is, the rocksare
not "sealedandunbroken." Elsewherecertainpipes
showa strong
localization
by faultsandjoint systems,
as, for example,the Childs-Aldwinklepipe in the
CopperCreekdistrict(Kuhn, 1941); CopperBasin,
Arizona (Johnstonand Lowell, 1961); Cuajone,
Peru (Lacy, 1958); SanJuan,Argentina(Llambias
andMalvicini,1969); andBagdad,Arizona (Ander-
son,Scholzand Strobell,1955). Sillitoeand Saw-
kins (1971) illustratesheeting(fractures) both in
the walls and roof of their typicalpipe. Further-
more, in all pipesreferred to above,the authors
mentionstrongwall rock alterationextendinginto
the wallsand into the piperoof. In somedistricts,
the wall rock alteration has an areal extent several
timesthat of the pipe. Intensefragmentand wall
rock alteration (replacement)associated
with the
pipesrequires
thatthe system
bean openone. For
thesereasons
it seems
clearthat the pipes,though
restricted,
werebynomeans
sealed.
The recognition
that suchpipeshavecharacter-
isticsthat do not contradicta solutioncollapse
origin doesnot, of course,establish
the "precise
mechanism
by which solutionof the granitichost
rock" took place (Sillitoe and Sawkins, 1971).
Nevertheless,Sillitoe and Sawkins considerthe field
evidence
in support
of fluidcorrosion
to becompel-
ling, as is the evidence that I have documented
aboveregardingthebrecciamatrix.
Generallywe are inclined
to consider
replacement
to be a volume-for-volume
process
and yet there is
no reasonwhy it shouldalwaysbe so. An excellent
illustration
of the intensely
corrosive
effectof hydro-
thermalfluidsisprovided
bythehundred
of pipesin
granite in easternAustralia (Blanchard, 1947).
These contain no breccia but most have cores of
quartz; some have been mined for bismuth and
molybdenum,some for wolframite, and some for
cassiterite. Vugs are absentfrom the tin-bearing
pipesbut arecommon
in otherpipeswheretherugs
may be very large, with quartz crystalsup to 10
inchesin diameter. I wouldagreewith McKinstry
whenhe writes,referringto theseAustralianpipes,
that he findsit "difficultto explainthesepipesas
otherthantheresultsof corrosion
followedby filling
and replacement
accomplished
by fluids (whether
liquid or gaseous)originatingfrom the graniteand
makingtheirwayupward"(McKinstry, 1955).
The first fluids to permeatethe fracture zones,
nowmarkedby mineralized
brecciapipesand their
adjacentmineralizedvein and sheeting
zones,must
havebeenvery corrosive. During alterationthere
was a considerable
excessof solution over dep-
ositionso that open spaces
were createdand en-
larged with concomitantand subsequent
collapse
formingthe breccia. Smallfragments
generated
by
collapse
and attritionwerereadilydissolved
and the
effluentescaped
into the fracturedroofs. Replace-
ment of fracture walls by alteration mineral as-
semblages
continued
with furthercollapse
aslongas
solutionexceeded
deposition. Later, lesscorrosive
fluids deposited
quartz and sulfidesand associated
minerals,partly or completely
filling the spacebe-
tweenthebreccia
fragments.
JOSEPH
W. MILLS
DEPARTMENT
OFGEOLOGY,
WASHINGTONSTATEIJNIVERSITY,
PULLMAN,WASHINGTON99163,
February2, 1972

3. DISCUSSIONS 535
REFERENCES
Anderson,C. A., Scholz, E. A., and Strohell, Jr., J. D.,
1955, Geology and ore depositsof the Bagdad area,
Yavapai County,Arizona: U.S. Geol.SurveyProf. Paper
278, 103 p.
Armbrust,G. A., 1969,Hydrothermalalterationof a breccia
pipe deposit, Tribag mine, Batchawana Bay, Ontario:
Ecoa. GEOL.,
V. 64, p. 551--563.
Blanchard,Roland, 1947, Some pipe depositsof eastern
Australia: Ecoa. Gv.o%v. 42, p. 265-304.
Johnston,
W. P., andLowell, J. D., 1961,Geologyandorigin
of mineralizedbreccia pipes in Copper Basin, Arizona:
Ecoa. GEOL.,
v. 56, p. 916-940.
Joralemon,I. B., 1952, Age cannotwither or varieties of
geologicexperience:
Ecoa. Gv.
oz..,v. 47, p. 253-256.
Kents, Paul, 1964,Specialbrecciasassociated
with hydro-
thermal developments
in the Andes: Ecoa. GeoL.,v. 59,
p. 1551-1563.
Kuhn, T. H., 1941,Pipe deposits
of the CopperCreek area,
Arizona: Ecoa. Gv.
oz..,v. 36, p. 512-538.
Lacy, W. C., 1958,Porphyry copperdeposit,Cuajone,Peru:
Mining Eng., v. 10, p. 104-107.
Llambias, E. J., and Malvicini, Lidia, 1969, The geology
and genesisof the Bi-Cu mineralized breccia-pipe,San
Francisco de los Andes, San Juan, Argentina: Ecoa.
Gv.
ot.., v. 64, p. 271-286.
Locke, Augustus,1926, The formation of certain orebodies
by mineralizationstoping:Ecoa. Gv.
ox..,v. 21, p. 431-453.
McKinstr.
y, H. E., 1955,Structure of hydrothermalore de-
posits:Ecoa. Gv.o[.50th Anniv. Vol., p. 170-225.
Perry, V. D., 1961, The significanceof mineralized breccia
pipes. Mining Eng., v. 13, p. 367-376.
Richard,Kenyon,and Courtright,J. H., 1958,Geologyof
Toquepala,Peru: Mining Eng., v. 10, p. 262-266.
Sillitoe, R. H., and Sawkins,F. J., 1971,Geologic,rainera-
logic and fluid inclusionsstudiesrelating to the origin of
copper-bearingtourmaline pipes, Chile: Ecoa. GEOL.,V.
66, p. 1028--1041.