ECONOMIC GEOLOGY

VoL. XXXVII MARCH-APRIL, 1942 No. 2

THE ORURO SILVER-TIN DISTRICT, BOLIVIA.

DONALD F. CAMPBELL.

ABSTRACT.

Paleozoic (Devonian (?)) shales were intruded by Tertiary

(Miocene or Pliocene) igneous rocks of acid to intermediate
composition. The intrusives are shallow-seatedand some were
extruded. The tin-silver veins, of Pliocene (?) age, occur in
rhyolite porphyry, shale, and explosive breccia near intrusive
bodiesof quartz-monzoniteporphyry. The fracturing was caused
by the intrusionof the quartz monzonite;the larger outcrops
were areas of uplift, between them, relaxation occurred. The
fracturedzone as a whole has been uplifted so that today the
vein outcropsare found in hills. The ore fluids came from the
samemagmabodyas did the quartzmonzoniteporphyry,and min-
eralizationtook place in two stages,an early quartz-pyrite-cas-
siterite stageand a late silver-sulpho-salt
stage. Ore deposition
took placeat relativelyhigh temperatures at a depthof •,oooto
2,0o0feet; the veinsmay be classed as xenothermal,
relatively
high temperaturedepositsformedat shallowto moderatedepths.
The averagelower limit of ore that can be minedunderpresent
conditionsis about 460 metersbelow the averagelevel of the
higher vein outcrops.
CONTENTS.

Introduction ................................................... 88
Petrographyand petrology..................................... 90
GeologicColumn for theOrurodistrict....................... 92
Paleozoic (Devonian(?)) sedimentary rocks................ 92
Tertiary(Pliocene(?)) igneous andrelatedrocks............ 94
Alluvialandlakedeposits ................................... 98
Structural
Geology............................................ 99
Districtstructures ......................................... 99
Pre-mineral fracturingin theminearea...................... 99
Post-mineral fracturingin the minearea ..................... Io5
Economic geology............................................. Io5
Mineralogy............................................... IO5
Origin of the deposits...................................... I IO
Practical considerations.................................... I I2
Acknowledgments
.......................... ,................... I•5
ø 87
88 DONALDF. CAMPBELL.

INTRODUCTION.

O}•ux•ois an old miningdistrictand,althoughmanyreferences

to
it have been made in the literature most of them have dealt with
the mineralogyand there has beena tendencyto baseinterpreta-
tionsof the generalgeologyon inadequatefield work. The most
completepaperspublishedto date are those by Kozlowskiand
Jaskolskix and Chace. 2 After two yearsof work as geologistat
the Oruro minesthe writer hasarrivedat an interpretationof the
districtgeology,whichis presented in this paper,and whichdif-
fers from that of previousworkers. The presentpaper bears
chieflyon the generalgeologyand structure;the mineralogyhas
beenadequatelydescribed previouslyand only a brief review of
it will be given here.
Location.--The Oruro district is locatedon the Bolivianhigh
plateau2oo km southeastof LaPaz and about x5 km west of the
easternrangeof the Andeswhich forms the easternboundaryof
the plateau. Oruro, oneof the principalBoliviancities,liesat the
easternfoot of the isolatedrange of hills in which the minesare
situated. The campis at an elevationof about3,8o0 meters.
History.8--It is probablethat the first miningin the Oruro dis-
trict was doneby the nativesduring the Inca period (before the
SpanishConquest). In x595 the outcropof an ore-bodywas
discoveredby a SpanishPadre and mining was soonbegun. A
revolt by the nativesin x78x causedthe minesto be closed;they
were abandoneduntil •846 when someof them were reopened.
The CompafiiaMinera de Oruro was organizedin x885; today
this companyownsandoperates all the activeminesin the district.
In •931 Lindgren• estimatedthat Oruro hadproducedapproxi-
mately 8,ooo tons of silver with a value of 200 million dollars.
x Kozlowski, R., and Jaskolski, S.: Les gisementsargentostanniferesd' Oruro en
Bolivie. Arch. de Mineral. de la Soeiete des Sciences de Varsovie, 8: x-xoo, x932.
:•Chace,F. M.: Tin-silver veins of Oruro, Bolivia. Doctor's Dissertation,Harvard
University, x938.
8 Wiener, P.M.: Les mines d'argent d'Oruro, Bolivie. Ann. des Mines, Set. 9,
5:5II-52o, x894.
4 Lindgren, W., and Abbott, A. C.: The Silver-tin deposits of Oruro, Bolivia.
ECON. GEOL., 26: 453--479, I93I.
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. 89

From 192oto 1939 Oruro produced

489,543kilogramsof silver
and'16,658 metric tons of fine tin.*
RegionalSettin#.--The arid "altiplano" (high plain) with its
isolatedrangesof hills and playasis suggestive of the basin-and-
rangeprovinceof westernUnited States. The Cerrosde Oruro
are oneof the groupsof hillsrisingabovethe flat surfaceof the
plateau. This groupis about6 km longin a north-southdirection
and from I to 3 km wide its highestpoints being about 30o
metersabovethe surrounding.plain.
In the vicinityof Oruro the "altiplano" is coveredby a veneer
of alluvialdeposits. Theseare underlainby Paleozoicsedimen-
tary rocksconsistingof slightlymetamorphosed shalesand local
sandstonebeds. In the vicinity of the Cerros de Oruro the
regionalstrike'isapproximately north-south anddipsare steep.
The Cerrosde Oruro owe their topographic expression to the
intrusionof igneousmasses, uplift, and differentialerosion. The
hills are elongatedparallelto the regionalstrikeof the sedimentary
beds.
The ageof the sedimentary rocksis uncertainbecause of lackof
fossilsbut they are thoughtto be Devonian. Fossilsfound at
Viacha, 195 km north of Oruro, are considered to be Silurian or
Devonian. 5 In the Llallaguadistrict,75 km southeast of Oruro,
Koeberlin6 found fossilsthat he consideredto be of Devonianage.
The igneousactivity with whichthe mineralizationof the dis-
trict is associated
tookplaceduringthe Tertiary; it is assigned to
Miocene or Pliocene. 7
During the Pleistocenethe Cerrosde Oruro and other ranges
of hillsof this part of the "altiplano" stoodfor a time as islands
in a fresh-waterlake; this is evidencedby terracesof travertine
containingfossilsof a freshwatergastropod. 8 In the vicinityof
Oruro no well definedshorelines remain;their positionhasto be
inferred from the location of the travertine terraces.
* Data from Annual Reports of Compafiia Minera de Oruro.
• Lindgren, W.: op. tit.
• Koeberlin, F. R.: Private Report to Cia. Minera de Llallagua, x9x9.
? Berry, E. W.: Fossil plants from Bolivia and their bearing upon the age of
uplift of the Eastern Andes, Proc. U.S. Nat. Museum, 54: •o3-•64, •9•7.
s Kozlowski, R., and Jaskolski, S.: op. tit., p. xS.
9ø DONALD F. CAMPBELL.

The detailedgeologyof the mine area is shownin Fig.

FIG. I.
THE ORURO SILVER-TIN DISTRICT, BOLIVIA. 91

' o

SAAI JOS• MIA/œ AREA

FIG. I (continued).
92 DONALD F. CAMPBELL.

GEOLOGIC COLU!VIN FOR TI{E ORURO DISTRICT.

Quaternary
fRecent Alluvial Deposits
[Pleistocene Alluvial and Lake Deposits
'P o r p h y r i t i c Quartz Latite-
Younger Lavas
Cenozoic Breccia associated with the
youngerlavas; composed'offrag-
ments of quartz lat{te and shale;
of explosiveor{gin
Tertiary Pliocene (?) Explosive Brecc{a; composedof
fragmentsof rhyolite in a ground-
massof fragmental shale
Quartz Monzon{te Porphyry;
shallow {ntrus{ves
Rhyolite Porphyry; shallow intru-
sires and lavas

Paleozoic Devonian(?) Shale; •vith re{nor amounts of

sandstone,arg{ll{te,and quartzite

Paleozoic(Devonian (?)) SedimentaryRocks.

Shale.--Thesedimentary rocksare hereclassified
as shale;some
workershave calledthem slate,but the slaty portionsare better
describedas argillite since secondarycleavageis lacking. In
the immediatemineareathereare only a few sandylayersin the
shale;but in the IrocoHills, westof the mines,the sandybedsin
the shalelocallymaybe asmuchasfifty percent. The attitude
of the shalebedsin the vicinity of the igneousbodieschanges
abruptlyovershortdistances andcannotbe'usedas a guideto
regionalstructure.
The shaleoutcropneartheminesis small(Fig. I) but in the
mineworkings theareaincreasesdownward
(Figs.2, 3, 4, and5).
Overlargeareasin thelowerworkingscontact
metamorphism has
changed the shaleintoa lightgray,massive,
rockcomposed of
sericite
andquartz. In placesnearigneous
contacts,theorganic
matter of the shalehas beenconcentratedinto areas about 3 mm
in diameter,whichgivetherocka spotted appearance.Silicifica-
tion of the shalenear the major veinsand igneouscontactsis
common butnotextensiveexceptonthelowerlevels,wheresome
tourmalinizationalso occursnear igneouscontacts.
THE ORURO SILVER-TIN DISTRICT, BOLIVIA. 93
94 DONALD F. CAMPBELL.

Tertiary (Pliocene(?)) I•lneousand RelatedRocks.

RhyolitePorphyry.--The oldestigneousrockin the districtand
the mostexposedin the mine workingsis a mediumto dark gray
porphyrywith about40 per centof phenocrysts, of whichthree-
fourths are white altered feldspar and one-fourth are quartz.
Pyrite is universallydistributedthroughthis rock, and averages
aboutx to 2 per cent,but locallyalongfracturesmay reach35 per
cent of the rock. Hydrothermalalteration is so extensivethat
fresh specimens are rare. Disregardingalterationproducts,the
approximatemineralcomposition appearsto havebeen6o per cent
feldspar(2• orthoclase and • albite); 25 per centquartz;and 15-
per cent biotite. It is classifiedas a rhyolite porphyry.
Hydrothermal alteration may be divided into the following
stageson the basisof decreasingtemperature: (a) Development
of tourmalineand allanite.--Tourmalineis widespreadin the
Itos mine and in the lower levels of the other sections. Allanite
(tentative identification)was developedlocallyin the Itos mine
and in a few placesin the lower levelsof the othermines. Some
pyritemayhavebeenformedduringthis stage. (b) Development
of epidote from the feldspars,chlorite from the biotite, and
pyrite.--Epidote is more widely distributedthan tourmalineand
allaniteand is not confinedto the lower levels. Somepyrite was
f•)rmedduringthis stage.(c) Development
of sericite
and
quartz from feldspars,and of talc from the chloriteand pyrite.--
This stageof alterationwasthe mostintensiveand extensiveand
affectedall of the rhyoliteporphyrybodies. At the end of the
hydrothermalalterationconsiderable pyrite had beenintroduced.
The rhyoliteporphyryformeddikesthat widentowardthe sur-
face,and spreadout as surfaceflows. This conclusion is based
uponthe followingevidence:(I) The smallarea of shaleout-
cropsand the undergroundevidencethat rhyolite porphyryout-
cropsare underlainat shallowdepthby shale;(2) the decrease in
thesizeof therhyoliteporphyrybodieswith increasing depth;(3)
porphyritictexture and the presenceof flow structureand a
slightlyfinergrain in the rhyoliteporphyrynear the surface,al-
thoughnot direct evidence,favor the aboveconclusion.The
 THE ORURO SILFER-TIN DISTRICT, BOLIVIA. 95

tl

/ E VœL ZE'RO

FIG. 4.

rhyolitebodiesmight be erodedlaccoliths,but it is thoughtthat

therehasnot beenenoughmaterialremoved by erosionto support
this theory. The outcropof the rhyoliteporphyrygivesno in-
dicationof the shapeof the bodiesat depth. At the horizon
of the lowermine levelsthe rhyoliteporphyrydikeshavea width
of from zooto 500 metersin an east-west directionanda length
exceeding800 meters (Figs. • and 5). Small dikes and sills
penetratethe surroundingshalesalongthe main contacts.
Alongsomeof therhyoliteporphyry-shale
contactsthereis some
contactbrecciaconsistingof shale fragmentsin a porphyry
groundmass,whichdiffersfrom the explosive
brecciawhichlatter
is composed
of fragmentsof rhyoliteporphyryin a crushedshale
groundmass.
Other I•7neousand RelatedRocks.--The remainingrocksof
thedistrictexceptthealluvialdeposits,
arepartof a groupformed
duringthe sameperiodof igneous activityat approximately
the
sametime. They include:'
96 DONALD F. CAMPBELL.

(I) Quartz monzoniteporphyry, which is a medium, dark

greenish-gray, porphyrywith about 5o per cent of phenocrysts.
About six-tenthsof theseare feldspar,two-tenthsare quartz,and
two-tenthsare biotitealteredto chlorite. Perhaps1.5 per cent
of the rock is composed of large sanidinephenocrysts averaging
I cm in lengthbut with a maximumlengthof I I cm.
This rock originallyconsistedof 65 to 7o per cent feldspar,
nearlyequallydividedbetweenorthoclase and andesine;15-2o per
centbiotite; and aboutx5 per centquartz. It is coarsergrained
thantherhyoliteporphyryandcontainsmorephenocrysts.Under
the microscope thegroundmass showsa fairly well'developedmon-
zonitictexture,whichis lackingin the groundmass of the rhyolite
porphyry. For thesereasonsit has been classifiedas a quartz
monzoniteporphyry.
Hydrothermalalterationconsisted of the development of sericite
from the feldsparsand of chloritefrom the biotite.
The quartz monzoniteporphyry was intruded as small stocks
and large dikes(Fig. I), the largestbodybeingabout x.2 by x.8
km across. A few small dikes have been encountered under-
ground,but in generalthe workingsare in olderrocksaroundthe
peripheryof the quartzmonzonitebodies. Dikes of quartzmon-
zonite porphyrycut rhyoliteporphyrybodies;this may be seen
bothon the surfaceand in the mine (Figs. I, 3, 4, and 5).
(2) Explosivebreccia.mThisrock is composed of light gray
rhyoliteporphyryfragmentsin a darkgraygroundmass of crushed
shale. The porphyryfragmentsvary in diameterfrom several
millimetersto morethana meterand may makeup from lessthan
I per centto more than 8o per centof the volumeof the breccia,
and averageabout2o per cent. In mostof the brecciathe por-
phyryfragmentsare angularbut in someof the brecciadikesthey
havebeenrounded. The fragments are highlyalteredto sericite,
quartz, and pyrite. Under the microscope the texture is seento
bethesameasthat of therhyoliteporphyryof the adjacentigneous
bodies;the monzonitictexture of the groundmassof the quartz
monzonite
porphyryis notablyabsent.Microscopic
study.con-
firmsthemegascopic
determination
of thecharacter
of theground-
 THE ORURO SILVER-TIN Dt•STRICT, BOLIVIA. 97

massof the breccia. In generalthe amountof porphyryfrag-

mentsin the brecciadecreases away from the rhyoliteporphyry
contacts,and the outer contactbetweenbrecciaand shaleis grada-
tional.
The explosivebrecciaoccursaspipes,dikes,and irregularbodies
that conformin a generalway to the rhyoliteporphyry-shale
con-
tacts. No brecciabodiesoccur in the shale or in the rhyolite
porphyryfar from their mutualcontact,but not all partsof the
contactshave brecciabodies. As far as is known, brecciabodies
were not formedalongcontactsbetweenquartz monzonitepor-
phyryandshale,but mayextendup to it (Fig. 4). The horizontal
extent of the breccia bodies is at a maximum near the surface and
decreases downward. There were two stagesof brecciaforma-
tion, sincenormalbrecciais cut by dikesof finer grainedbreccia
that containwell-rounded
fragmentsof rhyoliteporphyry.
The nature of the breccia indicates that it must have been
formedby explosive agencies,'andsincetherhyoliteporphyryfrag-
ments are angular, brecciationmust have been subsequent to
solidification.The shapeof someof the brecciabodiesis the same
as that assumedby intrudedmagmas;they form dikescutting
rhyoliteporphyry.It is possible thattheshalymaterialmakingup
thegroundmass of thebrecciamayhavebeenmadesoftby igneous
heat,permittingit to behaveunderpressure as a mud. Probably
the explosiveenergyand possible heat necessary for the breccia
formationresulted fromthesameigneous activitythatgaveriseto
thequartzmonzonite. Brecciaformationis thoughtto havetaken
placeshortlyafter the quartz monzoniteintrusionbecausesome
brecciabodiesenclosemajor veins and becausea similar breccia
outcropping on Cerro Serrato (Fig. x, coordinates N. x5o to 4oo,
E. 60o) containsfragmentsof quartzlatite,a volcanicphaseof
the monzonite. Avenuesof relief for the explosiveenergythat
causedthe breccia. were localizedby the contactsbetweenrhyolite
porphyry
andshale.,It is nocoincidence
thattheexplosive
brec-
cia bodiesand the veinsoccurin nearly the samearea. If all
of the igneousrocksof the district are assumedto be of the same
98 DON•ILD F. C•IMPBELL.

age,as has beenassumedin the past,it is hard to explainsatis-

factorilythe formationof the explosivebreccia.
The formation of explosivebrecciatook placebefore the for-
mation of the fracturesthat later becameveins, becausein many
placesveinsoccurin the explosivebreccia.
(3) Brecciaassociated with the youngerlavas.--At the eastern
sideof the minearea (Fig. •, coordinates N. x5o to 400, E. 60o)
there is a dike-likeoutcropof brecciathat consistsof a mixture
of quartz latite and shale fragmentsaveragingabout 5 mm in
diameter;roughlytwo-thirdsof the fragmentsare quartz latite.
Immediatelyto the east of this outcropflows of quartz latite cap
the hill. This brecciais undoubtedlyof explosiveorigin and was
probablyformed along the channelwaythrough which the lavas
were extruded. It is thoughtto be slightlyyoungerin age than
the intrusivesof quartz monzoniteporphyry.
(4) Younger Lavas.--Two closelyrelated types have been
identified,quartzlatite and dacite,bothporphyritic,and extrusive
equivalentsof the quartz monzoniteporphyry. Most of these
lavas are thoughtto be contemporaneous with the quartz mon-
zoniteporphyry;but someof themin the area north of the mines
are regardedas slightlyearlier,because:(a) theyare alteredmore
thanotherlavasof similarcomposition; (b) in placestheyappear
to becutby dikesof quartzmonzoniteporphyry;(c) in placesthe
contactbetweenthemand the quartzmonzoniteis gradational,the
sequence observed beingpurequartzlatite,nearlysquareblocksof
quartzlatite definedby jointsabouta meterapartand concentri-
callyalteredinwardfromthe joints(caused by solutions
from the
quartzmonzonite),quartz latite of whichthe joints are occupied
by material closelyresemblingthe quartz monzonitein color
(thoughtto havebeenformedfrom the quartzlatiteby solutions
from the quartzmonzonite),purequartzmonzonite. ß

Alluvial and Lake Deposits.

Theseare of interestbecause in placesthe•ycontaineconomic
tin placers. The placersoccuron bedrockin canyons that lead
out of the hills; the depositswere built up wherethe intermittent
 THE ORURO SILFER-TIN DISTRICT, BOLIFI•t. 99

streamsfrom the mineralizedportionsof the hills enteredthe

Pleistocene
lakethat formerlyoccupied
thispartof the"altiplano."
There is an abruptchangein the streamgradientsat this point.
The placertin is nowcoveredby from 5 to 15 metersof worthless
alluvium.

STRUCTURAL GEOLOGY.

District Structure.

The intrusionsof the igneousbodiesmaking up the Cerrosde

Orurotookplacealonga zoneof steeplydippingbedsof Paleozoic
sediments. The Iroco Hills, west of the Cerros de Oruro, consist
of a faulted anticline overturned to the eastward. Shale ex-
posuresin the Cerrosde Oruro havebeenso disturbedby nearby
intrusionsthat the local structureof the sedimentaryrocks pre-
vious to the intrusionscannotbe determinedexactly,nor is the
relation of the Cerros de Oruro to regionalAndean structureap-
parentfrom thelocalgeology. The north-south elongation
of the
hills is due to the regionalstrike of the sedimentarybeds,which
has localizedand elongatedthe intrusives. This is especiallytrue
of the ore-enclosing rhyoliteporphyry,but the youngerquartz
monzonziteporphyrybodiestend to be more equidimensionaI.
Locally the attitude of the shalebedshas had but little effect on
the shapeof the intrusives.

Pre-Mineral Fracturing in the Mine ,4rea.

Location.--Thehighlymineralizedpart of the districtconsists
of an area of older rocks lying betweentwo large quartz mon-
zoniteporphyryintrusives. One of theseintrusivesoutcropsin
the San Felipearea (Fig. • ). The other outcropsin the San
Pedro-Vizcachani area in the northernpart of the district(only
part of whichis shownin Fig. I ).
The mineralizedarea may be dividedinto five major fracture
systemsthat persistto appreciable depths;they are the Purisima,
Bronce(the SantoDomingovein is part of the Broncesystem),
IOO DONALD F. CAMPBELL.

San Josd,Grande-Moropoto,and San Luis. Fig. x showsthe

locationof thesesystems. The Purisima (Fig. 5) and Bronce
are regardedas separatefracture systems,becauseon the lower
mine levelsthe differencebetweentheir averagestrikesis nearly
9oø. The GrandeandMoropoto(Figs. 4, 5), fracturesareparal-
lel and about •5 o metersapart; they are consideredas members
of one systembecausethey have apparentlybeen formed by the
samestresses.Thesemajorfracturesystems
mayindividually
be
perpendicularto, or parallelwith, major rhyoliteporphyrydikes.
In addition to thesemajor systemswith their related smaller
fractures,thereare severalminor groupsof veinsthat havebeen
worked near the surface. These occur at some distance from,
and are independent of, the major fracture systems,but are also
thoughtto havebeenformedby the stresses setup by the intrusion
and solidification
of the quartzmonzoniteporphyry.
GeneralCharacteristics of FractureSystems.--(• ) Fracturing
is morecomplexnearthepresentsurfacethanit is at depth(Figs.
3, 6). (2) The dip of someof the main fracturessteepens with
depth. (3) The fracturesystemsare locatedaroundthe edgesof
and betweenthe monzoniteporphyryintrusions. (4) The exact
location and attitude of the main fracture zones were influenced
by the locationand attitude of large rhyolite porphyry dikes in
shale,as well as by the locationand shapeof explosivebreccia
bodies. The largerorebodiesseemto havebeenformedwherethe
individualfracturesare bestdefinedand correspond to the average
trend of the main fracturezone. Zoneswherecrackingand brec-
ciationwere mostextensiveare the sitesof the largestore bodies:
(5) A fracture systemconsistsof a main fracture or fracture
zoneand numerousassociated minor fractures. Theselatter may
be: (a) conjugateand parallelcausedby the samestressesthat
formedthe main fracture; (b) branchfractures,causedby ad-
justmentsbetweenpartsof the larger blocks,and slumpingin the
hangingwell of the main fractures;and (c) fracturescausedby a
combinationof causesa and b. There may thus be mineraliza-
tion in the walls of the main vein as in the San Luis fracture, Itos
mine.
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. IOI

The Purisima and Bronce systems,in contrastto the others,

do not steepen
with depth.(Figs. x, 2, 5, 6, 7)- In som.
e places
where they cut inclined igneouscontacts,displacementsuggests
that reversemovementhas takenplace. The displacement of a
contactbetweenrhyoliteporphyryand shaleindicatesthat normal
faulting movementwith a verticalcomponentof about90 meters
has takenplacealongthe Grandefracturesystem.
The San Luis fracturesystemoccursin rhyoliteporphyrynear
the shalecontact. Practicallyall of the mineralizationis in the
porphyry;the fracturesdie out rapidlywherethey penetratethe
shale. In the rhyoliteporphyrynear the San Luis fracture sys-
tem there are minor fractures,mostlyseveralmetersapart but in
placessocloselyspacedasto form sheetedzones. The main frac-
turesin the porphyry,normalto the contact,convergein the direc-
tion of the contact. 'It is probablethat vertical movementwas
greatestat the contactand decreased away from it into the por-
102 DON.4L.D F. CAMPBELL.
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. lO3

phyry. Under theseconditions,rotationalmovementalong the

main non-parallelfractureswould set up stressesthat probably
would causethe developmentof the complexminor fractures.
The ore bodiesof the Itos mine were indirectlylocalizedby the
eastcontactof the large rhyoliteporphyrybody; for this reason
stepsare beingtakento locateandprospect the westcontact.
Characteristicsof Individual Fractures.--Theseare: (•) The
veinmineralization
involves
deposition
alonga seriesof parallel,
curving,and overlappingcracks. The curvingparts of the frac-
tures departing from the main zone rarely contain more than a
thin stringer of the ore minerals. (2) Individual fractures are
commonlyirregularly "S" shapedor arc like. (3) Fractures
generallyare longestdown the dip and may overlapin plan and
section. (4) The movementalong fracturesis thought to have
beenrelativelysmall.- (5) Striae in placesindicatehinge move-
mentalongpre-mineralfractures. (6) Fracturesdo not intersect,
but die out before this occurs,and no bonanzasoccur in the vicin-
ity of such approach. (7) Fractures do not offset one another
appreciably. (8) Brecciation of wall rocksis present,
alongmost
of the larger veins near the surface, and slickensidesin lower
levels. Numbersx to 5 are from Chace.9
Relationof Fracturesto Wall Rocks.--Eachwall rockfractured
differentlyand the locationand shapeof the fractureswere in-
fluencedby the wall rock assemblage.
Thustherhyolite
porphyry
fractured
asa brittle
material
ar/d
was conduciveto the formationof persistentfractures.
Shale behavedlike a semi-plasticmaterial and hencedid not
yieldpersistent fractures;consequently
wherefracturespassfrom
rhyolite into shalethey die out shortly. In one case,however,
a strong vein, the Grande, extendeddown into the shale from
--5 ø to --29o meters becausethe rock was locally metamor-
phosedand hencemore brittle.
The explosivebrecciaalsofracturedlike a semi-plastic
material,
but lessso than the shalebecauseof includedrhyoliteporphyry
• Chace, F. M., op. cit., pp. 52-57.
104 DONALD F. CAMPBELL.

fragments. In addition, becausethe explosivebrecciaswere

zonesof weakness, they influencedthe loci of fracturingto some
extent,as for example,the San Josd.
Contactsbetweenrhyoliteporphyryand shalewere not favor-
able vein loci.
Origin of theFractures.--When the mineralizedareaas a whole
is considered, the fracturesseemto be haphazardlyarrangedand
their origin cannotbe explainedby stresses
of the sametypeacting
in the samedirectionover the entire district. However, when the
quartz monzoniteporphyrybodiesare taken into consideration,
the fracturesare seento be more or lesssystematically
arranged
aroundthem;therefore,
the fractures
arethought
to bedirectly
related to the porphyry bodies. The larger quartz monzonite
porphyrybodies(i.e. CerrosSan Felipeand W. Vizcachani)are
considered to beareasthat havebeenupliftedby igneousintrusion.
The area betweenthem (i.e. CerrosTodos Santosand Colorada,
Fig. •) is an area that collapsed. Thus, the fractures of the
Purisimaand Broncesystemsmay be interpretedas reversefaults
dippingtoward the area of uplift from near its margins,caused
either
bytheupthrust
of theintruding
magma
or itsvapor
pres-
sure. The fractures
of theSanJosd,Grande-MoroP0to
, andSan
Luis vein systems,may be interpretedas normal faults aroundan
area that collapsed becauseof non-support. This explanationis
advancedonly for the initiation of the major fractures;the de-
tails of fracture formationundoubtedlybecamemore complexas
the processcontinued. Local detailsof structurewere influenced
by the physicalcharacteristics of the intrudedrocks.
In the opinionof the writer not all of the fracturesystems could
havebeen-formed directlyby gaspressuregeneratedby the cooling
magma< The Purisima and Broncefracture systemscouldhave
beenformedin this way; but the vaporpressurehypothesis alone
couldnot applyto the San Josd,Grande-Moropoto, and San Luis
systemsbecausethey partially surroundand dip toward an area
that fieldevidenceindicatesto be comparatively' low in the roof of
the stock. If it is assumedthat thesefracturesystemspartially
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. lO5

surroundand dip towardthe regionalhigh pointof the stock,it

becomes
difficultto understandwhy the Purisimaand Broncesys-
temsshowsucha closerelationto the outcropof the San Felipd-
Lulicanchointrusivebody,comparatively
smallfrom a regional
point of view.
Vapor,pressure of the coolingmagmaand shrinkagedue to
crystallization
of the magmamay haveplayedan importantpart
in accentuating the fracturesthat wereinitiatedby the upthrust
of the intrudingmagmaand slumpingof areasbetweenthe high
pointsof the roof of the intrusive.
There were two stagesof mineralizationin the district (see
Mineralogy). After the earlystage(pyrite-cassiterite) the veins
becamesealedand were reopened,with resultingbrecciationof
early stagevein material,at the beginningof late stage (sulpho-
salt) mineralization. Reopeningof the veins may have been.
causedeitherby renewedfault movementor by v&porpressureof
the magmawhichbuilt up after the veinsbecamesealedfollowing
the early stagemineralization.

Post-Mineral Fracturing in the Mine Area.

No faults have been observed in the mine area that can be estab-
lishedas post-mineral
in age. There are numerouscasesin which
a fracturemay be followedalongthe strike from ore into barren
gouge. However, this conditionis attributed to the fact that the
gougeexcludedmineralizingsolutionsfrom someparts of the
fracture.'

Economic CEOLOC¾.

Mineralogy.
Numerousmineralogicalstudieshave beenmade of the Oruro
deposits, anda largenumberof mineralshavebeendegcribed.A
list of thosewhichhavebeenreportedfrom the districthas been
compiledby Chace•oand is givenbelow:
xoChace, F. M.: op. cit., p. 8z.
106 DON.4LD F. C.4MPBELL.

Hypogene
Gangue Oxidized
Hypogene Sulphide Minerals Minerals Zone Minerals

Pyrite Plagionite Quartz Limonite

Cassiterite Wurtzite Sericite Jarosite
Arsenopyrite Hematite Kaolinite Melanterite
Tetrahedrite Pyrrhotite Alunite Marcasite
Andorite Boulangerite Tourmaline Metastibnite
Zinkenite Meneghinite Augelite Chalcedony
Jamesonite Chalcostibite Wavellite Hydrohematite
Bournonite Semseyite Apatite Anglesite
Franckeite Stibnite Barite Cervantite
Galena Wolframite Siderite Cerussite
Stannite Ruby Silver Ankerite * Corellite
Sphalerite Stephanite Cerargyrite
Chalcopyrite Freieslebenite
Miargyrite Marcasite
* This mineral has been observed by the writer in the wide veins west of Cerro
San Pedro; it has not been seen in the veins in the Cerros de Oruro.

The hypogenemineralizationof the district has taken placein

two distinct stages,separatedby a structuralbreak. The min-
eralizationof the early stage consistsof pyrite, quartz, and cas-
siterite. The mineralizationof the late stage is complex and
consists chieflyof a largenumberof sulpho-salts.
If the outlying parts of the district are considered,it may be
possibleto add a third stageof mineralization,althoughevidence
for this is inconclusive. In the Iroco Hills, west of the mines,
there are numerousquartz veinssomeof whichcarry native gold.
In the smallhills westof San Pedro (northwesternpart of the dis-
trict) thereare two large quartz-ankeriteveinsthat containsmall
amountsof silverand tin. The locationof theseveinsaway from
the mainareaof mineralizationsuggests that theymay represent a
comparatively low te.mperaturestageof the samemetallogenetic
epochof whichthe tin-silverveinsare earliermembers. The fact
that there are a few narrow quartz veinsin the outlyingparts of
the mineralizedareasof the Cerrosde Oruro is regardedas evi-
dence in favor of this conclusion.
Sta#esof Hypo#eneMineralization.--(a) Early Sta#e. The
importantmineralsof this stageare pyrite,quartz,andcassiterite,
named in order of abundance. Of the total, pyrite makes up
roughly85 per cent,quartz x5 per cent,and cassiteritemay con-
 THE ORURO SILI/ER-TIN DISTRICT, BOLI[/I/I. Io7

stitute5 per centof the vein materialin the ore-shootsbut much

less in the veins as a whole. Most of the recoverable tin of the
veins, presentin the form of cassiterite,was depositedduring
this stage. The tin present in the late stage tin minerals,
franckeiteand stannite,presentssucha difficultrecoveryproblem
that they do not constitutean importantsourceof tin evenwhere
they occurin appreciable quantities.
It is estimated that about two-thirds of the vein material of this
stagewas depositedby replacement of wall rocksand one-thirdby
fillingof openspaces
in the fracturezone.
Microscopic
studyindicatesthat the depositionof quartz and
pyrite began before, and continuedafter, cassiterite. However,
thereare manyareasof nearlybarrenpyrite,thusindicatingthat
cassiteritedepositionwas not nearly so extensiveas pyrite and
quartz deposition;cassiteriteand later pyrite mineralizationtook
placealongmore localizedzones.
KozlowskiandJaskolski
• in additionto stagesequivalent
to
the early and late stageshere described,recognizedan earlier
highertemperature stagecharacterized
by tourmaline. However,
the writer regardsthe developmentof tourmalineas beingmore
closelyrelated to rock alteration than to vein mineralizationbe-
causeof the widespreadoccurrence of tourmalineaway from the
veins in rocks of the Itos section and in the rocks of the lower
levelsof the other sections,and the scarcityof tourmalinein the
vein filling.
(b) Late Sta#e. The importantmineralslistedin their order
of abundancein the ore now being mined are jamesonite(or
zinkenite), tetrahedrite (freibergite), galena, franckeite, an-
dorite, bournonite,and stannite. The other primary minerals
from the list of minerals reported from the Oruro veins are
presentin negligibleamountsin the ore minedat present,although
it is possiblethat they were presentin appreciable
amountsin the
bonanza ore-shoots that were mined at earlier times. The chief
sourceof silver in the ore is freibergiteand andorite; but the
mineralsjames0nite,
galena,andbournonite
carrysomesilver,as
xx Kozlowski, R., and Jaskolski, S.: op. cit., p. 63.
•'o8 DON.4LD F. C.4MPBELL.

hasbeenprovedby assays of individualspecimensof thesemin-

erals. Sincesomeof theminerals of thisstagelookmuchalike,it
is difficultto estimate
theamountof eachpresent in theore; the
following
percentage
estimate
mustberegarded
asveryapproxi-
mate:jamesonite 67, tetrahedrite•3, galena,4, franckeite
4, an-
dorite2, bournonite2, stannite
2, allothersulphide minerals6 per
cent. It is estimatedthat thevolumeof veinmaterialdeposited
duringthelatestageis abouthalf of that deposited duringthe
early stage.
Replacementof wall rock or older vein mineralswas much less
predominantduringthisstagethanduringthe earlystage;it is
estimated
that onlyabout35 per centof late stagematerialwas
deposited
by replacement
of earliermineralsand wall rocks,the
remainderbeingdeposited
by fillingof openspaces.
The orderof deposition
of thecommon late-stage
mineralsas
determinedby Chace• is stannite,freibergite,bournonite,an-
dorite,galena,and franckeite.
Although slickensided vein material suggeststhat minor
structuraladjustmentstook placealongthe fracturesfrom time
to time during vein formation,the two stagesof mineralization
wereseparated by a distinctstructuralbreak. The late-stage min-
eralizationtookplaceafter a reopening of the fracturesoccupied
by early-stageminerals. This conclusion is basedon the follow-
ing evidence:there has beenextensivebrecciationof the early
minerals;the reopenedfracturesdo not alwayscorrespond with
theearlierfracturesthathavebeenfilledandsealed with thepyritic
ore of the earlystage. Fracturesoccupied by late-stageminerals
are sometimesfound at one side of the early-stageveins. At
othertimeslate-stageveinsare separatedfrom the early pyrite-
cassiteritevein by a horseof waste.
Zoning.--In the district as a whole lead-silvermineralization
(later late-stage)is the mostwidespreadareallyalthoughthe total
amountof vein material depositedwas not great. In the mines
themselves
thequartz-pyrite
mineralization
is themostwidespread.
x2 Chace, F. M.: op. cit., p. 89.
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. 109

The silverand tin ore occursas shootsdefinedby assaylimits in

the more extensivepyritic veins.
The late-stage(chieflysilver) mineralizationis moreextensive
horizontallyand vertically than the tin mineralization(early
stage). Veins of argentiferousgalenaare knownon the surface
outsideof the areaoccupied by the tin veins. In places,the San
Luis and San Josdveinsmay be followedalong the strike away
from productiveore-shoots comparatively high in both tin and
silver,to stringersthat containa little silverandno tin. Available
data indicatesthat the ore of the lower levels,in generalbelowthe
--300 m level,was minedchiefly for its silver content.
It is thoughtthat the wider distributionof silver-bearingmin-
erals occurredbecausethe silver mineralizationtook place at 'a
lower temperaturethan the tin mineralization. In the earlier
stagesof the silvermineralization,depositionpersistedlaterally
farther from the center of influx of the ore-forming solutions
thandid the tin deposition.This wasbecause
the lowertem-
peraturesilver-bearingsolutionswere not so sensitiveto decreas-
ing temperatureas the high temperaturetin-bearingsolutions.
During the later stagesof the silver mineralizationas the tem-
peraturerecededdownward,the silver couldbe depositeddeeper
and depositionpersistedto greater depths than did the tin
deposition..
Oxidation.--The silverhas beenleachedfrom the outcropsso
that the oxidizedoresbeingminedat presentare workedentirely
for their tin content.
The major veinshavebeenoxidizedto as muchas I5O meters
verticallybelowthe 9utcrop. Minor veins,which are narrower
and morecompact,in placesare not oxidizedmorethan 25 m be-
low the outcrop. The veins are oxidizeddeeperthan the wall
rock.
The mineralsof the oxidezoneare limonite,quartzand chalce-
dony,jarosite,cassiterite,
and cerussite. Thoroughness
of early
silver mining near the surfacehas beensuchthat it is difficult'to
get exact informationaboutsupergene silver enrichmentbut it is
knownthat a zoneof secondaryenrichmentdid exist. During the
iio DON,4LD F. CztMPBELL.

periodthe writer has been familiar with the deposits,no silver

minerals have been identified in the oxide zone; but old miners
say that silver chlorideswere formerly mined near the surface.
There hasbeena residualconcentration of cassiteritecausedby
leachingof otherconstituents.It is possible
that somesecondary
cassiterite
hasbeenformedby re-deposition of tin released
by the
breakdown of stannite and franckeite.

Origin of the Deposits.

Sourceof the Ore-FormingFluids.--The depositsare directly
related to the quartz monzoniteporphyry. The intrusion and
solidification
of the quartzmonzoniteporphyrybodiesare thought
to have caused the formation of the vein fractures. The ore-
formingfluidscamefrom the samemagmabodywhoseupperpart
terminatesin the quartzmonzoniteporphyryoutcrops. This view
is held because the tin-silverveinsare systematicallyarrangedin
older rocksnear small quartz monzonitestocks,and becauseno
productivetin veinsare knownto occurin the quartzmonzonite.
Temperatureof Formation.--Tourmaline, which is widely de-
velopedin someparts of the mine, especiallyin the lower levels,
is indicativeof high temperature. However, sinceit is rare in
the vein filling, and its occurrenceis considered
to result from pre-
mineral wall rock alteration, its presencemay not indicate the
temperatureexisting at the time the vein mineralswere formed.
It is probablethat vein mineralizationtook place at somewhat
lower temperaturethan is required for the formation of tour-
maline.
The early-stagemineralizationis known to have taken place
at a higher temperaturethan the late stage,as is indicatedby the
minerals,and the fact that replacementof wall rock was more
intensein the early than in the late stage. Cassiteriteis formed
chieflyat high or relativelyhigh temperatures.
•a For this reason
the quartz-pyrite-cassiterite
mineral assemblage of the early stage
mineralizationis thoughtto have been formed at relativelyhigh
xa Ferguson, H. G., and Bateman, A.M.: Geologic features of tin deposits. Ecoa.
GEOL., 7:209--262, I9 I2-
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. 'r'r 'r

temperatures. The walls of veins or stringers occupiedexclu-

sivelyby early-stagemineralstend to showa gradationfrom vein
material to wall rock; this is not true of fracturesoccupiedex-
clusivelyby late-stageminerals. Accordingto Lindgren•4 tem-
peraturesof formation of the group of minerals of which the
secondstagemineralsare membersmay have rangedfrom • 75o to
•3oo ø C.
D.epthof Formation.--Directtopographic evidenceas to the
depthat whichmineralizationtook placeis lacking. The maxi-
mum relief in the districtis nearly 300 meters;so it seemslikely
that at least300 m of rockhasbeenerodedsincethe formationof
the deposits.
Geologicevidenceis presentbut its exactinterpretationis more
or lessa matter of opinion. The explosivebrecciabodiesin the
mineshave their maximumdevelopment relativelynear the sur-
face; below the --200 m level there is lessexplosivebrecciathan
on the--5 ¸ m level. The fracturingis complexnearthe surface
and below the --200 m level it is relativelysimple,but on the
--300 m levelthe main veinshave only a few branches. At the
surfacealong someof the major veins there are large bodiesof
brecciated wall rock that have been mineralized; on the lower
levels mineralized breccia is unknown. The grade of tin in the
major veins beginsto decreasemarkedly at about the --200 m
level. All this evidenceindicatesthat mineralizationtook place
relativelynear the surface.
Since all of the changesnoted take placewithin about 300 m
of thehighestveinoutcrop,it seemsreasonable to estimat•that the
mineralmatter of the presentvein outcropswas depositedat a
depthof around I,OOOto 2,000 feet.
Previousestimates of depthof formationwere:aslittle as I,OOO
feet by Lindgren;•5 and I,OOOto 4,000 feet by Chace?
Classificationof the Deposits.--Lindgren•7 has classifiedthe
Oruro veins as mesothermal. If the classification is based on
x4 Lindgren, W.: Mineral Deposits, pp. 599-53o , 4th edit., I933.
x• Lindgren, W., and Abbott, A. C.: op. cit.
•6 Chace, F. M.: op. cit., p. 117.
x? Lindgren, W.: op. cit., pp. 579-580.
II2 DONALD F. CAMPBELL.

mineralassociation
alone,the veinsproperlybelongin this group.
If Graton'smodificationof Lindgren'szones•8 is considered,
the
early stage quartz-pyrite-cassiteritemineralization would be
classedas mesothermal,whereaslate stagesulpho-saltmineraliza-
'tionwould
beclassed
asleptothermal.
If classified
onthebasis
of
depthalone,theveinswouldfall in the epithermalgroup.
Buddingtonxøhas given the name "xenothermal" to veinsin
which high temperaturemineral associations are found relatively
nearthe surface,i.e., at a depthof 2,000 to 3,000 feet,and at a
temperatureof 300ø to 500ø C. In view of the relativelyhigh
temperaturemineralassociationin the Oruro veinsand the shallow
depthat which they formed, it is the opinionof the writer that
the veins should be classed as xenothermal.

PRACTICAL CONSIDERATIONS.

Distributionof Tin and Silver.--There is a myth aroundold

miningcampsthat ore minedlong ago musthavebeenvery high
grade. Some of it undoubtedlywas but certainlynot all of it.
There is evidencein the Oruro minesthat someof the old stopes
wereminedon ore that cannotbe workedat a profitunderpresent
conditions. It is probablethat low-gradeore from suchstopes
was mixed with highergradeore from otherpartsof the mine so
as to increasethe tonnagemilled.
Sincethe upperparts of the major veinswere mined out long
ago, it is difficult to determinethe location of the horizons of
highestgradetin and silverore. It is probablethat the silverore
was more consistentin grade throughoutthe vertical range ex-
posedby mining than was the tin ore. High-grade silver ore
from the lower levelsdoesnot necessarily indicatethat the silver
contentincreases with depthbecause not enoughis known about
the silvercontentof the ore minedearlierfrom the higherlevels.
It is the writer's opinionthat the silvercontentof the ore did not
:•8Graton, L. C.: The hydrothermal depth-zones. Ore Deposits of the Western
States, Lindgren Volume, A. I. M. E., pp. x86-x89. x933.
•0 Buddington, A. F.: High temperature mineral associations at shallow to mod-
erate depths. ECON. GEOL., 30: 205--222, I935.
 THE ORURO SILVER-TIN DISTRICT, BOLIVIA. I 13

increasewith depththroughoutthe mine area as a whole. The

tin contentof someof the minor veinssuggests that rapid deposi-
tion tookplacein themnearthe surface;it is possiblethat the same
thing occurredin the major veins. In them the mostproductive
zone was from the zero to the --xoo m level.
When the veins are followed downward it is found that the tin
contentdecreases soonerand more sharplythan doesthe silver
content. In the veins that have beenworked and exploredex-
tensivelybelowthe --200 m level (San Luis, Grande,Bronce,and
Purisima) it was found that the tin contentbegan to decrease
sharplyat a higherlevelthan the corresponding decreaseof the
silvercontent. In the San Josdvein, DeWijs s0found that the tin
contentdroppedrathersharplybelowthe --Ioo m level (elevation
3,700 m) but the corresponding decrease in the silvercontentwas
at the--x 5¸ m level(elevation3,650 m).
It is known that ore from the lower levels of the Grande vein,
betweenthe --200 m and --290 m levels,was mined primarily
for its silver content. The few data still available from the lower
levels of the San Luis and Puri•ima veins indicates that those ores
were alsominedchieflyfor the silverthey contained. The lower
levelsof theseveinswere mineda numberof yearsago and the
workingsare now flooded. The deeperveinscarry tin to greater
depths,for example,the Bronceand San Luis veinshave been
workeddeeperthan any otherof the major veins. In thesetwo
veinsgoodtin ore extendeddownwardfarther than in veinsthat
were not workedso deep.
The ore-shoots of all partsof the mineare laterallydefinedby
assaylimits; bodiesof workableore can be followedalongthe
strike into ore too low gradeto be workedunderpresentcon-
ditions.

In generalit canbe saidthat silverore is morepersistentlat-

erallyandverticallythanthe tin ore. Depositionof tin seemsto
havebeenmoreclosely controlledby depththansilverdeposition,
probablybecausethe higher-temperature, tin-bearingsolutions
s0DeWijs, H. J.: Private Rgport to Cia. Minera de Oruro, I937.
 DONALD F. CAMPBELL.

were moresensitiveto decrease of temperatureas they spreadout

laterally or approachedthe surface.
There are numerous minor veins tha.t were worked near the
surface for their tin; many contain no silver-bearingminerals.
Where veinsof this type havebeencut from 50 to xoo m below
their outcrop,they were barren but veins containingappreciable
silver-bearingmineralsshowa lessertendencyto becomebarren
abruptlyat suchshallowdepth. In general,veins of this type
occuron the hangingwall side of the major veins. Such veins
indicatethat part of the tin mineralizationmust have beenclosely
controlledby depth. It is thought that at the beginningof tin
mineralizationrapid depositionoccurredas the solutionsneared
the surface. As tin mineralizationcontinued,the temperatureof
theore-formingsolutions decreasedenoughto permitdeposition at
increasingdepth. During this time someof the minor fractures
encounteredby the first solutionsbecameclosed,and no further
depositioncould occur.
Depth of Ore.--It is estimatedthat the lower limit of ore mine-
ableunderpresentconditions is aboutthe--360 m level (elevation
3,440 m), 460 metersbelow the averageelevationof the higher
outcrops. This figureis arrivedat by weightingthe veinsaccord-
ing to their importance,
whichof coursecanbe onlyapproximate,
especiallysince the lower levels have been flooded for a number
of years.
Favorable Areas for Future Prospectint7.--Althoughseveral
possibilities
worthy of explorationexist in the immediatevicinity
of the mines,the mostfavorableareasfor prospecting are in the
outlyingpartsof the hills in areasof olderrocksnearquartzmon-
zonite porphyry intrusives. Since such areas are for the most
part coveredby alluvial deposits,geophysical prospectingwill be
ne'cessary
to localizefavorablespotsso that somemoreexact
methodof prospecting can be efficientlyused. The gravelsof
severalof the canyonsshouldbe prospected for tin placers,just
below the Pleistocene lake level.
 THE ORURO •ClL["ER-TIN DISTRICT, BOLI["I.4. I 15

ACKNOWLEDGMENTS.

The writer wishesto thankMr. W. V. DeCamp,GeneralMan-

agcrof Mines,MauricioHochschild,S. A.M. I. for permission
to
publishthis paper. Gratefulacknowledgment is due Professors
H. Ri½sandC. M. Nevinof CornellUniversityfor readingof the
manuscript.
COMPAfiIAMIr•ERADE ORu½o,
ORURO,BOLIVIA,
July 2, I94r.