Economic Geology
Vol. 53, 1958, pp. 737-753
PORPHYRITIZATION
TOWNSHIPS,
IN DESTOR AND DUPARQUET
ATIBI
WEST
COUNTY,
QUEBEC, CANADA •
R. BRUCE
GRAHAM
CONTENTS
PAGE
Abstract ..............................................................
Introduction
..........................................................
737
738
General geology .......................................................
Duparquetsediments................................................
Post-Duparquet intrusives ..........................................
Quartz-feldsparporphyry ...........................................
Developmentof the theory of porphyritization ...........................
Contactrelationsof quartz-feldsparporphyrywith Duparquetsediments..
Effect of porphyryon bouldersin Duparquetconglomerate..............
Contactrelationswith porphyry-conglomerate
breccia ..................
Replacement ..........................................................
Modus operandi ......................................................
The theory of porphyritization..........................................
First stage .........................................................
Secondstage .......................................................
Third stage ........................................................
Fourth stage .......................................................
Fifth stage ........................................................
Summary of conclusions...............................................
759
759
741
741
742
743
745
745
748
749
750
750
750
751
751
752
752
References ............................................................
752
ABSTRACT
In Destot and Duparquet townships, Precambrian quartz-feldspar
porphyryintrusiveshavebeeninjectedinto Keewatin-typelavasand pyroclastics,Timiskaming-typeconglomerateand arkose and post-Keewatin
type, pre-Timiskaming type basic intrusives.
Surroundingthe quartz-feldsparporphyry intrusivesare haloscontain-
ing numerouswell developed
quartzcrystals. Theseare presentin all types
of rock with which the quartz-feldsparporphyryis in contact.
The presenceof quartz crystals in conglomeratebouldersadjacent to
the porphyryand the presenceof quartz-feldsparporphyryfragmentsand
conglomerate
bouldersalongbrecciatedcontactsbetweenconglomerate
and
quartz-feldsparporphyry had led previous workers to the erroneousconclusionthat the Timiskaming-type
sediments
are youngerthan the quartzfeldspar porphyry.
A studyof the abovefeaturesas well as the halosof quartzcrystals
Presentedwith the permissionof the Deputy Minister, Departmentof Mines, Quebec.
737
738
R. BRUCE GRAHAM
developed
in otherrocktypesadjacentto the quartz-feldspar
porphyry
indicatesthat thesephenomena
are due to porphyritization.
Porphyritization
is the development
of a porphyriticand porphyroblastictexturein a previously
formedrockbypartialmagmatic
replacement
and by metastomatism.
The processaccompanyingporphyritizationhas been divided into five
stages:
(1) The injectionof quartz-feldspar
porphyry.
(2) Permeationof the moreporousrock typesat the contactby a
tenuousphaseof the quartz-feldsparporphyryliquid fraction.
(3) Brecciationand fracturing.
(4) Injectionof a highlysiliceous
fractionof quartz-feldspar
porphyry
into zonesof fracturing and brecciation.
(5) Injectionof the finalliquidresidueof the cooledquartz-feldspar
porphyry into zonesof fracturing and brecciation.
Golddeposition
tookplacein thesezones,at this stage,and is genetically
relatedto the quartz-feldspar
porphyry.
INTRODUCTION
PORPHYRITIZATION
is a phenomenon
of partial magmaticreplacementor
metasomatism
associated
with certainquartz-feldspar
porphyryintrusivesin
Destotand Duparquettownships. Field observations
and materialfor laboratory study were obtainedduring the summersof 1944 and 1945 while the
writer wascarryingout detailedgeological
mappingfor the QuebecDepartmentof Mines. The laboratorywork was carriedout at the Universityof
Toronto. The area mappedis comprisedof some25 squaremilessituated
20 milesnorth of Noranda,Figure 1, and 335 milesnorth of Toronto,Ontario.
Geologicalwork in the regionhas beencarriedon from time to time since
1872 (see references).As a resultof this work, the age of a seriesof
porphyryintrusives
relativeto the Duparquet-Timiskaming
type sedimentary
formationhas beenthe subjectof muchdebate. The preponderance
of detailedfield evidenceleaveslittle doubtthat the porphyriesare youngerthan
the Duparquetsidements;
yet thereare certainanomalous
relationships,
such
as the presenceof porphyry"boulders"in the conglomerate
adjacentto the
porphyryand the presence
of localareasof intermingled
porphyryfragments
and conglomerate
bouldersalong zonesof faulting called "porphyryconglomerates." Becauseof this, a careful study was made of the contactsof
the Duparquet sedimentswith the porphyries. It was found that these
anomalous
relationships
wereconfinedto the vicinityof onetype of porphyry,
namely,quartz-feldspar
porphyry. Further, at the contactsof this porphyry;
the "porphyryconglomerate,"
the sediments
and the porphyrybouldersthey
containexhibit featuresthat cannotbe explainedas beingderivedfrom an
older porphyryterrain. Additionalstudy has also found thesefeaturesto
be presentin all other rock typesin closeproximity to the quartz-feldspar
porphyry.
As a result of thesestudies,the theory of porphyritizationhas been advancedasan explanation
of the relationships
betweenquartz-feldspar
porphyry
and conglomerate.This theoryalsooffersan explanationfor the association
of goldwith the porphyriesin the generaldistrict.
PO RPHYRITIZ.Xl TION
GENERAL
739
GEOLOGY
The consolidated
formationsare all Precambrianin age (Fig. 1). The
oldestis a Keewatin-typegroupthat hasbeensubdividedinto the Older Volcanics,the Cl•ricy Sedimentsand the YoungerVolcanics.
The Older Volcanicsconsistpredominantlyof a lower formationof andesitic and trachyticlavaswith somerhyoliteand basaltflows succeeded
conformablyby an upper formationconsistingpredominantlyof basalticflows.
Followinga period of gentlefolding and erosion,the Cl•ricy Sediments,
comprisedof slate and graywacke,were depositedon the Older Volcanics.
The YoungerVolcanicsand relatedintrusiveswere hid downconformably
on the Cl•ricy Sediments. These consistof a lower formationcomposed
predominantly
of basalticflowswith someandesiteand trachyte,and an upper
formationconsisting
predominantly
of andesiticand trachyticflowswith some
basalt,agglomerate,and tuff.
In the easternpart of the map-area,two tabularbodiesof gabbro,quartz
gabbro and pyroxenitewere intruded into the Keewatin-typeformations.
Theseare knownas the Pre-Duparquetintrusives.
Following this period of intrusion,there was a period of folding and
erosion. The Duparquetsedimentsconsistingof conglomerate
and arkose
were thenlaid downand are preservedas a belt that extendscentrallyacross
the area. After depositionof the sediments,there was folding resultingin
the formationof the L•pine lake syncline. Subsequently,
a periodof gentle
foldingtookplace. Regionalevidencesuggests
that this periodof foldingis
relatedto the forcesthat producedthe Destor-Porcupine
fault and its subsidiaryzones. This fault,trendingslightlynorthof west,comprises
the north
or lower contact of the Cl•ricy sediments. Where these sedimentshave
pinchedout, it forms the line of separationbetweenthe Older Volcanicsto
the northof the fault andthe YoungerVolcanicsthat lie to the south.
Towardsthe closeof the faultingbodiesof feldsparporphyryand quartzfeldsparporphyry,graniteand aplite were intrudedas well as small dikesof
lamprophyre.Theseoccuralongandadjacentto thevariouszonesof faulting.
They are knownasthe post-Duparquet
intrusives.
The areawasthensubjectto a second
periodof widespread
faultingalthoughon a smallerscale. Finally, many small dikes of diabase,gabbro,
diorite and somepyroxenitewere introduced. Their orientationwas apparentlycontrolledby the fracturingdevelopedduring the secondperiod of
faulting.
Sincethis paper is mainlyconcerned
with the relationships
betweenthe
Duparquetsediments,
and certainmembersof the post-Duparquet
intrusives,
theseformations
will be described
in greaterdetailthan that providedin the
precedingparagraphs.
DuparquetSediments.--The
Duparquetsediments
outcropcentrallyacross
the map-area. Conglomerate
comprises
the greaterpart of thesesediments.
Typically,it is poorlysortedbut localexposures
of pebbles
of a fairly uniform
sizewere observed. Roundedto slightlyelongatedpebblesand bouldersup
to 3 feetin diametermakeup 85 to 95 percentof the rock. The matrix con-
740
R. BRUCE GRAHAM
PORPHYRITIZ•t TION
741
sistsof arkosicmaterial. Bouldersin the conglomerate
consistof graywacke,
argillite,gabbro,diabase,gray-granite,chert,basalt,andesite,jasper,rhyolite,
quartz-feldsparporphyryand amphibolite.
Arkose forms a minor constituentof the Duparquet sediments. A few
slaty bandsare presentwith the arkose,but theseare local in distribution.
Arkosehorizonsare foundmainly alongthe trace of the synclinalaxis which
passesthroughthe sediments. Beddingwithin individualhorizonsis variable
and local. It may be lens-like,or it may consistof regular bandsup to
severalfeet in width. Scour channelling,crossbeddingand grain grading
are presentin places.
Post-DuparquetIntrusives.--The post-Duparquetintrusivesare distributed, for the most part, along a zone, with a maximumwidth of about
miles,crossing
centrallythroughthe areain a directionslightlysouthof east.
This zonefollowsthe zoneof fracturingandfaultingthat comprises
the DestorPorcupinefault. A few isolatedbodieslie outsidethis zone both to the north
and south.
The porphyry intrusivesoccur as lens-like bodiesassociatedwith shear
zones and at the intersectionsof such zones. In detail, these bodies are ir-
regularin outline,pinchingand swellingin bothhorizontaland verticalextent.
Where numerous,more or lessparallel,shearsform a zone severalhundred
feet wide,the porphyrymay be intrudedin the form of manysill-likelenses
with local cross-cuttingrelations. These intrusives contain inclusionsof
conglomerate,of membersof the pre-Duparquetintrusivesand of 'sheared
volcanics.
Included in the post-Duparquetintrusivesare micrographicfeldspar
porphyry, feldspar porphyry, quartz-feldsparporphyry, aplite and albite
granite. There are two ages of feldspar porphyry. One, older than the
quartz-feldspar
porphyry,comprisedpredominantlyof albite phenocrysts
in
an albiticmatrix, and the other,youngerthan the quartz-feldspar
porphyry,
containing
phenocrysts
of albiteandmicrocline
in a matrix of albiteandquartz.
The apliteis a phaseof the olderfeldsparporphyry. The ageof the micrographicfeldsparporphyry and the albite granite with respectto the other
membersof the group is not known.
Quartz-Feldspar
Porphyry.--Thequartz-feldspar
porphyryis of particular
interestbecause
of its bearingon the phenomena
of porphyritization.
Typical exposuresof quartz-feldsparporphyryweathervariousshadesof
gray to pink and are massivein appearance.On the freshsurface,it is light
greenish-gray,
seagreenor pink. It has a fine-grainedcompacttexture with
well developedsquarishquartz phenocrysts
up to % inch in diameterand
well developed
'lathsof feldsparfrom •6. to • inchin length. Generallythe
feldsparphenocrysts
makeup about40 percentof the rock and alongwith
the quartz phenocrysts,
impart an apparentcoarse-grained
texture.
In thinsection,
boththequartzandfeldsparphenocrysts
arewelldeveloped.
The feldsparphenocrysts
are of albiteand vary in sizefrom 0.25 to 4 mm in
length. They are speckledand seamedwith white mica. Quartz phenocrystsare present,but not abundant,generallyformingonly 1 percentof the
Note: In Figure 1 "Younger Volcanies•Upper Formation" is No. III,
IV.
Diagram not shown in legend.
extendingbelow
742
R. BRUCE GR,,IH,,IM
rock. They may be embayed,with knife-edgecontactsagainstthe matrix,
or they may have lace-like borders. They are larger than the feldspar
phenocrysts,
averaging4 to 5 mm in diameter.
The groundmassconsistsof a intimatelyassociatedaggregateof quartz
and white mica. The averagegrain size of this aggregateis 0.02 mm or
lessin diameter. Chloriteplatesare characteristicof the porphyryand occur
as well developedbooksfrom % to • inch across. Apatite is also a characteristic accessorymineral, commonlyforming from 2 to 5 percent of the
section. One crystal was noted 0.5 mm in length, but generallythey are
0.2 mm or less. The apatite is presentin the groundmass,
in the chlorite
platesand in someof the more highly altered feldsparphenocrysts.Leucoxenealteredfrom ilmenite,pyrite, calciteand magnetiteform the remainder
of the matrix.
Below is listedthe chemicalanalysisof a specimenof the typicalquartzfeldsparporphyry.• With these resultsand the known mineralogicalcompositionof the specimen
as a basis,the modeof the rockwas calculated.
Mode
SiO,,
AltOa
69.37%
17.05
Quartz
Albite
28.08%
43.24
CaO
1.04
Anorthite
MgO
F•Oa
1.20
0.59
Sericite
Paragonlte
0.27
10.64
6.89
FeO
1.09
Chlorite
3.18
I•O
Na•O
TiO,,
MnO
P•Os
H,O
1.93
5.53
0.28
0.02
0.12
1.28
Apatite
Calcite
Ilmenite
Magnetite
Pyrite
2.60
1.40
0.45
0.69
2.14
CO,,
0.64
HtOTotal
0.05
Total
•05.81
i00.1J
An averageof 3 Rosiwalanalysesof the phenocrysts
in the specimengave
the followingresults:albite as phenocrysts,
40.58%; quartz as phenocrysts,
0.93%. These resultscheckedwith the proportionof quartz and feldspar
occurringas phenocrysts
in other specimens
of typicalquartz-feldspar
porphyry. Thus, approximately94 percentof the total feldsparcontentand
only 3 percentof the total quartz contentoccur as phenocrystsin typical
quartz-feldsparporphyry.
DEVELOPMENT
OF
THE
THEORY
OF
PORPHYRITIZATION
Surrounding
bodiesof quartz-feldspar
porphyry,thereare haloscontaining
numerouswell developedquartz crystals. The halosdo not generallyextend
beyondthe quartz-feldspar
porphyrymorethan 80 feet, but in rare cases,they
may attain a width of 200 feet. The intensityof the developmentof the
quartz crystalsdecreases
proportionately
as the distancefrom the porphyry
increases.
• Locality: 800 feet south of the Beattie road and 200 feet west of the Destor-Duparquet
township line. Analysis by the Quebec Dept. Mines.
PORPHYRITIZA TION
743
Thesehaloshavebeenrecognized
in the Duparquetsediments,
quartzcrys-
tals beingdeveloped
in both the matrix and bouldersof the conglomerate;
theyhavealsobeenobserved
in brecciazonesalongthe edgesof the porphyry
bodies,in older feldsparporphyry,in gabbro,in chert,in basalt,in trachyte
andagglomerate.The mostdetailedstudywascarriedout alongthe contacts
of quartz-feldsparporphyrywith the conglomeritic
and non-conglomeritic
membersof the Duparquetsediments
and alongthe brecciazones.
ContactRelationsof Quartg-FeldsparPorphyry with DuparquetSediments.--A typicalexampleof quartz-feldspar
porphyryin contactwith the
Duparquet sedimentslies about 1,600 feet west of the Destor-Duparquet
townshipline immediatelysouth of the Beatfie road.
The arkose is well bedded.
Several of the beds contain numerous
stout
quartzcrystalsthat projectfrom the weatheredsurface. The contactbetween
the bedswith the quartzcrystalsand thosewithoutis not well defined. A
specimenfrom one of the bedscontainingquartz crystalswas examinedin
thin sectionand found to be a normal quartz-feldsparporphyrywith no trace
of a clastictexture. The thin sectionwas comparedwith a thin sectionfrom
a bodyof massiveporphyry50 feet to the southwestand the two were found
to be identical. Thus, it is consideredthat these particular beds represent
sill-like bodiesof quartz-feldsparporphyry intruded into the arkose.
Severalspecimens
from the contactbetweena fine-grainedgray-weathering
bed of arkose and one of the porphyry sills from the above locality was
examinedin thin section. The fine-grainedarkose consistsessentiallyof
quartz, feldsparand sericite. The quartz and feldspargrains have angular
outlinesand average0.1 mm in diameter. Also presentare a few larger
grainsaveraging0.5 to 0.6 mm in diameter. The rock is similar in texture
and compositionto a normal sediment.
As the porphyrysideof the sectionis approached,
lens-likepatchesappear,
consistingof a very fine-grainedaggregateof quartz,.sericiteand feldspar,
similar to the groundmassof the quartz-feldsparporphyry. Some of the
lensescontainwell-developed
quartzphenocrysts
in somecaseswith embayed
outlines,but no feldsparphenocrystswere observed. Occupyingthe area
betweenthe lensesare angularfragmentsof highly sericitizedfeldspar,a few
angular fragmentsof quartz and somewhite mica. Closer to the porphyry
the lensesincreasein number, the amount of white mica increasesand that
of detrital quartz decreases.The feldspar fragmentsare here completely
altered to white mica. The detrital quartz and altered feldspar fragments
are surroundedby a coatingof white mica. Still closerto the porphyry,the
lensesbecome
sonumerous
that theyare separated
onlyby selvages
of white
mica,and quartzand feldsparfragmentsare no longerpresent. In the lenses,
inclusionsof schlieren-likepatchesof white mica resemblingthe selvagesare
not uncommon. Finally, individuallensesof porphyry groundmasscan no
longer be distinguishedand feldsparphenocrystsappear for the first time.
The rock is now a normal quartz-feldsparporphyrywith a few schlieren-like
patchesof white mica,whichdie out into the porphyry(Figs. 2-4).
The abovedescriptiondemonstrates
the gradationalnature of the contact
of a sill-like body of quartz-feldsparporphyryintrudedinto an arkosicsedi-
744
R. BRUCE GRAHAM
Photimicrographsillustrating gradationalcontactbetweenporphyry and graywacke. Figures 2, 3, and 4 from the samethin sectionpassingfrom the sediments
to porphyry.
Fxc. 2. Normal sediment,note size and shape o• quartz grains. X-nicols,
x 12.9.
Fx•. $. Normal sedimentat the top of the photograph. Towards the bottom,
"phenocrysts"
of quartz appear. These are considerablylarger than the detrital
quartz of the sedimentsand have not the angularoutline. Towardsthe bottomof
the picture may be seen closelypacked lensesof typical fine grained porphyry
groundmass. X-nicols, X 13.6.
Fxc. 4. Quartz feldspar porphyry. l•lote the numerousangular quartz •ragmerits includedin the groundmass. X-nicols, X 13.6.
PORPHYRITIZA TION
745
ment. In this particularlocalitythe contactzonevariesfrom severalinches
to 10 feet wide.
Contactsshowingidenticalfeaturesto the above,both on the outcrop
and in thin section,may be seenin the followinglocalities:Three-quarters
of a mile southof the Beattieroad and 3,600 feet westof the Destor-Duparquet
townshipline; in lot 39, westrange,Macamieroad,just .westof the rangeline.
Effect o] PorphyryonBouldersin DuparquetConglo•erate.---Adjacent
to
quartz-feldsparporphyry,many bouldersin the conglomerate
containquartz
eyesirrespectiveof their composition.Severalspecimens
of bouldersshowing thesefeatureswere examinedin thin section.
A pebbleof sedimentary
origincontained
in the conglomerate
wasexamined
in thin section. It consistsof 60 percentquartz as grains,many of which
are angular,15 percentsanssuritized
and recrystallizedfeldspartoo altered
for identification,
10 percentoligoclase,
10 percentepidoteas grainsscattered
throughoutthe section,2 percentchloriteand 2 percentmagnetiteand pyrite,
as grains averaging0.6 to 1 mm in diameter. This rock has the texture of
a sedimentthat has beensomewhatrecrystallized.The specimenis cut by
quartzfilamentsthat containminutespherules
of quartz. Scatteredthroughout the sectionare "phenocrysts"
of quartz. The quartz veinletscommonly
join in with the "phenocrysts"
and wherethey do, the "phenocrysts"
are enlargedby growthsof quartzaroundtheirmargins. Thesehavethe samecrystallographicorientationas the "phenocrysts."
A specimenof a boulderof tuff in the conglomerate
is seamedwith long,
narrow,lens-likestringersof fine grainedquartz. Theseare locallyenlarged
to formelongated
quartzcrystalswith fuzzyindefinitelace-likeborders.
A bouldersimilarto a rhyoliteformationoccurringalongthe northborder
of rangeVIII in Duparquettownshipcontainsphenocrysts
of albiteclouded
with saussurite,
someof whichare mildly fractured. The quartz phenocrysts
are of two types. One hassharp,clear-cutbordersand is stoutand squarish
in outline.
The other is more euhedral in outline and has delicate lace-like
borderssuggesting
growthby replacement.The groundmass
is composed
of
quartz, feldsparand sericite. The quartz is very fine grained,less than
0.01 mm in diameterand generallyhas a stringyor wanderingnature with
delicatelace-likeborderssimilar to that surroundingthe phenocrystsof the
secondtype describedabove.
ContactRelationswith Porphyry-Conglomerate
Breccia.--In manyplaces,
the porphyryand adjacentcountryrockare brecciated
alongzonesof faulting
and there is considerable
interminglingof fragments. Where such breccia
zonesoccuralong the contactbetweenconglomerateand porphyry,porphyry
fragmentsinterminglewith conglomerate
bouldersgiving rise to porphyryconglomeratebreccia. A typical exposureof such a zone will grade from
goodconglomerate
througha mixedzoneof conglomerate
and porphyryfragmentsinto porphyrybrecciaand from there into massiveporphyry.
Several specimenswere collectedfrom a porphyry-conglomerate
breccia
zone betweenconglomerate
and quartz-feldsparporphyry 600 feet west of
the Duparquet-Destor
townshipline and 600 feet southof the Beatfie road.
The fragmentsare generallylessthan one inch acrossand the brecciated
?46
R. BRUCE GRAHAM
structureis visible only in thin section. In the field, the zone appearsas
an intimatemixture of porphyriticmaterial with conglomerate
pebblesand
boulders. The phenocrystsare quartz and occurin the pebblesas well as in
the matrix. No feldsparphenocrystswere observedin this zone.
The fragments,as observedin thin section,consistmainly of quartzfeldsparporphyrywith a uniformfine-grainedmatrix, someconglomerate
and
somefractured and granulatedquartz phenocrysts. The matrix betweenthe
fragmentsconsistsof quartz with a little sericiteand is coatsetgrainedthan
the groundmass
of the quartz-feldsparporphyryfragments(Fig. 5). The
larger crystalsapproachthe size of phenocrystsand many are lath-shaped.
These differ from typical phenocrystsfound in the porphyry in that they
have delicatelace-likebordersand marginalzonescontainingmany minute
inclusions(Fig. 6). The quartz in thesemarginalzonesis in opticalcontinuity with the rest of the crystal. The hth-shapedquartz crystalsare generally larger than the surroundingfragments. They containa few inclusions
of porphyryfragmentsand,in places,they penetrateand passthroughbreccia
fragments,and henceare regardedas later than the fragmentedportion of
the breccia(Fig. 7). These quartz crystalsare megascopically
visiblein
bothmatrix and fragmentsof the brecciazones. The smallerquartzcrystals
of the matrix have indefinitewanderingoutlinesand are also characterized
by delicatelace-likeborders,frequentlycrowdedby minuteinclusions. The
matrix fills the fracturesin the fragmentsand cementsthe granulatedquartz
crystals(Figs. 5, 6). There is no indicationof crushingor granulationof
the matrix in any of the sectionsof brecciaexamined.
Brecciazonesexhibitingfeaturessimilarto thoseof the zonejust described
may be seenin the north part of range IV, 2,400 feet west of the DestorDuparquettownshipline.
Several other porphyry-conglomerate
breccia zones on examinationin
thin section were found to exhibit
some additional
features.
Such a zone
outcropson the west side of the Macamicroad in lot 42. The north side
of the outcropis goodconglomerate
gradingtowardsthe southinto a mixed
zoneof porphyryfragmentsand conglomerate
bouldersand theninto massive
quartz-feldsparporphyry.
In the thin sections
examined,the fragmentspresentare feldsparporphyry,
quartz-feldspar
porphyry,chertand crushedquartz granules. The matrix is
similar to that in the brecciaspreviouslydescribed. However, they are distinguishedfrom the other brecciazonesby the presenceof numerousquartz
veinlets,and veinletscontainingcarbonatewith a brownishstain, hematite,
pyriteand quartz. Theseveinletsclearlycut.boththe matrix and the fragments. Locally, the carbonate,hematite,pyrite, and quartz cementsthe
brecciafragments.
Anotherexampleshowinga differentvarietyof this type of mineralization
occursin lot 37, 400 feet north of the railway in Destor township. At this
localityan outcropof conglomerate
is in contactwith quartz-feldspar
porphyry
to the north. The contactbetweenconglomerateand massiveporphyry is
gradational
overa widthof 80 feet. Near the edgeof the massive
porphyry
are angularinclusions
of greenstone.Theseincreasein numberas the con-
PORPHYRITIZA
TION
747
•6
FIG. 5. Fragment ot5fine grained quartz-feldsparporphyry in silicifiedmatrix
of porphyry-conglomeratebreccia. Quartz similar to that in the matrix seams
the fragment. X-nicols, X 11.7.
Fic. 6. Matrix of porphyry conglomeratebreccia filling fractures in quartz
"phenocrysts"in the centerof the picture. Note irregular shapeand size of quartz
grains and to upper left a quartz lath with lace-like inclusionrim suggestinggrowth
by replacement. X-nicols, X 11.2.
Fic. 7. Quartz crystal in porphyry conglomerate
breccia. Note upper part
of crystal penetratinga fragment. Surroundingfragmentsare smaller than quartz
crystal. X-nicols, X 22.8.
Fi•. 8. Photomicrograph
of crushedquartz phenocrystcementedby matrix of
porphyry conglomerate breccia. X-nicols, X 56.
glomerateis approached. The porphyry assumesa brecciatedappearance,
and chert fragments,characteristicof the conglomerate,appear. Progressively nearer the conglomerate,the number of conglomeratefragmentsincrease,jasper œragments
also characteristic
of the conglomerate
appearand
the amountof porphyriticmaterialdecreases.Finally, 80 feet away from
the massiveporphyry,the exposureis good conglomerate
with a chloritic
matrix. The bouldersare composed
mainlyof angularfragmentsof chloritic
748
R. BRUCE GRAHAM
basaltwith somechertand jasperpebbles.The matrix is chloriticand contains numerousquartz eyes.
In thin sectionthe quartz-feldspar
porphyryis well fractured. The fracturesare filledwith sericite,chloriteand quartz,and they cut the quartzand
feldsparphenocrysts
as well as the groundmass.
A specimen
of the contactzonetaken50 feet from the porphyrywas
examinedin thin section. The matrix surroundingthe chert and greenstone
fragments
appears
to be normalquartz-feldspar
porphyryand is fractured
to the sameextentas the porphyrypreviously
described.Well developed
phenocrysts
of quartzandfeldspar
arepresent.The quartzphenocrysts
have
deep,smoothly
roundedembayments
and stoutcrystaloutlines.Over 90
percentof the feldsparcontentand lessthan 10 percentof the quartzcontent
occursas phenocrysts.
This is a characteristic
featureof normalquartzfeldspar
porphyry.Chloriteis moreabundant
thanusual,occurring
aslaths,
thecentralportions
of whichare replaced
by carbonate
containing
shredsof
sericite. Minute veinlets of chlorite and carbonate fill the fractures, but the
hematiteandquartzobserved
in the brecciapreviously
described
is absent.
REPLACEMENT
The quartz-feldspar
porphyry-sediment
contactshavebeenshownto be
gradational.Theporphyritic
materialin thecontact
zones,
although
identical
to the groundmass
of normalquartz-feldspar
porphyryandcontaining
quartz
phenocrysts,
differsfromthe porphyryin that it contains
no feldsparphenocrysts. This is a significant
featurewhenit is recalledthat approximately
40 percent
ofthenormalporphyry
iscomposed
offeldspar
phenocrysts
whereas
onlyabout1 percentis composed
of quartzphenocrysts.
If thecontact
zones
represent
part of an ancientregolithderivedfromthe porphyry,ashasbeen
suggested
by someprevious
workers,whereare the feldsparphenocrysts?
Why are they not presentin the porphyrylensesin the contactzonesas are
the quartzphenocrysts
? Furthermore
the feldspathic
materialof the sedimentsin the contactzoneprogressively
becomes
morealteredto whitemica
as the porphyryis approached.This micaceous
alterationproductand a
few grainsof detritalquartzoccuras inclusions
in theporphyryat the edges
of the contactzonesand in the porphyrymaterialin the contactzones.
The inclusions
of sedimentary
materialin the porphyryandthe increasing
alterationof the sediments
as the porphyryis approached
further preclude
the possibility
of the contactzonebeingof detritalorigin. Rather,theyare
indicativeof intrusionof the sediments
by the porphyryand of reactionof a
magmaticphaseof the porphyrywith the sediments.
Alterationof feldsparto whitemicain a hydrothermal
mediumhasbeen
producedby Gruner (8) in the laboratory;consequently,
someof the white
micafoundin the quartzfeldsparporphyrywas probablyderivedfrom the
feldspathic
materialin the sediments.The quartzof the sediments
occurs
in places
asinclusions
in boththeporphyritic
materialin thecontactzoneand
the porphyryat the edgeof the contactzone. However,the greaterproportion has apparentlybeenrecrystallized
and incorporated
into the siliceous
PORPHYRITIZ,,t TI O1V
749
porphyrygroundmass.The chloritein the sediments
hasals.oto someextent
beenincorporated
into the porphyry. This is indicatedby the fact that it
is noticeably
richerin chloritenearchlorite-rich
facesof the sediments.The
process
of replacement
that accompanied
the introduction
of the porphyryis
apparently
oneof makingoverandfinallyof incorporating
sedimentary
material into the porphyryproper.
MODUS
0PERANDI
After the crystallizationof mostof the feldsparas phenocrysts,
the proportionsof mineralsthat formedfrom the still liquidphasewere enrichedin
quartzandwhitemicaandthe remainingaccessory
minerals,andimpoverished
in feldspar. The high proportionof mica observedin the thin sectionsindicatesa "wet" phaseof the magmaat this stage. The originalmagmaticpressureconsequently
wouldhave increasedconsiderably
and the magmawould
alsobe more fluid. The motivatingforceprovidedby the increasedinternal
pressureis believedto havecausedthis mobilemagmaticfractionto moveout
from the porphyry proper, forming an aureol of premeationin the country
rock. This fractionwas apparentlytoo low in feldsparcontentto permit the
formationof feldsparphenocrysts.The processof replacement
preferredthe
matrix of the conglomerate
and manylocalitiesmay be seenwhereboulders
of conglomerate
are surrounded
by a matrix partiallyor completelyreplaced
by porphyry.
Subsequent
to this periodof replacement
therewas a periodof brecciation
that formedthe porphyryconglomerate
breccias. The phenocrysts
in the
partiallyreplacedzoneswere crushed,as shownin Figures6 and 8, and the
porphyryand sedimentswere brecciated.However, there is no trace of
brecciatedstructurein the matrix. On the contrary,the quartz,whichforms
over 90 percentof the matrix, has wandering,stringyoutlinesand fills the
fracturesin thebrokenquartzphenocrysts
(Figs. 6 and8). The largerquartz
grainsapproachthe size of the phenocrysts
in the porphyryand are larger
than the majority of the surroundingfragments. They containinclusionsof
brecciafragmentsand penetrateand passthroughfragments. Their lace-like
borders,which are crowdedwith minuteinclusions,indicateoutward growth
by replacement(Fig. 6). The phenocrysts
in the porphyryproper, which
containthe lace-likebordersor growthrims,are believedto representenlargementsof normal phenocrysts
by later quartz. Some of the larger quartz
crystalsin the brecciazonesalsoundoubtedlyrepresentenlargementof quartz
grains,perhapsoriginal fragments,to form "phenocrysts."It is thus apparentthat a later periodof replacement
occurredafter brecciation.
The replacingmaterialdiffersfrom that of the primary porphyryin that
it is composed
almostentirelyof quartzandis coarsergrained(Fig. 5). The
contrastbetweenthe grain size of two types of matrix is well illustrated
in the quartzosematerialsurroundingthe large fragmentsin Figure 6.
This replacementis confinedto, or near the marginsof, bodiesof quartzfeldsparporphyry. It is thus believedto representa further continuationof
the processof replacement
by solutionsemanatingfrom the porphyry.
750
R. BRUCE GRAHAM
In additionto the high magmaticpressuresprevailing,the porphyry was
still subjectedto the final stageof stressthat implementedthe precedingperiod
of brecciation. This stresswouldprovidean additionalmotivatingmechanism
wherebythe residualsolutionswere squeezedout of the porphyryby a process
of filter pressing,into adjacentbrecciazones.
Followingthis periodof replacement,veinletscomposed
of quartz,chlorite,
carbonate,hematite and pyrite were formed. Locally these further replace
the matrix around the fragmentsand are clearly later than the later siliceous
replacement. The gold mineralizationof the Duquesne,Golcondaand Pitt
depositsoccursin and adjacentto theseveinlets. The closespatialrelationship of these depositsto the porphyry and the presenceof quartz, chlorite,
carbonate,hematite,pyrite and gold in areas of siliceousreplacementin and
adjacentto the porphyrysuggesta geneticrelationshiprepresentinga further
differentiationof the solutionsderivedfrom the quartz-feldsparporphyry.
THE
THEORY
OF
PORPHYRITIZATION
The "phenocrysts"
of quartz,whichhavebeendeveloped
duringthe process
of replacement,form the halos of quartz crystalsthat surroundbodiesof
quartz-feldsparporphyry. The replacementaccompanyingtheir formation
has beenattributedto progressively
changingsolutionsthat are believedto
representstagesin differentiationof a coolingporphyrymagma. The modus
operandihas beendividedinto five stages. Actual porphyritizationis, however,confinedto stagestwo, four, and to a minorextentthe fifth stage.
First Stage.---Duringthis stagethe intrusionof the porphyrypropertook
place. The modeof intrusionwas forceful. This is indicatedby the presence
of numerousinclusionsof the countryrock near the contacts,the numerous
dikesfoundin the Keewatincomplex,the occasional
sill-like body of porphyry
found within the Duparquetsedimentsand the bowingof schistosity
where
the porphyry intrudesinto shearzones. Apparentlycrystallizationhad not
begunin the porphyryat the time of intrusionas there is no flow structure
in the phenocrysts
alongthe contacts.
Second3'tage.--The secondstage began with the crystallizationof the
quartz-feldsparporphyry. The feldsparphenocrystswere the first to form
and continuedto crystallizeuntil all but approximately6 percentof the feldspar had formed. At this stage,magmaticvapor pressurehad increasedconsiderablyand the residual liquid was sufficientlytenuousto permeatethe
porousarkoseand arkosicmatrix of the conglomerate
underthe high pressure
existingat the time. The feldsparcrystalswere left behindby a processof
filter pressingandthe liquidfolloweda sinuouscoursereplacingthe sediment
and swellinginto lens-likeforms where the nature of the rock permitted.
Quartz crystallizedas phenocrysts
bothin the lensesand in the porphyry,and
the residualfeldsparcrystallizedas fine crystalsin the matrix. Finally, white
mica,residualfeldspar,and someof the quartzcrystallizedfrom the remaining
liquid. These occur as a fine-grainedmosaicthat is characteristicof the
groundmass
of the porphyry. The countryrock in the zoneof soakingmust
havebeenheatedto nearly the sametemperatureas the porphyryduring the
PORPHYRITIZ.xl TI ON
751
periodof soakingand replacement.If it had beennoticeably
cooler,the
porphyrylenseswouldhavebeencooledtoo quicklyto permitthe formation
of phenocrysts.
An alternativeto the idea of directcrystallization
from the magmain the
formationof white mica is the possibilitythat it resultedfrom an alteration
of the feldsparof the groundmass.In this case,the alterationwould have
resulted from deuteric reaction as there is no evidenceof dynamic metamorphism. The feldsparphenocrysts
are only slightlyalteredto white mica
and the quartzphenocrysts
are not fractured. The smallerfeldspargrains
in the groundmass
would exposea greater area to attack explainingthe
preference
of whitemicafor the groundmass
andthe lackof alterationin the
feldsparphenocrysts.The alterationof feldsparto white mica would releasequartzandenrichthegroundmass
in thismineral. However,the manner
of formationof the white mica doesnot affectthe generalprocessof differentiation and porphyritization.
Third Stage.--The quartz-feldspar
porphyrybecameincreasinglyrigid as
crystallizationproceeded.Eventually, minor movementalong pre-existing
faults resultedin brecciation. Bouldersin the conglomeratewere fractured
alongzonesadjacentto the porphyry. Fracturingand brecciationprovided
channelways
for solutionsto penetratethe more imperviousrocks.
Fourth Stage.--Subsequent
to brecciationthe residualliquid in the porphyry was predominantly
silicawith minor amountsof volatileconstituents
from which carbonate, some chlorite and sulfides and oxides of iron were
finally deposited.
The fourth stageis markedby the introductionof thesesolutionsinto the
brecciazonesin the quartz-feldsparporphyryand adjacentrocks. Their introductionwas aided by the remaining incrementof stressthat causedthe
precedingperiod of brecciationand which squeezedout the solutionsby a
processof filter-pressing. During this stagequartz crystallizedbut the other
constituentsremainedin the fluid state. In the porphyrythis resultedin the
healingof fracturesand in the healingof the brecciatedphenocrysts
by veinlets of quartz. These veinletsare locallyterminatedby, and have satellitic
aggregates
of irregularquartz "phenocrysts"
crowdedby inclusionswith lacelike borderscharacteristic
of growthby replacement. They are distinctfrom
the primary quartzphenocrysts,
whichare clearand haveknife-edgeborders.
Also presentare growthsof lace-likequartz surroundinglarge clear quartz
phenocrysts.These commonlyhave a slightlydifferentopticalorientation
from the clear nuclear portion and are attributed to the growth of quartz
during this stage. In the brecciatedrocks adjacent to the porphyry the
brecciamatrix is replacedby quartz and quartz "phenocrysts"occur in the
replacedmatrix and in fracturesin the fragments. These have the characteristicsof growth by replacementpreviouslydescribed.
Quartz "phenocrysts"
were developedin this manner in the matrix of
brecciazonesand in fracturesin the fragmentsand conglomerate
boulders.
This development
of quartz "phenocrysts"
by replacement
occurredthrough
a process
that wasessentially
hydrothermalratherthan magmatic.
752
R. BRUCE GR.dH.dM
Fifth 5'tage.--The fifth stagecompletesthe crystallizationof the porphyry
andis markedby deposition
in veinletsof chlorite,carbonate,hematite,pyrite
and a small amountof quartz. Depositionof gold also occurredat this time.
These effectsare not as marked as those of stagestwo or four. There is
some overlappingof this stage with the fourth stage as small amountsof
carbonate,hematiteand pyrite may be presentin quartz veinletsof the fourth
stagewhile quartz may be presentin veinletsof the fifth stage.
SUMMARY
OF
CONCLUSIONS
Porphyritizationis the developmentof a porphyriticand porphyroblastic
texture in a previouslyformedrock by partial magmaticreplacement
and by
metasomatism.
The "porphyryconglomerates"
are the result of brecciation,replacement,
and porphyritization,
whichtook placeduringthe solidification
of the quartzfeldsparporphyryintrusives.
Porphyritizationhas been developedto varying degreesin other formationsthat occurin contactwith quartz-feldsparporphyry. Theseare gabbro,
chert,basalt,trachyte,agglomerate
and olderfeldsparporphyry.
Porphyritizationin arkose,graywacke,older feldsparporphyryand some
phasesof the quartz-feldspar
porphyryis of the secondstage. That in basalt
wasof the fourth stagealongwith subordinate
secondstageporphyritization.
Porphyritizationin bouldersin the conglomerate,in gabbro and in chert,
was of the fourth stage.
In the agglomerate,
the "phenocrysts"
are comPosite
and are correlated
with the fifth stage.
TORONTO,ONTARIO,
Mar. 25, 1958
REFERENCES
1. Ambrose, J. W., 1941, Cl•ricy and La Pause Map-Areas, Quebec: Canada Geol. Survey
Mere. 233.
2. Bannerman, H. M., 1940, Lipinc Lake Area, Destor Township, Abitibl County: Quebec
Dept. Mines, Rept. 4.
3. Buffam, B. S. W., 1925, Destor Area, Abitibi County, Quebec: Canada Geol. Survey Suture.
Rept., Pt. C.
4. Cooke, H. C., 1922, Canada Geol. Survey Suture. Rept., Pt. D.
5. --,
James, W. F., and Mawdsley, J. B., 1931, Geology and Ore Deposits of the RouynHarricanaw Region, Quebec: Canada Geol. Survey Mere. 166.
6. Geology of Quebec, Vol. II: Quebec Dept. Mines, Geol. Rept. 20.
7. Graham, R. Bruce, 1945, Preliminary Report on Duquesne Map-Area, West Part of
Destor Township, Abitibi County: Quebec Dept. Mines, P. R. 189.
8.
, 1946, Preliminary Report of Lanaudi•re River Map-Area, East Part of Duparquet
Township, Abitibi West County: Quebec Dept. Mines, P. R. 193.
9.
, 1947, Duparquet Lake Map-Area, Duparquet Township, Abltlbl West County:
Quebec Dept. Mines, P. R. 206.
10. Gruner, J. W., 1944, The Hydrothermal Alteration of Feldspar in Acid Solutions Between
300 ø and 400 ø C: EcoN. Guov.., v. 39.
11. Johnston,J. F. E., 1901, Eastern Part of Abitibi Region: Canada Geol. Survey Ann. Rept.,
v. XIV.
12. Lang, A. H., 1932, Palmarolle and Taschereau Map-Area, Abitibi County, Quebec: Canada
Geol. Survey Sumre. Rept., Pt. D.
PORPHYRITIZ•I
TION
753
13. McOuat, W., 1872-1873, Report of an Expedition Between Lakes Timisle•mlng and Abihq•i:
Canada Geol. Survey Rept. of Prog.
14. Norman, G. W. R., 1944, Preliminary Map, Rouyn-Malartic-Destor Region, Quebec:
Canada Geol. Survey Paper 44-27.
15. O'Neill, J. J., 1932, The Beattie Gold Mine, Duparquet Township: Quebec Bur. Mines
Ann. Rept., Pt. C.
, 1933, Beattie-Galatea Mines Map-Area, Duparquet Township: Quebec Bur. Mines
Ann. Rept., Pt. C.
16. Wilson, M. E., 1913, Kewagama Lake-Area: Geol. Surv. Canada Mere. 93.
•.,
1918, Timiskaming County: Geol. Surv. Canada Mere. 103.
17. Wright, D. G. H., and Segsworth,W. E., 1924, Extension of the Porcupine Gold Belt
into Quebec: Eng. and Min. Jour.-Press, May 10, 1924.