Lateral and Vertical Alteration-Mineralization Zoning
Jul 16, 2015
ECONOMICAN'D
GEOLOGYTHE
BULLETIN
OF
THE
SOCIETY
OF
ECONOMIC
GEOLOGISTS
Vo..
65
JuNE-JuLY, 1970
No.
4
Lateral and Vertical Alteration-Mineralization Zoning in PorphyryOre Depositsj. DAWDLOWELL AND JOHN M. GUmBERTAbstract
The geologichistory of the San Manuel-Kalamazoodeposithas provided an opportunity for the examination of vertical and horizontal zoning relationships in a porphyry coppersystem. Precambrian Oracle "granite," a Laramide monzoniteporphyry, and a Laramide dacite porphyry are hosts to zones of potassic,phyllic, argillic, and propylitic assemblages shown to be coaxially arranged outward from a potassiccore through phyllic, argillic, and propylitic zones. Alteration zones at depth comprise an outer chlorite-sericite-epidote-magnetiteassemblageyielding to an inner zone of quartz-K4eldspar-sericite-chlorite. Mineralization zones are conformable to the
alterationzones,the ore zone (with a 0.5% Cu cutoff) overlapping the potassic andphyllic zones. Occurrenceof sulfideschangesupward and outward from dissemination at the low-grade core of the deposit through microveinlet to veinlet and finally vein occurrenceindicating the progressivelyincreasing effect of structural control. Several aspectsof San Manuel-Kalamazoo geology suggestthat it is exemplary of the porphyry copper depositgroup. To test that idea and to evolve three-dimensional aspectsof these deposits, table of geologiccharacteristics 27 major porphyry dea of posits is presented. Considerationof the table indicatesthat the "typical" porphyry copper deposit is eraplaced in late Cretaceous sediments and metasedimentsand is associated with a Laramide (65 m.y.) quartz monzonitestock. Its host intrusive
rock is elongate-irregular, 4,000X 6,000 feet in outcrop,and is progressively differentiated from quartz diorite to quartz monzonite in composition. The host is more like
a stockthan a dike and is controlled regional-scale by faulting. The orebodyis oval to pipelike, with dimensions 3,500 X 6,000 feet and gradational boundaries. of Seventy percentof the 140 million tons of ore occursin the igneoushost rocks, 30 percent in preore rocks. Metal values include0.45% hypogeneCu with 0.35% supergene Cu,
and 0.015% Mo. Alterationis zonedfrom potassic the core (and earliest) outward at through phyllic (quartz-sericite-pyrite), argillic (quartz-kaolin-montmorillonite), and propylitic (epidote--calcite-chlorite), propylitic zone extending2,500 feet beyond the the copperore zone. Over the sameinterval, sulfidespecies vary from chalcopyritemolybdenite-pyrite throughsuccessive assemblages an assemblage galena-sphalerite to of with minor gold and silver values in solid solution, as metals, and as sulfosalts. Occurrence characteristics shift from disseminations through respective zonesof microveinlets (crackle fillings), veinlets,veins, and finally to individualstructures the on peripherywhich may containhigh-grademineralization. Breccia pipes with attendantcrackle zones are common.
Expressionof zoning is affectedby exposure,structural and compositional homogeneity, and postore faulting or intrusive activity. Vertical dimensionscan reach
10,000feet, with the upperreaches the porphyryenvironment of perhaps only at subvolcanicdepthsof a few thousand feet. The vertical and lateral zoning described is repeated with sufficient constancy that depthsof exposure many deposits at can be citedagainst the model of San Manuel-Kalamazoo.373
374
.t. D. LOWELL AND J. M. GUILBERT Severallines of evidence suggest relativelyshallowdepthsof formationand significant variations in water contentin the porphyry environment. Shallow emplacement is consistent with the appearance breccia of pipesassociated with ring and radial diking and with vertically telescoped zoning. Models of the sourceof altering-mineralizingfluids are considered.
ContentsPAGE
Introduction
.................................
374
Genetic modelsof porphyry deposits........... Geology the San Manuel-Kalamazoo of deposit..Fresh rocks ................................ Alteration zones ........................... Mineralization zones ........................
375 376378 381 385
acteristicswhich link Bingham Canyon and many otherdeposits the generalporphyrycopperdeposit to type. There appearsto have been little published effort specifically compareand contrastthe porto phyry deposits a group. as The first portion of this paper describes bothlateral and vertical alteration-mineralization relation-
Comparison porphyry deposits.............. of Deposits (column 1) ....................... Preore host rock (column 2) ................ Igneoushost rock (columns3-11) ........... Orebody (columns12-19) .................. Hypogene alteration (columns 20-27) ........ Hypogene mineralization (columns 28-35) .... Occurrence sulfides(columns of 36-42) ....... Supergene sulfides(column43) .............. Genesisof porphyry deposits ..................Conclusions ..................................
386 386 386 386 399 400 402 403 403 403404
shipsat San Manuel-Kalamazoo. The exploration model includedand substantiated approximately 70 degrees of postmineralization tilting. Thus this geologic system provides information concerning both vertical and horizontalaxes of a porphyry deposit. A three-dimensional synthesisis given of hydrothermal alterationmineralogy and assemblages, of the distributionand quantitativeaspects suloffides, and of the structural occurrenceof sulfide andoxide minerals. Vertical treatment of alteration and
Acknowledgments.............................References ...................................Introduction
406406
Exv.o1,ToN the Kalamazoo of portionof the San Manuel-Kalamazoo district,Pinal County,Arizona, has presented unparalleled an opportunity for the studyof a porphyrycopperdeposit three dimenin sions. The coaxialsymmetry alteration of and mineralization zoneswhichwas the basisof the exploration model has been verified in the exploratory drilling(Lowell,1968) of the Kalamazoo portion of the district and in exploitationof the San Manuel portion. As explorationproceeded, becameinit creasinglyapparent that many elementsof mineralog-y, occurrence, geometry otherporphyry and of copperdeposits were explicitly represented San at Manuel-Kalamazoo. Zoning patternsthere can be considered refinedbasefor the studyof mineralizaa of and descriptions a of tion and alterationrelationships other porphyry sideration many deposits in copperdeposits, this is the subjectof the study "typical" one. and reported here, with compilationof data from 27 A porphyrydeposit here defined a copper is as major porphyrycopper and molybdenum deposits and/or molybdenum in sulfidedepositconsisting of North and SouthAmerica. Most significant the disseminated and stockwork veinlet sulfide mineraliis in emergence from the many descriptions a more zationemplaced varioushost rocksthat have been of generally applicableunifying theme of large-scale alteredby hydrothermal solutions roughlyconinto alteration-mineralization zoning in these large de- centric zonal patterns. The deposit is generally posits thanhasgenerally beenrecognized.Stringham large, on the scaleof several thousandsof feet, al(1953, p. 990) statedthat "a review of hydro- though smaller occurrences are recognized. The thermal studiesof porphyrycopperdeposits shows relativelyhomogeneous commonly and roughlyequias many dissimilarities similarities the hydro- dimensional as to deposit is associated with a complex, thermal features at Bingham Canyon." We now passively emplaced stockof intermediate composition take the opposite position that there are many char- includingporphyry units. It containssignificant
mineralization geometryis still tentative,but some vertical zoning changescan be identified. Comparison other major porphyrybase-metal of deposits San Manuel-Kalamazoo means pubto by of lisheddata assembled Table 1 permitsdevelopin ment of a generalizedlateral and vertical zonation modelfor the depositgroup. Finally, that modelis used to examine the genesisand environmentof formationof the porphyrydeposits. The data suggest that it is sometimespossibleto estimate the position of the present erosion surfacesof other porphyrydeposits with respect their originalcolto umns of mineralization. Depth parametershave beenassigned nine deposits, to and it is hopedthat both scientific and explorational use can be made of three-dimensional alteration-mineralization zoning. The porphyrycopperand molybdenum deposits, hereaftercalled"porphyries," must first be defined. A necessarily flexibledefinitionemerges from con-
.4LTERATION-MINERALIZATION
ZONING
IN PORPHYRY
ORE DEPOSITS
375
containsomechalcopyrite.Many deposits contain crackle, stockwork,and brecciatedzonesin the coolerrecoverablequantities of both minerals, either in marginswhere,augmented diffusioneffects, by alter-
feet. The modeldepends a melt derivedat some on greaterdepth,probably near the mantle-crust boundary, which becomes saturated with water as it approaches upper surface. Releaseof that water the may occur when internal vapor pressuredeveloped by supersaturation exceeds lithostaticload presthe sureor whenthe intrusive system rent by external is stresses. Crystallization then proceeds presumably tion. alongthe linesof Emmons'(1933) cupola R. H. or cupoladevelopment. The grade of primary mineralization typical Sales'ssub-hood in As described Nielsen (1968), the sequence by of porphyry copper deposits ranges to 0.8% Cu and up eventscan be paraphrased intrusion,early maras 0.02% Mo, andporphyry deposits whichmolybin whichproduces solidshell,and a deniteis the chief economic mineralhave grades ginalcrystallization porphyritic-aphanitic ranging to 0.6% Mo and 0.05% Cu. All por- ruptureof that shellto produce up crystallized rocks. Volatiles phyry copper deposits contain at least traces of texturesin subsequently by migrateoutwardthrough molybdenite, all porphyry and molybdenum deposits released the quenching
amounts pyrite, chalcopyrite, of molybdenite, quartz, and sericiteassociated with other alteration,gangue, and ore mineralsand metals includingminor lead, zinc, gold, and silver. Mineralizationand alteration suggesta late magrnatic-mesothermal temperature range. The deposit is generally associated with brecciapipes,usually with a large cracklebrecciation zone,and is surrounded peripheral by mineral deposits suggestive lowertemperature of mineraliza-
to depthsapparently the order of 3,000-5,000 on
occurin response gradito separate orebodies in ore with approximatelyation and mineralization or temperatures the center at equal copperand molybdenum dollar values. Al- ents"from near magmatic in though typical porphyry copper deposits differfrom of the stock to relatively cool temperatures the wall rocks" (p. 37). Silicate sulfide reactionsof typicalmolybdenum deposits some in respects, the by existence gradational of characteristics metalliza- the typedescribed HemleyandJones(1964) prein vail. Other authorswould not necessarily limit the tion suggests common a origin. This definition somewhat is generalized because separationof volatiles to the period of quenching, it evolution the hydroof mustpermitconsideration manydeposits of whose but rather would consider thermal fraction a quasi-continuous separationof localgeologic circumstances as expressed vary by volatilesin response the many variablesrelated to their geometries physical and characteristics. We to and pressure. The loss of volatiles believe porphyry the deposits be a petrological- temperature toof positsare bestinterpreted greateror lesser as de- upward and outward replenishment mineralizers from greaterdepths. partures from the unifying model of the above Fournier (1968) suggests that the initial deep definition elaborated as uponbelow.
portions a melt may permitthe of mineralizational andindividual class, porphyry de- from near-surface
depths lessthan about4,500 feet, and that rupof Several genetic models havebeen proposed re- ture by faultingwouldcausesudden, to evenexplosive late the characteristics porphyrycopperand loss of water and supercooling the silicatemelt. of of molybdenum deposits. of the models All recognizeCrystallization would then abruptlyhalt the upward the important involvement porphyritic of intrusive progress the now dry melt. Subsequent of "extenrocks withoredeposition, all arefundamentally argillicalteration and sive shown mostporphyry by copmagmatic-hydrothermal, differingin the sequences deposits probablydue to a superimposed per is cirof events, depths intrusion, timingof deriva- culatinghot-springsystem, mainly by meteoric of the fedtion of fluids, and the sourceof fluids. The models and cormate water" (p. 101).Fournier's model of intrusion of a water undersaturated melt, and the White model of multilevel
Genetic Models of Porphyry Deposits
porphyry copper melt was unsaturatedwith water at one to three percent, that it was intruded to
considered here are the orthomagmatic model,
circulation brinesadjacent a heat source. of to 5%-to 40% NaC1, are responsible many basefor The orthomagrnatic modelhasbeenbestdescribed metal deposits. Such brines may be producedin
White (1968) in a particularlystimulating paper suggests circulation sulfur-deficient that of Na-Ca-C1 brines, with salt contentsgenerally equivalentto
in therecent writings Burnham of (1967) andNiel- porphyry systemsby deuteric reactionof residual sen(1968). It is the genetic model tacitlyadopted liquids with earlier formed plagioclase and ferroin mostdeposit descriptions, for example, as those magnesian mineralsto achievehigh contentsof caldescribed TitleyandHicks(1966). It sometimesciumandbasemetals. AlthoughWhite in his paper ininvolves penetration the source levelsas shallow doesnot develop specific of to a space-time modelfor the
at 1,500feet (Nielsen, 1968),but morecommonlyporphyry deposits,he implicitly developsa model
376
.r. D. LOWELL AND J. M. GUILBERT
Kalamazoo portion moved about involvingmultilevelcirculation deuterically of metal- portions.Theupper enriched or cormate-meteoric sulfur-deficient metal8,000 feet in a down-dip,S55W direction. Small, high-angle, northwest-trending normal faults lizing solutions underthe influence thermalgradiof bothhalves the original of orebody, ents established an adjacent or subjacentmag- later displaced by stripped mostof the Gila Conglomerate matic heat source. The model differs importantly and erosion from the orthomagrnatic model in that the source from the east end of the presentSan Manuel oreof the solutions,and perhaps the metals, is almost body (Fig. lb). The original, unfaulted orebody, defined a as by completely externalto the magmaticsystem, with limit, formeda slightlyflattened or convective overturn of circulating solutions pro- 0.5% copper cylinder whichwasat least7,700feetlong ducingalteration-mineralization envelopes zones. elliptical and and from 2,500 to 5,000 feet in diameter. The top at Geology of the San Manuel-Kalamazoo Deposit of thecylinder, the eastendaftertilting,mayhave been rounded,with the bottom,at the west, having The San Manuel-Kalamazoo deposit (Lowell, an irregularshape. The centerof the orebody is 1968), located in Pinal County, Arizona, is here poorlymetallized, that ore actually so formsa holaccepted the type porphyrycopperdeposit, as and low cylinderor cylindrical shell. The shell surits geologyand other characteristics presented rounding low-grade are the center varies fromabout 100 for comparison contrastwith others (Table 1). to 1,000 feet in thickness. Mineralization and and alterPrecambrian quartz monzonite of the Oracle ation zones are approximately coaxial. Granite batholith in the San Manuel area was inThe alterationassemblages the San Manuelin truded in Laramide time by swarmsof monzonite Kalamazoo deposit formregular, smoothly bounded porphyry dikes and irregular masses monzonite zones,which, as in most porphyries, of are locally porphyry, more properly termed biotite latite por- gradational difficult place and to withina hundred phyries, althoughlong-established "monzonitepor- feet,although theyarewelldefined a broad on scale. phyry" terminology will be followedhere. Closely The boundaries more clearly definedthan they are related in time and spaceto the activity was a are in mostporphyry deposits, presumably because porphyrycopper mineralization eventthat produced the mineralizing fluidsaffected intrusive, essentially the San Manuel-Kalamazoo orebodyand its associ- homogeneous, isotropic plutonic hypabyssal and host ated concentric alterationzones. The hydrothermal rocksof intermediate composition. Theserocksresystem appears havebeencentered the middle sponded the indicated to in to alkalichemistry without of the monzonite porphyrydike swarm,and metal- important gains losses.No marginal or sediments, lization is almost equally distributedbetweenthe compositionally contrasting intrusive rocks,planar monzoniteporphyry and the Oracle Granite host rock fabrics,or prominenttectonicelements prorocks(Fig. 1). duced steep physical chemical or gradients influto Followinghydrothermal mineralization and alter- ence the uniform zoning and symmetry. ation (Fig. la), the whole districtwas tilted to the Mineralogic zoning Kalamazoo elsewhere at and northeast, and the block includingthe San Manuel- suggests at least that fouralteration assemblages are Kalamazoo orebody probably was relatively elevated. easily discernibletheporphyry in copper molyband Erosion of this block exposed the top of the ore- denum deposits. The terms potassic, phyllic, argilbody, and supergene activity formed a thin chal- lic, and propylitic havebeenadapted adopted or cocite enrichmentblanket. At this time, the long fromthe literature(Burnham, 1962;Creasey, 1966; axis of the orebodymay have plunged at about Meyer and Hemley, 1968) to describe four the65 SW. Shortly thereafter,terrestrial sediments principal assemblages. terms"argillic"and Thebeganto coverthe deposit.
"propylitic" well knownand widelyaccepted, are
quartz-kaolin-montmorilloniteFurther tilting, perhaps15 followeddeposition broadly describing , and chlorite-calcite-epidote-adulariaof the lowermost Cloudburst Conglomerate. An chlorite-biotite alteration assemblages, respectively. "Phyllic" erosion surface formed on the Cloudburst sediments albite is here appliedto the assemblage quartz-sericitewas later coveredby the Gila Conglomerate. Athird-stage of about30 gavethe Gila Conglomtilt erateits present inclination brought originand the ally verticalaxis of the San Manuel-Kalamazoo orebody into a 20 southwest-plunging attitude. The San Manuelfault thendiagonally offsetthe original, nearly cylindrical orebody into two roughlyequal-
pyritewithless than5% kaolin, biotite, K-feldor spar,and "potassic" suggested is (Guilbert and Lowell, 1968)to include introduced recrystallized or
K-feldspar biotite, and withminorsericite highly and variable persistent generally but and minoramounts of anhydrite. Each of theseassemblages be will morefully described below, especially theyoccur as at Other assemblages sized pieces, SanManuel the Kalamazoo San Manuel-Kalamazoo. the and
.4LTERATION-MINER.4LIZATION
ZONING IN PORPHYRY
ORE DEPOSITS
377
pCqm
OREBODY
12
sw
.
SAN MNUEL FAULTCc -
NE
qml { KALAMAZ
SAN
_ ..'
SEAMEN
0lb
1000'IApproximofe Scole
I
Fxa. 1. Schematic drawingof structural historyof SanManuel-Kalamazoo deposit. (a) at time of emplacement and (b) at present. Note the umbrella-like flare of dikeswarmandthe chalcocite enrichment zone(CCa). pqm-Oracle Granite,TKrnp= monzonite porphyry,Tcb= Cloudburst Formation, = Gila Conglomerate. Tgc
rarelyencountered the porphyry in environment are The alterationzoneswere separated during Kalathe advanced argillic (Meyer and Hemley, 1968) mazoo exploration as follows. The inner limit of and pegmatoid, respectively involvingquartz and the propylitic zonewasplaced wherethe total quartzpyrophyllite, traces dickire kaolinire, with of or topaz, montmorillonite,quartz-kaolin, or quartz-sericite and zunyite, and quartz-coarse sericite-K4eldspar, content in plagioclase sites exceedsthe total of with or withoutcarbonate, anhydrite, and apatite. chloriteand epidotereplacingmafic minerals; here Hydrothermalalterationassemblages the San the color usually changesfrom green to light gray. in Manuel-Kalamazoo deposit summarized Fig- The argillic zone, in which kaolin or montmorillonite are in ure 2, which showsalterationchanges mineralby predominates plagioclase in sites and chlorite remineraland assemblages AKF-ACF diagrams. placesbiotite, was not generallymappedseparately on Supergene activity is limited to a 200-foot thick and is least significantquantitatively. The inner zonenear the top of the deposit. limit of propylitic alteration is locally the outer
378
J. D. LOWELL AND J. M. GUILBERT
SHALLOW-MODERATE DEPTH ASSEMBLAGES'PORPHYRIES
FRESH IM,Quartz OrthoclaseMicrocline Plagioclase(An35.45)....
PROPYLITIC ZONENoChange NoChange Tr. Mont, flecks& granules ep,
ARGILLIC ZONEAugmented Flecked Sericite withMontmorilloniteKaolin -,
PHYLLIC ZONEAugmented SericitizedSericitized
POTASSIC ZONEAugmented Recrytallized, inpartreplaced byalteration K-felclspar-quartz Fresh completely to replaced bybrn-grn alt'n biotite,K-spar,ser.Fresh or recrystallized ta sucrose
zois, car, chlorite, kaol.
Biotite Chlor,zois, car, leucoxene Hornblen_d..e.._. car,mont, Ep, chlor(2 types)Magnetite trocepyr ire
Chloritized, leucoxene, + qtz ChlaritizedPyritized
Sericite,pyrite,rutile Sericite, pyrite,ruffle(?)Pyritized
brn-grn granules, chlorite + Biotite,+ chlorite rutilePyritized
A-K-C-F
Ac.(kaol} r
A.kool
A
A..
VeinletFillings
A=A' ,aa ?Cp 7[,,e active stock> dike Qd';Gd; /p, QIp all + seds
'"iitii'' ..................................................................................................................... '6'6i ................................................................................. passive> active stock 'Qp' k;' 'feld Qp' all + seds(both altered)
4000 x 4000
passive> active
stock> dikes
Qd.4 Dacp .4 I_p Iph .4
And, Qd, Dacp,/p
150,000 350,000 x
passive
batholith
Qm(apl, peg)-4 Qmp
all
8000 25,000 xcluster
passive> active
stock dike >
Dio,Gd,sy,G.4'QI Eb
all + seds
'"J'l'l''''J[ ..... passive
stockdike >
Gd Qm QmpG.4Gp Db .4 .4 .4 .4sodo .4Qd Dio Qm Gd .4 .4 .4Rhyp .4 apl p .4 Gp
Qm, & Db Gpall(?)all
'"'C'""(JJJ'- passive ...... > active(?) stockdike >4- 3000x 3000 active stock> dike
................................................................................................................................... 9.....p.! ..... .o...a.....a..p. ............................................................
'"/6i'/i';/"i'i/i///'//'/ ....... ;';';i'; ..............................................................................Gp stock Qm'.......alsk ,"-; ; apl ; .......... ....... Db ................................................. all
'"//'';"i;'6/// .................. ............................................................................. i;';';';i';; ......... ................................................................... stock> clikes,sills Gd'-; Gdp; 'Qp' alllarge,elongate EW passive stock dikes,sills > M, Qmp complex all + seds
stock> dike Qm.4 G '......... reid i; alsk ; Q '"//61iJiJii";,"i'ii6i;"c/6'6 ......... ;;';i;;;;.*' .................................................................................... all ............. ..................................................
'";/;'i:i";,";/ii ....................... '"; ................................................ ...................... pass, .......................................................................................... stock> dike Qm.4 Dio; Qmp.4Andp all '-seds&voles
60,000 all '-7/;';;;i '"/6/ci:i'; .......................................................... ...................... i;;';i';; ;,';;'';"/ii; i5i;";"'''5':-;"i5i; ..................................... .....
'"6///i';i'/// ............................... :'........................................................................................................................................................... pass, ve stock Qd.4 Qmp 'Qp'.4 'Qp'+ Q .4 Qmp 'C)p' +__4000 4000 x passive sill > stockin wall
................................................................................................................................................................................ ?...p.2+...9. ................Qmp all + seds
'"'i;'666';' b;'6'66 i; ; i; "'';i ................i" ii'ij"ili' g ............. ; ';' ;' ;i ii ;; il; .............';"-7 i;"-7 "6i; ii' "o;;;i5';,"-; ; ......................;ii;; 6';T' ........... };'stock5 dike Mp, Gp, bio G'; apl, apl i;; voles "i''6i:iii;,"//iii i;';;';'i;; ................ ........................................ ....................................................... d[l:7...................... ................ ......................
..................................................................................................................................... .a.....y....p. ..................................................................................8000x 15,000 passive stocks dikes > Qd.4 Db.4 Qmp And Qmp .4 .4 .4 all + metaseds
2000x 4000;4000 7000(?) x
passivepassive
dikeswarmstock dike >
QIp,Rhy,Dac,Qd,Gd Dac .4 .4Mp.4 Qmp Db .4
all + valesall
..................................................................................................................................... 9.!,...k,...a....y. .........................................................................'";/'66/i';F// ............................... " .................................................................................... & bi Gd'; ..................................... Qd, Gd+ seds ............................................ pass,ve stock> dike Dio ';Qd+ hbl Gd
......................................................................................................... 9.m. ..........................................................................> 10,000NE x30, oooNW
passive
stock siII > dike >
alsk.4 Dacp.4 Andp .4 Omp
all + seds
"i'g66'"''6'6' ;/:'ilk;; ...................... ................................. ................. .................................................... /;;/'i;i'';'"/li'i/g''g"/; i;ii"'i;;Fg",'g'i;"'4000x 6000 passive stock dikes > Dio.4 Qm Qmp 'Op' .4 .4 all + seds
390
.t. D. LOWELL AND .L M. GUILBERT
OREBOD'Y
D E P OSI T
Outward Shape
Boundaries
Percent inIgneousHost
Percent inPreore Rocks
(12)
(13)
(14)
(15)
Aio
ArizonaArizona
oval, elongate NW
original faults &
80?
20?
......................................................................................................................................................... '4: ................... elongate oval original +_90 10
"ifii;'d4 ................................................................................................................................................................................. steep, elliptical cylinder original -t-50 +50British Columbia
BinghamUtah
pear-shaped, elongate WSW original
75
25(incl. bx)
Arizona
......................................................................................................................................................................... ................. elongateEW,oval original& faults + 30 + 7'6'hollowcircular cylindero..........................
Braden.............
original& postore breccia
25100 + 90 100
75
Chile
0 ...........
pipecrudely domical pipelike original originalbrecciapipeo ..................................................................................................
..............................................0 + 10 0
Butte
MontanaSonora
Cananea, ......................... 0,oo,
..............................................................................................
CastleDomeArizona
oval, elongate w NE
original& NWfault
Chile.0 ..... , ...............
......................,,,,o,0,, o,,,, ...................... o ................. , .......................,.,,,,o,,0,,o.,00 .... ,o, o,.o ....... , .........................
...............original ......... 0 .............................................
................................... .......................... ............,., ........... , ......... , ....
............................................................................................
Climaxo0,.o,,, ...........
Colorado
nested,inverted ones
40(?), ..................... ,...., ......... 0 ..... 0.,,,., ............................
60(?),,,o,.,
Copper Cities.o[,,
Arizona.........
oval, elongate NW, .....................
Original NE & N faults &original original with faultsabove & below
100, ...........
0.,.o ................
, .....
,..,0 o..,,,o0,0 ..... , ......
o.......................................................................................................................
El SalvadorChile......... 0 ........... 0,.,, ..... 0 ..............
oval pipe, lowergradecenter ..... ............................................................................................................................
70(?),.,.0, ....... ,,., ....................
30(?)0 .........
Ely.........................
Nevada0 ,,
?flat cylinder
80
20
Endako.... 0,
elongate oval,o ................... ,,, ......................................................................................................................................................................................
original originaloriginal & fault
100 60(?)50+ 100 , ..... , .........
0 40(?)500, ......... o ....
British. Columbia............
EsperanzaArizonaoo., ............... ,.0.o.o .....
elongate oval NW ................................................................................................................................................................................................
InspirationArizona0 ......... .o ..... , ......... ,,,.o,,.,,o., ........
flat cylindercrescent, convexSWo ....... , ............. 0 ........................................................
Mineral ParkArizona,, ....... o .................
, .........................................
, ............................................
Mission-PimaAri zo n a
oval
original & fault
+ 10
+_90
...................................... :................................................................................................................................................................ .................. +_ 70 Morenci oval original+ faultArizonao.,,,o,o,0..,,.,.,,, o0.,...,.. ..... 0.0 ........................................ , ................................................................. o ........................................... 0 .......... 0 ......................
QuestaNew Mexi co,,..., 0,.0.,.,,0 ................... , ..... 0 ......
irregular0o. .............
original
70(?)
30(?)
, ..............................................................................................................................................................
.,0o,,,,
Rarizon a....... ,..,0,0,0.o,0.o0
........
, ......
, .....
irregular oval, EW original elongate &fault, ............. , ....... 0 ........... , ...........................................................
, ....................
0 ......................................
20
80, ...........
0 ....
SaffordAri zon a,,.0o, ..... , ........... ,,0 ........
oval, dippingpipe0 ............................................................
original ................................................................................
200 .............................
800., ...............
SanManuel-Kalamazoo hollowoval cylinderArizona,0 ................. , ...............
original original
50 +_70
50 +_30
SantaRitaNew Mexi co....... , ...................
oval elongateNW
Silver Bello, .....
elongate oval mineralbelt,,,,0 .............................................
original0......
70 .................
30, ..................................
Arizona , ...........................
, ..............................................................................
Toquepala
oval, elongate NW
original: brecciapipe
70
30 (walls
Typical PorphyryCopper
oval, pipelike
original& postore faults
70
30
ALTERA TION-MINERALIZA
TION ZONING
IN PORPHYRY
ORE DEPOSITS
391
0 R E BODY-
Continued
Dimensions (feet) (16)4000 x 7000
Total Ore Tonnoge (million) (17)< 500
Grade Hypogene+ Supergene (18)0.75% Cu
Grade Hypogene Only (19)0.75% Cu
1000 x 5000
< 100
2000 x 30005000 x 7000 WSW 2000 x 2000
< lr00> 500 < 100
0.76% Cu + 0..025% Mo 0.6% Cu0.75% Cu 0.05% Mo 0.81% Cu
+ 0.5% Cu + 0.025% Mo
0.6% Cu0.75% Cu 0.05% Mo + 0.55% Cu
...!?..r.?.:..y.! ...................................................................................................................................................................
5000 x 5000
> 500
2.25% Cu
1.00% Cu
....o. !).o...w...:..y.!!?..4?..r '................................................................................... .O....O..5....a..o. ........................................ ................. .0. :..O..5....a..O..5000 x 10,000 EW250 x 1200
> 500> 500
0.8% Cu0..8% Cu
0.2% Cu0.5% Cu
....r..?...:.s..a..p. ?....................................... ......................................................................................................................... !!?.!.:.t.?_ 1500 x 3000 < 100 + 0.70% Cu + 0.5% Cu (?)
2500 x 10,000
> 500
+ 1.7%.
+ 1.2% Cu
"';ii:Ji:JiS' .................................... ........................................ ":ii:Sti/5 5:'6i:i "6'.'/qo' .................................... '/,i ''i3i;i'qo"// ..................1500 x 2000 < 100 + 0.60% Cu + 0.4% Cu
> 500 1.5% Cu ND '"iSiti' ................................. ,";/ti/5/5 ................................................... ' ......................................................................................
+ 1000 x 3000 x
< 500
+ 0.9% Cu
+ 0.1% Cu
........ .1..o..-...2..o.,..o..o..o. ............................................................................... ................................?..o..m...m..o. !.l....2....:.o...m..m...o.?.! !.0.:.4... ?..! ...............1200 x 60002300 x 4200 2500 x 8300
> 100
+ 0.09% Mo0.51% Cu 0.028% Mo 0.90% Cu
_ 0.09% Mo+ 0.3% Cu 0.028% Mo 0.15-1.20% 0.007% Mo 0.04% Mo 0.8% Cu
.( 100 < 500
Cu
.... iJiS'"';i6 ................................................................................................................................................................................ < 100 0.5% Cu 0.1-0.15% Cu0.04% Mo 5000NW x 7000NE ) 500 0.8% Cu
6000 x 13,0007000 x 7000
) 500) 500?
0.88% Cu0.15-0.18% Mo
0.1-0.15% Cu0.007% Mo
0.15-0.18%
Mo
3000NS x 10,000 EW+ 4000 x 5000
( 500> 500
0.80% Cu0.50% Cu
0.10-0.80% Cu+ 0.2% Cu
cross section: 2500 x
> 500
4- 0.75% Cu
-t- 0.75% Cu
5000 x 4- 8000 high 5000 x 7000NNW
< 500
0.015% Mo 0.97% Cu
0.1-0.2% Cu(intr)0.8% Cu (tactite)
.... 2000x 2500&1500 x 25004000WNW x 5000NNE
< 100 500
............................................................ J':'"6l'"E'iig;i .... 0.75% Cu0.8% Cu (tactite)0.9% Cu 0.3% Cu
3500 x 6000
150
0.80% Cu 0.015% Mo
0.45% Cu 0.015% Mo
392
.r. D. LOWELL
AND .1. M. GUILBERT
HYD E P O S I T Known Extent
POG
EN
E
ALTERATION
Beyond Ore (ft)(20)
Peripheral Zone(21)
Outer Zone(22)
Intermediate Zone(23)
Ajo
Arizona
+ 5000500 + + 300Columbia
?chl, ab, zo, ser, Q, ankND ND not reported Q, chl, ep not reported Q, kaol, mont
BagdadArizona
BethlehemBritish
Bingham BisbeeArizona
3000 + 7000?
chl, talc, kaol, ep,
Q, chl, kaol, cal, ep chl, ep, zo, cal, ser ? kaol, ser(?)
........ ........................................................... .u..t ..r..,..m..?...,.....y..x. ...........................................................................................................Braden .............................................................. '4:6i:3 g'i;'37};7'; ''i:;;;,7' i;;';ii ,;'"'"',';; ........ 3; ......... i7;;,'i;' ..................Chile tm
ButteMontana
1000 + 5000
Q, chl, ep, cal chl, ep
Q, mont, kaol Q, ser, kaol
CananeaSonora
Castle DomeArizona
3000 few hundred
chl, ept py, ser, cal &c l zo TiOx
mont kaol ) ser
ChuquicamataChileColoradoAri zon a
chl, ep, cal, spec, hm,
Climax 2000? ............................................................................................................. 7'i'ii7;'C.,'i ................... ';''';''' .................................
CopperCitiesChile
5000 +
ep, cal, clzo, ser
mont, Q
.......................................................................................................................................................... El Salvador 1000 + py, chl ;&';;ii'ii ....................................Nevada..........................................................................................................................................................................................................
............................................................................................................. %;-g..iii;; ........................................................................ Ely 2000EndakoBritish Columbia
2000 + (?)
kaol weak, Q, cal
kaol moderate, Q, chl
EsperanzaArizona.0ooo ........... , .....
ND 1500 +10,000up to 5000
not reported chl, epchl, ep, clzo, Q, ser, 'clay'skarn, tactitc, hornfels
Q, kaoI, mon t, ................
, ...................................................................................................................................................................
InspirationArizona
Q, ser, kaolQpresent
MineralParkMission-PimaAr i zo na
....... ..r!.z..o.?.?. ............................................................................ !!.t?..t.! .(...!!..!! ..............................................................................MorenciArizona.........................................................................................................................................................................................................
) 5000
skarn on SE
chl, ep
Q, mont
QuestaNew Mexico Arizona
2000 + (?) 1000-15,000
ser, car, kaol, ep,chl
ser, Q, py -3_ cal, kaol,i l I, fl
ser, Q, py -3_ cal, kaol, ill
...........................................................................................................................................................................................................
Ray
chl, ep, ab, cal, montto20,000 x 30,000
SaffordArizonaArizona
-3_ 12,000
ep, chl
"chloritic"
.... g'''''"''''g ....5555"''6'6 ............................................... ' i4i';';',"fi'i ..................... /7fi;'i'gf;i .................................. 'a'"i ..................... 5000 +New Mexi co
tactite
tactitetacti te
ahl,ep (Argillic)tacti te
....ii;;"8'gl'' ...................................... ............................................................................................................................................. -3_ 32,00 ;5000 chl, cal, ser, mont Q, ser, kaolArizona a Iteration zone
ToquepalaPeru
mi nor; cp > mbpy cp py, cp
py,mb,magpy > cp > mbpy cp mb py, cp, mb
........ ....ce.. ........................................... ................ .).!! !.m. ! ..py->Au cp-> py->(gal, sl, Au, Ag)i ....................................................
mb at depth cp-> py. ....................................................
.... .!...o.:.!.... ............................................................................................................................................................... ....!py (1%); cp(1-3%)py:cp= 10:1..................................................
low total sul; py1%;
pycpmbbnpy cp mb bn sl py+__= cpbn, mb
ore zone & (cp, mb)->py->
low.gradecenter-> annular
py zone contracts& py:magincreases
.......................................................................................................... !...,..?..!,..?.! !...............................................................................py, cp, bn, tt, mb,-sl Q, tm+minor sul cp-> py->(Ag, gal, sl) (Q, tm)-> cp->py ND not observedanhydrite at depth
py cp?mb bn; low(3%) tot sul; py:p =3:1
py cpmbbn
(cp,mb)-py- (gal, sl, Ag, Au)
(cp,mb)-py
396
.t. D. LOWELL AND J. M. GUILBERT
OCCURRENCE
OF
SULFIDES
DE POSI T S
PeripheralAlteration Zone
OuterAlteration Zone
IntermediateAlteration Zone
InnerAlerati6n Zone
(36)
(37)
(38)
(39)
AJrizon aBagdadBethlehemBritish Columbia
veinletsvns & massiveveins
diss /vltsvlts dissveinlets veinlets
diss /vltsdiss vltsveinlets
....... .!.z..o.n. ......................... .r..p.!?.m..?.t. ............................................................................................................................BinghamBisbeeArizona
veins &vns, vlts,mass. repl.
vns, vlts, dissND patches & vltsvn, vltveinletsveinlets
vlts, dissND vlts & patchesvn, vltvlts, diss, mass.diss vlts
diss) vltsvns, vlts, diss vlts patchesvlt, vn, dissvlts dissdiss vlts0 ..................................
........ ............................... .u..t.. r...!.m..?..t .......................................................................................................................................BradenChile
veinsvn, vltveinveins, ..........................................................................................................................................
ButteMontana
CananeaSora
Castle DomeArizona............................
ChuquicamataChile..............
veinsvns & dikes
vns & vlts
vlts dissvlts ,,..
vlts dissvlts diss
, ...........................................................................................................................................................................................
ClimaxColorado................. , .......
diss
, ....................................................................................................
......................................................................
CopperCitiesArizona
veins
veinlets
di ss vlts
di ss vlts
"'ii"'i'; ....................... ,'i.............................. i';5'ii"' .................... ;i'';''ii". .................... i'";i' ................ Chile.............................. ,, ..................................................................................................................... , ...................................................
ElyNevada...................... , ....... , ................ , ..... , ................................................................................................
diss vlt, ....... , ..........
diss v.lt, ....... 0 ........................
EndakoBritish...............
vlt dissColumbia, ........... . ............................................................................................................................................................................
vlt diss
vlt --
vlt
EsperanzaArizona.,,.,
veins
vns & vlts
vlts
diss vlts
.......................................................................................................................................................................................................
Inspiration....................
Arizona
veins,., ...... ,.. ........................... ..,
vns & vlts.................................................................
vlts ' diss ................................................................
vlts dissvlts, vns stkwkvlt, diss &massive
, ...........
Mineral ParkMission-PimaArizona.......................... ,.,.,o,o,,,0 ......
veinsvn & vlt, ..... 0 ............. . ...................................
vlts vns, stkwk
vlts vns, stkwk
....... .r.!.z..o.?.? .................................................................. ................... ?. ................. ?..P. ........... Z.q.i..?..P.?.?.! :.7...?.P. ?...."... :.'.. ?..?..?.... ................................... . ....................... 0 ........... , .............. ,,.,,..0,.,0 .....
Morenci......................................
vns, Is repl.. ..... , .................... . .................
vlts diss ..................................................
ND: ..................................................................
vns, vlts, diss,..,
Ari zon a
QuestaNew Mexi co........................................................
veins,., ........... . ..............
paint, .......................................................................................................................
vltsvns, vlts, diss. .............................................
vns & vltsvlts,, diss, vns, ............................................
RayArizona..................................................................................................
veinsveins....... 0..,,, .,,0..... ,.. o,,0.,,0 .... , ........
vns, vlts, diss0 ......... 0 .....
SaffordSanManuel-KalamazooArizona.................... , ..... . ............................
in shears, vns,
in shears, vns,,.., J ...............
in veins, vlts, diss, ............................
....... !.z..o.?. ... ......................Santa RitaSilver BellArizona
eikes.......
........
.o...
..........
dikes
, ...................................
veins ......................................
vlts, .......... . ..............................................................
vlts diss
vlts diss, ........................ , ..........
New Mexico
veinsvns & tactite
vns & vlts,
vns & vlts.................. . ...............................................
vlts,/vlts diss, ...............
.........................................................................................................................
vns & tactite
vlts di ss
vlts diss
........................................................................................................................................................................... ii''':'i' ................ Toquepala veins di ss vltsPeru bx vug fillings bx vug fillings
Typical PorphyryCopper
veins
vns& vlts
veinlets
vnlts diss
.4LTERATION-MINERALIZATION
ZONING IN PORPHYRY
ORE DEPOSITS
397
OCCURRENCEInnermost
OF
SUL.
FIDES-ContinzedSUPERGENE Crackle Zones SULFIDES
Alteratio0
Zone
(40)
Breccia Pipes (41) not reported
(42)
(43)minor co, cv
diss /vltsdiss Its
beyond ore limit
.... ;i;";';1'/;';' ................................ ;;;;;;;:i',";,;'/;';;;;;ii'';,';i ............i-;'f;; ';'/i T;';';;;",';;;,' ................. ........................................ ;,';';,';.... Ji'''";'i; ......................... gal,sl zone in...................................................... .....................................................................................
extends beyond sl gal,NE horsetail zone present
co,cvcc
....................................................... !................................................................................................................................ .p..,.important; 2 stages vlts patchesdiss vlt
postore with min. frag.none
cc cvcc, cv, dg
horsetail zone
vlts diss...........................................................
numerous& mineralized; .......................................
presenti- .................................................
co, cv. ................................................. ....,
present?
present
co, cv
vlts)
diss
large central pipeminor breccia, dikespresent
horsetail zonepresentpresent
co, cvnoneco, cv
irregular clots
diss,/J. vltsdiss)....
deep, central, mineralized presentpresent ..................................................
cc cvco, v, ................. ....................................................
vlt
present ................................
. .............................................
diss vlt diss vlt
not reported present
present present
none cv, cc
vlts diss (?)..................................................
not reported. ..................................................
present, ................................................. ,. .............
cc, .....................................
vlts, vns, stockwork
none
present
cc
....
............................................................................................................................................................ore N-S dike;breccia zones in pit
vlt, diss & massive
poorly developedextensive> ..................................................
cc thin zoneco, cv ........................................... . .........
...................................................... .p.:.....a.!.,..?.!:..t..t. ................................................................................................................................................................ , ......... ,.! ..................................................
vns & vlts...... , ................................... , .......
present, important! ..................................................
extensive ......................................................................................................
none cc cv, .................................. . ..................
vlts diss, vns
present & mineralized ..................... , ............................ ..........
present, ......................................
..........
. .......................................
vns, vlts, diss..................................................
present & mineralized ............................. , ....................
present ..................................................
co, cv .................................. , ................
diss vlts..................................................
not reported ..................................................
+ 5000 ft diameter..................................................
cc...................................................
vlts/J. vlts, diss..................................................
one 500x 2500 ft pipemineralized ..................................................
samearea.as intrusive ..................................................
cc cv.....................................................
ND
none
NW horsetail
zone.........
cc, ........................... ,, .............
.... J'':' '"' i'' ''Ji'i ............ i;'"''Ji''J; "'small:::::
mineralized:: ::::
............. "'';';,'' ' 'i'' ..... cc::::: :::::: :J::::;: :::::::: :;: ;::: :::; :::; :::::: :: t.'::; '-:: :::: :::: ::: ::;: :: ::::: ::;: ::::: ', :::: :::;::: :: ::: :::: :.
',; : :;::;::::
::: :::: :: ::::
::: :::::::;;:
;:::J:
'.:;::
:::::::
:: :::: :::,::
diss )//vlts
present& mineralized
present
cc) cv
398
I. D. LOWELL AND J. M. GUILBERT
associatedwith them, but evidence shows that ore
to be relatedto contemporaneous youngerfaultand ing and uplift. Table 1 showsthat most of the host igneous bodiesare somewhat elongateand that districts with strongstructuralcontroltend to include pronouncedly elongatestocks. Column7 lists the size of igneous host rock outcrops for each district, the numbershaving been taken from texts or measured from geologic maps. These dimensions in part subjectto the same are uncertainties the descriptions Column6. The as in years ago. indicate that the porphyry copper deTable 1 includes agesfor deposits British Co- dimensions in lumbia and South America as wall as southwestern positenvironment commonly was developed stocks in North America. Six of 27 depositsare of mid- or cupolas with crosssections well under a square of Tertiary age at 30-37 million years,17 are probably mile at the elevationof ore deposition. There apin the Laramide range of 59 to 72 million years, 3 pear to be two host-rock sizepopulations, group one are in the Jurassicrange of 122-143 million years, less than a mile squareand another smallergroup and 1 deposit has a 200 million year Triassicdate. of very large dimensions. Mode of Eraplacement(Column 8).---These enOf the Southwestdeposits includedin Table 1, all and tend to confirmthe are of Laramideage exceptthree mid-Tertiary de- tries adoptthe terminology of posits (Climax, Questa, and Bingham) and two conclusions Stringham (1966) regarding mode Jurassic deposits (Bisbeeand Ely), two of the mid- of emplacement. Stringham'scriteria are extended Tertiary onesbeingporphyrymolybdenum deposits. to includethe additionalporphyry copperdeposits The pattern for porphyry dates emerging in described here. Emplacement the porphyrycopof British Columbia seems be one in whichparallel, per deposithost rocks is shownto be almosttotally to overlapping, northwest-trending brits of mineraliza- passive. This passivitysuggests that replacement, were more importantprotion increase age from west to east. The single stoping,and assimilation in numericalage for a South American deposit in cesses than shouldering asideor othermanifestations the Table 1 is for Toquepala, Peru, at 59 millionyears. of forceful intrusion,and it also suggests likelizoning However, geologic relationships recentdatingby hoodthat both lateral and verticalpetrologic and than has been recognized. Chileangeologists indicatethat many of the South might be more common Comparison Column 8 with Columns41 and 42, of Americandeposits of mid-Tertiary age. are Controllin#Structures(Column 5).--Column 5 the latter reportingbrecciation and shatteringsperevealsthat brecciation lists attitudesof regional-scale structuresthought to cificallywithin the orebodies, with ore deposition in have controlled the emplacement the stocksand or shatteringare associated of of batholithsand hencethe porphyry deposits them- every porphyrydeposit,evenwhere emplacement selves. Considerationwas given to local structure the host stocksis passive. This disparitysuggests and shattering themselves are "passhownon published mine and district maps in pre- that brecciation be to paring Column 5, but many boundingfaults shown sive," and that they can commonly expected North on thesemapsare of postoreage or of multiple age be "blind," as they are at many southwestern and suchthat their preore importance cannotbe deter- Americanporphyrydeposits prospects.Forcebrecciation mined.Greater reliance therefore was placed upon ful intrusionand active, even explosive direct text statements than upon maps. Several as at Toquepalaand Braden are apparentlyrare. assimilation, metaand authors commentthat the specifics controlling Extensivemagmaticstoplng, of appearmechanically and kineticallyinconstructures were obliterated the intrusions by which somatism sistent with extremely shallow emplacement, but they guided. may be indicated. Shapeand Size (Columns and 7).--The shapes moderatelyshallowenvironments 6 Porphyry molybdenum depositsseem to show of intrusions (Column 6), like determinations of of than do portheir size (Column 7), are difficult to establish more evidence forcefulemplacement in consists of meaningfully,sinceboth have been affectedby in- phyry coppers general. This evidence ternal and externalvariables. Exposureof a pluton ring and radial dikes and doming of the layered overlie the deposits. is certainlyaffectedby original depth and by post- rocks which sometimes Stock-Dike (Column 9).--Column 9 indicates intrusion tectonic and erosional history. The Boulder batholithhas been exposed tens of miles, and a that stocks and stocks with subordinate associated for large southern Arizona batholith (Ettlinger, 1928) dikes are far more typical of porphyrycopperdethanare dikes,dike swarms, breccias or alone. has been inferred from the distributionof cupolas. posits was indicated Column6 in The shape sizeof porphyry and hostintrusions seem This samerdationship deposition was essentially contemporaneous inwith trusion within the precisionof the I(-Ar technique. Age dating of the Laramide-mid-Tertiary interval in the Southwestreported by Damon and Mauger (1966) has indicatedtwo distinct pulses,one of Laramide plutonic activitybetween and75 million 50 years ago and one of dominantlyextrusiveactivity duringmid-Tertiary time approximately million 30
ALTERATION-MINERALIZATION
ZONING IN PORPHYRY ORE DEPOSITS
399
below. Dioritic rockscommonly where porphyry deposits were shownto be equi- will be considered to oval rather than tabular or linear occurat intrusionmargins,as at Ajo and Mineral more K-feldspathic rocks bodies. Twenty-fourof the 27 deposits involveim- Park, with progressively portant stockdevelopment and a high ratio of stock inward, a relationshipnot apparent in the table. to dike forms. This distributionis consistent with apparentfelsicSequence Intrusions and Rock Types Mineral- component of enrichment accompanying potassic alterized (Columns10 and //).--The sequences in- ation near the central portions of some porphyry of trusion shownin Column 10 reinforceearly observa- copper deposits. tions (Buddington,1933) of the association copof Orebody (Columns12-19) per deposits with intermediate felsicigneous to rocks. Except for generally late diabasedikes, no rocks Outward Shape (Column /2).--The porphyry more mafic than diorite occur in the intrusions ascopper deposits almost havecircularor oval cross all sociated with porphyry copperdeposits. Granodi- sections.At leastfour deposits haveclearlydefined orite and quartz monzoniteand their aphaniticand low-gradecentersproducinga ringlike orebodyin hypabyssalequivalentsoccur in almost all of the plan. The vertical dimensions hypogeneminof porphyrycopperdeposits, with more felsicvariants eralization in most depositsare unknown; however, common to the porphyry molybdenumdeposits. the tabulatedhypogene mineral bodiesseemto fall Most papers consultedin preparing Table 1 give into three general configurations. specificsequences intrusive events and igneous of 1. Seventeendepositshave a steep-walledcylinrock compositions, uncertainfield relationships drical shape. Two deposits(Cananeaand Toquebut coupled with paucityof radiometric age determina- pala) approximately coincide with brecciapipes. tions seldom permit unequivocal identification the of 2. Sevendeposits have stubbycylindricalor flat, beginning and endingof the magmatic episode that conical forms,as do all three of the porphyrymolybinvolved ore mineralization. Much older and much denurndeposits. youngerrocks,as described the appropriate in refer3. Three deposits (Inspiration,Ely, and Safford) ences, are excluded. Column 11 shows that all of have a gently dipping,tabular shape, perhapsreprethe intrusive rocks of Column 10 are mineralized sentinga depositsimilar to (2) following a preore in 22 of the 27 deposits tabulated and the youngest structure postore or displacement, theymay repreor intrusiveunit is mineralizedin 2 of the remaining5. sent a separate type. Columns 10 and 11 show that the sequence is Boundaries (Column13).--In all of the deposits generally from dioritic to monzoniticrocks, com- studied, the orebody boundaries at least in part are monly with late latitic to rhyolitic or "quartz por- gradationalor "assaywall" boundaries. All have phyry" intrusions. Typically, all of theseare min- beenintersected a postore by erosionsurface. Eleven eralized, showingthat mineralization either accom- are boundedby at least one postorefault. Two paniedor briefly succeeded emplacement in- coincidecloselywith brecciapipeswhich are preore the of trusive rocks. The association porphyrycopper or contemporaneous of with ore, and one deposit deposits with intermediate plutonicrocksis impres- (Braden) forms a crude cylindricalshell surrounddimensional
sive but not as consistent as the association with
porphyry in all 27 districtslisted. There has been discussion recent years as to whether the name in "porphyrycopper"is appropriate the group of for deposits described this paper. The writers bein lieve that this association geneticrather than cois incidentaland feel that "porphyry copper" is an excellentdescriptive name for this unique and important group of ore deposits. The lamprophyreor "late diabase"event is less common the porphyry in coppers thanhasbeenpreviously thought (Spurr, 1925). Late diabasehas been reported in only 5 of the 27 districts. The general trend, clearly, is from dioritic plutonic toward more felsic hypabyssal rocks with all rock typesusuallymineralized. The degreeto which the shift from dioritic throughgranodioritic monzoto nitic rocks may reflect K-feldspar enrichmentby meansof potassicalteration (Peters et al., 1966)
ing a postorebrecciapipe. Percent in I#neous Host and Preore Rocks
(Columns 14 and 15).--In severaldeposits,100 percentof the ore mineralization in igneous is host rocks (Butte, CastleDome, Copper Cities, Endako, and Mineral Park). All containsomeore in igneous host rocks, but most ore at Bisbee, Braden, Mis-
sion, and Ray is in wall rocks. Something like 30 percentof all ore mineralizationassociated with porphyriesoccurs wall rocks,again suggesting in cupola or at least high-levelenvironment the porphyry for deposition. Dimensions (Column16) .--Horizontal dimensions of the tabulateddepositsrange from 250 x 1,200
feet for the La Colorada pipeat Cananea 6,000 x to 13,000feet for the Morenci deposit. Fringesof the difficult-to-limitButte district may reach to dimensionson the order of 20,000 x 50,000feet (only the "porphyryequivalent" Butte is cited in Column for
400
]. D. LOWELL AND J. M. GUILBERT
16). The averagedepositsize deduced from pub- genesilicate alteration phases. Nonetheless, superlisheddescriptions mapsis a perhaps and surpris- gene sericitehas been reported. Supergene effects ingly small 3,500 x 5,000 feet. havebeeneliminated from Table 1 whereveroriginal Total Ore Tonnageand Grade (Columns17, 18, authors provided descriptions whichwouldpermitit. and 19.)--Of the 27 deposits tabulated, are esti13 Known Extent BeyondOre (Column20).--Colmated to containover 500,000,000tons of ore, 6 fall umn 20 records the stated or mapped extent of between 100,000,000 and 500,000,000 tons, and 8 alteration beyond outerboundary the orebody the of containless than 100,000,000tons. These tonnage itself. These distances are somewhat uncertain since estimates mustbe considered only approximate. differentobservers drew the outer line on differing Included in these figures are several deposits criteria. External alteration is narrow around the whoseore grade dependson secondary chalcocite Bethlehem, B.C., deposit, characteristic many a of enrichment.Averagegradeof copperore is 0.80% of the Canadian porphyrydeposits. Other deposits Cu, and averagegrade of hypogene mineralization, show alteration extendingthousands feet, averof where this information is available,is 0.45% Cu. aging approximately 2,500 feet. The higher numTwelve copperdeposits containat least 0.5% Cu in bers probably represent merging of hydrothermal hypogenemineralizationand 10 contain less than with low-rankregionalmetamorphic effects, two the 0.5% Cu. Molybdenumdepositsaverage 0.17% being distinguished only with difficulty. SignifiMo in grade. cantly,detectable alteration extends laterallyan average of half a mile beyondthe orebodies, perhaps Hypogene,dlteration(Columns20-27) more, since some authors drew the outer limit on The next three sections,Hypogene Alteration the basisof "bleaching" and the presence sericite, of (Columns20 through27), HypogeneMineralization phenomenathat probably do not mark the truePeripheralZone (Column 21) .--Alteration is deganization so that the columnsfor each zone in a scribedin this zone for only five deposits. It is givendeposit haveidenticalheadings. For example, generallyalongwell-developed structures and is selthe innermost alteration zone at San Manual-Kaladom well described with respectto associated minmazoo consistsof quartz, K-feldspar, biotite, and eralization. Where alteration mineralogyis given minoranhydrite(Column25), andthe ore minerals it is of mixed affinity, dominantlypropylitic,with at in (with amounts) are pyrite, chalcopyrite, molyb- sericitementioned Questa. Skarn is described denite, and trace bornite (Column 32). The sul- this zone at Morenci and Santa Rita. Skarn or fides occur more commonlyas disseminations than tactite development not as well reported in the is as veinlets (Column 40). literature as are hydrous silicatealteration assemIt should be restated here that the table is based blages. It is well known that skarn zonesproject as completely possible as upon publisheddescrip- into and apparentlydistort more normal zoningretions, and these are hardly uniform in approach, lationships, and that many porphyrydeposits might deposits. detail,or eventerminology.Several deposit descrip- also be describedas contact-metamorphic tions were based on temporal rather than spatial Skarn can also apparentlypersistto the centersof relationships; thesedeposits were enteredas earliest orebodies. equalsinnermost,and so on outward. Several deOuter Zone (Column 22).--Mineralogic notation posit descriptions involved separateand poorly re- is given for 20 of the 27 deposits, with "propylitic" lated descriptions alteration, mineralization,and citedfor Ely, Nevada. Of these,18 includechlorite, of occurrence. We have made every effort to match 17 epidote, and 13 a carbonate (calcite in 11). appropriatespatial and mineralogical data. Ques- Quartz is cited 7 times, sericite6, zoisite-clinozoisite tion marks in the table generallydenoteuncertainty 5, kaolin 3, specularite2, montmorillonite2, and of placementof the information rather than un- albite,hematite, magnetite, tourmaline, and ruffle(?) certainty in the data. once each. By far the most commonassemblage is(Columns28 through 35), and Occurrenceof Sulfides (Columns36 through 42), have parallel orouter limit.
The problem distinguishing of between supergene chlorite-epidote--calcite. Mentionis seldom madeof and hypogene effectsis important. Hemley and the replaced minerals,but the chief onesare amphiJones(1964) curvesindicatesericitestabilityonly bole,biotite,and plagioclase (Fig. 12). This assemat moderately high K+/H + ratios at low tempera- blagehas affectedby far the largestvolumeof rock. tures, an environment consistent with (but not re- The chlorite-epidote-calcite propyliticassemblage is quiring) high pH. The extremdy low pH pre- alwaysoutsidethe ore zone and beyondthe phyllic sumed activesupergene for enrichment zones argues and argillic zoneswhere theseare present. Sericite againstimportantdevelopment supergene of sericite is commonlyreported even in outermostalteration and indicatethe kaolin mineralsto be stablesuper- assemblages. Whether this mineral varies import-
ALTERATION-MINERALIZATION ZONING IN PORPHYRYORE DEPOSITS
40i
Innermost Zone (Column 25).--This column is antly in composition, hencein stabilityfield and and alterdistribution,is yet to be shown. It has been ob- perhapsthe most surprisingof the hypogene served, however, in amountsranging from trace to ation data block. Potassicalteration, though relain at moderate, and chieflyreplacingplagioclase, some tively subordinate the literature,occurs mostof in as outer zonesnot reportedin Table 1. The distribu- the porphyrydeposits eitheran early or an inneror tion with respect verticalzoningwill be discussed most assemblage both. It is reportedas simple to below. quartz, K-feldspar,and biotite(?) only at Endako; Intermediate Zone (Column 23).--This column as quartz, K-feldspar,biotite,and sericiteat 7 dedescribes predominantly argillicassemblages. Silici- posits,and as quartz, K-feldspar,biotite with chlorfication is clearly more important here than in the ite, albite, fluorite, anhydrite, or tourmaline at 8 are reported outer zone, and the dominant minerals are quartz, more. Quartz, K-feldspar,and sericite kaolin, montmorillonite,and sericite. Argillic as- at Silver Bell, and quartz with only K-feldsparocsemblages discernible 22 of the 27 deposits, cursat Mineral Park and Questa. Quartz, phlogoare in if quartz-sericite-kaolinite occurrences) in- pite, and tourmalineoccurat Cananea,but the zone (4 be cluded as argillic. Quartz is cited first in most may not be innermostthere. Quartz, sericite,bioassemblages. Kaolin is citedsinglyor beforemont- tite, and anhydriteoccurat Braden. Anhydrite at in morillonite in 17 of the 22 assemblages which several localesis given in parentheses Table 1 for in dataare given. Three deposits havemontmorillonite whereit hasnot beendescribed print. Specimens zonally beyondkaolin, and 7 involve sericite. No of anhydritefrom Esperanza,Questa,San ManuelKalamazoo, and Santa Rita have been observed to argillic assemblage reportedin 5 deposits. is at Inner Zone (Column 24).--Most of the quartz- swell the publishedoccurrences Butte, E1 SalAjo, and Braden. sericite (and pyrite) assemblages, chief ore vador, Toquepala, the The common occurrence anhydritein the poof bearers the porphyrycopperdeposits, in this of fall inner zone column. The zone is reportedunequi- tassiczone indicatesthat (1) redox potentialsare vocally to have a pervasivequartz-sericite assem- considerablyhigher in the late magrnatic-deuteric of sulfur speblageat 19 porphyrydistricts, quartz-majorseri- fluidsthan the prevalence unoxidized a of cite-minorK-feldspar array at 3 more,and a quartz- cies would indicate; (2) a high percentage the may be present major sericite-minorkaolin assemblage 3 more. total sulfur in the porphyrysystem at hydrothermal At Bradena quartz-sericite-biotite-anhydrite inner- as sulfate; and (3) high-temperature reactions involving silicates, oxides, and sulfides zoneassemblage gradesinto strongersecondary biowith equilibria involving tite in the innermostzone. Only at Esperanza is must concernthemselves a quartz-K-feldspar pair reportedzonallyoutside of higher total sulfur than the net sulfide contents that the conan unusual quartz-K-feldspar-biotite assemblage. would indicate. It is also noteworthy clusionof Lutton (1959) concerning depositional Creasey (1966) indicatesthat K-feldspar can be continuum from pegrnatoid into "porphyry"condipart of his quartz-muscovite assemblage found at and that the elements grouped Bagdad, Bingham, and Chuquicamata. Creasey tions are supported states(1966, p. 62) "quartz-sericite-pyrite without by Ringwood(1955) as "complexformers"of high either a clay mineral or K-feldspar associated a ionic potentialare preciselythose found in major is decommon assemblage doesnot fit into any of the and trace minerals in the porphyry base-metal that posits,especially the potassic in alterationzone. three previouslydescribed alterationtypes. If clay Other characteristics the potassic zone are of were present [as at Endako, Inspiration, and Misby sion-Pima,wherekaolin is reported],the assemblage briefly described Meyer and Hemley (1963) and occurs wouldbelong the argillicalteration, to and if K-feld- Guilbertand Lowell (1968). Ore commonly at the interfacebetweenpotassicand phyllic alterspar were present [as at Bagdad, Bingham, and zone is generallycentral Chuquicamata],it would belong to the potassic." ation zones. The potassic or is it Sincethe assemblage appears far mostcommonly or deepest, if a time sequence discernible, is by earliest. as quartz-sericite-pyrite, term "phyllic" is herein the Zoning Sequence from Center and Bottom (Colurged as a specific term. Advancedargillic alterumns26 and27).---The upwardzoning and outward ation, involving chiefly pyrophyllite,dickite, and zoningof alteration assemblages seldom are reported topaz (Meyer and Hemley, 1968), is associated with as such,but their systematic entry by description or phyllic assemblages Butte and Bisbee. It is not from map or diagramrevealsa significant at sequence. reportedelsewhere may have escaped but detection. Seven,possiblyeight (the positionof phyllic alThe phyllicassemblage Column24 is the inner- teration at E1 Salvador is uncertain), of the deof most exposedalterationassemblage at least six positsshow alterationassemblages the sameoutin in districts. ward sequence: potassic, phyllic, argillic, and pro-
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1. D. LOWELL AND ]. M. GUiLBERT
pylitic. Even wherecertainassemblages not reare ported, the remainingassemblages in the same fall order. Two deposits, possiblythree, show only potassic and phyllic zones,four lack only argillic, and six start with phyllic and includeargillic and propylitic. For a few deposits sequence unthe is
Outer Alteration Zone (Column 29).--This zone generally corresponds the propylitic alteration to zone, and mineralizationis generally restrictedto pyrite, although sparse chalcopyriteis generally present along with variable amounts of bornite, molybdenite, magnetite,specularite, rhodochrosite, known. sphalerite, galena,and rhodonite. Intermediate Alteration Zone (Column 30).-Vertical sequence zonationis generallymuch of roughly to the argillic alteration less well known, so assignments be made in This corresponds can is Column27 only for Butte, Climax,E1 Salvador, and zone,and the bulk of mineralization usuallypyrite ratios San Manuel-Kalamazoo.Except for uncertainty at with highpyrite-to-chalcopyrite whichaverage E1 Salvador, the order is consistent with lateral 23:1 in depositsfor which figures are available. of tennantite, zoning. Outwardand upwardzoningof the 27 de- Variableamounts bornite,molybdenite, galena,enargite, chalcocite, huebnerand posits mostconsistent the sequence potas- sphalerite, is with of ite have been found in this zone. Hypogeneoresic,phyllic, argillic, andpropylitic assemblages. may overlapinto this zone,but An alterationassemblage beennotedin sev- grademineralization has this zoneis outside orebody. the eral localities whichconsists K-feldspar, of biotite, generally Inner Alteration Zone (Column 31).--This zone coarsesericite, chlorite,and albite,accompanied by corresponds the phyllic alterationzone to moderate pyrite and chalcopyritemineralization. commonly and typicallycontains abundant pyrite and high total This groupdoesnot readilyfit the classification outtogether with pervasive sericitization. Pyrite linedin Table 1, nor do the deposits generally reach sulfides for ore grade. The writers are of the opinionthat this contentis not reportedquantitatively most debut it appears average to about 10 percent by represents deep assemblage a whoserelationship to posits or the mainporphyrysystem not beenexposed has for weightfor the 27 deposits, about 16 percent,exgroup,whichare study because the geometryand large vertical cludingthe porphyrymolybdenum of dimensions involved. relatively low in pyrite. Pyrite-to-chalcopyrite ratiosaverage12.5:1. This zonecommonly constitutes the ore zone, especially those deposits in in Hypogene Mineralization (Columns 28-35)which chatcocite enrichment has occurred. The
As has long beenknown,hypogene sulfide-oxide principal"ore" mineralis pyrite, whichoccurs with mineralassemblages closely are relatedin time and chalcopyrite, molybdenite, variable generally and but space with silicate alteration mineralassemblages small amounts of bornite, chalcocite,sphalerite, in porphyrydeposits. The designation pyrite and enargite,and magnetite. ofmagnetite as ore minerals rather than alteration
minerals,for example,appears be largely arbi- zoneis generallyequivalent the potassic to to alteration trary. zone and is usuallythe central zone. Total sulfide In Table 1, sulfide-oxide mineralassemblages have contentis low to moderatewith an averagepyrite been described in Columns 28-35 with reference to content of about one percent and a pyrite-to-chalthe same alteration zones as are described in Colcopyriteratio of 3:1 in the deposits tabulated. This umns20-27. The consistent sequence througheach zonemay reachore gradeand probablyaccounts for zone and from one assemblage another outward most ore in solelyhypogene to ore deposits. It also from the centeris again significant. formsthe "low-grade center"in five deposits.The PeripheralAlteration Zone (Column 28).roThis sulfidemineral assemblage chalcopyrite, is pyrite, column describes metal occurrences that form a disand molybdenite. continuous ring normallynear the outer edgeof the Overall Abundance ol Major Ore Minerals propyliticzone. The deposits tend to be small to (Column 33).In the porphyry coppers, pyrite is mediumsize, althoughlarge lead-zincdeposits with by far the most commonsulfide,followed in order or without preciousmetals occur in this zone at by chalcopyrite, bornite, enargite,and molybdenite. Santa Rita, Bingham, and Butte. At least minor Molybdeniteis presentin all 27 deposits, fact not a peripheralmineralization found in all 27 deposits previously is recognized. studied. Arcuateclusters mines prospects of or surZoning Sequence from Center (Column 34) and round 23 deposits. Minerals commonin this zone fromBottom(Column 35).Grading outward from are sphalerite, galena,silver,chalcopyrite, gold, and the centerof the deposit, typicallateral mineralithe pyrite, and less commonly,specularite,enargite, zation sequence appearsto be the assemblages (1) famatinite,tetrahedrite,barite, varioussulfosalts, and chalcopyrite, pyrite,bornite,molybdenite; pyrite, (2) manganese and vanadiumminerals. chalcopyrite, molybdenite, bornite; (3) pyrite, chal-
Innermost Alteration Zone (Column 32).roThis
,4LTERATION-MINERALIZATION ZONINGIN PORPHYRY OREDEPOSITS
403
copyrite; and (4) sphalerite, galena, silver, gold. itic alteration. Crackle texture is often less distinct if alteration Apparent reversalswere noted in only three camps. near the center,particularly a potassic Information as to vertical zoning is extremely zone is present. limited. Most deposits have beenexploredby mine Supergene Sulfides(Column43) openingsor drill holes only to depths which are shallowas compared with the probableoriginal verTwenty-three deposits contain supergene sulfides,tical dimensions. Tentative evidence from 13 de-
positssuggests that typicallya pyrite-chalcopyrite- marginal ore gradein 10. Supergene chalcocite molybdenite assemblage gradesupward into pyrite. (andprobably secondary also digenite djurleite) and An apparent reversalof this order hasbeenreported is present wherever secondary sulfides occurand in two deposits.Occurrence Sulfides (Columns36-42) ol
and secondary enrichment was requiredto reach
alwaysconstitutes chief enrichment the mineral. Covellite reported 12 deposits, is in generally lowin the enrichment blanket.
Hypogenesulfidesin porphyry deposits typically form veinlets or disseminated grains. This habit is probablyrelated to the fact that cracklebrecciation is presentthroughoutthe volumeof mineralization. Broadly,the porphyries seemto be masses homoof geneous rock penetrated reticulatefracturesand by mineralized fluids which soakedthe massrather by than beingconstricted tabularmasses replaceto orments.
Porphyry Deposit GenesisThe data of Table 1 and the inferences drawn
from them, from the field, and from the detailed geologyof the San Manuel-Kalamazoo depositap-
pear to support orthomagmatic the modeldescribed earlier, althoughthe nature of the data and thescalefactorsare not suchthat the problemscan be conclusively resolved. The formational modelwhich appears most generallyapplicable one of a difis
Occurrence Sulfidesby Zones (Columns36of 40) .--A progressive gradationin sulfidedistribution ferentiationcontinuumas suggested many years ago is noted in almost every deposittabulated. This by W. H. Emmons (1933) in his description of sequence progresses from veins in the peripheral cupolaformation. Near-surface intrusionof a melt zoneto veinletsin the outer zone,veinletsand minor whichproduces rocksof intermediate granitoidcomdisseminated grains in the intermediate zone,vein- positionis either a passiveintrusion as at Butte, lets approximately equal to disseminations the Santa Rita, and Ajo, or a dike swarm as at San in inner zone, and predominantdisseminations the Manuel-Kalamazoo in and Safford. Response wall of innermost zone. The tendency for the increasing rocksto this intrusiondepends upontheir composiresult from metasomatism recrystallization the intrusive melt. Cooling begins from the surface or of rockand healing veinlets. The absence promi- downward,and gentle thermal gradientsare estabof of nentveinsin mostalteration zones may indicate that lishedfrom higher temperatures depth to slightly at a crackle brecciation zonebehaves an incompetent lower ones nearer to the surface and outward. Minas masswhich can not supportthrough-going fissures eralization and alteration chemistries are established and veins. with respect thesegradients, to chemistries that reBrecciaPipes and CrackleZones (Columns41 flect essentially deuteric late magmatic to conditions, and 42).--Breccia pipesare presentin 20 and are with potassicalteration yielding upward and outmineralized 18 deposits.Toquepala Cananea ward through the phyllic zones (or the "zone of in and are mineralized breccia pipesin whichore limits are feldspar destruction," Robertson, 1962) into the nearly coextensive with the pipes. Toquepala, in zones of more typical hydrothermal alteration reparticular,showsevidence that the surrounding al- sponses. These gentle gradients presumablyhave terationzones havebeentelescoped a relatively a direct bearing on the large dimensionsof the into thin halo,and alteration assemblages within the ore- porphyriesand the coarselygradationalalterationbody' overlap. The Bradenorebody apparently con- mineralization boundaries which they show. sists a verticalcylindrical of deposit whichhasbeen We thus reaffirm on the basis of the published penetrated alongits verticalaxisby a postore breccia record that the porphyry copper depositsare the pipe. results of a physical-geochemical continuum from A well-developed cracklezone is presentin 26 low-temperature magmaticto "conventional" hydrodeposits is largely absentin the skarn of the thermal conditions. The gradientsare reachedas a but Mission-Pima orebody. Cracklezones usually result of cooling in an intrusive mass, and the are circularin outline and are alwayslarger than the alteration-mineralization zonal boundary interfaces orebodies, typically fadingout in the zoneof propyl- appear to have been established standingforms as
importance dissemination of towardsthe core may tion, their structural fabric, and the nature of the
404
.r. D. LOWELL AND J. M. GUILBERT
surfacesfor several porphyry copper depositsare shownin Figure 13. Morenci is placedhigh in the hypotheticalvertical section becauseof the wide exposureof the phyllic zone without exposureof potassicassemblages.Several aspectsof Morenci geologymbreccia zones, the broad-scalealteration symmetry, and the occurrenceand distribution of sulfidessuggest that potassic alterationwill be encountered depth under the existingopen pit. at It is alsonoteworthyhere that phyllic zonealterhowever, that an inner zone need not have been ation assemblages, with their high pyrite contentand preceded the mineralogyand assemblages an their profusion of veinlets and microveinlets,are by of for of outer zone in a systemof decline,of lowering tem- chiefly responsible the extensivedevelopment peratures, or of shallow upward gradients. Vari- supergeneoxidation, leaching, and enrichment of ation in the differentiation index of the intrusion southwestern North American deposits. This high may well dictate whether copper or molybdenum level of exposureappearsto be the most common, deposits. predominates the ultimate deposit,molybdenite especiallyin supergene-enriched in tending to be associated with more silicic variants. Recentpublications the Chinodeposit Santa on at Rita, New Mexico, showthat an islandof low-grade Conclusions material is being left in the center of the northern portion of the pit area. This island of low grade The foregoing summary forcefully demonstrates is symmetrically and centrally disposed with respect that the porphyry copper-molybdenum deposits dis- to secondary K-feldspar,chalcopyrite, pyrite disand play important unifying geologiccharacteristics in- tribution as reportedby Nielson (1968, Figs. 6, 7, cluding various lateral and vertical zones. The fact and 9). This "low grade island" may represent of zoning is not new, but severalimportantaspects, the croppingout of a low-grade barren zone analosuch as sulfide species,detailed alteration assem- gous to the central core at San Manuel-Kalamazoo. blages,and the characteristic occurrences the sul- Lastly, Gilluly's (1946) description the Ajo deof of fides, is far more widespread than has previously posit involves much the same K-feldspar-biotitebeenrealized. Indeed, a "typical"porphyrycopper chlorite-sericiteand magnetite-chalcopyrite assemdepositcan be hypothesized from Table 1 and is blagesand zonal characteristics those encountered as includedalong the bottom of the table. at depth in San Manuel-Kalamazoo. It appears It is especially noteworthy that many,and perhaps possible, therefore,to assigna third dimension to most, porphyry depositshave coaxially cylindrical at least severaldeposits,and many others may be alterationzones. Factorsthat limit the development assigneddepth parametersas further information of discernible symmetry porphyrydeposits in include develops. For example,brecciation and ring diking the following: may have significance regard to depth of formainrather than as upward and outward advancing mega envelopes. Application of the Hemley-Jonesmodel of potassiumsilicate stabilities and alteration, as modified by Fournier (1967) and Meyer and Hemley (1968), permits passagefrom essentially magmaticconditions depth to areas of higher at hydrogenion concentration and lower K+/H + and lower temperatures either with time at a given point deep in the systemor through spaceupward and outwardat a given time. It i
