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Economic GeologyVol. 70, 1975, pp. 993-995

DISCUSSIONS

GEOLOGIC, FLUID INCLUSION, ./tND ST./tBLE ISOTOPE STUDIESOF THE PISTO B UENO TUNGSTEN-BISE METILORE DEPOSIT, NORTHERN PER U3'ir: The paper by G. P. Landis and R. O. Rye(1974) presents ome nteresting ew data on PastoBueno, but it containsenoughvagariesand mislead-ing generalizations o that the average eader shouldhavean opportunity or a more balanced iew.Figure 2 of the Landis and Rye paper includesMundo Nuevo-Tamboras-Compaccha-La ictoria,and page 1026 lists Mundo Nuevo (and presumablythese others) as subordinatesettlements n t.hePastoBuenominingdistrict. n onesensehis s mis-leading since Mundo Neuvo-Tamboras-Compaccha-La Victoria are all part of one large zoneddistrict.Compacchas a molybdenum or. hyry, while theseother mentionedareas (mislocatedon their fig. 2)are in copper,zinc-, or lead-richparts of this district,in a typically zoned arrangementaround the heatsource. From the molybdenumporphyry outwardthe base metal zoning (in tungsten-bearing uartzveins) is copper (as sulosalts),phalerite,andgalena. In terms of geographic ize and volumeof

vein material, Compaccha nd its zoned district ismore important than Pasto Bueno, which is a dis-tinct, separate, nd smallercenterof mineralization.Compacchas described n page1034of the Landisand Rye paper as a low-grade molybdenitepor-phyry . Deep oxidation at Compacchahas de-stroyed much o he near-surfacemolybdenite,andthe oxide molybdenum ineralsare difficult o detectvisually.Grade is not, however, he decisive aram-eter in determining he economicsf t'hisporphyrymolybdenum eposit.La Victoria and Tamboras are stated to be min-eralogically imilar o Mundo Nuevo on page1034.This statementoverlooks he obviousmetal zoningpresent in the district, and it makes one wonder ifLandisand Rye examined he mine workings.Neither Compacchanor the tungsten-bearingquartz veins around t 'havea prominentgreisen.Tourmaline,which appears o be a characteristic fthe Pas{o Bueno greisen, s almostentirely absentin Compaccha nd its zoned district. The verylarge .quartz-sericite-pyritephyllic) zone coex-tensivelydeveloped ith the Compaccha eposit lsois noteworthy y the rarity with whic,hluorinemaybe identified s a distinctelement resent n mineral

assenlblages,nless,of course, t occurs estrictedto the sericite lattice. Fluorite, topaz, and othersuch fluorine minerals typically sought n greisenare either not presentor are too rare to permit useof the term greisen n the Compacchahylliczone.A few commentsconcerning he Pasto Buenogreisen re also n order. PastoBueno s a distinctcenter of mineralizationseparate rom Compacchaand its zoned district. The phyllic zone a.t PastoBuenooccurs nly n the wallsof someof the veins.The cross section shown in the Landis and Ryefigure 4 is in reality a longitudinalprojection orveins passing hrough the Consuelo-MariaOfeliaarea. The walls of veins in this area have the dis-trict's best developed hyllic zone. Had the sectionbeen drawn half a kilometer to the east or west, theamountof pervasivequartz-sericite-pyritelterationwould have been significantlyess. The absence falterationzoning n the Landis and Rye figure 3geologic lan may let the casual eaderassumehatthe phyllic zone is more pervasive n the districtthan it really is. Volumetrically he phylliczone smostsignificantn and near Sec.A-A (fig. 4); it isnot a pervasiveeature ypifyinga porphyrydeposit.Within the fairly restrictedarea where phyllicalterationexists,greisendoesoccur. However, sincegreisen omprises nly part of the phyllic zone,andthe phyllic zone is not everywhere present per-vasively n the walls of all tungsten-bearingeins,the greisen s a disappointmento the geoc.emistwho links tungstenwith large volumesof fluorine.At the times I visited the district (from 1955 to1962) the inadequacy f the exposedgreisen n thedistrict was more noteworthy than its presence.Landis and Rye couldhave helped he reader withthis point by showingalteration zoning in the wallrocks of their Chabuca and Consuelo-Manuelita veinlongitudinal projection (mislabeled cross sectionallevelmaps n their fig. 5.)The mention of MagistraI on page 1034 as ascheeli.te eposit s so casual as to be misleading.Magistral is a chalcopyrite-molybdeniteineralizedskarn and associated intrusive center and constitutesthe third major centerof mineralization after Com-pacchaand Pasto Bueno) for this region. Scheelite

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994 DISCUSSIONSis present ut,volumetricallyndeconomicallyun-lessdemonstratedtherwise y drilling), evaluationof thesurfacendicatest to bea chalcopyrite-molyb-denite eposit.The questionf why a chalcopyritedeposit hould xistsoclose o copper ulfosalt resdominantn CompacchandPastoBueno, speciallywhen he sizeof Magistral s coupledwith its moretypidal orphyryopper ineralogyas againstporphyrymolybdenumffinitiesor bothCompacc.haand PastoBueno), becomes ne of broad nterest.Compaccha,astoBueno,and Magistralare alllinked by a branchingand multistranded extralfault. Why this fault should avebeena majorregional ontrol or tungsten n'dporphyrymolyb-denurn-type ineralization t both CompacchandPastoBueno,but not for a more ypicalporphyrycoppermineralizationt Magistral,s a questionhat

Landis and Rye coul.dhave attacked or the benefitof all in the profession.Their speculationsn thismatter would still be of interest.The data supplied by Landis and Rye on thePasto Buenoarea do add considerablyo our under-standingof this type of ore deposit,and it is hopedthat eventually ther workersmay complementheireffort.

V. F. HOLLISTERDUVAL INTERNATIONAL CORPORATION

355 BUVaARD STREETVANCOUVER, .C., CANADAV6C 2G8December 27, 1974

REFERENCESLandis, G. P., and Rye, R. O., 1974, Geologic, fluid inclu-sion, and stable isotope studies of the Pasto Bueno tung-sten-basemetal ore deposit, northern Peru: EcoN. GFOL.,v. 69, p. 1025-1059.

GEOLOGIC, FLUID INCLUSION, AND STABLE ISOTOPE STUDIESOF THE PASTO B UENO TUNGSTEN-BASE METAL OREDEPOSIT, NORTHERN PERU .4 REPLYSir: V. F. I-Iollister's ommentsre appreciated.A pointby pointreplydoesnot appear o be justi-fiedas we feel he vagariesndmisleadingen-eralizations re clearlyspecifiedn the paper. Forexample,La Victoria, Tamboras,and Mundo Nuevoare mineralogically imilar, in spite of the obvious

zoning, n contrast o Pasto Bueno. T.he processand product, greisenization nd greisen, are termsused as defined by Shcherba,and constructionoffigure 4 is subject o well-specifiedestrictions p.1029). Majistral is not mentioned asually s ascheelitedeposit (p. 1034). T,he existenceof anorthwest-trendingranching nd multistranded ex-tral fault linking Compaccha, asto Bueno,and Ma-j istral is basedupon rather tenuous ssumptions.Although the possibleexistenceof this fault wasconsidered,he first author ('GPL) was unable odocumentield evidence uggestinghis fault duringfour months of field work (1969-1970). Com-paccha nd Pasto Buenoare occurrenceseoehemi-callyand emporallyelated o the quartzmonzonitephaseof the CordilleraBlanca ntrusivecomplex.Majistral is associated ith an earlier granodioriteemplacementhase. Approximately50 kilometersto the southwestnear Quebrada Rio Manta existthree additionalknown W-Cu deposits eportedly

similar o Pasto Bueno (fig. 6, p. 1033). A similarargument also can be made for structural controlhaving localized Pasto Bueno and the threesimilar deposits o the southwest. Possiblegeneticrelationsbetweenmore typicalporphyrycoppermin-eralization (Majistral) and tungsten-porphyrymo-lybdenum mineralization (Compacchaand PastoBueno) remain an important question. But struc-tural controlcannotbe considered f primary geneticsignificance.Deep structural eatures n the vicinityof the Consuzo uartz monzonite tockappear o bean essential part of the hydrothermal circulationsystemallowing magmatic and meteoricwaters tomix. But primary genetic mplication or the styleof mineralization remains unclear. We 1.ook orwardto ruttirepublications ealingwith Compaccha.

GARY P. LANDISROBERT O. RYE

G. P. L.DEPARTMENT OF GEOLOGYUNIVERSITY OF NEW MEXICO

ALBUQUERQUE,EW MEXICO87109R. O. R.

U.S. GEOLOGICAL SURVEYDENVER,COLORADO0225February 20, 1975