議灘議

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Michiqu皿ay is a major undeveloped porphyry copper deposit which is loc耳ted in the
Peruvian Cordillerajust east ofthe Continental Divide（Fig．1）．Thedepositis situated in themiddle
of the sedimentary terrain that extends60m the Ecuadorian border down to the S130parallel
（B肌LIDo and DE MoNTREuIL，1969）。：Recently detailed study on mineralization ofthis district has
been made by one ofthe authors（UcHIDA，1975，1976a，1976b）．In an attempt to reveal the time of
mineralization，we have carried out K−Ar age determin我tions on granodiorite porphyry in the
Michiquillaydistrict。Thispaperreports andbrieHydiscussestheresults ofagedeterminations．We
are indebted to Mr，S．Uchiumi fbr technical assistance．
2． Geo且ogic鴇且set電量聡9
1n the Michiquillay district，Lower（P）Cretaceous quartzite and shale，and Middle
Cretaceous limestone widely occur，repeating gentle fbldings which have WNW−ESE axes．The
Michiquillaystock ofTertiaryageintruded these Cretaceousstrata（HoLLlsTER，1974）．Thestockis
described as granodiorite porphyry in this paper fピom both the petrographic and chemical aspects
（UcHIDA，1975，1976a），though it has been described as quartz monzonite porphyry in the previous
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Fig．1Location map ofthe Michiquillay deposit．
＊Geochemistry and Technical Service Dep翫rtment
＊＊Explor呂tion and Mining Enginee血g Department，Sumitomo Metal Mining Go，，Ltd．
33一（745）
一￥、
地質調査所月報（第27巻第11号）
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Fig。2Geological map of the Michiquillay deposit and its a両acent
area．Sur魚ce geology is modi丘ed諭er AsARco（unpub。1969）。
＊The outline of the ore grade mineralization approximately
coincides with that of“original，，potassic alter＆tion on the
same leve1．
re驚rences。Mineralization occurs at about the center ofthe stock，showing a NE−SW elongation
（Fig．2）．
Granodiorite porphyry in and adjacent to the ore body is rem翫rkably suf琵red f坐om both
hydrothermal and supergene alteration．Hypogene alteration minerals which have been recognized
so far are biotite，sericite，epidote，chlorite，quartz，k−fddspar，carbonate，kaolinite and andalusite．
Neitheranhydritenorgypsumhas been observed as yet。A conspicuous but uniquealteration zoning
can be observed。Five zones are clearly discernible based on the petrographic observation，the
structural and textural relationship of the constituent miner論and veiエ11et systems，chemical
properties（UGHIDAラ1975，1976a），and minor elements in magnetite（UcHIDA，1976b）。These are
potassic，phyllic，propylitic，retrogradedpotassic−1，and一箕亙（Fig．2）．Thefbrmer three are considered
to have been concentrically zoned originally and to欝epresen慮汎n earlier episode。On the other hand，
the latter two are considered to have been fbrmed through the superimposition of a later stage
alteration on the earlier alteration products．The alteration zones fbrmed through this episode are
in蝕red to have been fンom center outward，（1）quartz−sericite−andalusite，（2）quartz−kaolinite一
34一（746）
K−Ar Ages ofAltered：Rocks丘om the Michiquillay Mine，Peru（Ken SHIBATA and Kinsuke UcHIDA）
（chlorite），and（3）quartz−kaolinite−carbonate一（chlorite）・By the super…mposition of this alteration，
the pre一｛brmed potassic zone is suf琵red fピom the retrogressive alteration to fbrm the retrograded
potassic zones．The retrograded potassic−I stands for the zone that is moderately affected and it
approximately corresponds to the above（2）with some relict minerぬof the potassic assemblage．
The retrograded，potassic−II represents the stro鷺gly af琵cted zone and it probably corresponds to the
above（1）．In this zone，most of the alteration minerals of the earlier stage have been completely
obliterated and the original texture is hardly observed（UcHIDA，1975，1976a）。
Majorhypogenporemineralsarepyrite，chalcopyrite，magnetite，hematiteandmolybdenite．
Besides these，fbllowing accessory minerals have been recognized so傭l cubanite，pyrrhotite，val−
1eriite，enargite，tetrahedrite（？），sphalerite and galena。These hypogene ore minerals are considered
to have been fbrmed both temporally and spatially related with the alteration zoning to fbrm the
mineralization zones．Through the earl三er stage min6ralization，which is considered contem−
poraneous with the earlier stage alteration，an outward zoning“magnetite−chalcopyrite−molybd−
enite．（pyrite）’，was formed．coaxally and coextensively with the alteration zoning“potassic−phyllic．
propylitic”（UGHIDA，1975，1976a）。This is quite similar to the model proposed by：Low肌L and
GuILBERT（1970）．During the later stage mineralization，which is considered contemporaneous with
the later stage alterationラ another n諺n6ralization zoning “molybdenite・sphalerite・galena−
pyrite−gold−tetrahedrite・enargite，’appears to have been fbrmed f士om the center outward（UcHIDA，
1975，1976a）．Thus the presently observed mineral zoning symmetry that apparently shows a
complex pattem can be accounted．fbr by the superimposition ofthe two episodes ofhydrothermal
activity．
翫Amlyzedsamplesandana且yt置calme伽ds
K−Ar age determinations were carried out on two samples丘om the Michiquillaydepositin
order to reveal the time dif琵rence between the two episodes of hydrothermal activity。The
localities ofthe samples areshown in Fig。2。Sample Michi．43is asecondarybiotite separated丘om
an altered granodiorite porphyry fヒom the potassic alteration zone of the earlier episode，whereas
Michi−23is a whole−rock sample of altered granodiorite porphyry taken fヒom the retrograded
potassic−II zone that is considered to represent the central part of the later episode．The details of
the rock samples are given in the fbIlowing．
Michi−43（DDH core）
Locality：About26。2m from the collar of DDH I−21−B，Michiquillay mine，Enc面ada，
Cajamarca，Peru（7。03／Sラ78020！W）
Rock： Altered granodiorite porphyry，consisting mainly of secondary biotite，chlorite，sericite，
quartz，plagioclase（relict），secondary：K−fddspar，associated，with small amounts ofpyrite，
chalcopyrite，magnetite（hematite），leucoxene，sphene．
Michi−23（Chip sample）
Locality：About420m from the portal ofthe3500m level adit，Michiquillay mine，Enca肱da，
Cajam乱rca，Pem（7。03／S，78。20’W）
Rock：Altered granodiorite porphyry，consisting mainly of quartz and sericite，associated with
35一（747）
地質調査所月報（第27巻第11号）
sma11＆mounts ofandalusite，pyrite，chalcopyrite．
Argon extraction and puriGcation were made in a pyrex high v批cuum system。Samples were
血sed in a molybdenum crucible at about1300◎C fbr30minutes，and argon was purified with hot
titanium sponge．Isotopic ratios of argon were measured on a Reynolds type mass spectrometer．
：Potassium was determined by atomic absorption乱nalysis．Constants used fbr age calculation are：
λβ・＝生72×10−10／y，λ、＝o．584×10−10／y，40K／K＝o．0119atom％．
4． Res魍岨重S我置且dl d遥SC胆SS量0孤
The results ofK−Ar age determinations are given in Table1。A secondary biotite separated
丘om the potassic zone，where the ef琵ct ofthe superimposition ofthe retrograded potassic alteration
was considered to be minima1，gives an ag60f28．2±4．9m．y．The age may indicate the time ofthe
potassic alteration．A whole−rock sample ofaltered granodiorite porphyry taken fピom the probable
center ofthe retrograded potassic alteration，where the effect ofthe second，stage alteration has been
maxima1，gives an age of18。7士L4m．y．This rock consists mainly ofquartz and sericite，and almost
all ofpotassium in the rock is contained in sericite．Therefbre the obtained age may represent the
time of sericite fbrmation and accordingly the time of the retrograded potassic alter乱tion。
：LAuGHLIN6渉α1。（1968）reported two K−Ar datesfヒom the Michiquillaydistrict．A K−Ar age
of46．4m。y．was obtained fbr homblende ffom a small homblende granodiorite stock occurring
about three miles northeast ofthe Michiqu皿ay ore body．An age of20．6m．y．was obtained for
biotite f｝om a su1丑de−bearing rock exposed in an explanatory adit．This sample must have been
taken f｝om the pot批ssic alteration zone and must certainly be a secondary biotite according to their
description』LAuG肌IN6飽」．（1968）consider that the homblende age is anormalously old owing to
the incorporation ofexcess40Ar，and that the intrusion ofgranodiorite could be as late as20m．y．as
indicated by the mineralization age．Eowever，the possibility ofexcess40Ar incorporation in horn−
blende in shallow．emplaced rocks such as granodiorite porphyry倉om the Michiquillay district，
seems to be very Iittle．Moreover potassium content of the analyzed homblende is not very low
（0・7％K）。Accordingly，it ismoreprobable that the hornblende乱ge represents the time ofintrusion
fbr granodiorite porphyry，although the possibility of excess40Ar cannot completely be ruled out．
The dif断ence in theageofbiotitef士om thepotassic alteration zonel20．6and28。2m。y。，may
beinterpreted as either（1）the biotite datedby L・AuG肌IN6勧1．mayhave been thermally af驚cted to
give an apparently younger age，as the effect of later stage alteration is more or less observed
throughout the tunnel where Laughlin6渉α乙sampled，or（2）the potassic alteration occurred in two
stages・
Using all the available age dat乱，the history ofthe igneous and．hydrothermal activities in the
Michiqui11批y district is summarized as fbllows，
Table l K−Ar ages ofaltered rocks丘om the Michiquiilay mineラPeru。
Sample No。
Minera1
K20
（％）
40Ar rad Atmospheric
（10−6ccSTP！g） 窪oAr（％）
Age
（m．y．）
Michi−43
biotite
7，69
7．21
88．1
28．2ニヒ4．9
Miぐhi−23
whole rock
3．86
2．40
73．4
18．7士1．4
36一（748）
K−ArAges ofAltered Rocks肋m the Michiquillay Mine，Peru（Ken SHIBATA and Kinsuke UcH亙DA）
Unaltered porphyry hornblende 《’46．4m．y。
Potassic alteration biotite 28。2（to20．6）m．y．
Retrograded potassic alteration sericite 18．7m．y．
Although therestillremainssome doubtasto the varidity ofthese age data，as LAuG肌INθ緬乙
（1968）pointed，out，the above history well accords with geological and chemical observations。
Re蛋￠re鴎ces
BELHDoフE．and DE MoNTREuIL，L（1969）Aspectos generales de la metalogenia del Peru。
G80Jog毎E60箆07痂α，：N’o．1タServicio de Geologia y Mineria del Peru。
HoLLlsTER，V。F。（1974）The Michiquillay porphyry copper deposit。M傭α」伽Dψ052渉α，voL9，
p．261−269．
LAuGHLIN，A．W．ンDAMoN，P．E．and WATsoN，B。N．（1968）Potassium−argon dates丘om
Toquepala and Michiquillay，Peru．E60π．080乙，voL63，p．166−168．
Low肌L，J．D．and GuILBERT，J。M。（1970）：Lateral and vertical alteration mineralization zoning
in porphyry ore deposits・E60π・060乙，voL65，P・373−408・
UcHIDA，K．（1975） 乃6励6」げα伽675」αg6勉盟α伽漉on on Jh6読67観oη一雁n67読z漉oηzo痂gゼηα
ρoψ妙び6吻87勿0526．Unpublished D。Sc．Thesis，：Fac。Sci．Hiroshima Univ。
（1976a）一一一一一 （resume）。ハ46η2．Eα6．3厩研705hJη2αU物。（in preparation）．
（1976b）Minorelementsinmagnetite丘omunalteredandalteredrocksattheMichiqui1−
1ay porphyry copper deposit．ル勧痂g O60乙（Tokyo）（in preparation）．
ペルーM量c賊噸戯星臨y鉱山産変質岩の藍一Ar年代
柴田 賢・内田欽介
要 旨
ペルーのポーフィリー・カッパー鉱床Michiquillay鉱山の変質した花闘閃緑斑岩から分離した黒雲
母のK−Ar年代は，28．2土4．9m・y・であり，これはpotassic alterationの時期を示すものと考えられ
る．一方，セリサイト石英岩の全岩年代は18・7士L4m・y・で，この年代は後期の変質作用の時期を示
すものであろう，
（受付；1976年6月7日；受理：1976年6月17目）
37一（749）