Bulletin of the Geological Survey of Japan，vol．49（6），p．249－255，1998

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Satoshi MuRAo＊，Danjindorjiin DoRJGoTov＊＊and Tsagaanbilegiin TsEDEN＊＊＊

Satoshi MuRAo，Danjindorjiin DoRJGOTOv and Tsagaanbilegiin TSEDEN（1998）KAr dating of granitoids and hydrothermal micas from the northempart of Kherlen Depression，Mongolia．B％ll． G601．Sz67zノ．ノ¢クα％，vo1．49（6），P．249－255，6figs．，1table。

Abstr＆et；We have detemined the：K－Ar ages for micas of granitoids and hydrothemal deposits in the Kherlen Depression，one of the prospective regions of eastem Mongolia．According to geo－ 10gic evidences，the granitoids belong to Triassic，Jurassic or Carboniferous in age，and are ac－ companied by many hydrothermal deposits．Some large stocks are considered to have developed during the Carboniferous or Triassic Period，while small plugs show evidences of Jurassic－time in－ trusion．Among the hydrothermaI deposits in the depression，the Ond6r tsaggan（Mo－W－Be）and Mδngδn－Ondδr polymetallic（Zn－Sn－Pb－Ag）deposits within the Ond6r tsagaan ore field were se－ 1ected for the age determination．The result showed that a granitic plug near the Ond6r tsaggan ore field has191Ma，while hydrothermal micas from Ondδr tsagaan deposit and Mδngδn－Ond6r deposit show the age of174and175Ma respectively．In addition to these，two ages for igneous mica from a Carboniferous complex were obtained to be273and3Q8Ma． Mineral deposits in the depression are distributed along the boundary between Devonian sedi－ mentary rocks and Carboniferous granitoids，which implies that mineralization mainly took place during the Carboniferous Period being related to large－size stocks．But our geochronology indi－ cates that several mineralization episodes were existed in the Kherlen Depression and especially Jurassic magmatism was important for po1ゾmetallic mineralization． Also it is noteworthy that the Mo－W．Be and Zn．Sn－Pb－Ag zones（deposits）of the Ond6r tsagaan ore field have mostly the same age，174and175Ma．This fact indicates that two zones in this ore field are probably a product of single pulse solution from a felsic magma．In addition geologic set－ ting indicates that a polymetallic ore field，10－20km2in dimension，could be formed around the apex of such small plug．In eastem Mongolia，fractionated small plugs must be examined with at－ tention to explore polymetallic an（i other type of mineral deposits．

this region（iue to the paucity of geochronologic in－ 1．亙虚rod盟ctio脆 formation．Thus we have surveyed the area，collected The Kherlen Depressionin eastem part of Mongolia some rock and ore specimens，and determined the KAr is one of the most prospective regions for rare metal ages for representative specimens．The aim of this deposits．Especially its northern part including the note is to present the（iata，an（1discuss exploration Onddr tsagaan ore field hosts many polymetallic de－ strategy based on the geochronology． posits with high－grade ore of Mo，W，Be，Zn，Sn，Pb and Ag。The depression has a dimension of80km by 2．紬er亘e盟De置essio髄 150km，and is situated in the Khentii Aimag，ca．300 km east from Ulaanbaatar，the capital of the nation The Kherlen Depression is a basin－1ike structure in （Fig．1）．The projected Ulaanbaatar－Ond6rkhaan eastem Mongolia which is mainly composed of highway will improve the infrastructure of the region． Devonian sedimentary unit，Kherlen Group，and In spite of such advantageous position in economy， granitoids of Carboniferous，1ate Triassic and Jurassic mineral exploration has not been progressed much in ages（Fig．2）．These granitic rocks intrude the：Kherlen Group（Tseden6厩乙，1992）．A granitic complex of the ＊Mineral and Fuel Resources Department，GSJ Carboniferous age，designate（1“Tsenkher gol Com一 ＊＊MONRUD Co．，Ltd．，Barmash Mineral House，Ching－ gis Avenue，Khan－Uul District，Ulaanbaatar－36，Mon－ Keywords；Mongolia，Kherlen Depression，M6ng6n－Ond6r golia deposit，Ond6r tsagaan deposit，Tsagaan chuluut deposit， ＊＊＊MONGOLYN ALT Corporation，Ulaanbaatar，24，P． Tsenker gol Complex，Bor－Ondδr Complex，radiometric 0．Box－268，Mongolia age，monoascendent zoning，polymetallic ore．

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Fig。1Representative mineral deposits in Mongolia．Gold deposits are not shown in the figure． κ一∠47伽惣聰欄づ耀s側伽編，κh6伽Z）吻召ss乞・n醐％名α・癩乙ノ

Dund Shaikhanii Omn6delger Kh6ndii Ond6rtsagaan M6ngOn6nd6r O825－3（273Ma） O824－9（191Ma） O－20（174Ma） C221－5A（175Ma）

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Fig．2Map showing the geology of the Kherlen Depression and the locality of K－Ar dating（modified from Tseden6鳳1992）・LOnd6rtsagaandep・sitl2：M6ng6nOnd6rdep・sit；3：Mandaliinkhureedep・sitl 4：Tsagaan chuluut deposit l5：Khoyor zotig deposit l6：0khaash（leposit l7：unnamed l81Uuliin rashaan deposit l9＝umamed l10－13：Nergui area l14：Ulaan－Ondδr depositl15：Tsagaan－Ovoo de－ posit l161Tuntger deposit；17：01ziitbulag deposit l18：unnamed l19：Tugalgatain－nuruu deposit l20＝ unnamed．

plex”，and another of late Triassic to early Jurassic， about his experiment and specimen are not recor（1ed called“Bor－Ondδr Complex’7，are the representative in his report．In our study，two specimens were collected large－size stocks in this depression．In addition to from the phase（2）to determine the age of the most these stocks，small granitic plugs，mainly of Jurassic active period of the magmatism． age，are often observe（i in the depression． The Bor－Ond6r Complex is represented by granite， The Tsenker gol Complex is composed of hom－ granosyenite，1eucogranite，alaskite，and granite por－ blende gabbro， diorite， grano（liorite， and granite． phyry．According to Borzak：ovskiiαα乙（1973），these Three phases can be recognised in this complex：（1） rock types show high silica and high alkaline chemistry gabbro and dioritel（2）biotite－hornblen（le granodiorite， （74．6－76．8wt。％and8．6－9．8wt．％respectively）l it is tonalite，a（1amellite an（1homblende－biotite／biotite characterized by high concentration of Zn，Sh，Ta granite l and（3）biotite leucogranite．Borzakovskii6渉 and Nb；and the KAr ages of biotite range from 磁（1973）determined the K－Ar ages for biotite from 194to235Ma．No other geochemical work has been the complex between263and318Ma，but details performed on this complex．

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The northem part of the depression includes sev－ aplite veinlet of a few centimeters wi（ie cuts the rock． eral ore fields which seem to be distributed along the The granite is medium－grained and equigranular in boundary between the Kherlen Group and Tsenkher texture．Homblende is subhedral grain，homogenous gol Complex（Fig．2）．Among the ore fields，the Ond6r and shows pale green in Z－axial color．Biotite is also tsagaan ore field and Tsagaan chuluut ore field were subhedral and the X二Y axial color is greenish selected for this pilot study．The Ond6r tsagaan ore brown to tan． field is of rare metal bearing polymetallic minerali－ The sample from Dund Shaikhanii Kh6ndii，No． zation，while the Tsagaan chuluut ore field is of sim－ 0824－9 （Fig．4），is biotite－hornblen（ie tonalite．It is ple molyb（1emm mineralization． medium－grained and equigranular．Homblende is The6nddr tsagaan ore fie1（1is composed of grani－ subhedral in shape，and shows bluish green in Z－axial toids of Paleozoic and Mesozoic ages，Devonian color．Biotiteisalsosubhedralinshapean（1theX二 green rock an（i sedimentary rocks，an（1Cretaceous Y axial color is greenish brown to tan．Plagioclase sedimentary rocks．Petrological features of the rocks shows weak zoning．Epidote，apatite，zircon and have not（iescribed yet in detail in this region．In this opaque minerals occur as accessory phases．Most of ore field，mineral deposits are distributed around a the quartz is dynamically recrystallized，and all of granitic plug which intmded the Kherlen Group and the biotite grains show strong wavy extinction．This the zoning on the order of ore ：field can be estab－ suggests that the rock was deformed after crystalli－ 1ished：Mo－W－Be zonel Pb－Zn－Sn zonel Pb－Zn－Ag zation． zone l and Ag zone from the center outwar（i．The The specimen from Omn6（1elger，No．0825－3，is Mo－W－Be zone is of porphyry type and is designated two－mica bearing granitic rock．This specimen shows the Ond6r tsagaan deposit，while Pb－Zn－Sn，Pb－Zn－Ag granoblastic texture，and is mainly composed of and Ag zones are of polymetallic vein type．The quartz，muscovite，plagioclase，and biotite．Muscovite boundary between the three zones is artificial and occur as large grains with wavy extinction（Fig．5）， each zone grades each other．The area which include ranging from l to l．5mm in diameter。Biotite occurs these three zo血es designates Mδng6n－Ondδr deposit． as small euhedral grains of X＝Y axial color in dark The Ondδr tsagaan deposit is mainly hosted by a brown to gre6n．Plagioclase is usually observed as strongly altered porphyritic granite（Plug）an（i partly saussul’itize（i subhedral grains．Zircon and apatite by the Kherlen Group，while the M6ngδn－Ond6r ore－ areaccessoryminerals． bodies are embedded in the Kherlen Group． The Tsagaan chuluut deposit，10cated at17km 4．K－Ar d就亘遡g o£翻鍛s south of the Ondδr tsaggan area，is of stock：work molybdenum miheralization．The ore bodies are The ore specimens from the Ondδr tsagaan ore found in greisenized granitoid of Carboniferous age． field were roughly crushed in an iron mortor．Phlo－ Withinthis granitoid，the molybdemm mineralization gopite，polytype3T，of the sphalerite－galena ore and is found in and around small granite－porphyry dykes muscovite，polytype3T，of the stockwork were care－ which crosscut the granitoi（1． fully hand picked under a binocular microscope．To decide the ages of granitoids，the rock chips were crushed and muscovite and biotite were concentrated 3．S＆醗B且eδes歴凶膿 with an isodynamic separator．After this procedure， Samples were collected from the Ondδr tsagaan， the quality of each concentrate was examined under M6ngdn－Ond6r，and Tsagaan chuluut deposits，and a binocular microscope：the purity of the concentrate from the granitic outcrops at Dmd Shaikhanii Kh6ndii was estimated to be more than99％．AH of the micas and Omn6（1elger．The outcrop at Dund Shaikhanii were fresh having K content more than6％。The Kh6ndii represents Jurassic plugs in the region，while ages were determined l74±9for Ond6r tsagaan and that of Omn6 delger is a part of the Carboniferous 175±9Ma for M6ng6n Ond6r．The biotite of the Tsenkher gol Complex． Tsagaan chuluut granite gave an age of308±15Ma． At Ond6r－tsagaan，samples were collected from The tonalite from Dun（i Shaikhani kh6ndii was stockwork of quartz－muscovite－bery1．The sample dated to be191±10Ma，while the two－mica－granite No．O－20contains veinlets of a few millimeters to a from Omn6delger，273±14Ma（Table l and Fig．6）． few centimeters in width，and carries molybdenite， wolframite，beryl，muscovite and small amount of 5． 1）董scuss量o亙亙 sphalerite．At M6ngδn（）nddr，sphalerite－galena ore， No．C221」5A，was sampled．The specimen is a veinlet The granite samples from Omn6delger and Tsagaan of a few centimeter wi（ie in slate in which quartz chuluut belong tQ the Tsenker gol Complex．Their and phlogopite are coexisted with sulfides． radiometric ages are included in the range of the The granite sample from the Tsagaan chuluut，No． complex，263－318Ma，that was reportedby Borzakovski 0824－1（Fig．3），is homblende－biotite granite．A sma11 6砲乙（1973）．Molybdenum dissemination at Tsagaan

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chuluut deposit is closely asscociated with this grardte and thus it must be distinguished from the molybdenum mineralization at Ond6r tsagaan． On a geologic map around the Kherlen Depression， mineral deposits are distributed along the boundary between sedimentary rocks and Carboniferous grani－ toids．But the geologic setting and radiometric ages for micas of M6ng6n－Ond6r and Ond6r tsagaan re－ vealed that the mineralization took place in the mid－ dle Jurassic time at the apex of a small plug．When carefully examined the geologic map，some small plugs of Jurassic age at the vicinity of polymetallic deposits are noticed（Fig．2）．Our stu（iy clarified that one of them，a small plug at Dund Shaikhanii kh6ndii， has on age of191Ma．This age corresponds to those Fig．3 Photomicrograph of a granite from Tsagaan chuluut．Sample No．0824－ll P： plagioclasel H： of mineralization ages，174and175Ma，within ana－ homblende；Q：quartz．Field of view，ca．2mm． 1ytical errors．Usually poymetallic deposits are found at or close to the apex of highly fractionated aci（iic intrusions（e．g．，Murao6砲1．，1991，1995）．Thus such small intrusion of middle Jurassic age is considered 撚灘 to be a possible source for polymetallic mineralization． One noteworthyfactisthatthe Mo－W－Bezone（Ondδr－ tsagaan deposit）andZn－Sn－Pb－Ag zone（M6ng6n（）nd6r deposit）have mostly the same age，and thus the zonality at On（1dr tsagaan seems to be of monoascen－ dent type．There have been controversies on zonal ar－ rangement of polymetallic ore bodies（e．g．，Kutina， 19651Nakamura and Miyahisa，19761Murao，19881 Furmoε厩乙，1992）．The observation this time implies that such zonality can be （ieveloped mostly at the same time in a hydrothermal histroy and that an ore field of10－20km2in dimension can be formed around a small felsic plug．

Fig．4 Photomicrograph of a tonalite from Dund 6． Co亙亘c亘腿s量o髄 Shaikhanii Khδndii．Sampe No．0824－91P：plagioclase l H：homblende；Q：quartz．Field of view，ca．2mm． Mineralization at Ondδr tsagaan ore field is of po－ 1ymetallic mineralization and the deposits are possi－ bly the product of single pulse of hydrothermal solu－ tion。The zoning from the Mo－W－Be zone to the Zn－ 欝糠 Sn－Pb－Ag zone is consi（iered to be of monoascen（ient feature． Jurassic granitic rocks are small in size and have less－distinct surface expression compared to Carbon－ iferous granitoids，but they could be more important for the rare metal exploration．As stated before，in the Kherlen Depression，many deposits seems to be distributed along the boundary between Carboniferous granitoids and Devonian sedimentary rocks but the real distribution might be controlled not only by the boundaries but also by localities of small Jurassic plugs。It might be necessary to inspect the apex of highly fractionated small stocks of Jurassic age to explore such ore bodies rather than to work on a Fig．5 Phtomicrograph of two－mica－bearing granitic large batholith or to simply extend the prospect from rockfromOmnδdelger．SampleNo．0825－31M：musco－ an exiSte（i mine Site． vite l Q：quartz．Fie1（i of view，ca．2mm． Based on the geochronology in this region，there

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Table l Potassium－argon age determinations on micas from granitoids and ore samples．Analyst：Tele（lyne Isotopes。

Sample No． Locality Material analyzed Isotopic age（Ma） 40Ar＊ ％40Ar＊ ％K （scc／gm×10一5）

0－20 Ond6r tsagaan muscovite 174±9 5．60 97．1 7．84 5．53 96．7 7．85 C221－5A M6ng6n Ond6r phlogopite 175±9 5．06 97．2 7．17 5．11 96．7 7．11 0824－9 Dun（1Shaikhanii biotite 191±10 5．50 94．2 7．13 Kh6ndii 5．65 94．6 7．09 0825－3 Omnδdelger muscovite 273±14 8．89 97．6 7．73 8．93 96。8 7．80 0824－1 Tsagaan chuluut biotite 308±15 8．00 96．5 6．04 7．80 96．6 6．06

40Ar／36Ar at・m・sphere＝295．5：Steiger and Jager（1977）． Equation to calculate the age：Dalrymple and Lanphere（1969）．

Polymeta”ic Mo mineralizatiOn mineraliZatiQn？

OndOr－tsagaan 一

2 M6ngOn Ond6r ←一一←

3 Dund Shaikhani khondii ←一一一一

4 0mno－delger

5 Tsagaan－chuluut

100 150 200 250 300 M：a

Late Middle Early Late Early Cretaceous Triass圭c Permian → ← Jurassic Jurassic Jurassic Carboniferous Carboniferous，

Fig．6Summary of K－Ar ages of igneous and hydrothermal mica minerals from eastem Mongolia

are at least two mineralization events，one of Mo，W， Potassium－Argon Dating l Principles，Tech－ Sn，Be，Zn，Pb，Ag at Jurassic time and the other of niques，and Applications to Geochronology． Mo dissemination at Carboniferous time．Thus sys－ W．H．Freeman and Co．，San Francisco，258p． tematic geological survey and radiometric dating of Furuno，M．，Itoh，K．and Mariko，T．（1992）Po． rocks are vital to perform effective exploration and 1ymetallic and gold－silver mineralizations in exploitation in this region． and around the Akenobe ore deposits，southwest Japan．ノ匠吻zσ％g G60乙，42，33－46 （in Japanese Ack遡ow且edlgments：Secretarial assistance and．advise with English abstract）． for the field survey was provided by Dr G Dejidmaa． Kutina，」、（1965）The concept of monoascen（1ent Dr S Harayama gave SM technical advise on K－Ar and polyascen（ient zoning．P706乙Pos云ηzα8％z6z． （iating Of mineralS． Z）の．，豊，47－55． Murao，S．（1988）Mineralization stages and some orders o穏onality in the Akenobe polymetallic Refere亙且ces ore deposits，SWJapan．Doctral Dissertation， Borzakovski，Y．A．，Mihailov，E．V．and Khasin， Hiroshima University，137p． K．A。（1973）B6r－Ond6r intrusive Complex． Murao，S．，Furmo，M。and Uchida，A．C．（1991） 肋：Geology of Mongolian Peoples Republic， Geology of indium deposits－a review．〃’翻％g 2，Nedra，Moscow，201－210and360－371（in G60乙，41，1－13． Russian）． Murao，S．，Sie，S．H．，Takagi，T．，Seetharam，R． Dalrymple，G．B．and Lanphere，M．A．（1969） and Naito，K．（1995）Comparative proton and

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electron probe study of two representative decay constants in geo－and cosmochronol－ indium－bearing deposits．丑4CRπ41951417－422． ogy．Eσπh P彪％6浜So∠五6あ．，36，359－362． Nakamura，T．and Miyahisa，M．（1976）Hypogene Tseden，T．，Murao，S．and Dorjgotov，D．（1992） zoning and role o：f geologic structures．ノ％勿z勿zg Introduction to geology of Mongolia．B％1乙 G召o乙．5メ）86．焉sz昭，no．7，37－57（in Japanese G60乙S解∂．ノ勿鷹，43，735－744． with English abstract）． Steiger，R．H．and Jager，E．（1977） Subco㎜ission Received January l4，1998 on geochronology：convention on the use of Accepted March18，1998

モンゴル国ケルレン陥没帯の花嵩岩類および熱水性雲母のK－Ar年代測定

村尾 智・D．Dorjgotov・T．Tseden

要 旨

モンゴル東部ケルレン陥没帯に産する花闘岩類と鉱床の熱水性雲母類の年令をK－Ar法によって測 定した．域内には地質学的な産状から三畳紀，ジュラ紀，石炭紀と判断される花闘岩類が分布し，多数 の鉱床を随伴している．一般的に三畳紀，石炭紀花商岩類は規模が大きく，ジュラ紀のものは小さい． 本研究では陥没帯内オンドルツァガーン地域の小規模花商岩体，その近傍に存在するMo－W－Be及びZn－ Sn－PbAg鉱床の雲母類を測定し，それぞれ191Ma，174Ma，175Maの結果を得た．また石炭紀花闘 岩類の黒雲母を測定し273Maと308Maの結果を得た．以上より，ケルレン陥没帯の地質図では一見 デボン系と石炭紀花商岩類の境界付近に鉱床が集中するようにみえるが，実際にはジュラ紀の小規模花 闘岩体が探査対象として重要であると結論した．

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