Geological structure，composition of ores and age of the
Bamsk gold deposit，Amur region，Russia
Vitaly A．Stepanov
A mur Research Integrated Institute FEB RAS，Blagoveschensk 675000，Russia
Abstract：The Bamsk gold—ore deposit is located in the Amur region，the Far East of Russia．It is confined to the Early
Cretaceous volcanic-plutonic uplift of central type， located in the Stanov folded-clumpy Pre-Cambrian system．The
depo sit is presented by a series of gold-bearing quartz and carbonate-quartz vein-stringer zones． They are confined to the super．intrusive zone of the Nevachansk subvolcanic intrusion of sienite—porphyric compo sition． The mineralization is
being controlled by zone of fractures developed in the autochthone of the Bamsk fault．Quartz，carbonates and sericite prevail in the compo sition of ores．The quantity of ore minerals doesn’t exceed l％ ～5％ ．Pyrite，chalcopyrite，galena and native gold are widely spread．Sheelite，gold and silver tellurides,sulphobismuthites，acanthite，sphalerite and cinnabar are less developed．Four stages ofmineralization have been distinguished．Gold-sulphide-sulphosalt ore with tellurides of gold and silver is productive for gold．The following set ofelements is typomorphic for the ores of the deposit：Au，Ag,Cu，Bi，Mo，Pb and Sb(W1，Pb1，Mo)-Cu-(Ag，Bi，Sb，Mn，W2，Pb2)form the vertical series of zoning．Rocks，enclosing the mineralization are exposed to the processes of listvenitization-beresitization． The processes of gumbeization and argillization are less manifested． The age of the gold mineralization， determined by Rb-Sr method on ore-accompanying minerals，is 1 30．6 Ma．
Key words：Bamck gold deposit；fault；quartz veins；Amur region；Russia
Introduction
The Bamsk gold-ore deposit is located in the northwestern part of the Amur region， the Far East of Russia， in the the upper course of the ChuylbangroRiver， the left tributary of the
Nizhnyaya Larba River．It was discovered in 1979．By the sum of reserves and prospected resources of silver and gold the deposit is the largest in the Amur region．Nowadays it is being exploited．
The deposit is confined to the Stanov foldedclumpy area， framing the south of the Siberian platform．Gneisses，crystalline schists and amphibolites of the Archean age，broken by the Early Proterozoic large granitoid rock masses (the Chyubachinsk complex)，are developed here． The Mesozoic magmatic fomlations， presented by the Late Jurassic—Early Cretaceous granitoid intrusions， subvolcanic intrusions and the effusive rocks as well as by a series of the Early Cretaceous dikes．are developed to less degree(Fig．1)．
The Mesozoic stage of the area activization has led to the formation of gold， silver and molybdenum mineralization，concentrated within the eastern branch of the North Stanov metallogenic zone of the Priamurye gold-bearing province(Stepanov．1 997)．The zone is located along the large lineament，traced by the subvolcanic intrusions and the fields of the Early Cretaceous volcanites．Directly to the southwest of it a series of the Late Jurassic—Early Cretaceous large granitoid intrusions is located． Within the zone in the knots of large faults with NW and NE orientations the ore areas(Togunassk， Bamsk and Apsakansk)have been distinguished(Fig．1)．The volcano-plutonic structures of the central type： positive (Bamsk， Apsakansk)and negative(Togunassk)answer the ore areas。
Fig．1 Geology structure position of Bamsk gold ore deposit
1． Archean gneisses and crystalline schists； 2．Early Proterozoic granites ofthe
Chubachinsk complex；3． Late Jurassic—EarIy Cretaceous granites； 4． Early
Cretaceous effusive rocks； 5． Early Cretaceous syenite-porphries of the
Nevachinsk complex；6 faults；7．the boundary of ore knots：1）-Togunassk，2）—
Bamsk，3）—Birkagchansk；8．the bo undary of Northern-Stanov mineragenic zone；
9．the Bamsk deposit；10．ore shows of：gold(1-Atugey，2-Denisk，3-Verkhneye，
4-Seerik，5-Des，6-Yemichnoye，7-Sivakan，8-Apsakan，10-Dominikan，1 1一
Kluchikan，12-Nakhodka，13-Glubokoye，14-Zagadka)，molybdenum 【15一
Chubachy．16-Strelka)and silver一(8-Kluych)．
1 Bamsk ore area
A volcano-plutonic uplift of the central type answers the Bamsk ore region with gold-silver special-ization． The area of the region is about 400 km2 ． The
Bamsk gold deposit， showing of gold of D’yes，Sivakan，Yernichnoye，of silver-Kluych，of molibdine—Strelka are in this region． The uplift is formed by subvolcanic intrusions and the Early Cretaceous dykes． The enclosing rocks are granitoids of the Chubachinsk Early Proterozoic massive， containing numerous blocks and xenoliths of the Archean crys—talline schists．The Archean metamorphites are widely dexeloped at the periphery of the ore area，framing it from the south and south—west．At the south—eastern and south-western margins of the area the intrusions of Late Jurassic—Early Cretaceous granitoids are also located．
The Archean metamorphic rocks are represented by gneisses， plagiogneisses amphibolitic crystalline schists， amphibolites with rare quartzites and calciphyres． Their thickness is not less than 2 500 m． The equiponderous mineral association of plagioclase+ clinopyroxene+amphibole+biotite+orthoclase
+quartz in rocks indicates their metamorphysm in the amphibolitic facies． The primary rocks were mainly volcanites，such as trachybasalts，basalts and also andesite—basalts，andesites with rare interlayers of carbonate and siliceous rocks．
The Early Proterozoic granitoids of the Chubachinsk complex form a large multistage pluton，the major part of which is located to the north of the ore knot．Quartz—syenites，granite and leucogranites of normal and hyperalkalinity take part in the pluton’s structure． By petrographic and petrochemical characteristics the granitoids are similar to the I-type formations． The absolute age of the quartz syenites determined by Rb—Sr method in the Laboratory of Isotopic Investigations of the Far East Institute of Mineral Resources(Khabarovsk City)is 1 924 Ma．
Along the western slope of the uplift in the north-western direction the Nevachansk subvolcanic intrusion extends．It is formed by the Early Cretaceous
quartz syenite—porphyries and quartz syenites．The intrusion has a form of a sill tilted to the east，to the center of the volcano—plutonic uplift．In the structure of intrusion dominate the quartz syenite-porphyries， containing insets of feldspar， quartz， biotite and homblende at the background of micropoikilitic and microgranite matrix．Hypsometrically lower，they change into quartz syenites with micropegmatitic structure．
A small intrusion of the Early Cretaceous monzodiorite—porphyfies， which is apparently a root part of the neck， is confined to the central part of the volcano—plutonic structure． Close by it the eruptive breccias are fixed．The subvolcanic dykes of the same age，varied in composition， are the most developed withinthe structure． They are represented by granite porphyries，granodiorite- porphyries，quartz monzodioriteporphyries，diorite porphyrites and lamprophyres．
The volcano-plutonic structure is expressed in the relief as a positive morphostructure of centra1 type． It is interpretated on the aero-and cosmic photographs of the Earth’s surface． Besides,it is clearly fixed in the
geophysical fields．Thus，on the gravimetric maps the Bamsk ore knot，formed mainly by subvolcanic rocks of acid composition，is distinctly expressed as a round negative anomaly。
2 Structure of the deposit
The Bamsk gold deposit belongs a system of ore-bearing carbonaceous-quartz veined ore bodies．It structurally confined to an echelon-like system of fractures in the lower side of the sublatitudinally oriented Bamsk fault(Fig．2)．
Fig．2 The structure of the Bamsk deposit(by A．A．Cheremisin)
1．
granite gneissic complex；2．syenite-porphyries ofthe Nevachansk complex；3．dykes ofdiorite porphyrites and lamprophyres,4．ore bodies and their numbers；5．the Bamsk fault；6．other fault；7 inclination ofthe be nds ofthe Bamsk fault plane
The Bamsk fault zone consisits of a series of parallel， interfluent and diverging fractures． Its width is several hundreds meters,the length一4 km．The strike
of the fractures in the central part of the deposit is sublatitudinal．To the western and eastern parts of the zone they bend to the south，in consequence of that the
zone acquires the from of an arch． The angles of the dip are southern。they fluctuate from 20。to 80。(in average 30。～40。)．
In the zone of deformations under consideralion3-4 subparallel faults are distinguished． They envelope the lens-shaped blocks of little-disturbed rocks with thickness from tens meters to the first hundreds meters．It bounds the spread of the ore bodies，developed in the underlying rocks，up the dip and performs
the role of the ore-screening system．
The ore-enclosing structure of the deposit is represented by a system of fractures of the north-eastern,submeridional and sublatitudinal strike．It confines to the lying side of the Bamsk fault and is represented by the plate of dislocated and hydrothermally changed rocks with thickness up to 300 m． The plate has the underscreen position．In the investigated part of the deposit it's traced along the Bamsk fault over 4 km.Within it the ore bodies，represented by quartcarbonate veins and streaky zones with impregnation and nests of sulphides， have been revealed． The predominant extent of the ore bodies of the western flank of the deposit is NE． In the central part of the deposit the ore bodies have near-1atitudinal strike． In the eastern part of the deposit the ore bodies change the submeridional and east—north—eastern strike line to the south—eastern．
Distribution of the mineralization within the mineralized space is uneven．The areas of thickening of the ore bodies are clearly outlined． They are usually
accompanied by increasing the thickness of mineralized rocks to 300～400 m． The boundaries of the areas are conventional enough． They are oriented in the
north-eastern direction．The areas answer the separate coulisses of the complex system of ore bodies of the deposit，with SE slope．
3 Form and structure of ore bodies
Within the deposit 14 major ore bodies are observed on a series of sections．Several small ones have been revealed．The contours of commercial ore bodies are determined by sampling data． The ore bodies are zones with streaky silicification． In some cases they coincide with carbonate—quartz veins． These types of ore bodies replace each other both along the strike and down the dip．
By principle of structural connection with faults of dirent orientation A． A． Cheremisin has distin-guished three groups of ore bodies (Stepanov &
Cheremisin，1 996)：
- located in the subscreen position， parallel to the Bamsk fault；
-connected with feathering faults of NE strike；
- controlled by the latitudinally oriented flagments of ore—enclosing faults．
The ore bodies of the first group are developed as fragments from the side of the Bamsk fault．There is no distinct attachment of the ore bodies to the fault
plane． Usually they are dexeloped not far away from its plane in the direction of the lower block． The ore bodies are often blind．Their pitch fluctuates from 30°to 40°。
The ore bodies in the system of feathering fractures of NE strike are developed in the lower side of the Bamsk fault． Their azimuth of strike fluctuates mainly from 40°to 60°. On the SW flank of the deposit the ore bodies acquire submeridional strike． The dip of the ore bodies of this group is from 30°to 55°averaging 40°-45°.The increase of the thickness is observed in the areas of the ore bodies’ bends along the dip and in the subvertical parts of the veins．
The ore bodies in the system of the sublatitudinal fractures and the fractured zones are developed insignificantly．They are bent at angles 40°～55°．Along the strike the ore bodies of this system smoothly change their orientation on NE．
Morphologically the ore bodies are elongated the ribbon-and lens-shaped vein zones．They are complicated by the monoclinal folds in the strike and the dip.As a rule，the ore bodies are characterized by significant alteration of thickness， which varies from tens of centimeters to 3 m．and in some places l0～l l m．The average thickness of the ore bodies is 1．5～2．5 m． for individual bodies 3～5 m (Fig．3)．
The character of distribution of gold commercial concentrations in the ore bodies is determined by complex combination of streaky silicification and veins．The volume of the quartzy rocks usually excels the ore bodies in parameters．The ore body is often formed by the separate discrete imbricate veins and small streaky silification zones among hydrothermally changed rocks．That resulted in appearance and interchange of areas with different productivity.
4 Mineral composition of ores
The ores of the occurrence are from low-sulphide to moderate—sulphide composition．They have massive， streaky，streaky-disseminated or brecciated texture．In the composition of non-metalliferous components quartz，carbonates，sericite prevail，barite and fluorite are recorded in small quantities．Among ore minerals，which usually don’t exceed 1～5％，pyrite，chalcopyrite，galena and gold are widely developed．Sphalerite， sheelite， hessite， matildite， acanthite and petzite are rare．Gold-silver ratio in ores is 1：3．L．G．Poritskaya distinguishes five stages of mineralization：pre-ore one (essentially quartz)， three ore ones(early-quartz-sheelite-pyritic,middle-polysulphidic,late-gold-sulphide-sulphosalt with tellurides)and post-ore one (quartz-carbonate) (Table 1)． Each of them is confined to the independent deformation stages within the common cycle of ore-hydrothermal activity． The pre-ore stage of enclosing ores silicification changed into the development of the early quartz-pyrite-sheelitic metasomatites with rare sedcite and carbonate．At this stage the concentration of insignificant amount of added or mobilized gold，enclosed in pyrite，took place．
For the formation of the middle ore stage of beresite-like metasomatites wide development of sericite-muscovite(chlorite)，dolomite-ankerite，quartz and polysulphides is typica1．Pyrite and chalcopyfite and grey ore are widely developed． The gold content in sulphides doesn't exceed(20～40)×10-6
The next productive ore stage of mlnerogenesls is characterized by wide development of Au， Ag,Bi tellurides，Ag，Pb,Cu sulphobismuthides，sulphides
proper-galena，pyrite，less developed chalcopyrite ore and considerable mass of free gold．The gold is fine，its purity varies from 730 to 952．Intergrowth of gold with
chalcopyrite and galena is often observed． Sometimes gold an d hessite infill the small cracks in the crystals of early pyrite．But more often the gold microparticles are disseminated in the sericite-quartz matrix．
5 Geochemical peculiarities of the ores
The higher contents of Au-Ag-W-Cu-Bi-Mo-Pb-Mn， ranged by decrease of the concentration coeficient， form geochemical spectrum of the ore bodies．Episodically the higher content of antimony is noted． The elements， enumerated above， are
characterized by their uneven distribution． Their variation achieves 1 00-200 and more per cents， that is typical for the zones with the intensive matter redistribution．
By relative accumulation of the elements in the ore bodies of various sections of the deposit the following series of lateral zoning： W-Mo-Pb-Bi-(As)-Sb-Cu， Ag has been determined．This zoning series reflects the change from the west to the east the relatively high-temperature gold-tungsten mineralization by gold-sulphides，and then by the relatively more low—temperature gold sulphosalts．The obtained zoning series coincides on the whole with a generalized
zoning series of gold-tungsten type deposits(Rundkwist& Nezhinsky．1 975)．
In the near-surface parts of the deposit the relative accumulation of bismuth，silver,manganese，zinc and，particularly intensively，of antimony are going on．
Gold-silver ratio monotonously grows with the depth．That proves the relative accumulation of silver at the upper leve1． The content of molybdenum increases
with the depth．W，Pb and Cu behave themselves in peculiar way．W and Pb have , wel1-defined concentration maximums on the upper and lower level and Cu on the middle one． Thus,in the section of the deposit the elements are in the following order (upwards)：(W1，Pb1，Mo)．Cu一(Ag，Bi，Sb，Mn，W2,Pb2)．
The vertical zoning of the ore body N7，which is not cropping out，was studied in details．The ore body，its supra-ore and under-ore aureoles were crossed by
holes．When processing the analyses of geochemical samples the following series of vertical zoning has been obtained(upwards)：Mo-(Au，Ag，W ，Pb，Cu，Zn,Bi,Ni,Co)-(Mn，Ba)．It indicates the relative accumulation of manganese and barium in the supra-ore part an d molybdenum-in the under-ore one．
6 Circum—ore metasomatites
The circum-ore metasomatites have been studied by I．P．Scherban．He distinguished：listvenites-beresites，gumbeites and argillizites in the most widely
developed processes of beresitization-listvenitization of the rocks enclosing the mineralization．The earliest by formation time and high-temperature is a formation
of gumbeites．The process of gumbeitization is developed restrictively on one of the rocks，enclosing the mineralization—granites． The following scheme of
zoning has been distinguished：unaltered granite-zone of sericitization-zone of gumbeitization．With this process the evacuation of aluminium， supply of potassium,iron and carbon dioxide occued．
The widely developed process of listvenitization-beresitization covers both granites and metamorphic form ations(gneisses，amphibolites)．The thickness of
zones varies in wide limits：from several centimeters to 1～ l 0 m．seldom even more．The metasomatic column of alterations consists of the following zones：non-al-
tered granite (gneiss)-sericitization zone-carbona-tization zone,silicification zone． In the process of listvenitization．beresitization the role of calcium，
potassium，silica and carbon dioxide increases with decrease of Na，Mg，Fe and Al role．
The argillites are spread restrictly．They form zones with thickness from l O～2O cm to several meters． Argillitization zones are formed by loose clay
minerals with abundant(up to 1 0％ )pyrite impregnations．A typical metasomatic column has the following structure：non-altered granite (gneiss)-chloritization
zone-sericitization zone-steadiness zone of clay minerals．In the process of argillization the major components are evacuated，except of silica and carbon dioxide．
7 Age of mineraIizati0n
The formation of the Bamsk deposit was conditioned by the formation and development of the Bamsk volcano-plutonic uplift． Paragenetic connection of gold-silver mineralization with volcanic intrusive complex is obvious． It is defined by the fact that the ore—bearing zone of the Bamsk deposit is actually in the above—intrusive zone of the Nevachansk pluton．The mineralization is evidently younger than the intrusion of syenite-porphyries， since gold—bearing quartz veins and beretization zones，analogous to ore—beating ones， have been discovered within this intrusion west to the deposit．At the same time the morelate dykes of
diorite porphyries and the Early Cretaceous lamprophyres cross the ore bodies of the Bamsk deposit．This determines the upper age boundary of the mineralization． The position of the gold—silver mineralization in the age fork ofthe intrusive and dyke of the Early Cretaceous complexes determines its age．
Isotopic-geochronological study of the monomin-eral samples of muscovite，calcite and fluorite from the ores of the Bamsk deposit and the ore showing of the
D’yes were carried out by Rb—Sr method in the Department of Isotopic Geology of VSEGEI(the analyst Yu．P．Shergina)．The determination of Rb and Sr content
as well as the isotopic composition of strontium was done with the instrument ME-1 20 l T．For the initial isotopic composition of strontium in the minerals，the
main productive stage of the Bamsk deposit，the value of isotopic composition of Sr in the sample of calcite PT一5 0．708 49±0．008 was taken (Table 2)． In this
case the formation time of ore—accompanying muscovite in the sample T-3d from the Bamsk deposit is appeared to be 130．6 Ma．(Moiseyenko et al.1997)．The age of muscovite in the D’yes showing，calculated with the use of the same initial isotopic composition of strontium,is l26．8 Ma． The common isochr one for both objects with parameters:T=l29±3．6 Ma．was calculated.
8 Conclusion
The Bamsk gold deposit is a system of gold-bearing quartz and carbonate—quartz vein zones．They are located in the superintrusive zone of the Nevachansk subvolcanic intrusion of syeniteporphyries and confined to the echelon．1ike formed system of
fractures．developed in the lower side of the Bamsk fault．Quartz,carbonates and sericite prevail in the composition composition of ores． Among the ore minerals,which don’t exceed 1％ ～5％ the most abundant are pyrite， chalcopyrite， galena，sheelite，native gold．Less developed are gold and silver tellurides， sulphabismu—thides，acanthite，sphalerite and cinnabar． A series of mineralization stages from which the most productive for gold is gold-sulphide sulphosalt stage with tellurides of gold and silver was distinguished．A series of elements typomorphic for ores of the deposit was revealed．The lateral and
vertical geochemical zonation of the mineralization was determined．(Wl，Pb1，Mo)一Cu一(Ag，Bi，Sb，Mn，W2，Pb2)make the vertical zonation series(upward) .The rocks enclosing mineralization are widely exposed to the processes of listvenitization and beresitization．The processes of gumbeization and argillization are manifested in less extent．The age of gold mineralization，determined by the Rb—Sr method on ore—accompanying minerals is 130．6 Ma． The deposit is related to gold-sulphide-quartz type， of subvolcaniclevel of localization．
References
Moiseyenko V G，Stepanov V A，Shergina Yu P．1 997．The age of gold mineralization of the Bamsk ore knot．Reports of RAS．355(3)：369.371．
Rundkwist D V，Nezhinsky I V．1 975．Zonation of Endogenic Ore deposits．Leningrad：Nedra，1-57．
Stepanov V A， Cheremisin A A． 1 996． The Bamsk gold ore knot in Priamurye．Ore and ment1．1：64-72．
Stepanov V A．1 997．Gold and Mercury deposits in the middle Amllrregion．Geology of Ocean，13：11l 9．1132
