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Geology of the Sepon Copper and Gold Deposits Laos
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The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. characte, eristics with the, t Carlin ty, ype of minera. alization it may, m be class, sified as disstal dissemin. nated Au , Ag beca, ause of the association, a with. w a possib, ble zoned po, orphyry intrus, sive centeredd system . K, Key words Sepon , S sedim, ment hosted A, Au distal dis.
sseminated jasperoid , j exxploration, duction, 1 Introd Mt 1 3 percent Cuu for 330Kt Cu at. 25M, Th, he Sepon prroject comprises a 1247 km The. engkham So, outh 1 0 perrcent Cu cutt off and, contractt area and. d is owned, d by Chin, nese 29M, Mt 1 5 percent Cuu for 430Kt Cu at. domicile, ed MMG which operates, s locally as L, Lane The.
engkham North 1 0 perceent Cu cut offf , Xang Miinerals Limite. ed LXML Gold, G and cop, pper The Sepo, on project iss located 40 km north. depositss at Sepon contain, c over 4 4 Moz of g, gold of the town of. o Sepon i n the Sava, annakhet, and 2 0 Mt of contained copper Seven gold and pro. ovince of south central Laaos at appro, oximately.
opper deposiits exist with, three co hin a distancce of 105. 5 59 east longitude and 16 58 north, h latitude, 5 km an. nd comprise, e the Sepon mineral disstrict Fig, g 1 Laos is. s a landlockked country in central, SMD The Sepon gold min. ne commen, nced Indochina sharing bordeers with Thailand .
T, ons in late 2002, operatio 2 and pro, oduced 165 000 Mya. anmar Chin, na Vietnam and Cambo, odia The, oz of go. old in its first year Expan, nsion of the g, gold topography of the Sepon area is mo. oderately, plant an, nd constructtion of the adjacent. a cop, pper und, dulating at an elevationn of around.
d 250 m, plant we, ere completted in late 2004, 2 and e. early abo, ove sea level Veggetation co, omprises. 2005 resspectively Current, C gold resources sttand sec. condary fore, est punctuate, ted by rice paddies . at 78 Mtt 1 7 g t Au, A for 4 4 Moz Au 0 5 g t Au slas.
sh and burrn agriculturre and patches of,165, cut off w. while copper resources comprise 44, 4 Mt prim, mary rain fo. orest The area has a typical, 2 9 p, percent Cu for. f 1 23 Mt Cu, C at Khano, ong monsoonal clim, mate with ann annual ra. ainfall of, Fig 1 Loca, ation of the S, Sepon projec.
ct area in sou, uthern Laos , 2200 mm and can be divided up into 3 three priority areas in southern and central Laos. seasons a dry cool period from October to were identified for assessment Of particular. February a hot humid period from March to interest were the Nam Kok and Nam Sengi. June and a rainy monsoon period from July to areas which had been the focus of Soviet. September alluvial mining between 1983 and 1985 Soviet. Access to the project from the provincial reports and mapping indicated the presence of. center of Savannakhet is via a 3 hour drive on a quartz stockworked sub volcanic intrusive. sealed highway followed by a 1 hour drive on an complexes and gold mineralization in silicified. all weather unsealed public access road The and sericitized contact zones Bakulin 1985 . company runs a 6 times weekly 90 minute CRA Exploration geologists first visited. charter flight to and from the capital Vientiane the Sepon area in December 1990 Gregory . The project is located on the southern portion of 1991 Reconnaissance traverses verified the. the historically infamous Ho Chi Minh Trail a presence of altered porphyry intrusives and. major North Vietnamese supply line into multi phase stockwork veining as described in. southern Vietnam during the Vietnam War As the Soviet reports This coupled with the. such the area was heavily bombarded by U S presence of pannable gold and outstanding rock. forces trying to stem the flow of North chip results 17 samples over 200 m averaging. Vietnamese troops and war machinery into the 18g t Au in strongly silicified sediments . southern Vietnam s theater Effective safety highlighted district scale potential for intrusive. systems developed for the clearance of residual related mineralization Manini et al 2001 A. 166, ordinance contamination remain an ongoing part rapid review of the available Landsat TM. of daily operations imagery confirmed regional prospectivity and an. application for a 5000 km area was submitted,2 Exploration History. to the Lao government After a lengthy, 2 1 Discovery of Sepon. negotiation exploration and development rights, Although the Sepon area had been.
were granted to CRA under a Mineral,sporadically worked by both local artisans and. Exploration and Production Agreement MEPA ,Soviet alluvial miners no assessment of the. in 1993 ,hard rock potential of the area had ever been. undertaken prior to the involvement of CRA Rio Following the grant of title in 1993 . Tinto Exploration in 1990 Manini and Albert surface mapping and rock chip sampling. 2003 Following the end of the cold war and the generated immediate drill targets On the. subsequent opening of new exploration and Discovery prospect a 700 m long boulder train. development frontiers CRA Exploration made a of highly mineralized silicified and brecciated. strategic decision to expand its search into Laos sediments on the south side of the Nam Kok. Manini et al 2001 Very little was known at river was delineated and subsequently drill. the time about the geology and mineral potential tested Drill holes eventually intersected in situ. of the country so a reconnaissance visit was silicified and mineralized calcareous shale . made to assess the opportunities there providing impetus for scaled up activities in the. Following reviews of the geology politics skills district . base infrastructure and logistics of the country , Vientiane Capital Lao PDR. The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. Upon conclusion of the 1993 drilling discovered ore bodies with 44 000 m of drilling. program systematic geological mapping and completed Oxiana acquired an 80 percent. sampling were conducted focused on stake in the project in early 2000 with the Rio. delineating additional in situ mineralization and Tinto retaining a 20 percent holding Upon. enhancing the geological understanding of the finalization of the asset sale Oxiana. immediate area As a result similar silicified immediately commenced feasibility and. sediment was discovered along 1500 m of strike environmental social impact studies for a two. immediately to the east of the Discovery stage development of the Sepon gold and. prospect Simultaneously broader geological copper projects Manini and Albert 2003 . prospecting was carried out identifying Following a 12 month construction period the. additional areas of gold mineralization at Vang Sepon gold mine poured first gold in late 2002. Ngang and Namkok West and copper gold and has been operating successfully since . mineralization at Khanong on the flanks of the, 2 2 Ongoing Exploration.
Padan porphyry intrusion Fig 2 Manini et al , Exploration re commenced in 2003 and. 2001 Mineralization in the Sepon area was, additional resources were delineated at both. identified as having affinities to the Carlin type. Luang and Vang Ngang Coupled with near , sediment hosted gold deposits of Nevada. mine resource extensions and before depletion, Sillitoe 1994 . the gold resource base rose from 3 5 to 4 2 Moz, By the end of the 1994 drilling campaign in the first year of renewed exploration activity.
potential for a 1 Moz resource at Discovery Manini and Albert 2003 . 167, was confirmed In addition systematic mapping, With detailed resource drilling grade . and sampling resulted in the recognition of six, control drilling and the commencement of. new sediment hosted gold targets and a second, mining in 2003 understanding of the geological. porphyry system with associated skarn and, framework and controls on mineralization in the. carbonate replacement copper mineralization at, SMD was significantly upgraded Enhanced.
Thengkham Fig 2 Manini et al 2001 , geological understanding of the economic gold. Regional exploration of the broader Sepon, deposits is being used to generate new. MEPA commenced in 1994 and programs, prospects in the SMD and a prospectivity matrix. comprising detailed regional stream sediment, for prioritization of existing prospects has been. and rock geochemistry mapping airborne, developed Five key exploration criteria common.
magnetic and radiometric surveys and remote, to all known ore bodies were identified 1 ENE. sensed Landsat and photo interpretation were, and or WNW structures 2 Rheological . completed over the entire 5000 km Stream, chemical contrasts 3 Presence of rhyodacite. sediment geochemistry clearly outlined a, porphyry dykes and sills 4 Presence of. standout high order district scale multi , mapped jasperoid and 5 Strong multi element.
element geochemical signature in the Sepon, Au As Sb geochemical signature in soils Smith . mineral district which has been the focus of, 2003 These key criteria were applied to the. gold and copper exploration since , entire district and 36 priority targets identified . Between 1994 and 1999 drill delineation, work by RioTinto continued on all of the newly. Fig 2 SSummary ge eological maap of the Seepon minera al district main Fig annd inset showing the 168. location of the Au an, nd Cu deposits with respe, ect to the su.
ub economic porphyry stoockwork systtems and,Padan a and Thengkh ham On main Fig A A L. Location of Discovery, D Colluvial cross section Fig 5 B B . Locationn of Nalou cross section Fig , 6 , Exxploration programs. p in late 2003 and wever blanke, how ets of transpported alluviu. um up to,2004 ha, ave been dessigned prima, arily to test th.
hese 10 m in thicknes, ss appear too be masking, g bedrock. 36 iden, ntified targe, et areas Several, S of the ochemical signatures As a consequ. geo uence a,prospeccts have y, yielded high, h grade dri lling disttrict wide regolith geomoorphology map. m was,intercepts most nottably Nakach, han and Pha. avat pro, oduced based on air photos and field, Fig 2 Many of the identified targets posssess trav.
verses identtifying areas that may be, e covered, mical signatture . little or no surficial geochem by transported, d alluvium An extensive RAB. Vientiane Capital Lao PDR, The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. Rotary Air Blade drilling program has been represent basin forming normal faults and. planned in an effort to more effectively sample northwest oriented structures represent original. bedrock transfer structures Coller 1999 Smith 2003 . While exploration at Sepon has always The timing of events in and around the. utilized a wide spectrum of techniques and Sepon basin is poorly constrained Limited. technologies programs have always maintained paleontological data suggest sedimentation took. a strong geological focus with an emphasis on place from Devonian to Early Permian Oxiana. basic prospecting geological mapping and Ltd unpublished data consistent with a Late. drilling Manini and Albert 2003 Devonian age for the onset of extension The. timing of igneous activity in the basin is similarly. 3 Geological Setting, poorly constrained The metamorphic grade is. 3 1 Regional Setting, sub greenschist and away from discrete.
The SMD is located within the informally, structures or high strain zones no penetrative. defined Sepon basin The Sepon basin is, fabrics are developed . situated within the Truongson belt which forms, part of the Indochina terrane of Southeast Asia 3 2 Stratigraphy of the Sepon basin. Metcalfe 1996 The Indochina terrane formed The stratigraphy of the Sepon basin has. part of northern Gondwanaland during the been studied in detail only in the vicinity of the. Precambrian and may have rifted off in the Late mine area where detailed drilling has allowed a. Devonian Late Paleozoic and Mesozoic faunas robust stratigraphic interpretation Away from. indicate amalgamation with Southern China the mine area the general relationships appear. 169, during the Late Paleozoic Metcalfe 1996 to hold true but generally scarce outcrop and. The Truongson belt extends in a NW lack of detailed drilling have prevented. orientation from central Vietnam to northern comprehensive development of a basin wide. Laos and consists of a metamorphic complex scheme . the Kontum block with successor basins The Sepon mine area stratigraphy. including the Sepon basin of poorly comprises eight major informal units Highway. constrained age possibly ranging from Silurian formations to Phabing with one unit Nalou. to Permian Lepvrier 1997 Overlying the formation comprising 3 informal members. Paleozoic basins is the Mesozoic Khorat basin Morris 1997 Fig 2 and 3 Sepon Highway. of Thailand and Laos formation is a thin 100 m unit consisting of. The Sepon basin has an anomalous E W basement derived conglomerate minor. character within the overall NW trend of the calcirudite calcarenite and green claystone . Truongson belt and the tectonic fabric of this This unit passes up to the 300 m thick heavily. part of South East Asia Fig 2 This has been bioturbated fine quartz lithic sandstone which. interpreted to indicate development of the dominates Upper Highway formation Minor. Sepon basin as a pull apart basin in response to interbedded mudstone siltstone and calcarenite. sinistral strike slip transpression on the exist throughout this formation Upper Highway. Truongson Belt Coller 1999 Smith 2003 In Formation passes conformably up to. this interpretation near east west faults VangNgang formation which is 300 m thick and. 170,Fig 3 S, Schematic strratigraphic coolumn for thee Sepon mine area Loca.
ation of Au deeposits is als, so shown,with thicckness of bla. ack bars relative to the a pproximate Au, A endowme ent of each sstratigraphic position . Abbrevia ations as for Fig 2, Vientiane Capital Lao PDR. The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. consists of pyritic black shale black laminated formation Namkian consisting of interbedded. chert and laminated siltstone black chert and calcareous shale Phabing. KengKeuk Formation is the Formation is a poorly defined laminated . stratigraphically lowest carbonate dominated calcareous and non calcareous black shale with. unit of the Sepon basin and is known from interbedded turbiditic calc arenite unit that. drilling in the mine area It is 100 m thick and marks the top of the currently known. consists of dark bedded carbonaceous stratigraphy . limestone and calcareous shale KengKeuk 3 3 Diagenesis. Formation is conformably overlain by the The Sepon basin carbonate sequence. dolomite dominated Nalou formation which has has a number of recognizable diagenetic stages. been subdivided into 3 informal members that provide constraints on the timing of other. Members 1 and 3 consist of medium gray events within the basin The earliest. bioclastic dolomite and minor limestone that recognizable diagenetic features are early. are generally 30 m thick These units bracket diagenetic pyrite and two minor calcite vein. the light gray laminated stromatolitic stages that all pre date the majority of bedding. dolomite and limestone of formation 5 member parallel pressure solution Swarms of non . 2 Nalou Formation has a total thickness sutured bedding parallel pressure solution. exceeding 120 m Nalou Formation member 3 features developed in the carbonaceous. has a gradational contact with the overlying dark calcareous units concentrating carbonaceous. gray to black bedded calcareous shale and material and other non calcareous components . 171, limestone of the Discovery formation The Lesser bedding parallel stylolites formed within. contact is gradational over approximately 10 20 the organic poor carbonates of the Nalou. m and is marked by decreasing bioclastic formation . component up section Discovery Formation has, A major basin wide dolomitization event.
a thickness exceeding 200 m and is, occurred primarily affecting the relatively. characterized by a well developed bedding , organic poor limestone of the Nalou formation . sub parallel lamination developed by, However it is also observed to affect the. concentration of residual carbonaceous matter, Discovery formation black calcareous shale . along swarms of non sutured pressure solution, both at the lower contact with the Nalou.
features Kengkeuk Nalou and Discovery, formation and less commonly higher up in the. Formations form the main carbonate package of, Discovery formation Dolomitization ranges from. the Sepon basin , relatively fine grained texture preservative to. Discovery Formation passes conformably coarse grained sparry and texture. upward to Namkian formation which marks the destructive . return to siliciclastic sedimentation Namkian, Cross cutting bodies of vuggy sparry. Formation consists of laminated black to dark, dolomite are also present and have been.
gray chert pale colored pyritic siltstone shale, interpreted as reefs Morris 1997 although. and lesser black shale with a total thickness of, some may also represent cross cutting. 600 m The formation Discovery Namkian, hydrothermal dolomite zones Smith 2003 . contact is gradational with the lower 10 m of, Dolomitization post dates much of the bedding . parallel pressure solution which together with 4 Gold Mineralization. its relatively coarse grain size and occasional The principal gold deposits in the Sepon. saddle form is interpreted to indicate a late district Table 1 are hosted in carbonate rocks. diagenetic timing for most of the regional of the Discovery formation with lesser amounts. dolomitization Smith 2003 The dolomitization in the Nalu and KengKeuk formations Figs 2. event fundamentally altered the rheology and and 3 The only economic deposit hosted within. chemistry of the Sepon basin a factor that is the siliciclastic dominated sequences is Vang. interpreted to have been important in the Ngang which is a minor component of the total. subsequent development of the gold deposits gold endowment of the district Table 1 . see discussion below , 4 1 Mineralogy Alteration, 3 4 Igneous Rocks Ore within the Sepon sediment hosted.
The Sepon basin sequence was intruded gold deposits is hosted predominantly by. by a widespread quartz rich dike set oriented jasperoid and decalcified shale with minor ore. along the dominant near east west structural also present in RDP intrusions Jasperoid. trends Fig 2 The dikes are locally referred to typically hosts the highest grade ore and. as rhyodacite porphyry RDP and generally consists of massive to weakly banded intensely. have a relatively narrow range of phenocryst silicified carbonate rock with 1 to 10 percent. populations Large up to 1 cm rounded and disseminated and fracture controlled pyrite Fig . embayed quartz phenocrysts generally 4 Jasperoid ranges from massive to highly. constitute 10 25 percent of the rock with the fractured and brecciated Brecciated jasperoid is. 172, remainder comprising altered K feldspar typically monolithic however rare examples of. plagioclase and 10 percent former mafic polylithic jasperoid breccia have been noted . minerals A strong basin wide phyllosilicate Vein and breccia cement is dominated by white. sericite illite pyrite quartz alteration has to gray quartz with lesser dolomite and calcite. affected all the dikes Fig 4 Jasperoid mimics parent rock textures. RDP intrusions do not have peperitic with jasperoid developed after calc shale. margins and are universally later than late typically dark colored and massive to weakly. diagenetic dolomitization The dikes have been banded In contrast jasperoid developed after. affected by the same regional fault sets that dolomite is typically mottled gray to black and. dominate the structural fabric of the basin and banded Fig 4 . most of the RDP sediment contacts are marked Decalcified shale is similar in hand. by shears rather than intrusive contacts specimen to the unaltered calc shale with. Therefore it is interpreted that intrusion occurred sanded textures very rarely developed . after lithification and late diagenetic alteration of Contacts between decalcified shale and. the sediments but prior to conclusion of the unaltered calc shale are typically gradational. major compressional event The only other and can be highly irregular . igneous rocks known in the Sepon basin are Variable degrees of alteration have. rare andesite dikes and conglomerate units resulted in zones of patchy jasperoid and. within Formation 2 that contain andesite cobbles decalcified shale at the centimeter to meter. scale A characteristic feature of these mixed, Vientiane Capital Lao PDR. The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. Tabe 1 T, Total gold re, esources pre, e mining and. d current forr the Sepon mineral, m distriict at 0 5 g t Au cutoff . 173, zones iis their hig, gher strain compared with or whereas the gold pooor base mettal zones.
colo, adjacentt host rockss The deca, alcified mate. erial gen, nerally have pale coloredd sphalerite . typicallyy displays a conspicuous cleavage and Mineralization withinn decalcifie. ed shale, small sccale folds an, nd shears Ea, arly veins w ithin zon. nes i e with, hout jasperoi d is best de, eveloped. jasperoid, d are in som, me cases fau, ulted and follded in the Discove.
ery West ddeposit but is also, Fig 4 esent in othe. pre er deposits Fig 4 In, n several, Pyyrite is the. e dominant sulfide in the dep, posits minor ore exists in calc shale that is. mineralizzed zones However sp, phalerite lessser not obviously altered . a retaaining its ca, alcareous, galena a, and minor stibnite exist in.
n some depo, osits com, mponent , e g Na, alou Discove. ery Main Fig 4 The b, base, 4 2 Geometry and S, Structural. metal sulfides are associated with pyrite and Ass. sociations, typicallyy form veins and fracture, e fill The zo. ones The gold, d deposits ppossess a range of, with elevated ase. ba metals are typiccally geo, ometries and, d some depoosits include.
e several, us although, auriferou h a few thin, n zones 5. 5 m diffe, erent shapes, s within the same ore body The. with perrcent levels of zinc and lead are kno, own typical geometries are . to be ba, arren of gold, d The sphalerite associa. ated, with gold mineraliza, ation is typic, cally dark re.
ed in, Shallow to moderate, m dip, pping sheets that ometry indic. geo cating that the intersec, ctions of, may or mayy not be con. nnected to stteep stee, ep faults are not a conntrolling facto. or in the, faults e g Discovery Collu vial form, mation of the. t Sepon ore bodies, s at the, Discovery West , W Discov.
very Main Na, alou dep, posit scale , Fig 5 Fig 6 Fig 7 Several key fault associatio. ons are, Steep fault controlled sheet like, s dies. bod app, parent in th, he Sepon gold system, ms The. continuous in both strike and down, n dip Dis, scovery Wes. st deposit iss dominantly, y hosted, directions e g .
e Discovery Colluvial Fig with, hin a WNW, W striking steep NNE. E dipping, 5 Fig 7 norrmal fault whereas the Nalou Namkok. dep, posits appea, ar to be assoociated with a WNW . Strike continuous dip limited ribb, bon , like bodiess e g Discovery. D M, Main striking steep south dipping, s g strike slip fault .
f The, scovery Main, Dis n Discovery C, Colluvial tren. nd exists, Nalou Namkok East and, d West Fig 7 , along a steep ENE striking. E g fault that ap, ppears to, All of these geometries relate in so. ome, ve normal movement, hav m in the Discove, ery Main. way to tthe interactio, on between steep structu, ures.
are, ea and reverrse movemeent in the Discovery, D. and a secondary sh, hallow dippin, ng control N, None. Collluvial area F, Fig 5 , eposits have,of the de e pipe like steeply plung. s ging, 174,Fig 4 P, Photos of Sep pon Au mineralisation A Dark gray massive. m jasp, peroid develooped after ca alcareous, shale wiithout significcant textural modificationn B Dark graay quartz veined and breecciated jaspperoid C .
Two exa amples of ja asperoid illusstrating the differences between jas speroid deveeloped after dolomite. lower ssample and massive bla ack jasperoid d developedd after calcareous shale D Mixed jasperoid. j, massive e and deca alcified shale, e thin doma ains with strrong cleavagge Thin quuartz pyrite ssphalerite. veins exxist and have e small scalee fault offsett E Mineralised decalciffied shale saamples F Discovery. D, West ramp showing the Discove ery West Fau ult with weatthered decalcified shale in the fault zone. z and, the adjacent minerallised transitioon between tthe Discoverry and Namkian formationns . Vientiane Capital Lao PDR, The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. 175, Fig 5 DDiscovery Coolluvial cross, s section shoowing the lo.
ocation of jassperoid dom inated mineralisation, within th. he steep souuth dipping Discovery, D Fa, ault and on the dolomite e calcareous shale conta. act in the, hangingwall of the sttructure Jas, s jasperoid. d dcs deca, alcified shale , Att Discovery Colluvial the south dipp. ping bstantial pos, sub st mineralizaation fault offfset had.
fault sh, hows an offset of the, e formation, n 5 occ. curred thes, se two oree zones would, w be, formatio. on 6 contacct and a lo, ower RDP that disp, placed along. g the fault , indicatess approximately 40 m of appa. arent All of the depossit scale structural, s, reverse movement Fig 5 Ja.
asperoid hossted ass, sociations fa, aults and foldds are also important. mineralizzation is devveloped both, h within the ffault regional orienttations withh major ba. asin wide, zone and at the calcc shale dollomite contacct in uctural patte. stru erns interpreeted from mapping , m, nging wall of. the han o the fault No significcant aerromagnetics aerial photoography and. d satellite, mineralizzation existss at the same, e contact on.
n the data mimicking, g the patterrns observed in the. footwall side of th, he fault Mineralization, M n is dep. posits Lane Xang Minerrals Ltd unp, published, interpretted to have occurred during or afterr the data Fig 2 This. T is interppreted to indiicate that, reverse movement of. o the fault because, b a diirect the Sepon basin has not uundergone any major.
continuitty exists between the fault hosted, f and defformation eve. ent after min eralization , dolomite, e contact h. hosted ore Fig 5 If, Th, he overall trend of the depositss is the overall WNW. W trend of thhe deposit fo, orms only,typicallyy the same as trends in. n internal grrade e of two interrnal trends W, one Within the de.
eposit the,and thicckness High, h grade zones in Discovvery higher grade zones. z definee separate ENE, E and, West Discovery Colluvial Namkok and NW trends that exist in th e hinge zone. WN e of folds,Discovery Main typiccally follow the. t overall trrend with, h sub vertica, al axes trendding ENE an. nd WNW ,of these deposits In, n the Nalou deposit .
d howe, ever ,Fig 6 C, Cross section, n through the Nalou dep posit On this section the deposit is concentrate. ed on the, 176,dolomite, e calcareous shale contact with lesse. er mineralisattion develope, ed on RDP ccontacts . Fig 7 S, Schematic illuustration of the, t geometryy and locatio. on of the Sep, pon gold depposits with re, espect to.
structure, e stratigraph, hy and geollogical contaacts Csh calcareous s shale Dol dolomite dcsh . decalcifiied shale DW, D Discov very West D, DC Discovvery Colluvia. al DM Diiscovery Main NE ,Namkokk East NW Namkok We est . Vientiane Capital Lao PDR, The 2nd Lao Thai Technical Conference on Geology and Mineral Resources January 17 18 2013. 4 3 Location of Mineralization With underlying calc shale and in the stratigraphic. Respect to Geological Contacts transition zone between the Discovery and. There is a strong association between Namkian formations Fig 4 . mineralization and the dolomite calc shale , 5 Discussion.
RDP dolomite calc shale and calc shale , 5 1 Timing of Gold Mineralization. chert contacts Fig 3 The most important of, Several observations constrain the. these is the dolomite calc shale contact that, timing of gold mineralization in the. exists close to the Nalou formation Discovery, Sepon deposits . formation stratigraphic contact Figs 5 and 6 , This is a major control at Nalou Discovery Gold mineralization occurred after late.
Colluvial Discovery Main Namkok West and diagenetic dolomitization. Namkok East Dolomitization within the upper Gold mineralization exists within altered. part of the Discovery formation is evident at RDP. Discovery West and exists in the immediate The structures that host gold deposits. footwall of the deposit Mineralization associated cross cut the RDP dikes and locally. with the dolomite calc shale contact is host ore within the RDP. developed in both calc shale and dolomite , In certain areas ENE and WNW faults. however it is much more extensive in the calc , are known to have undergone reverse. shale Jasperoid and decalcified shale hosted, and or strike slip movement before or. ore zones typically extend for tens of meters, synchronous with development of. 177, above the contact into the calc shale but only.
jasperoid and gold mineralization, several meters into the underlying dolomite . Thickness and grade is in some cases, Gold grade is also controlled by host rock. controlled by ENE and WNW oriented, lithology with grades typically higher in the. fold hinges Locally these folds have, altered calc shale than in the altered dolomite . deformed the RDP dikes , The RDP dolomite and RDP calc .
The ore zones include small scale faults , shale contacts are lesser controls on orebody. folds and cleavage that developed after, formation but are important at Nalou Discovery. jasperoid and decalcification, East and Discovery Main with approximately 20. percent of the resource located along these Given these constraints the Sepon gold. contacts The RDP contact controlled zones are deposits post date or are broadly synchronous. characterized by strong shearing of the RDP with compressional movement on certain faults . and mineralized calc shale Although fold development and RDP intrusion The. mineralization typically exists in both the calc increased strain and development of small scale. shale and the RDP it is best developed in the compressional structures within the ore zones. calc shale indicate that compression continued after. mineralization , The third major geological contact to host. ore is the calc shale chert Discovery Our preferred timing is syntectonic. formation Namkian formation contact At development of the gold systems within faults. Discovery West mineralization exists in the and folds during basin inversion which was. broadly synchronous with RDP intrusion In this porphyry stockwork systems These porphyry. interpretation gold mineralization developed stockwork systems consist of extensive quartz. within active folds and faults that were stockwork in host sediments and rhyodacite to. propagated or reactivated during basin quartz diorite porphyry Extensive silicification. shortening and these structures continued to be and lesser potassic K feldspar and biotite . active after initial alteration and mineralization alteration are known to be associated with the. to develop the small scale structures in the ore porphyry systems and may be equivalent to the. zones It is acknowledged that the timing of the prograde skarn developed in carbonate hosts . small scale structures is not well constrained The porphyry systems are predominantly Mo. and could be unrelated to the folds and faults systems with weakly elevated Cu and Bi Sillitoe. that control mineralization If this is the case 1994 and Manini et al 2001 have proposed. the gold deposits could have formed at any time that the Sepon district is zoned from the quartz. after development of the faults and folds stockwork porphyries at the core outward. However this is considered less likely because it through the Cu rich skarn and replacement. is interpreted that no major structural event has bodies to the Au Ag dominated sediment . affected the basin post mineralization as hosted deposits . discussed above , 5 3 Controls on Location of Gold.
Deposits, 5 2 District wide Metallogenesis, The gold deposits are controlled by both. In addition to the economic gold systems, regional and local scale features including the. a number of other styles of mineralization exist, 178. regional ENE and WNW to NW oriented steep,in the Sepon district Principal among these are. faults The intersections of the ENE and WNW,the copper deposits at Khanong and.
structures with the more NW oriented segments,Thengkham South Fig 2 Hypogene. may exert a regional scale influence over the,mineralization consists of massive sulfide . locations of deposits Fig 2 , hematite and silica sulfide hematite . replacement zones associated with skarn The spatial association of the gold. alteration of dolomite and locally siltstone deposits with the magmatic related Mo Cu. Prograde garnet pyroxene skarn is overprinted systems at Padan and Thengkham suggest the. by retrograde amphibole chlorite epidote possibility of a genetic link and therefore. calcite hematite sulfide magnetite skarn that location with respect to these systems should be. grades into massive sulfide hematite and important However major gold deposits range. silica sulfide hematite ore The mineralization from 100 m Discovery Main to 4 km away. at both deposits is characterized by a Cu Ag Bi Nalou from known copper mineralization . Au association At Phavat Fig 2 high sulfide Therefore proximity to porphyry stockwork or. ore is characterized by a strong Au Cu Ag Bi skarn occurrences is not a useful exploration. association that is geochemically intermediate criterion within the district . between the Cu dominated and Au dominated Local controls are a combination of. deposits structural and chemical influences The, The copper deposits are considered to be structural control consists of the interaction of. related to the nearby Padan and Thengkham steep faults and shallow dipping lithological.

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