proterozoic contraction extension tectonics of the southern sao francisco region minas gerais brazil the southern sao francisco region comprises the southern portion of the sao francisco craton which is underlain by archean basement and its fringing proterozoic orogenic belts included in this region are three geologic provinces the quadrilatero ferrifero qf the sao francisco basin and the cordilheira do espinhaco we synthesize preliminary results of ongoing structural analysis reconnaissance field studies and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its proterozoic evolution interpretation of superimposed folds faults and foliations leads to the conclusion that the southern sao francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the lower proterozoic minas supergroup the first event resulted in formation of a northwest verging fold thrust belt the second event led to development of high angle reverse faults open folds and basement uplift the third event is manifested in the qf by the formation of normal faults and by the intrusion of mafic dikes these extensional structures may be associated with formation of the sedimentary basins east and south of the sao francisco craton the final event resulted in formation of a west verging fold thrust belt that affected the qf the cordilheira espinhaco and the sao francisco basin the events that we describe can be tentatively associated with regionally recognized orogenic periods trans amazonian uruacuano brasiliano

nature and origin of the bif hosted sao bento gold deposit quadrilatero ferrifero brazil with special emphasis on structural controls abstract the orogenic banded iron formation bif hosted au mineralization at sao bento is a structurally controlled hydrothermal deposit hosted by archean rocks of the rio das velhas greenstone belt quadrilatero ferrifero region brazil the deposit has reserves of 14 3 t au and historical underground production of 44 6 t au between 1987 and 2001 the oxide facies sao bento bif is mineralized at its lower portion where in contact with carbonaceous pelitic schists particularly in the proximity of sulfide bearing quartz veins shear related au deposition is associated with the pervasive hydrothermal sulfidation mainly arsenopyrite of the fe rich bands of the sao bento bif auriferous sulfide and quartz rich zones represent proximal alteration zones they are enveloped by ankerite dominated haloes which reflect progressive substitution of siderite and magnetite within the bif by ankerite and pyrrhotite respectively the sao bento bif was intensely and extensively deformed first into open upright folds that evolved into tight asymmetric isoclinal folds the inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts mineralized horizons at sao bento result from early structural modifications imposed by major transcurrent and thrusts faults comprising the conceicao barao de cocais and sao bento shear zones dextral movement on the sw ne directed conceicao shear zone may have generated splays at a compressional side stepping zone such as the sao bento shear zone which is the structural locus for the sao bento gold mineralization relaxation of the conceicao shear zone under more brittle conditions resulted in the development of dilatational zones where gold sulfide quartz veins formed these structures are considered to have been generated in the archean geochronological data are scarce with pb pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single stage growth curve at 2 65 ga a later compressional ductile deformation of unknown age overprinted rotated and flattened the original n60e directed structure of the whole rock succession with development of planar and linear fabrics that appear similar to proterozoic aged structures fluid inclusion studies indicate low salinity aqueous fluids with or without co 2 and or ch 4 with extremely variable co 2 ch 4 ratios of probable metamorphic origin fluid evolution shows a paragenetic decrease in the carbonic phase from 10 15 to 5 and increase in the h 2 o co 2 ch 4 and co 2 ch 4 ratios suggesting important interaction with carbonaceous sediment trapping conditions indicate a temperature of 300 c at 3 2 kbar

generation of gas and oil from coal and other terrestrial organic matter abstract late carboniferous permian sedimentary rocks contain the world s largest coal reserves which have generated and released gas but little if any oil in comparison organic matter deposited in lacustrine fluvial deltaic or brackish water sediments has sourced large quantities of waxy oils in countries such as china australia the u s a argentina venezuela and indonesia plus gas and condensate in deltas worldwide why is more oil coming from land derived organic matter disseminated in sediments than from the carboniferous permian coal beds which also are land deposits the answer appears to be in the hydrogen content and the ease of migration when organic matter and coals have high hydrogen contents relative to carbon they can form oil when the hydrogen is low mainly gas is formed most of the world s coal is low in hydrogen whereas much of the terrestrial organic matter disseminated in sediments is higher in hydrogen so it forms more oil also both gas and oil are adsorbed on coal more strongly than on the disseminated kerogen of sedimentary rocks this makes migration of oil out of the coal more difficult compared to migration from conventional shale source rocks

pb sr and nd isotope constraints on the archaean evolution of gneissic granitoid complexes in the southern sao francisco craton brazil abstract three archaean gneissic complexes bonfim belo horizonte and campo belo in the southern part of the sao francisco craton were studied by means of u pb zircon and titanite monazite and sm nd rb sr and pb pb whole rock methods in the bonfim metamorphic complex the period 2780 2700 ma is marked by in situ anatexis of gneisses intrusion of tonalites mafic dikes and late granites and deposition of the rio das velhas greenstone belt sm nd tdm model ages on its gneisses amphibolites and granites range from 2800 to 3000 ma indicating involvement of older sialic crust in the 2780 2700 ma event in the belo horizonte metamorphic complex u pb zircon geochronology has yielded ages of 2860 2776 and 2712 ma and rb sr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800 2750 ma with large uncertainties and 87sr 86sr initial ratios of 0 700 0 710 the highest initial ratio 0 710 obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution existence of older crust is suggested by u pb zircon ages of 3030 2920 and 2880 ma from inherited grains in metavolcanics and orthogneisses rb sr ages of 2250 2130 ma on granitoids and u pb titanite monazite ages from 2320 to 2030 ma on granitoids felsic veins and amphibolite enclaves indicate reworking of this complex in the palaeoproterozoic in the campo belo metamorphic complex granite and greenstone lithologies have yielded ages of 3380 3000 2900 and 2650 ma by u pb zircon and rb sr and pb pb whole rock methods tdm ages on its granites gneisses and granulites have yielded ages from 3070 to 2780 ma indicating involvement of older sialic material during the 2650 ma event these complexes indicate major events during the period 2860 2700 ma including reworking of established sialic crust with components up to 3380 ma old late granites were injected between 2700 2600 ma preceding final assembly and stabilization of the archaean crust

two billion years of evolution enclosed in hydrothermal rutile recycling of the sao francisco craton crust and constraints on gold remobilisation processes abstract the quadrilatero ferrifero in the sao francisco craton brazil is known for its world class gold deposits hosted by the rio das velhas greenstone belt rdv as in other greenstone belts gold is also concentrated in late archean early paleoproterozoic supracrustal units in this case within the moeda formation this unit has been compared to the witwatersrand basin where there has been a long term debate over the nature of the gold and whether it is detrital placer hydrothermal or hybrid modified placer presently links between gold in the moeda formation and rdv are tenuous to enlighten this area of ambiguity a new approach using chemical isotopic and mineral inclusion data from detrital rutile found in the gold bearing horizons of the moeda formation is presented grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile related to as fe cu sb pb w rich mineralising fluids fe nb and v indicate that the likely source of the gold related rutiles is the archean banded iron formations within the rdv where most of the lode gold occurs average nb ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and or competing mineral phases u pb data yield primary crystallization ages older than deposition ages 2 58 ga supporting the detrital nature of these rutiles rutiles record either resetting or new growth at 2 1 ga and at ca 500 ma during which gold was remobilised hence this study supports a modified placer model for the gold bearing horizons of the moeda formation these fluid assisted gold remobilisation events can be found in other correlated paleoproterozoic gold bearing horizon units that represent intracratonic basins related to the rifting of the kenorland supercontinent this study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance

shear zone related gold mineralization in quartz carbonate veins from metamafic rocks of the bif hosted world class cuiaba deposit rio das velhas greenstone belt quadrilatero ferrifero brazil vein classification and structural control abstract the cuiaba deposit located in the quadrilatero ferriferro district qf in the southeastern part of the minas gerais state is the largest underground mine in brazil 5 78 moz resources of 10 26 g t au 1 18 moz of ore reserves and 6 moz of overall production this world class orogenic gold deposit is part of the rio das velhas greenstone belt a typical association of mafic volcanic rocks banded iron formation bif carbonaceous and micaceous phyllites metamorphosed to the greenschist facies conditions historically economic gold production at cuiaba derives from bif hosted replacement style sulfide pyrite and pyrrhotite ores more recently economically significant shear zone related gold mineralization in quartz carbonate veins systems hosted in metamafic rocks were identified they are associated with distal chlorite intermediate carbonate and proximal sericite dominant hydrothermal alteration zones structures related to the gold bearing quartz carbonate veins were generated during a single progressive ductile to brittle deformational event d1 with the compositional banding sb and the flexural foliation sf created in the early stages of deformation as deformation progressed shear zones isoclinal folds the mylonitic foliation sm the mineral lineation lmin and the intersection lineation lint were formed simultaneously to the main stage of hydrothermal fluid input flanking folds and flanking shear bands represent the late stage of deformation and the final hydrothermal fluid pulse four quartz carbonate vein systems are identified v1 v2 v3 and v4 the v1 mineralized system is an sm controlled fault fill type associated with the principal gold bearing fluid input stage the v2 veins are also mineralized controlled by the sf foliation and subdivided in three sub types according to internal crystal orientation v2a extensional v2b oblique extensional and v2c a fault fill vein the extensional array v3 veins are also controlled by the sm foliation whereas the extensional breccia style v4 system has veins associated with flanking structures the structural analysis of veins indicates a shortening vector rotation during d1 accompanied by hydrothermal fluid input since all vein systems are related to the d1 event each one defining an incremental deformational stage they provide information about the hydrothermal fluid evolution at cuiaba a proposed evolutionary model for the vein systems suggest that the ore forming fluid responsible for gold mineralization was channeled along shear zones and dispersed out into the sb and sf structures the internal vein features and the trapping temperatures of quartz from each vein system indicate a progressively decrease in tectonic activity

guidelines for assessing the petroleum potential of coaly source rocks using rock eval pyrolysis abstract coaly source rocks are sufficiently different from marine and lacustrine source rocks in their organic matter characteristics to warrant separate guidelines for their assessment using rock eval pyrolysis the rank threshold for oil generation is indicated by the increase in bi s1 toc at rank sr 9 10 tmax 420 430 c ro 0 55 0 6 and the threshold for oil expulsion is indicated by the peak in qi s1 s2 toc at rank sr 11 12 5 tmax 430 440 c ro 0 65 0 85 the pronounced rank related increase in hi s2 toc prior to oil expulsion renders the use of immature samples inappropriate for source rock characterisation a more realistic indication of the petroleum generative potential and oil expulsion efficiency of coaly source rocks can be gained from samples near the onset of expulsion alternatively effective hi values i e his near the onset of expulsion can be estimated by translating the measured his of immature samples along the maturation pathway defined by the new zealand or other defined coal band coaly source rocks comprise a continuum of coaly lithologies including coals shaly coals and coaly mudstones determination of the total genetic potential of coaly source rock sequences is best made using lithology based samples near the onset of expulsion

a biogeochemical influence on the secular distribution of orogenic gold orogenic gold deposits are unevenly distributed in geologic time the richest period of orogenic gold deposit formation was the phanerozoic which followed a 1 b y period of relatively meager deposit generation during the proterozoic this proterozoic low gold period developed despite continent building episodes which should have been appropriate for orogenic gold genesis here a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed it is suggested that the extensive oxygenation of the deep oceans during the second great oxidation event over the period 635 to 510 ma produced conditions where gold was soluble in the deep oceans from that point on regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold promoting its uptake into pyrite and allowing formation of metallogenically enhanced sedimentary sequences these sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism this biogenic influence on source region enhancement may explain the association during the phanerozoic eon between some of the world s most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks

smaller gold deposits in the archean rio das velhas greenstone belt quadrilatero ferrifero brazil abstract the quadrilatero ferrifero is a classic gold province on a global scale with hundreds of individual gold occurrences in the archean greenstone belt comprising the rio das velhas supergroup there are numerous small gold deposits including faria esperanca iii bicalho bela fama juca vieira brumal boa vista fernandes moita roca grande bico de pedra and pari as well as the world class deposits of morro velho and cuiaba all these deposits whether large or small are structurally controlled and related to either shear zones or folds extensive down plunge continuity is a consistent feature of the deposits in the rio das velhas greenstone belt six main styles of gold mineralization are recognized these are deposits hosted within 1 lapa seca e g morro velho bicalho bela fama 2 banded iron formations e g cuiaba sao bento raposos faria brumal roca grande 3 quartz veins e g juca vieira fernandes 4 disseminated sulfides with quartz veinlets e g moita 5 amphibolites e g pari and 6 disseminated to massive base metal sulfides e g bico de pedra the first four types of deposits are epigentic orogenic gold deposits similar to those found in greenstone belts worldwide the last two are unusual types of gold deposit peculiar to the quadrilatero ferrifero bico de pedra is a polymetallic au ag zn pb cu deposit related to an aplite intrusion whereas pari is a stratiform au bearing banded iron formation metamorphosed to epidote amphibolite metamorphic facies

comparison of three series of organic matter of continental origin abstract three series of sediments containing land plant derived organic matter were studied they include wells from mahakam delta indonesia miocene douala basin cameroon upper cretaceous and eastern paris basin france paleozoic all of the kerogens and coals isolated from these series can be related to type iii in a van krevelen diagram however they obviously differ in terms of chemical structure as reflected by their pyrolysis gc patterns and by their bitumen composition these observations indicate the variations in the nature of land plant derived sedimentary organic matter and thus differences in quantity and composition of the hydrocarbons which are are released upon maturation based on hydrocarbons potential of the insoluble organic matter and on pyrolysis gc there is no reason to disregard coals and to favor shales as potential source rocks in these two deltaic sequences moreover we assume that the bitumen depletion in coal samples is a reflection of loss by migration which is favored in coals when compared to shales the gc patterns of the residual saturates compared to the pyrolysis gc patterns of the related insoluble coals suggest that migration of hydrocarbons proceeds in these coals without any significant alteration of their distribution at least in the c15 range

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

magmatic hydrothermal evolution of highly fractionated granites evidence from the kuiqi miarolite in fujian province se china abstract the kuiqi miarolite a highly evolved alkaline granite pluton is situated in fujian province southeastern china quartz crystals from the kuiqi miarolite contain abundant inclusions which have preserved a complete record of the magmatic hydrothermal evolution five inclusion type were identified i e the melt m type melt fluid ml type daughter mineral bearing s type vapor rich v type and aqueous l type inclusions integrated petrographic microthermometric and laser raman spectroscopy studies demonstrate that the whole magmatic hydrothermal evolution at kuiqi comprises three stages i e the magmatic magmatic hydrothermal and hydrothermal stages the m type inclusions minimal trapping temperatures ca 710 800 c represent the melts entrapped at the magmatic stage of crystallization while the occurrence of euhedral prismatic arfvedsonites in these melt inclusions indicates that the granitic magma was probably volatile rich at the magmatic hydrothermal stage both the ml type inclusions t h ca 500 710 c and coexisting m and v type inclusions reflect the presence of late magmatic fluid circulation and melt volatile immiscibility exhibiting a transition toward the hydrothermal stage at the hydrothermal stage the pressure and temperature 700 485 c drop may have led to oversaturation of the primary saline fluids resulting in the phase separation a highly saline and a volatile rich fluid occurred accompanied by the widespread vapor brine immiscibility the l type inclusions in the miarolitic quartz display bimodal salinity distribution the t h and salinity of the inclusions at the bottom l 1 miarolitic quartz are ca 303 427 c and ca 6 6 17 9 wt nacl equiv respectively while that of the inclusions at the top l 2 are ca 165 232 c and ca 0 2 5 7 wt nacl equiv respectively this indicates a drop of fluid temperature and salinity during the miarolitic cavity formation moreover the low homogenization temperatures and salinities of the l type inclusions suggest an influx of meteoric water during the late hydrothermal evolution stage

the archean bif hosted cuiaba gold deposit quadrilatero ferrifero minas gerais brazil abstract the cuiaba gold deposit is located in the northern part of the quadrilatero ferrifero minas gerais state brazil the region constitutes an archean granite greenstone terrane composed of a basement complex ca 3 2 ga the rio das velhas supergroup greenstone sequence and related granitoids 3 0 2 7 ga which are overlain by the proterozoic supracrustal sequences of the minas the stratigraphy of the cuiaba area is part of the nova lima group which forms the lower part of the rio das velhas supergroup the lithological succession of the mine area comprises from bottom to top lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks the gold bearing cuiaba banded iron formation bif upper mafic metavolcanics and volcanoclastics and metasedimentary rocks the metamorphism reached the greenschist facies tectonic structures of the deposit area are genetically related to deformation phases d 1 d 2 d 3 which took place under crustal compression representing one progressive deformational event e n the bulk of the economic grade gold mineralization is related to six main ore shoots contained within the cuiaba bif horizon which range in thickness between 1 and 6 m the bif hosted gold orebodies 4 ppm au represent sulfide rich segments of the cuiaba bif which grade laterally into non economic mineralized or barren iron formation transitions from sulfide rich to sulfide poor bif are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide poor portions the deposit is gold only and shows a characteristic association of au with ag as sb and low base metal contents the gold is fine grained up to 60 m and is generally associated with sulfide layers occurring as inclusions in fractures or along grain boundaries of pyrite the predominant sulfide mineral 90 vol gold is characterized by an average fineness of 0 840 and a large range of fineness 0 759 to 0 941 the country rocks to the mineralized bif show strong sericite carbonate and chlorite alteration typical of greenschist facies metamorphic conditions textures observed on microscopic to mine scales indicate that the mineralized cuiaba bif is the result of sulfidation involving pervasive replacement of fe carbonates siderite ankerite by fe sulfides gold mineralization at cuiaba shows various features reported for archean gold lode deposits including the 1 association of gold mineralization with fe rich host rocks 2 strong structural control of the gold orebodies showing remarkable down plunge continuity 3 km relative to strike length and width up to 20 m 3 epigenetic nature of the mineralization with sulfidation as the major wall rock alteration and directly associated with gold deposition 4 geochemical signature with mineralization showing consistent metal associations au ag as sb and low base metal which is compatible with metamorphic fluids

petroleum geology of benue trough and southeastern chad basin nigeria cretaceous cyclic sedimentation in the southern benue trough together with unconformities provide a tripartite subdivision of the sedimentary succession into 1 the albian asu river group 2 the late cenomanian to early santonian cross river group new name comprising the marine nkalagu formation new name and interfingering marginal marine sandstones and 3 the post santonian coal measures sequence most of the albian to eocene marine shales in the benue trough and the turonian shales in the southern chad basin contain well over 0 5 total organic carbon with values of up to 7 4 in turonian anaerobic shales based on the high content of soluble organic matter thermal maturity and the predominantly terrigenous character of the late cretaceous shales mostly natural gas was probably generated in both basins the late santonian folding and uplift would have disrupted petroleum reservoirs also crude oil accumulations which were not dissipated by tectonism would be relocated at relatively shallow depths and hence become accessible to invading meteoric waters

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

rift basins of interior sudan petroleum exploration and discovery in early 1975 chevron overseas petroleum inc commenced what became a major petroleum exploration effort in previously unexplored interior sudan with the cooperation of the sudanese government chevron has acquired a vast amount of geological and geophysical data during the past 12 years these data include extensive aeromagnetic and gravity surveys 36 040 mi 58 000 km of seismic data and 86 wells this information has defined several large rift basins now recognized as a major part of the central african rift system the sedimentary basins of interior sudan are characterized by thick nonmarine clastic sequences of jurassic cretaceous and tertiary age over 45 000 ft 13 716 m of sediment was deposited in the deepest trough and extensive basinal areas are underlain by more than 20 000 ft 6 096 m of sedimentary rocks the depositional sequences include thick lacustrine shales and claystones floodplain claystones and lacustrine fluvial and alluvial sandstones and conglomerates those lacustrine claystones deposited in a suboxic environment provide good oil prone source rocks reservoir sandstones have been found in a wide variety of nonmarine sandstone facies the extensional tectonism that formed these basins began in the jurassic early cretaceous movement along major fault trends continued intermittently into the miocene this deformation resulted in a complex structural history that led to the formation of several deep fault bounded troughs major interbasinal highs and complex basin flanks this tectonism has created a wide variety of structures many of which have become effective hydrocarbon traps during the past eight years several important oil discoveries have been made significant accumulations have been delineated in the heglig and unity areas where estimated recoverable reserves are 250 300 million bbl of oil

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

the geology of the morro velho gold deposit in the archean rio das velhas greenstone belt quadrilatero ferrifero brazil abstract the morro velho gold deposit quadrilatero ferrifero region minas gerais brazil is hosted by rocks at the base of the archean rio das velhas greenstone belt the deposit occurs within a thick carbonaceous phyllite package containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites considering the temporal and spatial association of the deposit with the rio das velhas orogeny and location in close proximity to a major nnw trending fault zone it can be classified as an orogenic gold deposit hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates sulfides chlorite white mica biotite albite and quartz as described in other archean lode type gold ores two types of ore occur in the deposit dark gray quartz veins and sulfide rich gold orebodies the sulfide rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies the sulfide assemblage comprises by volume on average 74 pyrrhotite 17 arsenopyrite 8 pyrite and 1 chalcopyrite the orebodies have a long axis parallel to the local stretching lineation with continuity down the plunge of fold axis for at least 4 8 km the group of rocks hosting the morro velho gold mineralization is locally referred to as lapa seca these were isoclinally folded and metamorphosed prior to gold mineralization the lapa seca and the orebodies it hosts are distributed in five main tight folds related to f1 the best examples are the x main and south orebodies in level 25 which are disrupted by ne to e striking shear zones textural features indicate that the sulfide mineralization postdated regional peak metamorphism and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed lead isotope ratios indicate a model age of 2 82 0 05 ga for both sulfide and gold mineralization the lapa seca are interpreted as the results of a pre gold alteration process and may be divided into carbonatic micaceous and quartzose types the carbonatic lapa seca is subdivided into gray and brown subtypes non mineralized gray carbonatic lapa seca forms the hanging wall to the orebodies and is interpreted as the product of extreme co 2 metasomatism during hydrothermal alteration this dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite the brown carbonatic and micaceous lapa secas are the host rocks to gold these units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and or volcanic rocks of varying composition from dacitic to andesitic forming various types of schists and phyllites the high grade massive sulfide orebodies occur at the base of the gray carbonatic lapa seca both disseminated mineralization and quartz veins are hosted by micaceous lapa seca the data are consistent with a model of epigenetic mineralization for the lapa seca from a hydrothermal fluid derived in part from the archean basement or older crust material

brazil s premier gold province part i the tectonic magmatic and structural setting of the archean rio das velhas greenstone belt quadrilatero ferrifero rocks of the rio das velhas supergroup comprise one of the most significant archean greenstone belt successions in brazil in both their appreciable mineral productivity and extensive mineral potential a large part of this greenstone belt is contained within the quadrilatero ferrifero iron quadrangle region minas gerais state southeastern brazil which occupies the southernmost portion of the sao francisco craton the nova lima group rocks at the base of the rio das velhas greenstone belt host important orogenic gold deposits the group contains lithological associations from bottom to top as follows 1 mafic ultramafic volcanic 2 volcanic chemical 3 clastic chemical 4 volcaniclastic and 5 resedimented rocks rocks of the resedimented volcanic chemical and mafic ultramafic volcanic associations mainly host the most important gold deposits an early compressional deformation occurs in the rocks of the rio das velhas greenstone belt and basement gneisses with tangential thrusting from the north to the south or southwest structures generated during a second compressional deformation encompass nw striking thrust faults and sw vergent tight to isoclinal folds inferring a general southwest transport direction in the central portion of the quadrilatero ferrifero the paciencia lineament which strikes northwest and dips to the northeast in the south or strikes northeast and dips to the southeast in the north is a thrust related oblique ramp fault that hosts important gold deposits the convergence of these two trends in the nova lima region is accommodated by roughly e w striking transcurrent faults which are the most favored sites for large gold concentrations intracratonic extension in late archean to early paleoproterozoic times and nw vergent trans amazonian compressional deformation post date gold deposition late extension during the paleoproterozoic led to basin formation and the prominent dome and keel architecture of the quadrilatero ferrifero region deformation related to the west vergent thrust and fold belts of the brasiliano orogeny is recognized at the eastern margin of the quadrilatero ferrifero region defining structures as archean trans amazonian and brasiliano in age is still difficult although it is accepted that the gold related archean structures are best preserved in the central and western parts of the quadrilatero ferrifero the principal source for the rio das velhas sedimentary rocks was probably the trondhjemite tonalite granodiorite crust that formed in multiple episodes after 3 500 ma and was widely metamorphosed and intruded at 2 880 2 850 ma felsic volcanism at 2 772 6 ma formed the rio das velhas greenstone belt the volcanic succession was a source for some overlying sedimentary rock units as indicated by the presence of detrital zircons dated at 2 777 2 771 ma strongly foliated granitic plutons range between 2 712 5 4 and 2 698 18 ma the age of gold mineralization is inferred between 2 698 and 2 670 ma a metamorphic overprint during the trans amazonian orogeny is estimated at 2 050 ma there is evidence of local isotopic disturbances because of post trans amazonian tectonic events at 1 8 1 7 and 0 6 ga

fluid signature of the shear zone controlled veio de quartzo ore body in the world class bif hosted cuiaba gold deposit archaean rio das velhas greenstone belt brazil a fluid inclusion study the world class cuiaba gold deposit of the archaean rio das velhas greenstone belt in brazil is hosted in banded iron formation containing carbonaceous matter and carbonate within the reclined isoclinal cuiaba fold mineralised quartz veins are hosted in andesite in the stratigraphic footwall of the banded iron ores and form some of the more recently discovered ore bodies fluid inclusion data of the quartz vein associated veio de quartzo ore body are obtained from four quartz types qz1 qz2 qz3 qz5 in gold mineralised v1 shear vein and v2 extensional veins barren v3 extensional vein array and v4 breccia style veins all developed during the archaean d1 event three fluid types are distinguished i aqueous fluids of low salinity 1 8 3 8 wt nacl equiv homogenisation into liquid at 220 to 230 c ii aqueous fluids of moderate salinity 5 3 12 7 wt nacl equiv and homogenisation at 250 to 290 c and iii aqueous carbonic fluids of moderate salinity 6 0 15 1 wt nacl equiv with 30 91 mol co2 8 41 mol ch4 and up to 28 mol n2 and decrepitation into vapour at 280 to 310 c based on an independent pressure estimate a pressure correction was applied to aqueous fluid inclusions resulting in minimum trapping temperatures at 360 c for v1 veins 330 c for v2 veins 300 c for v3 veins and 270 c for the late stage v4 veins ion chromatography analyses reveal a br cl ratio of 0 7 10 3 in qz1 v1 from 1 4 to 1 5 10 3 in qz2 v2 0 3 to 0 4 10 3 in qz3 v3 and 0 7 to 0 9 10 3in qz5 v4 veins zinc pb and cu are relatively enriched with 100 to 1000 ppm in aqueous and aqueous carbonic fluid inclusion assemblages in all vein and quartz types which is similar to other orogenic gold deposits hosted in the rio das velhas greenstone belt the fluid inclusion data are consistent with a model invoking a metamorphic origin for the mineralising fluid a two step model of hydrothermal fluid flow and gold enrichment is suggested to have developed during the archaean d1 event with an early aqueous carbonic fluid pulse of relatively high temperature from v1 up to v3 and an evolved aqueous carbonic fluid pulse of lower temperature v4 breccia style veins the rio das velhas greenstone belt is dominated by regionally metamorphosed metasedimentary rocks resulting in a complex hydrothermal fluid evolution and related gold mineralisation such as the shear zone controlled veio de quartzo ore body

re os dating of polymetallic ni mo pge au mineralization in lower cambrian black shales of south china and its geological significance a discussion sir thirty years ago fan et al 1973 reported an entirely new type of ore from cambrian black shales of south china containing percent values for ni mo as and in some cases zn together with substantial amounts of pt pd and au 100 1 000 ppb subsequently chen et al 1982 and fan 1983 described additional ni mo ore bed occurrences in a 1 600 km belt between kunming and hangzhou mao et al 2002 p 1057 describe the deposits as a thin layer having a wide lateral extent over hundreds of kilometers in southern china that contrasts with the lenticular shape of exhalative deposits which are usually located near hydrothermal vents and are continuous over only a few kilometers at most actually the known shale hosted ni mo deposits of china do consist of restricted lenticular occurrences that are clustered in eight relatively small areas lott et al 1999 near zunyi the principal but now exhausted deposit on tianeshan measures approximately 150 by 1 000 m in plan view and gaps occur between it and outliers at tuanshanbao and xintugou lott et al 1999 these deposits in turn are separated by 5 to 7 km from the newly described mineralized zone at huangjiawan mine mao et al 2002 the thicknesses of ni mo ore beds or lenses are extremely variable typically ranging from 1

yield estimation of the 25 may 2009 north korean nuclear explosion we collect nine vertical component broadband seismograms from the 25 may 2009 north korean nuclear explosion for a regional seismic network in which eight stations also recorded the 9 october 2006 north korean nuclear test comparing the observed waveforms and spectra from the two events we estimate that the amplitudes of the records from the second event are approximately five times those from the first one additionally we use 599 vertical broadband seismograms from 82 regional events recorded on the regional network between december 1995 and may 2009 to calibrate the network lg wave magnitude the calibrated network is used to calculate the lg wave body wave magnitude m b lg 4 53 for the 25 may 2009 north korean nuclear explosion based upon 15 first arrivals from the two north korea nuclear explosions the regional pn velocity in the northeast china north korea region is calculated to be 8 0 km s this result along with the regional geological structures suggests that the north korean test site nkts is located at a relatively stable continental region we thus use a modified fully coupled magnitude yield relation to estimate the explosion yield and the result shows that the yield of the 25 may 2009 north korean nuclear test is approximately 2 35 kt under the minimum burial depth assumption online material epicentral parameters computed m b lg and corrected m b lg for 82 events used in the study

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

the gold conveyor belt large scale gold mobility in an active orogen the southern alps of new zealand are part of an active collisional orogen where metamorphism hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing the southern alps are forming due to transpressional collision between continental crust fragments on the pacific and australian tectonic plates the plate tectonic rates and geometries the sources of fluid and broad scale fluid pathways in the hydrogeological system and the geochemical compositions of the torlesse terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the southern alps of new zealand can be calculated advection of a 10 km wide x 5 km deep section of torlesse rock through the orogen at tectonic rates 0 01 m yr that is then metamorphosed up to amphibolite facies causes mobilisation of over 1127 t au 10 1mt as 47000 t hg 560000 t sb and 14000 mt h2o in 1 myrs the masses of elements mobilised at the same rate along the length of the southern alps 200 km for 5 myrs would be more than 100 times greater the metals were mobilised by the metamorphic fluid produced during the orogenic processing of the torlesse terrane rocks and the concentrations of au as hg and sb in this fluid are calculated to be 0 08 711 3 and 40 mg kg respectively the mobilised metals form the orogenic gold deposits that occur in the southern alps different styles of gold deposits form contemporaneously during the active orogenesis of the southern alps including those with a fluid temperature rock temperature that may appear have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path the mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits there is clear potential for large gold deposits occurring in the yet to be uplifted parts of the southern alps if there have been efficient enough fluid focussing and metal precipitation mechanisms occurring under the southern alps

raman microspectrometry of fluid inclusions abstract for many kinds of fluid inclusions the coupling of microthermometry and raman microspectrometry is still the only viable option to obtain compositions of single fluid inclusions a review is given on the basis of 16 years of experience and helped with about 120 references of the instrumentation analytical conditions and methodology of the application of raman microspectrometry to gaseous aqueous and hydrocarbon inclusions and their daughter minerals

metal source and fluid rock interaction in the archean bif hosted lamego gold mineralization microthermometric and la icp ms analyses of fluid inclusions in quartz veins rio das velhas greenstone belt brazil the lamego orogenic gold deposit 440 742 oz gold measured reserves and 2 4 million t measured resources with an average grade of 5 71 g t au and a cut off grade of 2 15 g t au anglogold ashanti corrego do sitio mineracao s a aga personal communication 2014 is located in the 5 km long trend that includes the world class cuiaba deposit it is hosted in the neoarchean metavolcano sedimentary rocks of the rio das velhas greenstone belt quadrilatero ferrifero brazil mineralization is associated mainly with metachert banded iron formation bif and carbonaceous phyllites in the reclined lamego fold in which the cabeca de pedra orebody represents the hinge zone mineralization is concentrated in silicification zones and their quartz veins as well as in sulfide minerals product of bif sulfidation hydrothermal alteration varies according to host rock with abundant sulfide carbonate in bif and sericite chlorite in carbonaceous phyllite quartz vein classification according to structural relationships and host rocks identified three vein systems the v1 system mainly composed of smoky quartz qtz i and pyrite is extensional crosscuts the bedding plane s0 of bif and is parallel to the fold axis the v2 system of the same composition is represented by veins that are parallel to the s1 2 foliation and s0 this system is also characterized by silicification zones in the bif carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds the v3 system has milky quartz qtz ii veins which result from the recrystallization of smoky quartz located mainly in shear zones and faults these veins form structures en echelon and vein arrays the most common ore minerals are pyrite as pyrite and arsenopyrite fluid inclusion fi trapped in all quartz veins present composition in the h2o co2 ch4 nacl system fluid evolution can be interpreted in two stages i aqueous carbonic fluid trapped in qtz i of low salinity 2 equiv wt nacl and ii carbonic aqueous fluid of moderate salinity average 9 eq wt nacl hosted in qtz ii both stages are characterized by decrepitation temperatures in the range of 200 to 300 c and suggest a fluid of metamorphic origin applying an arsenopyrite geothermometer the calculated formation temperature for the cabeca de pedra orebody is 300 to 375 c the vertical intersection of the isochors allows a minimum pressure calculation of 2 6 kbar the composition of individual fis of this orebody obtained by la icp ms analyses compared with results of fis for the carvoaria velha deposit corrego do sitio lineament highlights a standard composition typical of metamorphic fluids with na k ca mg which increase or decrease in concentration as a function of salinity in both deposits trace elements vary according to fluid rock reactions and are directly related to the host rock composition the comparison of data sets of the two deposits shows that the cabeca de pedra fis have a higher enrichment in zn while cu as and sb are richer in carvoaria velha suggesting influence of the host rock geochemistry the suggested mechanisms for gold precipitation at the cabeca de pedra orebody lamego gold deposit are i hydrolysis of the carbonaceous matter of phyllite and bif affecting fo2 destabilizing sulfur complexes and enhancing gold precipitation ii replacement of bif iron carbonates by sulfides and iii continuous pressure changes that lead to silica precipitation and free gold other than playing the long recognized role of the carbonaceous phyllites as a fluid barrier the data highlight their importance as a source of metals

a carbonaceous sedimentary source rock model for carlin type and orogenic gold deposits this paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for au and as in sediment hosted orogenic and carlin type gold deposits and develops a corresponding genetic model in this two stage basin scale model gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis stage 1 and concentrated to ore grades by later hydrothermal structural or magmatic processes stage 2 in reduced continental margin basin settings organic matter sedimented under anoxic to euxinic conditions immobilizes and concentrates gold arsenic and a range of trace elements particularly v ni se ag zn mo cu u present in marine bottom waters into fine grained black mudstone and siltstone of slope and basin facies during early diagenesis gold and certain other trace elements ni se te ag mo cu pge are preferentially partitioned into arsenian pyrite that grows in the muds these processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic commonly from 5 to 100 ppb au and 10 to 200 ppm as rare organic and sulfide rich metalliferous black shales may contain up to 1 to 2 ppm au and over 1 000 ppm as present as refractory gold in arsenian pyrite and nanoparticles of free gold during late diagenesis and early metamorphism stage 2 the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations and the organic matter is cooked to bitumen under higher grade metamorphism lower greenschist facies and above arsenian pyrite in carbonaceous shales is converted to pyrrhotite these processes release gold arsenic sulfur and other elements sb te cu zn mo bi tl and pb from the source rocks to become concentrated by hydrothermal processes locally to produce gold ores in structural sites such as fold hinge zones shear or breccia zones within or above the black shale sequence la icp ms analyses of diagenetic pyrite in carbonaceous sediments both associated and not associated with gold deposits suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits application of this sedimentary source rock model enables a systematic exploration approach for sediment hosted gold deposits based on the distribution composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite

brazil s premier gold province part ii geology and genesis of gold deposits in the archean rio das velhas greenstone belt quadrilatero ferrifero orogenic gold deposits are hosted by rocks of the archean rio das velhas greenstone belt in the quadrilatero ferrifero region minas gerais state brazil one of the major gold provinces in the world the gold deposits occur at the base of the mafic ultramafic succession with the most important orebodies controlled by e w striking strike slip faults the main mineralization styles are 1 structurally controlled sulfide replacement zones in banded iron formation bif 2 disseminated sulfide minerals and gold in hydrothermally altered rocks along shear zones and 3 auriferous quartz carbonate sulfide veins and veinlets in mafic ultramafic and felsic volcanic rocks and also in clastic sedimentary rocks the most common host rocks for ore are metamorphosed oxide and carbonate facies banded iron iron rich metachert formations e g the cuiaba sao bento and raposos deposits and the lapa seca unit which is a local term for intensely carbonatized rock e g the giant morro velho mine with 450 t of contained gold metabasalts host most of the remaining gold deposits mineralogical characteristics and fluid inclusion studies suggest variations in the h2o co2 ratio of a low salinity near neutral reducing sulfur bearing ore fluid the presence of abundant ch4 rich inclusions is related to reduction of the original h2o co2 fluid via interaction with carbonaceous matter in the wallrocks oxygen fugacity was close to that of graphite saturation with variations likely to have been influenced by reaction with the carbonaceous matter carbon rich phyllites and schists which commonly bound ore bearing horizons seem to have played both a physical and chemical role in localizing hydrothermal mineral deposition microtextural studies indicate that gold deposition was mainly related to desulfidation reactions and was paragenetically coeval with precipitation of arsenic rich iron sulfide minerals carbon isotope data are compatible with dissolution of co2 from pre existing mantle derived carbonation zones and indicate fluids of metamorphic origin a major episode of hydrothermal fluid introduction into different rock types caused epigenetic gold formation and wallrock alteration at about 300 to 400 c during the late stages of regional deformation and metamorphism of the greenstone belt the age of gold mineralization is constrained to be younger than 2 698 and perhaps closer to 2 670 ma

does gold in orogenic deposits come from pyrite in deeply buried carbon rich sediments insight from volatiles in fluid inclusions the origin of volatiles in fluid inclusions was reviewed for testing the involvement at depth of carbonaceous pyritic sedimentary rocks as the source for orogenic gold mineralization fluid inclusions from selected deposits were analyzed by solid probe mass spectrometry fluids are mostly aqueous carbonic with variable amounts of n 2 ch 4 c 2 h 6 ar h 2 s h 2 and he for fluids with ch 4 and c 2 h 6 their ratios c 1 c 2 range from 2 6 to 25 5 indicating that c 2 h 6 is sourced from thermally degraded organic matter proportions of co 2 ch 4 c 2 h 6 and h 2 are highly variable and can be explained by hydrothermal reactions where c 2 h 6 is degraded to co 2 by water consumption such reactions may account for the problematic co 2 rich h 2 o poor fluids associated with some of the richest gold districts conditions needed for c 2 h 6 degradation are also fundamental for forming gold deposits such as hs enriched fluids for carrying gold and local weakly oxidizing conditions for promoting gold precipitation the c 2 h 6 content is recorded in fluids from mesoarchean to cretaceous gold deposits providing support for a general model where fluids and gold were sourced from deeply buried carbon rich and pyrite gold bearing sedimentary rocks

geology and alteration in the golden mile kalgoorlie the giant kalgoorlie gold deposit 1 200 metric tons au recovered is surrounded by a wide wall rock alteration halo 1 x 3 km this alteration primarily represents the selectively pervasive formation of carbonate chlorite and localized intense pyrite and muscovite alteration which all formed synchronously with gold mineralization the carbonate alteration halo is interpreted as epigenetic because of evidence that carbonate chlorite assemblages overprint regional metamorphic fabrics furthermore the geometry of the alteration zone boundaries is controlled by ductile shear zones formed during regional deformation and the geochemistry of all the alteration zones suggests that one fluid was responsible for all assemblages there is no evidence of intense premetamorphic carbonation in the kalgoorlie area kalgoorlie is situated in the highly auriferous eastern goldfields province of the yilgarn block within a narrow corridor rift zone that contains several large gold deposits regional metamorphism has produced upper greenschist facies assemblages but has retained original rock textures at kalgoorlie the most important gold host is the golden mile dolerite 80 of gold produced a differentiated tholeiitic sill intruded between a lower mafic ultramafic pile including the paringa basalt the next most important gold host and overlying felsic rocks and metasediments the kalgoorlie syncline is the dominant tectonic structure in the main gold producing area of the golden mile adjacent to this infold of metasediments the golden mile dolerite and paringa basalt are the site of a number of ductile shear zones which formed synchronously with the syncline these zones and the many smaller shear zones between them have provided fluid channelways throughout the golden mile and are the site of most economic gold mineralization three alteration zones have been mapped in the golden mile dolerite on the basis of the mineralogic siting of fe i e the chlorite carbonate and pyrite zones the additional actinolite zone representing the unaltered regional metamorphic mafic assemblage has been converted to chlorite zone assemblages throughout the mined areas of the golden mile the carbonate zone bleached dolerite can be traced for 2 km through the main mineralized parts of the golden mile and to the full depth of lake view mine the boundary between the chlorite and carbonate alteration zones is locally sharp and controlled by ductile shear zones but is generally strata bound and preferentially developed in the more fe rich parts of the dolerite the pyrite zone which includes all economically gold mineralized areas is the site of the strongest muscovite alteration and is most intense and continuous in ductile shear zones the chlorite zone chlorite calcite or ankerite forms from mild carbonation of actinolite zone assemblages the carbonate zone ankerite siderite is a more local expression of this carbonation even after extensive carbonate alteration relative major element abundances are little changed and the main added components are limited to co 2 s k and rb the pyrite zone i e the gold lodes has ankerite muscovite quartz albite pyrite and minor siderite bearing assemblages it represents significant addition of sulfur the three alteration zones not only reflect differing degrees of alteration intensity but are also strongly influenced by host rock fe mg and this ratio largely determines the stability of chlorite favored by low fe mg and thus the distribution of the carbonate zone favored by high fe mg progressive deformation within the shear zones reworked existing mineralization kalgoorlie fits a metamorphic replacement model of gold formation by auriferous metamorphic fluids interacting with fe rich host rocks its great size is a function of a very well developed fluid channelway system via ductile shear zones and later fault zones and the favorable chemical nature of the host rocks especially the golden mile dolerite the full width of carbonation chlorite and carbonate zones reflects overlapping alteration envelopes around the closely spaced fluid pathways the processes responsible for the formation of most of the gold at kalgoorlie were essentially similar to those at several other archean gold deposits but on a larger scale

burial metamorphism thermal maturation in cretaceous sediments of the southern benue trough and anambra basin nigeria organic matter reflectance illite crystallinity and fluid inclusion techniques have been used to evaluate burial metamorphic conditions for the lithostratigraphic successions that accumulated during the cretaceous in the southern benue trough and anambra basin of southern nigeria these successions were invaded by intrusives volcanic rocks and vein type lead zinc minerals especially in the albian cenomanian section of the abakaliki anticline the sequence includes lower maastrichtian subbituminous coals in the anambra basin in exposed cretaceous deposits on a northwest southeast section from enugu to abakaliki mean random vitrinite reflectance rom in oil ranges from 0 55 to 0 67 in the lower maastrichtian coals and shales approximately 0 91 in the coniacian shales 0 97 in the turonian shales and up to 4 31 in the albian shales a corresponding increase in the illite content of the illite smectite mixed layer clay fractions is reflected in the low values of illite crystallinity indices coupled with a decrease from 32 to 0 in the percentage of smectite from the northwest southeast maastrichtian albian section fluid inclusion pressure corrected temperatures from vein quartz in the albian shales range from 170 to 250 c the results of this work show that thermal maturation in the cretaceous successions increases from the post santonian campanian maastrichtian anambra basin into the older benue trough where strong diagenetic to anchimetamorphic i e very low grade metamorphism conditions were reached the data suggest that these sediments at the present outcrop levels originally were buried at higher maturity levels maximum erosion appears to have taken place on the axis of the abakaliki anticline the presence of bitumen in fractures and pores of the exposed maastrichtian units in the anambra basin suggests that the matured sediments generated some unknown quantity of petroleum this finding coupled with reported gas finds and some oil in previous exploration wells of the anambra basin enhances the possibilities of cretaceous targets in the downdip regions

paleoenvironments source rock potential and thermal maturity of the upper benue rift basins nigeria implications for hydrocarbon exploration abstract the upper benue rift comprising the gongola and yola basins in nigeria consist of the aptian albian bima formation the yolde formation cenomanian turonian gongila pindiga dukul formation turonian coniacian and gombe formation campanian maastrichtian to evaluate the maturity and source rocks potential vitrinite reflectance rock eval pyrolysis and infrared spectroscopy were carried out on 52 shale samples collected from boreholes mine quarries and outcrop sections in the gongola basin mean random vitrinite reflectance rom values range from 0 45 in the gombe formation to 0 69 in the pindiga formation and to 0 82 in the bima formation reflectance values in the yola basin also increase with stratigraphic age ranging from 0 73 in the dukul formation to 0 94 in the yolde formation and up to 1 37 in the bima formation total organic carbon toc values in the pindiga and gongila shales are between 0 4 to 2 4 averaging 0 75 toc contents from 0 10 to 12 9 averaging 1 2 are contained in the yolde formation of the yola basin tmax values from the pyrolysis of shales in the gongola basin range from 419 to 435 c whereas for shales in the yola basin they range from 431 to 442 c plots of hi vs tmax for kerogen classification indicate the prevalence of type iii kerogens in the gongila and pindiga shales although there are some indications of type ii iii kerogens in the yolde shales of the yola basin our preliminary data suggest that cretaceous successions in the gongola basin are thermally immature to marginally mature whereas source rocks in the yola basin are thermally mature with respect to hydrocarbon generation the predominance of type iii kerogens in the gongola basin suggest their potential to generate gas in the deeply buried sections the dukul and yolde formations with type ii iii kerogens may have generated some quantities of oil and gas in the deeper non emergent sections

u pb evidence of late archean tectono thermal activity in the southern sao francisco shield brazil the sao francisco craton in eastern brazil is one of the major shield areas in south america in the quadrilatero ferrifero area southern sector of the craton the archean rio das velhas greenstone belt is surrounded by granite gneiss terrane and is overlain by proterozoic sedimentary successions the bonfim metamorphic complex is the only area of the granite gneiss terrane adjacent to the greenstone belt that has been mapped it comprises two gneissic units tonalites and late granitic dykes which crosscut the regional north south shear fabric samples of gneiss two tonalites and a granitic dyke were dated by u pb a zircon core from the alberto flores gneiss yielded a minimum age of 2920 ma whereas the overgrowth is concordant at 2772 6 ma two tonalites from the vicinity of serra da moeda are old and a late dyke yielded an age of these data together with previously published u pb ages show that i greenstone belt volcanism was coeval with granitoid intrusion and with metamorphism of older

the interaction of migrating grain boundaries and fluid inclusions in naturally deformed quartz a case study of a folded and partly recrystallized quartz vein from the hunsruck slate germany abstract we studied the microstructure of a folded and partly recrystallized quartz vein from the hunsruck slates in germany focusing on the morphology and distribution of fluid inclusions in the old and new grains and along the different types of grain boundaries blocky vein quartz grains show undulose extinction and develop subgrains new grains form by subgrain rotation and grain boundary migration with a bimodal size distribution the old deformed grains contain numerous complex h2o co2 graphite inclusions with significant differences in fluid inclusion setting along subgrain boundaries the new grains have a lower content of h2o rich inclusions than the old grains and do not contain graphite and there is a significant difference in volume and density of fluid inclusions between the large and small new grains grain boundaries between old and new grains are irregular containing similar fluid inclusions as the old grains but no enrichment in graphite while grain boundaries between new grains are smooth and can be inclusion free or contain arrays of fluid inclusions we interpret these structures to have formed by a series of complex interactions between grain boundaries migrating at different velocity and the fluid inclusions differences in mobility and grain boundary velocity can result in different variations of drag and drop interaction while chemical differences lead to phase separation during grain boundary fluid interaction migration of grain boundaries into the old grains was accompanied by significant redistribution of fluids and graphite along the grain boundary together with oxidation of graphitic inclusions to co2

multistage mineralization at the hypozonal sao sebastiao gold deposit pitangui greenstone belt minas gerais brazil abstract sao sebastiao is a recently discovered archean gold deposit at the northwesternmost part of the quadrilatero ferrifero qf southern sao francisco craton brazil the gold ore is strata confined in two levels of banded iron formation bif of the basal stratigraphic unit in the pitangui greenstone belt 2 8 ga pitangui is correlated to the widely known rio das velhas greenstone belt which hosts several orogenic gold deposits such as the world class cuiaba the gold mineralization at sao sebastiao is associated to replacement style sulfidation over magnetite where the main developed phases are pyrrhotite chalcopyrite arsenopyrite pyrite and arsenian pyrite native au occurs as inclusions in pyrrhotite formed at dilatation domains e g breccias fold hinges during compressional events rio das velhas orogeny at 2 8 2 75 ga but mainly as inclusions in late to post kinematic pyrite and arsenopyrite as fracture infills in arsenopyrite and in contact with gangue minerals electron microprobe analyses in arsenopyrite reveal a 465 560 c precipitation temperature interpreted from as atomic percentage and indicate that high temperature fluids were active after the deformational event the temperature increase caused the incorporation of la icp ms detected lattice bound ni and co in arsenian pyrite and arsenopyrite related to the mafic ultramafic signature of boundary rocks high temperature conditions are supported by pyrite and iss intermediate solid solution in the cu fe s system formation at 600 c and melting derived textures resulting from the precipitation of bi and as melts typical of amphibolite facies environments with high fs2 whole rock geochemical assays of drill core samples display two clear trends on ag bi cu vs au binary plots and on principal component analysis results the trends are a product of the heterogeneous spatial distribution of melts and high temperature fluids melting features and superposition of mineralization styles are common in hypozonal gold deposits experiencing higher heat influx at sao sebastiao the thermal effect is likely related to the 2 7 ga granitic intrusions that surround the pitangui belt sao sebastiao shows distinct characteristics than those described for the mesozonal gold deposits in the rio das velhas greenstone belt and may depict the deeper roots of the qf gold system

modeling petroleum generation in the southern muglad rift basin sudan maturity and petroleum generation have been modeled in a 120 km long northeast southwest cross section of the southern muglad basin sudan the section passes close to the unity and kaikang areas which provide well control on the structure interpreted from gravity and seismic reflection profiles modeling of geotherms from corrected bottom hole temperatures indicates present heat flows around 60 mw m2 in the well control areas using measured vitrinite data as calibrant maturity can be modeled equally well with a constant paleoheat flow similar to the present day or with a complex heat flow pattern related to rifting a geologically realistic complex heat flow model was employed to calculate the timing of generation and expulsion from the lower cretaceous lacustrine source rocks of the abu gabra and sharaf formations kinetic easy ro calculation of vitrinite maturity indicates a projected oil preservation depth limit around 4000 m shallowing to around 3500 m northeast of unity near the basin margin

interpreting rock eval pyrolysis data using graphs of pyrolizable hydrocarbons vs total organic carbon kerogen the major organic component of sedimentary rocks is commonly analyzed by rock eval pyrolysis plotting the data on a graph of s 2 vs total organic carbon toc and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content with the s 2 vs toc diagram the organic component of different suites of samples may be compared and their petroleum generation potentials established as an example the diagrams are used to evaluate the sedimentary environments and petroleum potential of the paleogene evaporitic sediments of the mulhouse basin alsace france

depositional and structural styles in chad basin of northeastern nigeria seismic reflection data in the maiduguri area of the chad basin reveal a significant juxtaposition of faults and folds the primary structural orientation of which is northeast southwest and parallel to the surface structural pattern in the contiguous benue trough the faults in the basin consist of both basement involved faults and detached faults the folds are simple symmetrical structures restricted to the deeper parts of the basin concomitant compressive structures are notably scarce the structural pattern is complicated further by the presence of intrusions that are presumed to be the subsurface extension of volcanic outcrops that rim the southeastern part of the maiduguri subbasin seismic reflection data also reveal distinguishable seismic sequences that correl te with the established stratigraphic sequence of the basin the apparent structure and the seismically derived stratigraphy suggest a rift origin for the basin furthermore the seismic stratigraphy sedimentologic implications and seismically determined structural features suggest that the maiduguri area of the chad basin has good prospects for hydrocarbon plays in cretaceous rocks with high potentials for both structural and stratigraphic traps

macflincor and its application to fluids in archean lode gold deposits abstract macflincor was developed to process laboratory data gathered on fluid inclusions and to calculate isochores for geologically important fluids composed of h2o c02 ch4 nacl and n2 within the program interactive diagrams are available to describe those chemical systems which cannot be adequately constrained numerically the user can choose from various published equations of state describing fluid behavior and can easily compare the results obtained by using different equations the program once the basic fluid parameters have been established allows the data to be used to determine the p t trapping conditions to evaluate possible pressure corrections and to compare different depths of formation based on the pressure calculations and given geothermal gradients for illustrative purposes macflincor has been utilized here to constrain different physical chemical parameters of fluid inclusions trapped in ore related quartz veins and breccias of archean lode gold deposits in western australia inferred pressures from individual deposits show a wide range of values which may be due to post entrapment modifications and or observational uncertainties including estimation of liquid vapor ratios

sedimentology organic geochemistry and petroleum potential of jurassic coal measures tarim junggar and turpan basins northwest china lower and middle jurassic coal bearing strata occur widely throughout central asia and are well developed in northwestern china where their thicknesses in the southern junggar northern tarim and turpan basins exceed 2500 2300 and 1500 m respectively examination of these strata along 13 transects across basin margin outcrop belts indicates that they are entirely nonmarine meandering fluvial deposits with local development of braided fluvial and lacustrine deltaic facies chinese subsurface data suggest that regional jurassic lacustrine facies are present down depositional dip consistent with predictions from global circulation modeling of early and middle jurassic monsoonal precipitation laboratory analyses of coals and organic rich shales show a dominance of terrestrial higher plant components visual kerogen analysis indicates that vitrinite inertinite and exinite are the dominant macerals and elemental analysis characterizes most kerogens as type iii rock eval analyses yield moderate hydrogen index values 50 300 and very low oxygen index values 20 jurassic source rock extracts are characterized by odd over even normal alkane distributions high pristane phytane and high hopane sterane ratios dominance of c29 sterane homologs local abundance of diterpenoid compounds and low abundance of tricyclic terpanes geochemical correlation with four petroleums from the junggar tarim and turpan basins strongly suggests that the jurassic coaly deposits and their lacustrine equivalents downdip are petroleum source rocks sterane and hopane distributions of petroleums and extracts of their putative jurassic source rock are similar and can be easily distinguished from published distributions of these compounds in other source rock layers additional correlation parameters include high pristane phytane low abundance or lack of tricyclic terpanes but similar distributions where present and lack of gammacerane with one exception and carotanes compounds that characterize permian and ordovician source rocks and their respective petroleums pyrolysis gas chromatography of selected jurassic samples s ggests that they possess potential for liquid hydrocarbon generation expulsion of c15 hydrocarbons from jurassic source rocks appears likely despite the traditional view that bituminous coals are incapable of expelling long chain hydrocarbons end page 929

petroleum geology of the southern bida basin nigeria the southern bida basin is located in central nigeria and is a major sedimentary area with a 3 5 km thick sedimentary fill however it is the least understood of nigeria s sedimentary basins because serious oil and gas exploration has not been undertaken in the basin the surrounding precambrian basement rocks experienced severe deformation during the late panafrican phase 600 plus minus 150 m y and developed megashears that were reactivated during the late campanian maestrichtian the ensuing wrenchfault tectonics formed the basin the sedimentary fill which comprises the lokoja formation are chiefly if not wholly nonmarine clastics these have been characterized into facies that rapidly change from basin margin to basin axis and have undergone only relatively mild tectonic distortion subsurface relations of the lokoja formation are postulated from outcrop study the potential source rocks are most likely within the basinal axis fill and have not been deeply buried based on vitrinite reflectance of 0 65 these findings with the largely nonmarine depositional environment suggest gas and condensate are the most likely hydrocarbons alluvial fans and deltaic facies that interfinger with lacustrine facies provide excellent reservoir capabilities potential traps for hydrocarbon accumulation were formed by a northwest southeast trending campanian maestrichtian wrench system with associated northeast southwest oriented normal faults more the traps include strata in alluvial fans fractured uplifted basement blocks and arched strata over uplifted blocks however the size of hydrocarbon accumulations could be limited to some extent by a lack of effective hydrocarbon seal because the dominant seals in the formation are unconformities less

metabasalts as sources of metals in orogenic gold deposits although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits the mobility of au and related elements as sb se and hg from these rocks during alteration and metamorphism is poorly constrained we investigate the effects of increasing metamorphic grade on the concentrations of au and related elements in a suite of metabasaltic rocks from the otago and alpine schists new zealand the metabasaltic rocks in the otago and alpine schists are of morb and wpb affinity and are interpreted to be fragments accreted from subducting oceanic crust gold concentrations are systematically lower in the higher metamorphic grade rocks average au concentrations vary little between sub greenschist 0 9 0 5 ppb and upper greenschist facies 1 0 0 5 ppb but decrease significantly in amphibolite facies samples 0 21 0 07 ppb the amount of au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in otago arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element the concentrations of sb and hg decrease between sub greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered morb protoliths and therefore unaltered oceanic crust cannot be a net source of sb and hg in a metamorphic environment the concentrations of au as sb and hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism we suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles

geochronological framework of the quadrilatero ferrifero with emphasis on the age of gold mineralization hosted in archean greenstone belts abstract archean terrains of the quadrilatero ferrifero comprise a greenstone belt association surrounded by granitoid gneiss complexes mainly composed of banded ttg gneisses whose igneous protoliths are older than 2900 ma this early continental crust was affected by three granitic magmatic episodes during the neoarchean ca 2780 to 2760 ma 2720 to 2700 ma and 2600 ma dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 ma contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the quadrilatero ferrifero lead isotopic studies of lode gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 ma proterozoic evolution of the quadrilatero ferrifero comprises deposition of a continental margin succession hosting thick lake superior type banded iron formations at ca 2500 to 2400 ma followed by deposition of syn orogenic successions after 2120 ma the latter is related to the transamazonian orogeny the western part of the quadrilatero ferrifero was also affected by the brasiliano orogeny 600 to 560 ma

fluid inclusion and sulfur and oxygen isotope studies on quartz carbonate sulfide veins of the carvoaria velha deposit corrego do sitio gold lineament quadrilatero ferrifero minas gerais brazil abstract the corrego do sitio lineament is defined as a 16 km long ne sw trending ductile shear zones which controls fourteen gold deposits including the carvoaria velha deposit the dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the archean rio das velhas greenstone belt which host gold mineralization metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence all these rocks exhibit alteration to quartz carbonate and sericite besides sulfides and sulfosalts the main gold mineralization styles at the carvoaria velha deposit and at corrego do sitio as a whole are quartz carbonate sulfide sulfosalt veins of varied distribution the veins are classified as v1 veins quartz ankerite pyrite berthierite gold parallel to the main regional foliation s n v2 veins quartz ankerite pyrite developed at extensional crenulation cleavage s n 1 and rarely gold mineralized v3 veins quartz ankerite filling s n 3 fractures usually free of sulfides and sulfosalts v4 veins quartz calcite of restricted occurrence in metamafic dikes and sills the latter crosscut the metasedimentary sequence are extensional and display no preferential orientation the most common ore minerals in all vein types are arsenopyrite pyrite berthierite and pyrrhotite microprobe analyses reveal the presence of metallic phases rich in sb pb and co such as stibnite ullmanite tetrahedrite galena cobaltite which commonly overgrow the sulfides fluid inclusion studies trapped in quartz from v1 v2 and v4 veins have identified a h 2 o co 2 ch 4 nacl mineralizing fluid that may contain kcl mgcl 2 e fecl 2 the presence of ch 4 in fluid inclusions of the v1 and v2 veins reflect interaction of the fluid with the corrego do sitio and santa quiteria carbon bearing metapelitic host units based on the microthermometric data the hydrothermal fluid is interpreted to have evolved in at least two stages i an early fluid stage trapped in smoky quartz of moderate salinity 8 5 eq wt nacl and minimal trapping temperature of 330 17 c and ii a late stage fluid trapped in recrystallized quartz with lower salinity 4 6 eq wt nacl and a minimal trapping temperature of 365 33 c isotopic values of 18 o fluid 7 9 and 13 0 33 s 0 3 to 3 5 and 34 s 2 9 to 6 1 suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the carvoaria velha deposit must have had a metamorphic origin and interacted with metasedimentary sequences during its ascending path the addition of ch 4 during fluid rock interaction may have caused some decrease in o 2 of the fluid which as a consequence destabilized gold bearing sulfur complexes liberating s 2 for the formation of fe sulfides and sulfosalts arsenopyrite pyrite pyrrothite and berthierite etc and outcome gold deposition

bulk microanalysis of assemblages of small fluid inclusions by la icp ms methodology and application to orogenic gold systems abstract microanalysis of individual fluid inclusions by laser ablation inductively coupled mass spectrometry la icp ms is a powerful tool for reconstructing the composition of hydrothermal fluids but it demands a sample quality that is unattainable in many cases in orogenic gold deposits the need for direct fluid microanalysis has been present for several decades but due to the high fluid flux and prolonged hydrothermal and tectonic history that typifies these systems most samples do not meet the criteria of fluid inclusion size and distribution that allow la icp ms analysis of individual isolated fluid inclusions to overcome this difficulty a method has been developed and tested whereby areas of quartz densely populated with fluid inclusions e g growth zones secondary planes are analyzed along a single continuous laser ablation profile the generated signals are subsequently converted to time slice datasets and plotted as element ratios in ternary diagrams to reconstruct specific major and trace element ratios the estimated fluid compositions are in good agreement with previous analytical results of the same material microthermometry evaporate mound and conventional la icp ms and are shown to be geologically viable on a boarder scale when compared to literature data from similar ore environments the method has high spatial and chemical resolution which allows the reconstruction of micro scale fluid chemical changes such as significant fluctuation in the relative element concentration e g k rb ba sr and v versus na li and as during crystal growth as observed in one of the test samples the significance of this bulk la protocol is that it allows the quick easy and cost effective microanalysis of samples that are typically inundated with fluid inclusions such as those in orogenic and epithermal systems which would otherwise not be amenable for further quantitative analysis to constrain fluid chemistry

structural control and hydrothermal alteration at the bif hosted raposos lode gold deposit quadrilatero ferrifero brazil abstract in the raposos orogenic gold deposit hosted by banded iron formation bif of the archean rio das velhas greenstone belt the hanging wall rocks to bif are hydrothermally altered ultramafic schists whereas metamafic rocks and their hydrothermal schistose products represent the footwall planar and linear structures at the raposos deposit define three ductile to brittle deformational events d 1 d 2 and d 3 a fourth group of structures involve spaced cleavages that are considered to be a brittle phase of d 3 the orebodies constitute sulfide bearing d 1 related shear zones of bif in association with quartz veins and result from the sulfidation of magnetite and or siderite pyrrhotite is the main sulfide mineral followed by lesser arsenopyrite and pyrite at level 28 the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping bif define a gross zonal pattern surrounding the ore zones metabasalt comprises albite epidote actinolite and lesser mg fe chlorite calcite and quartz the incipient stage includes the chlorite and chlorite muscovite alteration zone the least altered ultramafic schist contains cr bearing mg chlorite actinolite and talc with subordinate calcite the incipient alteration stage is subdivided into the talc chlorite and chlorite carbonate zone for both mafic and ultramafic wall rocks the carbonate albite and carbonate muscovite zones represent the advanced alteration stage rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock in the case of the ultramafic schists the precursor may have been peridotitic komatiite the eu anomaly of the raposos bif suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor the ore fluid composition is inferred by hydrothermal alteration reactions indicating it to having been h 2 o rich containing co 2 na and s since the distal alteration halos are dominated by hydrated silicate phases mainly chlorite with minor carbonates fixation of h 2 o is indicated the co 2 is consumed to form carbonates in the intermediate alteration stage in halos around the chlorite dominated zones these characteristics suggest variations in the h 2 o to co 2 ratio of the sulfur bearing aqueous carbonic ore fluid which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration