discriminating fluid source regions in orogenic gold deposits using b isotopes abstract the genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions however there is considerable debate as to the ultimate source of these fluids and the metals they transport tourmaline is a common gangue mineral in orogenic gold deposits it is stable over a very wide p t range demonstrates limited volume diffusion of major and trace elements and is the main host of b in most rock types we have used texturally resolved b isotope analysis by secondary ion mass spectrometry sims to identify multiple fluid sources within a single orogenic gold ore district the loulo mining district in mali west africa hosts several large orogenic gold ore bodies with complex fluid chemistry associated with widespread pre ore na and multi stage b metasomatism the gara deposit as well as several smaller satellites formed through partial mixing between a dilute aqueous carbonic fluid and a hypersaline brine hydrothermal tourmaline occurs as a pre ore phase in the matrix of tourmalinite units which host mineralization in several ore bodies clasts of these tourmalinites occur in mineralized breccias disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation tourmalines show a large range in 11 b values from 3 5 to 19 8 which record a change in fluid source between paragenetic stages of tourmaline growth pre mineralization tourmaline crystals show heavy 11 b values 8 19 8 and high x site occupancy na ca 0 69 1 apfu suggesting a marine evaporite source for hydrothermal fluids syn mineralization and replacement phases show lighter 11 b values 3 5 to 15 1 and lower x site occupancy 0 62 0 88 apfu suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments the large overlapping range in isotopic compositions and a skew toward the opposing population in the 11 b data for both tourmaline groups reflects continual tourmaline growth throughout mineralization which records the process of fluid mixing a peak in 11 b values at 8 largely controlled by tourmalines of syn to post ore timing represents a mixture of the two isotopically distinct fluids this paper demonstrates that b isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems the importance of b as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes b isotope analysis a powerful tool for testing the level of source region variability in these fluids and by extension that of metal sources

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

slab derived fluids and quartz vein formation in an accretionary prism otago schist new zealand regional quartz vein formation and the fluxes flow paths and sources of metamorphic fluids were investigated in the mesozoic accretionary prism of new zealand by using a new chemical mass balance analysis of outcrops samples were collected at meter or submeter intervals along outcrop length traverses and combined to obtain average chemical compositions of whole outcrops mass balance analysis used zr as an immobile reference frame and as a monitor of sedimentary sorting processes sio2 zr systematics produced by sedimentary processes differ greatly from those caused by metasomatic mass transfer of silica allowing evaluation of vein formation mechanisms relatively undeformed metasedimentary outcrops of low metamorphic grade mostly prehnite pumpellyite facies are nearly unveined and characterized by sedimentary compositional trends more deformed outcrops of higher metamorphic grade mostly greenschist facies contain 10 30 vol quartz veins these outcrops underwent mass addition of externally derived silica into quartz veins accompanied by addition of na and removal of k and w average silica additions suggest a time integrated fluid flux of 104 105 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 3 fluid end document batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 2 rock end document for fluids ascending through the prism dehydration of spilitized oceanic crust subducting beneath the prism is the most probable source for this large fluid flux and could also have caused the na k metasomatism the w removed from deep levels of the prism may have been deposited in focused retrograde au w quartz veins at shallow levels by ascending fluids transfer of sio2 from subducting slabs into accretionary prisms is a plausible mechanism for long term bulk silica enrichment of the continents beyond that possible by magmatic differentiation

boron isotope fractionation between tourmaline and fluid an experimental re investigation the fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700 c at 200 mpa and at 500 c 500 mpa tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess in one series of experiments b fluid b tour was 9 after the run in another series it was 0 1 all experiments produced tourmaline as the sole boron bearing solid along with traces of quartz and talc powder xrd and rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline 11b always preferentially fractionated into the fluid for experiments where b fluid b tour was 9 a consistent temperature dependent boron isotope fractionation curve resulted approximated by 11b tour fluid 4 20 1 000 t k 3 52 r2 0 77 and valid from 400 to 700 c no pressure dependence was observed the fractionation 2 7 0 5 at 400 c and 0 8 0 5 at 700 c is much lower than that previously presented by palmer et al 1992 experiments where b fluid b tour was 0 1 showed a significant larger apparent fractionation of up to 4 7 in one of these runs the isotopic composition of handpicked tourmaline crystals of different size varied by 1 3 this is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline thus indicating larger fractionation than observed at equilibrium the effect is eliminated or minimized in experiments with very high boron excess in the fluid we therefore suggest that values given by the above relation represent the true equilibrium fractionations

geodynamic settings for paleoproterozoic gold mineralization in the svecofennian domain a tectonic model for the faboliden orogenic gold deposit northern sweden abstract northern sweden is currently experiencing active exploration within a new gold ore province the so called gold line situated southwest of the well known skellefte vms district the largest known deposit in the gold line is the hypozonal faboliden orogenic gold deposit mineralization at faboliden is hosted by arsenopyrite rich quartz veins in a reverse mainly dip slip high angle shear zone in amphibolite facies supracrustal host rocks the timing of mineralization is estimated from field relationships at ca 1 8 ga the gold mineralization is hosted by two sets of mineralized quartz veins one steep fault fill vein set and one relatively flat lying extensional vein set ore shoots occur at the intersections between the two vein sets and both sets could have been generated from the same stress field during the late stages of the svecofennian orogen the tectonic evolution during the 1 9 1 8 ga svecofennian orogen is complex as features typical of both internal and external orogens are indicated the similarity in geodynamic setting between the contemporary svecofennian and trans hudson orogens indicates a potential for world class orogenic gold provinces also in the svecofennian domain the swedish deposits discussed in this paper are all structurally associated with roughly n s striking shear zones that were active at around 1 8 ga when gold bearing fluids infiltrated structures related to conditions of e w shortening

tourmaline b isotopes fingerprint marine evaporites as the source of high salinity ore fluids in iron oxide copper gold deposits carajas mineral province brazil the carajas mineral province in northern brazil contains a variety of world class 100 mt ore iron oxide copper gold iocg deposits including the only archean examples of this deposit class e g the igarape bahia alemao and salobo deposits tourmaline of schorldravite composition a common gangue mineral in these deposits precipitated shortly prior to and after the ore assemblage a boron isotope study of texturally different tourmaline from three iocg deposits igarape bahia salobo and sossego using secondary ion mass spectrometry sims provides new evidence in the long standing debate of magmatic versus non magmatic sources for the high salinity up to 50 wt nacl equiv of ore fl uids in these deposits values of 11 b from 14 to 26 5 for the igarape bahia and salobo deposits confi rm marine evaporite derived brines in the ore fl uids whereas lower 11 b values for the igarape bahia deposit 5 8 to 8 8 suggest that these fl uids may have mixed with an isotopically different hydrothermal fl uid or one that had a mixed boron source more variable and isotopically lighter boron in tourmaline from the sossego deposit 8 to 11 is attributed to mixed sources including light boron leached from felsic intrusive and volcanic host rocks and heavy boron derived from marine evaporites the boron isotope data indicate that the characteristic high salinity of the ore fl uids in the carajas mineral province was acquired by the interaction of hydrothermal fl uids with marine evaporites for iocg deposits that contain tourmaline as a common gangue mineral boron isotopes offer a valuable tool to constrain the high salinity source problem which is a critical issue in metallogenesis of iocg deposits worldwide

physical chemical properties and metal budget of au transporting hydrothermal fluids in orogenic deposits we use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits the most common ore fluid is a low salinity 0 4 6 5 wt nacl twoto three phase aqueous carbonic fluid its th total range is 200 400 8c and total homogenization occurs by bubble or liquid disappearance a less common aqueous fluid of similar bulk salinity and t h total is also documented microchemical data show that the ore fluid in the five deposits is chemically uniform and made of na subordinate k and b and minor to trace amounts of cu as li sr rb ba cs sb and au range 0 5 5 mg g thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition and that its vapour phase was distinctly enriched in au b as and sb the proposed mechanism of gold precipitation is a combination of fluid decompression and boiling combining these results with 3d geometrical reconstructions of one of the studied deposits sigma we estimate that a minimum ore fluid volume of c 0 1 1 0 km is sufficient to generate a world class orogenic deposit orogenic deposits provide about a third of the global gold production goldfarb et al 2001 frimmel 2008 these deposits are formed within active continental margins in orogenic belts where major tectonic boundaries juxtapose metamorphosed volcanic plutonic and sedimentary terranes the ore bodies consist of au mineralized vein networks filled with quartz carbonates and other silicates that in places crosscut the stacked terranes of the continental margins the veins are dilated portions of vertically extensive brittle ductile to ductile fracture networks that form due to the advection of aqueous hydrothermal fluids within the continental crust ridley et al 1996 groves et al 2003 these ore fluids are generated in still undefined source regions deep in the crust and transported to shallower levels 3 15 km depth through the vein networks along which they deposit gold a number of studies kerrich et al 2000 ridley diamond 2000 groves et al 2003 showed that although these networks do not occur systematically in all orogens they have a set of important geological features in common which include 1 formation during accretionary orogenic events 2 occurrence relatively close to regional tectonic lineaments especially along lower order splays of these structures 3 spatial association with granitoid intrusions 4 occurrence within some metamorphic belts with gold precipitation being synto postmetamorphic in age 5 relatively low sulphide abundance within the veins 6 carbonate sericitealbite sulphide hydrothermal alteration of host rocks postdating metamorphism 7 poorly developed mineral and metal zoning within the ore bodies and 8 genesis from low to moderate salinity ore fluids over a large temperature range 250 700 8c but mostly within 300 350 8c these common traits occur systematically in deposits from different terrains having distinct ages suggesting that a common geological process and au transporting fluid are responsible for their formation here we present a database of fluid inclusion properties from five typical orogenic deposits which show most of the geological characteristics listed above the database was constructed with the aim of determining as accurately as possible the properties of the au transporting fluid in this geological environment in particular laser ablation inductively coupled plasma mass spectrometry la icp ms of single fluid inclusions paragenetically coeval with vein gold provides the concentration of a suite of metals including au in this fluid this dataset when combined with 3d geometrical reconstructions of one of these deposits offers new constraints on the volume of the ore from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 8 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics by guest on april 3 2014 http sp lyellcollection org downloaded from

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

flash propagation and inferred charge structure relative to radar observed ice alignment signatures in a small florida mesoscale convective system a series of vertical cross sections taken through a small mesoscale convective system mcs observed over florida by the dual polarimetric smart radar were combined with vhf radiation source locations from a lightning mapping array lma to examine the lightning channel propagation paths relative to the radar observed ice alignment signatures associated with regions of negative specific differential phase kdp additionally charge layers inferred from analysis of lma sources were related to the ice alignment signature it was found that intracloud flashes initiated near the upper zero kdp boundary surrounding the negative kdp region the zero kdp boundary also delineated the propagation path of the lightning channel with the negative leaders following the upper boundary and positive leaders following the lower boundary very few lma sources were found in the negative kdp region we conclude that rapid dual polarimetric radar observations can diagnose strong electric fields and may help identify surrounding regions of charge

reactivity of evaporites during burial an example from the jurassic of alabama the jurassic louann salt was the first significant sedimentary unit to accumulate in the gulf of mexico sedimentary basin br cl and sup 87 sr sup 86 sr ratios of halite from a single core into the top of the formation record the evaporation of normal seawater to bittern stage the bittern zone today consists of intergrown halite and sylvite the br and rb contents of the solid phases along with sup 87 sr sup 86 sr ratios and rb sr systematics are inconsistent with precipitation of the existing phases from seawater evaporated in jurassic time rather petrography and fluid inclusion and solid phase chemistry from the bittern zone is consistent with postdepositional water rock interaction which diagenetically modified a marine bittern assemblage to halite sylvite the chemistry of the br and rb rich saline formation waters characteristic of this area today likewise may reflect water evaporite interaction during burial

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

surface water influx in shallow level archean lode gold deposits in western australia archean shallow level lode gold deposits in western australia are located in rocks of prehnite pumpellyite to lower greenschist facies and are characterized by 1 predominantly brittle shear zones that formed well after volcanism and late in the structural history 2 breccia hosted lodes 3 low temperature ore and gangue minerals such as silver sulfosalts stibnite and precursor chalcedony and 4 vertical metal zonation of the orebodies ore fluid temperatures ranged from 150 to 325 c and a depth of em placement of 5 km is inferred isotopic analyses of alteration minerals such as quartz sericite and chlorite indicate higher dfluid 6 9 relative to smow 1 n 19 and lower 18ofluid 4 4 2 3 relative to smow 1 n 167 values compared to such values for hydrothermal fluids that deposited au in other archean lode gold deposits with similar structural timing the fluid inclusion and stable isotope data and the presence of chalcedony in this deposit style are best explained by the influx of surface water at shallow levels and interaction of that water with advecting metamorphic magmatic fluids

metamorphosed plio pleistocene evaporites and the origins of hypersaline brines in the salton sea geothermal system california fluid inclusion evidence abstract the salton sea geothermal system ssgs occurs in plio pleistocene deltaic lacustrine evaporite sediments deposited in the salton trough an active continental rift zone temperatures up to 365 c and hypersaline brines with up to 26 wt tds are encountered at 1 3 km depth in the sediments which are undergoing active greenschist facies hydrothermal metamorphism previous models for the origins of the na ca k cl brines have assumed that the high salinities were derived mainly from the downward percolation of cold dense brines formed by low temperature dissolution of shallow non marine evaporites new drillcores from the central part of the geothermal field contain metamorphosed bedded evaporites at 1 km depth consisting largely of hornfelsic anhydrite interbedded with anhydrite cemented solution collapse shale breccias fluid inclusions trapped within the bedded and breccia cementing anhydrite homogenize at 300 c identical to the measured downhole temperature and contain saline na ca k cl brines some of the inclusions contain up to 50 vol halite sylvite and carbonate crystals at room temperature and some halite crystals persist to above 300 c upon laboratory heating the data are consistent with the trapping of halite saturated na ca k cl fluids during hydrothermal metamorphism of the evaporites and accompanying solution collapse of interbedded shales we conclude that many of the salt crystals in inclusions are the residuum of bedded evaporitic salt that was dissolved during metamorphism by heated connate fluids therefore the high salinities of the salton sea geothermal brines are derived in part from the in situ hydrothermal metamorphism and dissolution of halides and caso 4 from relatively deeply buried lacustrine evaporites this fact places important constraints on modeling fluid flow in the ssgs as brines need not have migrated over great distances the brines have been further modified to their present complex na ca k fe mn cl compositions by on going sediment metamorphism and water rock interaction

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

geochemical evolution of tourmaline in the darasun gold district transbaikal region russia evidence from chemical and boron isotopic compositions the darasun gold district transbaikal region eastern russia comprises three deposits teremkyn talatui and darasun where gold bearing quartz veins are hosted in metagabbro and granitoids tourmaline is a common gangue mineral in these deposits and a useful indicator of fluid source the tourmaline compositions are oxy dravite povondraite dravite and schorl we report here in situ b isotope analyses by secondary ion mass spectrometry sims on tourmaline from veins in metagabbro and k rich granodiorite as well as from a breccia pipe at the margin of granodiorite porphyry the b isotope composition of tourmalines from the darasun gold district as a whole covers a very wide range from 15 7 to 11 2 with distinctive differences among the three deposits the 11b values in the teremkyn tourmalines are the most diverse from 15 7 to 2 5 tourmaline core compositions yield an inferred 11b value of the initial fluid of ca 12 suggesting granitic rocks as the b source whereas the heavier rims and late stage grains reflect rayleigh fractionation the 11b values of tourmaline from talatuiare 5 2 to 0 9 taking into account fluid inclusion temperatures from vein quartz ca 400 c the inferred 11b value of fluid is heavy 2 5 to 2 2 suggesting a b source from the host metagabbro at the darasun deposit tourmaline from the breccia pipe is isotopically uniform 11 b 6 to 5 and suggested to have precipitated from a 10b depleted residual fluid derived from granitic rocks the darasun vein hosted tourmalines i and ii 11b from 4 4 to 1 5 may have crystallized from strongly fractionated residual granitic fluid although mixing with heavy boron from the metagabbro rocks probably occurred as well the boron isotopic composition of tourmaline iii 11 2 is attributed to a less fractionated fluid possibly a recharge from the same source

hydrothermal gold mineralization at the hira buddini gold mine india constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline we determined the boron isotope and chemical compositions of tourmaline from the hira buddini gold deposit within the archean hutti maski greenstone belt in southern india to investigate the evolution of the hydrothermal system and to constrain its fluid sources tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization the hira buddini tourmaline belongs to the dravite schorl series with variations in al fe fe mg ca ti and cr contents that can be related to their host lithology the total range of 11b values determined is extreme from 13 3 to 9 0 but 95 of the values are between 4 and 9 the boron isotope compositions of metabasalt hosted tourmaline show a bimodal distribution with peak 11b values at about 2 and 6 the wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources which entered the hydrothermal system separately and were subsequently mixed the estimated 11b values of the hydrothermal fluids based on the peak tourmaline compositions and a mineralization temperature of 550 c are around 1 and 10 the isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies whereas the 11b rich fluid is attributed to degassing of i type granitic magmas that intruded the greenstone sequence providing heat and fluids to the hydrothermal system

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

mineral precipitation in the quartz reefs of the bendigo gold deposit victoria australia gold precipitation at bendigo postdates cleavage development and was initiated during folding increased during a reverse faulting stage in bedding parallel quartz veins and culminated in quartz reefs associated with a strike slip faulting event some gold was remobilized along carbonate rich cataclasites associated with later brittle faults understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis from source to sink in this study a petrographic approach closely tied to macroscopic structural observations along with stable isotope and fluid inclusion analyses places constraints on the relative timing of gold mineralization with respect to deformation early sulfide assemblages in bedding and cleavage parallel veins are dominated by pyrite pyrrhotite siderite and are free of visible gold later assemblages in reactivated bedding parallel veins and other fold related veins are characterized by the presence of arsenopyrite ankerite gold fault related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold but lack pyrrhotite small amounts of late stage antimony bearing minerals occur in many vein types and postdate the precipitation of gold decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins stable oxygen isotope data show that quartz from all vein types 18 o 15 9 19 0 homogenized with the host rocks whereas carbon 13 c 14 0 to 4 1 and oxygen 18 o 4 5 24 0 in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature this corresponds with early siderite pyrrhotite and anhedral pyrite and with later ankerite and ferroan dolomite associated with arsenopyrite galena and euhedral pyrite that are crosscut by calcite veins fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide with smaller proportions of nitrogen and methane particularly in the later strike slip faults the sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the bendigo orogenic gold deposits we believe there is little direct or deposit scale evidence for a relationship between original metal content in the host rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites it is suggested that deep seated faults act as conduits for fluid flow and the source for the gold at bendigo the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites

boron isotopic composition of marine and nonmarine evaporite borates abstract here we present the results of the boron isotopic analysis of borates from some well known marine and nonmarine evaporite deposits combining new analyses with analyses from the literature yields a mean 11 b value for nine marine evaporite borate analyses of 25 4 permil 11 b 10 b 4 15 0 02 and a mean for twenty five nonmarine evaporite borates of 7 10 permil 11 b 10 b 4 02 0 04 previous studies have shown that the boron isotopic composition of seawater 11 b 40 permil 11 b 10 b 4 20 is about four percent heavier than the boron isotopic composition of most crustal rocks and minerals we suggest that our results for marine and nonmarine evaporite borates reflect this difference thus it should be possible to use boron isotopic composition as a tracer for boron of marine evaporitic origin applications include borate bearing evaporite deposits and perhaps borate and or borosilicate bearing metamorphic rocks

genesis of the auriferous c quartz tourmaline vein of the siscoe mine val d or district abitibi subprovince canada structural mineralogical and fluid inclusion constraints the c quartz tourmaline vein of the siscoe deposit is a classic example of a high grade archean gold lode and averages 45 g t au locally attaining 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear vein which formed during the development of a late to post d2 reverse fault the vein is composed of alternating tourmaline and quartz rich layers with minor amounts of rutile scheelite apatite chlorite muscovite carbonate mainly calcite pyrite chalcopyrite tetradymite and native gold vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening mineral deposition and plastic and brittle deformation the vein geometry was controlled by local rheologic heterogeneities or by a west northwest east southeast compressive event the wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite the timing of gold deposition and the gold fineness in the c quartz tourmaline vein differ from those of the wall rock in the vein native gold 4 7 7 3 wt ag was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers whereas in the wall rock gold 7 9 13 2 wt ag started to precipitate just before or synchronously with pyrite consequently the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments in the wall rock gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite this reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition however sulfides are very rare in the c quartz tourmaline vein and gold deposited after the main episode of vein filling suggesting that its precipitation was not controlled by pyrite deposition the occurrence of aqueous carbonic ac fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore forming process two types of aqueous carbonic fluid inclusions were identified h2o nacl co2 bearing salinities between 4 and 7 wt nacl equiv and h2o nacl co2 ch4 bearing salinities varying between 7 and 9 wt nacl equiv both types homogenize at t 190 c based on structural textural and fluid inclusion evidence we propose that gold deposition in the c quartz tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids

orogenic gold mineralization at the chah bagh deposit muteh gold district iran abstract the chah bagh gold deposit in the muteh gold district is located in the central part of the sanandaj sirjan zone ssz iran gold mineralization at chah bagh is hosted by a paleozoic sequence of rocks that is dominated by deformed schist metarhyolite and amphibolite that exhibits a greenschist to lower amphibolite facies metamorphism three deformation events are recognized in the chah bagh area d1 d2 and d3 the major nw trending n280 n290 dextral strike slip shear zone in the area was formed during d2 ductile events gold mineralization at chah bagh occurred over a prolonged deformation history but is closely related to alteration retrograde greenschist facies assemblages and ductile and brittle deformation during d2 and d3 the geometry of the au bearing quartz veins indicates that they are temporally related to the s2 foliation and therefore to the d2 flattening and shearing some veins however are spatially and temporally related to d3 brittle normal faults and are brecciated and boudinaged during the associated shear movement the presence of deformed au bearing quartz veins and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone indicates continuous mineralization during the d2 and d3 episodes the au hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones this gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein hosted au mineralization the epigenetic au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for chah bagh deposit similar to qolqoleh and kervian in the northwestern part of the ssz this model is consistent with a dextral transpressional deformation kinematic model along the ssz and refutes the previous intrusion related model suggested for the muteh gold deposits metamorphic devolatilization and fluid flow within a ca 30 m y long period in late cretaceous and early tertiary is suggested for the genesis of the gold occurrences in both ductile and post transpressional brittle structures

chemical and boron isotopic compositions of tourmaline from the archean big bell and mount gibson gold deposits murchison province yilgarn craton western australia abstract tourmaline is a commonly found gangue mineral in archean greenstone belt lode gold deposits worldwide in this paper we report major trace and rare earth element ree and boron isotopic compositions of tourmaline from two major amphibolite grade archean gold deposits big bell and mount gibson murchison province yilgarn craton western australia in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization tourmalines in these two major deposits all belong to the dravite schorl series and tend to be mg rich where closely associated with gold and sulfide ores the ore related tourmalines also contain high concentrations of ore metals such as au ag pb and cu in the big bell deposit tourmalines occur along the foliation in lode schists and show low fe fe mg ratios of 0 11 0 30 tio 2 contents of 0 15 0 73 wt and negligible mno 11 b values of 17 7 to 15 0 in the mount gibson deposit the tourmalines closely associated with the sulfide ores are the most mg rich with low fe fe mg ratios of 0 19 0 22 while tourmalines from garnet bearing biotite muscovite schists that overly the lode gold orebody are more fe rich fe fe mg 0 30 0 63 tourmalines from an amphibolite schist are mg rich with fe fe mg ratios 0 27 0 34 that lie between the above two groups of tourmalines the compositional trend from fe rich tourmalines in the sulfide poor schists to mg rich in the sulfide ores at mount gibson is similar to that observed in massive sulfide ore deposits worldwide alternatively the mg rich nature of the sulfide related tourmalines may be due to metamorphic reactions that are accompanied by fe incorporation into sulfides leaving coexisting silicates more mg rich the ree geochemistry of the tourmalines from the mount gibson ore zone schists displays strong hree enrichments and positive eu anomalies with eu eu ratios up to 2 4 which are similar to tourmalines associated with massive sulfide deposits the ore related tourmalines disseminated in the host schists show 11 b values of 18 6 to 17 0 whereas one coarse grained tourmaline vein has a lower 11 b value of 21 9 to 21 4 these data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal rich hydrothermally altered seafloor horizon that is typical of vms style mineralization with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation

no mass independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for archean gold deposits the source of the auriferous fluids that formed archean greenstone gold deposits remains controversial metamorphic fluids fluids derived from felsic magmas mantle fluids and meteoric water have all been suggested we report the results of ion microprobe sensitive high resolution ion microprobe stable isotopes multiple sulfur isotope analyses 32 s 33 s 34 s conducted to constrain the source of sulfur in four archean gold deposits the analyses are of pyrite samples that are genetically related to the gold mineralization from the bellerophon victory and wallaby deposits in the eastern goldfield superterrane of the yilgarn craton western australia together with sulfide from black shale from the golden mile deposit and st ives gold camp ore associated pyrite from the beattie gold deposit of the abitibi greenstone belt of the superior craton in canada was measured to test whether the results obtained from the yilgarn craton apply to other archean gold deposits sulfur analyses from sulfide from shale show mass independent fractionation mif with 33 s as high as 1 8 whereas those from the 4 archean gold deposits are dominated by mass dependent fractionation with only a minor contribution of mif sulfur in this respect archean gold deposits contrast with archean nis deposits which are dominated by mif sulfur the absence of mif in the pyrite from the four archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source but inconsistent with the metamorphic and meteoritic fluid hypotheses

an epigenetic origin for the passagem de mariana gold deposit quadrilatero ferrifero minas gerais brazil abstract the gold tourmaline quartz vein deposit of passagem de mariana in the southeastern part of the quadrilatero ferrifero produced more than 60 tonne of gold from the end of the 17th century until 1954 the mine has not operated since 1985 orebodies are veins composed of white quartz 60 vol carbonate ankerite tourmaline sericite and sulfides tourmaline dravite up to 10 vol of the vein occurs as subhedral coarse commonly zoned crystals and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins tourmaline is present in all rock types in the mine but the chemical composition of the host rocks determinates the intensity of tourmalinization with the alteration being greater in sericitic phyllites graphite sericite phyllites and calcareous rocks the most abundant sulfide is arsenopyrite which is normally associated with pyrite and pyrrhotite minor amounts of chalcopyrite galena lollingite berthierite and maldonite are present throughout the deposit sulfides are concentrated at veins boundary or are dispersed in the veins arsenopyrite is associated most commonly with calcareous rocks and graphite sericite phyllite pyrrhotite is usually found at the base of itabirites gold abundance is directly proportional to sulfide concentration hydrothermal alteration associated with the veins includes silicification tourmalinization and sulfidation the mineralized zone is a shear zone associated with a bedding parallel thrust fault that juxtaposes the itabirite lower proterozoic minas supergroup over other units this shear zone thrust fault extends for tens of km beyond the passagem mine and hosts numerous gold deposits the richest orebodies are along the itabirite footwall contact and within the graphite sericite phyllite main orebody although many lithologic units were mineralized the graphite sericite phyllite appears to have been most favorable for gold deposition the area underwent three phases of deformation d1 d2 and d3 mineral assemblages indicate upper greenschist to lower amphibolite conditions of regional metamorphism retrograde metamorphism characterized by chloritization of biotite and chloritization and biotitization of garnet developed locally the gold bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within or are in contact with all lithotypes field and laboratory data indicate that gold mineralization at passagem de mariana is epigenetic gold deposition occurred after the peak of metamorphism within the late to post d2 period of deformation which is correlated with second set of structures of trasamazonian age of alkmim and marshak alkmim f f marshak s 1998 transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero precambrian research 90 29 58 indicating that the gold mineralization occurred between 2 124 and 2 04 ga we choose to regard passagem de marina as an orogenic gold deposit as defined by groves et al groves d i goldfarb r j gebre mariam m hagemann s g robert f 1998 orogenic gold deposits a proposed classification in the context of their crustal distribution and relationship to other gold deposit types ore geology reviews 13 7 27 i e an epigenetic structurally hosted lode gold vein system in a deformed metamorphic terrane

endomycorrhizae in miocene paleosols implications in biotite weathering and accumulation of dolomite in plant roots sw madrid basin spain abstract this work provides evidence of arbuscular mycorrhizae or endomycorrizhae and their role in mineral weathering and soil carbonate production from miocene paleosols formation of the paleosols bearing mycorrhizae took place in marginal lacustrine and distal fan deposits in the madrid basin endomycorrizhae fossils are preserved as carbonate in biotite filled rhizoliths fossilized mycorrhizae are morphologically identical to those of living counterparts and consist of two linked parts the extraradicular segment surrounding the root is represented by a calcitic mycelium comprising spores and two types of hyphae that are seen to attach and to corrode the mineral grains the intraradicular part is made of a network of branched filaments hyphal coils arbuscules and likely vesicles that are mixed with the cortical root cells of the plants fossilized as dolomicrosparite aggregates the preferential formation of the dolomite aggregates on biotite grains in rhizoliths is interpreted to reflect a focused interest of the plants on this mineral and indicates the mineral may have been used as a source of plant nutrients the close association of the carbonate with the silicate further suggests that the precipitation of dolomite in the root cells and the weathering of the biotite resulted from interrelated processes main mechanisms to produce mineral dissolution include organic acids and chelate secretions by the plant roots and fungi which would remove nutrients these results confirm that carbonate paleosols provide an excellent reservoir of information on ecological interactions and biogeochemical cycling and expand the range of biogenic processes and their resultant products involved in the formation of carbonate soils this approach has broad applications given the abundance of carbonate paleosols and the endomycorrhiza record dating back 400 million years

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

archean gold relation to granulite formation and redox zoning in the crust the archean was the principal period for gold mineralization these rocks compose 12 of the exposed crust but have produced more than half of the world9s gold most major lode deposits formed at 2 7 0 2 ga this corresponds to the principal time of crustal thickening and stabilization an essential part of which was formation of granulite the anhydrous refractory base for the crust upward streaming of mantle co 2 along shear zones in the lower crust is one means for dehydrating amphibolite to form granulite and is accompanied by removal of large ion lithophile lil elements to higher crustal levels archean lode gold deposits were formed along major shear zones at intermediate crustal depths they are enclosed by zones of co 2 metasomatism many of them kilometres in extent 13 c data indicate that the co 2 is juvenile some major archean deposits have been shown to have formed from relatively oxidized hydrothermal fluids co 2 streaming with lil element depletion could only have occurred under relatively oxidized conditions that permitted a free co 2 vapor to exist in the lower crust such conditions would also have favored dissolution of gold and associated sulfide and their transfer to the mid crust in a co 2 h 2 o fluid the h 2 o coming from dehydration of amphibolite

a carbon oxygen hydrogen and sulfur isotopic study of the gold mineralization at loulo mali abstract the isotopic compositions of oxygen hydrogen carbon and sulfur have been determined on silicate carbonate and sulfide samples from auriferous tourmalinized sandstone its footwall and hanging wall of birimian age 2 1 ga at loulo in mali the sandstones have undergone two phases of deformation the samples were taken from three different orebodies loulo 0 loulo 3 and p64 for loulo 0 18o values for vein quartz are consistently between 16 1 and 16 6 smow for oxygen with one exception at 11 6 the 18o values for detrital quartz grains from the tourmalinized sandstone are also 16 1 tourmaline has a high 18 o of 13 9 the 18o values for total rocks are also relatively homogeneous at 13 2 to 15 7 the lowest values 13 2 and 14 being those for the footwall samples dolomites from loulo 0 and p64 have 18o values between 15 1 and 19 5 smow for oxygen and 13c values of 13 5 to 2 0 pdb with the exception of one sample from the hanging wall which is of sedimentary origin 13 c 0 5 18 o 22 2 the sampled dolomites are in veinlets or veins or disseminated in rocks and were deposited by hydrothermal fluids the 34s values of samples from the three orebodies range between 6 0 and 12 9 cdt they fall into different groups according to location but pre and post schistosity sulfides within each group are indistinguishable the results are interpreted as indicating the existence of two hydrothermal systems enriched in 18o at different times one of these gave rise to tourmalinization silicification and dolomitization of the sandstone and the deposition of sulfides the high d values of the tourmalines in agreement with petrological data indicate that the fluid of this system was of marine origin the two phases of folding intervened between the activity of this hydrothermal system and the initiation of the second system which gave rise to the stockwork this fluid rich in 18o was possibly of metamorphic origin with a low water rock ratio it remobilized the pre schistosity sulfides concentrating them into post schistosity veins the partly remobilized silica and carbonates were deposited under temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone the vertical extent of this hydrothermal system is as yet poorly known it is very extensive horizontally and strongly pervasive in the tourmalinized sandstone but in the vertical sense appears to be confined to fractures in the footwall

on kalgoorlie australia timmins porcupine canada and factors in intense gold mineralisation abstract the golden mile gold deposits of kalgoorlie eastern goldfields province yilgarn craton australia 46 moz au and the gold deposits in the timmins porcupine camp southern abitibi province superior craton canada 63 7 moz au are earth s two biggest archaean lode gold camps while similarities at the camp scale are dominant they differ in that the golden mile is a compact camp of one major deposit with two nearby subordinate deposits separated by less than 10 km along strike whereas timmins porcupine consists of a relatively large camp with 30 important mines distributed over 20 km strike length seismic data show a detachment fault at the base of the supracrustal rocks at kalgoorlie at between 5 and 9 km but none is identified in the timmins porcupine camp map scale patterns ages and lithologies are superficially similar the gabbros in kalgoorlie and conglomerates in timmins porcupine are distinctive rock types but served similar competence contrast functions late orogenic sediments are an important host at timmins porcupine for competence contrast reasons but are apparently absent in the golden mile area gold deposit formation in both camps is of the same age 2 67 ga with a long duration of mineralisation porphyries albitites and lamprophyres are characteristic features but played no major genetic role both camps have early elements of mineralisation that have an epithermal character in cu au w v 3 and f o 2 and both camps lie in belts locally characterised by the abundance of telluride minerals however none of these characteristics seem to be necessary or sufficient for intense gold mineralisation syn mineralisation transpressive deformation in both camps involved the imbrication of thrust slices followed by strike slip movement along a major crust scale fault with a dilatational curve in its trend in the centre of both camps the various deposits in each of the two camps were formed from immediately before to just after strike slip deformation and opening of late orogenic sedimentary basins both areas are characterised by an early episode of intense thrust faulting extensive intersecting fracture arrays likely developed as thrusting gave way to strike slip faulting these arrays may have enabled access to fertile source rock and also served as fluid conduits throughout deformation moreover both terranes are characterised by a relatively short pre mineralisation crustal history thin ned subcontinental lithospheric mantle and input of juvenile mantle derived material the pre mineralisation crustal history and evolution of the host terranes thus may have been the most important single factor in the formation of giant orogenic gold deposits at kalgoorlie and timmins porcupine

the massawa gold deposit eastern senegal west africa an orogenic gold deposit sourced from magmatically derived fluids the massawa gold project is situated on the senegalese side of the highly prospective productive palaeo proterozoic birimian kedougou kenieba inlier which hosts several world class orogenic gold deposits districts in western mali e g loulo and sadiola the massawa ore body has a strike length of at least 4 km and a current resource of 3 61 moz at a grade of 2 8 g t 1 the ore body is structurally controlled and located within a package of low grade regionally metamorphosed volcaniclastic sediments agglomerates tuffs and ash tuffs quartz feldspar and lithic wackes carbonaceous shales hydrothermal breccias and gabbro and porphyry sills these rocks have undergone pervasive silica alteration followed by a sericite ankerite chlorite alteration event related to mineralization two major styles of mineralization are recognized at massawa from field and laboratory studies the first stage of sulphide au mineralization is associated with disseminated arsenopyrite pyrite which follows shear zones in the sedimentary and volcano sedimentary host rocks the second stage consists of quartz stibnite tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization the two stages of gold mineralization are separated by a phase of quartz molybdenite veining a distinctive base metal trace assemblage is linked to stibnite formation including multiple sb phases such as chalcostibite zinkenite roshchinite aurostibite jamesonite and robinsonite secondary ion mass spectroscopy based gold deportment data indicate that up to 90 of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite stable isotope data yield 34ssulphide values of between 0 and 4 1 and 18oh2o values of 5 5 10 9 for all stages of mineralization suggesting a magmatic fluid influence this is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets that mineralization occurred at shallow 6 km depths is suggested both by the presence of stibnite and by fluid inclusion studies low temperature homogenization temperatures between 150 c and 230 c h2o nacl fluids 6 wt nacl equiv and coeval co2 ch4 inclusions observed in both phases of mineralization indicate trapping conditions of 220 315 c at 1 1 65 kbar a combination of phase petrology fluid inclusion and stable isotope data suggests deposition of gold from low salinity magmatic fluids most probably released from felsic rocks similar to those emplaced into the massawa sequence during mineralization

boron isotope composition of tourmalinite and vein tourmalines associated with gold mineralization serra do itaberaba group central ribeira belt se brazil abstract important concentrations of tourmaline occur as gold bearing stratiform tourmalinites and in mineralized quartz tourmaline veins at the tapera grande and quartzito gold prospects in the mesoproterozoic serra do itaberaba group central ribeira belt sao paulo state se brazil the main rock types in both prospects constitute the volcanic sedimentary morro da pedra preta formation which formed in a submarine back arc setting at tapera grande the volcanic sedimentary sequence is composed of metabasic and metavolcaniclastic rocks graphitic and sulfur rich metapelites banded iron formation metandesite metarhyolite calc silicates tourmalinites and metahydrothermalites derived from mafic and felsic rocks the mesoproterozoic rocks at quartzito prospect are lithologically similar but they have been affected by neoproterozoic faulting and shearing and by the emplacement of granitic rocks resulting in the formation of tourmaline rich quartz carbonate veins with gold and base metal mineralization we conducted a chemical and b isotope study of tourmalines in order to better understand the origin of the stratiform tourmalinites in the morro da pedra preta formation and their relationship with gold mineralization the overall range of 11b values obtained for the tourmalinite and vein tourmalines is between 15 and 5 with the tourmalinites falling at the low end of this range 15 to 8 such values are typical for continental crust and inconsistent with a primary marine boron signature as expected from the submarine exhalative model for the gold prospects we conclude from this that tourmaline formed or recrystallized from crustal fluids related to the amphibolite grade metamorphism which affected the serra do itaberaba group and that gold deposition occurred syn to post peak metamorphism by phase immiscibility as attested by fluid inclusions in tapera grande tourmalinite tourmaline and quartz the vein hosted tourmalines at quartzito have isotopically variable boron signatures with heavier 11b values of 5 to 8 for acicular green tourmalines and lighter values 15 to 7 for light blue ti free tourmaline from quartz carbonate veins we attribute the heavier boron to fluids derived from the volcano sedimentary rocks of marine affinity whereas the lighter boron was contributed by crustal fluids related to the granitoids or metasediments in the continental crust

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

metallogenesis of the paleoproterozoic piaba orogenic gold deposit sao luis cratonic fragment brazil piaba is an orogenic gold deposit 3 5 moz of the sao luis cratonic fragment in north northeastern brazil the deposit is epizonal mesozonal and associated with the development of a subvertical strike slip fault that cut across rhyacian metavolcano sedimentary rocks and granitoids the metavolcano sedimentary sequence comprises carbon bearing schists andesite dacite ultramafic rocks and felsic tuffs formed at 2240 5 ma to 2227 33 ma that were intruded at 2214 3 ma by fine grained subvolcanic granophyric granodiorite whole rock geochemistry indicates that the hosting metavolcano sedimentary sequence formed in a subduction related arc back arc setting which has previously been interpreted as the early arc stage of an accretionary orogen 2240 2214 ma that was followed by voluminous subduction related calc alkaline arc magmatism 2168 2145 ma and by a collisional phase 2100 15 ma that produced several bodies of peraluminous granites the hydrothermal alteration of the host rocks produced early distal carbonization and hematitization and proximal ore related chlorite sericite carbonate sulfide alteration two styles of mineralization occurring in association are present 1 a network of thin quartz sulfide veinlets and subordinate narrow breccia veins with refractory to free milling gold dissemination in hydrothermally altered host rocks and 2 subordinate low sulfide au quartz veins with free milling gold fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced log fo2 30 to 35 neutral to slightly alkaline ph 5 to 6 2 low salinity average 5 nacl aqueous carbonic xco2 5 to 59 mol xh2o 40 to 93 mol xn2 and xch4 1 mol metamorphic fluid 18oh2o 5 5 to 8 9 dh2o 8 to 69 and suggest that gold mineralization occurred at 250 to 330 c and 1 25 to 2 8 kbar in response to phase separation and fluid rock interactions that produced sulfidation and minor carbonatization sulfur 34s 2 7 to 3 8 is from an undetermined source probably magmatic oxidized or sedimentary lead isotope compositions of gold ore related pyrite and k feldspar from regional rocks suggest that collision related granitoids of ca 2100 15 ma are the most likely sources for pb in the ore although mantle and lower crustal sources are also observed 40ar 39ar in hydrothermal sericite yielded a plateau age of 1980 13 ma which is similar to the pb pb model age of gold of 1959 ma these are considered the minimum age for gold mineralization whereas the maximum age of 2009 11 ma is given by a small gold hosting intrusion in the same region

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

the relationship of mantle derived fluids to gold metallogenesis in the jiaodong peninsula evidence from d o c s isotope systematics abstract the largest gold district in china is the jiaodong peninsula where three types of gold deposits are recognized quartz vein fracture altered and breccia types the first two developed along a group of ne trending faults and are hosted by granitic intrusions dated at 160 to 150 ma biotite granite and 130 to 126 ma granodiorite and by metamorphic rocks of the precambrian crystalline basement the breccia type gold system is mainly located around the northern margin of the jiaolai cretaceous basin where mineralisation is controlled by both detachment fractures and ne trending faults this study is based on stable isotope determinations from ten gold deposits including linglong jiaojia sanshandao cangshang wang ershan dayigezhuang denggezhuang pengjiakuang fayunkuang and dazhuangzi as well as the linglong jurassic biotite granite guojialing cretaceous granodiorite and archean gneiss the stable isotope systematics reflect the style of the three types of gold deposits but also show that they belong to the same metallogenic system in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids the ore forming age is later than both the jurassic biotite granite and cretaceous granodiotite but overlaps with the 121 to 114 ma ages of lamprophyre and diabase dykes the hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar and represent a co 2 and potassium rich system this fluid system is interpreted to be the consequence of cretaceous lithospheric thinning asthenospheric upwelling and mantle degassing in eastern china

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event