Stable Isotope Thermometry Research Articles

The ikaite to calcite transformation: Implications for palaeoclimate studies

Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures.Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.

Pressure-Temperature-Fluid Constraints for the Formation of the Halo-Shakiso Emerald Deposit, Southern Ethiopia: Fluid Inclusion and Stable Isotope Studies

ABSTRACT The Halo-Shakiso emeralds were discovered near the town of Shakiso in southern Ethiopia in 2016. They are gem quality, Cr-dominant emeralds hosted within ultramafic rocks and associated with Cambrian pegmatite intrusions of the Adola Belt. Aqueous-carbonic primary fluid inclusions hosted within emerald have a composition of approximately 3.0 wt.% NaCl eq. and an XCO2 of 0.06, with minor amounts of N2, CH4, and H2S. Stable isotope thermometry of contemporaneous quartz and emerald yields temperatures in the range of 420 to 470 °C. Combined stable isotope and fluid inclusion data are consistent with emerald precipitation at pressures ranging from 2.0 to 3.0 kbar, corresponding to depths of 5.9 to 8.9 km. Additionally, emerald channel water δD and calculated δ18O isotope values are consistent with an igneous origin for the fluids responsible for emerald precipitation; these fluids are also responsible for the metasomatization of the host rocks in and near the pegmatite, forming the phlogopite schist that is host to the Halo-Shakiso emeralds. The isotopic signatures, combined with the occurrence of adjacent pegmatites, support the classification of the Halo-Shakiso emerald deposit as a Tectonic-Magmatic-Related emerald deposit.

Paragenesis of gold mineralization at the Kiyuk Lake Project, Kivalliq Region, Nunavut, Canada

ABSTRACT Exploration for gold in Nunavut has been primarily focused on Archean greenstone belts in the north and coastal regions of the territory, resulting in large areas of underexplored terrain in the south. The Kiyuk Lake property is located in the underexplored southwest corner of the Kivalliq Region of Nunavut within the Hearne domain of the ∼1.9 Ga western Churchill Province. The property is hosted by Proterozoic calc-silicate and clastic sedimentary units of the Hurwitz Group (<2.4–1.9 Ga) and the unconformably overlying Kiyuk Group (1.9–1.83 Ga). Gold mineralization in Proterozoic sedimentary rocks is rare in the Canadian Shield, so the Rusty Zone at Kiyuk Lake presents a unique opportunity to study the enigmatic gold mineralization hosted in such sedimentary rocks. Mineralization at the Rusty Zone is hosted by an immature lithic wacke cut by thin intermediate dikes that are associated with hydrothermal breccias composed of Fe-carbonate, calcite, calcic-amphibole, Fe-sulfide, Fe-oxide minerals, and gold. Textural and timing relationships suggest that the gold mineralization is post-sedimentary and syn- to post-intrusion of intermediate dikes. Stable isotope thermometry suggests that mineralization took place between 450 and 600 °C, and geochronological studies indicate that the intrusion and mineralization occurred before or about 1.83 Ga. Using basement breaching thrusts faults as conduits to the surface, over-pressurization along a later normal fault is thought to be the primary cause for the localized breccia pipe that controls gold mineralization. The hydrothermal fluids are postulated to be volatile-rich aqueous solutions exsolved from a source of cooling magmas at depth. Although sub-economic at present, the occurrence of high-grade gold in a Paleoproterozoic basin such as Kiyuk Lake could signal a new opportunity for exploration for gold in the Canadian Shield.

DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO‐TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN: CONTROLS ON RESERVOIR QUALITY

Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo‐Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow‐marine ramp or epeiric platform and, according to previous studies, underwent intense near‐surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage‐reflux models. The early dolomites then re‐equilibrated or re‐crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are significantly decreased.As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric‐retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment.Low‐temperature dolomitization is commonly fabric‐selective and partially fabric‐retentive. Whole rock stable isotope thermometry indicates that fabric‐destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric‐retentive dolomites and associated anhydrites formed in surface and near‐surface conditions. Fabric‐destructive dolomite or dolomite neomorphism post‐date fabric‐retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric‐retentive dolomite and anhydrite fabrics are traversed by stylolites.

Mass Mortality Of Fossil Decapods Within the Monte LeóN Formation (Early Miocene), Southern Argentina: Victims Of Andean Volcanism

Four exposed planar surfaces within the type area of the Monte León Formation (early Miocene) of southern Patagonia, Argentina, enclose significantly different fossil assemblages positioned in close geographic and stratigraphic proximity to one another. The exposed paleosurfaces were mapped in planar view using a quadrat grid system. Precise fossil position and orientation data, stable isotope thermometry and petrographic analyses, and petrologic and taphonomic evidence suggest an inner-shelf, shallow water habitat, above storm wave base, with a well-oxygenated benthos and weak offshore bottom currents. The rate of sedimentation was generally low, interspersed with periods of non-deposition and sporadic, higher-energy pulses of sediment input. Stable isotope analyses of foraminiferans indicate bottom water temperatures consistent with a modern mid-latitude coastal setting. Two distinctly different assemblages were observed: 1) a relatively diverse, normal marine benthic fauna consisting of bivalves, gastropods, bryozoans, echinoids, and decapods; and 2) a unique assemblage consisting solely of numerous, exceptionally preserved, fully-articulated crabs. These assemblages occur in a one meter interval within the lower-most beds of the Monte León Formation. The occurrence and preservation of large numbers of decapods within the fossil record are rare, making these deposits remarkable. The crab-rich assemblage was stratigraphically positioned below the surfaces containing the normal marine assemblage. The crabs are contained within a slightly compacted, argillaceous volcanic tuff, consisting mostly of volcanic glass shards and euhedral plagioclase grains. Biogenic fragments are noticeably absent from the deposit, unlike sediments collected from the upper surfaces. Most of the crabs were preserved with their third maxillipeds in an open, gaping posture. This is consistent with postures observed in extant crabs suffering from respiratory distress. From the above evidence it is inferred that the fossil crabs were suffocated, killed, and rapidly buried. Supradjacent layers record the re-establishment of normal marine conditions. The apparent faunal disparity observed on the four paleosurfaces within the Monte León Formation is interpreted as representing the initial decimation and eventual re-establishment of the benthic marine fauna following a catastrophic volcanic event.

Mineralogical and stable isotope studies of gold–arsenic mineralisation in the Sams Creek peralkaline porphyritic granite, South Island, New Zealand

At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ18Omagma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO2) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO2) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO2-CH4 + liquid H2O ± CO2 vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ18O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ18O (+12 to +16‰) and δ13C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ13Cbulk carbon values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ18O = +25‰ and δ13C = 0‰). Calculated δ18Owater (+8 to +11‰, at 340°C) and \(\delta^{13}{\text{C}}_{{\rm CO}_{2}}\)(−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ34S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ34S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits.

Stable isotope geochemistry and formation mechanisms of quartz veins; extreme paleoaltitudes of the Central Alps in the Neogene

Quartzveins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the Central Swiss Alps. In some cases, the veins are completely filled with milky quartz, while in others, sometimes spectacular void-filling quartz crystals are found. The style of vein filling and size is controlled by host rock composition and deformation history. Temperatures of vein formation, estimated using stable isotope thermometry and mineral equilibria, cover a range of 450°C down to 150°C. Vein formation started at 18 to 20 Ma and continued for over 10 My. The oxygen isotope values of quartz veins range from 10 to 20 permil, and in almost all cases are equal to those of the hosting lithology. The strongly rock-buffered veins imply a low fluid/rock ratio and minimal fluid flow. In order to explain massive, nearly monomineralic quartz formation without exceptionally large fluid fluxes, a mechanism of differential pressure and silica diffusion, combined with pressure solution, is proposed for early vein formation. Fluid inclusions and hydrous minerals in late-formed veins have extremely low δD values, consistent with meteoric water infiltration. The change from rock-buffered, static fluid to infiltration from above can be explained in terms of changes in the large-scale deformation style occurring between 20 and 15 Ma. The rapid cooling of the Central Alps identified in previous studies may be explained in part, by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more. An average water flux of 0.15 cm3 cm−2yr−1 entering the rock and reemerging heated by 40°C is sufficient to cool rock at 10 km depth by 100°C in 5 million years. The very negative δD values of

Stable Isotope Thermometry: There is More to it Than Temperature

Stable Isotope Thermometry: There is More to it Than Temperature

Stable isotope thermometry: There is more to it than temperature

Stable isotope thermometry: There is more to it than temperature

Stable Isotope Characterization of the Thermal Profile and Subsurface Biological Activity during Oxidation of the Great Australia Deposit, Cloncurry, Queensland, Australia

The application of stable isotope thermometry and phase relations was used to examine the thermal evolution of oxidation and secondary mineralization at the Great Australia Cu-Au-Co deposit, Cloncurry, Queensland, Australia. Results show that early secondary mineralization consisted of djurleite crystallization at temperatures <93°C, followed by calcite mineralization at 66° to 75°C. The secondary mineralization paragenesis continued with azurite that formed at about 51°C, followed by malachite crystallization at 41° to 38°C, and malachite after azurite at 34°C. The final stage of secondary mineralization consists of calcite that formed at 25° to 30°C, followed by goethite. The general trend observed is for early oxidation and supergene enrichment at apparent temperatures less than 93°C and between 66° and 75°C, cooling to ambient surface temperatures late in the period of mineralization. A correlation is observed between decreasing isotope thermometry temperatures and increasing δ 13C values as oxidation and supergene enrichment progressed. This is interpreted to result from exothermic sulfide oxidation and associated subsurface thermophilic bacterial oxidation within the oxidation front. Over the period of oxidation and supergene enrichment, temperatures decreased and δ 13C values increased, recording in the secondary minerals the passing of the oxidation front and a diminishing subsurface bacterial carbon source.

Stable Isotope Thermometry at High Temperatures

Determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by Urey. In theory, this should be simple. Calibrated mineral pairs are common, analysis is rapid, and there is no significant pressure correction. However, in spite of widespread application, the promise of reliable thermometry has been elusive. Stable isotope temperatures in metamorphic and igneous rocks are often controversial; is the fractionation between two phases a thermometer, speedometer (rate dependant), hygrometer (P(H2O)-dependant), or chimera? A number of factors have contributed to this uncertainty including: incomplete and sometimes conflicting calibration of isotope fractionation factors; limited understanding of the kinetics of mineral exchange; and the lack of microanalytical techniques. This situation has improved dramatically over the past decade, permitting reliable and detailed thermal histories to be inferred. In a landmark paper, Bottinga and Javoy (1975) empirically calibrated the oxygen isotope fractionation factors among ten rock-forming minerals at T > 500°C. Their compilation shows that 84% of analyzed samples contain at least three minerals meeting a concordance test for equilibrium. They concluded “the great majority of igneous and metamorphic rocks …. has conserved a state of oxygen isotope exchange equilibrium.” If correct, this bodes well for thermometry. However, this conclusion was contested by Deines (1977) who critically examined the existing data and concluded that “less than half of the rocks analyzed to date would yield concordant 18O-derived temperatures.” While seeming irreconcilable, both conclusions are based on fact. All rocks contain some degree of disequilibrium, but many also preserve a thermometric record. It is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted. The organization of this paper will be to discuss the assumptions …

Hydrothermal Alteration and Fluid Chemistry of the Endako Porphyry Molybdenum Deposit, British Columbia

Hydrothermal alteration and fluid chemistry data of the early Cretaceous Endako porphyry molybdenum deposit, British Columbia, provide new information on the hydrothermal fluids associated with low-fluorine molybdenite mineralization. Molybdenite mineralization and hydrothermal alteration occur as early quartz ± molydenite stockwork veins with K feldspar-bearing selvages and paragenetically later quartz-molybdenite ribbon veins with sericite-bearing selvages. Late hydrothermal alteration is associated with the development of kaolinite and postore (Tertiary age) calcite veins. Fluid inclusions in early-formed quartz ± molybdenite stockwork veins with K feldspar-bearing alteration assemblages are dominated by moderate-salinity (5 to 15 wt % NaCl equiv), liquid-rich (type 1) and rare high-salinity (30 to 45 wt % NaCl equiv), halite-bearing (type 3) fluid inclusions. Type 1 and type 3 fluid inclusions in early veins homogenize between 390° and 430°C and 375° and 420°C, respectively. Secondary fluid inclusions (type 2) of low salinity (1 to 5 wt % NaCl equiv) in these early veins are minor, and homogenize between 130° and 285°C. Fluid inclusions in quartz-molybdenite ribbon veins with sericite-bearing alteration assemblages are dominated by moderate-salinity, liquid-rich (type 1) inclusions, with minor type 2 fluid inclusions. Type 1 fluid inclusions of ribbon veins homogenize between 360° and 400°C. Fluid inclusions in postore calcite veins are of only type 2 fluid inclusions, which homogenize at 209°C. Hydrothermal fluids recorded by type 1 and type 3 fluid inclusions in early veins were trapped under lithostatic to hydrostatic conditions between 0.3 and less than or equal to 2.0 kbar, and 360° and 560°C. Postore fluids recorded by type 2 fluid inclusions were trapped under conditions less than or equal to 0.5 kbar, and between 190° and 300°C. Quartz stockwork and ribbon veins possess δ 18O values of 8.4 ± 0.2 ( n = 9) and 8.4 ± 0.6 ( n = 13), respectively. Hydrothermal K feldspar and biotite from K feldspar alteration assemblages possess δ 18O values of 6.8 ± 0.4 ( n = 7) and 3.5 ± 0.8 ( n = 8), respectively. Oxygen isotope geothermometry of quartz-biotite and quartz-K feldspar pairs from K feldspar alteration assemblages yield temperatures between 200° and 490°C, which is similar to the trapping temperatures of hydrothermal fluids determined from fluid inclusion studies associated with molybdenite mineralization, the development of kaolinite, and calcite veins. The oxygen isotope temperatures of the quartz-biotite and quartz-K feldspar pairs suggest that K feldspar and biotite either record the approximate 18O composition of hydrothermal fluids associated with K feldspar alteration or have undergone 18O exchange with late-stage hydrothermal fluids. Hydrogen isotope composition of quartz stockwork and ribbon veins fluid inclusion waters range between –105 and –173 per mil. Solute chemistry studies of fluid inclusion waters indicate that ore-forming fluids from Endako have low Br/Cl and Br/Na ratios, and high I/Cl and I/Br ratios in comparison to Porgera (epithermal), Babine Lake (porphyry Cu), and St. Austell, Capitan Pluton (vein) deposits associated with magmatic processes. Na/K ratios of fluid inclusion waters yield temperatures (308° to 429°C) similar to those determined from type 1 and type 3 fluid inclusions and stable isotope thermometry. Results from fluid inclusion and solute chemistry studies indicate the involvement of hydrothermal fluids exsolved from a crystallizing melt in the formation of the Endako molybdenum deposit. However, oxygen and hydrogen isotope values deviate from the generally accepted magmatic compositions, which suggests the early involvement of meteoric water in the ore-forming fluids and ore genesis.

Changes in the [formula omitted] ratios of fluids as evidence for different metamorphic episodes in high grade gneisses from the Konovalov Mountains area (Rayner Complex, East Antarctica)

Rocks of the Rayner complex in East Antarctica have suffered a complex multiphase metamorphic evolution. Radiogenic age constraints taken from the literature suggest at least two metamorphic events, the `main Rayner event' accompanied by charnockite emplacement 963 Myr ago and an amphibolite facies overprint 500–550 Myr ago. Some authors have suggested that a second high temperature metamorphic event was connected with the widespread intrusion of granitic magmas at about 770 Myr. Petrological and stable isotope thermometry is consistent with three metamorphic events, (i) a first granulite facies metamorphism, as preserved in the cationic equilibria of minerals in enderbitic gneisses yielding 810°C and 6.9 kbar; (ii) a regionally pervasive overprint, also at high temperatures (730°C and 5.9 kbar), previously regarded as the `main Rayner' event and; (iii) an amphibolite facies overprint followed by a greenschist facies retrogression, which is especially pronounced in late shear zones. We show that the two older events can be distinguished by the isotopic composition of the metamorphic fluids. During the first granulite facies event, the rocks preserved their inherited isotopic signatures, whereas the second granulite facies overprint was accompanied by pervasive flux of fluids of magmatic (possibly mantle) origin. During the third, amphibolite grade event, the fluids trapped during the second event were remobilised and led to partial reequilibration.

Very low grade metamorphism of the Taveyanne formation of western Switzerland

The andesitic early Oligocene Taveyanne metagreywacke of the Helvetic nappes of western Switzerland shows an increase of metamorphic grade from zeolite facies through lower greenschist facies. Electron microprobe analysis, fluid inclusion thermometry, stable isotope analysis, coal rank, illite and chlorite crystallinity and thermodynamic calculations were carried out to determine metamorphic conditions. Evaluation of all techniques used in this study suggest that only combinations of different parameters yield reliable information to constrain very low-grade metamorphic conditions. Electron microprobe analyses are presented for actinolite, chlorite, epidote, phengite, laumontite, prehnite, pumpellyite, and titanite. With increasing metamorphic grade, chlorite is enriched in tetrahedral Al, pumpellyite becomes poorer in Fetot and more homogeneous in chemical composition, and titanite tends to incorporate Ti at the expense of Al and Fe3+. Metamorphic P-T conditions were determined by a combination of fluid inclusion microthermobarometry, stable isotope thermometry on quartz-calcite veins, chlorite “geothermometry” and thermodynamic calculations. Peak temperatures range from 210–250 °C for zeolite facies to 270–300 °C for prehnite-pumpellyite facies to 300–360 °C for pumpellyite-actinolite facies. An evaluation of 289 chlorite analyses indicates that the tetrahedral Al content is negatively correlated with the saponite component. Temperatures derived from chlorite “geothermometry” match maximum temperature conditions mentioned above. Illite crystallinity data for shales and slates intercalated with the Taveyanne metagreywacke indicate that the diagenetic zone correlates with the zeolite facies, the upper anchizone with the prehnite-pumpellyite facies, and the lower epizone with the pumpellyite-actinolite facies. A comparison of coal rank and illite crystallinity data (n=12,r=0.91) yielded R max values of 2.9 and 5.5% for the lower and upper boundary of the anchizone, respectively.

Metamorphism and gold mineralization in the Blue Ridge, southernmost Appalachians

Lode gold mineralization in the Blue Ridge of the southernmost Appalachians is hosted by metavolcanic rocks (e.g., Anna Howe mine, AL; Royal Vindicator mine, GA), metaplutonic rocks (e.g., Hog Mountain mine, AL), and metasedimentary rocks (e.g., Lowe, Tallapoosa, and Jones Vein mines, AL). Most gold occurs in synkinematic quartz + or - plagioclase + or - pyrite + or - pyrrhotite + or - chlorite veins localized along polydeformational faults that juxtapose rocks with significantly different peak metamorphic mineral assemblages.Mineralogy, chemistry, and O and H isotope studies suggest that the three types of host rocks have undergone differing amounts and types of alteration during mineralization. Limited wall-rock alteration in metavolcanic- and metasediment-hosted deposits, and relatively extensive wall-rock alteration in granitoid-hosted deposits, suggests that most deposits formed from fluids that were close to equilibrium with metavolcanic and metasedimentary rocks. Stable isotope compositions of the fluids calculated from vein minerals and vein selvages are consistent with a predominantly metasedimentary fluid source, but vary from deposit to deposit (-22 to -47ppm delta D, 4-5ppm, delta 18 O, and 5-7ppm delta 34 S at Anna Howe and Royal Vindicator; -48 to -50ppm delta D, 9-13ppm delta 18 O, and ca. 19ppm delta 34 S at Lowe and Jones Vein; and -22 to -23ppm delta D, 8-11ppm delta 18 O, 9-10ppm delta 34 S, and -6 delta 13 C at Hog Mountain). Silicate mineral thermobarometry of vein, vein selvage, and wall-rock mineral assemblages indicate that mineralization and regional metamorphism occured at greenschist to amphibolite facies (480 degrees + or - 75 degrees C at Anna Howe, 535 degrees + or - 50 degrees C at 6.4 + or - 1 kbars at Lowe, 530 degrees + or - 50 degrees C at 6.9 + or - 1 kbars at Tallapoosa, and 460 degrees + or - 50 degrees C at 5.5 + or - 1 kbars at Hog Mountain). Oxygen isotope fractionation between vein minerals and selvage minerals consistently records equilibration temperatures that are similar to or slightly lower than those estimated from silicate thermometry.Auriferous veins contain numerous fluid inclusions that were emplaced in several stages and can be subdivided into five compositional types based on salt and CO 2 concentrations. Fluid inclusion isochores for early formed inclusions from these veins intercept the pressure and temperature conditions estimated from silicate mineral thermobarometry and stable isotope thermometry, and are compatible with entrapment at those conditions. These fluids exhibit significant variation in salinity (X (sub NaCl equiv ) ) = 0.0-0.2) and CO 2 (X (sub CO 2 ) = 0.0-0.2), suggesting variation in fluid-wall-rock interaction that accompanied gold deposition during declining temperatures. Less abundant and later fluids within the veins are dominantly CO 2 .The association of gold mineralization with structurally controlled concordant and discordant quartz sulfide veins, and the temperatures and pressures of wall-rock alteration and regional metamorphism indicate that the present distribution of gold is a result of metamorphism during progressive D 2 -D 3 deformation. Isotopic data for alteration envelopes date this event as Alleghanian: 279 + or - 14 Ma (K-Ar whole rock) and 343 + or - 18 Ma (K-Ar biotite) at Lowe; and 315 + or - 18 Ma (Rb-Sr whole-rock isochron; 87 Sr/ 86 Sr i = 0.7061 + or - 0.0008) and 294 + or - 16 Ma (K-Ar whole-rock) at Hog Mountain. Available data are compatible with development of the lodes during early Alleghanian overthrusting of allochthons over sedimentary rocks of the autochthonous North American margin. The implication is that the fluids were derived from metasedimentary and/or metavolcanic formations in the lower parts of the crystalline thrust stack (or possibly from underlying autochthonous sedimentary formations), ascended along permeable fault zones, and were emplaced as veins into dilatent areas in and adjacent to the fault zones.

Fast grain boundary: A FORTRAN-77 program for calculating the effects of retrograde interdiffusion of stable isotopes

Exchange of stable isotopes between coexisting minerals is recognized widely as an important factor in the interpretation of stable isotope geochemistry of plutonic and high-grade metamorphic rocks. Where retrogression has occurred without major recrystallization events, the rate limiting step for stable isotope exchange will be diffusion. The mathematics of diffusion are well known for many problems, but no analytical solution, including that for closure temperature, adequately describes the complex and highly variable controls of rate and mass balance that will dominate many diffusion processes in rocks. We have implemented a model describing diffusional exchange for rocks in which grain boundary diffusion is sufficiently rapid that a representative volume of rock (typically millimeter to centimeter) is able to have mutual equilibration of all grain boundaries for the time scale of cooling. This Fast Grain Boundary model explicitly links intracrystalline diffusion rates and abundances of all minerals in a rock, and allows study of the impact of rock type on stable isotope thermometry, retrogression, and zonation. The FORTRAN-77 program for the Fast Grain Boundary model presented here can be used with a personal computer to solve typical problems in minutes. Input includes the grain size(s), model abundance(s), diffusion coefficient, and fractionation factor for each constituent mineral, and a cooling rate for the rock. Output includes the diffusion profile and integrated (bulk) composition of every mineral in a rock, as well as the apparent temperatures that would be observed by applying bulk-mineral stable isotope thermometry to such a rock.

A new look at stable isotope thermometry

Interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks. The Fast Grain Boundary (FGB) model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion ( Eiler et al., 1991, 1992). This model assumes that grain boundary diffusion is much faster than volume diffusion, and it accounts for exchange among all minerals in a rock. Previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks. Modeling results are described for amphibolites and hornblende granites and gneisses; biotite granites, schists, and gneisses; pelitic and semi-pelitic rocks; garnet peridotites; anorthosites, gabbros, pyroxenites, and related rocks; and calc-silicate rocks. Examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase-pyroxene in pyroxene bearing anorthosites and garnet-quartz in garnetiferous quartzites. In contrast, the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures. Closed-system processes are capable of producing a variety of patterns of stable isotope resetting, discordance, mineral zonation, and fractionation reversals. Examples include large reversals of quartz-feldspar fractionations in micaceous rocks, and oscillatory zonation in feldspar from some quartz-rich rocks. These results permit reinterpretation of many studies of stable isotope thermometry, speedometry, and retrograde alteration history. FGB modeling of mineral zonation provides an important new guide to applying recently developed micro-analytical tools to slowly cooled rocks. Application of the FGB model to quartzo-feldspathic gneisses from the Adirondack Mountains, New York, demonstrates the usefulness of diffusion modeling in discriminating closed-system, diffusion controlled retrogression from open-system retrogression, and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity.