Edward T. C. Spooner

Fluid chemistry of Archean seafloor hydrothermal vents: Implications for the composition of circa 3.2 Ga seawater

Seafloor hydrothermal vents of mid-Archean age (ca. 3230 Ma) have been identified and mapped in the Barberton greenstone belt, South Africa and are known as the Ironstone Pods. Fluid inclusion homogenization temperature data, when combined with gas chromatographic data, provide a minimum calculated water depth for the pods of 982 m. Ironstone Pod hydrothermal fluid endmember concentrations (Mg = 0) of various dissolved components derived from bulk fluid inclusion crush-leach experiments, include: Cl(730 mmol/L), Br (2.59), I (0.058), Na (822), NH4 (11.4), K (21.5), Ca (42.6), and Sr (0.15). This hydrothermal fluid also contains up to 1.07 mol% CO2, 0.03 mol% N2, 0.02 mol% CH4, 262 ppm COS, and minor amounts of C2–C4 hydrocarbons. Hydrothermal endmember Ca, Sr, and NH4, in particular, and to a lesser degree K, I, and CO2, commonly plot on, or very close to, modern vent fluid trends. By contrast, endmember Na and Br concentrations are distinct (higher) from modern vent fluids. High I and NH4 concentrations are consistent with contributions from sediments and/or organic matter. Calculated δ18OH2O values for the pod hydrothermal endmember fluid define a narrow range from 0.9 to 1.6‰ very similar to that of modern vent fluid values (0.4–2.1‰). A best estimate for the Ironstone Pod seawater endmember composition is Cl (920 mmol/L), Br (2.25), SO4 (2.3), I (0.037), Na (789), NH4 (5.1), K (18.9), Mg (50.9), Ca (232), and Sr (4.52). Barberton seawater components are commonly within an order of magnitude of modern seawater values, with the exception of significantly higher 1, NH4, Ca, and Sr in the inclusions. Sulfate concentrations are minimum estimates for Barberton seawater. Fluid inclusion samples containing the greatest amount of seawater component have higher N2 (up to 0.1 mol%) and low CO2, when compared to samples dominated by the hydrothermal endmember fluid. Barberton ambient seawater is considered to have been an evaporative brine of NaClCaCl2 composition during the time of pod deposition. Ironstone Pod fluid inclusion seawater endmember Br/Cl and I/Cl values of 2.45 × 10−3 and 40.2 × 10−6, respectively, are within error of bulk Earth (2.38 × 10−3 and 190 × 10−6) and are consistent with the chemistry of 3.23 Ga Barberton seawater being buffered by the mantle.

Early Archean (>3.2 Ga) Fe-oxide-rich, hydrothermal discharge vents in the Barberton greenstone belt, South Africa

Ironstone pods (Fe 2 O 3 = 72.5-97.2 wt%), interpreted to have formed by early Archean, sea-floor-related hydrothermal activity, are described from the ca. 3.5-3.1 Ga Barberton greenstone belt, South Africa. Most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy. They rest on silicified ultramafic rocks and are overlain by ferruginous shales, silicified sand-stones, other coarse clastics, and barite horizons. The ironstone pods grade along strike into laminated iron-oxide facies banded iron-formation that is inferred to represent periodic discharge of iron-oxide- and silica-rich flocculates from the pods. The massive texture of the iron-stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor; relict hydrothermal Discharge chimney structures have been recognized. Honeycomb-like cavities and possible fluid-flow channel textures attest to primary porosity. The iron-stone pods are dominated by massive, locally coarse-grained, specular hematite and goethite, with lesser amounts of quartz and an X-ray amorphous Fe-AI-bearing silicate. The pods are distinct from typical massive sulfide gossans in total trace-element concentrations, including precious and base metals, indicating a different origin. In contrast, the ironstone pods have similarities in major oxide and trace-metal concentrations to iron-oxide deposits presently forming, in part, from low-temperature hydrothermal fluids on the sea floor (for example, Red Seamount iron-oxide deposit). Fluid-inclusion studies on quartz show dominant type I primary fluid inclusions with salinities of 4.7 to 15.8 wt% NaCl equiv. and homogenization temperatures (T h ) of ∼90 to 150 °C; no evidence for boiling is seen. Type I fluid inclusions are most likely dominated by NaCl-CaCl 2 -H 2 O, although their measured eutectic temperatures are consistent with the presence of FeCl 2 ; these inclusions represent the end-member hydrothermal fluid. More saline (24-29.6 wt% CaCl 2 equiv.), lower temperature (T h = 33-109 °C), type II and Ila fluid inclusions represent pulses of a hydrothermal fluid of NaCl-CaCl 2 -FeCl 2 (or MgCl 2 )-H 2 O, or possibly FeCl 3 -bearing composition, mixing with ambient sea water in the case of type II inclusions. The various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine. The lowest salinity measurement for a type I inclusion of 3.1 wt% NaCl equiv. provides a constraint on the salinity of unenriched (by evaporation) sea water. The T h data give a minimum trapping pressure of ∼7 bars, which equates to a minimum sea-water depth of ∼60 m. The lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods. Quartz δ 18 O analyses average 17.3 ± 0.6‰ (1σ n = 5). Calculated δ 18 O H 2 O , for two samples are -1.4‰ and -1.2‰, respectively. These estimates are within error of the postulated Barberton early Archean sea water value of ∼0‰ Quartz/hematite-goethite mineral pairs yield temperatures inconsistent with fluid-inclusion T h data, suggesting isotopic disequilibrium.

Field, geochemical and U-Pb isotopic constraints from hypabyssal felsic intrusions within the Barberton greenstone belt, South Africa: Implications for tectonics and the timing of gold mineralization

Abstract Field mapping, geochemical and U-Pb zircon isotopic data are reported for two undeformed feldspar-quartz porphyries from within the central region of the Barberton greenstone belt (∼ 3450 Ma) and from a porphyry at the deepest level of one of the largest gold mines in the Barberton area, Fairview Mine. One porphyry, the Mudpools porphyry, has an age of circa 3230 Ma and provides a minimum age on unusual BIF/ferruginous shale-hosted mudpool structures. Field mapping constraints and similar geochemical signatures between the Mudpools porphyry and a second porphyry, the Ironstone pod porphyry, suggest that 3230 Ma can also be considered the minimum age of the equally unusual Ironstone pods. The two porphyries cross-cut Fig Tree sediments which are regionally deformed (D 2 ), and intrude up a thrust fault(s) suggesting pre-3230 Ma ages for D 2 deformation and thrusting. Geochemical data (trace elements, REE) indicate that these two porphyries are high-level, co-magmatic equivalents of the nearby 3227 Ma Kaap Valley tonalite, which in part intrudes the lower sequences of the greenstone bell. Since the edge of the Kaap Valley pluton is also deformed by the D 2 deformational event, either D 2 is diachronous, or D 2 in the center of the belt is not equal to D 2 at the edge of the greenstone belt. Zircon from the Fairview Mine porphyry indicates a maximum age of 3126 ± 21 Ma (2σ). A gold-mineralized shear which cross-cuts the Fairview porphyryl is responsible for hydrothermal alteration of the porphyry. U-Pb analysis of hydrothermal rutile (associated with gold) from the porphyry gives an age of 3084 ± 18 Ma (2σ)_and may represent the age of gold mineralization associated with this late shearing. It appears that ∼ 3230 Ma is a important age in early Barberton greenstone belt history, signifying intrusion of the Kaap Valley tonalite adjacent to the greenstone belt, together with the emplacement of high-level, co-magmatic equivalents (porphyries) of the Kaap Valley tonalite along regional thrust faults (shear zones) within the greenstone belt. Gold-bearing hydrothermal fluids have utilized similar shear zones ∼ 150 Ma after emplacement of both the porphyries in the central region of the greenstone belt, and the Kaap Valley tonalite.

U-Pb and Pb-Pb age constraints on Paleoproterozoic magmatism, deformation and gold mineralization in the Omai area, Guyana Shield

Abstract The Omai intrusion-centred Au-quartz vein system, the largest Au producer presently operating in the Guyana Shield, was sampled for detailed U-Pb and Pb-Pb geochronology and petrological investigation. The age of a metavolcanic/sub-volcanic unit in the host rock sequence is 2120±2 Ma. Zircon analyses from the main body dioritic rocks give U-Pb ages of 2094±6, 2092±11 and 2096+11/−10 Ma. Magmatic titanite and apatite that grew in hornblende-rich peripheral phases of the intrusion define a consistent Pb-Pb age of 2094±1 Ma, in agreement with the zircon data. Colourless titanite and rutile from strongly altered phases of the intrusion, along with low-U apatite and feldspar, define a significantly younger Pb-Pb isochron age of 2002±5 Ma. The igneous ages agree with data from similar units in the Guyana Shield and West Africa, showing that ∼2100 Ma was a time of significant intrusive activity. The ages obtained for the deformed metavolcanic and undeformed intrusion at Omai define a 26±2 Ma bracket for Trans-Amazonian deformation in central Guyana. Previous fluid inclusion studies indicate that the mineralizing solutions at Omai were too CO 2 -rich to form titanite, and the titanite-bearing sample is unmineralized, suggesting that it was not altered by gold-bearing solutions. Therefore, the 2002±5 Ma age is interpreted as a late hydrothermal overprint that formed titanite and reset rutile. Zircon and baddeleyite from a thick gabbro dyke of the Avanavero Suite, which cuts the Omai pluton, define an age of 1794±4 Ma, ruling out the dyke as a source for the late thermal effects. The hydrothermal age may record the passage of fluids released by deep crustal metamorphism due to late-stage tectonic underplating as previously proposed for the Superior province.

LASER step-heating 40Ar39Ar age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events

Abstract Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40 Ar 39 Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca K and Cl K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T0 (original blocking age) = 2673 ± 3 Ma (1σ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1σ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T0 ages of ∼2350–2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025–2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km2) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40 Ar 36 Ar ratios

Fluid inclusion volatile analysis by heated crushing, on-line gas chromatography; applications to Archean fluids

Eighteen fluid inclusion volatile peaks have been detected and identified from 1–2-g samples (quartz) by gas chromatography using heated (∼ 105°C) on-line crushing, helium carrier gas, a single porous polymer column (HayeSep R; 10′ × 1/8″; 100/120#; Ni alloy tubing), two temperature programme conditions for separate sample aliquots, micro-thermal conductivity (TCD) and photoionization detectors (PID; 11.7 eV lamp) and off-line digital peak processing. In order of retention time these volatile peaks are: N2, Ar, CO, CH4, CO2, C2H4, C2H6, C2H2, COS, C3H6, C3H6, C3H4 (propyne), H2O (22.7 mins at 80°C), SO2, ±iso-C4H10±C4H8 (1-butene) ± CH3SH, C4H8 (iso-butylene), (?) C4H6 (1,3 butadiene), and ±n-C4H10 ±C4H8 (trans-2-butene) (80°C and −70°C temperature programme conditions combined). H2O is analysed directly. O2 can be analysed cryogenically between N2 and Ar, but has not been detected in natural samples to date. H2S, SO2, NH3, HCl, HCN and H2 cannot be analysed at present. Blanks determined by crushing heat-treated Brazilian quartz (800–900°C/4hrs) are zero for 80°C temperature programme conditions, except for a large, unidentified peak at ∼ 64 mins, but contain H2O, CO2 and some low molecular weight hydrocarbons at −70°C temperature conditions due to cryogenic accumulation from the carrier gas and subsequent elution. TCD detection limits are ∼30 ppm molar in inclusions; PID detection limits are ∼ 1 ppm molar in inclusions and lower for unsaturated hydrocarbons (e.g. ∼ 0.2 ppm for C2H2; ∼ 0.3 ppb for C3H6). Precisions are ∼±1–2%, except for H2O (∼±13%). Major fluid inclusion volatile species have been successfully analysed on a ∼50 mg fluid inclusion section chip (∼ 7 m × ∼ 10 m × ∼ 100 μm). Two distinct end-member Archean fluids, one internal and one external, have been found related to the Tanco zoned, granitic pegmatite, SE Manitoba. The former is an H2O (∼96%)-CO2 (∼40%)-CH4-N2 fluid (S species not included) with a moderate salinity of 6.6 ± 1.3 eq. wt.% NaCl which is interpreted to be magmatic in origin, whereas the latter is an H2O (∼97%)-CH4 (∼2%)-CO2 (∼0.4%)-C2H6-N2 fluid with a distinctly higher salinity of ∼10–20 eq. wt% NaCl which is interpreted to be of metamorphic/lower crustal (∼2,900 bar/∼10km) origin. The volatile compositions of H2O (∼87–94%)-CO2 (∼6–13%)-CH4-N2 fluids with ∼5–6 eq. wt.% NaCl from primary inclusions from three structurally controlled, mafic-ultramafic rock hosted Archean Au-quartz vein deposits in the Barberton greenstone belt, southern Africa (n=9) are distinctly different from the composition of the Tanco external fluid, but similar to the composition of primary fluids of interpreted

The Cl−Br−I− composition of ∼3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry

Fluid inclusion leachates obtained from vug and vein quartz samples from an Archean (∼3.23 Ga) Fe-oxide hydrothermal deposit in the west-central part of the Barberton greenstone belt, South Africa, were analyzed by ion chromatography for chloride, bromide, and iodide. The deposit, known as the ironstone pods, formed by seafloor hydrothermal activity and fluid discharge. Quartz is dominated by type I liquid-vapor, aqueous inclusions with a bimodal salinity distribution (0–0.25 MCl− and 0.9–1.8 MCl−). Bulk analytical salinities range from 0.45 to 0.99 MCl− represent averages of type I inclusions. Bulk fluid inclusion bromide and iodide concentrations are 1.44–3.32 mM and 0.01–0.12 mM, respectively. For comparison, modern seawater has halogen contents of 590 mM chloride, 0.9 mM bromide, and 0.5 μM total iodine. In the fluids from the ironstone pods, bromide and iodide are enriched relative to chloride, when compared with modern seawater. Approximate Br−Cl− and I−Cl− ratios of 3.2 Ga Barberton seawater are 2.5 × 10−3 and 40 × 10−6, respectively. Dispersion to higher values was caused principally by reaction with organic sediments whose trends are similar to those seen for modern vent fluids at unsedimented and sedimented ridges, relative to modern seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromine and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values.

A discussion of the Jahns–Burnham proposal for the formation of zoned granitic pegmatites using solid-liquid-vapour inclusions from the Tanco Pegmatite, S.E. Manitoba, Canada

Jahns and Burnham (1969) proposed that the internal evolution of zoned granitic pegmatites could be explained by crystallisation from water-saturated melts which evolved to produce systems with a melt plus a separate aqueous fluid. Examination of microthermometric properties, chemical compositions and gas contents of solid-liquid-vapour inclusions from a number of the zones of the Tanco rare element granitic pegmatite places constraints on fluid evolution within the framework of the crystallisation history of the pegmatite, and contributes to an examination of the Jahns–Burnham proposal. Initial crystallisation at Tanco was from the wall rock inwards, producing the relatively unfractionated wall zone (potassium feldspar–quartz-albite-muscovite). Textural evidence, and an upward increase in the level of geochemical fractionation, indicate that much, but not all , of the subsequent crystallisation of the pegmatite was from the base upwards. Inclusions trapped by wall zone and metasomatic wall rock tourmaline indicate that the pegmatite was intruded as a 2 phase alumino-silicate melt/fluid mixture at ∼720°C, with an initial fluid composition of ∼98mol.% H 2 O (containing 2 equiv. mo1% NaCl) and 2 (containing 4 ). These observations indicate that both melt and fluid were present from the start of crystallisation (Jahns & Burnham 1969), but show that CO 2 and dissolved salts were important additional components of the fluid phase. The bulk of the pegmatite then crystallised in the range 600-470°C from melts and fluids with continued low levels of CO 2 (2-3mol.%) and approximately constant salinity (∼7 equiv. wt.% NaCl dissolved in the aqueous phase). Crystal-rich inclusions, which may represent trapped alumino-silicate melts, are present throughout pegmatite crystallisation down to temperatures as low as ∼262°C. The final stages of crystallisation resulted in the formation of the beryl fringe at 291 ± 33°C and the lower part of the quartz zone at 262 ± 29°C. By the later stages the fluid had cooled through an H 2 O-CO 2 – dissolved salt solvus resulting in H 2 O-CO 2 phase separation. Gas chromatographic analysis of the fluid components in the vug quartz, beryl fringe and lower part of the quartz zone shows that the inclusions contain H 2 O, CO 2 , CH 4 , N 2 , CO, Ar, and trace C 2 H 6 in the beryl fringe. Measured CH 4 :CO 2 ratios of 0·0060 (±0·0015) for the beryl fringe (twenty crushes on five samples) and 0·0042 (±0.0021) for the quartz zone (thirty crushes on six samples) yield f O 2 estimates of 1×10 −36 and 2 × 10 −38 , respectively, which are just above QFM at these temperatures.

Fluid regime and ore formation in the tungsten(–yttrium) deposits of Kyzyltau (Mongolian Altai): evidence for fluid variability in tungsten–tin ore systems

Abstract The Kyzyltau ore field is located in the northern part of the Mongolian Altai Mountains. W(–Y–Be–Mo) mineralization occurs in veins and stockworks in granites, felsic volcanics, basaltic flows, and conglomerates. The main ore minerals in the veins are wolframite, fluorite, beryl and minor molybdenite. Inclusions in hydrothermal quartz, fluorite and beryl from veins, as well as in magmatic quartz in vein-hosting granites of the Ulaan uul, Buraat uul and Tsunkheg deposits were investigated using microthermometry and laser Raman spectroscopy. Pseudosecondary/primary inclusions in the Kyzyltau ore veins are predominantly liquid-rich aqueous inclusions, containing variable amounts of CO 2 as the main gas component, and showing N 2 /CH 4 ratios >1. Secondary inclusions contain no or only trace concentrations of gases, as indicated by clathrate melting. The T h values of pseudosecondary/primary inclusions in veins are between 180 and 433°C. Fluid inclusion data indicate that phase separation processes leading to fluid immiscibility occurred in vuggy quartz II and green fluorite II in the Ulaan uul and Buraat uul ore veins. Phase separation pressures of 100–350 bars were estimated. Fluorite I from the Kyzyltau ore field shows a strong enrichment of HREE and a strong negative Eu anomaly common for rare metal-bearing ore systems. Fluorite II from Buraat uul and Tsunkheg is characterized by a change in the incorporation of rare earth elements (REE) with decreasing HREE contents and a decreasing Eu anomaly. The REE distribution patterns in fluorite II from Ulaan uul remained unchanged compared with fluorite I despite a strong increase in the total REE content. A review of the literature shows that from high-temperature, Fe-rich, K-dominated brines cassiterite ores can precipitate in quartz veins together with Fe–chlorite and Fe–tourmaline (Bolivian type). If phase separation of a gas-rich, low-salinity and Fe-rich fluid occurs, cassiterite–wolframite ores may be deposited (Cornwall/Devon type). The deposits of Kyzyltau are characterized by low-Fe alteration assemblages, a predominance of sodium over potassium, high contents of REE and Y, and a lack of extended tin mineralization despite the tin potential of the ore systems. Fluid inclusion data as well as geochemical and geological indications suggest formation of the tungsten deposits near the tops of Li–F-rich sub-volcanic intrusions. We interpret the pH of the mineralizing fluid to be the main factor controlling wolframite precipitation in the Kyzyltau ore field. Fluid–wall rock interactions, a lowering of the temperature and unmixing processes in the ore fluid generated contributions to neutralization and buffering of the acid CO 2 -bearing fluid into a pH range where tungstates were precipitated. It can be inferred that formation of tungsten ores without precipitation of extended tin mineralization is possible in deposits characterized by high potential tin and tungsten. According to this inference, tin mineralization may be found in a more favourable setting in the vicinity of the Kyzyltau deposits.

Integrated cation–anion/volatile fluid inclusion analysis by gas and ion chromatography; methodology and examples

Abstract Combined gas and ion chromatographic analysis of well characterized, small (∼1 g) fluid inclusion-bearing samples is a powerful, but simple, means for obtaining integrated fluid concentrations of major and trace, volatile and ionic fluid constituents without using microthermometrically determined salinity for normalization. The methodology, which is described and assessed in detail, involves crushing a carefully cleaned sample at ∼105°C in a stainless steel crusher on-line to a gas chromatograph. After volatile analysis, the crushed sample is removed and leached with deionized water to produce a leachate solution, which is filtered and analyzed by ion chromatography, and other methods. For example, detailed procedures are given for I− analysis with a low detection limit of 0.5 ppb using a trace anion concentrator column and pulsed amperometric, rather than electrochemical, detection. The data are combined (calculation procedure given) to give whole fluid analyses in concentrations of mol% or mmol/l; this procedure removes a large systematic error of approximately a factor of 2, if separate volatile and hand crush cation/anion analyses are linked through sample weights. Results indicate a mean crushing efficiency of 82±6% for a mean sample mass of 0.97 g, based on 97 determinations. Tests on the efficiency of the leaching process showed that >90% of univalent ions are removed. Based on 145 analyses, charge balances are found to be close to 1.0 when all major positive and negative species are included, both analyzed and calculated (e.g., carbonate species). Analytical errors (coefficients of variation; percentage) for species in moles vary from 1.1 (Br−) to 17.9 (I−), from 0.02 (H2O) to 0.21 (I−) for species in mol%, and from 13.1 (F−, Br−) to 22.1 (I−) for species in mmol/l. Application of the combined GC/IC technique to well characterized sample sets from diverse settings, including the Tanco Li–Cs–Ta pegmatite, Archean Au–quartz vein systems, the Polaris MVT Pb–Zn deposit, and the ∼3.2 Ga Barberton sea-floor Fe-oxide deposits (ironstone pods) has demonstrated its utility for constraining fluid sources, identifying fluid types, and determining the effects of processes such as H2O–CO2 phase separation and fluid–wall rock interaction.