1,721,024 research outputs found
Active near-surface mobilisation of slab-derived geochemical signatures by hyperalkaline waters in brecciated serpentinites
No full textUnusual hyperalkaline meteoric groundwaters on Troodos massif, Cyprus, issue from highly deformed and completely serpentinized ultramafic brecciated rock masses of the Artemis Diapir and have high salinity (25–30% seawater total dissolved solids) and some of the highest recorded pH values (11−13) for natural waters. These waters have elevated dissolved Na, K, Li, B, Ba, Rb, Cs, Cl and SO4 with ion/chloride substantially above seawater ratios, overprinting minor contributions from marine aerosols. For example, K concentrations are similar to seawater values. Water stable isotope ratios imply extensive water-rock interaction and 87Sr/86Sr is rock-dominated (0.705) and significantly lower than seawater. These rare fluids contrast with surface and shallow ground waters draining the ultramafic Troodos Mantle Sequence that have pH of 8.5–9, meteoric stable isotope ratios, 87Sr/86Sr similar to early Miocene seawater (0.7085) and low dissolved salts albeit with ion/chloride ratios also greater than seawater. The combination of high pH and salinity is unusual in ophiolites and these hyperalkaline waters are most similar to end-member fluids emanating from serpentine mud volcanoes in the Mariana forearc. Rainwater rapidly transiting terraces of asbestos mine tailings on Troodos mostly resemble the surface waters but show slight contamination by a saline component. We propose that inclusions within the serpentinite that contain highly soluble salts have been made accessible by tectonic deformation during uplift and diapirism, or comminution during mining activity. A multistage process is proposed whereby alkali and other signatures have been released from the stalled subducting slab beneath the Troodos massif and have infused the mantle wedge. Some of these rocks have been carried to the surface by diapiric uplift and erosion, where tectonic deformation or mining activities have enabled the near-surface mobilisation of slab-derived geochemical signatures by modern meteoric waters
Radiogenic and stable Sr isotope ratios (87Sr/86Sr, d88/86Sr) as tracers of riverine cation sources and biogeochemical cycling in the Milford Sound region of Fiordland, New Zealand
No full textThis study reports radiogenic Sr isotope ratios (87Sr/86Sr), stable Sr isotope ratios (?88/86Sr), and major ion concentrations for river, rock, sediment, soil, and plant samples collected from the Cleddau and Hollyford catchments in the Milford Sound region of Fiordland, New Zealand. The catchments primarily drain gabbro, but some tributaries access limestone and volcanogenic sediments. The goal of the study was to understand controls on riverine ?88/86Sr values in a landscape with multiple factors that may influence chemical weathering, including dense vegetation, high rainfall, and abundant, freshly-eroded Holocene fluvio-glacial and landslide debris.Rivers draining gabbro have higher ?88/86Sr values than bedrock, by as much as ?0.14‰, and the ?88/86Sr values strongly correlate with molar Ca/Sr ratios (R2 = 0.69). Leaching of rocks and sediment reveals no evidence for the preferential dissolution of minerals having high ?88/86Sr values and Ca/Sr ratios. In-stream Sr isotope fractionation seems unlikely because comparison against 87Sr/86Sr and Ca/Sr ratios demonstrates that riverine ?88/86Sr values conservatively trace water-mass mixing. The riverine data are best explained by the input of soil water, which is distinct from potential bedrock end-members (i.e., silicates and carbonates) based on ?88/86Sr but indistinguishable in terms of Ca/Sr and 87Sr/86Sr. While strontium isotope fractionation during secondary mineral formation and pedogenesis is possible, clay mineral formation is minor and most soils are poorly developed. Instead, soil water ?88/86Sr values more likely reflect plant uptake. Plant samples yielded a wide range of ?88/86Sr values, but on average, they are lower than those for bedrock, consistent with the expectation that plants preferentially incorporate lighter Sr isotopes. Mass-balance constraints, together with 87Sr/86Sr ratios, indicate that soil water ?88/86Sr values are ?0.30‰ higher than bedrock ?88/86Sr values, and mixing calculations show that the plant-fractionated soil water pool contributes ?27% of the riverine Sr. For tributaries accessing limestone and volcanogenic sediments, Ca/Sr and 87Sr/86Sr ratios appear consistent with two-component mixing between silicate and carbonate weathering, but ?88/86Sr values reveal a third contribution from soil water inputs, similar to gabbro catchments.The results of this study suggest that Sr isotopes behave conservatively during water mass mixing and stream transport but non-conservatively in soil, where plant uptake can elevate soil water ?88/86Sr values relative to bedrock. Plant uptake, or related biogeochemical processes, such as ion-exchange on organic matter surfaces, also appear to modify soil water Ca/Sr ratios. Many studies use 87Sr/86Sr and Ca/Sr ratios to apportion riverine solutes between silicate and carbonate weathering, but Ca/Sr ratios may be non-conservative in densely vegetated areas. The stable Sr isotope tracer shows promise for resolving riverine cation sources, as well as effects from biological cycling
The Gold Conveyor Belt: Large-scale gold mobility in an active orogen
No full textThe 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
Metabasalts as sources of metals in orogenic gold deposits
No full textAlthough 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
Features of seafloor hydrothermal alteration in metabasalts of mid-ocean ridge origin from the Chrystalls Beach Complex
No full textThe Taieri Mouth locale of the Chrystalls Beach Complex (CBC) in the South Island of New Zealand includes well preserved to strongly deformed pillow lavas and flattened veins of epidote, quartz and chlorite intercalated with basalt flows and volcanoclastic breccias. The tectonic affinity for this rare igneous portion of the predominantly sedimentary CBC has not been well established in the context of its regional metamorphic geology. New field, petrographic, geochemical and isotopic evidence suggest a mid-ocean ridge origin for the Taieri metabasalts. Further, paleo-vertical networks of epidote-quartz-chlorite veins and cross-cutting faults provide a record of seafloor fracturing and fluid-flow. Altered pillows and epidote separates have δ18O isotope values ranging from 9.3 to 13.1‰. This indicates slightly enriched δ18O fractionation resulting from seafloor weathering and low-temperature (<250°C) exchange between seawater and hydrothermal fluids in basaltic fractures. Age-corrected 87Sr/86Sr ratios between 0.704135 and 0.70624 show low temperature fluid-rock interactions where the altered pillows and veins did not succumb to major mineralogic changes or isotopic re-equilibration after formation. In contrast, compressed s-fold epidote and coarse quartz veins near metasediments are suggestive of the elevated temperatures and pressures during accretion. We differentiate between episodic seafloor venting and accretional wedge-related alteration recorded within these metabasalts
Multiple episodes of serpentinite alteration revealed by progressive leaching experiments
No full textSerpentinized mantle rocks typically experience multiple alteration events, but to date, identifying distinct serpentinization episodes remains difficult because whole rock geochemical and isotopic analyses reflect the cumulative sum of fluid-rock interactions. Here we unravel the alteration history of serpentinized mantle rocks by undertaking multi-stage progressive leaching experiments to isolate distinct geochemical and 87Sr/86Sr signatures hosted within serpentinized peridotites from the Troodos Mantle Sequence and Limassol Forest Complex, Cyprus that have different geological histories. Whole rock powders underwent an initial 10 % acetic acid solution leaching step before a subsequent 3 M HCl treatment. The remaining residue was then completely dissolved and analysed following a standard HF and HNO3 digestion. For comparison an untreated sample of whole rock powder was also completely digested and analysed. In the Troodos Mantle Sequence, an initial serpentinization event with relatively primitive 87Sr/86Sr ratios (0.705 to 0.706) is recorded by the rock residues and is consistent with alteration by fluids sourced from the dehydrating downgoing Cyprus slab. A more radiogenic 87Sr/86Sr (0.7086–88) signal, consistent with Cyprus Messinian seawater and evaporites, is mobilised by the initial leaching step. These contrasting signatures in the mantle rocks are similar to the Sr-isotopic compositions of Troodos Mantle Sequence groundwaters. Although previous field observations within the Limassol Forest Complex indicate some serpentinization by Cretaceous seawater, evidence from progressive leaching experiments of pervasive serpentinization on the Cretaceous seafloor is not forthcoming. Our results show that most samples from the Limassol Forest Complex yield a radiogenic 87Sr/86Sr signal (0.7087), most consistent with mid-Miocene Messinian seawater and broadly coeval with significant mid-Miocene uplift of the Limassol Forest Complex. These results demonstrate that progressive leaching approaches can reveal distinct alteration episodes with contrasting 87Sr/86Sr ratios and geochemical signatures that contribute to the overall integrated bulk rock signal.</p
Transformation of graphite by tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand
No full textGraphite is a material with one of the lowest frictional strengths, with coefficient of friction of 0.1 and thus in natural fault zones it may act as a natural solid lubricant. Graphitization, or the transformation of organic matter (carbonaceous material, or CM) into crystalline graphite, is induced by compositional and structural changes during diagenesis and metamorphism. The supposed irreversible nature of this process has allowed the degree of graphite crystallinity to be calibrated as an indicator of the peak temperatures reached during progressive metamorphism. We examine processes of graphite emplacement and deformation in the Alpine Fault Zone, New Zealand's active continental tectonic plate boundary. Raman spectrometry indicates that graphite in the distal, amphibolite-facies Alpine Schist, which experienced peak metamorphic temperatures up to 640 ◦C, is highly crystalline and occurs mainly along grain boundaries within quartzo-feldspathic domains. The subsequent mylonitisation in the Alpine Fault Zone resulted in progressive reworking of CM under lower temperature conditions (500◦C-600◦C) in a structurally controlled environment, resulting in spatial clustering in lower-strain protomylonites, and further foliation-alignment in higher-strain mylonites. Subsequent brittle deformation of the mylonitised schists resulted in cataclasites that contain over three-fold increase in the abundance of graphite than mylonites. Furthermore, cataclasites contain graphite with two different habits: highly-crystalline, foliated forms that are inherited mylonitic graphite; and lower-crystallinity, less mature patches of finer-grained graphite. The observed graphite enrichment and the occurrence of poorly-organised graphite in the Alpine Fault cataclasites could result from: i) hydrothermal precipitation from carbon-supersaturated fluids; and/or ii) mechanical degradation by structural disordering of mylonitic graphite combined with strain-induced graphite localisation. The lack of published systematic studies of mechanical modification of the structure of graphite inhibits further conclusion to be drawn. Thus, we performed laboratory deformation experiments during which we sheared highly crystalline graphite powder at room temperature, normal stresses of 5 MPa and 25 MPa and sliding velocities of 1 μm/s, 10 μm/s and 100 μm/s. The degree of graphite crystallinity, both in the starting and resulting materials, was analysed by Raman microspectroscopy. Our results demonstrate consistent decrease of graphite crystallinity with increasing shear strain. We conclude that: i) graphite 'thermometers' are unreliable in brittely deformed rocks; ii) a shear strain calibration of graphite 'thermometers' is needed; iii) fault creep is very likely responsible for the observed structural and textural characteristics of graphite in the Alpine Fault cataclasites. Finally, to investigate the possibility of hydrothermal origin for at least some of the graphite in the Alpine Fault cataclasites we will also present synchrotron FTIR and carbon isotope analysis of the Alpine fault rocks
Hydrothermal alteration within the alpine fault zone
Full text linkHydrothermal alteration, hosted in brittle structures, has been documented along the length of the Alpine Fault Zone. The along-strike profile has been divided into two geographical groups based on differing fault zone character. The central section, Havelock Creek to the Whataroa River, and the northern section, Whataroa River to the Taramakau River, host different late-stage hydrothermal assemblages. Alteration occurs as veins, cemented breccias, and partially or wholly recrystallised fault rocks throughout the ~1 km wide fault zone.
Geochemical investigations suggest that gouges, cataclasites, and mylonites within the hangingwall of the Alpine Fault Zone have been derived from a metabasite-poor, intermediate greywacke protolith of Torlesse affinity. Western Province derived fault rocks are not confirmed in the hangingwall. Cataclasites and gouges are retrogressed, hydrated equivalents of adjacent schist-derived amphibolite (upper greenschist) facies mylonite. CaO, MgO, Fe203 and SrO are mobile while TiO2, Al2O3, and K2O are immobile within the Alpine Fault Zone. Most base metals show no distinctive enrichment or depletion trends. Cu and As occur in anomalously high concentrations at Dickson River and tributaries of Wainihinihi River, respectively.
δ13C and δ18O results for vein and fault gouge calcites indicate a derivation from a meteoric fluid with a minor rock exchanged component (δ13C -7.0 to 1.6‰ ; δ18O 10.4 to 27.4‰; n=10). The analysed ankerite veins and breccias were derived from a mixed, meteoric rock-exchanged fluid (δ13C -7.6 to 3.4‰; δ18O 12.1 to 25.2‰; n=21). Along-strike variation in isotope signatures is negligible.
Retrogression within cataclasites and fault gouges is pervasive. Green mylonites have been thoroughly retrogressed under semi-ductile conditions while localised retrogression has occurred proximal to fault/ fracture systems within the Alpine Fault Zone.
Ankerite is the dominant alteration phase in the northern section while calcite is the major alteration phase in the central section. Alteration volume is greater in the northern section where three hydrothermal breccias and numerous vein networks have been reported. No hydrothermal breccias and minimal vein networks are reported from the central section of the Alpine Fault Zone. Throughout the entire strike length of the fault zone, increases in metallic mineralisation correspond to increases in dilatancy and fluid-rock ratios (i.e. cemented breccias contain highest amounts of metals).
Upper greenschist facies rocks, particularly chlorite-bearing, argillaceous-type mylonites, appear to be the best sources of ankeritic carbonate. Lowered geothermal gradients may also enhance ankerite stability and promote ferromagnesian mineralisation to the north.
Strain and seismicity are closely linked to hydrothermal mineralisation. Local fault-fracture networks, especially high-angle minor faults, have acted as permeable flow networks channelising fluids and providing favourable mineralising sites. Gouges and cataclasites have acted as fluid distributors and poor mineralising sites.
Active and passive porosity and permeability have been assessed for the Alpine Fault Zone. Passive estimates of porosity from several outcrops and specimens have permitted the extrapolation of passive and active porosity and permeability at depth during and shortly after seismic events. Using this approach, brecciated and fractured mylonite are found to be the most permeable mediums during passive (interseismic) periods. During active (co- and post-seismic) intervals, fluids are believed to be discharged from fluid sinks (cataclasites, gouges) and focused onto backbone networks (i.e. minor faults, large fractures) within fractured mylonites.
Brittle strain distribution is highest to the north, possibly associated with the crustal-scale Alpine Hope Fault junction. This increased fault/fracture distribution correlates with frequent shallow earthquakes and elevated levels of hydrothermal alteration. The Wainihinihi River area, lying near the fault junction, contains anomalously high arsenic which is thought to represent a deeper fluid source. It is suggested that enhanced shallow seismicity has created a crustal plumbing network which taps deeper, metal-rich metamorphically-derived fluids. The nearby Wilberforce Valley (Main Divide) is characterised by similar arsenic, isotope and seismicity signatures suggesting that the hydrothermal systems may be regionally related
The nature and origin of gold mineralization at Sams Creek, north-west Nelson
Full text linkGold mineralization is hosted by a peralkaline granite dyke which outcrops over about eight kilometers in the area of Sams Creek, a tributary of the Takaka River, north-west Nelson. The granite is riebeckite and acmite bearing, with antiperthitic alkali feldspars; it has A-type chemistry, implying an origin by partial melting at a temperature > 800°C. Associated with the granite is a suite of lamprophyre dykes; now highly altered, they probably range from original camptonites to quartz - bearing calc-alkaline types. A quartz- bearing lamprophyre has intruded along the footwall contact of the granite over much of the observed outcrop length; it may have been a source rock for gold. Granite intrusion is inferred to have occurred before or during the 0 2 deformation in the surrounding shales and quartzites, suggesting a minimum age of 400Ma. Deformation involving E-W compression has caused the granite to break into essentially rigid pieces, of the order of 50m in size, surrounded by ductilely deforming sediments. Strain has been concentrated into the footwall lamprophyre, which has also served as a conduit for hydrothermal fluids. Locally, small brittle - ductile shear zones occur within the granite; some of them post - date the mineralization. Tectonic imbrication of the granite and the development of fracture permeability have probably controlled the locus of gold mineralization.
Two styles of hydrothermal alteration are observed: in Main Zone, hydrolysis of riebeckite produced the assemblage magnetite+ siderite; an influx of Fe, As, S and associated Au then produced the assemblage quartz + albite + K-feldspar + pyrite + arsenopyrite. Gold is present as the native element, poikilitically enclosed by arsenopyrite. In the western part of the area, iron has been lost during the early stages of alteration to produce the assemblage quartz - albite - K-feldspar; with further reaction, feldspars alter to sericite, and pyrite and arsenopyrite are deposited. Gold values are lower than in Main Zone. The mineralizing fluids are inferred to have been metamorphic in origin. Geothermometry based on the assemblage arsenopyrite - pyrite - sphalerite suggests a temperature of 370 ±50 °C for Main Zone. Fluid inclusions in the western part of the area show evidence of immiscibility between H2O and CO2; their homogenization temperature directly yields an estimate of 260 ± 25°C for the hydrothermal fluid. Water-bearing inclusions from Main Zone, taken in conjunction with the arsenopyrite – sphalerite geothermometer, suggest a pressure of between one and two kilobars for the mineralization. In Main Zone, progressive hydrolysis of granite was accompanied by falling aO2 , which resulted in the precipitation of arsenopyrite; this in turn caused a localized fall in total. dissolved sulphur which caused gold to co-precipitate. The fluid pH was ~ 9. Further west, in the sericitically altered zone, the fluid pH was buffered to between 5 and 5.4 by the alteration of K-feldspar to sericite. The lower pH has been critical in limiting gold mineralization in the western part of the area
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