240 research outputs found
Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: Insights from the Troodos ophiolite
No full textThe Troodos ophiolite is an ideal location to investigate the relationships between the mobilisation of metals by hydrothermal fluids from the lower oceanic crust and the formation of volcanogenic massive sulphide (VMS) deposits. The ophiolite hosts the classic “Cyprus-type” Cu-rich VMS deposits as well as abundant zones of epidosite alteration in the lower sheeted dyke section that are significantly depleted in base metals including Cu and Zn, and are considered to be the source of the metals enriched in the overlying deposits. Previous research indicates that the Troodos VMS deposits are irregularly enriched in Au and related elements As, Sb, and Se, but the behaviour of these elements during the hydrothermal alteration of the Troodos ophiolite hitherto has been poorly investigated. Low detection limit whole rock analyses of fresh glass samples reveal that the Troodos primitive crust has a similar metal content and distribution to modern-day arc-related environments such as the Manus Basin. Compared to mid-oceanic ridge basalt (MORB), the Troodos primitive crust is enriched in As, Sb and Pb most likely due to addition from a subducting slab during crustal formation. During early stages of magmatic differentiation (9–3.5 wt% MgO) Au, As, Sb, Se, Cu, Zn and Pb behave as incompatible elements due to the sulphide-undersaturated nature of the melt. The onset of magnetite crystallisation, however, at ∼3.5 wt% MgO leads to sulphide segregation and depletion of strongly chalcophile elements (Au, Cu and Se) during continued differentiation (<3.5 wt% MgO) whereas poorly chalcophile elements (As, Sb, Zn and Pb) remain incompatible. These differences in metal behaviour can account for the Cu-rich, Zn-Pb-poor of the Cyprus-type VMS deposits as the source area rocks show high Cu fertility compared to Zn and Pb. Mobilisation of metals during hydrothermal alteration of the Troodos ophiolite is more extensive than observed in hydrothermally altered MORB. Mass balance calculations show that the epidosite zones are significantly depleted in Au (−88 ± 16%), As (−89 ± 23%), Sb (−60 ± 12%), Se (−91 ± 20%), Cu (−84 ± 18%), Zn (−63 ± 9%) and Pb (−60 ± 8%). Background altered diabase from outside epidosite zones shows similar metal depletions which suggests that the source areas of VMS are not restricted to epidosite zones but are extended to the lower sheeted dyke section. The masses of metals mobilised from a source area of 10.9 km3, (composed of a 5 km3 epidosite zone and 5.9 km3 of background altered diabase) in the Solea graben are 47 t Au, 21 kt As, 1200 t Sb, 3100 t Se, 2.4 Mt Cu, 1.8 Mt Zn and 27 kt Pb. Comparison of metal quantities mobilised from lower sheeted dike section in the Solea graben with those hosted in VMS deposits shows trapping efficiencies ranging from 4 to 37% indicating that most of the metals is lost by other processes
The behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization: evidence from the Otago and Alpine Schists, New Zealand
No full textMetamorphism is a major mechanism for the re-distribution of fluids and mass in the Earth's crust, with these processes most prominently highlighted by the occurrence of major gold resources within these terranes. However, although orogenic gold deposits have contributed over 20% of the global gold production, their origins remain controversial. The nitrogen concentration and isotopic composition of rocks and minerals are potentially powerful tracers of crustal metamorphism and mineralization, but there have been few detailed applications of this approach to date. Although nitrogen isotopes have recently been used to elucidate the source of fluids in some Neoarchean orogenic gold deposits and Proterozoic to Paleozoic mountain belts, due to their age and geological complexity of these terranes, major uncertainties as to the behavior of nitrogen remain. The Otago and Alpine Schists in the South Island of New Zealand comprise a large, comparatively young (< 190 Ma), metasedimentary belt with multiple generations of quartz ± carbonate veins, some of which are mineralized with gold. A range of rocks, with little primary compositional variation, is exposed from unmetamorphosed protolith to high-grade amphibolites and as such they present an ideal laboratory to investigate the mobility of nitrogen and potential nitrogen isotopic fractionations during metamorphism and mineralization. Here we present nitrogen concentrations and isotopic analyses of whole rock samples and mica separates from a number of crustal transects through the Otago crust.The range of ?15N values for mica and whole rock samples from the schists spans 0.2 to 7.0‰, and the nitrogen concentration from 23 to 3483 ppm. Sample provenance and rock type have minimal influence on the nitrogen concentration and isotopic value, which appears to have been inherited from the original sedimentary kerogen. There is no systematic variation between metamorphic temperature and ?15N or N concentration in micas, suggesting that there has been little discernible loss of 15N-depleted fluids from silicates with increasing metamorphic temperature. Comparison of fluid mobile alkali element (K, Rb and Cs), carbon and nitrogen whole rock concentrations, indicates that for the Otago and Alpine Schists, in rocks up to upper greenschist facies significant nitrogen remains hosted in phases other than micas, most likely poorly matured carbonaceous material. Samples from Macraes Flat, a major gold producer, have a similar range of ?15N values to the host terrane, but show distinctly higher nitrogen concentrations relative to unmineralized samples, due to the incorporation of nitrogen from the mineralizing hydrothermal fluid. This suggests that there is only a subtle metamorphic re-distribution of nitrogen during mineralization, albeit with minimal isotopic fractionation. In the case of nitrogen at least, the isotopic signatures of mineralized rocks support a metamorphic fluid source, and are inconsistent with mantle or meteoric fluid reservoirs. However, due to the high relative abundance of nitrogen in sedimentary rocks compared to other potential reservoirs, unless fluid fluxes are very large and well channeled, nitrogen signatures are not sensitive recorders of fluid inputs from mantle, magmatic or meteoric reservoirs. Conversely, the absence of a “sedimentary”-source nitrogen isotope signature similar to the host rock in an orogenic deposit would be a very strong indicator of an external, exotic source for the mineralizing fluids
Geochemical signatures of mesothermal Au-mineralized late-metamorphic deformation zones, Otago Schist, New Zealand
No full textHydrothermal processes along two regional-scale shear zones in the Otago Schist were dominated by structurally controlled fluid flow and mineralization in the host schist, with relatively minor quartz vein formation, and mineralized rocks are only subtly different from unmineralized rocks. Most Au in the shear zones is associated with sulphide minerals (pyrite and arsenopyrite) disseminated through the host schist or along microshears. Minor enrichment of Sb, Mo and Bi (ppm level) is detectable in the Hyde-Macraes Shear Zone (HMSZ). Hydrothermal muscovite is slightly more aluminous (1–2 wt%) than metamorphic muscovite in both shear zones. HMSZ muscovite averages >900 ppm N, in contrast to metamorphic muscovite that averages c. 200 ppm N. In both shear zones, rutile has replaced metamorphic titanite and epidote has altered to carbonate and phyllosilicates, but these reactions were nearly isochemical. Structurally controlled hydrothermal graphite in the HMSZ occurs in microshears (up to 3 wt%, above background <0.2 wt%). Alteration in the Rise & Shine Shear Zone (RSSZ) was accompanied by addition of abundant ankerite. The two shear zones have subtly different geochemical signatures and are not directly genetically related. However, As enrichment is a key exploration target for both shear zones. <br/
Detecting hydrothermal graphite deposition during metamorphism and gold mineralization
No full textFourier Transform IR analyses of carbonaceous material from the Otago and Alpine Schists and the Macraes gold deposit of the South Island, New Zealand, show the progressive effects of graphitization during metamorphism and suggest that the majority of carbonaceous material identified at the Macraes deposit precipitated from hydrothermal fluids. Given that the distinction of sedimentary carbonaceous material from fluid-precipitated graphite is a key to better understanding the redox state of crustal rocks, ore depositional processes and the extent of carbon cycling in the crust, this study highlights the importance of IR spectroscopy in the characterization of reduced carbon-bearing rocks
Method for ultra-low-level analysis of gold in rocks
No full textA new method for analyzing gold at ultralow concentrations (<10 pg/g) in geological samples has been developed involving HF-aqua regia acid digestion, chromatographic separation of Au from matrix elements using DIBK supported on an inert resin, and analysis by inductively coupled plasma-mass spectrometry (ICPMS). This method has an analytical detection limit of 2 parts per trillion (pg/g), significantly lower than most routinely used methods developed for analysis of ore samples with Au concentrations considerably higher than average crustal abundance (~2 ng/g). Such methods commonly have detection limits in the low nanogram per gram range. Many areas of geological research including ore genesis, crustal mobility and redistribution, planetary differentiation, and plume volcanism require quantitative analysis of geological materials with much lower Au concentrations. We present a rapid, easy to use method where Au is separated from matrix elements onto extractant primed chromatographic resin and analyzed by quadrupole ICPMS. The method is suitable for the relatively rapid analysis of a large number of samples and is reliable over a wide range of concentrations from picogram to microgram per gram level. Analysis of four different geostandards, GXR1, GXR4, CH-3, and SARM 7, yields concentrations within error of the published concentrations with accuracies of >95%. <br/
Golden plumes: Substantial gold enrichment of oceanic crust during ridge-plume interaction
No full textMantle plume events are increasingly implicated as the source of gold (Au) in regions of the Earth that show a high Au endowment. However, the process of enriching oceanic crust in Au by plume activity is poorly understood and unconstrained. We present the first systematic study of Au concentrations in oceanic basalts as a function of distance from a plume center. We show that the influence of the Iceland plume on the Mid-Atlantic Ridge progressively enriches the oceanic crust in Au along the Reykjanes Ridge by as much as 13 times normal levels, over a distance of ?600 km, and that the enrichment can be attributed to specific plume components. This Au enrichment by the Iceland plume implies a genetic relationship between deep mantle upwelling and major gold mineralization
Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D
No full textFluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations into the mineral reactions that release metals, such as sulphide leaching, would generate better understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from Ocean Drilling Program (ODP) Hole 1256D. The ODP Hole 1256D drill core provides a unique sample suite representative of a complete section of a fast-spreading oceanic crust from the volcanic section down to the plutonic complex. The sulphide population at Hole 1256D is divided into five groups based on mineralogical assemblage, lithological location and texture: the magmatic, metasomatised, high temperature hydrothermal, low temperature and patchy sulphides. The initiation of hydrothermal alteration by downward flow of moderate temperature (250–350 °C) hydrothermal fluids under oxidising conditions leads to metasomatism of the magmatic sulphides in the sheeted dyke and plutonic complexes. Subsequent increase in the degree of hydrothermal alteration at temperatures >350 °C under reducing conditions then leads to the leaching of the metasomatised sulphides by rising hydrothermal fluids. Mass balance calculations show that the mobility of Cu, Se and Au occurs through sulphide leaching during high temperature hydrothermal alteration and that the mobility of Zn, As, Sb and Pb is controlled by silicate rather than sulphide alteration. Sulphide leaching is not complete at Hole 1256D and more advanced alteration would mobilise greater masses of metals. Alteration of oxide minerals does not release significant quantities of metal into the hydrothermal fluid at Hole 1256D. Mixing of rising high temperature fluids with low temperature fluids, either in the upper sheeted dyke section or in the transitional zone, triggers local high temperature hydrothermal sulphide precipitation and trapping of Co, Ni, Cu, Zn, As, Ag, Sb, Se, Te, Au, Hg and Pb. In the volcanic section, low temperature fluid circulation (<150 °C) leads to low temperature sulphide precipitation in the form of pyrite fronts that have high As concentrations due to uptake from the circulating fluids. Deep late low temperature circulation in the sheeted dyke and the plutonic complexes results in local precipitation of patchy sulphides and local metal remobilisation. Control of sulphides over Au, Se and Cu throughout fast-spreading mid-oceanic crust history implies that the generation of hydrothermal fluids enriched in these metals, which can eventually form VMS deposits, is strongly controlled by sulphide leaching
Experimental study of blockage of random waves by counter currents
No full textFrom February to April 1999, wave-blocking experiments have been conducted at the Laboratory of Fluid Mechanics of the Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands as a part of the Ph.D. research of I.K. Suastika. Wave blocking is a special case of wave-current interaction. It can occur when waves are propagating against a counter of which the velocity is increasing in the wave propagation direction. Blocking occurs where the intrinsic wave group velocity (cg) is equal, but opposite in sign, to the mean velocity. The counter current velocity at the blocking point is called the blocking velocity. In this thesis the experiments are described and a part of the measurement data is analysed, especially the data concerning the blocking of irregular waves. The first objective of this research is to acquire quantitative data on partial and complete blocking. The second objective is to develop a model for wave blocking. The model should describe the wave field in a situation where blocking occurs. 80, for a given incoming (generated) wave field and counter current, the model should describe the wave field up to and at the blocking point. (And beyond, in cases of partial blocking.)Hydraulic EngineeringCivil Engineering and Geoscience
Experimental study of blockage of monochromatic waves by counter currents
No full textBlockage of waves by a current can occur if waves are propagating on a spatially varying opposing current in which the velocity is increasing in the wave propagation direction. The ongoing waves become shorter and steeper while they are propagating against the current. Blocking occurs at the location where the opposing current strength is the same as the effective wave energy transport velocity, the intrinsic wave group velocity. This implies that upstream of this location, the blocking point, there is no propagation of wave energy. A question that immediately arises is what happens with the ongoing wave energy. The theory suggests that the ongoing waves will break before reaching the blocking point due to steepening of the waves. However, other mechanisms than wave breaking may also play an important role in the wave energy dissipation such as energy dissipations due to wave interactions with the turbulence and due to viscous effects. Another possibility is that the ongoing wave energy may be partially reflected at the blocking point. Some earlier studies (e.g. Long et al [1993]) have reported some evidence of wave reflection in blocking situations. The present study aims to investigate experimentally the phenomena of wave breaking and/or wave reflection in blocking situations. For this purpose a novel experimental arrangement has been designed and implemented in a laboratory flume. Previous laboratory studies utilized a constant discharge (Q) and a varying cross section (A) to obtain a longitudinal velocity gradient. This has the disadvantage that the effects of non-uniform cross-section and non-uniform velocity are mixed. In the present study, it was decided to use a constant cross-section and a non-uniform discharge, to he obtained by withdrawal of water through a perforated false bottom. CONCLUSIONS The fitted model has shown to he capable to reproduce the wave field of monochromatic waves being blocked by a counter current reasonably well, allowing us to discriminate the reflected waves and the incoming waves. However, the model in its present form contains unknown calibration coefficients whose variation requires further study and most likely a reformulation of the dissipation model.Hydraulic EngineeringCivil Engineering and Geoscience
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