Fluid-rock Reactions Research Articles

Complex proterozoic to paleozoic history of the upper mantle recorded in the Urals lherzolite massifs by Re–Os and Sm–Nd systematics

Re–Os and Sm–Nd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions. The Re–Os and Sm–Nd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals. The oldest Re–Os age of 1250 ± 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over ∼ 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent. The younger Sm–Nd age of Mindyak peridotites (882 ± 83 Ma) and Re–Os age of associated gabbros (804 ± 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between ∼ 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian arc in the Southern Urals. A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at ∼ 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation. The combination of 187Re– 187Os and 147Sm– 143Nd systematics for peridotites, mafic–ultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation.

Hydrothermal alteration zonation and fluid chemistry of the Southern Ridge and North deposits at Mt Tom Price

The Mt Tom Price deposit is a world-class high-grade hematite deposit in the Hamersley Province of Western Australia with an original resource of 900 Mt of almost pure hematite, averaging 63·9 wt-%Fe. Petrological and geochemical studies at both the Southern Ridge and the North deposit at Mt Tom Price have identified three hypogene alteration zones between unmineralised banded iron formation (BIF) and high-grade iron ore: distal magnetite-siderite-stilpnomelane; intermediate hematite-magnetite-ankerite-talc-chlorite; and proximal martite-microplaty hematite-magnetite-apatite alteration zones. Fluid inclusions trapped in siderite within the distal magnetite-siderite-stilpnomelane alteration zone at the North deposit revealed primary high salinity (25·5 eq.wt-%NaCl–CaCl2) inclusions that homogenised between 107 and 142°C into liquid. Fluid inclusions trapped in ankerite within ankerite-microplaty hematite veins in the intermediate hematite-ankerite-magnetite-talc-chlorite alteration zone at the North deposit revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 22·4–25·4 equivalent wt-%CaCl2 and 22·9–25·9 equivalent wt-%CaCl2 respectively. Pseudosecondary inclusions homogenised between 153 and 449°C and secondary inclusions homogenised between 103 and 157°C. Fluid inclusions trapped in apatite within talc-microplaty hematite veins in the intermediate hematite-magnetite-talc-chlorite-apatite alteration zone at Southern Ridge revealed that primary, medium salinity (7·0–9·0 equivalent wt-%NaCl) inclusions homogenised between 181 and 257°C and primary high salinity (22·8–25·9 equivalent wt-%CaCl2) fluid inclusions homogenised between 118 and 257°C into liquid. Microthermometric analysis of quartz from quartz-hematite veins from the Southern Batter Fault, Southern Ridge shows a complex fluid inclusion history. Primary fluid inclusions consist of: low and high salinity H2O–NaCI inclusions trapped at temperatures of approximately 140–230°C; vapour-rich inclusions of unknown compositions; and 'complex' salt-rich (Ca, Mg, K and Na inclusions). Secondary inclusions consist of medium-salinity fluid inclusions trapped at temperatures of about 140–280°C. A two stage hydrothermal model is proposed for the formation of both the Southern Ridge and North deposits. Early stage 1a hypogene alteration involved the release of hydrothermal NaCl-CaCl2-rich (25·5 equivalent wt-%) basinal brines (110–150°C) from the underlying Wittenoom Formation and directed upward along normal faults and focused within the silica-rich rocks of the Dales Gorge Member, by the shales of the underlying Mt McRae Shale Member and overlying Whaleback Formation. Within the Dales Gorge Member hydrothermal basinal brines migrated laterally within large-scale folds with permeability controlled by shale bands and the NW trending dolerite dyke sets. Fluid rock reactions transformed unmineralised BIF to magnetite-siderite-iron silicate BIF, with subsequent desilification of the chert bands. Stage 1b hypogene involved an increase in temperature of the hydrothermal, CaCl2-rich saline (24 equivalent wt-%) basinal brines (250–300°C) resulting in formation of hematite-ankerite-magnetite- talc-chlorite alteration and the crystallisation of microplaty hematite. Late stage 1c hypogene alteration involved the interaction of low-temperature (∼120°C) basinal brines with the hematite-ankerite-magnetite mineral assemblage. At the Southern Ridge deposit this process was very intense (i.e. with a high fluid flux), therefore resulting in the almost total removal of ankerite and resulting in the increased porosity of the ore. Stage 2 supergene enrichment, during the Tertiary, resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.

Models of permeability contrasts in subduction zone mélange: Implications for gradients in fluid fluxes, Syros and Tinos Islands, Greece

Geological and geochemical evidence from previous studies suggest limited fluid flow through the interiors of many blocks of subducted oceanic crust within meta-ultramafic or metasedimentary mélange matrix on the islands of Syros and Tinos, Cyclades, Greece. This finding is provocative because metamorphic devolatilization of igneous and sedimentary rocks during subduction should have liberated substantial volumes of fluid. In an effort to address this problem, two-dimensional numerical models incorporating permeability anisotropy and devolatilization are used to investigate spatial patterns of steady-state fluid flow through subduction zone mélange under high-pressure/low-temperature (HP/LT) conditions. The modeling shows that as the permeability of the mélange blocks decreases relative to the mélange matrix, more flow is diverted around the blocks into the matrix. The ratio of the maximum flux (in matrix) relative to the minimum flux (in blocks) increases with the permeability contrast between the matrix and the less permeable blocks. Order-of-magnitude variations in permeability produce order-of-magnitude spatial variations in fluid fluxes between block interiors and surrounding matrix; flux variations of a factor of 10 or more can be present over very short length scales (as little as meter scale). The largest fluxes are produced in the matrix adjacent to block margins lying sub-parallel to regional foliation and flow direction. Fluid–rock reaction on block margins would be greatest in these areas, leading to asymmetrical distributions of reaction progress around blocks (illustrated herein by field mapping of a mélange block on Syros). However, syn-metamorphic deformation can rotate blocks and strip off reaction zones at their margins, so it is likely that reaction zone asymmetries are commonly disrupted in nature. The inferred absence of strong fluid–rock reaction within mélange blocks on Syros and Tinos could reflect low permeabilities in block interiors which acted to divert flow into mélange matrix. Consequently, fluxes of subduction zone fluids through the mélange matrix could have easily been very large, while low permeabilities shielded block interiors from the regional flow to varying degrees.

Development of spatial variations in reaction progress during regional metamorphism of micaceous carbonate rocks, Northern new England

Progress (ξ) of the infiltration-driven reaction, muscovite + ankerite + quartz + rutile + H₂O = biotite + calcite + plagioclase + CO₂, which occurred during Barrovian and Buchan regional metamorphism in east-central Vermont and south-central Maine, can vary by a factor of ten or more at all spatial scales down to that of adjacent lithologic layers <1 cm thick. Values of proxies for the activity of CO₂ and δ¹⁸O_fluid, K_s(6) ≡ [(a_phl)(a_an)(a_cal)²]/[(a_ms)(a_dol)³(a_qtz)²] and δ¹⁸O_Cal, are uniform within error of measurement over distances up to ≈1 m across layering. The conventional explanation of cm- to dm-scale variations in ξ in terms of layer-parallel channeled fluid flow cannot explain the uniformity in the proxies. Observed cm- to dm-scale variations in ξ are better explained by (a) mineral reactants and products that are solid solutions, (b) layer-by-layer variations in the amounts and compositions of minerals prior to reaction, and (c) uniformity of K_s(6) on a spatial scale larger than the scale of variations in ξ during subsequent infiltration and reaction. The m-scale uniformity in K_s(6) and δ¹⁸O_Cal is interpreted as homogenization of a_CO₂ and δ¹⁸O_fluid caused by the combined effects of intergranular diffusion and hydrodynamic dispersion. Reaction progress therefore was driven by the interplay of layer-parallel advection of chemically-reactive H₂O-rich fluid at decameter and larger scales and cross-layer transport of CO₂ and H₂O by diffusion/dispersion at scales of ≈1 m and less. Regardless of whether mineral reactants and products are solid solutions, the geochemical tracer considered, or the mechanism of fluid-rock reaction, the geometry of fluid flow can never be determined at a scale smaller than the one over which the concentration of the tracer is homogenized in the fluid within error of measurement by diffusion and dispersion.

Australian Zn-Pb-Ag Ore-Forming Systems: A Review and Analysis

Zn-Pb-Ag mineral deposits are the products of hydrothermal ore-forming systems, which are restricted in time and space. In Australia, these deposits formed during three main periods at ~2.95, 1.69 to 1.58, and 0.50 to 0.35 Ga. The 1.69 to 1.58 Ga event, which accounts for over 65 percent of Australia’s Zn, was triggered by accretion and rifting along the southern margin of Rodinia. Over 93 percent of Australia’s Zn-Pb-Ag resources were produced by four ore-forming system types: Mount Isa (56% of Zn), Broken Hill (19%), volcanic-hosted massive sulfide (VHMS; 12%), and Mississippi Valley (8%). Moreover, just 4 percent of Australia’s land mass produced over 80 percent of its Zn. The four main types of ore-forming systems can be divided into two “clans,” based on fluid composition, temperature, and redox state. The Broken Hill- and VHMS–type deposits formed from high-temperature (>200°C) reduced fluids, whereas the Mount Isa- and Mississippi Valley-type deposits formed from low-temperature (<200°C), H2S-poor, and/or oxidized fluids. The tectonic setting and composition of the basins that host the ore-forming systems determine these fluid compositions and, therefore, the mineralization style. Basins that produce higher temperature fluids form in active tectonic environments, generally rifts, where high heat flow produced by magmatism drives convective fluid circulation. These basins are dominated by immature siliciclastic and volcanic rocks with a high overall abundance of Fe2+. The high temperature of the convective fluids combined with the abundance of Fe2+ in the basin allow inorganic sulfate reduction and leaching of sulfide from the country rock, producing reduced, H2S-rich fluids. Basins that produce low-temperature fluids are tectonically less active, generally intracratonic, extensional basins dominated by carbonate and variably mature siliciclastic facies with a relatively low Fe2+ abundance. In these basins, sediment maturity depends on the paleogeography and stratigraphic position in an accommodation cycle. Volcanic units, if present, occur in the basal parts of the basins. Because these basins have relatively low heat flow, convective fluid flow is less important, and fluid migration is dominated by expulsion of basinal brines in response to local and/or regional tectonic events. Low temperatures and the lack of Fe2+ prevent in-organic sulfate reduction during regional fluid flow, producing H2S-poor fluids that are commonly oxidized (i.e., ∑SO4 > ∑H2S). Fluid flow in the two basin types produces contrasting regional alteration systems. High-temperature fluid-rock reactions in siliciclastic-volcanic–dominated basins produce semiconformable albite-hematite-epidote assemblages, but low-temperature reactions in carbonate-siliciclastic–dominated basins produce regional K-feldspar-hematite assemblages. The difference in feldspar mineralogy is mostly a function of temperature. In both basin types, regional alteration zones have lost, and probably were the source of, Zn and Pb. The contrasting fluid types require different depositional mechanisms and traps to accumulate metals. The higher temperature, reduced VHMS- and Broken Hill-type fluids deposit metals as a consequence of mixing with cold seawater. Mineralization occurs at or near the sea floor, with trapping efficiencies enhanced by sub-surface replacement or deposition in a brine pool. In contrast, the low-temperature, oxidized Mount Isa- and Mississippi Valley-type fluids precipitate metals through thermochemical sulfate reduction facilitated by hydrocarbons or organic matter. This process can occur at depth in the rock pile, for instance in failed petroleum traps, or just below the sea floor in pyritic, organic-rich muds.

Rock-Buffering of Auriferous Fluids in Altered Rocks Associated with the Golden Mile-Style Mineralization, Kalgoorlie Gold Field, Western Australia

The Kalgoorlie gold field contains structurally controlled, epigenetic gold deposits hosted by mafic rocks in the Archean Yilgarn craton of Western Australia. Its giant size has prompted much interest in the processes that led to its formation, particularly of the Golden Mile mineralization, which hosts over 70 percent of the gold in the Kalgoorlie gold field. It is generally agreed that the widespread presence of hematite and the moderately negative sulfur isotope composition of some of the pyrite (δ34S of −10 to −2‰) in the Golden Mile lodes and associated alteration indicate the presence of a relatively oxidizing ([SO2−4 ] ~ [HS−] + [H2S], with hematite stable) fluid during gold deposition and wall-rock alteration, but the origin and evolution of this fluid are not well constrained. A piece of evidence that has not been fully integrated into interpretations is the low variance of mineral assemblages in the alteration haloes of the Golden Mile lodes (e.g., coexisting magnetite-hematite-siderite-pyrite-ankerite-albite-muscovite-ilmenite ± rutile-quartz ± chlorite). Thermodynamic modeling, using HCh and a purpose-built code that facilitates investigation of systems that involve complex mineral solid solutions, CO2-rich fluids, and open-system chemical behavior was used to investigate the constraints that the low variance assemblages place on the source and evolution of mineralizing fluids. Results of the modeling show that fluid-rock reaction with decreasing temperature can drive pyrrhotite-magnetite assemblages, in equilibrium with a fluid that contains aqueous sulfide, to hematite-pyrite-magnetite assemblages in equilibrium with a fluid that contains aqueous sulfate. This modeled shift arises from cooling-driven oxidation of sulfides and reduced sulfur-bearing aqueous species by ferric iron in magnetite and formation of hematite and siderite from magnetite and CO2; there is no requirement for electron acceptors other than those provided by the rock. The implication of the model results for the Golden Mile mineralization is that hematite growth and sulfate-bearing fluids could have resulted from fluid-wall rock interaction without involvement of an externally derived oxidizing fluid. The change from aqueous-sulfide dominated to aqueous-sulfate–rich solutions would destabilize gold sulfide complexes in solution and lead to gold precipitation. Formation of aqueous sulfate species would also result in the precipitation of pyrite with negative δ34S. Mass balance calculations show that production of hematite in the carbonate zone of the Golden Mile mineralization could have occurred without any requirement for addition of fluid-derived electron acceptors although open-system behavior is not precluded. Overall, the characteristics of the carbonate zone alteration are consistent with electron redistribution caused by interaction between a reduced auriferous fluid and the host dolerite.

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boleo district of the Miocene Santa Rosalia basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boleo Formation is characterized by modified marine carbon (δ13CPDB=−6.0 to +4.4‰) and oxygen (δ18OSMOW=+19.5 to +26.2‰) isotope composition due to the influx of 13C- and 18O-depleted fluvial water. Sulfate sulfur isotope composition (δ34SCDT=+17.21 to +22.3‰ and δ18OSMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ34S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ34S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ13C=−11.6 to −3.2‰ and δ18O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ13C=−7.9 to +4.3‰ and δ18O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.

Evolution of fracture permeability through fluid–rock reaction under hydrothermal conditions

We report flow-through experiments on a natural fracture in novaculite under moderate effective stresses (∼ 1.4 MPa) and temperatures (20–120 °C) to examine the effect on flow and transport characteristics. The efflux of fluid and dissolved minerals were measured throughout the 3150-h experiment. After the experiment the fracture was imaged by X-ray CT, impregnated with Wood's metal, and a cast recovered of the Wood's metal-filled fracture. These measurements constrain the evolution of fracture structure, and the change in permeability that resulted from stress- and temperature-dependent dissolution at both propping asperities and fracture void surfaces. During the first 1500 h, the aperture of one fracture decreased from 18.5 to 7.5 μm, when it was loaded with constant effective stress of 1.4 MPa, at room temperature, and a flow rate decreasing with time from 1 to 0.0625 mL/min. This reduction is attributed to the removal of mineral mass from bridging asperities. After 1500 h the fracture aperture increased, ultimately reaching 13 μm. Apparently the dominant dissolution process switched from prop removal to etching of the void surfaces. We used X-ray CT images, digital radiographs, and fracture casts as independent methods to constrain the resulting architecture of the evolved fracture porosity, and developed a simple process-based model to examine the relative roles of asperity removal and free-face dissolution. The comparison of the model with the measurements identifies the relative importance of mass removal at fracture faces and at propping asperities. The experiments underscore the importance of dissolution in determining the sense, the rates and the magnitude of permeability-enhancement within rock fractures stimulated by chemical permeants in geothermal and petroleum reservoirs, and to a lesser degree under natural conditions pushed far-from-equilibrium.

Origin of coarse-crystalline calcite cement in Early Ordovician carbonate rocks, Ordos basin, northern China: Insights from oxygen and carbon isotopes and fluid inclusion microthermometry

The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low T h values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher T h values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in 18O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons.

ESTUDO DE INCLUSÕES FLUIDAS EM MINERALIZAÇÃO SULFETADA VULCANOGÊNICA HIDROTERMAL E DE OURO OROGÊNICO NA SERRA DO IPITINGA, AMAZÔNIA, BRASIL

The Ipitinga Group (2.27 Ga) at the Serra do Ipitinga consists of a meta-volcano-sedimentaryunit of mafic and ultramafic metavo1canics, cordierite-antophyllite-and quartzo-chlorite-bearing rocks under1ying chemical (oxide and silicatetype BIFs) and clastic metasedimentary rocks. Part of the mafic and ultramafic lavas was hydrothermally altered, which gave rise to quartz-chlorite-bearing rocks and hydrothermal syndepositional exhalative vo1canogenic sulfide mineralization, formed by pyrrhotite, pyrite, chalcopyrite and sphalerite with Au and Ag and traces of galena , molibdenite and silver telluride. Ali the meta-volcano-sedimentary unit was deformed by ductile-brittle shearing and transformed in present-day NW-SE trending shear belts with a subvertical - NE dip. During this defomational shearing episode, hydrothermal fluids yielded gold-sheared Iodes. Twelve types of fluid inclusions identified in the Ipitinga Group distinguish two fluid systems related to three peculiar hydrothermal events. The quartz of the quartz-chlorite-bearing roeks (hydrothermally - altered basalts), the main host of the volcanogenic sulfide mineralization, eontains eleven types of fluid inclusions. Eight of these oceur only in these rocks, which can be considered to have resulted from fluid-rock reactions related to the first hydrothermal event (1). This event involved an aqueous earbonic fluid consisting mainly of CH 4 and Hp, with traces of N 2 and H 2 S. CO 2 -bearing inclusions are subordinate and contain traces of CH 4 , H 2 S and N 2. The predominance of CH 4 over CO 2 in the aqueous earbonic fluid system is coherent to proposed models for the generation of exhalative volcanogenic sulfide mineralization in mid - oceanic ridge hydrothermal systems. The presence of CH 4 in the first hydrothermal event in the Ipitinga Group is evidence of an ocean-floor metarnorphic-hydrotherrnal process, in an environment similar to present-day ocean spreading eenters. Pressure and temperature values for the sulfide mineralization were considered to be in the range of 0.7 - 2.3 kbar and 250 - 450°C, respectively. Aqueous saline fluid related to the second hidrothermal event (II) oceurs only in the cordierite-anthophyllite rocks. Quartz veins are the products of a third hydrothermal event related to duetile shearing (III) and contain inclusions of an aqueous saline fluid which can also be found in quartz of the quartz-chlorite-bearing roeks. The fluid inelusion study also pointed out the relative chronology for the hydrothermal-metarnorphic events in the Ipitinga Group.

The Environment of Vein Formation and Ore Deposition in the Purisima-Colon Vein System, Pachuca Real del Monte District, Hidalgo, Mexico

The Environment of Vein Formation and Ore Deposition in the Purisima-Colon Vein System, Pachuca Real del Monte District, Hidalgo, Mexico

Comparing closed system, flow‐through and fluid infiltration geochemical modelling: examples from K‐alteration in the Ernest Henry Fe‐oxide–Cu–Au system

AbstractPotassic alteration of rocks adjacent to, and within the Ernest Henry Fe‐oxide–Cu–Au deposit is used here as a test case to investigate fluid–rock interactions using various equilibrium dynamic geochemical modelling approaches available in the HCh code. Reaction of a simple K–Fe–(Na,Ca) brine (constrained by published fluid inclusion analysis) with an albite‐bearing felsic volcanic rock, resulted in predicted assemblages defined by (i) K‐feldspar–muscovite–magnetite, (ii) biotite–K‐feldspar–magnetite, (iii) biotite–quartz–albite and (iv) albite–biotite–actinolite–pyroxene with increasing rock buffering (decreasing log w/r). Models for isothermal–isobaric conditions (450°C and 2500 bars) were compared with models run over a T–P gradient (450 to 200°C and 2500 to 500 bars). Three principal equilibrium dynamic simulation methods have been used: (i) static closed system, where individual steps are independent of all others, (ii) flow‐through and flush, where a part of the result is passed as input further along the flow line, and (iii) fluid infiltration models that simulate fluid moving through a rock column. Each type is best suited to a specific geological fluid–rock scenario, with increasing complexity, computation requirements and approximation to different parts of the natural system. Static closed system models can be used to quickly ascertain the broad alteration assemblages related to changes in the water/rock ratio, while flow‐through models are better suited to simulating outflow of reacted fluid into fresh rock. The fluid infiltration model can be used to simulate spatially controlled fluid metasomatism of rock, and we show that, given assumptions of porosity relationships and spatial dimensions, this model is a first‐order approximation to full reactive transport, without requiring significant computational time. This work presents an overview of the current state of equilibrium dynamic modelling technology using the HCh code with a view to applying these techniques to predictive modelling in exploration for mineral deposits. Application to the Ernest Henry Fe‐oxide–Cu–Au deposit demonstrates that isothermal fluid–rock reaction can account for some of the alteration zonation around the deposit.

Applications of neutron computed tomography in the geosciences

A research program that applies neutron computed tomography (CT) to geological problems has been developed at the McClellan Nuclear Radiation Center (UCD-MNRC) owned and operated by the University of California, Davis (UCD-MNRC). The high penetration of neutrons, their sensitivity to light elements, and their special sensitivity to hydrogen (in water or organic materials) make them an ideal tool for non-destructive examination of complex materials of geological interest. There are four research foci at UCD-MNRC related to geological material characterization: (1) quantification of textures of deformed crystalline rocks; (2) properties of multi-phase flow in porous rocks and sediments; (3) distribution of organic and inorganic carbon in silicate and carbonate rocks; and (4) CO 2 sequestration through fluid-rock reaction. Examples of these research activities will be presented. A new CT infrastructure is being installed at the UCD-MNRC to meet the needs of these four research programs. Among the upgrades are the development of a “micro” CT facility and standardized CT acquisition, compatible with X-ray CT. The latter will allow direct indexing between X-ray and neutron radiograph projections and subsequent CT reconstructions.

Petrographic and geochemical evidence for hydrothermal evolution of the North Deposit, Mt Tom Price, Western Australia

High-grade iron mineralisation (>65%Fe) in the North Deposit occurs as an E-W trending synclinal sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge Member and consists of martite-microplaty hematite ore. Three hypogene alteration zones between unmineralised BIF and high-grade iron ore are observed: (1) distal magnetite-siderite-iron silicate, (2) intermediate hematite-ankerite-magnetite, and (3) proximal martite-microplaty hematite-apatite alteration zones. Fluid inclusions trapped in ankerite within ankerite-hematite veins in the hematite-ankerite-magnetite alteration zone revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 23.9±1.5 (1σ, n=38) and 24.4±1.5 (1σ, n=66) eq.wt.% CaCl2, respectively. Pseudosecondary inclusions homogenised at 253±59.9°C (1σ, n=34) and secondary inclusions at 117±10.0°C (1σ, n=66). The decrepitation of pseudosecondary inclusions above 350°C suggests that their trapping temperatures are likely to be higher (i.e. 400°C). Hypogene siderite and ankerite from magnetite-siderite-iron silicate and hematite-ankerite-magnetite alteration zones have similar oxygen isotope compositions, but increasingly enriched carbon isotopes from magnetite-siderite-iron silicate alteration (−8.8±0.7‰, 1σ, n=17) to hematite-ankerite-magnetite alteration zones (−4.9±2.2‰, 1σ, n=17) when compared to the dolomite in the Wittenoom Formation (0.9±0.7‰, 1σ, n=15) that underlies the deposit. A two-stage hydrothermal-supergene model is proposed for the formation of the North Deposit. Early 1a hypogene alteration involved the upward movement of hydrothermal, CaCl2-rich brines (150–250°C), likely from the carbonate-rich Wittenoom Formation (δ13C signature of 0.9±0.7‰, 1σ, n=15), within large-scale folds of the Dales Gorge Member. Fluid rock reactions transformed unmineralised BIF to magnetite siderite-iron silicate BIF, with subsequent desilicification of the chert bands. Stage 1b hypogene alteration is characterised by an increase in temperature (possibly to 400°C), depleted δ13C signature of −4.9±2.2‰ (1σ, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisation of microplaty hematite. Late Stage 1c hypogene alteration involved the interaction of low temperature (~120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porous martite-microplaty hematite-apatite mineral assemblage. Stage 2 supergene enrichment in the Tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO 2-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist. Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from ∼0.1 to ∼1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing. Low-salinity, CO 2-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins. Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions. Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.

Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea

The ~20-million-ounce (Moz) Porgera gold deposit, Papua New Guinea, is hosted by 6-m.y.-old alkalic intrusions and Cretaceous sedimentary rocks in which the intrusions were emplaced. Gold-bearing veins occur in three stages: (1) magnetite-sulfide-carbonate ± quartz veins with minor gold (prestage I), (2) base metal-sulfide-carbonate ± quartz ± Au veins (stage I), and (3) quartz-roscoelite-pyrite-gold veins and breccias (stage II). Stage II veins are economically the most significant. Quartz-roscoelite-pyrite-gold veins form high-grade zones associated with the Roamane fault (a late normal fault that crosscuts the intrusive complex) and in the footwall to the fault (the North zone). The North zone mineralization is the main focus of this study. The quartz-roscoelite-pyrite-gold assemblage occurs in three texturally distinct styles: (1) thin (1–5 mm) veinlets in which roscoelite-pyrite-gold are more abundant than quartz and in which roscoelite and gold also occur in the wall rock; (2) veins (5 mm to 10 cm) in which roscoelite-pyrite-gold with minor quartz form a band at the vein edges, followed by coarse-grained quartz and the vein centers commonly filled with anhydrite and carbonate; and (3) breccia veins and breccias in which wall-rock fragments are rimmed by roscoelite-pyrite-gold and minor quartz, followed by vuggy quartz infilling. Fluid inclusions from quartz in these veins and breccias are mostly liquid rich, and average salinities in individual samples range from 7.5 ± 1.0 to 9.6 ± 0.2 wt percent NaCl equiv. In five of 27 samples, an additional cluster of salinities between 4.4 and 6.2 wt percent NaCl equiv was observed. These relatively low salinity inclusions occur toward the vein center and are less abundant than high-salinity inclusions that occur toward the vein margins. Three samples exhibit a continuous salinity trend from 4.5 to 10.2 wt percent NaCl equiv. For samples where CO 2 analyses were available average corrected salinities range from 5.1 to 8.0 wt percent NaCl equiv. Average homogenization temperatures (T h ) of individual samples range from 127° ± 12° to 167° ± 25°C. The average T h of the low-salinity inclusions (145° ± 9°C) is marginally lower but overlaps with that of all high-salinity inclusions (152° ± 17°C). Gas chromatographic analyses showed that the high-salinity fluid contains up to 2 mol percent CO 2 , 0.11 mol percent CH 4 , 0.065 mol percent N 2 , and traces of C 2 H 4 , C 2 H 6 , and COS. Concentrations of Cl – (310–609 mM/l), Br – (0.28–0.75 mM/l), Li + (1.25–8.80 mM/l), Na + (462–1126 mM/l), K + (0–81 mM/l), Mg 2+ (0–7.0 mM/l), and Ca 2+ (0–185 mM/l) were determined by ion chromatography. The δ 18 O values of quartz range from 13.9 to 18.3 per mil, and calculated δ 18 O H 2 O values range from –1.2 to 4.1 per mil. The δ D H 2 O values lie between –77 and –52 per mil. The calculated isotopic composition of the hydrothermal fluid lies between that of magmatic and that of meteoric water. The δ 18 O values of carbonates range from 15.2 to 16.6 per mil. Carbon isotopes were analyzed on carbonates ( δ 13 C = –3.3 to –2.4‰), altered and unaltered sedimentary rocks (–5.4 to –4.0 and –1.5 to 0.0‰, respectively), and organic carbon in shales (–23.6 to –16.8‰). The δ 34 S pyrite values range from –9.4 to 6.1 per mil, and δ 34 S anhydrite values range from 12.4 to 17.5 permil. The petrographic and analytical results suggest that an ascending fluid interacted with the sedimentary rocks and/or fluid hosted by the sedimentary rocks at depth (rather than at the site of ore deposition). This interaction is suggested by the presence of organic-derived volatiles and high \(NH_{4}^{+}\) concentrations in inclusion fluid. In some samples, two fluids were involved in stage II vein formation. Vapor-rich fluid inclusions indicate that the hydrothermal fluid boiled locally, and continuous salinity trends suggest that fluid mixing occurred in some stage II veins. The analytical data and paragenetic information were used to estimate element concentrations in the Porgera hydrothermal fluid that were used for thermodynamic reaction path modeling using the software CHILLER. This fluid was subjected to boiling, mixing with sedimentary formation water, cooling, and fluid-rock reaction. During boiling, quartz, pyrite, gold, and K-feldspar formed; mixing resulted in the deposition of pyrite, gold, and mica; reaction with diorite produced the observed wall-rock alteration assemblage; and cooling formed quartz plus minor mica, pyrite, gold, and kaolinite. Observations, analytical data, and modeling results suggest that more than one process was involved in stage II vein formation in different locations and at different times. Boiling, mixing, and fluid-rock reaction, all accompanied by cooling, occurred intermittently over the entire depth extent of the North zone, and the size of Porgera may be a result of all of these processes occurring at the deposit.

Fluid–rock interaction during progressive migration of carbonatitic fluids, derived from small-scale trace element and Sr, Pb isotope distribution in hydrothermal fluorite

Associated with the Cretaceous Okorusu carbonatite complex (Namibia) is a hydrothermal fluorite mineralization hosted in Pan-African country rock marbles, which resulted from fluid–rock reaction between the marbles and orthomagmatic, carbonatitic fluids expelled from the carbonatite. Yellow fluorite I was deposited in veins up to 5 cm away from the wallrock contact, followed by purple and colorless fluorite II, smoky quartz and barite, a Mn-rich crust on early calcite, and pure calcite. This clear-cut sequence of mineral growth allows an investigation into fluid–rock interaction processes between the marble and the migrating carbonatitic fluid, and element fractionation patterns between the fluid and subsequent hydrothermal precipitates. Fluorite I shows a progressive change in color from dark yellow to colorless with purple laminations over time of deposition. Subsequent fluorite I precipitates show an increase in Ca, and a continuous decrease in F, Sr, REE, Y, Th, U and Pb contents. The ratios (Eu/Eu*) cn, Th/Pb and U/Pb increase whereas Y/Ho, Th/U and (La/Yb) cn decrease. The Sr-isotopic composition remains constant at 87Sr/ 86Sr = 0.70456–0.70459, but with varying, highly radiogenic Pb ( 206Pb/ 204Pb = 32–190, 238U/ 204Pb = 7–63). Fluorite II has 87Sr/ 86Sr = 0.70454–0.70459, 206Pb/ 204Pb = 18.349, and 207Pb/ 204Pb = 15.600, and a chemical composition similar to youngest fluorite I. The Mn-rich crust on early calcite accumulated REE, Ba, Pb, Zr, Cs, Th and U, developing into pure calcite with a prominent negative Ce anomaly and successively more radiogenic Sr. The calculated degrees of fluid–rock interaction, f = weight fraction of fluid/(fluid + marble), decrease from fluorite I and most fluorite II (f = 0.5) to calcite (f = 0.2–0.3) and hydrothermal quartz (f ≪ 0.1). A crush-leach experiment for fluid inclusions in the hydrothermal quartz yielded a Rb-Sr isochron age of 103 ± 12 Ma. Crush-leach analysis for the carbonatitic fluid trapped in the wallrock yielded a trend from the fluid leachate to the host quartz ( 206Pb/ 204Pb = 18.224 and 18.602, 207Pb/ 204Pb = 15.616 and 15.636, respectively) extending from carbonatite towards crustal rocks. Calculated trace element distribution coefficients fluorite/fluid are below unity throughout, and increase from La to Yb. Elements largely excluded from fluorite (Ba, Pb, LREE relative to HREE) were incorporated later into the Mn-rich crust on calcite. The trace element patterns of the hydrothermal minerals are related to changing aCO 2 and aF − in the fluid during continued fluid–marble reaction. A predominance of carbonate over fluoride complexing in the fluid as reactions proceeded controlled the Y/Ho, Th/U and REE patterns in the fluid and the crystallizing phases. Deviations from these trends indicate discontinuous processes of fluid–rock reaction.

Carbon dioxide reaction processes in a model brine aquifer at 200 °C and 200 bars: implications for geologic sequestration of carbon

The reactive behavior of supercritical CO 2 under conditions relevant to geologic storage and sequestration of C is largely unknown. Experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid–rock reactions, in addition to carbonate mineral precipitation, that may occur in a brine aquifer–aquitard system that simulates a saline aquifer storage scenario. The system was held at 200 °C and 200 bars for 59 days (1413 h) to approach steady state, then injected with CO 2 and allowed to react for another 80 days (1924 h). In addition to magnesite precipitation, silicate minerals (quartz, plagioclase, microcline and biotite) in the aquifer and the aquitard display textures (etch pits, mineralization) indicating significant reaction. Changes in elemental abundances in the brine following addition of CO 2 include pH decrease and enrichment in Cl −, partly due to supercritical CO 2 desiccation of the brine. Geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of CO 2 and precipitation of carbonate. These reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored CO 2. An understanding of multi-phase equilibrium relationships between supercritical CO 2 and aquifer–brine systems also raises new questions for a variety of geologic systems. Multi-phase fluid equilibria may, for example, account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems, particularly in sedimentary basin sandstones.

Evolution of strength recovery and permeability during fluid–rock reaction in experimental fault zones

Physical and chemical fluid–rock interactions are implicated in controlling earthquake nucleation and recurrence. In particular, interseismic compaction, sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones. In contrast, these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults. Here, we present high-temperature, hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock, and also characterise the associated changes to porosity and permeability. Cores of Fontainebleau sandstone were initially loaded to failure in a high-pressure gas–medium apparatus. The failed specimens were then hydrothermally reacted at 927°C for variable duration under isostatic conditions, and subsequently re-fractured to determine the ‘interseismic’ strength recovery. In the most extreme case, hydrothermally induced gouge compaction, cementation and crack healing resulted in 75% strength recovery after reaction for 6 h. Isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability. Strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity, consistent with previous studies on brittle failure of porous aggregates. The experimental results show how hydrothermal reactions in fault zones may lead to two competing, time-dependent effects; fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented, low-permeability gouge layer.

Volcanoes, Fluids, and Life at Mid-Ocean Ridge Spreading Centers

▪ Abstract The recent recognition of a potentially vast, unexplored hot microbial biosphere associated with active volcanism along the global mid-ocean ridge network has fundamentally shifted concepts of how planets and life coevolve. Many processes intrinsic to the dynamics of the spreading center volcanic system provide partial or complete nutritional fluxes that support diverse microbial communities that thrive under extreme conditions on and beneath the seafloor. Mantle melting, volcanism, and fluid-rock reactions transport volatiles from the asthenosphere to the hydrosphere. Volcanic heat and exothermic reactions drive circulation of nutrient-rich fluids from which chemosynthetic organisms gain metabolic energy. In turn, many of these organisms symbiotically support macrofaunal communities that populate the vents. Long-term seafloor observatories will allow exploration of linkages between volcanism and this newly discovered biosphere. Such approaches may provide essential new information about our own planet while providing critically needed insights into how we can explore other planets for life.