Sedimentary Formation Waters Research Articles

Lithium enrichment processes in sedimentary formation waters

Reservoirs exploited for oil, gas, or geothermal energy in sedimentary basins may represent an alternative lithium resource to salars and hard rocks. Similarities in the geochemical characteristics of the basins were searched using a database of 2400 water analyses available from more than 40 sedimentary basins worldwide, notably a selection of representative geochemical trends on 13 basins. The objective was to understand better the factors controlling Li concentrations and the potentiality of such basins for the Li-recovery. Considering LiCl and MgCl relationships, major cation (Na/K, Na/Ca) and halogen (Cl/Br) ratios, Li concentrations are monitored in several basins by mixing processes between low salinity recharge waters, seawater and brines. Lithium concentrations span three orders of magnitude in brines between evaporated seawater that has passed saturation with halite and a particularly Li-enriched pole. In the latter, significant Li-enrichment is obtained when brines are primary, in equilibrium with halite, and then enriched in Li through fluid-rock interaction processes. Li-rich lithologies such as volcanic rocks, Li-enriched clay formations, or Li-phyllosilicates dispersed in siliciclastic rocks or the underlying crystalline basement probably act as the primary Li sources. Finally, Li extraction is promoted by the rise in temperature, which explains the high Li concentrations in the waters of geothermal systems. Therefore, favourable reservoirs for Li-rich brines are characterised by a brine collection from evaporites during tectonic events, such as compressive events or salt tectonics, at a relatively short distance from the present reservoir. They must be preserved from convection and surface water ingress via recharge zones to prevent dilution and Li depletion.

Formation of Fe-Cu-Au deposit in basin inversion setting in NW China: A perspective from ore-fluid halogen and noble gas geochemistry

The Qiaoxiahala deposit is a significant Paleozoic Fe-Cu-Au deposit in the northern margin of East Junggar, NW China, which has been confirmed to form in a basin inversion setting (dying back-arc basin). The composition of noble gases (Ar, Kr, and Xe) and halogens (Cl, Br, and I) extracted from fluid inclusions, hosted in quartz, calcite, and epidote from various mineralization stages of Qiaoxiahala, are investigated to trace the sources of ore fluids and discuss the ore genesis. Significant multistage mineralization at Qiaoxiahala is identified as magnetite mineralization (stage III), magnetite-pyrite mineralization (stage IV), and chalcopyrite mineralization (stage V). Syn-ore fluid inclusions show salinities varying from 13.2 to 15.3 wt% (NaCl eq.) associated with elevated I/Cl ratios (214 × 10−6 to 3950 × 10−6), uniformly low Br/Cl ratios (0.78 × 10−3 to 1.30 × 10−3), and low 40Ar/36Ar ratios (<700). The low 40Ar/36Ar ratios and Br/Cl ratios preclude the involvement of magmatic or metamorphic fluids (high 40Ar/36Ar), and sub-aerial evaporation bittern brines (elevated Br/Cl). The predominance of sedimentary formation water modified by organic-rich sediments is supported by the high I/Cl ratios, low 36Ar concentrations (less than or equal to that of Air Saturated Water, ASW), and the constant 40ArE/Cl slopes (10−5 to 10−4) of fluid inclusions from various mineralization stages in Qiaoxiahala. The elevated δ18Owater values (7.2–11.1‰) of the ore-forming fluids and the common presence of organic matter (e.g. C4H6, C2H2 and CH4) in fluid inclusions further indicated the involvement of organic-rich materials. However, different δDwater values and a significant gap between halogen and noble gas data in stages III and IV-V indicate that different sedimentary formation fluids are involved during these stages, i.e., extensive water-wall rock interactions between seawater and Beitashan Formation in stage III and formation waters formed by diagenesis and compaction in stages IV-V. Multistage mineralization, i.e., Fe-oxide and chalcopyrite (-gold), and the mixing of different non-magmatic fluids with variable origins in Qiaoxiahala are similar to many iron oxide-Cu-Au deposits (IOCGs) from the Central Andes, indicating an IOCG-like type deposit for Qiaoxiahala. These finding further suggest that non-magmatic fluids dominate the formation of IOCG deposits in the basin inversion setting at the northern margin of East Junggar.

Factors Controlling Hydrocarbon Accumulation in Jurassic Reservoirs in the Southwest Ordos Basin, NW China

AbstractThe sedimentary, paleogeomorphological and reservoir characteristics of the Jurassic Yan'an Formation in the southwestern Ordos Basin, northwestern China, were studied by means of casting thin sections, scanning electron microscopy, inclusion analysis and identification of low‐amplitude structures. A model for reservoir formation is established, and the controlling effects of sedimentary facies, paleotopography, low‐amplitude structures and formation water on oil reservoirs are revealed. There are significant differences in the sedimentary characteristics, structural morphology and paleowater characteristics between the reservoirs above the Yan 10 Member and those in the Yan 9 to Yan 7 Members. The Yan 10 Member contains fluvial sediments, whereas the Yan 9 to Yan 7 members contain delta‐plain anastomosing‐river deposits. The distribution of high‐permeability reservoir is controlled by pre‐Jurassic paleogeomorphology and sedimentary facies. Some of these facies exhibit high porosity and high permeability in a low‐permeability background. The main hydrocarbon accumulation period was the late Early Cretaceous, filling was continuous, and the charging strength altered from weak to strong and then from strong to weak. The Yan 10 reservoir is mainly controlled by the paleogeomorphology: hydrocarbons migrated upward at a high speed through the unconformity surface, and accumulated in the favorable traps formed by paleogeomorphic structural units, such as gentle slopes or channel island. Furthermore, groundwater alternation in these areas was relatively stagnant, providing good reservoir preservation conditions. The reservoirs in the Yan 9 and higher members are controlled by the sedimentary facies, low‐amplitude structure and paleowater characteristics. Hydrocarbons migrated through the three‐dimensional delivery system, influenced by favorable sedimentary facies and high‐salinity groundwater, then accumulated in the favorable low‐amplitude structural traps that formed during the hydrocarbon production period.

The geochemistry and origin of the hydrothermal water erupted at Lusi, Indonesia

The spectacular Lusi mud-eruption started in northeast Java the 29th of May 2006. Despite extensive research, the origin of the erupted water remains elusive and poorly constrained. Here we present a comprehensive study of the geochemistry of Lusi waters compared with those collected from surrounding areas, all collected between 2006 and 2013, including data from mud volcanoes and volcano-hosted hydrothermal springs. Within this broad context, the geochemical characteristics of the fluids expelled in the Lusi region suggest that we can classify the waters in three groups: 1) meteoric waters expelled in cold springs and artesian wells, 2) hydrothermal waters typically mixed with meteoric waters, and 3) formation water from marine sediments altered by diagenetic processes such as clay-mineral dehydration. Samples collected from the Lusi crater are Cl and Na dominated (up to 527 mM and 471.7 mM, respectively) similar to seawater indicating that altered sedimentary formation waters are predominant in this system. In addition they are enriched in Sr (up to 808.4 μM) and other elements commonly associated with hydrothermal systems, such as Li (up to 877.6 μM compared to 26 μM in seawater). Some of these elements are up to ten times enriched compared to seawater values. High-temperature fluid mineral interactions in the subsurface appear to have facilitated the transfer of Li and other mobile elements into the fluids. High temperature fluid-mineral interaction reactions are also supported by Si concentrations significantly higher compared to other sampled mud volcanoes in the island. Crater samples also show the highest δ18O values (+5‰ after correction for evaporation compared to +1‰ at the MV localities). 87Sr/86Sr ratios vary between of 0.7077 and 0.7083 and seem to reflect a general mixture of fluids from clay-mineral dehydration, carbonate recrystallization, alteration of volcanic rocks and hydrothermal imprint. Eight years of geochemical monitoring indicate that the composition of the deep-sourced Lusi fluids remain fairly constant through time. Thus our findings show that the Lusi crater waters represent a regional geochemical anomaly, and we suggest that a combination of high temperatures in the source region, and fluid-rock interactions with silicates and, possibly, carbonate-rich lithologies can explain the data. This is consistent with a model where the emitted gases migrate from a deep-seated (>4 km) source region, likely associated with the presence of hot igneous intrusions and/or high T reactions related to the presence of neighbouring active volcanoes.

Hydrothermal Fluid Origins of Carbonate-Hosted Pb-Zn Deposits of the Sanjiang Thrust Belt, Tibet：Indications from Noble Gases and Halogens

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization. Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 × 10 −3 ) and (2.86 ± 0.04) × 10 −3 , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) × 10 −3 . In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of ~0.8 × 10 −6 and extend from (2.1 ± 1.1) × 10 −6 to (506 ± 12) × 10 −6 . This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios. The fluid inclusions have 40 Ar/ 36 Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric 36 Ar and variable 129 Xe/ 36 Ar and 84 Kr/ 36 Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric ( 36 Ar, 84 Kr, 129 Xe) and radiogenic (e.g., 40 Ar ✼ ) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The 3 He/ 4 He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 ± 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived 3 He. The fluids with the highest 3 He/ 4 He also have 4 He/ 40 Ar ✼ close to the mantle value, suggesting the 3 He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( 40 Ar ✼ = radiogenic 40 Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H 2 S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.

Metal-rich fluid inclusions provide new insights into unconformity-related U deposits (Athabasca Basin and Basement, Canada)

The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium deposits. The scope of this study is to establish the pressure, temperature, and composition (P-T-X conditions) of the brines that circulated at the base of the Athabasca Basin and in its crystalline basement before, during and after UO2 deposition. These brines are commonly sampled as fluid inclusions in quartz- and dolomite-cementing veins and breccias associated with alteration and U mineralization. Microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data from five deposits (Rabbit Lake, P-Patch, Eagle Point, Millennium, and Shea Creek) complement previously published data for the McArthur River deposit. In all of the deposits investigated, fluid inclusion salinity is between 25 and 40 wt.% NaCl equiv., with compositions displaying a continuum between a “NaCl-rich brine” end-member (Cl > Na > Ca > Mg > K) and a “CaCl2-rich brine” end-member (Cl > Ca ≈ Mg > Na > K). The CaCl2-rich brine has the highest salinity and shows evidence for halite saturation at the time of trapping. The continuum of compositions between the NaCl-rich brine and the CaCl2-rich brine end-members combined with P-T reconstructions suggest anisothermal mixing of the two brines (NaCl-rich brine, 180 ± 30 °C and 800 ± 400 bars; CaCl2-rich brine, 120 ± 30 °C and 600 ± 300 bars) that occurred under fluctuating pressure conditions (hydrostatic to supra-hydrostatic). However, because the two brines were U bearing and therefore oxidized, brine mixing was probably not the driving force for UO2 deposition. Several scenarios are put forward to account for the Cl-Na-Ca-Mg-K composition of the brines, involving combinations of seawater evaporation, halite dissolution, mixing with a halite-dissolution brine, Mg/Ca exchange by dolomitization, Na/Ca exchange by albitization of plagioclase, Na/K exchange by albitization of K-feldspar, and Mg loss by Mg-rich alteration. Finally, the metal concentrations in the NaCl-rich and CaCl2-rich brines are among the highest recorded compared to present-day sedimentary formation waters and fluid inclusions from basin-hosted base metal deposits (up to 600 ppm U, 3000 ppm Mn, 4000 ppm Zn, 6000 ppm Cu, 8000 ppm Pb, and 10,000 ppm Fe). The CaCl2-rich brine carries up to one order of magnitude more metal than the NaCl-rich brine. Though the exact origin of major cations and metals of the two brines remains uncertain, their contrasting compositions indicate that the two brines had distinct flow paths and fluid-rock interactions. Large-scale circulation of the brines in the Athabasca Basin and Basement was therefore a key parameter for metal mobility (including U) and formation of unconformity-related U deposits.

Noble gas and halogen constraints on fluid sources in iron oxide-copper-gold mineralization: Mantoverde and La Candelaria, Northern Chile

The noble gas (Ar, Kr, Xe) and halogen (Cl, Br, I) composition of fluid inclusions in hydrothermal quartz and calcite related to the hypogene iron oxide-copper-gold (IOCG) mineralization at Mantoverde and Candelaria, Chile, have been investigated to provide new insights of fluid and salinity sources in Andean IOCG deposits. A combination of mechanical extraction by crushing and thermal decrepitation methods was applied and collectively indicate that fluid inclusions with salinities ranging from 3.4 up to 64 wt% NaCl equivalent have molar Br/Cl and I/Cl ratios of between 0.5 × 10−3 and 3.0 × 10−3 and I/Cl of between 8 × 10−6 and 25 × 10−6 in the majority of samples, with maximum values of 5.2 × 10−3 obtained for Br/Cl and 64 × 10−6 for I/Cl in fluid inclusions within individual samples. The fluid inclusions have age-corrected 40Ar/36Ar ratios ranging from the atmospheric value of 296 up to 490 ± 45, indicating the presence of crustal- or mantle-derived excess 40Ar in the fluid inclusions of most samples. The fluid inclusions have 84Kr/36Ar and 130Xe/36Ar ratios intermediate of air and air-saturated water. However, 40Ar/36Ar is not correlated with either 84Kr/36Ar or 130Xe/36Ar, and the fluid inclusion 36Ar concentrations of 0.2–3.5 × 10−10 mol/g (calculated from measured Cl/36Ar and thermometric salinity measurements) extend below the seawater value of 0.34 × 10−10 mol/g, suggesting that contamination with modern air is a minor artifact. The range of fluid inclusion Br/Cl and I/Cl ratios overlap those previously documented for the mantle and magmatic-hydrothermal ore deposits, and the fluids’ unusually low 36Ar concentration is consistent with the involvement of magmatic-hydrothermal fluids. Input of additional non-magmatic fluid components is suggested by the spread in Br/Cl and I/Cl to values characteristic of bittern brine sedimentary formation waters and near atmospheric 40Ar/36Ar. These data are compatible with mixing of magmatic-hydrothermal fluids and evaporated seawater, which was modified by interaction with back-arc basin sediments as the major control on ore formation. Low Br/Cl (<0.5 × 10−3) and I/Cl (<5.0 × 10−6) values that would provide evidence for evaporite dissolution as an important source of fluid salinity were not detected.

Noble gases (Ar, Kr, Xe) and halogens (Cl, Br, I) in fluid inclusions from the Athabasca Basin (Canada): Implications for unconformity-related U deposits

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150–200°C) at ∼1.6–1.5Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite–sudoite–dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25–35wt.% salts and high U concentrations of up to 600ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar–39Ar methodology) using a combination of noble gas extraction techniques.The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8×10−3 to 10.4×10−3, and molar I/Cl ratios of 1.8×10−6 to 8.2×10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb–Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localising U mineralisation.The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5–17.3×10−10molg−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200°C.Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.

On the occurrence of gold mineralization in the Pala Neoproterozoic formations, South-Western Chad

The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite–pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N–S to NNE–SSW or NW–SE and are interpreted as extensional shear fractures related to regional NE–SW-trending sinistral strike–slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike–slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.

External sulphur in IOCG mineralization: Implications on definition and classification of the IOCG clan

Although the sources of the ore metals remain problematic in most Iron-oxide Cu and Au (IOCG) deposits, external sulphur, either from surficial basinal brines and seawater (e.g., Central Andean and Carajás deposits) or from formation water and metamorphic fluids (e.g., the Cloncurry deposits), or introduced by magmatic assimilation of metasedimentary units (e.g., Phalaborwa), has been documented in many major Cu-rich IOCG centres. However, only the evaporite-sourced fluids yield diagnostically high δ34S values (i.e., >10‰), while sedimentary formation water or metamorphic fluids commonly have lower values and are less clearly distinguishable from magmatic fluids, as in the Cloncurry deposits in which the involvement of external fluids is revealed by other evidence, such as noble gas isotopes. On the basis of these arguments, IOCG deposits could be redefined as a clan of Cu (AuAgU) deposits containing abundant hypogene iron oxide (magnetite and/or hematite), in which externally-derived sulphur probably plays an important role for the Cu (AuAgU) mineralization. In this definition, all “Kiruna-type” magnetite deposits, hydrothermal iron deposits (e.g., skarn Fe deposits) and magnetite-rich porphyry CuAu and skarn CuAu deposits are excluded. Two subtypes of IOCG deposits are recognized on the basis of the predominant iron oxide directly associated with the Cu (Au) mineralization, whether magnetite or hematite. Neither magnetite- nor hematite-rich IOCG deposits show any preference for specific host rocks, and both range in age from Neoarchean to Pleistocene, within a broad tectonic environment.

On the origin of sellaite (MgF2)-rich deposits in Mg-poor environments

Sellaite (MgF2) forms from melts, fluids, and gases under variable temperature, pressure, fO₂, and fluid salinity conditions. It is typically associated with, but much rarer, than fluorite (CaF2). The Clara mine near Oberwolfach (Schwarzwald, Germany) is an extensive hydrothermal vein-type deposit, where sellaite occurs in huge quantities (thousands of tons) in veins of mostly Jurassic/Cretaceous age. The sellaite mineralization, occurring in gneisses altered prior to and during sellaite mineralization, represents a stockwork-like network of veins and fissures, which is overlain and sealed by sediments, preventing the inflow of and fluid-mixing with sedimentary formation waters. The occurrence of sellaite is unique among the more than 1000 hydrothermal vein-type occurrences of the Schwarzwald ore district. The favored formation of sellaite compared to fluorite requires the initial Ca/Mg-ratio of the mineralizing fluid to be unusually low. These conditions are possible if fluids equilibrate with pre-altered rocks that lost some or much of their Ca during an earlier hydrothermal alteration event. Indeed, calculations demonstrate that rock-buffered fluids of pre-altered rocks (i.e., gneiss around the Clara mine altered during prior hydrothermal events) show significantly lower Ca/Mg-ratios than fluids equilibrated with unaltered gneisses, because Ca-phases (e.g., the anorthite component of plagioclase) are more prone to hydrothermal destruction. Due to the network-like structure of the sellaite-bearing portion of the Clara fluorite vein, the fluid is shielded from sedimentary formation water, resulting in fractionation processes of the repeatedly ascending mineralizing fluid. In addition, fluid cooling and formation of water-bearing phases like illite that consume fluids, favor sellaite, and later fluorite precipitation. The rarity of this combination of prerequisites explains the limited occurrence of sellaite in hydrothermal vein-type deposits.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

Halogens and noble gases in sedimentary formation waters and Zn–Pb deposits: A case study from the Lennard Shelf, Australia

Halogen ratios (Br/Cl and I/Cl) and concentrations provide important information about how sedimentary formation waters acquire their salinity, but the possible influence of organic Br derived from sedimentary wall-rocks is rarely quantified. Here, it is demonstrated that Br/Cl versus I/Cl mixing diagrams can be used to deconvolve organic Br contributions; that organic matter has a limited range of Br/I ratios; and that organic Br is a more significant component in Zn–Pb deposit ore fluids than previously recognised. The significance of these findings is illustrated for the Lennard Shelf Zn–Pb deposits of Western Australia. Fluid inclusions related to Lennard Shelf Zn–Pb mineralisation have variable salinity and hydrocarbon contents. The halogen data from these fluid inclusions require mixing of three fluid end-members: (1) an evaporated seawater bittern brine (30 wt.% NaCl equiv.) with greater than seawater Br/Cl ratio; (2) a lower salinity pore fluid (⩽5 wt.% NaCl equiv.) with moderately elevated Br/Cl and I/Cl; and (3) fluids with Br/Cl ratios of ∼5 times seawater and extremely elevated I/Cl ratios of ∼11,500 times seawater. The first two fluids have 40Ar/36Ar of 300–400 and greater than air saturated water 36Ar concentrations that are typical of fluid inclusions related to Zn–Pb mineralisation. The third ‘organic-rich’ fluid has the highest 40Ar/36Ar ratio of up to 1500 and a depleted 36Ar concentration. Mineralisation is interpreted to have resulted from mixing of Zn-rich evaporitic brines and H2S present in hydrocarbons. It is suggested that aqueous fluids acquired organic Br and I from hydrocarbons, and that hydrocarbons exsolving from the aqueous fluid removed noble gases from solution. Interaction of variably saline brines and hydrocarbons could account for the variable Br/Cl and I/Cl composition, and 36Ar concentrations, recorded by Lennard Shelf fluid inclusions. The distinct 40Ar/36Ar signature of the fluid with the highest I/Cl ratio suggests the hydrocarbons and brines were sourced independently from different parts of the sedimentary basin. These data indicate the complementary nature of halogen and noble gas analysis and provide new constraints on important mixing processes during sediment-hosted Zn–Pb mineralisation.

Fluid Flow Events in the Southern Dabashan Fold-thrust Belt, Central China: Insight From Fluid Inclusion and Isotope Data in Calcite Veins

The authors systematically analyzed fluid inclusions in calcite veins from 33 samples collected from the fractures zones in 3 different units of southern Dabashan (SDBS) fold-thrust belt to study the characteristics of fluids in the different units and determine possible migration pathways of the fluids with particular reference to the petroleum exploration potential in the region. Homogenization temperatures and salinities of fluid inclusions and their stable isotope (δD, δ18O) showed a distinct zonation and variations of fluid characteristics in the different tectonic units of the SDBS fold-thrust belt. Homogenization temperature measurements and fluorescence observation results indicated that faults are the main conduits for fluids during their active periods. Oxygen and hydrogen isotopic compositions from the imbricate thrust belt to the fold-thrust belt and to the front detachment-fold belt shift from a mixing of surface meteoric water and sedimentary formation water box toward the magmatic water zone and metamorphic zone. The front detachment-fold belt has the best condition for petroleum preservation and is the potentially favorable area for future exploration.

The Origin and Evolution of Mineralizing Fluids in a Sediment-Hosted Orogenic-Gold Deposit, Ballarat East, Southeastern Australia

The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (~4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 × 10−3 and 2.9 × 10−3 and I/Cl values between 150 × 10−6 and 500 × 10−6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have 40Ar/36Ar ratios ranging from 322 (close to the atmospheric 40Ar/36Ar ratio of ~296) up to a maximum of 4503. 40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 × 10−4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm 40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3–2.7 ppb). Fluid inclusion 84Kr/36Ar and 130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma.

Deciphering fluid sources of hydrothermal systems: A combined Sr- and S-isotope study on barite (Schwarzwald, SW Germany)

Primary and secondary barites from hydrothermal mineralizations in SW Germany were investigated, for the first time, by a combination of strontium (Sr) isotope systematics ( 87Sr/ 86Sr), Sr contents and δ 34S values to distinguish fluid sources and precipitation mechanisms responsible for their formation. Barite of Permian age derived its Sr solely from crystalline basement rocks, whereas all younger barite also incorporate Sr from formation waters of the overlying sediments. In fact, most of the Sr in younger barite is leached from Lower and Middle Triassic sediments. In contrast, most of the sulfur (S) of Permian, Jurassic and northern Schwarzwald Miocene barite originated from basement rocks. The S source of Upper Rhinegraben (URG)-related Paleogene barite differs depending on geographic position: for veins of the southern URG, it is the Oligocene evaporitic sequence, while central URG mineralizations derived its S from Middle Triassic evaporites. Using Sr isotopes of barite of known age combined with estimates on the Sr contents and Sr isotopic ratios of the fluids' source rocks, we were able to quantify mixing ratios of basement-derived fluids and sedimentary formation waters for the first time. These calculations show that Jurassic barite formed by mixing of 75–95% ascending basement-derived fluids with 5–25% sedimentary formation water, but that only 20–55% of the Sr was brought by the basement-derived fluid to the depositional site. Miocene barite formed by mixing of an ascending basement-derived brine (60–70%) with 30–40% sedimentary formation waters. In this case, only 8–15% of the Sr was derived from the deep brine. This fluid-mixing calculation is an example for deposits in which the fluid source is known. This method applied to a greater number of deposits formed at different times and in various geological settings may shed light on more general causes of fluid movement in the Earth's crust and on the formation of hydrothermal ore deposits.

The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters

The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of X Ca (Ca/(Na + Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in X Ca with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio ( mol Ca/ mol Na 2) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Na-dominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains X Ca to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in X Ca and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and P CO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks.

Formation of the Wiesloch Mississippi Valley-type Zn-Pb-Ag deposit in the extensional setting of the Upper Rhinegraben, SW Germany

The Mississippi Valley-type (MVT) Zn–Pb–Ag deposit in the Wiesloch area, Southwest Germany, is controlled by graben-related faults of the Upper Rhinegraben. Mineralization occurs as vein fillings and irregular replacement ore bodies consisting of sphalerite, banded sphalerite, galena, pyrite, sulfosalts (jordanite and geocronite), barite, and calcite in the Middle Triassic carbonate host rock. Combining paragenetic information, fluid inclusion investigations, stable isotope and mineral chemistry with thermodynamic modeling, we have derived a model for the formation of the Wiesloch deposit. This model involves fluid mixing between ascending hot brines (originating in the crystalline basement) with sedimentary formation waters. The ascending brines originally had a near-neutral pH (around 6) and intermediate oxidation state, reflecting equilibrium with granites and gneisses in the basement. During fluid ascent and cooling, the pH of the brine shifted towards more acidic (around 4) and the oxidation state increased to conditions above the hematite-magnetite buffer. These chemical characteristics contrast strongly with those of the pore and fracture fluid residing in the limestone aquifer, which had a pH between 8 and 9 in equilibrium with calcite and was rather reduced due to the presence of organic matter in the limestone. Mixing between these two fluids resulted in a strong decrease in the solubility of silver-bearing sphalerite and galena, and calcite. Besides Wiesloch, several Pb–Zn deposits are known along the Upper Rhinegraben, including hydrothermal vein-type deposits like Badenweiler and the Michael mine near Lahr. They all share the same fluid origin and formation process and only differ in details of their host rock and fluid cooling paths. The mechanism of fluid mixing also seems to be responsible for the formation of other MVT deposits in Europe (e.g., Reocin, Northern Spain; Treves, Southern France; and Cracow-Silesia, Poland), which show notable similarities in terms of their age, mineralogy. and mineral chemistry to the MVT deposit near Wiesloch.

Sources of solutes and heat in low-enthalpy mineral waters and their relation to tectonic setting, New Zealand

Approximately 90% of the low-enthalpy spring systems in New Zealand are distributed along the present-day convergence of the Pacific and Australian plates, which varies from subduction to oblique plate collision. Forearc subducted waters rising in the Hikurangi Accretionary Prism consist of pore waters containing labile organic material, water of hydration from clays, and seawater. These subducted waters are traceable in the North Island Axial Ranges and the northern Marlborough Fault System in the South Island albeit modified by temperature, mixing with surface and heated meteoric waters and protracted interaction with the rock. The signature of subducted marine sediments in the Hikurangi Trench is retained in the chemical and isotopic compositions of the Taupo Volcanic Zone magmatic arc-type waters. The concentrations of B, Ba, Br, Ca, Cl, I, K, Li, Mg, Na, Rb and Sr—solutes found in high concentrations in forearc subducted waters—increase from northeast to southwest towards the incipient subduction zone in the Puysegur Trench southwest of South Island. Most thermal mineral waters in the South Island are expelled along the Alpine Fault Zone where plate collision results to rapid uplift of the Southern Alps, elevated geothermal gradients and fault-shearing of rocks. In the Alpine Fault Zone, variations in temperatures (175°± 30 °C) and mineral water compositions are affected by elevation, hydrological gradient, intensity of tectonic activity, permeability, fluid volume and rate of throughput, and rates of uplift. As the main trace of the Alpine Fault Zone is approached, concentrations of Cl and other solutes generally increase suggesting increasing contributions from ascending Cl-rich “metamorphic waters”. Outside the active convergent zone, low-enthalpy springs discharge diagenetically-modified ancient forearc waters; Na–Cl sedimentary formation waters and heated seawater; Na–HCO 3–Cl waters; and Na–HCO 3 deep-circulating heated meteoric waters. The total surface heat flow in the North Island low-enthalpy mineral waters is more than 10× higher than in the South Island, with projected subsurface temperatures as high as 250 °C. This is because of the widespread occurrence of shallow magmatic and mantle sources, associated with Pliocene to Holocene volcanism and back-arc continental rifting. The sources of heat and solutes in low-enthalpy mineral waters, outside the Taupo Volcanic Zone and Ngawha, are decoupled except for rapidly ascending hot pore waters in the Hikurangi Accretionary Prism and low-temperature serpentinization Ca–Cl waters in Fiordland.

Origin of Upper Triassic formation water in middle Sichuan Basin and its natural gas significance

Abstract The formation water of Upper Triassic Xujiahe Formation in the middle Sichuan Basin is of high salinity in most areas and of low to medium salinity locally. All the formation water lack sulphate ion implying good reducing and sealing conditions, but are rich in barium ion. The forming environment of the Xujiahe formation water is similar to that of the Jurassic formation water, but very different from that of the underlying marine formation water. The Xujiahe formation water has lower sodium (potassium) chloride coefficients and higher metamorphism coefficients. The water shows the characteristics of original continental sedimentary formation water reworked by later marine water of high salinity. In reworking the reservoirs, surface water of high sulphate ion content should be avoided, and also quick changes of formation temperature, pressure and chemical equilibrium should be avoided to prevent precipitates from blocking pores and throats.