Stages Of Hydrothermal Alteration Research Articles

Mineralogy and fluid inclusion investigations in the Reagan Porphyry System, Iran, the path to an uneconomic porphyry copper deposit

The Reagan Porphyry System in southeastern Iran (Kerman Province) is associated with diorite/granodiorite to quartz-monzonitic rocks of Miocene age. Three stages of hydrothermal alteration and associated mineralization have produced new minerals, created new textural relationships and in many cases obliterated the primary character of the rocks. Potassic, phyllic and propylitic are the main hydrothermal alteration types within the intrusive stock. The early hydrothermal alteration produced a potassic assemblage in the central part of the stock; propylitic alteration occurred contemporaneously with potassic alteration, but in the peripheral parts, phyllic alteration which contains the main copper mineralization, occurred later, overprinting the earlier alteration. Phyllic is also the major alteration in the Reagan system. The early hydrothermal fluids are represented by high temperature (487–598 °C), high salinity (up to 61.1 wt% NaCl equiv.) liquid-rich fluid inclusions, and high temperature (397–401 °C), low-salinity, vapor-rich inclusions, which are interpreted to originate from very hot orthomagmatic fluids. The brines are interpreted to have caused potassic alteration, producing first generation chalcopyrite. Propylitic alteration is attributed to a lower temperature, liquid- and Ca-rich, evolved meteoric fluid. Influx of meteoric water into the system, and mixing with magmatic fluid produced deep albitization, and shallow phyllic alteration. This influx also caused dissolution of early formed copper sulfides and remobilization of Cu into the phyllic zone. The following data indicate that the mineralizing fluid temperature was too high to produce an economic porphyry copper deposit at the studied area. Supergene alteration was restricted to a very thin blanket with uneconomic levels of Cu in the form of Cu-sulfides and oxides.

Poly-phase alteration history of the kaolinitized ‘Cava di Caolino’ volcanics (Lipari Island, southern Italy)

AbstractA 65 m thick altered volcanic profile was studied in the Cava di Caolino (Lipari Island) in order to (1) identify the alteration event(s), (2) model these events, and (3) propose estimates of the degree of chemical alteration (CIA, chemical index of alteration). Two mineral parageneses were identified: one comprising silico-aluminous phases, with well crystallized kaolinite (type 1), and the other comprising sulphates, with kaolinite as fracture infilling (type 2). The geochemistry of fluids analysed from a local hot spring (40°C, pH ≈ 8) allowed modelling of the observed silico-aluminous paragenesis. A later fumarolic event is suggested to be responsible for the sulphate paragenesis. The main stage of hydrothermal alteration lasted for ~50 ky, was marked by an increase in the CIA from the protolith (~52) to the alterites (~98), and resulted in the alteration of a 65 m thick series. Such data could be used to predict the alteration of volcanic rocks around underground nuclear waste repositories.

Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman

The late Cretaceous-lower Tertiary hydrothermal alteration of serpentinized peridotite in the Semail ophiolite has formed two distinct types of listwaenite. Type I is characterized by the presence of calcite (Type IA) or dolomite (Type IB)+fuchsite±spinel. Type II is dominated by silicate minerals (quartz, chlorite, fuchsite)±calcite+dolomite±magnetite±apatite±plagioclase. Most listwaenites occur as veins along thrust fault zones within the ophiolite mélange. High Cr and Ni contents, abundant occurrence of Cr-spinel within a matrix of red-brown ferruginous carbonates within a micro-vein network of goethite, and the relics of mesh texture indicate an ultramafic protolith. Type I and II listwaenites represent different stages of hydrothermal alteration. The mineralogical and chemical distinctions of both types are the response to the extent of the reactions between the protoliths and the solutions leading to different stages of metasomatic replacement. The hydrothermal fluids involved in the formation of Type I listwaenite were enriched in Ca, Mg, and CO 2, whereas Type II listwaenite bodies were formed from a hydrothermal fluid enriched in SiO 2. REE and trace elements in both listwaenite types were extracted in part from adjoining peridotite. No Au anomaly in the study areas has been detected.

田上花こう岩中のアルカリ長石の微細組織とその形成史

Alkali feldspars in the Tanakami granite, southwest Japan, contain a complicated fine-scale mixture of various micro-textures; lamellar crypto- and micro-perthites and micropore-rich patch perthites. Mineralogical examination mainly by TEM and SEM disclosed that there are two formational stages of these micro-textures, the subsolidus cooling stage and hydrothermal alteration stage. In the subsolidus cooling stage, lamellar crypto- and micro-perthite were formed by exsolution. Then nearly regularly arranged dislocations were formed at interfaces of Ab-rich lamellae and Or-rich feldspars. Hydrothermal alteration caused the dissolution of feldspar grains and re-precipitation of new feldspar grains mainly around the dislocations. These processes result in neo-formation of patch perthites that comprise irregular microcline, tweed orthoclase, numerous micropores and albite grains. Bulk compositions of cryptoperthite have excessive Or content (around Or90) in comparison to those expected for primary alkali feldspars. It is estimated that retrograde inter-crystalline Na-K exchange between alkali feldspars and plagioclases occurs in the subsolidus stage. On the contrary, bulk compositions of patch perthite have poor Or content (Or64-71) from those of the cryptoperthite. This feature indicates that the chemical compositions of alkali feldspars changed drastically as well as the micro-textures in the hydrothermal alteration stage. Coexisting plagioclases also underwent chemical change from the subsolidus cooling stage through the hydrothermal alteration stage. Especially in the hydrothermal alteration stage, alteration by percolation of hydrothermal water occurred in the internal areas of the plagioclase grains. These processes result in lower Or content (max Or2.7) and higher Ab content (>Ab90) of the plagioclases, and neo-formation of patchy zoning including micropores and fluorite subgrains. Summarizing all of these features, the relation between micro-textural development and chemical compositional changes in feldspars was discussed taking into consideration their formation processes such as inter-crystalline Na-K exchange and hydrothermal reaction.

Mineral-chemistry and stable-isotope constraints on the magmatism, hydrothermal alteration, and related PGE?(base-metal sulphide) mineralisation of the Mesoarchaean Baula-Nuasahi Complex, India

The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a “contact-type” mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary “ferritchromite” in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.

Fluid inclusion evidence for impact‐related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP‐Chicxulub drill core Yax‐1

Fluid inclusions studies in quartz and calcite in samples from the ICDP‐Chicxulub drill core Yaxcopoil‐1 (Yax‐1) have revealed compelling evidence for impact‐induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 °C), highly saline, metal‐rich, hydrocarbon‐bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre‐impact organic matter. The center of the crater is assumed to be the source of the hot quartz‐forming brines. Fluid inclusions in abundant newly‐formed calcite indicate lower cyrstallization temperatures (75–100 °C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite‐forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact‐induced conductive heating of the sediments.

Composition of impact melt particles and the effects of post‐impact alteration in suevitic rocks at the Yaxcopoil‐1 drill core, Chicxulub crater, Mexico

Abstract Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m‐thick suevitic sequence at the Chicxulub Yax‐1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post‐impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post‐impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene‐rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi‐stage post‐impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, Al2O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid‐rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post‐impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well‐crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax‐1 suevitic rocks show strong potassium metasomatism with hydrothermal K‐feldspar formation and whole rock K20 enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na‐bearing Mg‐rich smectite, among other minerals. This indicates a general evolution from a silica‐undersaturated fluid at relatively high potassium activities at an early stage toward a silica‐oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction.

Source of Iron for Sulfidation and Gold Deposition, Twin Creeks Carlin-Type Deposit, Nevada

This study was undertaken to determine the source of iron in Comus Formation sedimentary rocks that were sulfidized during deposition of gold in the Megapit area of the Twin Creeks Carlin-type deposit. Sedimentary rocks in and near the Megapit contain ferroan dolomite, largely as overgrowths on iron-poor dolomite. Iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks. These igneous rocks have undergone two stages of hydrothermal alteration. The first stage involved formation of albite and iron-rich chlorite, possibly caused by interaction with seawater. The second stage involved destruction of the iron-rich chlorite by illite or sericite, which released iron to form ferroan dolomite in the sedimentary rocks. Comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks. Relative to basalts, Comus Formation igneous rocks are enriched in iron and potassium. These results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron-bearing igneous rocks. Recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation.

Geology and Geochemistry of the Deep Star Gold Deposit, Carlin Trend, Nevada

Deep Star is a high-grade Carlin-type gold deposit located in the northern part of the Carlin trend. The deposit averages 34.0 g/t Au and by year end 2000 had produced 37.8 t (1,217,000 oz) gold with a remaining reserve of 16.0 t (513,698 oz) gold. The deposit is primarily hosted in brecciated calc-silicate rocks of the Devonian Popovich Formation, with a minor amount of gold in the Jurassic Goldstrike diorite. Intrusion of the syn- and postore Deep Star rhyolite constrains the age of the mineralization. The postore rhyolite is compositionally and mineralogically similar to the synore dike and yielded an average 40Ar/39Ar isochron age of 38.3 Ma. Eocene rhyolite dikes intruded active, dilatant north- to northeast-striking faults and/or fractures, providing an important age constraint on the local stress regime at Deep Star during mineralization. Essentially horizontal, west-northwest-directed Eocene extension (291°) is consistent with dextral-normal oblique slip observed on north-south-striking, east-dipping portions of the Gen-Post fault system and dilation and sinistral shear on dike-filled, northeast-striking structures. A right-stepping, releasing bend in the Deep Star fault at its intersection with northwest- and north-northwest-striking subsidiary structures created a deep-tapping dilatant conduit for gold-bearing hydrothermal fluids. Five stages of hydrothermal alteration are present at Deep Star. Stages 1 and 2 are related to thermal metamorphism, metasomatism, and possibly retrograde alteration associated with intrusion of the Goldstrike stock. Stage 3 consists of preore quartz-carbonate alteration. The carbonate minerals are zoned both laterally and vertically, with dolomite as the dominant mineral closest to ore. Stage 3 overlaps with stage 4 quartz-kaolinite alteration associated with the introduction of gold and is therefore interpreted to be part of the Eocene mineralizing event. Gold is hosted in As-rich rims on marcasite and pyrite within a kaolinite and quartz matrix. Postore stage 5 is dominated by calcite with lesser siderite and barite. A three-dimensional geochemical model of the Deep Star deposit and its environs reveals that Carlin-type systems may have a geochemical expression involving a much broader suite of elements than previously recognized. Elements with distribution patterns considered to be related to the mineralizing event include Ag, As, Au, Ba, Bi, Ca, Cd, Co, Cu, Fe, Hg, Mg, Mo, Mn, Ni, P, Pb, S, Sb, Se, Te, Tl, U, V, W, and Zn. The multielement anomaly associated with the Deep Star deposit is best described as a vertical plume that is (1) focused between the Deep Star and Post faults below the deposit, (2) present in the footwall of the Deep Star fault and the Deep Star-Post fault corridor at the level of the deposit, and (3) broadly dispersed above, and to the west of, the deposit under the influence of northeast- and northwest-trending structural fabrics. Most closely associated with gold deposition are enrichments in As, Sb, Hg, Tl, Ag, and Zn within a halo of Ca, Mg, Ba, and Sr depletion. The elements Fe, Mn, Co, Ni, and P are most elevated in the immediate hanging wall of the Deep Star fault and above the deposit in a region where secondary carbonate veins (ankerite, kutnahorite, and Mn-rich dolomite); open-space-filling carbonate minerals (siderite, calcite) have been observed. Some lateral zonation is evident at levels above the deposit, with increases of Pb and Bi on its eastern margin and more Mo, U, V, and W west of the deposit. Isotopic analyses of kaolinite are interpreted to indicate that gold-mineralizing fluids originated from either a magmatic or possibly a deep metamorphic source. Fluids range from near the magmatic water field for those near the center of the Deep Star orebody along a mixing path toward exchanged mid-Tertiary meteoric water as represented by the fluids near the eastern orebody margin.

Mineralogical studies of dumortierite from the Miryang clay deposit, Korea

Dumortierite occurs as blue veinlets in the Miyrang clay deposit, southeastern Korea, which was formed by hydrothermal alteration of andesitic tuff. Dumortierite chemistry shows a little excess in Al contents in the octahedral sites possibly due to common occurrence of the Al-rich minerals such as diaspore, dickite, pyrophyllite, and tourmaline. Cell-dimensions of dumortierite are determined as a=11.71 A, b=20.39 A, c=4.70 A, and volume =1129.48 A3. Dumortierite formed during the early stage of hydrothermal alteration and afterwards pyrophyllite formed by an addition of silica. The presence of dumortierite indicates that boron plays an important role in Al-rich clay mineralization. It is suggested that dumortierite formed by boron metasomatism of aluminous rocks as a result of the intrusion of biotite granite.

THE OCCURRENCE OF Pb Cl (OH) AND Pt Sn S COMPOUNDS IN THE MERENSKY REEF, BUSHVELD LAYERED COMPLEX, SOUTH AFRICA

An unusual phase rich in Cl (78.4 wt.% Pb, 19.2% Cl) and close in composition to penfieldite [Pb 2Cl3(OH)] was found as a ~5 m inclusion in chalcopyrite, in a spatial association with platinum-group minerals, in a sulfide-poor ( ≤5 vol.% of base-metal sulfides) enstatite orthocumulate of the Merensky Reef, Bushveld layered complex, South Africa. This seems to be the first reported occurrence of a Pb–Cl–(OH) compound in mafic-ultramafic rocks. The associated platinum-group minerals are members of the braggite series, cooperite (which forms large intergrowths with braggite: up to 0.5 mm in the longest dimension), member s of the rustenburgite–atokite and merenskyite–moncheite series, zoned laurite, and an unknown stannosulfide of Pt, the likely chemical formula of which is PtSnS. The stannosulfide probably formed at a hydrothermal stage from microvolumes of a latestage fluid or liquid. The Cl-rich phase precipitated from a late-stage solution rich in Cl, or formed as a result of replaceme nt of a precursor mineral (probably galena) by an aqueous hydrochloric solution at a very low temperature, at the final stage of hydrothermal alteration.

Fuchsite and other Cr-rich phyllosilicates in ultramafic enclaves from the Almadén mercury mining district, Spain

AbstractFuchsite and other Cr-rich phyllosilicates, paragenetic with dolomite, are present in some ultramafic enclaves from the ‘frailesca’ rock (a lapilli- to block-size pyroclastic lithic-tuff), in the Almadén mercury mining district, Spain. Analyses (EMPA and TEM) of fuchsite and Cr-chlorite showed a relatively large range in levels of Cr2O3. Petrographic relationships between these phyllosilicates and primary relics of Cr-spinel crystals, as well as their high Cr content, indicate that these Cr-rich minerals originated from primary chromian spinels through an early hydrothermal alteration stage. The hydrothermal fluids accounting for this early alteration would be of relatively high temperature, high aCO2 and aK, and variable aNa/K. In a later alteration stage, fuchsite was partially or totally replaced by illite and Cr-illite, giving rise to an argillitic alteration.

Clay mineral transformations and weakening mechanisms along the Alpine Fault, New Zealand

Abstract The formation of clay minerals within active fault zones, which results from the infiltration of aqueous fluids, often leads to important changes in mechanical behaviour. These hydrous phyllosilicates can (1) enhance anisotropy and reduce shear strength, (2) modify porosity and permeability, (3) store or release significant volumes of water, and (4) increase fluid pressures during shearing. The varying interplay between faulting, fluid migration, and hydrous clay mineral transformations along the central Alpine Fault of New Zealand is suggested to constitute an important weakening mechanism within the upper section of this crustal discontinuity. Well-developed zones of cataclasite and compacted clay gouge show successive stages of hydrothermal alteration, driven by the cyclic, coseismic influx of meteoric fluids into exhumed amphibolite-facies rocks that are relatively Mg rich. Three modes of deformation and alteration are recognized within the mylonite-derived clay gouge, which occurred during various stages of the fault’s exhumation history. Following initial strain-hardening and frictional melting during anhydrous cataclastic breakdown of the mylonite fabric, reaction weakening began with formation of Mg-chlorite at sub-greenschist conditions (<320 °C) and continued at lower temperatures (<120 °C) by growth of swelling clays in the matrix. The low permeability and low strength of clay-rich shears are suitable for generating high pore-fluid pressures during faulting. Despite the apparent weakening of the c. 6 km upper segment of the Alpine Fault, the upper crust beneath the Southern Alps is known to be actively releasing elastic strain, with small (<M 5) earthquakes occurring to 12 km depth. We predict that larger events will nucleate at c. 6–12 km along an anhydrous, strain-hardened portion of the fault.

Hydrothermal alteration and associated mineralization in the Freda-Rebecca gold deposit - Bindura District, Zimbabwe

The Freda-Rebecca Mine is currently the largest gold producer in Zimbabwe. The ore deposit is hosted by two main shear systems crosscutting the Rebecca diorite and Bindura granodiorite (2.65 Ga) as well as Shamvaian metasediments, which are affected by contact metamorphism. Following the intrusion of the Bindura granodiorite, intensive hydrothermal alteration developed preferentially in the dioritic part of the igneous complex (Rebecca diorite). The hydrothermal alteration started with an extensive K-dominated hydrothermal metasomatism in the whole Rebecca diorite. It was followed by less penetrative hydrothermal alteration developed preferentially near shear zones and veinlets. Hydrothermal metasomatism caused microcline and biotite formation, prevailing in the Rebecca diorite. Two main stages of post-metasomatic hydrothermal alteration and mineralization were distinguished, based on spatial relationships between different minerals and some geochemical aspects. In the first stage, an actinolite-tourmaline-arsenopyrite mineralization formed, which is characterized by Ni-Co arsenopyrite associated with actinolite, tourmaline and quartz. The second, lower temperature stage gave a polyphase gold-rich mineralization that developed in four phases which are distinguished by different parageneses including: Phase 1. Actinolite, chlorite, clinozoisite/epidote, quartz, calcite, arsenopyrite, pyrite, pyrrhotite, chalcopyrite. Phase 2. Chlorite, epidote, calcite, gold, native Bi, Bi-Pb sulfides, galena, chalcopyrite, fahlore, pyrite, Fe-gersdorffite. Phase 3. Epidote, calcite, (Ni, Co, Fe) As S phases, Co-Fe-gersdorffite. Phase 4. Chlorite, calcite, quartz. The gold mineralization is exclusively associated with phase 2 and developed in three sulfide parageneses: – Gold + Bi-Pb sulfides + bismuth + chalcopyrite + galena, associated with chlorite (Au 1). – Gold + galena + pyrite, associated with calcite and chlorite (Au 2). – Gold + pyrite + Fe-gersdorffite, associated with epidote, chlorite and calcite (Au 3). The hydrothermal alteration and mineralization formed after the consolidation of the Rebecca diorite and Bindura granodiorite, most probably in the postmagmatic cooling stage. The mineralization was emplaced either synchronously or subsequently to the shear zones which crosscut the consolidated pluton. Not all shear zones are mineralized and different shear zones show different amounts of mineralization and hydrothermal alteration. Thus, it is suggested that during the cooling stage of the pluton and subsequent to the formation of the K-metasomatic zone (microcline + biotite), hydrothermal fluids preferentially followed just forming or pre-existing shear zones. It cannot be excluded that this process developed in a plutonic porphyry copper-like environment, in which the classically hydrothermal zonation did not form due to synchronous tectonic disturbance, which preferentially drives the hydrothermal flow along shear zones. Mineral parageneses and data from chlorite geothermometry indicate that the different stages and phases of hydrothermal alteration reflect cooling stages of the hydrothermal system, from >300 °C in the first stage to about 150 °C in the last phases.

Formation and Transformation of Clay Minerals in the Hydrothermal Deposits of Middle Valley, Juan de Fuca Ridge, ODP Leg 169

Mineralogical, chemical, and isotopic characteristics of clays from Ocean Drilling Program (ODP) sites 1035, 856, 1036, and 858, drilled in Middle Valley at the northern end of the Juan de Fuca Ridge during Legs 139 and 169, were studied using X-ray powder diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray fluorescence, and inductively coupled plasma-mass spectrometry. Oxygen isotope measurements on authigenic clay minerals provide a record of mineral formation temperatures. Holes 1036A and 858B are located in the present-day hydrothermally active Area of Active Venting and reflect several stages of hydrothermal alteration. Smectite and some mixed-layer chlorite-smectite clays are the dominant phyllosilicates in the upper part of holes 1036A and 858B. Nearly pure chlorite-smectite mixed-layer clays and corrensite mainly occur more than 20 meters below the seafloor. Formation temperatures of these nearly pure chlorite-smectite mixed-layer clays and corrensite were calculated as 250° to 268°C. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. The high-temperature alteration stages resulted in gains (50–85%) of Mg and Zn, an almost complete loss of K, Ca, Cu, Rb, Sr, and Ba (> 90%), and depletion (20–40%) in Si, Fe, Na, and Zr. Alteration effects in hemipelagic and turbiditic sediments from hole 856H, drilled near the summit of the Bent Hill massive sulfide deposit, are similiar to those of sediments in hole 1035H, which was drilled 350 m south of hole 856H near the southernmost peak of the Ocean Drilling Program mound. At both holes, the clay fraction of the sedimentary sequence between 140 and 470 m below seafloor is composed of varying amounts of authigenic quartz and chlorite indicating high-temperature alteration. The chlorite formation temperatures calculated from oxygen isotope data lie at 290° to 320°C in hole 1035H and 250° to 264°C in hole 856H. Mineralogical data from hole 1035D, drilled 75 m east of hole 856H, and hole 1035A, drilled 77 m west of hole 856H, indicate lower temperatures of hydrothermal alteration in the upper 50 m of their sedimentary sequences. These sequences are partly altered to chlorite-smectite mixed-layer clays coexisting with several detrital minerals and with a dolomite formation temperature of 74°C. Oxygen isotope evidence indicates authigenic chlorite in the deeper sedimentary section of hole 1035A that formed at relatively low temperatures (180°C), whereas chlorites from hole 1035D formed at higher temperatures (277°–292°C). Chemical changes in the strongly altered clay fractions of hole 856H and 1035D include gains (60–75%) of Fe, Mg, Zn, and Cu, a nearly complete loss (> 95%) of K, Na, Ca, Rb, Sr, and Ba, and a strong depletion (30–70%) in Si, Ti, and Zr. All of the chondrite-normalized rare earth element patterns of chlorite are characterized by negative Eu anomalies and varying levels of light rare earth enrichment. The chloritization observed at the Area of Active Venting is similar to the Mg-rich chlorite and to chloritic minerals observed at sediment-covered spreading centers in Escanaba trough on Gorda Ridge and in Guaymas basin on the East Pacific Rise. This observation suggests intense Mg metasomatism in a mixing zone where hydrothermal fluids interact with seawater and sediment at temperatures above 200°C. The similarity of the Fe-rich chlorites from the Bent Hill massive sulfide deposit to Fe-rich chlorites from several ancient massive sulfide deposits indicates that they probably reflect hydrothermal fluid-rock interaction at high temperatures below the zone of seawater mixing.

Chlorine cycling during subduction of altered oceanic crust

Eclogitic rocks that have experienced devolatilization, with little or no interaction with external fluid sources, can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction. Thus they can be used to evaluate the extent of the recycling of volatile elements, such as chlorine. We report new oxygen isotope ratios of omphacite, and fluid inclusion data determined from eclogitic metagabbros, of the Rocciavré massif (Italian Alps). The data are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavré, Monviso and Franciscan Complex) or garnet (Cyclades). Salinity estimates of the inclusion fluids range from 32 to 45 wt% NaCl in Rocciavré, 17 to 21 wt% NaCl in Monviso, and are similar to seawater in other areas. Omphacite and bulk-rock δ 18O values of Rocciavré (5.1–6.8‰) and Monviso (3.0–5.3‰) metagabbros are markedly lower than those of Cyclades (6.8–14.3‰) and the Franciscan (6.7–13.1‰) metabasites. The fluid salinity–δ 18O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites. This suggests that high-pressure metamorphism, and associated processes, did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios, inherited from a stage of sea-floor hydrothermal alteration under low- (basaltic layer) and high-temperature (gabbroic layer) conditions, respectively. Extrapolating the estimated H 2O and Cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm Cl could be recycled into the mantle during subduction. This yields a Cl/H 2O ratio of 3.6 to 7.5×10 −2 for the subducted oceanic crust, which is similar to E-MORB. On the basis of available Cl isotopic data, we infer that a large proportion (70%) of the Cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid-ocean ridge basalt (δ 37Cl = 4.7‰).

Foreland basins and gold-bearing conglomerates: a new model for the Jacobina Basin (Sa˜o Francisco province, Brazil)

From a lithological, structural and metamorphic viewpoint, the Jacobina Basin in the Sa˜o Francisco province (Brazil) is representative of the foreland basins that mark a characteristic evolutionary stage in the development of the Palaeoproterozoic orogenic belts around 2000 Ma ago. In addition, it contains gold-bearing conglomerates of economic interest that are presently mined. The general structure of the basin is one of stacked detrital and schist formations along sinistral strikeslip/overthrusting shear zones, with exposed deep-seated units in the east and shallower and younger rocks in the west. New 40Ar/ 39Ar geochronological data give minimum ages of 1940-1910 Ma for the main thrusting and wrenching events. Shearing was accompanied by metamorphic recrystallization that indicates a high geothermal gradient and a progressive loading of the underthrust units. Despite this final major tectonic imprint, at least five stages of basin evolution are indicated by traces of tectonic instability, such as debris-flow deposits, chaotic breccias and early dyke swarms. In this context the conglomerate layers found in the basin formations correspond to periods of erosion of a preceding stage in the basin development. Multistage hydrothermal activity is shown by widespread alteration within mineralized conglomerates (sulphide, fuchsite, tourmaline, chromite, rutile), leaching of ultrabasic rocks, and phlogopite and/or emerald mineralization in contact metamorphic aureoles around late- to post-tectonic granites. This tectonic evolution is coherent with the development of the Jacobina Basin in a foreland position and its progressive involvement in the Transamazonian collision belt. Resedimentation processes, preservation of earlier stages of hydrothermal alteration in pebbles, extensive hydrothermal activity and a high geothermal gradient favour a model of epigenetic mineralization for the gold-bearing conglomerates with enrichment during the progressive propagation of the basin. Comparison with similar mineralized conglomerates shows that this model can be applied to other Precambrian foreland basins.

Carbon and oxygen isotope variations in southern African carbonatites

The C and O isotope composition of 54 carbonate samples from 18 carbonatite localities throughout southern Africa was determined. Sample frequency distributions indicate a dependence of the δ 13C and δ 18O values of southern African carbonatites on their carbonate mineralogy. The calcite carbonatite samples fall into the range −8% to −4% for δ 13C and 6% to + 10% for δ 18O, which is defined as the primary igneous carbonatite field (Taylor et al., 1967). Those samples with higher δ 13C and δ 18O values are either dolomite/ankerite carbonatites or contain dolomite/ankerite, possibly indicating the involvement of different magmatic or hydrothermal phases in the carbonatite formation. Emplacement level, type of country rock intruded and secondary alteration by e.g. hydrothermal fluids, seem to be the most important parameters which affected the C and O isotope patterns found in southern African carbonatite complexes. Magmatic processes like fractional crystallisation and liquid immiscibility are of minor importance in controlling C and O isotope ratios. An alteration model of at least two stages is proposed for the evolution of the C/O isotope patterns observed in southern African carbonatites. The first stage involved magmatic-metasomatic processes during or shortly after the formation of carbonatites. These processes affected both the C and O isotope composition of carbonatites by exchange with a H 2O/CO 2-rich fluid and led to higher δ 13C and δ 18O values when compared to primary igneous values. The elevated 18O content of carbonatite carbonates was introduced by a second stage (or more stages) of post-magmatic, hydrothermal alteration and/or interaction with ground water at low (<200–250°C) temperatures. These alterations seem to be accompanied by a growing mineralogical diversity, which reflects, to some extent, the degree of alteration affecting the carbonatites. Information on stable isotope heterogeneities of the igneous sources of southern African carbonatites is obscured. Small scale C isotope heterogeneities within the igneous sources of carbonatites may be found, provided that the effects of a first stage, magmatic-metasomatic alteration can be eliminated. Changes in δ 18O values of carbonatites by sub-solidus second stage alteration processes within the crust further complicate an estimation of stable isotope variations of igneous carbonatite sources.

Distribution of boron, lithium and beryllium in ocean island basalts from French Polynesia: implications for the B/Be and Li/Be ratios as tracers of subducted components

AbstractThe study focuses on the distribution of B, Be, Li, rare earth elements (REE), high-field-strength elements (HFSE), Th, U and Pb in fresh and hydrothermally altered ocean island basalts (OIB) from French Polynesia, and evaluates B/Be and Li/Be ratios as potential tracers of subducted components in the mantle. Hydrothermal solutions affecting the rocks during cooling were derived from meteoric water, sea water and magmatic fluids. The concentrations of REE, HFSE, Th and Be in the OIB were not affected by secondary processes except during advanced stages of subaerial hydrothermal alteration where saponite was a dominant secondary phase. This alteration modified the contents of these elements, changed REE patterns and produced a positive Ce anomaly. The subaerial and submarine hydrothermal alteration (T ∼ 70–100°C) may change U concentrations in OIB, whereas Pb was only marginally redistributed during alteration.Boron was enriched during submarine and subaerial hydrothermal alteration but was not noticeably affected in basalts altered by magmatic fluids at T &gt; 200°C. Like B, the mobility of Li during the alteration varies with fluid temperature. Lithium became enriched in the basalts during advanced stages of lower T hydrothermal alteration (&lt;100°C). However, this element was partly removed from the rocks during higher T alteration (&gt;200°C) by magmatic fluids. Boron, Be and Li behave as incompatible trace elements in basaltic magmas. Beryllium content in primitive mantle is estimated to be 0.07 ppm. Fresh Polynesian OIB have low abundances of B and Li and low B/Be (2–5) and Li/Be (2.5–5) ratios compared with volcanic arc rocks, marine sediments and altered oceanic crust. Various OIB including even those which have HIMU- and EM-affinities have similar overlapping B/Be and Li/Be ratios. Both B and Li are probably stripped from a lithospheric slab during subduction-related metamorphism and are, thus, not involved in deep mantle recycling. The mantle-normalized trace element abundances of MORB and OIB usually display patterns characterized by negative B, Pb and Li anomalies. The patterns of continental crust and crustal rocks have distinct positive anomalies for these elements whereas continental basaltic rocks have variable relative abundances of B, Pb and Li. The anomalies of these elements in basalts can be useful in discriminating their tectonic setting and constraining the mantle source regions of basalts.

High‐temperature alteration of oceanic gabbros by seawater (Hess Deep, Ocean Drilling Program Leg 147): Evidence from oxygen isotopes and elemental fluxes

Two major stages of high‐temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the Hess Deep Rift Valley at the East Pacific Rise (2°N, 101°W). The 154 m of plutonic rocks cored at site 894G show mineralogies and textures that suggest that they represent the roof of an approximate 1 Myr magma chamber. A late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite ‐ upper greenschist facies conditions. The hydration of the gabbros occurred locally through the development of well‐crystallized green amphiboles. Significant chemical fluxes were associated with the fluid flow and precipitation‐dissolution mechanisms. Oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the Hess Deep gabbros at temperatures above 500°C. Hydration was limited with only minute amphibole lamellae in pyroxenes that trapped Cl, Na, and minor K from seawater. Ca‐plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns. We propose that this high‐temperature event lowered the δ18O of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion. Although it is unclear whether the 18O depletion of Hole 894G gabbros is representative of the entire lower oceanic crust, it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans. During this stage, water rock ratios calculated using Sr isotopes are underestimated (0.1 – 0.5) because of insignificant diffusion of this cation in plagioclases. Better estimates of water rock ratios are provided by oxygen isotopes (0.2 – 1) which are important in unraveling the cooling history of the oceanic crust.