Infiltration Of Hydrothermal Fluids Research Articles

Early Pleistocene banded iron-rich sedimentary rocks at Cape Vani, Milos Island, Greece: A modern analogue of Precambrian banded iron formations?

The genesis of banded iron formations is commonly associated with anoxic and iron-rich (ferruginous) marine conditions throughout the Archean to Palaeoproterozoic, and occasionally in the Neoproterozoic. In recent studies, ~2 million-year-old banded iron-rich sedimentary rocks with alternating Fe- and Si-rich bands occurring in the Cape Vani sedimentary basin on Milos island (Greece) were interpreted to be a modern analogue of Precambrian banded iron formations. This interpretation is mainly due to the discovery of photoferrotrophic-like filamentous fossils in massive iron-rich cherty beddings of another subbasin about 450 m to the south. In this study, we re-examined the stratigraphy and lithology of the Cape Vani basin, carried out detailed petrographic, mineralogical and geochemical investigations of the iron-rich sedimentary rocks, as well as volcaniclastic/volcanic conglomerates and banded hydrothermal veins that are in close spatial association with the iron-rich sedimentary rocks. The Fe2O3 content of the banded iron-rich sedimentary rocks ranges from 0.41 to 11.02 wt% (2.73 wt% on average), far lower than that required per definition for banded iron formations. In addition, the banded iron-rich sedimentary rocks contain substantial clastic materials. Samples with stronger silicification, however, usually contain less clastic materials. The non-clastic components occur as siliceous matrix comprising the same mineral assemblage as the banded hydrothermal veins. These components occur in the interspaces of the clastic materials, within cavities, microfractures and veins crosscutting the layers. Fe2O3, principally reflecting hematite, is intimately linked with the presence of non-clastic materials. Therefore, the non-clastic materials are interpreted as post-depositional products associated with silicification during later stage hydrothermal activities, rather than being syn-depositional as the minerals in Precambrian banded iron formations. Consequently, the Milos banded iron-rich sedimentary rocks cannot be an analogue of Precambrian banded iron formations, seeing their lithological and genetical differences.The geochemical results reveal that both the banded iron-rich sedimentary rocks and the massive iron-rich cherty beddings have the same clastic provenance as the volcaniclastics/volcanic conglomerates, but the massive iron-rich cherty beddings contain more hydrothermal contributions due to more intensive fluid-rock interaction. Thus, the two types of iron-rich sedimentary rocks are considered as clastic sedimentary rocks with varying degrees of hydrothermal overprinting. The massive iron-rich cherty beds were generally subjected to stronger hydrothermal overprinting, and, therefore, contain more siliceous matrix and Fe2O3. The enrichment of Fe is due to post-depositional hydrothermal fluid infiltration to the primary sediment piles. The primary layer planes of the banded iron-rich rocks, their interconnected pores and fractures provided preferential pathways for the hydrothermal fluids, and resulted in denser precipitation of Fe(III) (oxy)hydroxides and SiO2 cement therein. The alternating bands in the banded iron-rich sedimentary rocks, therefore, reflect cyclic permeabilities of the layers (i.e., primary sedimentary planes and porosities), rather than cyclic precipitation of Fe- and Si-minerals. This study also sheds light on an alternative formation mechanism for banded structures in Fe- and Si-rich sedimentary rocks similar to those of Precambrian banded iron formations.

Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition

The changes in the physicochemical conditions of the carbonaceous material during progressive metamorphism in metapelitic rocks is widely used as a geothermometer with the aid of Raman spectroscopy. However, the application of this technique in carbonate rocks has not been established yet. Here, we compare Raman spectroscopy of carbonaceous material thermometry and carbon isotope thermometry in low- to medium-grade metacarbonate rocks from the Archean Chitradurga Schist Belt in the Dharwar Craton, India. The carbonates in the lowermost Bababudan Group have metamorphosed under lower amphibolite facies metamorphic conditions giving consistent estimates for both Raman spectra of carbonaceous material thermometry (460–592 °C) and carbon isotope thermometry (450–560 °C). Contrastingly, in the Vanivilas Formation, the carbonaceous material with very fine flaky morphology in the grain boundary has slightly lower crystallinity, when compared to the coarse-grained ones near the vein boundary. Nevertheless, the carbon isotope thermometry estimated a lower temperature around 400 °C. The inconsistencies between the temperature estimates are ascribed to the pervasive post-metamorphic aqueous hydrothermal fluid infiltration indicated by the presence of numerous criss crossing quartz veins. This is also corroborated by the lowering of the oxygen isotope ratio of the carbonates near the vein boundary by 2.3‰, but without much difference in the carbon isotope ratios. The coarsening of the carbonaceous material near the vein boundary signifies fluid assisted recrystallisation, that enhanced the crystallinity as evidenced in the Raman spectra. Moreover, the negative δ13C shift in the carbonaceous material (c. –8.5 to –13‰) was possibly due to recrystallization following partial CO2 degassing. In the Chitradurga Schist Belt, the Bababudan Group and the Vanivilas Formation have regionally metamorphosed under lower amphibolite facies condition, possibly related to the widespread granitic intrusion at c. 2.61 Ga and later affected by hydrothermal event at c. 2.5 Ga.

Using whole rock and in situ pyrite chemistry to evaluate authigenic and hydrothermal controls on trace element variability in a Zn mineralized Proterozoic subbasin

The mid-Proterozoic stratigraphy of the McArthur Basin (Australia) contains some of the most well-preserved sedimentary rocks of Precambrian age, which are also host to giant, clastic dominant (CD-type) massive sulfide Zn deposits. The most recently discovered CD-type deposit (the Teena deposit) is located in the Teena subbasin and hosted by the 1.64 Ga Barney Creek Formation. The Teena subbasin, therefore, provides the perfect natural laboratory for evaluating authigenic and hydrothermal controls on trace element (TE) variability, both of which contribute to paleoenvironmental reconstructions and ore deposit models. As the Teena deposit formed beneath the paleoseafloor, this also provides the opportunity to evaluate TE zonation around a fossilized subseafloor replacement hydrothermal system. In situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) has been used to define compositional end members in diagenetic and hydrothermal pyrite. The overgrowth of hydrothermal sulfides on diagenetic pyrite is associated with TE anomalism (Tl, Pb, As, Zn) that extends > 100 meters above the main high grade sulfide mineralization the Teena subbasin. The vertical zonation in TEs is consistent with the infiltration of hydrothermal fluids into overlying hangingwall sediments that were undergoing diagenesis. Bulk rock lithogeochemical data record covariation between total organic carbon (TOC) and a suite of TEs (Mo, Co, Ni, V). We suggest this was caused by local hydrographic factors during deposition of the Barney Creek Formation. High TOC/P molar ratios, resulting from regeneration of P in a euxinic water column, are associated with an interval overlying the main maximum flooding surface in the subbasin. The relationships between TOC, P and TEs resemble the redox architecture of a silled basin rather than an open marine margin. Sulfidic conditions developed during periods of high productivity, which were linked to nutrient supply that was enhanced by connectivity with surrounding water masses. The evidence of redox bistability, involving a delicate balance between ferruginous (anoxic, non-sulfidic) and euxinic (sulfidic) conditions, is consistent with recent models for other mid-Proterozoic sedimentary units. Nevertheless, there was a strong localised (101 km2) control on the authigenic and hydrothermal TE chemistry of the Barney Creek Formation in the Teena subbasin, which highlights a key challenge when extrapolating from data collected in partially restricted intracontinental marine settings.

Fluid-assisted localized embrittlement in an overall ductile fold-and-shear belt: A case study from Cap de Creus

The Cap de Creus massif in the eastern Pyrenees is a renowned late Variscan fold-and-shear zone belt characterized by partitioning of an overall ductile strike-slip-dominated dextral transpression into coeval folding, ductile shearing and localized embrittlement. The local partitioning of deformation mode (fracture vs. ductile flow) can be related to rheologically contrasting lithologies and syntectonic infiltration of hydrothermal fluids that were likely residual fractions of earlier pegmatite intrusion. In particular, the influx of hydrothermal fluids results in cm-to m-sized damage zones of localized fractures and veins with associated hydrothermal alteration across most lithological units. Such damage zones systematically formed at a low angle to the axial surfaces of contemporaneous folds, even in cases with minimal shortening. In higher strained areas, evident by tighter folding and shearing, cataclasites developed along these same damage zones. This study provides an example of contemporaneous ductile and brittle structures, in this case resulting from localized embrittlement due to fluid flow.

Ore genesis and tectonic implication of the Daeheung-Ryongyang magnesite deposits in the East Jiao-Liao-Ji Belt, North Korea

The Daeheung and Ryongyang magnesite deposits in the Dancheon Mineralized Belt (DMB) of South Hamgyong Province, North Korea, have attracted wide attention because of their large reserves and geological similarity with a number of magnesite deposits in the Dashiqiao magnesite belt of Liaoning Province, China. The study of the magnesite deposits in the DMB was performed to analyze and compare the genetic and geotectonic implications between the two districts. The Daeheung and Ryongyang deposits comprise sedimentary and metamorphic rocks of the Paleoproterozoic Bugdaecheon Series and intrusive and extrusive rocks of diverse ages. Magnesite ores comprise magnesite, talc, chlorite, quartz, calcite, and apatite, and based on the mineralogy, ores can be divided into four groups: pure magnesite, chlorite-magnesite, talc-magnesite, and chlorite-talc-magnesite. Rare earth element (REE) patterns can be classified into three types: middle REE-enriched, heavy REE-enriched, and light REE-enriched, implying multiple events in the formation of magnesite deposits in the DMB. The carbon and oxygen isotope compositions of magnesite ores and dolostones in the DMB range from −1.0 to 1.0‰ and from 8.7 to 23.0‰, respectively. The results suggest that the mineralization proceeded under low fluid/rock ratios at low XCO2 or large input of freshwater into hydrothermal fluids. Salinity and homogenization temperature of fluid inclusions in magnesites range from 0.5 to 34.1 wt% NaCl eq. and from 112 to 290 °C, respectively, which are similar to those of the Dashiqiao magnesite belt. Considering the ore mineralogy, REE patterns, stable isotope composition, and fluid inclusion characteristics, magnesite ores may have been formed by various processes involving the inflow of Mg-enriched brines into carbonate rocks and then hydrothermal fluid infiltration related to granitoid intrusions. These results indicate that the Daeheung and Ryongyang magnesite deposits in the DMB belong to the Veitsch-type sedimentary-metamorphic deposits and are genetically correlated with mineralization in the Dashiqiao magnesite belt of Liaoning Province in China.

Role of Hydrothermal Fluids in the Deterioration of Pictographs and Petroglyphs in Rock Shelters of The Gawilgarh Hills, Madhya Pradesh, India

India has one of the largest concentrations of rock-art sites. However, these rock shelters have deteriorated due to natural agencies. The present study was carried out in one such site in Central India, which is hosted by argillaceous and ferruginous sandstones, showing pro-minent chemical, biological and mechanical weather-ing. The results based on field investigations, petrography, XRD, FE-SEM-EDS and FTIR meas-urements have indicated that chemical weathering is caused due to infiltration of hydrothermal fluids through innumerable zones of weaknesses formed due to neotectonic activities in the Satpura Lineament Zone, especially along Gawilgarh and Salbardi Faults in this region.

40Ar/39Ar ages of Northwest Africa 7034 and Northwest Africa 7533

AbstractThe Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

Trace element associations in magnetite and hydrothermal pyrite from the Geita Hill gold deposit, Tanzania

Gold mineralization in the Geita Hill deposit is associated with pyrite formed along microfracture networks and sulfidation fronts together with K-feldspar and biotite. The sulfidation fronts are best developed in magnetite-bearing ironstone. The gold is present mainly as electrum and gold tellurides along grain boundaries, and as inclusions in pyrite, quartz, biotite and K-feldspar. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of pyrite and magnetite grains reflect complex fluid-host rock interactions. Magnetite textures and chemistry change with alteration intensity, indicating the progression of the alteration front into the host rock. Pyrite textures are uniform across all rock types and reflect late-tectonic growth linked to multi-staged infiltration of hydrothermal fluids. Trace element distribution patterns in pyrite are locally complex and influenced by host rock chemistry.Gold distribution patterns in pyrite correlate closely with Te, Ag, Bi and Pb, indicating that gold occurs in micro- and nano-inclusions of telluride minerals. This is especially so for gold in quartz veins, whereas gold in ironstone and diorite also occurs as electrum with an average Au/Ag ratio of 0.41. As, Co and Ni in pyrite are lattice bound and occur in high concentrations in ironstone and diorite where they show characteristic growth zoning patterns. Pyrite in quartz veins has As, Co and Ni concentrations that are low and variable. Cr, Cu, Mo, Mn and Zn are present in all rock types in isolated inclusions in pyrite grains, whilst Pb, Bi and Sb occur in more dispersed patches of fine clustered inclusions. The Se content in pyrite is typical for Archean gold deposits, and reflects an average temperature of ~340 °C for the mineralizing fluid. The Co/Ni ratio of pyrite grains varies between 0 and 5.2 in ironstone and diorite, and most likely reflects the equilibration Co/Ni ratio of the host rock. The Co/Ni ratio of pyrite grains in quartz veins varies between 1 and 12, and is consistent with a magmatic-hydrothermal origin for the ore fluid. Trace element distribution patterns in magnetite and pyrite indicate that As, Ni, Co, Cr, Mn and Cu were mostly locally derived, and remobilised into the pyrite during sulfidation of the host rock. The concentrations of these elements are strongly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Bi and Pb were externally derived, and closely correlate in all varieties of pyrite as well as strongly altered magnetite. The alteration footprint of the Geita Hill deposit is limited in extent, and does not involve As and Sb that are typically enriched in Archaean lode-gold systems. Instead, Te and Bi are most characteristic for the deposit and could be of use as path finder elements together with altered magnetite grains.

Quartz chemistry – A step to understanding magmatic-hydrothermal processes in ore-bearing granites: Cínovec/Zinnwald Sn-W-Li deposit, Central Europe

Quartz from granites, greisens and quartz veins from a 1596 m long vertical section through the Cínovec/Zinnwald Li-Sn-W deposit (Czech Republic) was studied using cathodoluminescence (CL) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP MS). The trace contents of Al, Ti, Li and the Ge/Ti and Al/Ti values in quartz reflect the degree of fractionation of parental melt from which primary quartz crystallized. From the biotite granite to the younger zinnwaldite granite, quartz is characterized by increasing contents of Al (from 136–176 to 240–280 ppm) and decreasing Ti (from 16–54 to 6–14 ppm), while the contents of Li and Ge are similar (15–36 and 0.8–1.7 ppm, respectively). Quartz of the greisen stage and vein stage is poor in all measured elements (26–59 ppm Al, 0.5–1.6 ppm Ti, 2–13 ppm Li, 0.8–1.6 ppm Ge). The youngest low-temperature quartz forming thin coatings in vugs in greisen and veins differs in its extreme enrichment in Al (>1000 ppm) and Li (∼100 ppm) and very low Ti (<1 ppm). Within the greisen, remnants of primary magmatic quartz should be distinguished from metasomatic greisen-stage quartz in their higher intensity of CL and relatively higher Ti contents. A part of primary magmatic quartz may by secondarily purified via infiltration of hydrothermal fluids and dissolution–reprecipitation processes. Such quartz parallels newly formed greisen-stage quartz in its chemical and CL properties; the share of greisen-stage quartz may by therefore overestimated.

Age of the granitic magmatism and the W-Mo mineralization in skarns of the Seridó belt (NE Brazil) based on zircon U-Pb (SHRIMP) and molybdenite Re-Os dating

Over five hundred W-Mo skarns have been reported in the Neoproterozoic Seridó belt in the northeastern Brazil. The origin of these mineralizations has been attributed to metasomatic reactions occuring after the infiltration of hydrothermal fluids that are mostly derived from the plutonic magmatic activity that ranged between approximately 600 and 525 Ma. Here we date molybdenite using N-TIMS on Re-Os analysis of three major scheelite deposits (Brejuí, Bonfim and Bodó) hosted in the skarn horizons of the metasedimentary sequence. Molybdenite is an integral part of the mineralizations that include scheelite in skarns and, in the Bonfim deposit, gold concentrate in late brittle faults. The Re-Os ages are 554 ± 2 Ma (Brejuí), 524 ± 2 Ma (Bonfim) and 510 ± 2 Ma (Bodó). The age of the Brejuí molybdenite, however, appears to be anomalous based on the local geology of the deposit, which is located next to the contact of a batholith dated ca. 575 Ma. In turn, the Bonfim molybdenite yields similar ages in replicated samples with variable high Re contents. New U-Pb SHRIMP ages of four biotite (leuco)granite plutons vary from 577 ± 5 Ma to 526 ± 8 Ma, which overlap with molybdenite crystallization. These results indicate a close connection between the W-Mo mineralizations and the plutonic activity that intruded the belt after the peak HT/LP metamorphism. The latest pulses of felsic magmatism, which were contemporaneous with the emplacement of Be-Ta-Nb-Li pegmatites, therefore constitute a potential guide in the Seridó belt for prospective W-Mo deposits.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

The Golden Mile in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, has produced 385 million tonnes of ore at a head grade of 5.23 g/t gold (1893–2016). Gold-pyrite ore bodies (Fimiston Lodes) trace kilometre-scale shear zone systems centred on the D2 Golden Mile Fault, one of three northwest striking sinistral strike-slip faults segmenting upright D1 folds. The Fimiston shear zones formed as D2a Riedel systems in greenschist-facies (actinolite-albite) tholeiitic rocks, the 700-m-thick Golden Mile Dolerite (GMD) sill and the Paringa Basalt (PB), during left-lateral displacement of up to 12 km on the D2 master faults. Pre-mineralisation granodiorite dykes were emplaced into the D2 shear zones at 2674 ± 6 Ma, and syn-mineralisation diorite porphyries at 2663 ± 11 Ma. The widespread infiltration of hydrothermal fluid generated chlorite-calcite and muscovite-ankerite alteration in the Golden Mile, and paragonite-ankerite-chloritoid alteration southeast of the deposit. Fluid infiltration reactivated the D2 shear zones causing post-porphyry displacement of up to 30 m at principal Fimiston Lodes moving the southwest block down and southeast along lines pitching 20°SE. D3 reverse faulting at the southwest dipping GMD-PB contact of the D1 Kalgoorlie Anticline formed the 1.3-km-long Oroya Shoot during late gold-telluride mineralisation. Syn-mineralisation D3a reverse faulting alternated with periods of sinistral strike-slip (D2c) until ENE-WSW shortening prevailed and was accommodated by barren D3b thrusts. North-striking D4 strike-slip faults of up to 2 km dextral displacement crosscut the Fimiston Lodes and the barren thrusts, and control gold-pyrite quartz vein ore at Mt. Charlotte (2651 ± 9 Ma).

Stratiform platinum-group element mineralization in the layered Northern Ultramafic Center of the Lac des Iles Intrusive Complex, Ontario, Canada

The Northern Ultramafic Centre (NUC) of the Lac des Iles Complex, Northwest Ontario hosts several platinum group element (PGE) occurrences, including the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched intervals exposed within the cyclically layered eastern flank of the NUC. Field relationships, mineral paragenesis and lithogeochemistry allowed for the identification of 14 cyclic cumulate sequences of two distinct types – Cyclic unit type A (CUA) and Cyclic unit type B (CUB). CUA-type and CUB-type units are interpreted to have formed from a Si-enriched and Si-poor parent magmas, respectively. PGE-enriched intervals occur in four of the CUA-type cyclic units (CUA-5, -6, -8 and -11). PGE enriched intervals are commonly associated with websterite, olivine websterite and gabbronorite containing primary disseminated sulfide (0.2–2vol%) which are dominated by pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, and troilite. In hydrothermally altered rocks enriched in PGE, primary sulfides are locally partially replaced by secondary chalcopyrite, sphalerite, heazlewoodite, and chalcocite. Palladium occurs either in solid solution with primary pentlandite or is associated with platinum group minerals (PGM) such as Pd-plumbide, Pd-telluride, and Pt-bismuthotelluride. PGMs commonly occur within primary sulfides, at contacts between primary sulfide–silicate minerals, or in association with secondary serpentine and actinolite. Gold and silver typically occur as electrum that exhibits similar textural characteristics and mineralogical associations as the PGMs.Two different chemostratigraphic patterns of PGE, Cu and S enrichment can be recognized among the mineralized CUA cycles: The first (top-loaded) occurs near the top of CUA cycles (CUA-6, -8 and -11) in websterite and/or gabbronorite, just below the levels at which CUB magmas were emplaced. The second (middle-loaded), occurs midway through the lower cycle (CUA-5) in the olivine websterite, which is overlain by CUA-6. Within the four mineralized intervals, PGE tenors average 643ppm Pd+Pt (in 100% sulfide), Pd/Pt and Pd/Ir ratios range from 0.9 to 3.5 and 35 to 537, respectively, and S/Se ratios range between 500 and 6000. The highest PGE tenors (4377ppm Pd+Pt) are found in the lowermost interval in serpentinized olivine websterite and have an average Pd/Pt ratio of 3.5 and a S/Se ratio of approximately 2000.It is proposed that orthomagmatic processes of fractional crystallization and dynamic magma recharge were the dominant mineralization processes triggering sulfide-saturation and PGE concentration at the Sutcliffe Zone. Textural relationships between PGM, sulfide minerals, and primary and secondary hydrous silicates suggest that late magmatic to postcumulus hydrothermal fluid infiltration occurred locally during and after sulfide mineralization of the PGE-enriched intervals. However, these fluids had a minimal effect on the distribution of PGE in the Sutcliffe Zone. The Sutcliffe Zone shares many similarities with classic stratiform PGE deposits in terms of Pd/Pt ratio, high PGE tenors, low abundance of sulfide, and PGM assemblages. However, it is distinguished from most stratiform PGE deposits by its tectonic environment and lithostratigraphic position and by the intimate spatial association of the two parental magmas that are interpreted to have been responsible for the observed chemostratigraphy and PGE enrichment.

Isotopic, Chemical, and Textural Evidence for Pervasive Calcite Dissolution and Precipitation Accompanying Hydrothermal Fluid Flow in Low-Temperature, Carbonate-Hosted, Gold Systems

Oxygen isotope ratios measured from microdrilled calcite in limestone wall rocks at the Banshee Carlin-type Au deposit show marked depletion proximal to Au mineralization, indicating isotopic exchange between wall-rock calcite and hydrothermal fluids. Isotopic alteration is spatially coincident with mineralization and puts minimum constraints on hydrothermal fluid infiltration outside of visible indicators (i.e., carbonate dissolution and silicification). Additionally, 18 O depletion is nearly homogeneous at hand-specimen scale indicating near-complete alteration of limestone protolith. The primary mechanism of isotopic exchange is coupled dissolution- precipitation leading to pseudomorphic replacement of calcite during hydrothermal fluid infiltration. Surface reactions between calcite and the hydrothermal fluids are evidenced by textural and chemical variations between altered and unaltered calcite in wall-rock limestone and limestone breccia. Cathodoluminescence (CL) reveals distinct changes in the luminescence of altered calcite relative to unaltered equivalent calcite, and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) data from altered and unaltered calcite show that observed CL responses are due to changes in calcite mineral chemistry. Altered samples exhibit bright CL responses in calcite with increases in Mn and Fe. In addition to wall-rock calcite alteration, calcite veins without clear paragenetic relationships can be related to hydrothermal alteration owing to 18 O depletion, CL response, and positive Eu* anomalies. Together, isotopic alteration of wall-rock calcite and evidence of hydrothermal calcite veins define the distal expression of low-temperature hydrothermal alteration in calcite-bearing rocks at the Banshee deposit.

Pseudotachylyte and Fluid Alteration at Seismogenic Depths (Glacier Lakes and Granite Pass Faults, Central Sierra Nevada, USA)

We present evidence for ancient seismicity in the form of tectonic pseudotachylyte and coeval, cyclic hydrothermal alteration, and cataclasis along fault zones exhumed from 2.4 to 6.0 km in the central Sierra Nevada, CA. The Glacier Lakes fault (GLF) and Granite Pass fault (GPF) are exhumed left-lateral to left-lateral oblique, strike-slip faults with up to 125 m of left-lateral separation exposed in Mesozoic granite and granodiorite plutons. Precipitation of epidote along fault slip-surfaces, chloritization of biotite, saussurite and sericite alteration of plagioclase, and quartz- and-calcite filled veins are present in the GLF and GPF zones. One difficulty encountered in studying exhumed fault zones is providing convincing evidence for a frictional melt origin of pseudotachylyte. Rocks in the field may preserve convincing evidence for frictional melt (i.e., aphanitic, dark, injection structures) that are later shown to be related to cataclasis or injection of hydrothermal fluids. Another challenge results from the low preservation potential of several of the microscopic features that are convincing evidence of a frictional melt origin (microlites, amygdules, and glassy matrix). Here we test the usefulness of grain shape and nearest neighbor distribution analysis of pseudotachylyte and cataclasites from the GLF and GPF to discriminate between these fault rocks and to determine a frictional melt origin for pseudotachylyte. Fabric analyses of the clasts within the pseudotachylytes examined are more circular and exhibit a random nearest neighbor clast distribution relative to adjacent cataclasites. With increased comminution and melting the mean clast circularity increases and the nearest neighbor distances approach a random distribution. We conclude that this observed pattern can be applied to other fault zones as an indicator of a frictional melt origin for fault-related rocks. Mutually cross-cutting zones of hydrothermal alteration and calcite deformation twins constrain the ambient temperature conditions in the fault zone to between 170 and 320 °C during pseudotachylyte formation. Based on previous thermochronologic studies, the temperature conditions of the country rock during faulting were between 110 and 220 °C. The overlapping to elevated temperatures in the fault zone can be explained by: (1) infiltration of hydrothermal fluids into the fault zone; or (2) residual elevated temperatures as a result of frictional heating in the fault zone during seismic slip.

Metallogenesis of the Zhibo and Chagangnuoer volcanic iron oxide deposits in the Awulale Iron Metallogenic Belt, Western Tianshan orogen, China

The Zhibo and Chagangnuoer iron ore deposits occur at the same volcanic edifice and are situated along the eastern segment of the Awulale Iron Metallogenic Belt in the Western Tianshan orogen of China. The former deposit is exposed near the caldera and the latter lies on the northwestern flank of the volcanic structure. The ore mineralization, the ore texture, the ore bodies structures, the trachytic to trachy andesitic host rocks and the geological characteristics of the Zhibo and Chagangnuoer deposits indicate a magmatic source for the iron oxide mineralization. This is supported by the observation that the vast majority of the Zhibo and Chagangnuoer magnetites have δ18O values between >+0.9‰ and <+4.00‰ consistent with a precipitation from ‘normal’ subduction type intermediate to felsic magmas or high-temperature magmatic fluids. However some magnetites with low δ18O values (<+0.9‰) are thought to have formed in equilibrium with a low to moderate temperature hydrothermal fluid as is evidenced by pervasive and vein-type controlled (skarn-type) alteration of the ore and host volcanic rocks. Furthermore, the trace element composition of the equilibrium ortho-magmatic magnetites was reset during this hydrothermal alteration event. Major and trace element characteristics indicate that the volcanic rocks, the granitoids and the iron ore bodies were formed in a continental arc setting during the Early Carboniferous. The ore-hosting volcanic rocks probably originated from melting of a mantle wedge that was metasomatized during subduction. Laser ablation-ICPMS (LA-ICPMS) U–Pb zircon ages of 350±2Ma and 361±7 Ma were obtained for zircon from the trachy andesitic host rock of the iron ore exposed near the caldera of the volcano and the highly altered fine grained garnet-dominated host rock on the flank of the volcano, respectively. A U–Pb zircon age of 321±2Ma was gained for the rhyolite overlying the ore bodies at the Chagangnuoer deposit, which were intruded by a granitic pluton with a U–Pb zircon age of 319±2Ma. Furthermore, the minerals of the skarn-type alteration zone at the Chagangnuoer deposit yield an Sm-Nd isochron age of 313±7Ma (Grt-Act-Epi-whole rock). Considering previous geochronological studies, a 350–330Ma age range as observed in the host rocks of the Zhibo mine is believed to display a period of continuous volcanic activity and associated ortho-magmatic, high-temperature iron oxide mineralization during subduction. The iron oxide mineralization in both deposits is thought to have been derived from the same magma source, as the ore-hosting volcanic rocks are similar to ‘Kiruna-type’ iron oxide ores. Subsequently, the volcanic host rocks and the iron ore bodies were variably hydrothermally altered during subduction (ca. 319–313Ma). The alteration zones were probably facilitated by the infiltration of hydrothermal fluids along faults, which were created by the intrusion of a granitic pluton.

Hydrothermal alteration associated with Mesozoic granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong Gold Province, China

The Sanshandao gold deposit (reserves of more than 200t Au and average grade of 3.96g/t), located at northwestern edge of the Jiaodong Peninsula, eastern North China Craton, is one of the largest gold deposits in the Jiaodong gold province. In this deposit, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids; mineralization and alteration are largely controlled by the regional Sanshandao–Cangshang fault. Host granitic rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Activities of the Sanshandao–Cangshang fault created structurally controlled permeability allowing for infiltration of hydrothermal fluids, leading to diffusive K-feldspar alteration on the two fault planes. Later, large scale diffusive sericitization symmetrically developed across the main fault, and partially overprinted the earlier K-feldspar alteration. Following the sericitization, relatively small scale silicification occurred, but now it is only retained in the hanging wall of the main fault. Subsequently, the fault gouge formed as a “barrier layer”, which is impermeable for later fluids to move upward. After that, strong pyrite–sericite–quartz alteration occurred only in the footwall of the main fault, and was accompanied by gold precipitation. The last stage carbonation and quartz-carbonate veins marked the waning of gold-related hydrothermal activity. Mass-balance calculations indicate complex behaviors of different types of elements during fluid–rock interaction. Most major elements were affected by intensive mineral replacement reactions. As expected, the fluid-mobile elements, LILE and LREE, generally show moderate to high mobility. It is notable that even the commonly assumed fluid-immobile elements, such as HREE and HFSE, tend to be changed to various degrees. In addition, Y–Ho, Zr–Hf and Nb–Ta fractionations are observed in altered domains. Studies on alteration assemblages and fluid inclusions suggest that the ore-forming fluids were characterized by low salinity (≤8.4wt.% NaCl equiv.), moderate temperature (300–400°C), weakly acidic (pH: 3–5), and relatively reducing (log fO2: ~–28) characteristics. In this type of fluids, gold was most likely transported as Au(HS)2− complex. With alteration going on, log (aK+/aH+) of fluids generally decreased due to significant formation of secondary K-bearing minerals. In addition, there might be a decrease of fO2 from pre-gold alteration stage to the main gold mineralization stage, and decrease of fO2 was probably one of the factors controlling gold precipitation. The Sr and Nd isotopic compositions of hydrothermal minerals, combined with previous H–O and He–Ar isotopic studies, indicate that the hydrothermal fluids were mainly derived from crustal sources (e.g., degassing of felsic magmas and meteoric water), but with involvement of mantle derived components. The gold mineralization event just coincided with reactivation of the North China Craton, as marked by asthenosphere upwelling, voluminous igneous rocks, and high crustal heat flow, which may have provided sufficient heat energy and fluid input required for the formation of the gold deposits.

Mineralogy and microchemistry of the Egat gold deposit, South Eastern Desert of Egypt

Gold-bearing quartz lodes from the Egat gold mine, South Eastern Desert of Egypt, are associated with pervasively silicified, highly sheared ophiolitic metagabbro and island-arc metavolcanic rocks. The mineralized quartz veins and related alteration haloes are controlled by NNW-trending shear/fault zones. Microscopic and electron probe microanalyses (EPMA) data of the ore and gangue minerals reveal that fine-grained auriferous sulfarsenides represent early high-temperature (355–382 °C) phases, with formation conditions as (fS2 = −10, and fO2 around −31). A late, low-temperature (302–333 °C) assemblage includes coarse pyrite, arsenopyrite, and free-milling gold grains (88–91 wt.% Au), with formation conditions as (fS2 = −8 and fO2 around −30). Gold was impounded within early sulfarsenides, while free-milling gold blebs occur along microfractures in quartz veins and as inclusions in late sulfides. Infiltration of hydrothermal fluids under brittle–ductile shear conditions led to mobilization of refractory Au from early sulfarsenide phases and reprecipitated free gold, simultaneous with silicification of the host rocks. The positive correlation between Au and As favors and verifies the use of As as the best pathfinder for gold targets, along the NNW-trending shear zones.

Using pegmatite geochronology to constrain temporal events in the Adirondack Mountains

U-Pb laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) ages have been determined from large zircon crystals separated from pegmatites of the Adirondack Mountains, New York. Emplacement and metamorphic ages ranging from 949 ± 10 to 1222 ± 12 Ma help constrain the timing of igneous, metamorphic, and deformational history of the region, and are associated with Shawinigan, Ottawan, and Rigolet orogenesis. Geologically reasonable ages were obtained from most zircon separates despite large size, a limited number of grains, high uranium and thorium contents, dark and opaque interiors, high density of fractures, and widespread areas of metamictization and Pb loss. However, few grains show zoning or differences in composition when viewed with the backscattered mode on the scanning electron microscope. Large, clear, internally featureless, U-poor grains yield the best constrained ages. U-Th-Pb monazite ages, determined by electron probe, vary from 874 ± 27 Ma to 297 ± 62; the younger age may reflect the timing of hydrothermal fluid infiltration related to late Acadian events. This study suggests that, with appropriate care, zircons from pegmatites are a reasonable target for LA-MC-ICP-MS geochronology, widening the current arsenal of sampling targets.

Alteration of the Oceanic Lower Crust at a Slow-spreading Axis: Insight from Vein-related Zoned Halos in Olivine Gabbro from Atlantis Massif, Mid-Atlantic Ridge

Incipient-stage alteration products in relatively fresh oceanic gabbros from deep boreholes provide critical information on hydration processes in the oceanic lower crust and their effect on lithosphere dynamics.We present the results of a petrographic study on the alteration of olivine-bearing gabbroic rocks recovered from the deeper parts of Integrated Ocean Drilling Program (IODP) Hole U1309D in the Atlantis Massif near the Mid-Atlantic Ridge at 308N. In these rocks, alteration is localized in proximity to fluid-infiltration veins or igneous contacts. It is most conspicuous in halos surrounding amphiboleþ chlorite veins or leucocratic veins in olivine-bearing gabbros, where coronitic fringes of tremolite, chlorite and talc occur around discrete olivine grains. Many of the halos exhibit a zonal pattern with systematic changes in mineral assemblage, generally consisting of three zones: tremoliteþ chlorite around relict olivine^ plagioclase contacts; talc pseudomorphs after olivine; and tremolite pseudomorphs after olivine. The tremoliteþ chlorite assemblage appears in increasing amounts and talc grows unevenly with increasing thickness toward the veins.The alteration minerals have highly magnesian compositions, reflecting the compositions of the precursor igneous phases.Within the zone closest to the veins, green hornblende with a relatively high Al content occurs, showing textures suggestive of its later formation than the coronitic tremolite and chlorite. Considering the mode of occurrence and chemical composition of the minerals combined with thermodynamic calculations of silica and water activities in a simplified system, we conclude that the zoned halos were caused by metasomatism owing to protracted or sequential infiltration of hydrothermal fluids at amphibolite-facies conditions (450^7508C, 1·5^2 kbar). Textural relationships clearly indicate that zoned halos formed earlier than serpentinization and clay mineral formation, and suggest that the high-temperature, amphibolite-facies alteration took place in a near-axis region before the exhumation of the lower crustal rocks. Recent results of seafloor drilling have provided supporting evidence for the predominance of gabbroic rocks in oceanic core complexes.The similarity in mineral association between zoned halos and schistose fault rocks suggests that preferential formation of talc and/or chlorite, rather than serpentine, at contacts between gabbroic rocks and peridotite plays an essential role in detachment faulting and tectonic exhumation of oceanic core complexes from lower crustal levels.

Dating of fault zone activity by apatite fission track and apatite (U-Th)/He thermochronometry: a case study from the Lavanttal fault system (Eastern Alps)

IntroductionFaulting in the upper crust may leadto thermal anomalies caused by fric-tional heating (e.g., Maddock, 1983;dAlessio et al., 2003) and⁄or infiltra-tion of hydrothermal fluids (Yamadaet al., 2007). Heating effects are ex-pected to be localised along faultzones and restricted to zones a fewcentimetres adjacent to the main faultsurface (dAlessio et al., 2003).In this study, we attempted to datethermal anomalies within fault zonesby comparison of cooling ages at thetransition from fault-related rocks tothe undeformed host rocks. For thispurpose, we chose the Lavanttal faultsystem. Due to underground excava-tion and cored drillings related togeological site investigations for amajor railway project in Austria, it isone of the best exposed fault zones inthe Eastern Alps. We used a combi-nation of fission track (AFT) and(U–Th)⁄He analysis on apatites. Thesensitivity intervals of these thermo-chronometers are often referred to aspartial annealing and retention zones(APAZ, HePRZ) and range from 120to 60 C and 80 to 40 C, respectively(Green et al., 1986; Wolf et al., 1996),assuming holding times over geo-logical time-scales (Wagner, 1979;Ketcham et al., 1999). The low-temperature sensitivity of these ther-mochronometers allows thermalanomalies along surfaces of fault slipor fluid pathways to be detected. Forsuch applications, however, annealingand retention depend not only onheating temperature but also on heat-ing duration (Laslett et al., 1987;Murakami et al., 2006). A combina-tion of forward and inverse modellingon apatite age, track length and Hedata allows quantitative analysis ofthe cooling behaviour of the hostrocks.Our new results underline theimportance of using multiple low-temperature thermochronometersin elucidating details of the thermalhistory of a fault zone.Geological outlineThe NNW-trending Lavanttal faultsystem, one of the major strike–slipfaults in the Eastern Alps (Fig. 1),separates the basement complexes ofthe Kor- and Saualm by dextral offsetof approximately 10 km (Reinecker,2000). It is assumed to have beenactive since Early Miocene times withpeaks in activity between 18–16 and14–12 Ma (Reinecker, 2000) as indi-cated by the evolution of adjacentsedimentary basins and the regionalstructural framework (e.g., Pischingeret al., 2008). Near its southern termi-nation, the Lavanttal fault offsets thePeriadriatic fault by about 20 km.Despite some models at the scale ofthe Eastern Alps describing the gen-eral evolution of confining fault zonesduring Miocene times (e.g., Peressonand Decker, 1997; Frisch et al., 1998,2000; Neubauer et al., 2000), littleremains known about the evolutionof the Lavanttal fault system. How-ever, these models include a relation-ship between the Lavanttal faultsystem and the uplift of the KoralmComplex (Neubauer and Genser,1990). Previous thermochronologicalstudies documented that the Koralmcomplex was already near the surfaceduring the whole Cenozoic period(Hejl, 1997). The timing of its finaluplift, however, is less constrained.Sample descriptionParts of the Lavanttal fault system arecharacterised by cataclasites and faultgouges up to several tens of metresthick. Thermochronological analyseswere carried out on drill core samples(diameter 10 cm) taken during thegeological investigations for theKoralm railway tunnel. DrillingABSTRACT