Unaltered Host Rocks Research Articles

Rare-metal halos of lithium pegmatite at the Wolfsberg deposit, Austria, and their implications for exploration

The Austroalpine Unit Pegmatite Province (AUPP) in the eastern European Alps hosts abundant Permian-aged, variably rare-metal-enriched pegmatite bodies. However, only a small number containing spodumene [LiAlSi2O6] have been discovered. Here, we characterize three drill cores from the AUPP’s largest spodumene pegmatite resource, the Wolfsberg deposit (Austria), to determine the extent of rare-metal halo formation within different host rock types generated by pegmatite-derived fluids and their applicability to spodumene pegmatite exploration. The halos in both mica schist and amphibolite host rocks, as quantified via whole-rock mass-balance calculations, are characterized by enrichments in Li, Cs, Sn, Rb and Tl, amongst others, elements which were highly mobile during initial interactions between pegmatite-derived fluids and host rocks. The concentrations of these elements within the halos can be several times that present in unaltered host rock. They are most enriched in altered host rock interbedded or at the contacts with pegmatite, and decrease with distance from pegmatite to a distance of at least 4 m in mica schist and <3 m in amphibolite. Lithium, and in mica schist, Cs, have the most consistent and furthest extending halos, with respective concentrations of >590 ppm and >40 ppm in mica schist, and >390 ppm Li in amphibolite indicating proximity to spodumene pegmatite. An ID-TIMS U-Pb titanite age of ca. 103 Ma derived from a rare-metal-enriched amphibolite within the alteration halo reflects recrystallization of the Permian protolith during the Cretaceous Alpine orogeny and indicates preservation of the original pegmatite signature through eclogite-facies metamorphism. Recognition of rare-metal halos surrounding pegmatite at Wolfsberg serves as a potential geochemical exploration tool for spodumene pegmatite within the AUPP and may be applicable to other regions.

Sediment‐hosted disseminated gold mineralization at the Gegalaw deposit in Central Myanmar

AbstractGegalaw is one of the gold deposits situated in the Sagaing Fault zone, which marks the boundary between the Central Myanmar Basin to the west and the Mogok metamorphic belt to the east. The gold mineralization is mainly confined along northeast striking faults between carbonaceous silty limestone of the Kywethe Chaung Formation and carbonaceous and calcareous shale of the Eocene Male Formation. Hydrothermally induced dissolution breccia associated with progressive silica replacement increase near the northeast trending faults. The hydrothermal alteration associated with the mineralization is silicification in addition to illite‐kaolinite argillic alteration, of which illite is dated by the K‐Ar method as 40.26 ± 0.91 Ma. Sulfide minerals contained in the ores are mainly pyrite and arsenopyrite, with minor to trace amounts of marcasite, chalcopyrite, and sphalerite, among which pyrite and arsenopyrite are the hosts of gold. Pyrite and arsenopyrite are divided into diagenetic and hydrothermal stages, based on their mineral textures. Diagenetic stage pyrites are framboidal (Py1A) and coalescent framboidal (Py1B). Those of the hydrothermal stage are zoned pyrite with porous cores (Py2) and compact rims (Py4). In addition, euhedral homogeneous pyrites with or without compact rim are present (Py3). Some pyrites show pseudomorphs of ilmenite (Py3i) and carbonate minerals, possibly ankerite (Py3c). Arsenopyrites are also divided into porous, sooty arsenopyrite (Apy1), and compact arsenopyrite (Apy2) that co‐exists with Py3 and Py4 pyrites. Electron probe microanalysis for sulfides indicates that Py1A and Py1B are deficient in Fe while Py2, Py3 and Py4 are deficient in S compared to stoichiometry of pyrite. Such deficiencies are compensated by metal ions such as Co2+ and Ni2+ for Py1A, As2+ for Py1B, and As1− for Py2, Py3, and Py4. Arsenic equilibrium temperature of Apy2‐Py3 pairs shows a mode range of 250–270°C. Detected gold concentrations (~650 ppm) in arsenian pyrite (Py2, Py3, and Py4) indicate that gold in pyrite are below the solubility, suggesting that gold is present as solid‐solution in pyrite. Sulfur isotopic values of the pyrites in the unaltered host rocks are −18.9‰ and −8.9‰, and those of the pyrites from the hydrothermal altered rocks range from −17.3‰ to −0.5‰. These similar negative δ34S ranges of the pyrites suggest that the source of sulfur is dominated in biogenic reduction origin. Nevertheless, the two relatively heavier sulfur isotopic values (−4.9‰ and −0.5‰) suggest a contribution of an external hydrothermal fluid source. The research results indicate that gold mineralization occurred by infiltration of auriferous reduced hydrothermal fluids into the carbonaceous silty limestone and carbonaceous and calcareous shale through northeast trending secondary faults of the dextrally moving Sagaing Fault system and the fluids reacted with framboidal pyrite as well as iron‐bearing minerals in the host rocks to precipitate gold‐bearing arsenian pyrite. The age of hydrothermal illite suggests a link to the right‐lateral strike‐slip movement of the Sagaing Fault that is related to the collision of the Indian and Asian plates.

ВОЗРАСТ И ИЗОТОПНЫЙ СОСТАВ ЗОЛОТО-СЕРЕБРЯНЫХ МЕСТОРОЖДЕНИЙ И РУДОПРОЯВЛЕНИЙ ЭВЕНСКОЙ ГРУППЫ (ОХОТСКО-ЧУКОТСКИЙ ВУЛКАНОГЕННЫЙ ПОЯС, СЕВЕРО-ВОСТОК РОССИИ)

Existing and recently acquired data on the isotope age and composition of volcanogenic Au-Ag deposits and occurrences of the Evensk group (Okhotsk-Chukotka volcanogenic belt, Northeast Russia) are summarized. The K-Ar isotopic and Rb-Sr isochron ages of Au-Ag ores from the deposits and occurrences studied is 82.7 ± 3–77.5 ± 3 Ma and 84.1 ± 2–79.7 ± 5, respectively. Au-Ag mineralization formed during a 5 million-year time interval. Au-Ag mineralization, typical of productive ore stages and practically unaffected by later thermal processes, is characterized by low values of primary 87Sr/86Sr(0) = 0.7055–0.7059 ratios close to primary 87Sr/86Sr(0) ratios in wall-rock metasomatites (0.7033–0.7082) and unaltered host rocks (0.7045–0.7048) indicative of a pronounced predominance of mantle Sr in the ores. The composition points of ore Pb on the 207Pb/204Pb – 206Pb/204Pb evolution diagram are grouped in close proximity to the Pb isotope composition points in the exhausted (depleted) mantle with a trend towards the volcanic rocks formed in subduction zones typical of the continental margin. A close genetic relationship between ore and magmatic processes is confirmed. It is assumed that this relationship is due to the functioning of a single mantle basaltoid chamber, a source of metal-bearing fluids and, probably, the primary source of Au and Ag.

Stream Sediment Datasets and Geophysical Anomalies: A Recipe for Porphyry Copper Systems Identification—The Eastern Papuan Peninsula Experience

Airborne magnetic and radiometric datasets have, over the past few years, become powerful tools in the identification of porphyry systems which may host economic porphyry copper–gold–molybdenum ore bodies. Magnetisation contrasts with the unaltered host rocks, coupled with the elevated radiometric signature, compared to the host rock, makes identification of large-scale porphyry copper systems possible. Integrating these two different datasets with stream sediment data and other geochemical exploration methods results in a higher degree of confidence. Stream sediment data were analysed to see the distribution of copper and gold elements throughout the study area, located within the Eastern Papuan Peninsula of Papua New Guinea. Airborne geophysics data over the same area were also processed for magnetic and radiometric responses. The processing of the magnetic data revealed several magnetic anomalies related to concealed intrusive rock units, with associated radiometric signatures. The distribution of gold and copper anomalism was correlated with the geology and geophysical signatures. Results indicate varying degrees of correlation, with some areas showing a strong correlation between gold/copper occurrence and geophysical signatures, compared to other areas. Some factors that we believe impact the level of correlation may include tectonic history, volcanic cover, and weathering patterns. We recommend caution when applying multi-data exploration for porphyry copper systems.

Early Paleozoic Orogenic Gold Deposit in the Cathaysia Block, China: A first example from the Shuangqishan Deposit

The Shuangqishan gold deposit (>20t gold) is located in a hitherto poorly-documented gold province in the southeastern Cathaysia Block, South China. It is hosted by Neoproterozoic amphibolite-facies metamorphic rocks and consists of quartz-sulfide veins with distinct alteration haloes. Three stages of hydrothermal activity have been identified, and gold is mainly associated with veins of Stage II (quartz-pyrite±chalcopyrite). Gold occurs both as refractory gold hosted by pyrite and free gold enclosed in, or filling, microfractures of pyrite and quartz. U-Pb isotope dating of hydrothermal apatite from the Substage IIa veins yielded an age of 436.8 ± 7.6 Ma. The ore fluid was of carbonic-aqueous composition and low oxygen fugacity. Total homogenization temperatures in Substage IIa and IIb range from 270° to 310°C and 250° to 295°C, respectively. Variable CO2/H2O ratios indicate fluid unmixing under fluctuating pressures. Stage I quartz veins show evidence of ductile deformation and have δ18Ofluid and δD values of 10.3 to 11.2‰ and −67to −66‰, respectively, consistent with a metamorphic fluid. Stage II veins have slightly higher δ18O values of 10.9 to 13.7‰, with calculated δ18Ofluid values of 4.8 to 8.0‰. The 3He/4He ratios of pyrite are elevated (0.06–0.23 Ra) relative to crustal He reservoirs (<0.01 Ra). Fluid inclusions in quartz yielded δ13CCO2values of −23.0to −11.4‰ (V-PDB), reflecting variable contributions of organic carbon. The sulfide δ34SV-CDT values range from −4.7to −1.0‰, consistent with a deep-seated sulfur source. The Pb isotope compositions of pyrite (206Pb/204Pb=17.141–17.871, 207Pb/204Pb=15.484–15.570) are more radiogenic than those of unaltered host rocks. All available data demonstrate that the Shuangqishan deposit is the first report orogenic gold deposit formed during the late Wuyi-Yunkai intracontinental orogeny in the Cathaysia Block.

The Jurassic Danba hypozonal orogenic gold deposit, western China: indirect derivation from fertile mantle lithosphere metasomatized during Neoproterozoic subduction

The genesis of hypozonal orogenic gold deposits is highly controversial, with both crustal metamorphic fluids and variously derived sub-crustal fluids suggested as fluid and metal sources. The Lower Jurassic Danba gold deposit, a rare Phanerozoic hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China, provides an opportunity to resolve these controversies. Danba is characterized by biotite–amphibole alteration and pyrrhotite-dominant ore assemblages formed at ~ 500–600 °C. The δ34S values of ore-related pyrrhotite range from + 3.1 to + 9.9‰, in sharp contrast to those of syn-sedimentary pyrrhotite from Devonian host rocks which are between − 6.8 and − 9.5‰. The Pb isotope compositions of ore-related pyrrhotite, with 206Pb/204Pb = 17.85–18.25 and 207Pb/204Pb = 15.48–15.67, are less radiogenic and more variable than those of unaltered host rocks. These differences, combined with the hypozonal feature of the deposit, indicate that ore metals and sulfur were derived from a sub-crustal, rather than a local crustal source. The PGE patterns of ore-related pyrrhotites are similar to those of sulfides in Baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of Pd and Ru but significantly differ from those of magmatic intrusions and related magmatic–hydrothermal deposits. This suggests that the metals that formed the Danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source. The δ18O of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12‰. The anomalously high S and O isotope ratios and variable Pb isotope ratios, together with PGE data, support a model in which the inferred mantle lithosphere source was significantly hydrated, metasomatized, and fertilized by fluid derived from subducted oceanic sediments with elevated δ34S and δ18O values. Geological evidence suggests that this subduction event was Neoproterozoic in age. This study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction, but it was over 500 million years later, during later Lower Jurassic asthenosphere upwelling, that metal- and sulfur-rich fluid was released into the crust to form the Danba gold deposit.

Imaging porphyry copper alteration using aeromagnetic data at Highland Valley Copper, British Columbia, Canada

ABSTRACTWe evaluated imaging alteration associated with porphyry copper style mineralisation using aeromagnetic data in the Highland Valley Copper (HVC) district as part of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Mining Innovation Council (CMIC) Footprints project. The first step of the investigation involved construction of a 3D geological model based on surface and regional information, and petrophysical data of the host rocks. From the petrophysics, it was observed that hydrothermal alteration affected the magnetic susceptibility of some batholithic host rocks at HVC, and low susceptibility values were associated with the altered rocks. From this observation, an alteration index was devised, differencing, based on the magnetic susceptibility, between unaltered and altered batholith host rocks. Three-dimensional inversions were conducted using geological constraints obtained from surface and borehole geology as well as physical property data. Then the alteration index methodology was tested using 3D inversions and residual models within five overlapping blocks covering portions of the batholith. The indices inferred from inverted susceptibility were compared to surface mapped alteration. For some felsic facies of the batholith, the alteration index derived from inverted magnetic susceptibility corresponds to the mapped alteration and delineates the known porphyry Cu system district. However, it is not true for others where alteration was less magnetite-destructive and more focused in structures.

Metallogeny of the Zoujiashan uranium deposit in the Mesozoic Xiangshan volcanic-intrusive complex, southeast China: Insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions

The Zoujiashan U deposit in the Mesozoic Xiangshan volcanic-intrusive complex is one of the largest volcanic-related type U deposits in China. Chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of U and to further constrain the origin of U and ore-forming fluids of the Zoujiashan deposit. Illitization, hematitization, and U mineralization successively occurred within rhyolite and porphyritic lava. Three types of hydrothermal apatite are identified. Apatite Type 1 (Ap1) and apatite Type 2 (Ap2) are both euhedral-subhedral crystals found in hematitization zone, while only Ap2 shows evident irregular chemical zoning along its rims. Apatite Type 3 (Ap3) occurs as anhedral aggregates closely associated with U-minerals in U ore veins, implying synchronous precipitation of Ap3 and U ores. Comparable chemical compositions of Ap1 and cores of Ap2, including similar high Cl content, indicate both of them were precipitated from a Cl-rich fluid that induced hematitization. The elevated F contents of Ap3 and rims of Ap2 implies that a F-rich fluid was involved and altered Ap2 during the main stage of U deposition. Ap1 and cores of Ap2 have lower Mn contents than Ap3 and rims of Ap2, indicating an oxidized character for the Cl-rich fluid and a reducing character for the F-rich fluid.Metal contents of fluid inclusions in fluorite and quartz occurring in U ores suggest that two end-members of fluids, including a U-rich and a U-poor, were involved in the main stage of U mineralization. Mixing between these two fluids led to the deposition of U- and Th-minerals through oxidation-reduction reactions and destabilization of Th-F complexes, respectively. The U-rich fluid is depleted in Th and Sr compared with the U-poor fluid. Based on comparison of chemical compositions of apatite and fluid inclusions, we suggest that the Cl-rich fluid precipitating Ap1 and Ap2 corresponds to the U-rich end-member fluid, while the F-rich fluid precipitating Ap3 represents a mixture between the two end-members. The high U (12.7–58.5 ppm) in the U-rich end-member fluid is in agreement with a relatively high oxygen fugacity and a surface-derived origin. The U-poor end-member fluid has lower Rb/Cs than the U-rich end-member and the unaltered host rocks, indicating the U-poor fluid was derived from a mantle wedge that experienced addition of Cs by fluids generated from the devolatilization of the subducted paleo-Pacific slab.

Geochemical pattern recognition through matrix decomposition

This study compares the use of joint singular value decomposition and semi-discrete decomposition (JSS) and non-negative matrix factorisation with univariate analysis of raw data, to detect multi-element patterns in soils related to geochemical dispersion from Mississippi Valley-Type Pb-Zn deposits in the Irankuh area of central Iran. Joint singular value decomposition and semi-discrete decomposition clustering of the data identified a suite of mineralisation-related variables and corresponding sample clusters that are spatially associated with mineralisation or variations in parent lithology. Non-negative matrix factorisation generated three main factors and sample clusters that relate to the main zones of sulphide mineralisation, variations in clay mineralogy and the composition of unaltered host rocks. These two matrix decomposition techniques deliver similar results, though JSS delivers a significantly higher sample classification accuracy. Both methods result in delineation of a contiguous cluster of samples above an extensive zone of blind mineralisation at the Tappe Sorkh Pb-Zn deposit and define new potentially mineralised targets between this deposit and the Gushfil Pb-Zn deposit. More conventional univariate methods to detect anomalous populations of Pb and Zn in the soil geochemical data, such as number-size fractal analysis, were less effective at defining these targets. Joint application of practical matrix decomposition techniques, utilising noise removal and modelling of the underlying geochemical patterns associated with ore-forming processes, has led to more reliable prospectivity mapping of the Pb-Zn deposits in Irankuh.

Ore genesis of the Bake gold deposit, southeastern Guizhou province, China: Constraints from mineralogy, in-situ trace element and sulfur isotope analysis of pyrite

The Bake gold deposit is exceptional for its high native gold abundance relative to the other orogenic gold deposits in the southeastern Guizhou district. The grades of gold can be up to 2 to 3 wt% in bonanza ore, and native gold preferentially occurs within voids and fractures in pyrite, arsenopyrite and quartz. Gold content can be up to 6.9 ppm in hydrothermal pyrite, however, the majority of the pyrite analyses have Au contents less than 1 ppm (mean = 0.32 ppm) based on LA-ICPMS analyses. Meanwhile, EMPA reveals that arsenopyrite has a maximal Au content of up to 0.14 wt%, but most analyses are below detection limit (0.01 wt%). Moreover, native gold grains in cataclastic pyrite and arsenopyrite are commonly high-purity with fineness ranging from 949 to 977. Given that paragenetically early hydrothermal pyrite and arsenopyrite contain variable but relatively high gold concentrations than later generations of sulfide, it is suggested that early invisible gold may have been remobilized to form at least part of the microscopic visible gold. Post-mineralization shear zones, which cut the vein-bearing anticline, may have led to the deformation of the lode quartz vein system on the macro level, and microscopically, sulfide fragmentation and elemental remobilization. Although diagenetic pyrite from unaltered host rocks is enriched in invisible gold with mean values at 0.83 ppm, evidence supporting the release of Au from them is weak. Corresponding evidence comes from the distinct sulfur isotope signatures in different pyrite generations. Diagenetic pyrite has δ34S values in the narrow range of +9.27‰ to +10.75‰, whereas hydrothermal pyrite shows a broad range (+6.81‰ to +17.42‰), based on LA-MC-ICPMS analyses. The Mesoproterozoic Lengjiaxi Group underlying the Xiajiang Group is most likely the source region, on the basis of geological background and assuming the metamorphic devolatilization model.

Palaeokarst Deposits in Caves: Examples from Eastern Australia and Central Europe

Palaeokarst deposits are most commonly found in excavations, drill holes and naturally exposed at the Earth’s surface. Some caves however intersect palaeokarst deposits. This occurs in large hypogene caves in the USA, thermal caves in Hungary and in many caves in eastern Australia. Palaeokarst deposits in caves respond to cave forming processes in the same way as hostrock: the palaeokarst deposits form cave walls. A range of palaeokarst deposits is exposed in caves including; filled tubes, walls composed of flowstone, large-scale bodies, breccia pipes, dykes, volcaniclastic palaeokarst and crystalline palaeokarst. As well as being exposed in cave walls, palaeokarst deposits can wholly or partly form speleogens (speleogens made from palaeokarst). Records of geological events not preserved elsewhere can occur in palaeokarst deposits in caves. These can be difficult to correlate with conventional geological histories. It is important to be able to distinguish between palaeokarst deposits, relict sediments and phantom rock (in-situ weathered rock, also called ghost rock; Vergari & Quinif 1997). Relict sediments can be distinguished from palaeokarst deposits because the cave walls bound relict sediments while palaeokarst deposits form the cave walls. Palaeokarst can be distinguished from phantom rock, as palaeokarst is unconformable with the hostrock, with structures in the hostrock not continuing across the boundary into the palaeokarst. Hostrock structures and textures do continue across the boundary between unaltered hostrock and phantom rock. Similarly, cave sediments are unconformable or disconformable with the hostrock while phantom rock is conformable with hostrock containing hostrock structures and textures. It has been difficult to explain why palaeokarst occurs in some caves and not others. One explanation worth considering is that palaeokarst deposits are not intersected by caves or sections of caves that contain large perennial streams and/or have undergone large-scale vadose fluvial development capable of escaping from the bounds of structural guidance, such as the caves in the Classical Karst. Key words: palaeokarst, caves, Australia, relict sediments, phantom rock.

Ore-forming elements diffusion and distribution in the altered host rock surrounding the Koktokay No. 3 pegmatite in the Chinese Altay

Petrography and geochemistry of the altered and unaltered host rocks surrounding the Koktokay No. 3 pegmatite revealed that the unaltered amphibolite is mainly composed of hornblende, plagioclase, and ilmenite. Beyond these primary components, the altered host rocks contain a few newly formed minerals, including biotite, tourmaline, chlorine, and muscovite. The alteration zone surrounding the Koktokay No. 3 pegmatite is limited to 2.0 m, characterized by biotitization, tourmalization, and chloritization. In the altered host rocks, the contents of SiO2, MgO, MnO, Na2O, and TiO2 did not vary greatly. However, Al2O3 showed a weak decreasing trend with the increasing distance from the pegmatite contact zone, while Fe2O3 and CaO showed an increasing trend. The contents of Li, Rb, and Cs in the altered host rocks were much higher than those in the unaltered host rocks, decreasing with distance from the contact. The chondrite-normalized rare earth element (REE) pattern of the altered and unaltered host rock was right-inclined from La to Lu, but enriched in light REEs over heavy REEs after hydrothermal alteration. An isocon plot shows that some oxides migrated in with an order of P2O5 > K2O > TiO2 > Al2O3 > SiO2 > MnO ≥ MgO, while others migrated out with an order of Na2O > CaO > Fe2O3. For REEs, the migration ratios are positive values with Cs > Rb > Li > Nb > Ta > Be, signifying that all REEs migrated from the exsolved magmatic fluid into the altered host rocks. It was concluded that diffusion was the only mechanism for migration of ore-forming elements in the alteration zone. The effective diffusion coefficients (D eff ) of LiF, RbF, and CsF were estimated under a fluid temperature of 500–550 °C. Using a function of concentration (C (x,t)) and distance (x), the order of migration distance was determined to be LiF > CsF > RbF, with diffusion times of (3.39 ± 0.35) × 106, (3.19 ± 0.28) × 105 and (6.33 ± 0.05) × 105 years, respectively.

Cation migration in hydrothermal clays: The problem of mineralization criteria in gas-hydrothermal fluids of hydrothermal fields in Southern Kamchatka

Based on a comprehensive study of the hydrothermal clay layer that occurs in geothermal fields, the conditions of formation of cation composition in argillitized rocks are discussed. Under the influence of gas-water fluids and pore solutions, micro- and nano-mineral mixtures are formed in hydrothermal clays; these mixtures include crystalline, amorphous, and transitional mineral phases. A considerable role in their composition belongs to cations of several metals (Fe, Al, Ti, Na, Mg, Ca, K, Mn, and Ba), as well as Si, C, N, S, and volatiles (F− and Cl−). The sources of cations and other elements are unaltered host rocks, newly formed hydrothermal-metasomatic rocks, hydrothermal clays, salt deposits, siliceous, carbonate, and other sediments, as well as deep fluids. In the structures of geothermal anomalies and deposits the “hydrothermal metasomatic rock—gas-water fluid—newly formed mineral chemical compounds” united system is formed. Each of the elements of this system takes part in the transportation, accumulation, and redistribution of metals. This approach to studies of the geochemistry of present-day geothermal systems may serve as a foundation for developing criteria for the presence of mineralization in metasomatites, gas-hydrothermal fluids, and new mineral associations.

Trace-element mobilization during Ca-metasomatism along a major fluid conduit: Eclogitization of blueschist as a consequence of fluid–rock interaction

In the subduction complex of the Tianshan mountains, western China, massive blueschist is cross-cut by an eclogite-facies major fluid conduit surrounded by a reaction zone which is mainly composed of omphacite and garnet. Petrological as well as geochemical evidence suggest that formation of the vein and the eclogitic selvage around the vein was caused by fluid infiltration under peak metamorphic conditions of 21 ± 1.5 kbar and 510 ± 30 °C. The combination of whole-rock with mineral trace-element data as well as mass-balance calculations indicate that substantial differences exist between the unaltered host rock and the part of the system which was altered by fluid–rock interaction. These differences include: (1) depletion of mainly large-ion lithophile elements (LILE) and Li of up to 60% relative to their concentrations in the unaltered host rock; (2) an extreme enrichment of CaO (∼115%), Sr and Pb (>300%) in the altered parts of the vein-wall-rock system; (3) redistribution of heavy rare earth elements (HREE) from partly replaced rutile and recrystallized titanite in the blueschist–eclogite transition zone into newly grown garnet rims in the eclogitic selvage around the vein; (4) transformation of high Nb/Ta rutile into low Nb/Ta titanite which is associated with preferred mobilization of Nb over Ta; and (5) decoupling of Zr and Hf from Nb and Ta; the latter are depleted by ∼30% relative to the unaltered blueschist host rock whereas the former are depleted by only ∼10%. The prerequisite for the transformation of Ca-poor blueschist (6–7 wt.% CaO) into Ca-rich eclogite (up to 13 wt.% CaO) was the infiltration of a Ca-rich fluid. The release of trace elements can be attributed to partitioning of these elements into the passing fluid phase during dissolution–reprecipitation processes in the course of eclogitization. The reactivity of the precursor mineral assemblage and the chemical gradients between the reacting and passing fluid of the conduit are mainly responsible for trace-element mobilization in the studied samples. The suite of trace elements released upon fluid-induced eclogitization of the reactive wall-rock resembles that in island arc magmas showing strong enrichment of LIL elements, strong depletions in HFS elements and intermediate concentrations of REE.

Relationship Between Physical Properties and Alteration at the St Ives Gold Mine, Western Australia

I use a simple rock physics model, based on linear combinations of the properties of unaltered and altered rocks, to predict the physical properties of altered rocks at the St Ives Gold Mine, Western Australia. Previous studies of mineral systems demonstrate that alteration can produce physical property contrasts with respect to unaltered host rocks and thus produce geophysical signatures in various datasets such as gravity, magnetics and seismic.

Isotope geochemistry of the Huize Zn-Pb ore field, Yunnan Province, Southwestern China: Implication for the sources of ore fluid and metals

The Huize ore field, which is the most famous high-grade Zn-Pb ore field in China, comprises the Kuangshanchang and Qilinchang deposits. The Zn and Pb reserves of these two deposits are more than 5 Mt with ore grades ranging from 25% to 35% in weight. Lead, sulfur, carbon, oxygen, hydrogen and strontium isotope geochemistry is reported to help understand the sources of the ore fluid and metals. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of the ores range from 18.251–18.530, 15.663–15.855 and 38.487–39.433, respectively. These values are similar to those of the wall rocks. The pyrites disseminated in the wall rocks have indistinguishable Pb isotope composition with the ores. These data indicate that the wall rocks provided metals to the ore fluid. Most δ34S values of the ores range from 13 to 17 per mil. The sulfur of the ores originated by in situ reduction of sulfate. Three kinds of gangue calcite from the ores have similar isotope compositions, which have δ13C values in the range of −2.1 to −3.5 per mil with respect to PDB and δ18O values in the range of 16.8 to 18.6 per mil with respect to SMOW. The δDFI values of fluid inclusions in the three kinds of gangue calcites have a narrow range of −50 to −60 per mil and the δ18OH2O values calculated from δ18O values of calcite range from 7.0 to 8.8 per mil at 200°C. These data suggest that the ore fluid was a basinal brine that passed through shale, clastic rocks and mudstone underlying the host rock. Initial 87Sr/86Sr values of the pyrite, sphalerite and calcite from the ores range from 0.714 to 0.717. The initial 87Sr/86Sr values of unaltered host rock (0.7083–0.7093) are lower than that of the altered host rock (0.7106). It suggests that the ore fluids have higher initial 87Sr/86Sr values than the wall rocks. These high initial 87Sr/86Sr values may be due to the reaction between the ore fluid and the shale, clastic rocks and mudstone underlying the host rock or the fluid might have originated from these rocks.

Albite vein formation during exhumation of high‐pressure terranes: a case study from alpine Corsica

AbstractThe formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.

URANIUM IN IRON OXIDE-COPPER-GOLD (IOCG) SYSTEMS

The Olympic Dam iron oxide-copper-gold (IOCG) deposit is currently the world's largest producer of Other IOCG deposits generally have anomalous, though uneconomic, uranium grades. The fluids that generate IOCG deposits are thought to be saline and highly oxidized and would be capable of leaching and transporting significant uranium. The reasons for uranium enrichment in certain IOCG deposits remain speculative but could include the composition of mineralization-related magmas, of the wall rocks altered by hydrothermal fluids or of the fluids mixing at the site of mineralization, There is sparse data available on uranium abundance in IOCG ores; there is even less reliable data on the uranium content of unaltered wall rocks for the deposits. However, the data that is available suggest that the uranium grade of IOCG deposits may be primarily related to the uranium content of their unaltered host rocks. Many IOCG deposits appear to have approximately 10 to 40 times enrichment in the ore compared to unaltered host rocks. This suggests that exploration for uranium-rich IOCG deposits should focus on areas with anomalously high uranium contents in host-rock sequences.

Magma-driven hydraulic fracturing and infiltration of fluids into the damaged host rock, an example from Dronning Maud Land, Antarctica

Excellent outcrops in Dronning Maud Land, Antarctica, provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust. The fluids are liberated from pegmatitic veins and give rise to alteration halos. In the alteration halos, the conspicuous change in colour is correlated with (1) hydration mineral reactions, and (2) high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude. The field relations indicate that the veins originated as melt-driven hydraulic fractures, sealed by pegmatite and aplite crystallising from volatile-rich melts, with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures. It is proposed that (1) the size of the alteration zone and the width of the vein are correlated, resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses; (2) the damage zone is characterised by a transient state of high permeability which was short-lived due to rapid healing and sealing of microcracks; (3) the infiltration and retrogression of the high-grade rocks can be considered as a quasi-instantaneous process on geologic time scales with a duration of hours to weeks.

Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

formation at a mid-ocean ridge. Cooling rates in the range of 1°C/ geochemistry; Bismarck Archipelago‐Papua New Guinea Ma were calculated from zoning of Ca in olivine using the Lasaga algorithm. Subsequent to this cooling, a strong metasomatism affected the mantle peridotites when metasomatic agents emerged from the underlying slab of a subduction zone, which was stalled about 15 my ago. This resulted in the formation of orthopyroxene-, INTRODUCTION clinopyroxene-, phlogopite- and hornblende-bearing veins crosscutting Evidence for metasomatic processes in the Earth’s mantle spinel peridotites and olivine clinopyroxenites, as well as pervasively is provided by trace element and isotope variations in metasomatized plagioclase lherzolites. The metasomatic xenoliths volcanic rocks (e.g. Hawkesworth et al., 1987), by metareveal strong chemical disequilibria between the metasomatic minerals somatized sections of ultramafic massifs (e.g. Bonatti et and the adjacent, unaltered host rock minerals, which are especially al., 1981) and by metasomatized mantle xenoliths of prominent in the veined samples. Temperatures during the meta- mainly peridotitic composition, which are transported to somatic overprint, estimated using spinel‐olivine thermometry, range the surface by mafic or ultramafic magmas (e.g. Jagoutz between 660 and 950°C. Oxygen barometry reveals an elevated et al., 1979). Two different styles of metasomatism are oxygen fugacity, with log(fo2) FMQ values of 0·4 to >4. A geo- recorded in these rocks. Some ultramafic rocks show chemical study of the ultramafic rocks shows that all types of xenoliths cryptic metasomatism, resulting in trace element enhave been metasomatized. Pervasively metasomatized plagioclase richment without any visible mineral or textural changes lherzolites and cumulate olivine clinopyroxenites have high contents (Dawson, 1984). The second type of metasomatism is of middle and heavy rare earth elements compared with veined characterized by petrographically well-recognizable reperidotites. Cryptic metasomatism, indicated by increased light rare placement textures and/or the presence of secondary, earth elements and Nd concentrations, results from LREE-rich hydrous minerals (e.g. phlogopite, hornblende) as well as secondary clinopyroxene, plagioclase, and accessory hydrous fluid circulation. The investigated peridotites underwent a