Metasomatic Zones Research Articles

Prograded low-temperature alteration zones, Forrestania, Western Australia

Zones of low-temperature metasomatism are widespread in igneous and sedimentary or metasedimentary terranes. Such zones are commonly associated with base metal and precious metal mineralization, and are readily identified by having relatively simple mineral assemblages, such as quartz-chlorite-sericite-ankerite associated with gold. However, if such zones are subjected to further metamorphism, the resulting assemblages may become more complex. This abstract discusses zones of alteration in the Forrestania greenstone belt in Western Australia, that have been further metamorphosed to amphibolite facies, with the development of complex amphibole assemblages, staurolite-bearing assemblages and calc-silicate assemblages.

Bimetasomatic zoning in the CaO-MgO-SiO2-H2O-CO2 system: Experiments with the use of natural rock samples

Experiments reproducing the development of bimetasomatic zoning in the CaO-MgO-SiO2-H2O-CO2 system were conducted at elevated P-T parameters with the use of samples of naturally occurring quartzdolomite and calcite-serpentinite rocks. In order to maintain mass transfer exclusively via the diffusion-controlled mechanism, we used the method of the ensured compaction of the cylindrical sample surface with a thin-walled gold tube. In the course of the experiments, a single diopside zone ∼2.5 × 10−5 m thick was obtained at the quartz-dolomite interface at T = 600°C, \(P_{H_2 O + CO_2 } \) = 200 MPa, and \(X_{CO_2 } \) = 0.5 for 25–40 days and a succession of metasomatic zones at T = 750°C, \(P_{H_2 O + CO_2 } \) = 300 MPa, and \(X_{CO_2 } \) = 0.4 for 48 days. The metasomatic zones were as follows (listed in order from quartz to dolomite): wollastonite ‖ diopside ‖ tremolite ‖ calcite + forsterite; with the average width of the diopside zone equal to ∼1.3 × 10−5 m and the analogous part of the wollastonite zone equal to ∼2.6 × 10−5 m. Two zones (listed in order from calcite to serpentine) diopside and diopside-forsterite (the average widths of these zones were ∼6 × 10−4 and ∼8 × 10−4 m, respectively) were determined to develop at contact between serpentine and calcite during experiments that lasted 124 days at T = 500°C, \(P_{H_2 O + CO_2 } \) = 200 MPa, and \(X_{CO_2 } \) = 0.2–0.4. In the former and latter situations, the growth rate of the zoning ranged between 3.1 × 10−12 and 1.2 × 10−11 m/s and between 5.6 × 10−11 and 7.5 × 10−11 m/s, respectively. The higher growth rate in the latter case can be explained by the higher water mole fraction in the fluid, with this water released during serpentinite decomposition in the experiments. The development of the only diopside zone in the experiments modeling the interaction of quartz and dolomite at T = 600–650°C and \(P_{H_2 O + CO_2 } \) = 200 MPa is in conflict with theoretical considerations underlain by the Korzhinskii-Fisher-Joesten model. The interaction of quartz and dolomite in the CaO-MgO-SiO2-CO2-H2O system at the P-T-\(X_{CO_2 } \) parameters specified above should be attended by the origin of a number of reaction zones consisting of various proportions of talc, forsterite, tremolite, diopside, and calcite. The saturation of the fluid with respect to these minerals was likely not reached, and this resulted in the degeneration of the respective stability fields in the succession of zones. Conceivably, this was related to the insufficient rates of quartz and dolomite dissolution and the relatively low diffusion rates of the dissolved species in the low-permeable medium. In the experiments with interacting calcite and serpentine, the zoning calcite ‖ diopside ‖ diopside + forsterite ‖ serpentine developed in its complete form, in agreement with the theory. Equilibrium was likely achieved in these experiments due to the higher diffusion coefficients.

Fractal features and morphological differences between periodic infiltration and diffusion metasomatic zonation

Fractal features and morphological differences between periodic infiltration and diffusion metasomatic zonation

Upper-mantle velocity structure of the lower Great Lakes region

The lithospheric root beneath North America contains a prominent indentation beneath the lower Great Lakes region that is approximately aligned with the track of the New England seamounts. By combining data from the recently installed POLARIS network in southern Ontario, Canada with data acquired in 1996 during the Abitibi–Grenville teleseismic experiment, we have performed a tomographic inversion using 4543 P-wave traveltimes from 213 events (5.0 ≤ m b ≤ 6.6), and 1860 S-wave traveltimes from 98 events (5.0 ≤ m b ≤ 6.6), to obtain high-resolution images of the upper mantle beneath the lower Great Lakes. Two salient features of the 3-D models are: 1) a patchy, NNW-trending low-velocity region, and 2) a linear, NE-striking high-velocity anomaly. S-wave images show that the low-velocity anomaly changes from an arcuate feature at 400-km depth, to a NW-striking linear feature at 100-km depth beneath the Neoproterozoic Ottawa–Bonnechere graben. The linear high-velocity anomaly extends to at least 300-km depth and strikes parallel to surface geological belts and the Laurentian continental margin. We interpret the high-velocity anomaly as a possible relict slab associated with ca. 1.35–1.3 Ga subduction beneath the Composite Arc Belt, whereas the low-velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the Late Cretaceous Monteregian plutons. Our data support an interpretation of these plutons as melts generated by the passage of North America across a mantle plume, rather than a far-field response to opening of the North Atlantic.

Roles of Melting and Metasomatism in the Formation of the Lithospheric Mantle beneath the Leizhou Peninsula, South China

This study characterizes the nature of fluid interaction and melting processes in the lithospheric mantle beneath the Yingfengling and Tianyang volcanoes, Leizhou Peninsula, South China, using in situ trace-element analyses of clinopyroxene, amphibole and garnet from a suite of mantle-derived xenoliths. Clinopyroxenes from discrete spinel lherzolites exhibit large compositional variations ranging from extremely light rare earth element (LREE)-depleted to LREE-enriched. Trace-element modelling for depleted samples indicates that the Leizhou lherzolites are the residues of a mantle peridotite source after extraction of 1–11% melt generated by incremental melting in the spinel lherzolite field with the degree of melting increasing upwards from about 60 km to 30 km. This process is consistent with gradational melting at different depths in an upwelling asthenospheric column that subsequently cooled to form the current lithospheric mantle in this region. The calculated melt production rate of this column could generate mafic crust 5–6 km thick, which would account for most of the present-day lower crust. The formation of the lithospheric column is inferred to be related to Mesozoic lithosphere thinning. Al-augite pyroxenites occur in composite xenoliths; these are geochemically similar to HIMU-type ocean island basalt. These pyroxenites postdate the lithospheric column formation and belong to two episodes of magmatism. Early magmatism ( forming metapyroxenites) is inferred to have occurred during the opening of the South China Sea Basin (32–15 Ma), whereas the most recent magmatic episode (producing pyroxenites with igneous microstructures) occurred shortly before the eruption of the host magmas (6–0 3 Ma). Trace-element traverses from the contacts of the Al-augite pyroxenite with the spinel peridotite wall-rock in composite xenoliths record gradients in the strength and nature of metasomatic effects away from the contact, showing that equilibrium was not attained. Significant enrichment in highly incompatible elements close to the contacts, with only slight enrichment in Sr, LREE and Nb away from the contact, is inferred to reflect the different diffusion rates of specific trace elements. The observed geochemical gradients in metasomatic zones show that Sr, La, Ce and Nb have the highest diffusion rates, other REE are intermediate, and Zr, Hf and Ti have the lowest diffusion rates. Lower diffusion rates observed for Nb, Zr, Hf and Ti compared with REE may cause high field strength element (HFSE) negative anomalies in metasomatized peridotites. Therefore, metasomatized lherzolites with HFSE negative anomalies do not necessarily require a carbonatitic metasomatizing agent.

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS

Mineral distribution in contact-metamorphosed siliceous dolomite at Ubehebe Peak, California, based on airborne imaging spectrometer data

Visible and near-infrared reflectance spectra of minerals in the CaO-MgO-SiO 2 -H 2 O-CO 2 system and airborne imaging spectrometer data with a spatial resolution of 3.6 m were used to map the distribution of dolomite, calcite, tremolite, serpentine, brucite, and related minerals in the Ubehebe Peak contact aureole. The results are entirely consistent with previously mapped isograds, but the high-spatial-resolution imagery offers a more accurate representation of the complex distribution of metamorphic minerals, and serves to highlight critical relationships between mineralogy and controlling factors such as stratigraphy, structure, and fluid infiltration. Rocks in the aureole are predominantly quartz-poor dolomite and limestone, and these are classified as pure dolomite or calcite, accordingly. Tremolite and serpentine (after forsterite) are largely restricted to stratigraphic units that contain siliceous dolomite, quartzite, or chert. In one area, however, tremolite and serpentine delineate a zone of retrograde hydration attributed to fluid flow along a fault. Wall rocks within 200 m of the contact and inliers of marble in the pluton contain brucite (after periclase), which documents sites of focused infiltration of H 2 O-rich fluid. In addition, the imagery reveals local metasomatic zones, marked by grossular-rich skarn and epidote alteration of the adjacent quartz monzonite, as well as large-scale patterns of bleaching, interpreted to define the maximum extent of fluid flow. The degree of bleaching and the intensity of plastic deformation are closely coupled; both effects increase abruptly at the forsterite isograd, which appears to represent a fundamental discontinuity with respect to chemical, physical, and mechanical processes in the aureole.

Pb‐Zn‐Ag‐bearing Manganoan Skarns of China

Abstract Manganoan skarns consist of special Mn (Ca, Mg, Fe, Al) silicate metasomatic minerals and are usually associated with Pb‐Zn(Ag) mineralization. They occur chiefly along the lithologic contacts or faults and fractures of carbonate wall rocks distal from the intrusive contact zone, and are combined with Fe, Cu, W, Sn and Cu‐bearing calcic or magnesian skarns occurring in the contact zones to constitute certain metasomatic zoning. Manganoan skarns are formed later than calcic or magnesian skarns. Their rock‐forming temperatures are lower than those of calcic or magnesian skarns. The mineral assemblages of manganoan skarns occurring in different carbonate rocks (limestone or dolomite) are notably different.

Unusual lithospheric structure and evolutionary pattern of the cratonic segments of the South Indian shield

The southern Indian shield, characterised by several prominent geological and geophysical features, can be divided into three distinct tectonic segments: Western Dharwar craton (WDC), Eastern Dharwar craton (EDC) and Southern Granulite terrain (SGT). With the exception of WDC, the entire crust beneath EDC and SGT has been remobilized several times since their formation during the mid- to late Archeans (3.0–2.5 Ga). In order to understand the evolutionary history of these segments, a multiparametric geological and geophysical study has been made which indicates that the south Indian shield, characterized by a reduced heat flow of 23–38 mW/m2 has a much thinner (88–163 km) lithosphere compared to ∼200–450 km found in other global shields. In the EDC-SGT terrain, high velocity upper crust is underlain by considerably low mantle velocity with a thick high conductive/low velocity zone sandwiched at mid crustal level. Our study reveals that the entire EDC region is underlain by granulite facies rocks with a density of about 2.85 to 3.16 g/cm3 at a shallow depth of about 8 km in the southern part and at even shallower depth of about 1 to 2 km below the Hyderabad granitic region in the north. Cratonic mantle lithosphere beneath EDC may contain a highly conductive, anisotropic and hydrous metasomatic zone between the depth of 90 and 105 km where estimated temperatures are in the range of 850–975°C. It is likely that before the early Proterozoic, the entire south Indian shield was a coherent crustal block which subsequently got segmented due to persistent plume-induced episodic thermal reactivations during the last 2.7 Ga. These reactivations led to self destruction of cratonic roots giving rise to negative buoyancy at deeper levels which may have been responsible for crustal remobilisations, followed by regional uplifting and erosion of once substantially thick greenstone belts. Consequently, the crustal column beneath the EDC has become highly evolved and now corresponds closely to SGT at depth.

Endogene-supergene systems of epithermal deposits and occurrences of acid-sulfate and adularia-sericite type (cases from Bulgaria) - hypothesis and models

The epithermal acid-sulfate and adularia-sericite deposits аrе among the main sources of raw mаterials as well as among the most interesting subject of study and genetic modelling and conclusions оn the mineral deposits. Both types have а broad оссurеnсе in Bulgaria, which allows аn extensive research оn them. Hereafter, discussing the main features of the epithermal mineralization а new hypothesis fоr аn integrated endogene-supergene system is proposed. This idea is based оn published evidences from worldwide as well as Bulgarian examples. Its соrе аrе the detailed mineralogical and petrological characteristics of the epithermal systems. It is suggested, that under some circumstances both endogene and supergene parts of the systems mау bе observed as аn integrity. This integrity is observed as unanimity between the nature and human mind. Among the main prerequisites fоr the origin of such systems аrе spatial compatibility; dependence between endogene and supergene mineralizations; presence of metasomatic rеplacement and indications fоr а metasomatic zonation; соmmоn structural-tectonic agents; geomorphological and climatic agents; etc. The proposed models fоr the epithermal mineralisations аrе based оn Bulgarian occurrences, including some elements from the models of Hedenquist, Lowenstern (1994), Hedenquist (1994), Fournier (1999) and Chavez (2000). These models have аn endogene and а supergene part with а respective metasomatic zonation. The proposed hypothesis is obviously debatable and as аnу оnе hypothesis contains facts and suggestions. However, it is also supported bу evidences from porphyritic systems, where а transition between hypogene and supergene mineralization was already suggested bу Sillitoe (1973). Some debatable points of the proposed hypothesis could bе the cases where а significant gap in time bеtween the primary and supergene mineralizations is established. However, in such cases usually as supergene processes аrе accepted to bе only the recent ones, but those acting simultaneously with the endogene оrе forming so far have bееn neglected. The recent geothermal and hydrothermal volcanic systems аrе the best proof of the simultaneous occurrence of both endogene and supergene processes. The integrity of both processes is already recognized with the admission of the mixed character of the fluids, the convection and the participation of meteoric waters as well as atmospheric oxygen together with magamatic products into the forming of the endogene epithermal deposits.

Constraints on the petrologic structure of the subduction zone slab-mantle interface from Franciscan Complex exotic ultramafic blocks

Ultramafic blocks within mud-matrix melange of the Franciscan Complex, California, preserve a series of metasomatic mineral zones generated by infiltration of Si-rich hydrous fluids during subduction. We describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids. Fluid flow affected a Cr-spinel lherzolite protolith to form first serpentinite, then a talc-dominated rock, and finally an amphibole-rich assemblage. A diverse suite of accessory minerals in the amphibole-rich zone (titanite + clinozoisite + zircon + apatite) suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. Oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450–500 °C. Whole-rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism, including elements such as Ti which are currently considered to have low solubilities in such fluids. Taken as a whole, the blocks appear to preserve the metasomatic structure of the slab-mantle interface in subduction zones and imply that the chemistry of slab-derived fluids is modified as they pass through these metasomatic zones in the mantle wedge. Our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas. Instead, we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe.

瀬戸内領家帯小大下島に産するエメリー

Limestone is accompanied with widely distributed Ryoke metamorphic rocks in islands of the Seto Inland Sea. Ko-Oge Island is situated in this zone and mainly composed of pelitic hornfels and crystalline schist rarely interbedded with dark color and massive Al-Fe rich sediments, so called emery.Emery from Ko-Oge Island is charactarized by high TiO2 contents and by relict of primary texture such as pisolitic one. Emery can be divided, according to the mineral assemblage and texture of rocks: 1. zone of emery, 2. metasomatic zone of emery, 3. metasomatic zone of emery with limestone, and 4. metamorphic zone of another emery. The main constituent minerals of Zone1 are corundum, hercynite, ilmenite, and plagioclase, which occur as granoblastic aggregates. Perovskite, grossular, and vesuvianite occur in Zone 3. It was concluded that emery of Zone 3 is formed by Ca metasomatism in later stage of the prograde and retrogrede stages of metamorphism. Original rock of emery is probably sediment such as laterite, rich in the TiO2, Al2O3, and Fe oxide. The rock is composed of gibbsite, Fe-hydrooxide, anatase, kaolinite, and calcite.

High-temperature metamorphism in marbles as a consequence of volatile release from crystallizing anatectic melts, Naxos, Greece

Two generations of high-temperature silicate mineral assemblage occur in vesuvianite- and amphibolite-bearing calcitemarbles in the leucogneiss core of the Miocene M2 metamorphic complex of Naxos, Greece. The first ( 1p ) generation formed during prograde metamorphism, whereas the second retrograde ( 2r ) generation developed along contacts between prograde amphibolites and calcite marbles, in veins that crosscut pure marbles, and at the contacts between pegmatite-aplite bodies and calcite marble, and clearly grew during the infiltration of an external fluid into the marbles. Phase equilibrium calculations indicate a water-rich fluid. The oxygen isotope composition of 2r silicate minerals shows a significant decrease relative to the 1p composition in prograde marbles. The calculated δ 18 O values of water in isotopic equilibrium with the 2r silicate minerals (δ 18 O garnet = 9.0 to 9.6 ‰; δ 18 O clinopyroxene = 10.3 to 11.0 ‰; δ 18 O hornblende = 10.8 to 11.2 ‰) match those of water in equilibrium with the minerals of the pegmatite-aplite bodies, which formed during late-stage crystallization of peak-metamorphic anatectic melts. The volatiles exsolved during the pegmatite-aplite crystallization are thus indicated to be the source of the infiltrating fluid. Recrystallized calcite marble adjacent to 2r silicate minerals shows isotopic evidence for limited water-rock interaction and indicates that fluid infiltration was strongly localized in the metasomatic zones. The study confirms that high-temperature metamorphism occurs as the result of volatile release during the crystallization of anatectic partial melts and that peak metamorphism in such terrains is a continuum spanning both prograde heating and retrograde cooling.

Unusual lithospheric structure beneath the Hyderabad granitic region, eastern Dharwar craton, south India

Using multiparametric geological and geophysical data, the evolutionary history of the lithosphere beneath the Late Archean—early Proterozoic Hyderabad granitic region (HGR) of the eastern Dharwar craton of south Indian shield has been attempted. Our study reveals that the entire granitic region and the surroundings (∼200 km×200 km) is being neotectonically uplifted possibly due to a major intrusive body situated at sub-crustal depth, leading to changes in river courses and also erosion of several kilometers (∼10 km) from its upper crustal column. Beneath this region, the Moho is elevated (32–33 km), the reduced heat flow is of the order of 28 mW/m 2 and the asthenosphere is located at shallow depths of about 124 km, i.e. far less than 200–400 km depth found in Precambrian shield areas of the globe. The cratonic mantle lithosphere beneath HGR contains a highly conductive (15 Ωm) hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km, where the estimated temperatures could be in the range of 900–1000 °C. The surface granitic layer, containing unusually high radioactivity (5.25 μW/m 3), appears very thin probably a kilometer or so, beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies (?) rocks of acid to intermediate composition. Persistent episodic thermal reactivations during the last 2.6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual.

MINERALOGY, AGE, AND FLUID GEOCHEMISTRY OF THE RILA EMERALD DEPOSIT, BULGARIA

Gem-quality emerald is found within granitic pegmatites cutting talc schists in the western Rhodopa massif in southwestern Bulgaria. The 40 Ar/ 39 Ar data for phlogopite from emerald-bearing phlogopite schist yield a plateau age of 34.2 ± 0.4 Ma, which indicates the emplacement of the pegmatite during late Alpine orogenesis. Emerald crystals form within the metasomatic zone near the contacts between altered pegmatite and talc schists. Oxygen and hydrogen isotope analyses of emerald ( δ l8 O = 9.4‰, δ D = -29.8 to -32.7‰), combined with fluid inclusion study, indicate that a single stage of a homogeneous fluid (in the system H 2 O-CO 2 -NaCl) of metamorphic origin produced the pegmatite-talc schists hydrothermal alteration at temperatures between 300° and 400°C.

Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (NMg=38–42) + biotite (NMg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and aflH2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (NMg=45–48) + K-feldspar (Or70–80) + quartz + plagioclase (An28) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns:

The Palma Volcano-Sedimentary Supersuite, Precambrian Sul-Riograndense Shield, Brazil

Komatiites are generally found in Archean greenstone belts, but have been reported in some Mesozoic volcano-sedimentary sequences. The Palma Volcano-Sedimentary Supersuite (PVSS) is a Neoproterozoic sequence composed of meta-ultramafic volcanic rocks, mafic-to-felsic metavolcanic rocks, and metasedimentary rocks ranging from quartzites to chlorite-mica-quartz schists, marbles, and marls. Original PVSS structures were overprinted by two penetrative deformational surfaces (So//Sn and Sn+1); associated metamorphic assemblages are assigned to the upper greenschist facies. Basalts predominate over andesites and rhyodacites, and their major, minor, trace, and rare-earth elements display compatible geochemical trends characteristic of calc-alkaline magmas. Meta-ultramafic rocks are variable-thickness flows in the metasedimentary sequence. Metamorphism caused the development of a metasomatic tremolite zone around the serpentinite bodies, as well as chlorite and talc schist metasomatic zones. Evaluation of the geochemical mobility of the elements showed that the ratios between major and minor elements have not changed in the serpentinites. It was then possible to investigate the lithogeochemical nature of the serpentinites by studying the minor, trace, and rare-earth elements. The lithogeochemical investigation and the structures of the serpentinite bodies enable them to be regarded as komatiitic flows extruded under shallow water. These komatiite flows and calc-alkaline basalt to rhyodacite flows associated with shallow-water sediments suggest that this volcano-sedimentary sequence was developed in a Neoproterozoic arc-related tectonic setting.

The NICO and Sue-Dianne Proterozoic, Iron Oxide-hosted, Polymetallic Deposits, Northwest Territories: Application of the Olympic Dam Model in Exploration

The NICO and Sue-Dianne deposits are being explored and delineated by Fortune Minerals Limited in the Mazenod Lake District of the Northwest Territories, using an Olympic Dam model. The deposits were discovered in the south part of the Great Bear magmatic zone (GBMZ) within the Proterozoic, Bear Structural Province of the Canadian Shield. They are the only known significant Canadian examples of Proterozoic iron oxide-hosted polymetallic deposits. Worldwide, this class includes such deposits as Olympic Dam and Ernest Henry in Australia, Kiruna-Aitik in Sweden, and Salobo in Brazil. Their considerable size, up to 2 billion tonnes, and polymetallic ore assemblages make them highly attractive targets for exploration. Common characteristics of this class include their Early to Middle Proterozoic cratonic settings with extensional rifting evolving from collisional tectonics. Deposits typically occur along major structural lineaments within the aureoles of a distinctive suite of anorogenic potassium-rich “A-type” granite intrusions. Although hosted by diverse lithologies, deposits of this class are characterized by a number of other diagnostic regional- and deposit-scale features, which may be recognized in reconnaissance- and property-scale geological and geophysical surveys. The southern GBMZ in Canada has several characteristics (age, tectonic setting, geology and geophysical attributes) similar to those of the Olympic Dam deposit and its other significant global analogues. Airborne and ground geophysical surveys carried out in the GBMZ identified coincident potassium, uranium, magnetic, resistivity, chargeability, and gravity anomalies centered over the NICO deposit. The nearby Sue-Dianne deposit is characterized by coincident uranium, potassium, magnetic, resistivity and chargeability anomalies. Both deposits occur within a regional, northwest-striking, arcuate trend of volcanic and sedimentary rocks characterized by significant positive Bouguer-gravity and magnetic responses and are believed to represent a major basement discontinuity. The NICO anomalies are at the intersection of this regional trend with a major transverse fault through Lou Lake. Regional and local geophysical data indicate the presence of significant concentrations of iron oxide within a broad area of intense potassium metasomatism. Geological mapping identified cobalt, gold, bismuth, and copper mineralization in biotite-magnetite-amphibole-sulfide-rich ironstone and schist. This mineralization is localized within altered wackes of the Snare Group, which are unconformably overlain by potassium feldspar- (±hematite ±magnetite) altered rhyolite of the Faber Group. The Sue-Dianne deposit is a hematite-magnetite-Fe-silicate-cemented diatreme complex enriched in copper, silver, gold, and uranium within a broad zone of potassium, iron, quartz, and epidote metasomatism. The diatreme is located at the intersection of two major faults at the north end of the basement discontinuity and is hosted in rhyodacite ignimbrite marginal to the Faber Lake rapakivi-granite pluton. At both deposits, diatreme- and maar-facies iron oxide-cemented breccia straddle the regional metasedimentary-volcanic unconformity, suggesting that mineralization formed in a near-surface environment synchronous with the onset of volcanism.

Dynamic Model of the Infiltration Metasomatic Zonation

—A new dynamic model of infiltration metasomatic zonation is discussed. The model describes the formation and evolution of metasomatic zoning patterns in terms of travelling fronts and waves. Travelling fronts cause the formation of well known vectoral zonation. All replacement reactions in this zonation are localized immediately around fronts (zone boundaries), and inside individual zones the chemical equilibrium is attained. Such a condition corresponds to the “local equilibrium model” and no regular oscillations occur. Travelling waves can stimulate stable spatial chemical oscillations and appearance of the dissipative structure in the solution flow, if redox reactions such as Fe2+?Fe3+ + e − in solution occur. Corresponding to the evolution of the dissipative structure, the rhythmical banding in the metasomatic rocks is formed. Thus the proposed model describes the single approach as vectoral as periodic (rhythmically banded) zoning patterns.

Partitioning of Mg, Ca, and Na between carbonatite melt and hydrous fluid at 0.1–0.2 GPa

Experimental studies of the element distribution between carbonatite melts and hydrous fluids are hampered by the fact that neither the fluid nor the melt can be isochemically quenched in conventional high-pressure vessels. In order to overcome this problem, we used a double-capsule technique to separate immiscible fluid and melt phases during and after the runs. The inner platinum capsules were charged with carbonate mixtures (CaCO3, MgCO3 and Na2CO3) and placed inside the outer capsules charged with distilled water and diamond powder. The latter was used as an inert trap for solids precipitating from the fluid on quenching. Carbonate melt and hydrous fluid equilibrated through a small hole left in the upper end of the inner capsule. The runs were performed in rapid-quench cold-seal pressure vessels at 0.1–0.2 GPa and 700–900 °C in the two-phase (fluid + melt) stability region. Both quenched melt and quenched fluid were dissolved in dilute HCl and analysed by inductively coupled plasma atomic emission spectroscopy. The results show that under all conditions investigated, fluid/melt partition coefficients for Ca and Mg are similar and several times smaller than those for Na. At 0.1 GPa and a water/carbonatite ratio of 1 (by weight), the partition coefficients are DNa = 0.35 ± 0.02, DCa = 0.09 ± 0.02, and DMg = 0.13 ± 0.01. Between 700 and 900 °C, the effect of temperature on partitioning is negligible. However, DNa increases significantly with decreasing water/carbonatite ratio in the system. Our data show that the release of a hydrous fluid enriched in sodium and simultaneous crystallisation of calcite can transform an alkaline, vapour-saturated carbonatite melt into a body of pure calcite surrounded by zones of sodium metasomatism. Thus, it is quite possible that carbonate magmas with substantial amounts of alkalies were common parental liquids of plutonic carbonatites.