Metamorphic Biotite Research Articles

New experimental data on biotite + magnetite + sanidine saturated phonolitic melts and application to the estimation of magmatic water fugacity

New experimental data are presented that allows the biotite–magnetite–sanidine equilibrium to be used for estimating water fugacity (ƒH2O) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (aannite). Crystallization experiments were carried out on a representative sample of peralkaline, phonolitic obsidian of Montana Blanca (MB) pumice deposit, Tenerife, Canary Islands. Experiments were performed from 720–810 °C and 50–250 MPa. Redox conditions were varied between NNO (nickel + nickel oxide) + 1 (±0.2) and FMQ (fayalite + magnetite + quartz). The majority of the experiments were done under H2O saturation conditions (Pwater = Ptotal). Several experiments were done using a mixed H2O-CO2 s uid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive ƒH2O of the Montana Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive ƒH2O for the Fish Canyon tuff (753–2978 bars) and Bishop tuff rhyolite (1065–2440 bars) were also calculated, as well as ƒH2O for metamorphic biotite from Au Sable Forks (≈ 130 bars). The results of this study suggest that this geohygrometer can be used in any magmatic system in which biotite–magnetite–sanidine is a stable assemblage.

Monazite U–Pb dating and 40Ar– 39Ar thermochronology of metamorphic events in the Central African Copperbelt during the Pan-African Lufilian Orogeny

New SHRIMP U–Pb age data on metamorphic monazite, as well as step-heating 40Ar– 39Ar ages on metamorphic biotite, muscovite and microcline, from Katangan metasedimentary rocks of the Central African Copperbelt are presented. These rocks were deformed and metamorphosed during the Pan-African Lufilian Orogeny. Three samples of metamorphic monazite from the Chambishi structural basin give ages of 592 ± 22 Ma, 531 ± 12 Ma and 512 ± 17 Ma, which correspond respectively to the ages of eclogite facies metamorphism, high-pressure talc-kyanite whiteschist metamorphism, and of a regional metamorphism/mineralisation pulse elsewhere within the Lufilian Orogen. A biotite population from Luanshya gives a 40Ar/ 39Ar plateau age of 586.1 ± 1.7 Ma, coinciding with the oldest monazite age. Several samples from the Chambishi basin and the Konkola area give 40Ar/ 39Ar biotite plateau ages in the range of 496.6 ± 0.6–485.2 ± 0.9 Ma, and a muscovite plateau age of 483.6 ± 1.1 Ma, which are a manifestation of regional uplift and cooling that affected the whole Katangan basin. The youngest apparent 40Ar/ 39Ar ages obtained are from microcline at Musoshi and range from 467.0 ± 2.7 Ma to 405.8 ± 3.8 Ma, reflecting continued slow cooling of the Lufilian Orogen.

Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from 40Ar/ 39Ar geochronology

New 40Ar/ 39Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at ∼42 Ma to ∼10 kbar and ∼400 °C at ∼39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at ∼35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.

Frictional melt pulses during a ∼1.1 Ma earthquake along the Alpine Fault, New Zealand

Our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine-grain size of frictionally fused rock, and the lack of detailed high-resolution microscopy studies that describe features at the crystal lattice scale. In such a study, we here report the complex nature of melting and crystallization in a symmetrically layered, pseudotachylyte vein from the Alpine Fault, New Zealand. Two melt pulses are recognized, attributed to successive, but rapid, injections of frictionally generated material. The initial injection, preserved at the vein margins, was proximally derived and contains a high concentration of clasts and a Si-rich glass. This was quickly followed by a second generation of a more distally derived, melt-dominated fraction, which was injected into the weak vein center. Whereas fragments of wall-rock biotite are preserved in the marginal zones, neocrystallized microlitic biotite characterizes both margins and center. The vein biotite is different in composition, microstructure and polytypism from the metamorphic biotite of the wall rock. In all melt layers, newly formed biotite shows notable signs of syn-flow crystallization, strain features and erosion at crystal–glass contacts, with breakdown of neocrystallized microlites along both crystal edges and faces. These characteristics imply cyclic pulses of heating, melting and crystallization occurred during a single, large earthquake episode, and probably reflects the stick-slip propagation properties of coseismic faulting. 40Ar/ 39Ar total gas ages from the vein center give a 1.11±0.04 Ma date for cyclic melting which, based on current exhumation rates, occurred at a crustal of depth ∼11 km.

Relationships between contact metamorphism and magnetite formation and destruction in a plutonʼs aureole, White-Inyo Range, eastern California

Magnetic-petrologic studies on metasedimentary rocks from the low-pressure–high-temperature contact-metamorphic aureole of the Joshua Flat–Beer Creek pluton at the southern rim of the Inyo batholith, California, reveal the effect of contact metamorphism on magnetic minerals. The regional metamorphic biotite zone and three different contact-metamorphic zones (andalusite- cordierite, transition, and cordierite–K- feldspar) in the aureole are identified by silicate petrology and their rock magnetic behavior. Phengite barometry yields pressure data in the range of 0.2–0.3 GPa, and Na-in-cordierite thermometry indicates a peak temperature of ∼730 °C for the highest-temperature contact-metamorphic zone. Compositional changes of iron-magnesium silicates and iron oxides correspond to changes in magnetic susceptibility, temper ature dependence of magnetic susceptibility, and AF (alternating field) demagnetization of NRM (natural remanent magnetization) and can be correlated with the metamorphic history of the aureole. Magnetic susceptibility ranges from 0.2 × 10 –3 to 0.5 × 10 –3 SI in the biotite zone and is highly variable in the andalusite-cordierite and transition zones, for which the values range from 0.35 × 10 –3 to 28 × 10 –3 SI. In prograde metamorphism, the assemblage ilmenite + rutile ± hematite transforms to magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograd. In the cordierite–K-feldspar zone and in samples having undergone retrograde metamorphism, prograde magnetite reacts to ilmenite and hematite. This breakdown is associated with decreased magnetic susceptibility (0.2–0.8 × 10 –3 SI) and blurs the magnetite isograd that formed during prograde metamorphism in the outer contact aureole. Retrograde metamorphism in the contact aureole is defined by chlorite composition [ T : 280 and 420 °C, log f (O 2 ):−35 and−28]. Contact-metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite- series plutons. This fact has to be considered when interpreting aeromagnetic data of granitoid rocks.

Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite

Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange thermometry and average pressure calculations constrain the conditions of the overprinting metamorphism to about 660 °C at 6 kbar. Biotite compositions reflect chemical equilibrium in local domains defined by partly to completely pseudomorphed cordierite grains. They show a continuous range of Ti content from 0.0 to 4.5 wt% TiO 2 depending on the proximity of the domain to a source of Ti such as ilmenite. Thus, they allow an assessment of the Ti substitution that is independent of metamorphic grade variation. Significant correlations between Ti and microprobe analysis total indicate that Ti is accommodated largely through a deprotonation substitution Ti 4 + + 2O 2 - + H 2 → M 2 + + 2OH - (where M 2 + is a divalent octahedral cation) rather than by substitutions involving octahedral vacancies. Ti shows a positive correlation with K, and strong negative correlations with octahedral Al and the sum of divalent cations. Ti content also varies with molar Mg/(Mg + Fe) in biotite (X M g ), because the process of cordierite replacement involves the mass transfer of Fe into cordierite domains. However, the above trends are also shown by a set of analyses with uniform X M g , In detail, the correlations depend on the method of formula recalculation. If the deprotonation substitution is dominant, normalization should not be made to a constant number of oxygen atoms. A statistical treatment of biotite analyses normalized to 14 tetrahedral and octahedral cations per formula unit suggests that the introduction of Ti involves deprotonation of two OH groups per Ti, and that the replacement of some octahedral Al by Ti is balanced by increases in K and/or Si in other sites. An appropriate Ti-biotite end-member may be K 2 [Mg 4 Ti 2 ]Al 2 Si 6 O 2 4 , rather than the vacancy-bearing end-members proposed in other studies. In view of the fact that even Ti-free biotite appears to contain a small proportion of octahedral vacancies, a suitable recalculation scheme for microprobe analyses may involve normalizing to 22 + Ti O atoms (neglecting H 2 O).

Anorogenic alkaline granites from northeastern Brazil: major, trace, and rare earth elements in magmatic and metamorphic biotite and Na-mafic minerals

The anorogenic, alkaline silica-oversaturated Serra do Meio suite is located within the Riacho do Pontal fold belt, northeast Brazil. This suite, assumed to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional event. Preserved late-magmatic to subsolidus amphiboles belong to the riebeckite–arfvedsonite and riebeckite–winchite solid solutions. Riebeckite–winchite is frequently rimmed by Ti–aegirine. Ti-aegirine cores are strongly enriched in Nb, Y, Hf, and REE, which significantly decrease in concentrations towards the rims. REE patterns of Ti-aegirine are strikingly similar to Ti-pyroxenes from the Ilı́maussaq peralkaline intrusion. Recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe+Mg) ratios. Recrystallised amphibole is pure riebeckite. Magmatic Ti–Na-bearing pyroxene was converted to low-Ti aegirine+titanite±astrophyllite/aenigmatite. The reaction riebeckite+quartz→aegirine+magnetite+quartz+fluid is also observed. Biotite and Na-mafic minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 600°C between the NiNiO and HM buffers.

Radiometric evidence for Middle Devonian inversion of the Hill End Trough, northeast Lachlan Fold Belt

The publication of a new geological time‐scale by the Australian Geological Survey Organisation and radiometric dates from the Hill End goldfield have prompted the re‐examination of the timing of deformation of the Hill End Trough to determine whether it occurred in Middle Devonian or Early Carboniferous time. Palaeontological evidence from the western trough margin and the Capertee High dates the end of deposition in the trough as late Emsian or early Eifelian (385–382 Ma). After a mid‐Devonian hiatus of at least 15 million years, paralic sedimentation commenced on the Molong and Capertee Highs in late Frasnian or early Famennian time (367–363 Ma). No Upper Devonian sedimentary formations occur in the Hill End Trough. Structural relationships indicate that the oldest mineral veins at Hill End preceded cleavage formation in the deformed trough sedimentary rocks. Early vein muscovites have Middle Devonian 40Ar/39Ar dates of 380–370 Ma. Regional metamorphic biotites from Hill End have well constrained 40Ar/39Ar closing ages of 360–358 Ma (mid‐Famennian). The metamorphic (thermal) maximum which outlasted penetrative deformation, is estimated here by modelling to have been about 370 Ma (latest Givetian). This clearly places the earlier main deformation in the Middle Devonian. Deformation probably began by terminating trough deposition in latest Emsian to early Eifelian time and ended in early Givetian time at about 375 Ma ago. Published pressure and temperature data from the Hill End goldfield suggest that deformation thickened the 6 km sediment column to around 11 km. The thermal model suggests there was post‐deformation erosion of about 4 km and little if any further erosion occurred during Late Devonian to Early Carboniferous time. The shortening accompanying the inversion of the northern Hill End Trough may have been taken up in the region to the south, both east and west of the Copperhannia Thrust, and east of the southern termination of the Capertee High. The general conclusions are not affected by a new Devonian time‐scale discussed in Appendix 2.

四国西部肱川地域の秩父帯北帯における含黒雲母塩基性準片岩

Metamorphic biotites have been found from the pumpellyite-actinolite facies basic semischists in the northern Chichibu Belt, western Shikoku, Japan. Representative mineral assemblages of the northern Chichibu basic semischists in the study area are chlorite+phengite+calcite, riebeckite+phengite+chlorite+calcite, riebeckite+stilpnomerane+chlorite+calcite, biotite+riebeckite+chlorite+calcite and biotite+riebeckite+phengite+chlorite+calcite. Albite, titanite, hematite and quartz are common. The biotites occur in high-MgO basic semischists and are rich in phlogopite content. The mineral parageneses systematically change depending on MgO/(MgO+FeO) of bulk compositions. The bioitite-calcite-bearing basic semischists were metasomatically altered based on petrographical and geochemical data. The isocon diagram suggests that CaO and MgO were added in significant amounts and Na2O, K2O and P2O5 were removed. It seems that the increase in MgO due to metasomatism is a cause of the formation of biotite under the pumpellyite-actinolite facies.

Quantitative determination of the oxidation state of iron in biotite using X-ray photoelectron spectroscopy: II. In situ analyses

X-ray photoelectron spectroscopy (XPS) was used to determine Fe(III)/ΣFe in individual biotite crystals in thin sections of ten metapelites and one syenite. The in situ XPS analyses of Fe(III)/∑Fe in biotite crystals in the metapelites were compared with published Fe(III)/IFe values determined by Mössbauer spectroscopy (MS) for mineral separates from the same hand samples. The difference between Fe(HI)/ΣFe by the two techniques was greatest for samples with the lowest Fe(III)/ΣFe (by MS). For eight metamorphic biotites with Fe(III)/ΣFe = 9–27% comparison of the two techniques yielded a linear correlation of r = 0.94 and a statistically acceptable fit of [Fe(III)/ΣFe] xps = [Fe(IIl)/ ΣFe] ms. The difference between Fe(III)/1Fe by the two techniques was greater for two samples with Fe(III)/ΣFe ≤ 6% (by MS). For biotite in the syenite sample, Fe(III)/ΣFe determined by both in situ XPS and bulk wet chemistry/electron probe microanalysis were similar. This contribution demonstrates that XPS can be used to analyze bulk Fe(III)/ΣFe in minerals in thin sections when appropriate precautions are taken to avoid oxidation of the near-surface during preparation of samples.

Determination of high spatial resolution argon isotope variations in metamorphic biotites

The circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood. Textures and chemistry cannot always give direct information on the conditions attained in the successive events. We show in this study that 40 Ar 39 Ar laser studies can, in contrast, provide thermal history determinations and successfully discriminate between geological models. Two samples have been examined from the Seconda Zona Diorito Kinzigitica (IIDK) of the Sesia Zone, Italian Alps. A unit which preserves relict Hercynian metamorphic assemblages and is now located structurally above rocks which preserve Alpine eclogite and greenschist facies assemblages. The circumstances of this preservation remain controversial, with proposed models falling into two main categories. One hypothesis advocates that the IIDK was juxtaposed into its present position late in the Alpine orogeny and experienced no significant heating event. Alternatively the IIDK has been in its present position for the duration of the Alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics, it failed to reequilibrate and develop Alpine metamorphic minerals. A high spatial resolution 40 Ar 39 Ar study using the ultraviolet laser ablation microprobe (UVLAMP) technique was undertaken with the aims of reassessing the metamorphic history of the IIDK and investigating the behavior of argon within the unit during its thermal evolution. 128 analyses have been obtained from ten profiles, between 60 and 275 μm long, across biotite grains which show apparent ages ranging from 514 to 161 Ma within individual crystals. These apparent age variations are systematic according to location in the grain, with the oldest ages at the grain edges and the youngest in the cores. These apparent age variations can not be explained by chemical alteration and require diffusion of excess argon into the grains. The age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon. Time integrated diffusion values obtained for the profiles fall between 2 × 10 −6 cm −6 and 5 × 10 -6 cm 2; this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network. Diffusion, and hence profile development, initiated at different times within the biotites, consequently producing the range of time integrated diffusion values. The temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories. Using the assumptions of either a short thermal pulse event or continuous cooling from high temperatures, the models suggest that the profiles developed at temperatures less than 300°C. Possible geological models are proposed which document the thermal and excess argon history of the IIDK from the Hercynian through to the Alpine orogeny. The study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages.

Timing of mineralization and source of fluids in a slate-belt auriferous vein system, Hill End goldfield, NSW, Australia: Evidence from [formula omitted] dating and O- and H-isotopes

40Ar 39Ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the Hill End goldfield, N.S.W., Australia, has distinguished four major geological events, including the timing of gold mineralization. The earliest hydrothermal event occurred during the Middle Devonian Tabberabberan Orogeny (370–380 Ma) and resulted in the formation of quartz veins barren of Au. A second and major episode of vein emplacement occurred in the Early Carboniferous during the principal phase of metamorphism and deformation at 359–363 Ma. This was followed by Au accumulation in two stages: (1) after the major phases of quartz deposition, and (2) during and after the development of conspicuous internal vein laminations (~ 357 Ma and ~ 343 Ma, respectively). Two sources of fluid are proposed for vein and ore formation. The first is a local metamorphic fluid characterized by δ 18O H 2O values of 8.9 to 12.5 per mil and δD H 2O values of −87 to −90 per mil. The second is a mixed ore fluid with δ 18O and δD values in the range of δ 18O H 2O 8.4 to 11 per mil and δD H 2O of −49 to −36 per mil. Progressive entry of this second fluid, sourced from trough-fill or deeper crustal rocks, is linked closely to cycles of gold precipitation at Hill End.

Pre-ore hydrothermal alteration in an unconformity-type uranium deposit

Hydrothermal alteration associated with the formation of unconformity-type uranium deposits is manifested mainly by intense chloritic replacement of metamorphic mineral assemblages. At the Koongarra deposit (northern Australia), three stages of pre-ore alteration were identified in chloritized metamorphic muscovite using optical microscopy, the electron microprobe, and transmission and analytical electron microscopy (TEM and AEM). During the first stage metamorphic muscovite was altered to a mica with very low interlayer occupancy (K content as low as 0.35 compared to 1 for ideal muscovite), high octahedral cation content (up to 2.3 whereas ideal muscovite has 2) and high Si content. High resolution TEM images show that these changes in composition were accompanied by frequent layer terminations in contrast to the metamorphic muscovite, whereas selected area electron diffraction patterns indicate that the altered muscovite remained a two-layer polytype. The alteration of muscovite appears more extensive in the outer alteration halo and has been almost completely eradicated in the ore zone where chloritization is more complete. During the second stage of the pre-ore alteration the altered muscovite was replaced by Mg-rich serpentine (lizardite) and clinochlore. High resolution TEM images show two 10 A mica layers coalescing to one 14 A chlorite layer. This type of coalescence appears to involve transformation of one dioctahedral layer of the mica structure to one trioctahedral brucite layer in the chlorite structure through dissolution of the tetrahedral sheets and cation influx in the octahedral site. The final stage of the pre-ore alteration resulted in replacement of lizardite and clinochlore by Fe-rich chlorite. Comparison of the alteration observed in metamorphic muscovite with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization of these phases was also a pre-ore alteration event. Pre-ore alteration was principally characterized by the evolution of chlorite from a Mg-rich phase to an Ferich one.

Nappe tectonics, extension, and metamorphic evolution in the Indian Tethys Himalaya (Higher HImalaya, SE Zanskar and NW Lahul)

In the southern part of the Indian Tethys Himalaya between SE Zanskar and NW Lahul, nappe tectonics with transport toward the WSW during the main deformation phase has been ascertained. If the stratigraphy of the different units considered as nappes is combined, it results in a composite section ranging from lower Cambrian to Middle Cretaceous. A metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking. The Lower Miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the Oligocene, or at the Oligocene‐Eocene boundary. After the compressional phases, an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault. This interpretation is based on detailed mapping.

40Ar- 39Ar laser probe multi-dating inside single biotites of a Variscan orthogeneiss (Pinet, Massif Central, France)

We have developed a laser-probe technique in which we use the pulsed laser beam focused through a microscope lens on a polished slab of rock. This technique is especially suitable for the study of rocks that suffered a polyphase geological history resulting in a complex behaviour of Ar. Both the 40Ar- 39Ar step-heating and laser-probe in situ dating methods have been applied to sheared and unsheared Variscan granites in order to study the distribution of Ar between old magmatic and younger metamorphic biotites. The undeformed granite, with a well-preserved magmatic assemblages shows good agreement between laser-probe and step-heating plateau ages on biotites at 351 and 346 Ma, respectively. The sheared sample contains a more inhomogeneous population of biotites resulting in a disturbed age spectrum. Laser-probe ages do not document this inhomogenous distribution of Ar isotopes whatever the kind of analyzed biotites (magmatic and recrystallize biotites). This laser-probe technique may be considered as a very promising tool for the study of minerals that occupy different textural sites in a rock sample. The comparison of data inside each crystal and between the different grains is possible. Moreover, a plot of the data in an isotope correlation diagram demonstrates the homogeneity or heterogeneity of Ar composition in each analyzed single grain. In situ dating of minerals by the laser-probe techniques is likely to improve our knowledge of the behaviour of Ar in complex geological rock samples.

Structural geology and vein mineralisation in the Callery River headwaters, Southern Alps, New Zealand

Abstract Three phases of folding have been recognised in the greywacke-Alpine schist transition in the Torlesse terrane rocks of the Callery River headwaters, Westland. The first phase was synmetamorphic and isoclinal (F1). This deformation resulted in variable textural reconstitution of the rocks (textural zones 2 and 3). Subsequent deformation has disrupted the textural zone boundaries, so that the boundaries have been folded almost isoclinally (F2) and refolded by an upright antiformal structure (F3) correlated to Alpine Fault related structures. All these structures are crosscut by several generations of veins. Three late-stage vein types are recognised on the basis of mineralogy and fluid inclusion studies. Type 1 veins consist of calcite and quartz, and were formed by H2O-CO2 fluids (about 12 mole % CO2) at >270°C and >2–3 km depth. Type 2 veins consist of quartz and calcite with minor biotite and sulphides, and traces of scheelite and gold. These minerals were deposited where hot (>320°C) brines mixed with cooler, less saline waters. Metal deposition involved interaction with pelitic schist. Type 3 veins consist of quartz, calcite, and adularia deposited from low-salinity water which may have boiled at about 240°C. High regional uplift rates and shallow formation of veins indicate a maximum age of vein mineralisation of about 1.7 million years, and a minimum age for Type 3 veins of about 117 000 years. Mineralisation is believed to have resulted from passage of fluids, at least some of which are metamorphic in origin, induced by high heat flow during rapid uplift of the Southern Alps. The first appearance of metamorphic biotite in this greywacke-schist transition occurs at the textural zone 2–3 boundary which is at least partly tectonic and has been folded isoclinally. Biotite is also a hydrothermal mineral in Type 2 veins and vein margins. Hence mapping of a biotite isograd in this area was not attempted.

Geology of the Nabarlek uranium deposit, Northern Territory, Australia

The Nabarlek deposit is an unusually high-grade uranium orebody (1.84% U 3 O 8 , 12,000 metric tons) hosted by amphibolite and semipelitic schist of lower Proterozoic age. It is unusual in the Alligator Rivers uranium field in being hosted by amphibolite. The disposition of the ore is controlled by a prominent reverse fault, which is part of a larger zone of reverse faulting. The fault zone is nearly perpendicular to prominent metamorphic S 2 and S 3 fabrics; a geometry which might enhance the fluid-rock ratio. A striking feature of the deposit is the extent and intensity of hydrothermal alteration which extends more than 1 km from the ore. Alteration remote from the ore resulted in pseudomorphous replacement of metamorphic biotite, amphibole, and feldspar, under conditions of a low fluid-rock ratio and mass transfer. Ore is associated with a small desilicification zone within, and superimposed on, this larger envelope. The fluid-rock ratio and mass transfer, resulting mainly in silica loss, were high within this interior zone and produced intense pervasive alteration, leading to an assemblage of phengitic white mica, magnesian chlorite, and hematite. Large volumes of fluid were required to effect the massive desilicification (and deposit such a volume of uranium), given the current knowledge of the solubility of quartz and uraninite.Ore deposition (predominantly as uraninite) postdated intense phengitic white mica alteration. Chlorite, however, is the main gangue mineral, possibly reflecting a change in redox conditions accompanying ore deposition. A redox control on ore deposition is supported by the sparse ocurrence of host-rock graphite which evidently participated in CH 4 -forming reactions. Abundant hematite in the alteration halo suggests the presence of an oxidized fluid (log f (sub O 2 ) approx. -35 at 200 degrees C) which was thus suitable for transporting ore-forming quantities of uranium, probably as chloride complexes. Primary mineralization underwent incongruent dissolution to coffinite, in equilibrium with a vanadian illite, kaolinite, and secondary hematite assemblage (residual ore). Weathering of primary and residual ore produced yellow and green secondary uranium minerals, both pseudomorphous after earlier ore types and as crusting of open goethite-lined fractures.

Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism

Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively‐exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less‐well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.

Chemical remobilization of volcanic-hosted sulphide deposits at Rosebery and Mt. Lyell, Tasmania

Devonian hydrothermal activity has resulted in localised chemical remobilization of the Cambrian massive sulphide deposits at Rosebery and Mt. Lyell in the Mt. Read mineral province of western Tasmania. At Rosebery, part of the massive pyrite-sphalerite-galena-chalcopyrite-quartz-phyllosilicate lens has been replaced, volume-for-volume, by a pyrite-pyrrhotite-chalcopyrite-magnetite-phlogopite-tourmaline-quartz assemblage, with complete removal of Cambrian sphalerite and galena. The replacement is later than folding of probable Devonian age. The likely temperature, the presence of tourmaline, and the relatively reduced nature of the secondary assemblage, suggest that the related fluids were derived from Devonian granitoids which, on independent evidence (tourmaline veins, pyrrhotite deposits, metamorphic biotite, and shallow granites nearby), appear to extend beneath the Rosebery mice. Lead and zinc dissolved from the Cambrian sulphide lens may have been redeposited, as veins and replacements in younger rocks. The resulting lead is likely to yield anomalous isotopic ages, and be associated with sulphur having the isotopic characteristics of Cambrian massive sulphide deposits. At Mt. Lyell, altered felsic volcanics host large disseminated pyrite-chalcopyrite-quartz-phyllosilicate lenses and minor stratabound exhalative(?) massive sulphide bodies, all of probable Cambrian age. In the North Lyell fault zone, late Cambrian/early Ordovician siliciclastic sediments have been replaced by cherty quartz-bornite-chalcopyrite-chalcocite-sericite-pyrophyllite ores of likely Devonian age. This assemblage probably formed from acid (pyrophyllite), oxidized (barite) solutions that may have derived high a Cu a Fe by dissolution of Cambrian chalcopyrite. Disseminated pyrite-chalcopyrite ores of Cambrian age have a well-developed Devonian cleavage in which hematite has crystallized, thereby indicating circulation of oxidized fluids during or after cleavage development. Precipitation mechanisms for the bornite ores may have involved fluid-sulphide and/or fluid-sediment reactions. The fluids are unlikely to have been derived directly from Devonian granitoids, but they may have been modified groundwaters circulating either during cleavage growth or as a result of heating by post-kinematic Devonian granitoid plutons.

TEM observations of defects in biotite and their relationship to polytypism

A dislocation model, based on Frank's screw dislocation model is proposed to explain transmission electron microscope (TEM) microstructures associated with growth defects in natural 1M and 2M1 metamorphic biotites. Burgers vectors have a non-zero component parallel to (001). Each rotation of the dislocation ledge repeats a stacking fault (R). By exercising control over the pitch of the screw dislocation and the component parallel to (001), the observed biotite crystal defects (stacking faults, superlattices and twins) and polytypes based on ± 120° rotations about c* can be generated. This model adds flexibility to Frank's spiral growth model as applied to mica by Baronnet (1975) by providing a means for changes of microstructures with changes in the growing environment.