Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S₁). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D₂), open to isoclinal south–southeast-oriented F₂ folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D₂ Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D₂ history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.     

An investigation of the magnetic variation within two recent lava flows

Samples collected in vertical sections from two recent (1169 AD and 1971 AD) lava flows show significant variations in both the palaeomagnetic vector and the rock magnetic characteristics. The age and the short cooling time of the flows (both are less than 3 m thick) are such that the observed range of palaeomagnetic variation is more than can reasonably be explained by secular variation. The variations in palaeomagnetic and rock magnetic characteristics appear in part to be a response to the relative proportions of two coexisting ferrimagnetic phases, one that displays a primary coarsegrained and Ti-rich composition and a second that has finer-grained, Ti-poor characteristics. The use of AF demagnetization in this study means that the latter phase will have a disproportionate influence on the palaeomagnetic results. It has been suggested elsewhere that in such a magnetic system the Ti-poor phase may represent a CRM formed at some temperature below its final Curie temperature; if so, the results of this study imply that not only does it affect the palaeointensity estimate (which requires a pure TRM) but it appears also to influence the palaeomagnetic direction to an extent that far exceeds the normal secular change that might be expected to occur during delayed acquisition. Such a result has important implications for palaeomagnetic studies that use lavas, both in the selection of a suitable sampling strategy and in the geomagnetic interpretation of palaeomagnetic variation within single flow units.     

The 1669 eruption at Mount Etna: chronology,petrology and geochemistry,with inferences on the magma sources and ascent mechanisms

 Analysis of the petrochemical characters of the 1669 Etnean lavas shows that they can be grouped into two sets: SET1 lavas were erupted from 11 to 20 March and are more primitive in composition than SET2, erupted later until the end of activity. Both sets may be interpreted as the result of crystallization under different conditions of two primary magmas which are compositionally slightly distinct and which fractionate different volumetric proportions of minerals. To explain why more mafic lavas (SET1) were erupted earlier than more acid ones (SET2), we argue that new deeper magma rose up into a reservoir where residing magma was fractionating. Density calculations demonstrate that new magma is less dense and may originate a plume, rapidly rising through the residing magma which is cooler and more volatile-depleted than the new magma. Calculations of uprise velocity assuming laminar flow are consistent with this hypothesis. Received: 20 November 1995 / Accepted: 2 August 1996     

Structural geology of the Dugald River ZnPbAg deposit, Mount Isa Inlier, Australia

The sediment-hosted ZnPbAg deposit at Dugald River is situated 87 km northeast of Mount Isa, NW Queensland. It is a mid-scale base metal accumulation restricted to a black slate sequence of low metamorphic grade. The orebody is tabular and consists of fine- to medium-grained sulphides with a dominant mineralogy of sphalerite, pyrrhotite, pyrite, galena, quartz and muscovite. Three different ore types have been recognized based on mineralization textures; laminated, banded and brecciated. The present reserve stands at 38 million tons of ore averaging 13.0% Zn, 2.1% Pb and 42 g/t Ag. A structural investigation has revealed that six stages of deformation have affected the metasediments in the Dugald River area. The first four (D₁, D₂, D₃ and D₄) are characterized by the extensive development of folds and associated axial plane cleavage. They were all generated in a ductile regime and are of considerable significance for the structural evolution of this region as well as for the emplacement and localization of the sulphide mineralization. D₅ provides a transition towards brittle deformation developing strong kink folds with subhorizontal axial planes. D₆ was a brittle event, producing E-W-trending open folds and major NE and NW strike-slip faults crosscutting all the pre-existing structural elements plus segmenting the orebody. Correlation between the development of deformation and the formation of mineralization can be observed from macro- to microscales. Relationships of mineralization with folds and cleavage indicate a post-D₂ (dominant deformation event) and probably syn-D₄ deformation timing for the ZnPbAg mineralization at Dugald River, as suggested by the ubiquitous truncations of D₂ fabrics by ore mineral assemblages throughout the deposit.     

Addition of magmatic volatiles into the hot spring waters of loowit canyon,Mount St. Helens,Washington, USA

Geochemical studies on cold meteoric waters, post-1980 hot spring waters, fumarole emissions from the dacite dome, and volcanic rocks at Mount St. Helens (MSH) from 1985 to 1989 show that magmatic volatiles are involved in the formation of a new hydrothermal system. Hot spring waters are enriched in ¹⁸O by as much as 2 and display enrichments in D relative to cold waters. A well-defined isotopic trend is displayed by the isotopic composition of a>400°C fumarole condensate collected from the central crater in 1980 (-33 D, +6 ¹⁸O), of condensate samples collected on the dome, and of cold meteoric and hot spring waters. The trend indicates that mixing occurs between local meteoric water and magmatic water degassing from the dacite dome. Between 30 and 70% magmatic water is present in the dome fumarole discharges and 10% magnatic water has been added to the waters of the hydrothermal system. Relations between Cl, SO₄ and HCO₃ indicate that the hot spring waters are immature volcanic waters formed by reaction of rocks with waters generated by absorption of acidic volcanic fluids. In addition, the B/Cl ratios of the spring waters are similar to the B/Cl ratios of the fumarole condensates (0.02), values of ¹³C in the HCO₃ of the hot springs (-9.5 to-13.5) are similar to the magmatic value at MSH (-10.5), and the ³He/⁴He ratio, relative to air, in a hot spring water is 5.7, suggesting a magmatic origin for this component.managed by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400 with the US Department of Energy     

Sulphur dioxide,particle and elemental emissions from Mount Etna,Italy during July 1987

Sulphur dioxide emission rates were measured at Mount Etna, Italy during July 1987 while the volcano was undergoing relatively quiet activity. The SO₂ flux averaged 930 ± 587 (1) Mg/d, excluding 19 July when the flux was 3200 ± 1730 (1) Mg/d. Rising magma and/or an influx of less degassed magma could explain the increased SO₂ flux. The high SO₂ flux did not correlate with changes in observed volcanic activity. This suggests that SO₂ monitoring may be useful as an indicator of shallow magmatic activity, but not as a predictor of future eruptions of Mount Etna.Particles emitted from the two active craters, Bocca Nuova and Southeast Crater (SE), were composed of silicates, sulphates and dithionites. Chloride species were only observed in particles from SE. Different eruptive styles probably produced the differences in particle compositions emitted from each crater.Vapour — magma enrichment factors were calculated for many elements from both craters. Cl, Br and S were the most enriched elements in the sampled fumes. Similar enrichment factors at both craters suggest a common magma supply.Cl, S and F have the largest elemental fluxes emitted from Mount Etna. During quiet activity, the Cl flux represents 27% of the global anthropogenic emissions, but its effect is limited to the local region due to atmospheric removal processes. Mount Etna also exhaled significant amounts of Zn, Br, Mo, F and Cu compared with regional anthropogenic emissions.     

Volcanic hazards at Mount Semeru,East Java (Indonesia), with emphasis on lahars

Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity, and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results from the growth of two edifices: the Mahameru ‘old’ Semeru and the Seloko ‘young’ Semeru. On the SE flank of the summit cone, a N130-trending scar, branched on the active Jonggring-Seloko vent, is the current pathway for rockslides and pyroclastic flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5 to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941–1942. Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m³ each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit, was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment yield of 2.7 × 10⁵ m³ km⁻² and a denudation rate of 4 10⁵ t km⁻² yr⁻¹, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang can be affected by debris avalanches and debris flows if the steep-sided summit cone fails.     

Ground Thermal Profiles from Mount Kenya, East Africa

This paper presents and compares ground thermal regimes at 4200 and 4800 m a.s.l. on Mount Kenya's southern aspect. Temperatures were recorded using Tinytalk? data loggers, installed at the ground surface and at depths of 1 cm, 5 cm, 10 cm and 50 cm. Temperatures were logged at 2‐hour intervals over a period of 12 months (August 1998 to July 1999). The study is designed to demonstrate near‐surface freeze conditions, which would have implications for contemporary periglacial landform production. Although ground freeze at 4200 m a.s.l. occurs during most nights (c. 70% at 1 cm depth), freeze penetration is restricted to the top 2 to 3 cm, such that no freeze was recorded at 5 cm depth. At 4800 m a.s.l., the diurnal frost frequency at the surface is 365 days (100%), whilst that at 10 cm depth is 165 days (45%). The paper demonstrates that a greater longevity of contemporary thin snow cover at 4800 m a.s.l. permits progressive sub‐surface cooling with depth. However, the near‐surface ground temperature profiles suggest that conditions are not conducive to permafrost development at the sites.     

西藏玉龙铜矿带南段马牧普铜多金属矿床矿物学特征

西藏马牧普铜多金属矿床是玉龙铜矿带南段的重要组成部分，最新勘查进展揭示其铜、金资源量均已达到中型规模，但理论研究工作仍十分薄弱。本文基于系统的镜下鉴定和电子探针分析，对马牧普铜多金属矿床矿物学进行详细研究，进而探究矿床成因。马牧普矿床主矿体为中—厚层板状的角砾岩型矿体，矿石类型以角砾状为主，局部矿体呈透镜状产于角岩中。矿区发育广泛的绿泥石化、角岩化、金云母化和弱绢云母化蚀变。电子探针分析结果表明，矿区内矽卡岩矿物中辉石主要为透辉石，云母主要为金云母，角闪石则以透闪石为主。马牧普矿床成矿阶段和矿物组合为：Ⅰ进变质阶段，矿物组合为透辉石- 石榴子石- 磁铁矿±白钨矿；Ⅱ退变质阶段，矿物组合为透闪石- 金云母±磁铁矿±绿泥石±绿帘石±白钨矿；Ⅲ硫化物阶段，矿物组合为黄铁矿- 黄铜矿±辉钼矿±磁黄铁矿±辉铋矿±方铅矿±闪锌矿±萤石±石英，为主成矿阶段；Ⅳ碳酸盐阶段，矿物组合为赤铁矿±萤石±方解石±石英±玉髓。矿物学特征表明，矽卡岩演化经历了岩浆期后高温—中低温演化的过程，随着温度的降低，镁质矽卡岩矿物从进变质阶段的透辉石转变为退变质阶段的金云母；矽卡岩形成过程中经历了早期相对较为氧化的环境到晚期相对较为还原环境的转变。矿区角砾岩层为原沉积地层中存在的白云质、钙质碳酸盐夹层，广泛的热液交代作用使得大部分碳酸盐呈角砾状。含矿岩浆热液流体沿着碳酸盐岩产状侵位并发生交代作用，使得金属矿物在角砾岩层中沉淀并富集成矿。