Jb Gemmell

Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island are, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain are, and differ significantly from those of MORE-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na-Ca carbonates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2-H2O fluid, which is degassed significantly during magma fractionation. Hydrothermal fluids in the PACMANUS system may carry a direct contribution of the magmatic metal-bearing fluid, exsolved from the crystallising are-like magmas at this immature backarc basin, and are able to transport and concentrate major amounts of ore metals, particularly Cu.

Fluid bubbles in melt inclusions and pillow-rim glasses: high-temperature precursors to hydrothermal fluids?

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu–Mo–Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc–island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge–Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(±H2O±sulfur)-bearing vapor and contain significant amounts of amorphous (Na–K–Ca–Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions.

Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry

The Hellyer deposit is a classic, large tonnage, high-grade, mound style volcanic-hosted massive sulphide (VHMS) deposit in the Cambrian Mt Read Volcanic belt of western Tasmania. In the footwall directly underlying the deposit, there is an extensively altered pipe which cootains a well developed and preserved stringer zone. The vein paragenesis at Hellyer indicates that premineralization Stage I veins consist entirely of quartz, and occur throughout the alteration pipe. The synmineralization Stage 2 veins are the most abundant veins in the stringer zone and consist of three sub-stages: Stage 2A veins of crustiform quartz, pyrite, and carbonate with minor amounts of chalcopyrite, sphalerite and galena, Stage 2B veins with abundant base metal sulphides, minor quartz, carbonate and barite gangue and Stage 2C veins of coarsely crystalline barite with variable amounts of pyrite, sphalerite, galena and carbonate. Stages 3-6 veins are postmineralization veins and are related to the Devonian Tabberabberan Orogeny. Textural, petrographic and microthermometric investigations of fluid inclusions in the Hellyer stringer system indicate that Type I, primary, liquid-vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite. These inclusions are 10-15 µm in size, and yielded homnogenisation temperatures of 170-220°C in early 2A veins, 165-322°C in main-stage 2B veins and 190-256°C io late-stage 2C veins. These data suggest a waxing and waning thermal history. However, the average salinity remained between 8-11 NaCI equiv. wt% in all Stage 2 veins. Chalcopyrite-bearing primary fluid inclusions have been recognised in the base metal-rich Stage 28 veins. No evidence for presence of CO2 (e.g. formation of clathrates) was recorded by rnicrothermometry. However, Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (< 1 mole%) in the Stage 2B veins, and no detectable CO2 in 2A and 2C vein stages. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz indicates that the Stage 2B ore fluids have a significantly higher base metal concentration compared to the Stage 2A veins. The postmineralization Stage 4 veins have a variable but lower base metal content. In this study, there was no fluid inclusion evidence of boiling. Cation composition, higher salinities relative to seawater and the presence of CO2, suggest that recycled seawater alone cannot be the sole source of the ore fluids. This interpretation is in agreement with previous isotopic studies in the Hellyer stringer system. Although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data, such a contribution of ore fluids from a magmatic source cannot be ruled out. The possible input from the magmatic source may have occurred during the base metal-rich Stage 2B vein formation characterised by the intensifying temperature of deposition, higher base metals and CO2 contents.

Design, calibration and geological application of the first operational Australian laser ablation sulphur isotope microprobe

This contribution describes the setup and operating procedures of the first operational laser ablation microprobe for stable (sulphur) isotope analysis in Australia as well as some brief geological applications. A significant feature on this laser ablation microprobe is automated gas purification and analysis; operator control is only required to locate and ablate sample targets. As with other laboratories, samples were ablated in an oxygen atmosphere, producing a SO2/O2 gas mixture. SO2 was separated from this mixture by either of two techniques. In the first technique, SO2 was condensed into a liquid N2 trap by cryogenic pumping, and O2 was pumped away. This resulted in the collection of 60–70% of the produced SO2. In the second technique, SO2 was condensed into a liquid N2 trap as the SO2/O2 mixture was slowly bled away. This technique collected 90–95% of the SO2, with a small fractionation of 0.16%. Laser ablation and SO2 collection via the second technique required a mineral dependent, additive corre...

Microthermometry and chemical composition of fluid inclusions from the Mt Chalmers volcanic-hosted massive sulfide deposits, central Queensland, Australia: implications for ore genesis

Mt Chalmers is a mound-style, volcanic-hosted massive sulfide (VHMS) deposit in central Queensland, Australia. The ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of Early Permian age. The two ore lenses (Main lode and West lode) consist of Cu-Zn-Pb massive sulfide underlain by Cu-rich stringer mineralization. Textural and petrographic investigations of fluid inclusions indicate that primary Type I inclusions up to 25 microns are found in quartz from the stringer mineralized zone, and microthermometric studies of these inclusions yielded homogenization temperatures of 160-285 degrees C and salinities of 5-10.5 NaCl equiv. wt.%. Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (0.1-1 mol%)in the Mt Chalmers VHMS systems. Semiquantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt Chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids. Cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids. High base metal content and the presence of CO2 in the fluid inclusions imply that magmatic input of ore metals, copper in particular, accompanying seawater leaching of the footwall volcanic pile is a distinct possibility. In terms of fluid composition, the K-Ca-Fe variation of the Mt Chalmers ore fluid is comparable with those of typical epithermal deposits (e.g., Thames, New Zealand) and porphyry copper deposits (Bingham, UT). The Cu-Zn-Fe/10 plot of the Mt Chalmers ore fluids, indicates that there is significant copper in the system, comparable to copper enrichment in a porphyry copper system. The Mt Chalmers ore fluids also show similar copper content with the Cu-rich end-member ore fluid composition of the mineralized Stage 2B veins of the Hellyer VHMS deposit, whereas the Stage 2A veins of Hellyer are more enriched in lead and zinc. Overall, the ore fluids have a variable chemistry with a continuum of compositional data from VHMS to epithermal-porphyry style ore fluids. Shallow-water emplacement ( < 300 m) for the VHMS mineralization has been postulated for the Mt Chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies. However, fluid inclusion studies do not rule out a moderate to deeper submarine environment, as there is no fluid inclusion evidence of boiling. At Mt Chalmers, boiling of ore fluids would probably have occurred (as in most epithermal systems) if the ore fluid exhaled onto the seafloor under a shallow environment.

Precious metals in barite-silica chimneys from Franklin Seamount, Woodlark Basin, Papua New Guinea

Abstract Barite-silica chimneys with disseminated sulphides and elevated contents of gold and silver occur at two sites on the caldera floor of the basaltic andesite Franklin Seamount, situated near the western tip of the Woodlark spreading axis where it propagates into continental crust. The mineralogical residences of silver and gold in these chimneys are very different. Gold occurs in tiny electrum particles dispersed through very late-stage sulphide outgrowths from barite into cavities; these represent the fourth reported mineralogical occurrence of gold in seafloor hydrothermal deposits. Silver, by contrast, was deposited as real or occult sulphosalt inclusions in pyrites apparently formed throughout the local paragenetic sequence in each growth zone of the chimney walls. Correlations between silver, lead and antimony indicate that the main silver habitat is an unrecognized Ag-dominant PbSb sulphosalt, but unresolved inclusions of a pyrargyrite-like sulphosalt in colloidal pyrite may also occur. Electron and proton probe microanalyses have together established a balanced budget for many trace elements in minerals and bulk contents, except it is necessary to appeal to variations throughout the chimney in distribution of As, Cd, Sb and Hg. The residence of Mo (which correlates with gold) remains unknown. Isotopic studies indicate that the local volcanic pile, or its parental magma chamber, is the source of chalcophile metals in the chimneys, but neither the source rocks nor the tectonic setting offer definitive explanations for the enrichment of precious metals at Franklin Seamount. Chemical aspects of the probably small-scale and short-lived hydrothermal system appear the controlling factors, including low fluid/rock ratios leading to selective leaching of highly extractable constituents, and substantial subsurface mixing with seawater leading to pyrite deposition before venting.