Application of mineral equilibria modeling to constrain T and X CO2 conditions during the evolution of the Magdala gold deposit, Stawell, Victoria, Australia

Mineral equilibria modeling involving solid solution calculations has been combined with mineral assemblage information from the alteration zones associated with gold mineralization to determine the T and X _CO2 conditions for the formation of the Magdala gold deposit at Stawell, Victoria, Australia. Economic gold mineralization is primarily hosted within the stilpnomelane alteration zone of the Stawell Facies that is adjacent to the Magdala Basalt. Evolution of the Magdala gold deposit involved at least three fluid infiltration events: (1) a CO₂-bearing fluid during the D₂ deformation event produced carbonate spots throughout the chlorite zone; (2) a CO₂–S–K-bearing fluid, accompanied the D_3–4ab deformation and produced a muscovite zone and siderite rims on ankerite; and (3) a CO₂–K–S–Au-bearing fluid during the D_4c deformation event produced the stilpnomelane zone of the Stawell Facies, the proximal and distal alteration zones within the Magdala Basalt, and the main economic gold mineralization. Mineral equilibria modeling constrains the temperature of formation of the Magdala deposit to T = 345–390°C at 3kbar, substantially lower than indicated by other previous classical thermobarometry methods. Furthermore, this method has allowed the characterization of the mineralizing fluid and constrained its composition to X _CO2 < 0.08 at 3kbar. The timing and composition of the mineralizing fluids are similar to that of metamorphic fluid generated from devolatilization of a greenstone pile with peak of metamorphism occurring earlier and at deeper levels in the crust.     

An apparent ‘even-odd’ cycle distribution in Mt. Wilson ‘numbers of spots’ data

Mt. Wilson numbers of spots data (as defined in Howard et al., 1984) appear to be distributed according to even-odd cycle numbering. Linear fits of annual numbers of spots versus annual sunspot number for even- and odd-numbered cycles have slopes which are statistically different at the 5% level of significance. The existence of an even-odd split in Mt. Wilson numbers of spots data may be due either to a real difference in even- and odd-numbered cycles on the Sun or to a difference in weather at Mt. Wilson (perhaps, related to the 22-yr rhythm of drought in the western United States) during even- and odd-numbered cycles, or both. For cycle 22, an even-numbered cycle, the peak numbers of spots is estimated to be near 2600.     

Abundances in the interstellar medium

Summary Recent developments in the theory of element production and the chemical evolution of the galaxy are presented. Following this, observational data and their interpretation are given. A case by case analysis of results for D, He, Li and CNO isotope data in the disk and center of our galaxy is presented; previous results for element gradients are also summarized.The primordial abundances of D and He cannot be directly obtained from observations; corrections for stellar processing are discussed. From these data and the Li abundances, it appears that the abundance of the light elements is consistent with the standard big bang. In agreement with previous results, the range of, the baryon to photon ratio, is 5–8 10^–10. If the amount of non-baryonic matter is small, these results indicate an open universe, in the standard big bang model.New data show a gradient in the (¹²C/¹³C) and (¹⁶O/¹⁸O) ratios with galactocentric distance, D_GC. The presence of a gradient in the (¹⁴N/¹⁵N) ratio is less clear and there is no measurable gradient in the (³²S/³⁴S) ratio. In the interstellar medium near the sun, the carbon isotope ratio is –20 percent lower than the solar system ratio. This indicates that there has been only a moderate amount of enrichment of the nearby interstellar medium since the formation of the solar system. These results and previously determined galactic element gradients are interpreted in the framework of chemical evolution models. Delayed recycling of nucleosynthesis products is essential for the correct interpretation of the results. Comparisons of data with galactic evolution models are discussed.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991     

Elysium planitia,mars: Regional geology,volcanology, and evidence for volcano-ground ice interactions

Geological mapping of Elysium Planitia has led to the recognition of five major surface units, in addition to the three volcanic constructs Elysium Mons, Hecates Tholus, and Albor Tholus. These units are interpreted to be both volcanic and sedimentary or erosional in origin. The volcano Elysium Mons is seen to have dominated constructional activity within the whole region, erupting lava flows which extend up to 600km from the summit. A major vent system, covering an area in excess of 75 000 km², is identified within the Elysium Fossae area. Forty-one sinuous channels are visible within Elysium Planitia; these channels are thought to be analogous to lunar sinuous rilles and their formation in this region of Mars is attributed to unusually high regional topographic slopes (up to ~ 1.7). Numerous circumferential graben are centered upon Elysium Mons. These graben, located at radial distances of 175, 205–225, and 330km from the summit, evidently post-dated the emplacement of the Elysium Mons lava flows but pre-dated the eruption of extensive flood lavas to the west of the volcano. A great diversity of channel types is observed within Elysium Fossae. The occurrences of streamlined islands and multiple floor-levels within some channels suggests a fluvial origin. Conversely, the sinuosity and enlarged source craters of other channels suggests a volcanic origin. Impact crater morphology, the occurrence of chaotic terrain, probable pyroclastic deposits upon Hecates Tholus and fluvial channels all suggest extensive volcano-ground ice interactions within this area.NASA Summer Intern.     

Structural control on gold mineralization at Walhalla,Victoria

Gold mineralization associated with quartz reefs is related to the structural history of the Early Devonian, Walhalla Group. These reefs are situated in the Walhalla Synclinorium, developed during the Middle to Late Devonian Tabberabberan Orogeny. A pervasive north‐south‐trending axial planar cleavage and two styles of folding were produced during regional east‐west compression. The first are upright, open to close folds with sub‐horizontal fold axes. The second are plunging inclined, close to tight folds with fold axes that plunge steeply to the north and south. An extensional event is associated with the emplacement of the Woods Point Dyke swarm and a set of normal faults that offset all earlier structures. High‐angle reverse faults, which post‐date the folding and the emplacement of the dykes, were utilized as conduits for hydrothermal fluids and preferentially localize mineralization to laminated quartz veins. En echelon vein arrays formed during initial stages of reverse faulting became deformed during prolonged shearing to produce ptygmatic veins. Laminated quartz veins within high‐angle reverse faults contain arsenopyrite and pyrite in vein margins and gold in fractures that cross‐cut continuous quartz crystals. Gold, galena, chalcopyrite and sphalerite may also be deposited adjacent to and within fractured arsenopyrite and pyrite. Late‐stage, cross faults developed in a regime of north‐south compression and post‐date the laminated quartz veins and mineralization.