Fluid Inclusion Study of Epithermal Quartz Veins from the Kyaukmyet Prospect, Monywa Copper-Gold Ore Field, Central Myanmar

The Kyaukmyet prospect is located near the main ore bodies of the Kyisintaung and Sabetaung high-sulfidation Cu-Au deposits, Monywa copper-gold ore field, central Myanmar. Lithologic units in the research area are of mainly rhyolite lava, lapilli tuff and silicified sandstone, mudstone and siltstone units of Magyigon Formation which hosted to be polymetallic mineralization. Our field study recorded that epithermal quartz veins are hosted largely in rhyolite lava and lapilli tuff units. Those quartz veins show crustiform, banded (colloform), lattice bladed texture and comb quartz. The main objectives of the present research in which fluid inclusion studies were considered to conduct the nature, characteristics and hydrothermal fluids evolution from the epithermal quartz veins. In this research, there are three main types of fluid inclusions are classified according to their phase relationship (1) two-phase liquid-rich inclusions, (2) the coexisting liquid-rich and vapor-rich inclusions, and (3) only vapor-rich inclusions. Microthermometric measurements of fluid inclusions yielded homogenization temperatures (Th) of 148–282 °C and final ice-melting temperature (Tm) of -0.2°C to -1.4°C . The value of (Tm) are equal to the salinities reaching up 0.35 to 2.07 wt % NaCl equiv. respectively. Estimation formation temperature of the quartz veins provide 190°C and 210°C and paleo-depth of formation are estimated to be between 130m and 210m. Petrography of fluid inclusion and microthermometric data suggest that fluid boiling as well as mixing processes were likely to be happened during the hydrothermal fluid evolution at the Kyaukmyet prospect. According to the characteristics of many parameters including petrography of fluid inclusion, microthermometric data, paleo-depth, evidence of quartz vein textures and types of hydrothermal alteration from the Kyaukmyet prospect allows to interpret these data to be the low-sulfidation epithermal system.

Conduit system, degassing, and flow dynamics of a rhyolite lava: A case study of the Shiroyama lava on Himeshima Island, Japan

This study presents a description of a rhyolite lava-forming eruption, including the conduit system, degassing history during the lava flow dynamics. We examined the Pleistocene Shiroyama rhyolite lava on Himeshima Island, Japan. The lava is mainly characterized by locally developed obsidian. Based on the structural variation, the obsidian lithofacies correspond to the shallow conduit. The geological investigation and FTIR analyses showed that gas removal from the conduit magma proceeded via vesiculation, fracturing, and brecciation, allowing formation of the dense obsidian. Since the lava originally maintained some extent of water, the lava effervesced just after the effusion. This vesiculation resulted in pervasive bubble coalescence and the formation of abundant permeable pathways. The volcanic gasses escaped via those pathways, allowing collapse of the bubbles and deflation of the lava. AMS (anisotropy of magnetic susceptibility) results indicate that the lava spread concentrically.

Deformation of rhyolite lava crust associated with intermittent inner flow of lava: palaeomagnetic evidence

SUMMARY A palaeomagnetic study has been conducted to examine the deformation of thick crusts of rhyolite lava while its inner portions continue to flow. The Sanukayama rhyolite lava, which erupted in the Pleistocene in Kozushima Island, Japan, was chosen as the investigation site because of its well-exposed vertical lithofacies variations classified into three distinct zones (pumiceous, obsidian and crystalline). The targets of this study are the pumiceous and obsidian zones, which constitute the crust of the lava. Thermal demagnetization reveals three remanent magnetization components from the pumiceous and obsidian samples but only a single magnetization component from the inner crystalline rhyolite samples. Alternating field demagnetization is ineffective in isolating the magnetization components in the pumiceous and obsidian samples. The multiple components of remanent magnetization of the crust are interpreted to have been acquired during cooling as thermoremanent magnetizations. We suspect intermittent lava transport of the inner portions, the primary mode of rhyolite lava advancement, to be responsible for the presence of multiple components in pumice and obsidian of the lava crust. When the inner portions of the lava retain mobility to flow out of the crust, the solidified crust of the lava surface below the magnetite Curie temperature remains susceptible to deformation. Analysis of palaeomagnetic directions from the crust allows the deformation of the crust to be described in terms of rotation. Although the mode of rhyolite lava advancement is not well understood, because of its infrequent occurrence, our observations offer an important insight on how the mobile part of the lava is associated with the deformation of the crust during continued lava advance.