Masaharu Nakagawa

Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India

We report the morphological, textural and chemical characteristics of gold grains in stream gravels from the Siruvani River in Attappadi Valley, southern India. The placer gold deposits contain both primary grains with jagged grain contours and secondary grains with smooth grain margins. The primary and secondary gold grains are also distinguished by marked contrast in microtextures with the latter displaying a range of corrosion textures including striations, etch pits and chemical corrosion cavities that coalesce to form honey-comb patterns. Some of these cavities are filled with fine clay derived from lateritic weathering front. While the primary grains are characterized by high silver content (up to 35.77 wt.%) with marginal overgrowths of high purity gold, the secondary grains show exceedingly high fineness (1000 Au/Au+Ag) levels (up to 984) with no marked compositional variation indicating selective extraction of Ag and/or reprecipitation of Au. From morphological and chemical characteristics, we propose that the high purity gold grains were not derived directly from primary sources, but underwent chemical refinement in the weathering front before they were transferred to the fluvial systems. Our findings have important implications for gold exploration in the Attappadi Valley.

Proto-Japan and tectonic erosion: Evidence from zircon geochronology of blueschist and serpentinite

Blueschist facies rocks in the Shikoku island of southwest Japan were extruded from depth as blocks in the Kurosegawa serpentinite melange, where they occur in association with other tectonic blocks of tonalite-trondhjemite-granodiorite (TTG) rocks and calc-alkaline volcanics. Here we report magmatic zircons in the blueschist that yield 206Pb/238U mean ages in the range 505 ± 3 to 503 ± 3 Ma. These zircons crystallized at a mid-ocean ridge within basaltic rocks, and migrated to the continental margin of paleo-Asia. Subduction of the oceanic lithosphere into the mantle at a depth of ∼40 km resulted in metamorphism under jadeite-glaucophane facies conditions at 250 Ma. The blueschist facies rocks were then exhumed to the surface along a splay fault connected with the forearc region through tectonic extrusion. Zircons separated from serpentinite yielded a 206Pb/238U mean age of 152 ± 3 Ma, marking the timing of serpentine protrusion to the surface, consistent with geologic constraints. The serpentinites also contain minor ca. 500 Ma zircons, similar to those in the blueschist. The predominance of detrital ca. 150 Ma zircons in the serpentinite suggests extensive tectonic mixing of Jurassic trench turbidites with the mantle wedge. The Kurosegawa serpentinite melange belt extends along the strike of the orogenic belt for more than 800 km, parallel to the active trench. We propose that this belt marks the location of extensive tectonic erosion and provides an excellent case study for the destruction of continental crust along a convergent plate boundary.

Clay mineral associations and mineralogical properties of quartz in some pottery stones of western Kyushu, Japan

Abstract Mineral compositons and mineralogical properties of quartz in Amakusa, Izumiyama, and Hasami pottery stones of western Kyushu have been examined. These deposits were formed by hydrothermal alteration of intrusive rhyolites. The main constituent minerals of these ores are quartz, sericite and kaolinite. The sericite is pure dioctahedral mica or irregularly interstratified mica/smectite with small amounts of smectite layers (up to 10%). The polytype is 1 M , 2 M 1 and 2 M 2 . Dickite, rectorite and tosudite are also found in the Amakusa Sarayama deposit. X-ray diffraction peaks of the quartz are broad. The cell dimensions are considerably large with a=4.9136−4.9158 A and c=5.4048−5.4063 A at 18°C. The high-low inversion peak in the DTA curve is broad and the peak temperature is by 4–15°C lower than that of standard quartz. All over the quartz samples in the pottery stones, there is a good correlation between the cell dimensions and the high-low inversion temperature. The quartz occurring near the center of the hydrothermal alteration has smaller cell dimensions and higher inversion temperature as compared with the quartz in the outer alteration zone.