Geologic and Petrographic Investigations of Khao Phanom Pa Area, Phichit Province, Northcentral Thailand
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Abstract
Khao Phanom Pa area (1 km2) in Phichit Province, north-central Thailand has been a site of interest due to a gold rush since 1997. Two kinds of gold deposits were discovered – primary deposit in the hard – rock mountain and the secondary deposit in colluvial sediments. The purposes of this project are to document detailed geology and alteration and to provide petrographic investigation of the area which bears gold deposits. 105 rock samples were collected and 41 pieces of rock slabs were selected for petrographic analysis and thin section study. A new geological map shows that the Khao Phanom Pa study area is dominated by various kinds of igneous rocks. Pyroclastic rocks (tuff and lapilli tuff) of the latest Permian (250±6 Ma) were observed to have been intruded by microdioritic rocks. The microdiorite stocks were later cross-cut by trachytic andesitic dike. Metamorphic rocks were found in locally as calc-silicates and skarnoids. Altered volcanic rocks including retrograde skarnoids, silicified lapilli tuff and micro-diorite show strongly pervasive and selective alteration styles. The latter seems to be more common, and there are four kinds of vein – style alteration viz. epidote – chlorite – sulfide vein, quartz vein, quartz- calcite vein, and calcite vein. Quartz-carbonate and quartz veins are quite more abundant and they appear to be major host of gold. Unconsolidated colluvial and alluvial sediments are also observed to contain appreciable amounts of gold in the surrounding low-lying areas. Nonvisible gold was found only from ore assay analysis of altered rocks and quartz veins whereas the visible gold was found by panning around Khao Phanom Pa alluvial area. Gold deposit at Khao Phanom Pa can be classified as low sulphidation epithermal deposit. Gold may have occurred contemporaneously with the vein – style alteration. Volcaniclastic host rocks were first subject to metamorphism as calc – silicate rocks and followed by metasomatism as skarnoids. Subsequently, intense alteration associated with multiple episodes of epithermal alteration assemblages took place.
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Copyright © 2008 Department of Geology, Faculty of Science, Chulalongkorn University. Parts of an article can be photocopied or reproduced without prior written permission from the author(s), but due acknowledgments should be stated or cited accordingly.
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