Petrography and Geochemistry of volcanic rocks in Khao Sam Sip area, Sa Kaeo Province

Abstract

The volcanic rocks at Khao Sam Sip, Sa Kaeo province is part of the Loei-Phetchabun-Sa Kaeo Volcanic Belt. Based on field observation, petrographic and geochemical studies, the rocks in the study area can be divided into four rock units namely, 1) Basalt-andesite unit, 2) Polymictic andesitic unit, 3) Epiclastic unit, and 4) Clastic unit. Unit 1 is the lowest unit in sequence consisting of olivine-pyroxene-plagioclase basalt, pyroxene basalt, hornblende-plagioclase andesite, and plagioclase andesite. Unit 2 consists of polymictic andesitic breccia and polymictic andesitic sandstone. Unit 3 is epiclastic unit comprising of crystal-lithic sandstone and polymictic conglomerate. Units 4 includes fine-grained clastic rock and limestone.

In the geochemical study, the basalt-andesite unit was selected for geochemical analyzes including major, trace and rare earth elements. They are ranging in composition of trachyandesite and alkali basalt with few rhyodacite, andesite, and basanite and classified dominantly as alkaline affinity. Based on the results of trace elements and REE abundances, the rocks can be subdivided into four groups. All four groups have similar chemical characters of enrichment in high field strength elements (HFSE) and light rare earth elements (LREE) with distinct negative Eu anomalies, and this may suggest the rocks were formed by Early Triassic fractionated magma of volcanic arc system as a part of Loei Fold Belt. Gold has not been identified during this study. It could have occurred as refractory gold (in pyrite structure) or nano-size particles included in sulfide minerals. However, copper-rich veins/veinlets are hosted in coherent volcanic rocks and they may represent either skarn or epithermal style mineralization.