Stratigraphy, geochemistry and petrography of volcanic rocks in the chatree gold mine, changwat Pichit

Chatree gold mine in Phichit Province, north-central Thailand, situated within the Loei-Phetchabun-Nakhon Nayok volcanic belt. In the northern part of the Chatree gold mine, the Permo-Triassic volcanic rocks mapped by Department of Mineral Resources were collected from the C-H pit. Forty-four samples were petrographically classified and geochemically analyzed to remap for their stratigraphy. Two types of volcanic rocks are recognized, coherent and non-coherent units in which the former is older on the basis of stratigraphic succession. Several lines of evidence suggest that the studied volcanic rocks occurred nearby the volcanic edifices and were dominated by debris flows of submarine environment. Based on petrographic and field investigations, the coherent unit has the minimum thickness of about 80 m and is composed mainly of andesite and basaltic andesite. Both rocks contain largely plagioclase and hornblende, as major constituents, and have principally porphyritic and microlithic textures. Alterations are indicated by the presence of quartz, pyrite, chlorite and epidote. The non-coherent rocks have the average thickness of about 80 m and are classified into monomictic, polymictic, and fiamme breccias. Monomictic breccia (20 m-thick) is characterized by the only one kind of volcanic fragments whereas more than two kinds of fragments characterize polymictic breccia (30 m-thick). Fiamme breccia (30 m-thick) is represented by the presence of stretched chlorite, lenticular glassy material and plagioclase phenocrysts. Both of the coherent and monomictic units show strong hydrothermal alteration with associated Au-Ag mineralization. Geochemically, nineteen samples of the least altered and least weathered volcanic rocks were analyzed for major, minor, and trace elements by using XRF and ICP OES methods. Of these, three rock samples were analyzed for rare-earth element (REE) concentrations by using ICP MS method. Discrimination diagrams of minor and trace elements and REE pattern reveal that the volcanic rocks at Chatree mine are regarded as calc-alkaline strato-volcano occurring in the subduction zone complex of volcanic arc setting during Permo-Triassic Period.