Petrochemistry of probable gem-bearing basalts in Sop Prap-Ko Kha Area, Changwat Lampang : research report

Abstract

The project area covers approximately 300 km[superscript 2] encompassing parts of Amphoe Sop Prap, Amphoe Ko Kha, and Amphoe Mae Tha of Changwat Lampang. The area is mainly occupied by sedimentary (and metamorphic) rocks of Permian, Triassic, Tertiary and Quaternary ages. The Triassic rocks include the Phra That, the Pha Kan, and the Hong Hoi Formations of the Lampang Group. Igneous rocks comprise Permo-Triassic volcanics, Triassic granodiorite, and Cenozoic basalts. Sapphires are frequently found in alluvial and residual deposits, particularly in the northern basaltic area. Sapphires in the south terrain are also encountered in stream channels flowing from nearby Cenozoic basaltic crater. Sapphires are normally blue, dark blue, greenish blue, and dark brown, and occur as angular to subangular forms, ranging size from 0.5 to 6 cm. Basalts in the project area can be geographically subdivided into 2 terrains the north and the south. The northern terrain (Nam Cho Basalt) covers approximately 3 km[superscript 2] in Thambon Nam Cho, Amphoe Mae Tha. The basalts occur as one major flow layer, that flowed following the hill slope. The rocks are characterized by reddish brown vesicular rocks (top), and black dense rocks (bottom). They are generally present as fine-grained and porphyritic. Ultramafic nodules of lherzolite comprising olivine, pyroxene and spinel and megacrysts of olivine, and pyroxene, are often observed in this terrain. Microscopically, the northern basalts invariability occur as microporphyritic to porphyritic, aphanitic. Phenocrysts of olivine and pyroxene are generally surrounded by small plagioclase- microlite groundmass and glass. The basalts are typically composed of plagioclase, pyroxene, olivine, opaque minerals, and other accessories. Average grain size is relatively smaller than that of the south basalts. The southern terrain (Sop Prap-Ko Kha Basalt) can be subdivided into 5 flow layers. Good exposures are typically present at road cut quarries between kms 586-569 on the highway number 1, nearby the volcanic crater. The area covers approximately 55 km[superscript 2] between Amphoe Ko Kha and Amphoe Sop Prap. These five basaltic flows are similarly characterized by vesicular, or massive, microporphyritic-porphyritic, fine-grained to aphanitic rocks. Phenocrysts of oviline frequently occur in most flows. These basalt flows microscopically comprise similar characteristics, mostly they always contain plagioclase, pyroxene, olivine, opaque minerals, and other accessories. Intersertal, and subophitic textures are frequently present in these basalts. Ultramafic nodules are rarely found in this basaltic terrain. Geochemically, that Nam Cho and Sop Prap-Ko Kha Basalts can be clearly divided by difference in major and trace- element contents. Though several variation diagrams display similar trend. The Nam Cho basalts are mainly classified as basanite, where as the Sop Prap-Ko Kha basalts are dominantly alkaline-olivene basalt base on normative plagioclase and hyperstene and/or nepheline. Rare-earth element concentrations show similar chondrite-normalized patterns for most basalt groups. Calculated Mg-values indicate crystal fractionation process of the derivative magmas. [superscript40] Ar/[superscript 39] Ar geochronological data are clearly concluded that Sop Prap-Ko Kha Basalts, ranging in age from younger than 2.30 to 2.41 Ma, are older than Nam Cho Basalts (ca. 2.02 Ma). Therefore, it is tentatively inferred that basalts from both terrains were probably originated from derivative magmas, which are evolved by similar processes, as crustal fraction with subsequent crustal contamination process. However, these derivate magmas may have been derived from different originated sources and primary magmas with low degree of partial melting in mantle. The occurrence of sapphires possibly indicates partial melting of primary magma at high depth and pressure, and crustal fractionations of derivative magma route to 8-10 kbar. Then Nam Cho Basalt can be regarded as gem-related and formed different derivative magma from mantle source region.