Mineralogy and petrography of Skarn at Khao Phra Ngam area, Amphoe Muang, Changwat Lopburi

Abstract

The Khao Phra Ngam area is located at a contact zone between the Permo-Triassic diorite intrusion and Permian carbonate rocks of Khao Khad Formation. The intrusion had thermally metamorphosed the host rock into marble, minor diopsidic marble and reaction skarn rimming chert nodules and subsequent formation of metasomatic skarns. A systematic zonal arrange from the host rock side to igneous rock side are; marble with associated diopsidic marble, wollastonite skarn, garnet-clinopyroxene skarn, garnet skarn and contaminated diorite. The formation of skarn can be devided into 3 stages; 1) Metamorphic (isochemical) stage was taken place during the early period of diorite intrusion which transformed limestone into marble, diopsidic marble and reaction skarn rimming (bimetasomatism) chert nodules or beds. 2) Metasomatic stage was characterized by the formation of wollastonite skarn, garnet-clinopyroxene skarn, garnet skarn and contaminated diorite. 3) Retrograde alteration stage was probably formed during the declining temperature and composition of fluid was change into a meteoric dominated component. The fact that the prograde skarn were developed as a relative narrow shell and there is only minor development of retrograde alteration, and the bedding in the marble is nearly parallel to intrusive contact. It is likely that these skarn were formed at a relative deep level. The skarn at Khao Phra Ngam can be classified as calcic copper ± iron skarn deposit and probably related to oceanic subduction and island-arc environment.