Petrography of iron Skarn at Khao Lek, Amphoe Nong bua, Changwat Nakhon Sawan

Abstract

The occurrence of iron and secondary copper minerals (malachite and azurite) at Khao Lek area at Amphoe Nong Bua, Changwat Nakhon Sawan has attracted my attention during the Fieldwork II course work in 2009. This iron occurrence was partly open-cut at the foot of a hill and was reportedly penetrated by a series of vertical diamond-drilled holes for mineral exploration and evaluation by a private company. It was found that one drilled-hole intersecting the section of volcanics, skarn, iron mineralization and marble at a total depth of 60 meters is rather interesting and suitable for this senior project research. The objective of this project is to study petrography of the andesitic rocks, marble and type of skarn and mineralization. From logging of the drilled-core samples, four types of rocks can be separated in the field; andesitic lapilli tuff, marble, garnet skarn and epidote skarn. Andesitic lapilli tuff is greenish grey, hard and contains abundant rock fragments of andesite, rhyolite, and silicified shale varying in size from 0.1-3 cm in fine-grained plagioclase-dominated groundmass. Marble is characterized by mosaic interlocking grains of mainly subhedral calcite with minor clinopyroxene suggesting that it was transformed from a rather pure carbonate rock. Garnet skarn is composed mostly of garnet with minor pyroxene and formed earlier as prograde alteration. Two generations of garnet formation were recognized, the early isotropic garnet and late zoned anisotropic garnet overgrowth on the early isotropic garnet. Epidote skarn is the characterized by epidote, actinolite-tremolite, chlorite, calcite, and occurs as replacement or crosscutting of garnet skarn. The epidote skarn is therefore the retrograde alteration of early garnet skarn. The skarn at Khao Lek can be classified as calcic iron (magnetite) skarn deposit. The formation of iron probably took place during prograde alteration stage as indicated by the occurrence of magnetite in association with garnet.