Mineralogy and petrography of skarn at Khao Lek, Amphoe Nong Bua, Changwat Nakhon Sawan

Abstract

Khao Lek deposit is located at Amphoe Nong Bua, Changwat Nakhon Sawan. Skarn is better developed in limestone than volcanics protoliths. It shows mineralogical zonations represented by garnet skarn, pyroxene skarn, garnet-pyroxene, pyroxene-wollastonite. However, in volcanics protolith, pyroxene skarn is the only skarn type identified and it is characterized by pyroxene veinlets or pyroxene infilled vugs in volcanic unit mainly in the footwall. Mineralogically, skarn may vary within each skarn zone for example in garnet skarn in the western part of the zone is represented by dark brown, medium- to coarse-grained associated with calcite whereas, at the eastern part garnet is reddish brown to yellowish green closely associated with pyroxene. Skarn variation may reflect distance from intrusion or proximal to more distal. This is consistence with composition obtained from EPMA analyzes in which garnet at proximal has composition of spessartite-grossular-andradite series whereas, at the distal become andradite. These reflect source of fluid and type of protolith and proximal and distal from source intrusion. Similar to pyroxene where diopside represent pyroxene hosted in volcanic whereas, pyroxene hosted in or close to limestone protolith is represented by ferroaugite.Magnetite orebody is likely replacing the major endoskarn which was emplaced along NE-SW major fault as a dyke. This magnetite mineralization could well be formed during retrograde skarn formation. Retrograde skarn is better developed along major faults particularly the footwall (southwest of the pit). It characterized by epidote-chlorite ± calcite vein/veinlets. The skarn at Khao Lek can be classified as calcic skarn based on its mineralogy. When consider in terms of ore deposits that are hosted by skarns, this deposit can be classified as skarn deposit. It is classified as iron ± copper skarn deposit which is based on the dominant metal i.e., Cu, Au, Pb-Zn, Fe, Mo, W and Sn