| Summary: | Jiwo Hills area in the Bayat District, Klaten Regency, Central Java Province is a
relatively small area but with a complex geological condition. Some of the oldest
rocks in Java, that consist of metamorphic rocks and Paleogene sedimentary rocks,
are exposed in this area. Metamorphism is generally of a regional metamorphism
style characterized by the presence of mica schist. However, at Pagerjurang village
we found a contact metamorphism as indicated by the occurence of skarn rocks.
Skarn is likely to be formed by contact metamorphism and metasomatism by
hydrothermal solutions derived from magma (intrusion) that affected limestone and
siltstone. Characteristics of mineralogy and mineral paragenesis of the skarn are
studied by petrography, mineragraphy, and XRD analysis on rock samples
representing different zone of prograde and retrograde skarn. Skarn alteration and
mineralization focus at the bedding contact of limestone and siltstone, that has been
altered to marble and hornfels. Mineralization is controlled by the NE-SW and NWSE
trending geological structures, there are Melikan Dextral Strike Slip Fault and
Pagerjurang Sinistral Strike Slip Fault. The skarn can be divided into three zones,
namely 1) proximal prograde exoskarn zone characterized by the presence of
garnet (grossular) more abundant than clinopyroxene (wollastonite-augitediopside-
hedenbergite)
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