MINERALOGI DAN GEOKIMIA BATUGAMPING MERAH PONJONG, GUNUNGKIDUL, DAERAH ISTIMEWA YOGYAKARTA – INDONESIA

Abstract

Batugamping berwarna merah yang tersebar secara setempat-setempat dan berasosiasi dengan batugamping berwarna putih hingga abu-abu yang dijumpai di Daerah Ponjong, Kabupaten Gunungkidul, Daerah Istimewa Yogyakarta, termasuk dalam Formasi Wonosari-Punung. Batugamping tersebut perlu diteliti karakteristik mineralogi dan geokimianya, yang sangat diperlukan dalam memahami genesa batugamping di daerah tersebut. Hasil penelitian ini diharapkan dapat menambah data khususnya batugamping di daerah penelitian maupun Formasi Wonosari-Punung di daerah Pegunungan Selatan. Pengamatan petrografi dan XRD pada batugamping merah menunjukkan hadirnya mineral kalsit, kuarsa, siderit, hematit, dan titanit. Analisis geokimia oksida mayor batugamping merah memperlihatkan tren pengkayaan senyawa SiO₂, TiO₂, Fe₂O₃ dan MnO yang diinterpretasikan berhubungan dengan kehadiran mineral-mineral titanit (CaTiSiO₅), siderit (FeCO₃), hematit (Fe₂O₃), dan diduga rodokrosit (MnCO₃). Mineral-mineral tersebut mempunyai karakteristik warna coklat kekuningan, merah muda sampai merah sehingga dimungkinkan dapat memberikan warna merah pada batugamping. Ada tiga proses yang diinterpretasi berperan dalam genesa batugamping merah Ponjong yaitu pengaruh material terigenus yang mengandung oksida SiO₂, Al₂O₃, Fe₂O₃, dan TiO₂ saat pengendapan batugamping, proses diagenesis oleh air meterorik yang mengkayakan senyawa Fe₂O₃ dan proses bekerjanya larutan hidrotermal.

Red limestone, which is sporadically distributed and associated with white to grey limestone is located in Ponjong area, Gunungkidul District, Daerah Istimewa Yogyakarta. This limestone belongs to the member of Wonosari-Punung Formation. It is necessary to study the mineralogy and geochemistry chracteristics, which are important in understanding the genesis of the limestone. The result of this study might add the geological data for limestone in this study area and Wonosari-Punung Formation as well. The petrographical observation and X-ray diffraction results of red limestone indicated the presence of calcite, quartz, siderite, hematite and titanite. Major element analysis of the red limestone showed enrichment of SiO₂, TiO₂, Fe₂O₃ and MnO, which have considered to have relation to the presence of titanite (CaTiSiO₅), siderite (FeCO₃), hematite (Fe₂O₃), and rhodochrosite (MnCO₃) in the red limestone. The minerals are typically yellowish brown, pink to red in colour, and are therefore interpreted to be responsible in giving red colour of the limestone. There are three processes that are considered in the genesis of the Ponjong red limestone, which are: impact of terrigenous material when deposition of the limestone, diagenesis process of meteoric water that enriched Fe₂O₃, and processof hidrotermal fluid activity.