Achmad Bilal Rabbani, BSc

A. Rabbani¹, M. Abdallah¹, R. Nugroho¹, F. Hakim¹

¹Department of Geological Engineering, Universitas Gadjah Mada, Yogyakarta City, Special Region of Yogyakarta

Limestone is an industrial mineral commodity with significant mining potential in Indonesia and relatively easy extractability. However, its exploitation is often unplanned and carried out with limited attention to economic feasibility and overestimation sometimes occurs in resource calculations with arbitrary assumptions without correct geological characterization. To overcome this issue, an accurate resource estimation approach is necessary. One of the most critical aspects in limestone resource estimation is the determination of true bulk density, which is influenced by mineral composition and porosity. This study aims to present a systematic geological approach to resource estimation by calculating true bulk density of limestone, which combines several geological parameters such as physical, geochemical, and petrographic observations. Furthermore, this study attempts to compare this approach with conventional arbitrary calculations to answer the frequent problem of resource overestimation. The research was conducted in a dolostone quarry in Paciran, Lamongan, East Java. The investigation began with fieldwork and collecting samples. These observations were validated by physical tests. Representative samples were then selected for laboratory analysis. Petrographic analysis used to determine mineral composition and porosity, while geochemical analysis using XRF used to validate chemical composition. All of these methods were used to calculate true bulk density. The results revealed six layers consisting of both limestone and calcitic dolostone varieties. XRF showed that five layers identified as calcitic dolostone contained 17.45-18.93 wt% MgO, whereas two layers identified as limestone contained only 0.31-0.42 wt% MgO. XRF data indicated that calcitic dolostone consisted of 80.3-86.9% dolomite, with porosity ranging from 16.54-32.76% and bulk density values of 1.91-2.37 g/cm³. In contrast, limestone with 1.5%-1.9% dolomite displayed porosity of 16.06- 28.62% and bulk density values of 1.94-2.28 g/cm³. Using a block model , the resource potential was calculated for an area of 4,000 m². For the calcitic dolostone layer, with total thickness of 28.5 m, the total resource was estimated at 395,608 tons. In comparison, calculations that are often carried out in Indonesia with the assumption of a dolostone density of 2.5 g/cm3 (realistic) or 2.7 g/cm3 (optimistic) without considering porosity are 436,000 tons and 470,880 tons, representing a difference of 10.2% and 19%. These findings demonstrate that calculating true bulk density is affecting limestone resource estimation. Calculations should therefore employ bulk density values derived from porosity and composition rather than density assumption to achieve accurate and realistic results. By integrating geology, petrographic, and geochemical data, resource estimation can be performed more realistically.