Mineralogy, petrography, geochemistry and economic potential of carbonate rocks of Sri Lanka

Abstract

Sri Lanka comprises different paragenetic types of carbonate rocks such as Miocene limestone, crystalline limestone (marble), carbonatite and dyke like carbonate bodies. The purpose of this paper is to summarize documented studies on petrogenesis and economic potential of crystalline limestone, carbonatite and dyke-like carbonate bodies. Pure marbles, which are confined to the Highland Complex (HC) of the country are composed mainly of dolomite, while carbonatite occurrences at Eppawala consist mainly of calcite. Monomineralic calcite bodies with trace amounts impurities are found in the Southeastern part of the country. Detailed geochemical, stable isotope and petrological studies on marbles of HC have revealed that they have formed by granulite grade metamorphism of dolostones deposited in an open ocean. Pure marbles basically retain their primary isotopic signatures with only their  18 O values amenable to alteration due to 18 O/ 16 O exchange with percolating waters. Over a limited distance, geochemistry of contact zones of the marbles has been changed significantly due to metamorphic geochemical alterations. Eppawala carbonatites are unique among the other carbonatites in the world in terms of field setting, isotope and elemental geochemistry and textural features of carbonate minerals. Trace and rare earth element composition as well as stable isotope ratios from the Eppawala carbonatites are not comparable with most of carbonatites in the world. But they are within the broader range of carbonatitic rocks and do not show any meta-limestone signatures. Textural features and chemical composition of carbonate minerals suggest that present carbonatite bodies have been dislocated from the emplaced positions in the crust due to deformations experienced by the country rocks. Geochemically and petrographically the dyke-like carbonate bodies differ significantly from both carbonatites and crystalline limestone. Concentrations of Sr and Rare Earth Elements (REEs) of dyke-like bodies lie between those of carbonatites and marbles. However, the concentrations are much lower than those of carbonatites. Carbon and oxygen isotope compositions are similar to those of most meta-sedimentary carbonates suggesting that the carbonate dykes were formed as melts from marbles at crustal levels due to shearing and thrusting of HC and Vijayan Complex (VC) plates. Laboratory experiments carried-out in the recent past were able to synthesize value-added products such as Precipitated Calcium Carbonate (PCC), magnesium hydroxide [Mg(OH) 2 ], magnesium oxide (MgO), their nanoparticles, their polymer-nanocomposites, Amorphous Calcium Carbonate (ACC) and poly(acrylate)(PA-)-encapsulated Mg(OH) 2 ; Hollow Calcium Carbonate (HCC), hydrophobic PCC and hydrophobic Mg(OH) 2 from carbonate rocks of Sri Lanka. Results of those studies suggest that there is a high possibility to synthesize nanoparticles and nanocomposites of PCC, Mg(OH) 2 and MgO using Sri Lankan carbonate rocks with appropriate quality to meet industrial requirements and to fulfi...

Boron isotope compositions establish the origin of marble from metamorphic complexes: Québec, New York, and Sri Lanka

“…An analogous geological situation is present in Sri Lanka, where multi-colored, calciterich marble (term also adopted here), skarn, or dyke-like units occur with an unclear genesis, whether derived from remobilized marble or carbonatite-related fluids (Madugalla et al 2013;Madugalla and Pitawala 2015;Pitawala 2019). Previous work done on these carbonate units in Sri Lanka have mainly focused on the economic viability of these deposits (i.e., lime and carbonate-derived products; Mantilaka et al 2013Mantilaka et al , 2014a, however, a few studies contain information on their geochemical characteristics (e.g., mineralogy, stable isotopic signatures, and trace element abundances; Madugalla et al 2013;Madugalla and Pitawala 2015;Pitawala 2019). Pitawala (2019) suggested that their formation is due to marble remobilization as a result of shearing related to the assembly of Gondwana.…”

“…Boron-related investigations of geological samples are advantageous for many reasons, including its use as a paleo-pH proxy in marine carbonates (e.g., Deegan et al 2016;Rasbury and Hemming 2017;Rae 2018), isotopic sensitivity, especially in fluid-mediated processes (e.g., Spivack and Edmond 1987;Lemarchand et al 2000Lemarchand et al , 2002Gaillardet and Lemarchand 2018;De Hoog and Savov 2018), and limited isotopic fractionation during high-temperature metamorphism and/or hydrothermal activity associated with mantle-derived carbonates (e.g., Çimen et al 2018, 2019Kuebler et al 2020). Boron is a widespread trace element in natural carbonates (~1 to 100 ppm; Kowalski and Wunder 2018 and references therein); its incorporation depends on the conditions of the precipitating fluid (e.g., pH, temperature), the presence of other elements (e.g., Mg, Sr), and the type of carbonate present (biogenic or inorganic; Hemming and Hanson 1992;Hemming et al 1995;Sanyal et al 2000;Penman et al 2013;Rasbury and Hemming 2017;Sutton et al 2018).…”

“…In recent studies, boron has proven to be a powerful diagnostic tool even in complicated geologic settings. For instance, it has been possible to elucidate the mantle source region(s) of the Miaoya and Bayan Obo carbonatite complexes in China despite extensive hydrothermal activity or high-grade metamorphism or both (Çimen et al 2018, 2019Kuebler et al 2020). Furthermore, combining boron isotope values with C, O and Sr isotope compositions has established an effective means for identifying pristine (unaltered) mantle-derived carbonate samples that can then be used to decipher the chemical nature of their upper mantle source regions (Hulett et al 2016;Çimen et al 2018, 2019Kuebler et al 2020).…”

“…Overall, the marbles occur as layers or intercalations (10 -20 km along strike) with pelitic gneiss and mafic granulites that have been deformed, metamorphosed, and recrystallized under granulite facies P-T conditions and thus obscuring primary sedimentary features (Cooray 1984;Osanai et al 2000Osanai et al , 2006Pitawala 2019). The marble layers in the SW region of Sri Lanka trend northwesterly and have gneissose banding with both silicate-rich and pure marble layers (Pitawala 2019). The samples of This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…Previous work done on these carbonate units in Sri Lanka have mainly focused on the economic viability of these deposits (i.e., lime and carbonate-derived products; Mantilaka et al 2013Mantilaka et al , 2014a, however, a few studies contain information on their geochemical characteristics (e.g., mineralogy, stable isotopic signatures, and trace element abundances; Madugalla et al 2013;Madugalla and Pitawala 2015;Pitawala 2019). Pitawala (2019) suggested that their formation is due to marble remobilization as a result of shearing related to the assembly of Gondwana. In order to provide new insights into the provenance and formation model of the carbonate units in both the Grenville Province and Sri Lanka, this study investigates a suite of multi-colored carbonate-rich rocks from these regions This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”