Major oxide, trace element and rare earth element geochemistry of siyah inci marbles (Karakaya, Cüngüş, Diyarbakır)

The Siyah İnci marbles in the study area are located in the Hoya Formation belonging to the Midyat Group, around the Karakaya village of the Çüngüş district in Diyarbakır. Pütürge Metamorphites, Koçali Complex and rocks belonging to the Midyat Group outcrop in the region. Midyat Group consists of Gercüş, Hoya, Gaziantep and Fırat Formation from old to young. The Hoya Formation consists of chalky, dolomitic, clayey and fossiliferous limestones with reef character in places. This formation strikes N40oE and dips to 27o NW. It has color tones ranging from dark gray to black.In the drilling works, the unit has a massive thickness of more than 35 m, and production is carried out on the field with a minimum block thickness of 3 m. Major oxide analyzes of Siyah İnci marbles were made and average CaO 52.2%, CO2 42.2%, SiO2 2.59%, Al2O3 0.73%, Fe2O3 0.54%, MgO 0.92%, Na2O 0.21%, K2O 0.14%, TiO2 0.06%, P2O5 0.26%, It varies as Mn 0.03%, Cr2O3 0.01%. The total carbon content of these samples varies between 10.3-12.8% and the sulfur value between 0.24% and 0.39%. Siyah İnci marbles are very poor in terms of trace elements and no significant trace element enrichment is observed. Accordingly, when Siyah İnci Marble trace elements are compared with the trace element ratios in marine carbonates, it indicates enrichment in terms of V, Co, Ni, Rb, Sr, Nb, Ba, Hf and U, and a depletion in terms of Cr, Y, Zr and Th. Such enrichments also indicate the presence of a partial hydrothermal contribution to the sedimentation basin during the deposition of carbonates. This hydrothermal activity probably highlights that while the main elements such as V, Co, Ni, Rb, Sr, Nb, Ba, Hf and U were in solution in seawater during the carbonate precipitation, this precipitation was accompanied during the precipitation of Siyah İnci marbles. The total rare earth element (REE) content of Siyah İnci marbles ranges from 5.2 to 44.3 ppm (27.9 ppm on average), very close to typical marine carbonate values (28 ppm). When Siyah İnci marbles are normalized to PAAS, it shows a horizontal trend. However, it partially indicates an enrichment for medium (0.91) and heavy rare earth elements (0.87), and a depletion for light rare earth elements (0.44). Siyah İnci marbles show negative Ce and positive Eu and Y anomalies. The (La/Yb)n ratios of Siyah İnci marbles vary between 0.55 and 1.05 (average 0.87), and it is observed that they are more enriched by heavy REE. All samples in the study area have negative Ce anomalies between 0.42 and 0.70 (mean 0.54), as in sea water and autogenic carbonates. Except for two samples, the average Eu values of Siyah İnci marbles are above 1 and show a significant positive Eu anomaly. These anomalies indicate high oxygen fugacity in seawater and the presence of Eu2+ during the precipitation of carbonates. The average Y content in Siyah İnci marbles ranges from 1.90 to 11.5 ppm (average 7.46 ppm). All samples have positive Y anomaly. These results also indicated that the Hoya formation was partially deposited in a shallow marine environment, in an oxygen-rich suboxic zone. Although it is poor in trace elements, the presence of a partially hydrothermal contribution was also effective here. In addition, the high carbon content in these marbles indicates the presence of an intense creature living in a shallow environment. As a result of the decay and corruption of these animals, the intense organic carbon released in sea water precipitated together with the carbonates, causing the formation of Siyah İnci marbles in thick layers.