Age and source heterogeneities in the rocks of Lutzkan and Ruy plutons, Bulgaria: some thoughts about their relation to the Aumineralization at “Zlata” deposit

The Lutzkan magmatic complex (LMM) belongs to the Kraishte tectonic zone of Bulgaria. It crops out as large plutons (Lutzkan and Ruy) and few small bodies S and NW from the town of Trun and about 50 km W of the Bulgarian capital Sofia. The rocks of the magmatic complex intrude amphibolite facies metamorphic rocks with presumed Precambrian age and Lower Paleozoic low-metamorphic carbonaceous metasediments, meta-andesite and metabasalts deposited in a deep marine environment. The intrusive rocks of the Lutzkan complex are covered by Permian sediments and overlain and intruded by Paleogene volcanic rocks and dykes. The rock types of Lutzkan pluton are considered to range from gabbros and diorites to leucocratic aplite-granites whereas the granites and granodiorites being the most widespread variety. The Ruy pluton and its vein rocks are mainly granitic in composition. The age of the plutons is considered Ordovician-Silurian or Lower Carboniferous.
   Geochemical studies and U-Pb zircon/titanite conventional (ID-TIMS) and LA-ICP-MS dating of the plutons revealed that the gabbro-diorites of the Lutzkan pluton belong to the basement unit. They are Cambrian in age 537 ± 1.6 Ma (U-Pb zircon dating) with mantledominated island-arc geochemical characteristics (Ta-Nb negative anomaly; εHf-zircon values between +8.9 and +12.4). The rocks are calc-alkaline, metaluminuous, and A/CNK varies between 0.7 – 0.9. They show low fractionation of the REE with LaN/LuN of 4 and a weak Eu anomaly of 0.8. The gabbro-diorites have very low K₂O Rb, Ba, Cs, Sr, Th, U contents and flat HREE distribution, slight enrichment in LILE and LREE. These features are consistent with a subduction-related geodynamic setting.
   The granitoids of Lutzkan and Ruy pluton are dated at 334.1 ± 1.2 Ma by late magmatic titanites and at 332.57 ± 0.60 Ma by zircons, applying the “chemical abrasion” technique and the new double spike solutions of the Earth Time project ET2535. They show distinct geochemical characteristics: negligible or absent Ta-Nb anomaly; εHf-zircon values between +3 and -10. The granitoids of both plutons are mainly high-K calc-alkaline, meta- to peraluminuous, with A/CNK between 0.74 - 1.2, fairly enriched in K₂O, Rb, Ba, Cs, Sr, Th, and with important U content up to 21.4 ppm. Granitoids display fractionated trend of REE, LaN/LuN varying between 13 and 17, and moderate Eu negative anomaly (0.68-0.69). On spidergrams the granitoids show pronounced LILE enrichment. With the transition toward more compatible elements the trend becomes less fractionated. The geochemical characteristics and zircon inheritance imply melting of lower-middle crustal materials with mixed crust-mantle origin. A possible candidate might be the hosting amphibolite facies metamorphic rocks: the differentiated metagranites with MME there are dated at 588.3 ±1.6 Ma by U-Pb zircon method and reveal positive (age corrected) εHf-zircon values between +0.4 and +10.
   Gold has been mined since ancient times in Trun region and has been traditionally related to the Lutzkan and Ruy plutons. The Lutzkan pluton hosts the first gold deposit in Bulgaria, “Zlata”. It was mined from the beginning of the 20^th century by English companies and in 1939-1974 by Bulgarian companies. The EurOmax Company has been exploring a license in the region of the Lutzkan magmatic complex since 2004. EurOmax classifies these deposits as Intrusion Related Gold (IRG) deposits with gold mineralisation related to the CO₂-rich gold bearing fluids produced by the cooling of the intrusion at depth (www.euromaxresources.com/projects). Although the deposit is not directly dated based on the type and the characteristics of the Au-Ag±W mineralisation we suggest a link with the differentiated Carboniferous granitic intrusion of LMM.