Lead Isotope Systematics of the Orogenic Gold Deposits of the Baikal-Muya Belt (Northern Transbaikalia): Contribution of the Subcontinental Lithospheric Mantle in Their Genesis
А. В. Чугаев, В.А. Ванин, И. В. Чернышев, К. Н. Шатагин, Irina V. Rassokhina, A. S. Sadasyuk
Abstract
Orogenic gold deposits associated with metamorphic orogenic fold belts are the major sources of gold currently being mined in the world. The role of metamorphic and magmatic processes and crustal and mantle sources in the gold mineralization genesis are of much debate. The formation of orogenic gold deposit in Precambrian metamorphic belts is usually described by а metamorphic devolatilization model. Some of the orogenic deposits, however, are proved to be much younger than the peak of regional metamorphism. Geological features of these deposits suggest close relation of their gold-forming systems with deeper lithospheric processes. The Baikal-Muya Belt (BMB) of the Central Asian Orogenic Belt is an example of such a region. The BMB comprises ca. 20 orogenic gold deposits and occurrences of Permian age. Gold mineralization at the deposits is usually located in quartz lodes situated in the vicinity of fault zones in association with Late Paleozoic sub-alkaline and alkaline basic and acid intrusions. Using high-precision (0.02–0.03%, 2SD) MC-ICP-MS method, we studied in detail Pb isotopic composition of the Permian gold-bearing mineralization at six BMB deposits, as well as metamorphic and igneous rocks of Tonian to Permian age. The 46 studied sulfide samples representing BMB gold-bearing lodes are highly variable regarding their Pb isotopic composition: 206Pb/204Pb from 16.628 to 18.293, 207Pb/204Pb from 15.325 to 15.614, and 208Pb/204Pb 36.742 to 38.384. Individual Permian orogenic gold deposits and occurrences differ in Pb isotope ratios and degree of their Pb isotopic composition heterogeneity. The Pb isotope features weakly correlate with the geological setting of the deposits. Age-corrected Pb–Pb data for the 41 samples of the BMB metamorphic and igneous rocks are similar to those of the gold mineralization: 206Pb/204Pb from 16.89 to 18.57, 207Pb/204Pb from 15.34 to 15.65, and 208Pb/204Pb 36.67 to 38.4. The high variability of model parameters (µ2 = 9.02–9.78, ω2 = 35.2–42.5, Th/U = 3.84–4.46) and Pb-isotope mixing trends for the ore lead suggest different sources of the metal. One of the sources was the BMB Neoproterozoic metamorphic and magmatic rocks hosting the gold deposits. The obtained data also suggest a common source for these deposits which is identical in its Pb-isotope characteristics to the Permian alkaline and sub-alkaline igneous rocks. We propose that the metasomatized subcontinental lithospheric mantle (SCLM) as well as the SCLM derived alkaline mafic magmas could be the important source of metals (including gold) for the Permian orogenic gold mineralization of the BMB.