The Ada Tepe deposit: a sediment-hosted, detachment fault- controlled, low-sulfidation gold deposit in the Eastern Rhodopes, SE Bulgaria

摘要

The Ada Tepe gold deposit, 230 km SE of Sofia, formed in the eastern part of the Rhodope Mountains that underwent extension and metamorphic core complex formation, followed by normal faulting, basin subsidence, and silicic to mafic magmatism during the Maastrichtian-Oligocene. The region comprises numerous volcanic-hosted epi-thermal and base-metal vein deposits spatially and temporally associated with the Oligocene magmatism. Ada Tepe is a typical low-sulfidation epithermal gold deposit, unusual in that it is older than adjacent magmatic-related deposits and is hosted in Maastrichtian-Paleocene sedimentary rocks above a detachment fault contact with underlying Paleozoic metamorphic rocks. Gold mineralization is located in: (1) a massive, tabular ore body above the detachment fau and (2) open spacefilling ores along predominantly east-west oriented listric faults. The ores are zones of intensive silicification and brecciation synchronous with detachment faulting. Brittle deformation opened spaces in which bands of opaline silica and electrum, quartz, pyrite, massive and bladed carbonates were deposited. Mineralization is exclusively a Au system with Au/Ag ~3, trace As and no base metals. Alteration consists of quartz, adularia, chlorite, sericite, calcite, pyrite and clay minerals. Adularia and abundant bladed carbonates indicate boiling within the entire span of the deposit, whereas bands of opaline silica with dendritic gold suggest that silica and gold were transported as colloids. The physical setting of formation of the Ada Tepe deposit was very shallow and low temperature. The Sr and Pb isotope ratios of carbonates and pyrite reflect hydrothermal fluid signatures derived predominantly from the metamorphic rocks. The age of mineralization and association with the detachment fault suggest that gold mineralization at Ada Tepe is more closely linked to the Kessebir metamorphic core complex rather than to local magmatism.