Laser ablation ICP-MS titanite U-Th-Pb dating of hydrothermal ore deposits: A case study of the Tonglushan Cu-Fe-Au skarn deposit, SE Hubei Province, China

摘要

Titanite from the Tonglushan Cu-Fe-Au skarn deposit. Daye district, eastern China, has been successfully dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Hydrothermal titanite from both skarn ores and calcite veins cutting the ore bodies are characterized by having much lower REE and Th/U and Lu/Hf ratios compared to their magmatic equivalents from the ore-related Yangxin quartz diorite intrusion and a late-stage albitite dike. Titanite from the Yangxin quartz diorite and an albitite dike, which intrudes both the quartz diorite intrusion and ore bodies, yield weighted mean Pb-206/U-238 ages of 136.0 +/- 1.5 Ma and 120.6 +/- 2.3 Ma (2 sigma), respectively, in agreement within analytical uncertainty to their Pb-208/Th-232 ages. In situ analysis of epidote-enclosed hydrothermal titanite in thin sections of a skarn ore sample yields a mean Pb-206/U-238 age of 135.9 +/- 1.3 Ma and Pb-208/Th-232 age of 138.2 +/- 4.5 Ma, whereas titanite in calcite from a calcite-dominated vein cross-cutting the skarn ore body has consistent Pb-206/U-238 and Pb-208/Th-232 ages of ca. 121 Ma. The data thus reveal two independent hydrothermal events. The earlier event was responsible for the known Cu-Fe-Au skarn mineralization at Tonglushan and throughout the Daye district, and was caused by coeval magmatism that has resulted in the emplacement of the Yangxin quartz diorite. The later event, previously unrecognized, was associated with a second magmatic pulse as represented by the albitite dikes, and provides potential for discovering new mineral deposits at Tonglushan and possibly over the Daye district. This study represents the first direct dating of skarn deposits using titanite U-Th-Pb geochronometer and signifies the utilization of titanite for precise dating of hydrothermal ore formation. (C) 2009 Elsevier B.V. All rights reserved.