ELECTRON MICROPROBE DATING OF MONAZITE FROM THE PALEOPROTEROZOIC HOMESTAKE IRON FORMATION AT THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LABORATORY, SOUTH DAKOTA, USA: CONSTRAINING REACTIONS AND EPISODES OF MINERAL GROWTH WITHIN THE HOMESTAKE MINE

Electron microprobe (Ultrachron) analysis of monazite from the Homestake Iron Formation at the Deep Underground Science and Engineering Laboratory (DUSEL), reveals the ages of reactions and several episodes of mineral growth during the complex Paleoproterozoic tectonic history of the Black Hills. Samples from gold mineralized and unmineralized rocks and from different grades across the mine area were investigated. A core domain of ~1848 ± 16 Ma supports previously seen subordinate ages as an indication of growth during the early Trans-Hudson orogeny (THO) in the Black Hills, where terminal Wyoming-Superior collision did not occur until ~1750 Ma. A monazite to allanite reaction texture included in garnet preserves the age of this reaction as 1757 ± 30 Ma, and monazite included in garnet clusters around D₁ at ~1775 Ma. Within the matrix, ~1750 Ma monazite ages correspond to regional D₂. A matrix monazite grain with concentric zonation contains D₁ and D₂-age domains as well as a younger ~1670 Ma domain and later reaction texture, corresponding to Mazatzal-related mineral growth after the terminal Black Hills orogeny and Harney Peak Granite magmatism. No domains have yet been found corresponding to the ~1730 Ma shearing and ~1715 Ma Harney Peak Granite magmatic events, which have been considered upper and lower limits for mineralization and which are represented by a significant monazite population elsewhere in the Black Hills. In several grains exhibiting growth around S₂ textures, ages cluster around ~1300 Ma, which occurs during a record of slow cooling, and may reflect a Middle Proterozoic period of uplift.