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MINERAL CHEMISTRY AND SHRIMP U–Pb GEOCHRONOLOGY OF MESOPROTEROZOIC POLYCRASE–TITANITE VEINS IN THE SULLIVAN Pb–Zn–Ag DEPOSIT, BRITISH COLUMBIA

John F. Slack^1,, John N. Aleinikoff², Harvey E. Belkin³, C. Mark Fanning⁴ and Paul W. Ransom⁵

¹ U.S. Geological Survey, National Center, MS 954, Reston, Virginia 20192, U.S.A.
² U.S. Geological Survey, Denver Federal Center, MS 963, Denver, Colorado 80225, U.S.A.
³ U.S. Geological Survey, National Center, MS 956, Reston, Virginia 20192, U.S.A.
⁴ Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 0200, Australia
⁵ 9452 Clearview Road, Cranbrook, British Columbia V1C 7E2, Canada

E-mail address: jfslack{at}usgs.gov

Small polycrase–titanite veins 0.1–2 mm thick cut the tourmalinite feeder zone in the deep footwall of the Sullivan Pb–Zn– Ag deposit, southeastern British Columbia. Unaltered, euhedral crystals of polycrase and titanite 50–100 µm in diameter are variably replaced by a finer-grained alteration-induced assemblage composed of anhedral polycrase and titanite with local calcite, albite, epidote, allanite, and thorite or uranothorite (or both). Average compositions of the unaltered and altered polycrase, as determined by electron-microprobe analysis, are (Y_0.38REE_0.49Th_0.10Ca_0.04Pb_0.03Fe_0.01U_0.01)(Ti_1.48Nb_0.54W_0.04Ta_0.02)O₆ and (Y_0.42REE_0.32Th_0.15U_0.06Ca_0.04Pb_0.01Fe_0.01)(Ti_1.57Nb_0.44W_0.04Ta_0.02)O_b, respectively. The unaltered titanite has, in some areas, appreciable F (to 0.15 apfu), Y (to 0.40 apfu), and Nb (to 0.13 apfu). SHRIMP U–Pb geochronology of eight grains of unaltered polycrase yields a weighted ²⁰⁷Pb/²⁰⁶Pb age of 1413 ± 4 Ma (2) that is interpreted to be the age of vein formation. This age is 50–60 m.y. younger than the ca. 1470 Ma age of synsedimentary Pb–Zn–Ag mineralization in the Sullivan deposit, which is based on combined geological and geochronological data. SHRIMP ages for altered polycrase and titanite suggest later growth of minerals during the ~1370–1320 Ma East Kootenay and ~1150–1050 Ma Grenvillian orogenies. The 1413 ± 4 Ma age for the unaltered polycrase in the veins records a previously unrecognized post-ore (<1470 Ma) and premetamorphic (>1370 Ma) mineralizing event in the Sullivan deposit and vicinity. The SHRIMP U–Pb age of the polycrase and high concentrations of REE, Y, Ti, Nb, and Th in the veins, together with elevated F in titanite and the absence of associated sulfides, suggest transport of these high-field-strength elements (HFSE) by F-rich and S-poor hydrothermal fluids unrelated to the fluids that formed the older Fe–Pb–Zn–Ag sulfide ores of the Sullivan deposit. Fluids containing abundant REE, HFSE, and F may have been derived from a geochemically specialized magma such as those that form alkaline granites, pegmatites, or carbonatites. In an alternative model, preferred here, these fluids were associated with a rift-related, crustal metasomatic event in the region. Determination of a Mesoproterozoic age for the polycrase–titanite veins establishes the first known occurrence of pre-Grenvillian REE-rich mineralization in the Belt–Purcell basin.

Keywords: polycrase, titanite, veins, Sullivan deposit, Mesoproterozoic age, U–Pb geochronology, electron-microprobe compositions, genesis, British Columbia.