MESOPROTEROZOIC POLYCRASE-TITANITE VEINS IN THE SULLIVAN PB-ZN-AG DEPOSIT, SOUTHEASTERN BRITISH COLUMBIA: MINERAL CHEMISTRY, SHRIMP GEOCHRONOLOGY, AND METALLOGENIC SIGNIFICANCE

Planar polycrase-titanite veins up to 1 mm thick occur locally in the deep footwall of the Sullivan Pb-Zn-Ag deposit, British Columbia. The veins cut fine-grained tourmalinite 350 m below the massive sulfide ores of the vent complex and have alteration envelopes composed of zoned tourmaline aggregates. Subhedral polycrase grains (50-150 µm) contain both unaltered, oscillatory-zoned domains and altered, irregularly-zoned domains, all of which are variably replaced by titanite+albite+calcite +chlorite±allanite±epidote±thorite±pyrite±rutile. Galena occurs rarely as thin (<3 µm) fracture fillings in polycrase and titanite. Electron microprobe (EMP) analyses of the unaltered polycrase [AB₂O₆] indicate that the A site has high average Y (0.38 apfu), total REE (0.49 apfu), and Th (0.11 apfu), with low Ca (0.04 apfu) and U (0.01 apfu); the B site has high average Ti (1.47 apfu) and Nb (0.55 apfu), with low W (0.04 apfu) and Ta (0.02 apfu). The altered polycrase has higher U and Th and lower Ti. EMP data for vein-related tourmaline aggregates show dravite to Mg-schorl compositions very similar to those of disseminated tourmaline in the matrix of the host tourmalinite, suggesting that the vein-forming fluids were not B-rich and only recrystallized preexisting tourmaline in the adjacent wall rock.

Preliminary Pb-Pb SHRIMP dating yields ages of 1415±5 Ma for the unaltered polycrase and about 1400-1280 Ma for the altered polycrase. Limited SHRIMP ages for the titanite are 1.4-1.3 Ga and 1.0 Ga. The ages for the altered polycrase and titanite are consistent with mineral growth during the 1370-1320 Ma East Kootenay and the 1100-1000 Ma Grenvillian orogenies, both of which have been documented in the area by other geochronological studies. However, the 1415±5 Ma age for the unaltered polycrase records a previously unrecognized postore (<1470 Ma) and premetamorphic (>1370 Ma) hydrothermal event in the Sullivan deposit and vicinity. The distinctive Y-REE-Th-Ti-Nb signature of this event suggests petrologic affinity to Mesoproterozoic alkaline magmatism and/or carbonatites. Occurrence of these veins may also be related to regional-scale Mesoproterozoic faults like those elsewhere in the Belt-Purcell basin that are at or near large Th-rich veins such as Hall Mountain in northern Idaho and Lemhi Pass on the Idaho-Montana border.