2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 7
Presentation Time: 9:30 AM


SLACK, John F., U.S. Geol Survey, MS 954, Reston, VA 20192, ALEINIKOFF, John N., U.S. Geol Survey, MS 963, Denver, CO 80225, BELKIN, Harvey E., U.S. Geol Survey, MS 956, Reston, VA 20192 and RANSOM, Paul, 9452 Clearview Rd, Cranbrook, BC V1C 7E2, Canada, jfslack@usgs.gov

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.