CRETACEOUS GARNET GROWTH IN THE IDAHO COBALT BELT: EVIDENCE FROM LU-HF GEOCHRONOLOGY

Garnet geochronology for two samples from the northwest-trending Idaho Cobalt Belt (ICB), east-central Idaho, demonstrates that an episode of Cretaceous garnet growth is preserved in the Mesoproterozoic Apple Creek Formation. A well defined five point garnet-whole rock Lu-Hf isochron from a coarse-grained garnet amphibolite collected from the Salmon Canyon Co-Cu deposit, located at the northwest end of the ICB, yields an age of 112.8 ± 7.7 Ma (MSWD = 9.5) and produces an initial ¹⁷⁶Hf/¹⁷⁷Hf of 0.28312 ± 15 (ε _{Hf (t)} = +14.9). All four garnets analyses from this sample contain moderate amounts of radiogenic hafnium and have ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.284741 to .286143 (ε Hf of +66.5 to +116.1). A less well defined Lu-Hf garnet-whole rock isochron from a fine-grained garnet-biotite schist from the Blacktail pit of the Blackbird Fe-Co-Cu-Au-REE deposit, located at the midpoint of the ICB, produces an age of 151 ± 32 Ma (MSWD = 0.31) with an initial ¹⁷⁶Hf/¹⁷⁷Hf of .28229 ± 4 (ε _{Hf (t)} = -13.7). Little variation exists in the ¹⁷⁶Hf/¹⁷⁷Hf ratios of the three garnet fractions from this sample (0.282445 to 0.28255 (ε Hf -11 to -14.7)) or the ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.05239 to 0.09339). The lack of variation in the ¹⁷⁶Hf/¹⁷⁷Hf ratios may be due to the presence of Hf-bearing inclusions, which may not have been eliminated from the sample during the leaching and dissolution procedures, or may be related to the low amounts of Lu in garnet from this sample (1.5 to 2.2 ppm Lu). Rock fabric and garnet porphyroblast relationships in both samples are complex, but there is textural evidence to suggest that the episode of garnet growth reflected in the Lu-Hf ages was static and was superimposed on a pre-existing rock fabric. The principal ore mineral in these deposits is cobaltite, which occurs both as inclusions within garnet and in the rock matrix (Eiseman, 1988). There is isotopic evidence which indicates that Pb and other metals in the Co-Cu ores were leached from their Mesoproterozoic sedimentary host rocks either by syn-sedimentary basinal brines or by later metamorphic hydrothermal solution, which could have been activated contemporaneously with the generation of garnet during the Cretaceous (Panneerselvam et al., 2004).

Panneerselvam et al., 2004. Abs. w/ Prog. GSA. v 36; 5, p. 517. Eiseman, H.H., 1988. Colorado School of Mines M.S. Thesis.