STRUCTURAL EVOLUTION OF GOLD-BEARING COBALT-COPPER DEPOSITS OF THE BLACKBIRD MINING DISTRICT, EAST-CENTRAL IDAHO

Structural evolution of sediment-hosted, Au-bearing, Co-Cu deposits of the Blackbird mining district is elucidated by regional studies integrating the Mesoproterozoic depositional setting and stratigraphy with the subsequent tectonic history. Stratigraphic revisions show that the Blackbird deposits are unique to thick turbiditic, Fe-enriched successions of the Mesoproterozoic banded siltite unit of the Apple Creek Formation, upper part of the underlying coarse siltite unit of the Apple Creek, and basal part of the overlying Gunsight Formation. Regionally, exposure of mineralized host rocks is controlled by parallel northwest-trending thrust faults that are dated at 83 Ma by 40Ar/39Ar cooling data on white mica from cleavage. The Blackbird district lies within the Blackbird Mountain oblique thrust ramp where locally intense deformation resulted in three imbricate subplates that are stacked in inverted metamorphic sequence. Subsequent Eocene normal faults juxtaposed different structural levels side-by-side.

(1) The Indian Creek subplate is structurally highest, composed of compositionally layered chloritoid-garnet-biotite schist that reached 400-520o C and contains lowest Cl and K2O values. Mineral deposits are in sheared quartz-rich zones and contain relatively low sulfide and high As and Au. (2) The structurally central Blackbird subplate is characterized by bedded biotite phyllite and transposed biotite schist that was metamorphosed at 250-350o C. These rocks are characterized by intermediate Cl and K2O values. Co-Cu sulfides are in layered packages, quartz veins in fold hinge and shear zones, and ductility-contrast durchbewegung zones. Relative proportion of Co:Cu decreases and secondary sulfide mineral increases with greater evidence of metamorphism and deformation. (3) The structurally lowest Haynes-Stellite subplate has the youngest strata. These rocks are gently folded, lower greenschist facies, bedded biotite phyllite having highest Cl and K2O values. These contain high Co:Cu ratio and little evidence of secondary sulfide mineral formation.

Variations in metamorphic character, sulfide mineral paragenesis, and halogen contents in individual structural domains at Blackbird are products both of different mineral deposit origins and of heterogeneous tectonic processes.