The Babbitt Copper-Nickel Deposit: Part B: Structural Datums

Abstract

This portion of the investigation concentrated on determining specific structural and stratigraphic datums within the Babbitt Cu-Ni deposit of the Partridge River Intrusion, Duluth Complex, northeastern Minnesota. Structural datums and footwall lithologies were obtained by relogging the footwall portions of all surface drill holes (391) within the confines of the deposit. Specific lithologies recorded during relogging included depth to: 1) basal contact of the Duluth Complex with the footwall rocks; 2) distinctive units within the footwall Virginia Formation; and 3) top of the underlying Biwabik Iron-formation and the specific iron-formation submembers intersected in drill hole. In addition, 100 surface drill holes were relogged in detail and compared with 166 previously relogged drill holes. The top of Unit I, the main sulfide-bearing igneous unit of the Babbitt deposit, was determined for all 266 drill holes that were relogged. The top of the first significant sulfide-bearing zone (not always the top of Unit I) was also determined for the relogged drill holes. All these datum points are used to generate several contoured surface maps to gain a better understanding of the main structural features present within the Babbitt deposit. The contoured surface for the top of the Biwabik Iron-formation is an excellent means of displaying the major structural features, i.e., the Local Boy Anticline, Bathtub Syncline, and Grano Fault. These same features are evident in the contoured surface of the basal contact and indicate that pre-existing structures in the footwall were important to how the basal portion of the Duluth Complex was emplaced. Datum points within the upper portion of the Duluth Complex also suggest that these structures were reactivated throughout the emplacement history of the Duluth Complex (see discussion in Part C). Some of the structures were also important controls of Cu-Ni mineralization. A contoured surface of the bedrock ledge is also presented for the Babbitt deposit. Several structural features are outlined by the bedrock ledge. A crude subsurface geologic map is also portrayed for the bedrock ledge. In addition, an isopach map of glacial overburden thickness is included in this report.