RI-51 Geologic Setting and Descriptive Geochemistry of Archean Supracrustal and Hypabyssal Rocks, Soudan-Bigfork Area, Northern Minnesota: Implications for Metallic Mineral Exploration

Abstract

Geochemical data are presented for a suite of samples chosen to represent as nearly as possible the compositions of unaltered rocks within the Archean greenstone belt of the Soudan-Bigfork area in northern Minnesota. These data are interpreted within the stratigraphic and structural framework provided by geologic mapping, and are used to deduce probable depositional and tectonic environments for the volcanic and sedimentary sequences described. In addition, the data indicate regional differences in intrinsic mineral potential, as judged from the presence or absence of geochemically favorable rock types associated with economic deposits of metallic minerals elsewhere in the Superior Province. The greenstone belt as a whole has attributes consistent with development in an oceanic volcanic-arc setting that is broadly analogous to modern arcs of the western Pacific basin. Supracrustal rocks in the Soudan belt (southern portion of the Soudan-Bigfork area) include a lower volcanic cycle that consists of the upper parts of a calc-alkaline volcanic edifice which evolved with time from a submarine basaltic pile into an emergent or nearly emergent dacitic eruptive center, and an upper cycle that represents submarine fan deposits mingled in time and space with tholeiitic basalt of possible back-arc origin. The Newton belt (northern portion of the Soudan-Bigfork area) consists of several fault slices made up chiefly of tholeiitic basalt and pyroclastic and epiclastic dacitic rocks. Because of structural shuffling and imbrication, the rock sequences within the Newton belt cannot be reliably correlated from slice to slice, which handicaps paleotectonic interpretation. Geochemically they are consistent with a back-arc origin. Basaltic sequences in the Soudan belt are evolved to iron-enriched compositions and exhibit very low background values for gold. Basaltic sequences in the Newton belt are moderately more magnesian and auriferous, and therefore may be somewhat better candidates for harboring economic gold deposits. The dacitic rocks of the Soudan belt fall into the least favorable geochemical category for hosting massive- sulfide deposits, based on empirical associations throughout the Superior Province, whereas the geochemical attributes of dacitic rocks in the Newton belt remain inadequately characterized in this respect. Differentiated gabbro-pyroxenite-peridotite sills are absent from the Soudan belt but are characteristic of the Newton belt. Peridotite sills within the Deer Lake Complex contain subeconomic amounts of magmatic Cu-Ni sulfide minerals and are petrologically reasonable, but untested, candidates for hosting platinum-group minerals. On the basis of these broad lithogeochemical criteria, the mineral potential of the Newton belt would appear to warrant serious further investigation.