Geology and Mineralization of the Serpentine Copper-Nickel Deposit

Abstract

A geological model explaining the formation of mineralization at the Serpentine Cu-Ni deposit, located at the western edge of the Keweenawan (1.1 Ga) Duluth Complex near Babbitt, Minnesota, has been developed following detailed re-logging of drill core and compilation of geochemical (Cu-Ni-S) data. The model suggests that sulfide mineralization was influenced by a combination of factors, the most important being localized structural preparation of the Lower Proterozoic (1.8 Ga) Virginia Formation footwall. The drill core provides evidence that faulting and the emplacement of sills (both pre-and/or early-Duluth Complex) were responsible for this structural preparation. Both processes led to weakening and/or fracturing of the footwall, thereby allowing greater magma/footwall interaction to take place in a relatively confined area. This intimate interaction enhanced the genesis and distribution of Cu-Ni sulfide mineralization at the Serpentine deposit. A laterally extensive bedded pyrrhotite unit of the Virginia Formation is believed to have further contributed to the formation of semi-massive to massive sulfides present within the deposit by acting as a local source of additional sulfur (and minor additional Cu and Ni). Additional sulfur isotope work is necessary before an in situ sulfur source can be ruled out, however. A very general igneous stratigraphy has been determined for the Serpentine deposit, based on the idea that at least three distinct magmatic events and/or intrusive styles were responsible. The first (and earliest) contributed to the deposit's sulfide mineralization, due to its extensive interaction with the footwall; its rocks are a heterogeneous, gradational mixture of fine- to medium-grained troctolite, augite troctolite, and norite, loosely confined to the lowest one-half to two-thirds of the deposit. The second event produced rocks of greater homogeneity that are more plagioclase-rich, coarser grained, and relatively sulfide-poor; spatially, they tend to overlie those of the first. A third, but much less volumetrically significant, magmatic event and/or intrusive style probably occurred shortly after the second and was likely related to a faulting episode; it formed rocks that are predominantly ultramafic. Spatially, these ultramafic rocks are largely associated with the more homogeneous plagioclase-rich rocks, frequently occurring as linear sub-vertical bodies. Analytical work shows the platinum group element (PGE) potential of the Serpentine deposit is somewhat limited. The highest value (274 ppb rhenium) occurred in a massive sulfide sample. Only three samples contained more than 100 ppb palladium. Based on these results, the mechanisms and/or conditions for PGE enrichment that were present at the Local Boy deposit were lacking at the Serpentine deposit. A possible explanation is that the Serpentine deposit received additional sulfur from a local source, while the Local Boy deposit formed from a sulfide melt that: 1) formed elsewhere; 2) scavenged more PGEs from a more primitive(?) magma; and 3) moved an undetermined distance before being injected into structure zones, without the addition of local sulfur. Future investigations may shed more light on the apparent differences between these two deposits.