Minnesota’s PGE potential

Exploration for polymetallic deposits, including platinum group elements, in Minnesota has traditionally focused on mafic intrusions of the Mesoproterozoic Duluth Complex, as it contains known occurrences of PGEs associated with copper-nickel deposits that were discovered decades ago. By contrast, a new study, Geology, Geochemistry and PGE Potential of Mafic-Ultramafic Intrusions in Minnesota, Excluding the Duluth Complex, soon to be released by the Minnesota Geological Survey (MGS), investigates the many smaller intrusions and Archean layered mafic complexes scattered throughout the remaining four-fifths of the state.

The study contains location, lithologic and geochemical data from nearly 200 intrusions, as well as newly acquired assay results from 149 samples. The study is the first phase of an ongoing project funded by the Minnesota government.

The study involved compiling both written and geographic-information-systems-based inventories of mafic, ultramafic and alkalic intrusions using all available information on outcrops, drill holes and geophysics. The inventory of intrusions was augmented by conducting core logging, outcrop mapping, petrographic studies, geochemical analyses, and geophysical modeling on select intrusions. In addition, a small geochronologic program using 40Ar/39Ar analyses of magmatic hornblende and biotite separates has been conducted in order to constrain the general temporal framework of some intrusions and thereby contribute to mineral deposit modeling.

The accompanying picture shows the geologic setting of major mafic intrusions and intrusive complexes known to exist in Minnesota. Although uranium-lead-zircon geochronological data are sparse, the intrusions are inferred to range in age from Archean to Mesoproterozoic. This inferrence is based on emplacement relationships deduced from fieldwork and geophysical signatures, and from seven new forms of Argon analyses.

Many of the intrusions were discovered in the ongoing process of geologic mapping by the MGS and variously described in published maps, information circulars, and reports. Others were discovered by exploration companies in search of base metal or other mineral deposits. This pattern of discovery resulted in a reasonably thorough database of intrusions and associated sample materials, but scant details pertinent to their potential for PGEs. Indeed, prior to this endeavour, few of the intrusions had been thoroughly described and analyzed, and almost none had been analyzed for PGE content.

Some of the intrusions are lithologically similar to the so-called “Quetico” and “Atikokan” intrusions that are the targets of exploration in Canada. For example, the productive Lac des les PGE-copper-nickel mine, north of Thunder Bay, Ont., lies within one such intrusion. The geological settings of significant PGE deposits world-wide include mafic to ultramafic plutons associated with volcanic arc terranes in linear orogenic belts (Ural-type), differentiated sill complexes, ophiolitic complexes, syn- to late-orogenic layered complexes, and alkalic-to-subalkalic composite plutons. This project characterizes the various types of mafic intrusions in Minnesota in an effort to understand the state’s resource potential, and to encourage exploration for PGEs in similarly diverse terranes throughout the state.

The report contains an inventory of intrusions that are either exposed in outcrop or represented by archived drill core. It also includes facts from some of the thousands of diabasic dykes and other geophysical anomalies inferred to be mafic intrusions.

The project work conducted on each intrusion varied with the level of detail that existed but included some combination of geophysical delineation, outcrop mapping, drill core examination, petrography, and geochemical analysis.

The intrusions can be grouped into several major types, primarily according to the age of host rocks into which they were emplaced, and secondarily on the bases of perceived temporal, mineralogic, geophysical and geometric attributes.

The classification is as follows:

Archean host rocks, northern and western Minnesota:

— Layered mafic-ultramafic complexes.

— Subvolcanic mafic sills.

— Amphibolitic sills and dykes.

— Lamprophyric, pyroxenitic, peridotitic, and hornblendic intrusions.

— Sanukitoid composite stocks containing mafic and ultramafic enclaves.

— Gabbroic intrusions.

— Tonalite-diorite intrusions.

Paleoproterozoic and Archean host rocks, southern and eastern Minnesota:

— Gabbroic intrusions.

— Pyroxenite and peridotite plugs and stocks.

— Hornblendite, diorite, and granodiorite.

— Diabasic to gabbroic dykes (likely Paleoproterozoic in age).

— Diabasic to gabbroic dykes and irregular plugs (likely Mesoproterozoic).

Whole-rock and trace-element analyses and metallic element assays were acquired on 149 samples representing various intrusion types in an effort to characterize the intrusions and identify anomalous metal contents.

It should be noted that metallic assay results represent only a cursory geochemical evaluation. Nevertheless, the assay results show that values above background (10-30 parts per billion) are present in virtually every type of intrusion. The more anomalous results (less than 30 ppb combined platinum-palladium-gold) come largely from metagabbroic rocks (subvolcanic sills) and other gabbro intrusions in Archean rocks of the Wabigoon and Wawa subprovinces, and from mafic enclaves in alkalic plutons. Maximum values, in ppb, obtained are 84 platinum, 688 palladium, and 2,399 gold. The largest single content of combined platinum and palladium is from the previously explored Winterfire gabbroic intrusion in the Rainy Lake area of Wabigoon subprovince.

The report demonstrates that a number of intrusions contain PGE contents that are above background levels, some significantly so. Many of these deserve further investigation; however, intrusions that did not produce anomalous values cannot be summarily excluded from additional efforts (this phase of study was superficial). Furthermore, many of the intrusions are represented by the content of only a single drill core (or relatively few drill cores) and a few outcrop samples. Additional study of available data, acquisition of new outcrop and drill core samples, and inevitably drilling are necessary in order to define zones of potential mineralization, and also to conduct the more rudimentary definition of intrusion and phase boundaries, phase relationships, layering, and mineralogic content. Some of this work (excluding new drilling) will be conducted during the second phase of this study, which will be published in July 2005.

Free copies of the report can be obtained after Sept. 30 by visiting www.geo.umn.edu/mgs/

— The author is a geologist with the Minnesota Geological Survey.