Stratiform PGE Mineralization in Tholeiitic Layered Intrusions of the Midcontinental rift, Northeastern Minnesota: Known Examples and Potential Targets James D. Miller, Jr. Minnesota Geological Survey, University of Minnesota, 2642 University Ave., St. Paul, MN, 55114, USA e-mail: mille066@tc.umn.edu Zones of stratiform PGE mineralization (PGE reefs) have recently been identified in two well-differentiated tholeiitic layered intrusions associated with the 1.1 Ga Midcontinent Rift in northeastern Minnesota (Fig. 1). In both intrusions, PGE mineralization is predominantly orthomagmatic and is related to the saturation and segregation of sulfide melt from tholeiitic magmas at medial to late stages of differentiation. These known examples presage the potential for similar mineralization in other Midcontinent Rift-related intrusions. The Sonju Lake intrusion (SLI) is a 1.2- km-thick intrusion that is part of the Beaver Bay Complex (Fig. 1). It displays a very regular, unidirectional cumulus paragenesis and a smooth cryptic variation (Fig. 2) indicative of closedsystem fractional crystallization (Miller and Ripley, 1996). The SLI became sulfide-saturated after about 60% crystallization. Samples stratigraphically below the sulfide saturation level average 2 ppb Pt + Pd. Near the sulfide-saturation level Pt + Pd attains concentrations of at least 390 ppb and above the saturation level, Pt + Pd drops to below 0.2 ppb (Miller, 1999). The chemostratigraphic pattern of Pd (Fig. 2) is very similar to that of the Skaergaard (Anderson et al., 1998) and indicates that PGEs were efficiently scavenged from the magma by the initial saturation and exsolution of sulfide melt. Figure 1. Geology of the Duluth Complex and related rocks of northeastern Minnesota (after Miller et al. 2001). Welldifferentiated intrusions located in explorable areas (outside the BWCAW) are labeled: SLI - Sonju Lake intrusion, DLS - Layered Series at Duluth, BLI - Boulder Lake intrusion, GLI - Greenwood Lake intrusion, OLI - Osier Lake intrusion, CLLS - Cloquet Lake layered series, and HCT - Houghtaling Creek Troctolite.
Figure 2. Chemostratigraphic variations in Pd, Cu, Fo (in Ol) and En' (mg# in augite) through the Sonju Lake intrusion. maaa- meters above cumulus augite arrival. The Layered Series at Duluth (DLS) is a 4.5-km-thick layered intrusion that is part of the Duluth Complex. It too is composed of a unidirectionally differentiated suite of mafic cumulates (Fig. 3), though the transition from troctolitic to gabbroic cumulates is cyclical across a 1-km-thick interval. As many as six macrocycles make up this cyclic zone and imply that despite being well-differentiated, the DLS magma system was open to recharge and venting. The DLS magma reached sulfide saturation near the onset of cyclic zone crystallization. Anomalous S and PGE concentrations (up to 875 ppb Pd+Pt) occur near the upper boundaries of the lowest two macrocycles. The stratigraphic positioning of this mineralization is consistent with its being linked to decompression and devolatilization attending magma venting from a shallow chamber (Miller and Ripley, 1996). These stratiform PGE occurrences are similar to recent discoveries in the Skaergaard and other rift-related intrusions and lend support to the concept that PGE reefs in tholeiitic mafic intrusions constitute a unique type of ore deposit (Nielsen and Brooks, 1995, Prendergast, 2000, Miller and Andersen, this volume). Acknowledging arguments to the contrary (Boudreau and Meurer, 1999), examples of Skaergaard-type PGE reefs (see Miller and Andersen, this volume) have been generally interpreted to form by orthomagmatic processes whereby the saturation, segregation, and settling of sulfide melt from silicate magma cause scavenging of PGE from the magma. As such, the optimal conditions favoring the orthomagmatic formation of economic, stratiform PGE mineralization in any mafic layered intrusion can be simply stated to be: 1) The parent magma is initially sulfideundersaturated. 2) The parent magma has a high initial PGE tenor and/or experiences significant fractional crystallization that increases metal concentrations prior to sulfide saturation. 3) The initial segregation of sulfide melt is triggered by a process (e.g., crystallization differentiation, cumulus phase changes, magma recharge, country rock assimilation, magma venting) that promotes a high R-factor (silicate/ sulfide melt ratio).
Figure 3. Chemostratigraphic variations in Pd+Pt, Cu, mg# in augite and olivine (Fo), and An in plagioclase through the Layered Series at Duluth. Conditions 1 and 2 should be routinely met in tholeiitic mafic intrusions formed in plumeinfluenced, continental rift environments such as the Midcontinent Rift (Nicholson et al., 1997). Barring sulfide contamination en route, such magmas should arrive in the upper crust in a sulfide-undersaturated state because of high degrees of partial melting resulting in complete melting of mantle sulfide (Naldrett and Barnes, 1986) and increased sulfide solubility of decompressing mantle melt during adiabatic ascent (Mavrogenes and O Neill, 1999). A relatively high PGE tenor appears to be a primary characteristic of plumegenerated tholeiitic magmas (Nielsen and Brooks, 1995). The occurrence of most Skaergaard-type PGE reefs in medial parts of intrusions attests to the fact that sulfide saturation can be sufficiently delayed to promote the further enrichment of PGE in the magma. The third condition is the most difficult to predict. Yet, the manner by which sulfide becomes saturated (or oversaturated) in, segregates from, and settles through silicate magma is by far the most critical factor in orthomagmatically forming a stratiform PGE deposit of mineable grade and thickness (Naldrett, 1989). The potential effectiveness of possible saturation triggering mechanisms for tholeiitic intrusions are considered in a companion abstract (Miller and Andersen, this volume). These basic conditions required to generate stratiform PGE mineralization imply that all well-differentiated, tholeiitic mafic layered intrusions associated with the Midcontinent Rift can potentially host Skaergaard-type PGE reefs. Because PGE reefs are inherently low in sulfide (<1%) and are thus difficult to recognize, exploration for stratiform PGE horizons requires systematic geochemical profiling, especially of S, Cu, and PGE concentrations. Parameters such as Cu/Pd can help to indicate where sulfide saturation has occurred and provide a qualitative indicator of the efficiency with which PGE were scavenged from the magma (Maier et al., 1996). It is also important to have a keen understanding of the crystallization history of each intrusion, particularly in magmatic systems that were open to periodic episodes of recharge and venting. For intrusions that fractionally crystallized under generally closed conditions, like the Skaergaard and SLI (Fig. 2), exploration for stratiform PGE mineralization involves a fairly straightforward process of identifying the level at which the magma became initially saturated in sulfide. In open magmatic systems, like the DLS (Fig. 3) exploration should
also focus on locating the horizon of initial sulfide saturation. The first significant sulfide saturation/exsolution event has the greatest chance of encountering the most PGE-enriched silicate magma. In addition to the SLI and DLS, the Midcontinent Rift terrain of northeastern Minnesota contains a number of well-differentiated tholeiitic intrusions that are also inviting exploration targets for stratiform PGE mineralization. Unfortunately most of these intrusions are poorly exposed and exploration requires systematic drilling and perhaps acquiring geophysical data. These intrusions are delineated on a new geologic map of northeastern Minnesota (Miller et al., 2001) and are described in a companion report (Miller et al., 2002). The welldifferentiated intrusions that occur in explorable areas of northeastern Minnesota are noted in Figure 1 and include 1) the DLS-like Greenwood Lake intrusion in the upper central part of the Duluth Complex; 2) the small plug-like Osier Lake intrusion also in the central Duluth Complex; 3) the poorly known Boulder Lake intrusion in the southcentral part of the Duluth Complex; 4) the differentiated cycles of the Cloquet Lake layered series forming the western part of the Beaver Bay Complex; and 5) the differentiated northeastern part of the Houghtaling Creek troctolite macrodike. A geochemical and petrographic study of a series of recently acquired drill cores that profile the Greenwood Lake intrusion is currently underway will be published this summer. References Andersen, J.C.Ø., Rasmussen, H., Neilsen, T.F.D., Rønsbo, J.G., 1998, The Triple Group and the Platinova gold and palladium reefs in the Skaergaard Intrusion: stratigraphic and petrographic relations: Economic Geology, v.93, p. 488-509. Boudreau, A.E. and Meuer, W.P., 1999, Chromatographic separation of platinum-group elements, gold, base metals, and sulfur during degassing of a compacting and solidifying igneous crystal pile: Contributions to Mineralogy and Petrology, v. 134, p. 174-185. Maier, W.D., Barnes, S.J., De Klerk, W.J., Teigler, B., Mitchell, A.A., 1996, Cu/Pd and Cu/Pt of silicate rocks in the Bushveld Complex: implications for platinum-group element exploration: Economic Geology, v. 91, p. 1151-1158. Mavrogenes, J.A., and O Neill, H.S., 1999, The relative effects of pressure, temperature and oxygen fugacity on the solubility of sulfide in mafic magmas: Geochimica et Cosmochimica Acta, v. 63, p. 1173-1180. Miller, J.D., Jr., 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake Intrusion, Finland, Minnesota: Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA, in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Sci., p. 257-301. Miller, J.D., Jr., and Andersen, J.C.Ø., this volume, Attributes of Skaergaard-type PGE reefs. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map M-119, scale 1:200,000, 2 sheets. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 210p. w/ CD-ROM. Naldrett, A.J., 1989, Stratiform PGE deposits in layered intrusions, in Whitney, J.A., and Naldrett, A.J., eds., Ore deposition associated with magmas. Reviews in Economic Geology v. 4, p. 135-165. Naldrett, A.J., and Barnes, S-.J., 1986, The behaviour of platinum group elements during fractional crystallization and partial melting with special reference to the composition of magmatic sulfide ores: Fortschritte der Mineralogie, v. 64, p. 113-133. Neilsen, T.F.D., and Brooks, C.K., 1995, Precious metals in magmas of East Greenland: Factors important to the mineralization in the Skaergaard Intrusion: ECONOMIC GEOLOGY, v. 90, p. 1911-1917. Nicholson, S. W., Shirey, S. B., Schulz, K. J., and Green, J. C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Science, v. 34, p. 504-520. Prendergast, M.D., 2000, Layering and precious metal mineralization in the Rincón del Tigre complex, eastern Bolivia: Economic Geology, v. 95, p. 113-130.