Interaction between a Virginia formation xenolith and mafic magma at the Babbitt Cu-Ni deposit, Duluth Complex, MN.

Previous work on the Duluth Complex, Duluth, MN has shown the importance of the interaction between Virginia Formation xenoliths and tholeiitic magmas in the formation of sulfide mineralization. In this study the effects of a single xenolith on the surrounding igneous rocks were examined from drill core in order to assess: 1) in situ introduction of sedimentary sulfur was an important factor in the formation of the massive sulfide layer at the top of the xenolith, 2) if a flow path of a partial melt derived from the xenolith can be determined from major element gradients, 3) if sulfur and oxygen isotopic gradients were preserved at the xenolith contacts, and if so to model their origins using mass balance and diffusion calculations.

Sulfur isotopic values of disseminated and lenticular pyrrhotite in the xenolith vary from 12.4 to 17.4 ‰ with the variation thought to reflect primary layer to layer differences in the protolith (Fig. 1). The massive sulfide at the upper contact of the xenolith is characterized by d34S values of 12.0 to 12.4 ‰. In the igneous rock above the massive sulfide, d34Svalues range from 10.7 to 11.5 ‰. These values are strongly suggestive of a sedimentary source for the sulfur in the sulfide minerals' taken together with the pyrrhotite-rich nature of the sulfide in the d34S values are also consistent with an in situ origin of the massive mineralization at the xenolith border. Andrews and Ripley (1989) detected a negative correlation between whole rock S content and Fe/Mg ratios of orthopyroxene and biotite in Virginia Formation footwall rocks. This relationship was attributed to sulfur fixation in the country rocks and the sulfurization of Fe-bearing silicates. The opposite relationship was observed in ht xenolith from core B1-127, suggesting that sulfurization of Fe-bearing silicates was not a major process and that Fe/Mg ratios of minerals in the xenolith reflect differences in bulk composition between original sedimentary layers. The d34S values of the igneous rocks below the xenolith are very different from those in the igneous rocks above the xenolith, and have a range from 2.8 to 5.2 ‰.

Oxygen isotopic values also show very different trends above and below the xenolith (Fig. 1). d18O values of the xenolith vary from 9.8 to 11.7 ‰, with the lowest value near the upper contact. d18O values of the overlying igneous material gradually decrease to normal values of 6.2 ‰ over a 30 foot interval. A much sharper d18O gradient is found at the lower contact, with d18O values of 5.8 to 6.3 ‰ found in the igneous rocks within a distance of less than 1.5 feet.

Major element analysis of the xenolith indicate that it has undergone extensive partial melting, with the loss of a Si-rich, K- and Na-bearing component. Unmetamorphosed Virginia Formation typically contains between 55 and 65 wt. % SiO2, whereas xenoliths contain from 45 to 50. However, gradients in SiO2 content in the igneous rocks above the xenolith that might track the movement of a partial melt and also explain the d18O gradient are not detected. The absence of SiO2 gradients may be related to: 1) relatively rapid melt extraction and movement into the surrounding magma due to buoyancy, 2) homogenization in the magma due to convection, 3) destruction caused by thermal perturbations resulting from later magma pulses. Modeling of SiO2 transport into the surrounding melt via diffusion alone indicates that an initial SiO2 gradient could be homogenized within a maximum time of 10,000 years. We interpret the d18O profile at the top of the xenolith as being due to retrograde 18O-exchange (i.e. developed after the extraction of a granitic melt from the xenolith and homogenization of the enclosing magma). Diffusion modeling indicates that a retrograde oxygen isotopic exchange profile could be established in as little as 110 to 500 years, assuming an oxygen diffusivity of ~3 x 10-8 cm2/sec.

The very different isotopic profiles developed at the upper and lower contacts of the xenolith correlate with distinct differences between composition and mineralogy of the igneous rocks. The upper unit is marked by higher TiO2 and SiO2 contents, and Fe/Mg ratio relative to the unit below the xenolith (Fig. 2). This unit is an evolved norite to gabbronorite that is the most frequently mineralized rock type in the Partridge River Intrusion. In contrast the lower igneous unit is a more primitive troctolite with only sparse sulfide mineralization. The well developed oxygen and sulfur isotopic profiles in the gabbronorite are indicative of the preservation of a primary xenolith-magma interface. Geochemical data suggest that the troctolitic magma was a later pulse that may have intruded after sulfur and oxygen isotopic equilibration with mafic liquids had been established in the xenolith. Alternatively the troctolitic magma may have delaminated a portion of the Virginia Formation that formed the base of a magma chamber. Because of the thinness of the troctolitic magma sheet isotopic gradients did not develop due to rapid cooling.

Reference:

Andrews, M. and Ripley, E.M., 1989, Mass transfer and sulfur fixation in the contact aureole of the Duluth Complex, Dunka Road Cu-Ni deposit, Minnesota: Can. Mineralogist, v. 27, p. 293-310.

Figure 1. d18O and d34S values through a Virginia Formation xenolith and igneous rocks of the Partridge River Intrusion, drill core B1-127.

Figure 2. Variations in chemical compositions through xenolith and surrounding igneous rocks, Babbitt area, drill core B1-127.

INSTITUTE ON LAKE SUPERIOR GEOLOGY, 43RD ANNUAL MEETING; PROCEEDINGS; Proceedings and Abstracts- Institute on Lake Superior Geology, Annual Meeting; Vol 43, Part 1; p. 5-6