Movement of titanium across the Duluth Complex - Biwabik Iron Formation Contact at Dunka Pit, Mesabi Iron Range, northeastern Minnesota

Abstract

In the north half of the Dunka open pit iron ore mine (T60N-T61N, R12W) of northeastern Minnesota, the Biwabik Iron Formation (BIF) is in direct contact with the South Kawishiwi Intrusion (SKI) of the Duluth Complex. Progressive contact metamorphism of the BIF originally was thought to be isochemical (Bonnichsen, 1968), however this study presents evidence that titanium was introduced into the BIF from the SKI. Field and petrographic observations show that within 25 feet of the SKI the BIF has been highly altered. Textures include: (1) ilmenite intergrown in magnetite layers of the BIF, (2) layers of plagioclase, clinopyroxene, orthopyroxene and other commonly magmatic minerals in the BIF, and (3) symplectite of plagioclase-orthopyroxene and orthopyroxene- Fe-Ti oxide. Ilmenite forms partial pseudomorphs after magnetite in the altered BIF, as well as lamellae and rims around magnetite. Layers of magmatic minerals which are not found elsewhere in the iron formation in any abundance appear to be intergrown in magmatic textures. Within 25 feet of the iron formation, the SKI is also considered altered on the basis of: (1) increased amounts of magnetite, clinopyroxene, orthopyroxene, and amphibole, (2) loss of olivine, and (3) increased amounts of symplectite of plagioclase-orthopyroxene and orthopyroxene-Fe-Ti oxide. Magnetite in the altered SKI occurs in similar textures to the most altered BIF, with ilmenite partially forming pseudomorphs after magnetite. Loss of olivine and gain of pyroxene may be explained by the assimilation of quartz, which reacted with the olivine in the SKI magma to form pyroxene. Symplectites of orthopyroxene-plagioclase and orthopyroxene-Fe-Ti oxide are also present in the unaltered SKI, but in the altered SKI their abundance is much greater. Magnetite from altered BIF is higher in TiO2 (3.15 wt.% on average) than that in unaltered BIF (0.29 wt.% on average). Composition of titaniferous magnetite and ilmenite from both the altered BIF and SKI were used to calculate values of temperature of metamorphism/ intrusion and fO2 using the QUILF PASCAL program (Andersen, Linds.ley and Davidson, 1992). Values on a log fO2 - temperature plot appear above the fayalite-magnetite-quartz buffer curve, in the range of 550°C to 689°C and -15.2 to -18.8 log fO2 for both the most altered SKI and BIF. Whole rock analyses of altered BIF show gains of TiO2, V, Al2O3, CaO, Na2O, K2O, Ba, Rb, Sr, MgO, Cu, Ni, and H2O content, and loss of SiO2 and P2O5 compared to altered BIF. Present elevated amounts of ilmenite, plagioclase, sulfides, and actinolite correlate with these gains. Loss of SiO2 from the altered BIF correlates with the loss of quartz from the most altered BIF via assimilation into the SKI. Chemical components gained to the altered SKI include K2O, Rb, S, Fe2O3, and H2O. Gains in K2O, Rb and H2O correlate with increased biotite and amphibole content in the most altered SKI, especially hornblende, which is hydrous and contains some potassium. Ferric iron gained to the most altered SKI represents a change in oxidation state of the magma due in part to assimilation of some of the magnetite in the iron formation. Loss of TiO2 and MnO appear to be related to the formation of ilmenite in the most altered BIF. Oxygen isotope studies show that the altered BIF contains quartz which is lower in δ18O (δ18O = 10.82 parts per thousand) than quartz in altered BIF (δ18O = 15.21 parts per thousand, Perry and Bonnichsen, 1966). The altered SKI shows greater δ18O (7.00 to 9.40 parts per thousand) than the altered SKI (5.94 to 6.57 parts per thousand). Fingers of SKI magma assimilated quartz from the BIF, enriching the altered SKI in 18O, K2O, Fe2O3 , and LOI, whereas depleting the BIF of SiO2. Layers of magnetite in the altered BIF have been enriched in Ti, while preserving the original layering, indicating some diffusion or infiltration of titanium into the magnetite has occurred. Combination of infiltration of "fingers" of SKI magma into the BIF, and enrichment of BIF magnetite in titanium occurs only where the SKI is in direct contact with the BIF. None of the above textures, changes in mineralogy, or titanium enrichment occur where the Virginia Formation lies between the Duluth Complex and the Biwabik Iron Formation. Thus, textures, changes in mineralogy, and titanium enrichment in the most altered BIF must be caused by direct contact with the SKI of the Duluth Complex.