Igneous petrology of the Ni-Cu-PGE mineralized Tamarack intrusion, Aitkin and Carlton Counties, Minnesota

Abstract

The Tamarack intrusion is an unexposed mineralized cumulate ultramafic intrusion located near the town of Tamarack, about 80 kilometers west of Duluth, Minnesota. Rio Tinto Exploration (previously Kennecott Exploration) has been conducting exploration drilling of the Tamarack intrusion for Cu-Ni-PGE sulfide deposits since 2001. The intrusion was emplaced into black slates of the Paleoproterozoic Animikie Basin during the early magmatic stage of the 1.1Ga Midcontinent Rift. A new U-Pb baddeleyite age reported here yields an age of 1105.6 ± 1.2 Ma and confirms its association with the Midcontinent Rift. Drilling and geophysical data indicate that the Tamarack intrusion has a tadpole-like shape that is about 13 km long and is between 1 and 4 km wide. The narrow tail area of the intrusion, which is the site of greatest exploration drilling, is composed exclusively of ultramafic rock types. The wider “body” area at the southeastern end of the intrusion is composed of a wider variety of rock types ranging from lherzolite to granophyric gabbronorite. Core logging, petrographic observations, mineral chemical analyses, lithogeochemical analyses, and XRF scanning of drill core were employed on four drill cores from the Tamarack intrusion to evaluate its emplacement and crystallization history. Core logging and petrography show that the lherzolitic cumulates of the tail area can be subdivided into two texturally and modally distinct units – a lower Feldspathic Lherzolite Unit characterized by coarse olivine primocrysts, and an upper Lherzolite Unit characterized by medium-grained olivine. The contact between the two lherzolite units was investigated in three cores. In one core, the contact occurs across a zone of intense alteration; in another, it shows the two lherzolite lithologies irregularly interlayered; and in a third, the feldspathic lherzolite contains gabbroic inclusions of unknown origin. In the drill core from the body area, a lherzolite similar to the upper Lherzolite Unit of the tail area grades upsection to an intergranular olivine websterite, and then to a gabbronorite, which locally contains interstitial to irregular segregations of granophyre. Disseminated Ni-Cu-PGE sulfide mineralization is present throughout most of the lithologies studied here, but is particularly abundant in the tail area at the basal contact of the Feldapathic Lherzolite unit and in a zone straddling the contact between the two lherzolite units. Whereas mineral compositions and whole rock chemical analysis within the Feldspathic Lherzolite and Lherzolite Units found in the tail of the intrusion show little cryptic variation, the chemical attributes of lithologies in the body areas show evidence of extreme differentiation. Olivine composition ranges from Fo84 in the lowermost lherzolite and becomes progressively evolved to Fo10 in the uppermost granophyric gabbronorite. Other mineral and whole rock data both show smooth gradations from the lherzolite to the gabbronorite which are consistent with these lithologies having formed from a single mafic parental magma by fractional crystallization in a closed system. The main petrologic conclusions of this study are: 1) The two lherzolitic units in the tail area formed from a similar high-Mg olivine tholeiitic parent magma. The composition of this parent magma is estimated from subtracting 30% Fo89 olivine phenocryst composition from a chilled margin found at the basal contact of the Feldspathic Lherzolite unit. The resulting composition is comparable to other picritic basalt compositions found at the base of the MCR-related Mamainse Point Volcanics. 2) The emplacement of the lower feldspathic lherzolite preceded that of the upper lherzolite in the tail area. The differences in texture and modal mineralogy between the two lherzolite units are attributed to more rapid cooling of the earlier feldspathic lherzolite, creating an orthocumulate in contrast to the more adcumulate upper lherzolite. 3) The sulfide mineralization straddling the lherzolite contact in the tail area is attributed to country rock assimilation and sulfur contamination in the leading edge of the lherzolite parent magmas during the two main emplacement episodes. The sulfide mineralization in the upper part of the Feldspathic Lherzolite is thought to be related to downward infiltration of sulfide liquid from the overlying Lherzolite unit magma upon its emplacement into the semi-molten core of the Feldspathic Lherzolite. 4) Finally, the well differentiated lithologic sequence comprising the body area is interpreted to have resulted from closed-system fractional crystallization of the second magma pulse that created the upper Lherzolite Unit in the tail area.