T87 Geology and Mineral Deposits of the Midcontinent Rift I

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 101H-J

EMPLACEMENT AND CRYSTALLIZATION HISTORY OF THE TAMARACK INTRUSION, MINNESOTA

Paper number: T87-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

The Tamarack intrusion is an unexposed mineralized ultramafic intrusion located about 50 miles west of Duluth Minnesota. Rio Tinto Exploration has been conducting exploration drilling of the Tamarack intrusion for Ni-Cu-PGE sulfide deposits since 2002. The intrusion was emplaced into Paleoproterozoic black slates during the early magmatic stage of the 1.1 Ga Midcontinent Rift (MCR). A new U-Pb baddeleyite date yields an age of 1105.6 ± 1.3 Ma. Drilling and geophysical data indicate that the intrusion has a tadpole-like shape that is about 13 km long and 1-4 km wide. The narrow “tail” area of the intrusion, which is the site of greatest exploration drilling, is composed exclusively of ultramafic olivine cumulates comprising two lherzolitic units – a lower feldspathic lherzolite orthocumulate and an upper lherzolite adcumulate. The wider “body” area is composed of stratiform sequence of successively evolved rock types ranging from lherzolite to granophyric gabbronorite. Core logging, petrography, mineral chemistry, lithochemistry, and XRF core scans were acquired on four drill cores from the Tamarack intrusion to evaluate its emplacement and crystallization history.

The principal conclusions of this study are:

1) The greater abundance of postcumulus pyroxene and plagioclase in the lower feldspathic lherzolite orthocumulate compared with the more adcumulate upper lherzolite is interpreted to indicate that the lower unit cooled more rapidly and thus was emplaced first in the tail area.

2) Similar trace element ratios imply that the two lherzolite units in the tail area formed from a similar parent magma. A high-Mg tholeiitic composition with elevated Si, Ni and Cr is estimated from a fine-grained, olivine feldspathic lherzolite sample from the basal contact. This composition is comparable to picritic compositions found in early MCR volcanics.

3) The cumulus phase progression evident in the body area is Ol→ Opx+Cpx+Ol → Pl+Cpx+Opx → Pl+Cpx+Opx+FeOx → Pl+Cpx+Opx+FeOl. A smooth cryptic variation in mg# in Ol, Cpx, and Opx and An content in Pl compliments this phase layering. This well differentiated cumulate sequence 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