The Ore Petrology and Structural Geology of the Lower Precambrian Deer Lake Mafic-Ultramafic Complex, Effie, Itasca County, Minnesota

Abstract

The Deer Lake Complex is a 13 km long by 3 km wide belt of Lower Precambrian ultramafic and gabbroic rocks located near the town of Effie, Itasca County, Minnesota. Two layered sills, averaging 200 m in thickness, a 122 m gabbroic sill, and two ultramafic lenses ranging in thickness from 15 to 40 m have been investigated by detailed mapping and drill core investigation. The igneous bodies are intrusive into a sequence containing metagraywackes, slates, argillites and mafic metavolcanic rocks. The volcanogenic sequence overlying the sills is on the order of 400 m in total thickness. Metamorphism of the Complex and associated host rocks has produced mineral assemblages of the greenschist facies. The layered sills contain a succession of basal chilled margin, peridotite, orthopyroxene clinopyroxenite, porphyritic two-pyroxene gabbro, nonporphyritic gabbro, quartz diorite cap rock, and upper chilled margin. The general structure of the sills and their petrographic and chemical characteristics indicate that they are differientiated from a basaltic magma by in situ gravitational mineral accumulation and subsequent nonaccumulative crystallization. Phase chemistry studies utilizing mineral and trace metal compositions from the layered sills of the Conplex indicate that their initial temperature of crystallization was near 1010 C. Sulfide and primary oxide phases are disseminated throughout the layered sills. Textural evidence and phase relations indicate that sulfide and oxide minerals within basal chilled margins formed from an immiscible sulfide-oxide liquid which segregated prior to sill emplacement. Nickel-rich sulfide minerals are concentrated in the lower portions of basal chilled margins with copper sulfide and oxide minerals located in overlying chilled units. Phase relations indicate that sulfide-oxide crystallization began at approximately 1070° C and ended about 960° C. Re-equilibration within sulfide phases occurred at temperatures below 600° C. The primary sulfide phases in the differentiated portions of layered sills range from nickel-rich in peridotite units through copper-rich at intermediate levels to iron-rich in upper layered units. Textural evidence and phase relations suggest that sulfides in peridotite and clinopyroxenite units crystallized from an immiscible sulfide-oxide liquid which segregated during silicate differentiation, whereas sulfides and oxides in upper gabbro units within the layered intrusives crystallized directly from the silicate magma in response to an increased oxygen activity. Textural features in quartz diorite cap rocks indicate that sulfides replaced silicate phases. The Complex has undergone Lower Precambrian isoclinal folding, with a maximum principal stress orientation of N45W. Gravity faulting followed as a result of stress relaxation producing grabens. Subsequent strike-slip faulting along a N40E trend resulted in 1100 m right lateral separation of Complex units. The final phase of observed deformation in the Deer Lake area is evidenced by an additional 100 m of movement of Middle Precambrian dikes along the strike-slip faults. Five characteristics commonly used to evaluate Ni-Cu sulfide ore bearing ultramafic rocks indicate that the Deer Lake Complex has little ore potential. These characteristics are: 1) the presence of layering in the differentiated sills; 2) the Fe-rich, Mg-poor nature of the parent magmas which produced the Deer Lake layered sills and ultramafic lenses; 3) the low nickel content of peridotite units and olivines at Deer Lake; 4) the absence of pyrite in sulfide assemblages of chilled margins and peridotites and 5) the low metal content of sulfide phases in ultramafic zones and chilled margins.