The Lac des Iles Complex appears as a linear zone of mafic plutons that trend east to northeast and extends from Lake Nipigon to Atikokan in northwestern Ontario (Sutcliffe, 1986). The complex is situated in Archean granitoids that consist of gneissic tonalites, medium-grained hornblende diorites and quartz diorites. The Lac des Iles Complex occurs in a circular outcrop fashion that is approximately 30 km in diameter and is the largest of a series of mafic to ultramafic intrusions (Sutcliffe, 1986). The Roby Zone was the initial site of mining at the Lac des Iles Mine. The North Roby Zone is its narrow northward extension.
The North Roby Zone contains a narrow strip (<50 m) of anomalously high PGE and Au values associated with sparse sulfides called "Noseeum Ore". Five stripped outcrops approximately 50 x 10 m were studied in the North Roby Zone. Alteration of primary pyroxene to talc and pink coloration of recessively weathered plagioclase is strongly suggestive of hydrothermal alteration.
The five stripped outcrops reveal a northeasterly striking, steeply dipping sequence of leucogabbro, varitextured gabbro, pyroxenite and east gabbro. A total of 32 hand samples were collected and studied in thin section. From these 21 were selected for whole-rock and trace element analysis in order to compare chemistry of altered and unaltered samples.
Hydrothermal alteration appears to have affected the primary ortho-and clinopyroxenes of the host rocks, progressively converting them to talc. Other petrographic indications are obscure, as regional metamorphism has overprinted the Lac des Iles Complex and its mineralized rocks.
The grade of metamorphism is the albite-epidote subfacies of greenschist facies as evidence by incipient breakdown of plagioclase to sericite and clinozoisite, chloritization of pyroxenes and formation of tremolite-actinolite, as well as minor metamorphic albite. Chlorite coronas surround mafic minerals, and develops decussate assemblages of chlorite and tremolite-actinolite. Minor penetrative deformation is evident in undulatory extinction in plagioclase and development of weak schistosity.
The development chlorite coronas and general overprinting of mafic minerals by chlorite and sericitization of plagioclase are significant in distinguishing hydrothermal alteration from subsequent regional metamorphism.
Analysis did not generally reveal striking compositional variations in the host rocks; however, more detailed analysis did show chemical effects of alteration. Chondrite-normalized REE plots revealed that all REEs were depleted relative to unaltered rocks; however, in most altered leucogabbro and varitextured gabbro, minor to insignificant depletion of Eu relative to other REEs produced an apparent positive Eu anomaly in the plots. Other chemical changes, because of their subtle expression and obscurity owing the problem of closure, were examined by use of Pearce Element Ratios. Most notable was Na-depletion due to alteration, which can be distinguished from igneous fractionation effects in plagioclase.