Rocks native to the Southpass / Atlantic City / Greenstone Belt
Felsic gneiss and granite migmatite, interlayered with supracrustal rocks and intruded by granodiorite, is intercalated in places with the supracrustal rocks of the greenstone belt.
South Pass supracrustal rocks along with light-colored, metamorphosed leucodacite porphyry and quartz diorite. This unit is exposed in small areas near the Mary Ellen mine, just west of the Duncan mine, and in a 6-mile long north-trending intrusion east of Lewiston. A quartz vein included within the Mary Ellen stock carries visible gold, and a quartz stringer within sheared metaleucodacite porphyry dike contained disseminated arsenopyrite dominated by gray to dark brown feldspathic and biotitic metagreywack, and mica schist.
