New mapping near Iron Creek, Talkeetna Mountains, indicates presence of Nikolai greenstone

Detailed geologic mapping in the Iron Creek area, Talkeetna Mountains B-5 Quadrangle, has documented several intrusive bodies and rock units not previously recognized and has extended the geologic history of the area through the Mesozoic and into the Tertiary era. Greenschist-facies metabasalt and metagabbro previously thought to be Paleozoic are intruded by Late Cretaceous to Paleocene dioritic to granitic plutons. The metabasalts are massive to amygdaloidal, commonly contain abundant magnetite, and large areas are patchily altered to epidote ± quartz. They host numerous copper oxide–copper sulfide–quartz–hematite veins and amygdule fillings. These lithologic features, recognized in the field, suggested a correlation of the metamafic rocks with the Late Triassic Nikolai Greenstone, which had not previously been mapped in the Iron Creek area. Thin, discontinuous metalimestones that overlie the metabasalt sequence had previously been assigned a Pennsylvanian(?) and Early Permian age on the basis of correlation with marbles to the north, which yielded Late Paleozoic or Permian macrofossils, or both. Three new samples from the metalimestones near Iron Creek yielded Late Triassic conodonts, which confirms the correlation of the underlying metamafic rocks with Nikolai Greenstone. These new data extend the occurrence of Nikolai Greenstone about 70 km southwest of its previously mapped extent.

Five to 10 km north of the conodont sample localities, numerous microgabbro and diabase sills intrude siliceous and locally calcareous metasedimentary rocks of uncertain age. These sills probably represent feeder zones to the Nikolai Greenstone. In the Mt. Hayes quadrangle 150 km to the northeast, large sill-form mafic and ultramafic feeders (for example, the Fish Lake complex) to the Nikolai Greenstone in the Amphitheatre Mountains host magmatic sulfide nickel–copper–platinum-group-element (PGE) mineralization. This new recognition of Nikolai Greenstone and possible magmatic feeders in the Iron Creek area suggests a much greater potential for large PGE, copper, or nickel deposits in the Talkeetna Mountains than previous mineral resource appraisals of the area have suggested, and requires reevaluation of large-scale tectonic models for the area.