DETRITAL ZIRCON AGES FROM METASEDIMENTS IN THE FORTYMILE DISTRICT, EAST CENTRAL ALASKA

Drilling at the Fish Zn-Pb-Ag prospect (decimal degrees: N 64.2588,W 142.7051) during mineral exploration undertaken by Full Metal Minerals in 2006 was predominantly within a lower amphibolite-grade metasedimentary sequence. A crude stratigraphy identifies three units A,B and C. The upper unit A (>200 m thickness) consists of thinly laminated quartzites and chlorite schists, unit B (~100 m) is dominated by marble and dolomite, and unit C (> 100 m) contains biotite and chlorite schists with lesser quartzites. A 0.5 meter section of core from a sub-arkosic quartzite in unit C was processed for detrital zircons and using an ICP laser ablation system, detrital zircon ages were measured on 135 zircons by a commercial laboratory (Apatite to Zircon, ID USA). The range of ages contain a typical cratonic North American signature with well developed populations at 2.55 Ga to 2.8 Ga, a broad Paleoproterozic population (n=102) from 2.5 Ga to 1.5 Ga, a distinct break in ages between 1.5 Ga and 1050 Ma and a population from 1050 to 550 Ma (n=18). In general pattern, the distribution of ages looks very similar to the plot of the Tracy Arm assemblage, (fig 4. CJES 35:269-279); that is a typical Yukon-Tanana distribution of detrital zircons from cratonic provinces of northwestern North America, with the important distinction that there is no evidence of any Devonian-Mississippian (DM) arc magmatism. This was a surprising result as much of the available mapping in the area shows widespread DM-aged rocks. One consideration is that this younger zircon population exists, but has not been measured in the 135 grains sampled, although this is a statistical improbability. Alternatively, the preferred interpretation is that a pre-DM basement extends into Alaska, comparable to the Snowcap and Dorsey Complexes in the Yukon.