Abstract

The acceptance of sedimentational hypotheses on the origin of stratiform deposits has had a major impact on ore search. This impact is being felt in Alaska, and sedimentational hypotheses are playing a more important role in prospecting here than they did only a few years ago.

The use of a sedimentational approach in ore search in Alaska is justified empirically by the occurrence of deposits which fit a genetic stratiform model closer than they do previous concepts of origin. Theoretically, it is justified by existence of rocks which formed in environments deemed favorable for the formation of essentially syngenetic stratiform deposits. These environments include those of ancient volcanic-sedimentary island arcs and spreading centers.

Data are already sufficiently abundant to identify volcanogenic stratiform deposits in several parts of the Alexander Terrane of Southeastern Alaska, including the metamorphosed Wales Group, and less metamorphosed units of Devonian age. Barite-zinc deposits in a Paleozoic pillow basalt-rhyolite sequence near Haines give particularly clear evidence of syngenetic deposition associated with rhyolitic volcanics in a dominantly basaltic pile.

Other regions where stratiform deposits suggest volcanogenic affinities include (1) the Brooks Range copper belt, (2) Prince William Sound copper belt, (3) the Totalanika Schist belt, and (4) both the Taku-Skolai and Gravina-Nutzotin orogenic belts.

Not withstanding the value of the volcanogenic or other stratigraphic hypotheses, there are other stratiform deposits which are best explained by replacement, or by complex processes involving both early sedimentation and metamorphic mobilization. Rather than blindly calling any stratiform deposit “syngenetic” or sedimentary, the use of the criteria developed in the past to distinguish epigenetic and syngenetic deposits should continue to be used.