Geologic description
The Nelson prospect contains sphalerite, galena, and pyrite in veinlets subparallel to layering in marble and in stockwork-like fracture zones; it includes a galena vein 2 inches thick (Mertie, 1918; Cathcart, 1922). When the prospect was visited by Cathcart in 1920, mineralization was exposed along 6 feet of an open cut near a contact of marble and mica schist; the schist had a strike of N15W and a dip of 18 degrees west. The marble was bleached in the mineralized zone. An adit, reportedly 55-feet long (Mertie, 1918), was caved and inaccessible to Cathcart (1922). The prospect was visited in 1995 by C.C. Hawley (personal communication) but the exposures were badly sloughed and the nature of the mineralization could not be observed. The Nelson prospect is in the main marble mass of Mount Distin and includes thin, mica schist layers (Hummel, 1962 [MF 248]; Bundtzen and others, 1994). Layering in the marble and the schist generally dips north and the prospect is about one-half mile south of the east-trending axis of the Mount Distin syncline of Hummel (1962).
Till and others (2011) map the host rocks as part of the Devonian-Ordovician Nome Group that consists of mixed marble, graphitic metasiliceous rock, and schist. The rocks were subject to blueschist facies metamorphism, then retrograded to greenschist-facies in the Jurassic and early Cretaceous. Although earlier workers have classified this deposit as a lead-zinc replacement in marble, Slack and others (2011) classify this and several other prospect in the Nome Group (Aurora, NM140; Wheeler North, SO142, and Christophosen, NM141) as a sedimentary exhalative (Sedex) deposit. Whole-rock analyses of the schist interbedded with the marble indicates that they were originally clastic sedimentary rocks and they see no sign of volcanic protolithic rocks. Sulfur-isotope analysis of the sulfides also indicate a marine origin for the sulfur, as does isotopic analysis of barite in certain of these deposits. |
