Geologic description
The Eagle prospect, formerly called the Northern Light was developed by a tunnel 62 feet long and several small cuts (Ross, 1933). He reported that the country rock is chloritic and cherty argillite and tuff, with some lava in the vicinity. The average trend is northeast, but locally the beds trend northwest, probably because of faulting. The average trend of the mineralized zones is somewhat east of north, but there is much local variation, resulting in part from irregular deposition, in part from later faulting. The dip is generally steep to the east. If the pits are all on the same lode, it has a known length of about 300 feet, but it is doubtful if underground development would show continuous ore throughout this distance. The width of the mineralized shear zones varies in different exposures, ranging from 1 to 10 feet. The vertical range between the tunnel and the highest cut is roughly 185 feet. The shear zones generally contain bands of white, coarsely crystalline quartz from a few inches to 2 1/2 feet in width. Locally, the quartz contains numerous vugs. Most of the sulfides were deposited along fissures, and the best samples are reported to have been taken along the fissures that traverse the bedding at considerable angles. Most of the rock in and near the shear zones contains sparsely disseminated sulfides, mainly pyrite. There are a few lenses of massive arsenopyrite with a little pyrite, which are several inches wide. Small amounts of somewhat oxidized chalcopyrite are present at some places. Two samples were taken in the cut immediately north of the tunnel, which is reported to contain the best ore so far exposed. One of these, taken across a quartz lens somewhat more than 2 feet wide, yielded 0.20 ounce of gold and 6 ounces of silver to the ton. The other, taken across the full 4 feet of exposed lode, yielded 0.18 ounce of gold and 9.10 ounces of silver to the ton. A sample probably from the quartz lens in this pit contained 0.56 ounce of gold and 3.20 ounces of silver to the ton. Capps (1919) stated that at the time of his visit in 1917 the area of heaviest mineralization had been traced along the surface for a distance of about 800 feet, considerably farther than Ross (1933) could trace it in 1931. Capps (1919) noted the presence of arsenopyrite, pyrite, chalcopyrite, sphalerite, and a little stibnite. Thick Quaternary glacial deposits and Upper Tertiary sedimentary strata cover most of the older bedrock in this area. Regionally, the main geologic feature is the northeast-trending Chulitna fault, which separates a sequence of Upper Jurassic to Upper Triassic(?) crystal tuff, argillite, chert, graywacke, and limestone on the north, from Upper Devonian to Lower Triassic volcanogenic and sedimentary rocks on the south. Faulting and shearing of the rocks has occurred, along with intrusions of Late Cretaceous dikes and stocks. The Eagle prospect is a 3-foot wide quartz vein that contains pods of arsenopyrite, chalcopyrite, sphalerite, and stibnite up to 1 foot wide. The vein is spatially associated with one or more dikes. The sulfide pods contain substantial Au and Ag values. Seven chip samples of the veins averaged 0.23 ounce of gold per ton and 4.55 ounces of silver per ton (Hawley and Clark, 1974; Ross, 1933; Bundtzen, 1983). The deposit is discontinuous, owing to post mineralization faulting and to pinch-and-swell structure of the vein. The Eagle vein is subparallel and about 400 feet east of the Lucrata segment of Upper Chulitna fault. |
