NEW GEOLOGIC AND ALTERATION MAPPING OF THE SOUTHERN PORTION OF THE RATTLESNAKE HILLS, NATRONA COUNTY, WY

New 1:5,000 scale geologic and alteration mapping of the southern Rattlesnake Hills, Wyoming, has been completed to assist gold exploration of an Archean greenstone belt intruded by Eocene hypabyssal rocks.

Archean basement rocks consist of four major units, in ascending order: Sacawee orthogneiss, Barlow Springs Formation (quartzite, pelitic schist, banded iron formation, biotite to amphibolite schist), McDougal Gulch Metavolcanics (hornblende augen gneiss, olivine tholeiite metabasalt, and amphibolite gneiss), and UT Creek Formation (metagraywacke). Repetition of these units and the presence of sub-vertical NW-striking foliations document a map-scale isoclinal syncline.

Late Eocene alkaline and calc-alkaline hypabyssal intrusions are randomly dispersed throughout the area. An older set of linear, discontinuous quartz monzodiorite dikes strike NE across the area. A younger intrusive phase is represented by elliptical phonolite diatremes with steep concentric flow foliations, and by localized brecciated margins. Other intrusives include soda trachyte, trachyte, and latite plugs and domes. Some latite intrusions were previously mapped as Wagon Bed Formation. Finally, at least one narrow (15 cm) vertical carbonatite(?) dike intrudes Archean and Eocene units.

Except for the North Granite Mountains fault, no faulting with displacement >10 m was observed. Joints sets and veins within the Archean were noted but not measured.

Mapping and preliminary petrographic analysis indicates localized zones of propylitic, weak argillic, and moderate potassic alteration of Archean rocks. Gold concentrations are primarily within the tertiary intruded portions of the UT Creek metagraywacke, which is lithologically and stratigraphically similar to the mesothermal gold-bearing Miners Delight metagraywacke at South Pass in the Wind River Range.