Abstract

The Lake Owen Complex (LOC) is the easternmost and smallest of three large gabbroic intrusions in the 1,770 to 1,800-Ma Green Mountain accreted island-arc terrane of southern Wyoming and northern Colorado. The intrusion is virtually devoid of metamorphic and internal deformation other than a few faults. The LOC consists of subparallel rock layers with an average dip of about 75°. About 70 km of mapping traverse the layered sequence, and thin-section petrography on more than 400 samples demonstate that the intrusion contains at least 16 cyclic units defined by large-scale repetitions of two or more units in the lithologic sequence troctolite → olivine gabbro → gabbronorite → Fe-Ti-oxide-cumulate gabbronorite. Most cyclic units are a few hundred meters thick. "Marker" units as thin as 2 m have been found to be continuous with little variation in thickness over a strike length of at least 11 km. The intrusion is of crude funnel or bowl form — the lowest cyclic unit has a strike length of about 3 km; succeeding cyclic units on-lap gneissic floor rocks, and the uppermost cyclic units have an exposed strike length of about 12.5 km.

Variations in mineral assemblage, mineral proportions, and mineral compositions have been studied in substantial detail along traverses spanning the 6-km-thick layered sequence at an average sample spacing of about 30 m. More than 3700 electron-microprobe analyses of plagioclase, augite, and spinels in 206 successive samples along the stratigraphic column demonstrate that the aforementioned lithologic cyclic units coincide with cycles in mineral composition in which molar Ca/Ca+Na in plagioclase decreases up-section from as much as 0.83 at the base of a cyclic unit to as little as 0.52 at the top, while augite varies correspondingly in molar Mg/ Mg+Fe from 0.84 to 0.50, and spinels in the series chromitet-itanomagnetite span the range 43 to 0.01 wt% Cr₂0₃.

Cumulus sulfide mineralization occurs in at least 12 stratigraphic units. Four of them, referred to herein as "precious-metal zones" PMZ-I, -II, -III, and -IV in stratigraphic order, are known on the basis of geochemical reconnaissance grab sampling to contain laterally persistent Au+Pt±Pd mineralization in concentrations of a few hundred to a few thousand ppb. In PMZ-I, platinum arsenide and Pt-Pd tellurides are associated with sparsely disseminated chalcopyrite, pentlandite, pyrrhotite, pyrite, and gersdorffite in olivine gabbro at the base of a cyclic unit having a short strike length near the stratigraphic base of the intrusion. PMZ-II contains PGE tellurides and sulfides and Au-Ag alloys with bornite, chalcopyrite, millerite, and PGE-bearing carrollite in gabbronorite and magnetite-cumulate gabbronorite. The latter unit is remarkable for its intercalations of anorthosite, bronzitite, magnetite, and olivine gabbro as lenses and wisps generally less than 1 m thick, and a few meters or tens of meters long. The Au+PGE-anomalous zone is locally greater than 5 m thick, but mineralization within it seems to be lensy or spotty. PMZ-II has an identified strike extent of at least 2.1 km, but could be as much as 10 km. PMZ-III is a 1- to 3-m-thick interval markedly enriched in Au and Pt located 2 to 5 m above the base of a magnetite-cumulate gabbronorite layer capping a 500-m-thick cyclic unit directly overlying the PMZ-II platiniferous magnetite cumulate. In PMZ-III, the Pt+Au-anomalous zone is virtually devoid of sulfides (<0.1%), but it is consistently situated 1 to 3 m below the base of a PGE-poor, chalcopyrite-pyrrhotite-pentlandite mineralized layer in magnetite-gabbronorite.

The three mineralized zones just described occur within the lower third of the igneous stratigraphic column. The middle third consists largely of troctolite and olivine gabbro lacking recognized mineralization. About 25 to 50 m above the top of the thick olivine gabbro, capping the middle third, the onset of persistent precipitation of sparesely disseminated cumulus sulfides is associated with anomalous anorthosite and norite layers a few meters thick within the predominantly gabbronorite sequence. The base of the thick sulfide cumulate, PMZ-IV, contains PGE tellurides with chalcopyrite, pyrrhotite, and a little pentlandite. Random grab samples at localities 9 km apart along strike contain Au+PGE in the range 100 to 1,000 ppb.

A hornblende-oikocrystic olivine gabbro layer at the base of the next cyclic unit 450 m above PMZ-IV was found to contain PtAs₂ and PdSBTe in a thin section. The locality has not been geochemically sampled, and the strike persistence of the mineralization has not been investigated.

The Lake Owen Complex has thick vanadiferous magnetite-cumulate gabbronorite layers in 16 cyclic units that have 3 to 12.5 km of strike length, depending on position in the stratigraphic column. Of the 16, five have average V₂O₅ contents in the range 1.2 to 1.5 wt% in cumulus magnetite, and rarely attain 4.5 wt%, according to a systematic survey of stratigraphic trends in V₂O₅ content of magnetite by electron microprobe analyses of more than 1200 magnetite grains in 118 samles. The general coarseness of magnetite-ilmenite exsolution intergrowths suggests that it might be feasible to recover ilmenite as well as chalcopyrite as co-products of vanadiferous magnetite mining. Lake Owen ilmenite is exceptionaly Tirich, being nearly pure FeTiO₃ with 50 to 52 wt% TiO₂ in the approximately 400 crystals analyzed. The Fe-Ti-V-oxide cumulate layers are crossed by the Union Pacific Railroad, which could reduce potential development costs.