Analytical data from host rocks and ores, Schwartzwalder uranium deposit, Front Range, Colorado

Abstract

The Schwartzwalder uranium deposit is located in the foothills of the Front Range northwest of Golden, Colo., and is one of more than 20 uranium mines and prospects in the foothills (fig. 1). It is currently the largest known vein-type uranium deposit in the United States. This report provides the analytical data from host rocks and ores that have been (Wallace, 1983) and will be used in reports discussing the genesis of the deposit. Previous analytical data were presented by Young (1979).The deposit is in a complex fracture system that cuts a mixed raetasedimentary and metavolcanic terrane of Proterozoic age (Sheridan and others, 1967). The lithologies include regionally extensive mica schist and hornblende gneiss units with an intervening transition zone of garnet-biotite gneiss and quartzite (fig. 2). The protoliths for the units were probably mafic volcanic rocks with associated carbonate muds (hornblende gneiss), chert (quartzite), silicate iron formation (garnet-biotite gneiss), and iron-rich shale (mica schist) (Wallace, 1983). All rocks were metamorphosed to amphibolite grade during Proterozoic time.The major controlling fault system for fractures at the deposit is the Rogers fault, a northwest-trending system that, at the deposit, has two parallel segments, about 1,000 m apart, that dip steeply to the east. The Illinois fault forms a north-trending, cymoid-type fracture between the two segments. Tension or horsetail fractures (the 17-1, Titan, RB, and other fractures of fig. 3) developed in the hanging wall of the Illinois fault in response to movement along the latter structure.Ascending hydrothermal fluids invaded the entire fracture system 70.6 m.y. ago (K. R. Ludwig, written commun., 1982). The fluids altered the wall rocks and produced successive assemblages of carbonate-sericite and hematite-adularia. Major reductions in the confining pressure during repeated fault movement initiated the deposition of pitchblende, carbonates, adularia, and various sulfides during several stages of mineralization. Clastic dikes composed of remobilized fault gouge were injected into the fractures at several intervals.Numerous samples were collected from unaltered and altered wall rocks, from clastic dikes, and from veins in the three major fracture systems; all samples were grab samples from the faces, ribs, and backs of the stopes. The samples were analyzed for major and trace elements by emission spectroscopy, X-ray fluorescence, neutron activation, and optical spectroscopy. The results are listed in tables 1, 3, 4, 6-9, and 11-13. Tables 2 and 5 provide data from microprobe analyses of the major minerals in the hornblende and garnetbiotite gneisses. Table 10 is a summary of the average compositions of altered and unaltered hornblende gneiss and garnet-biotite gneiss, the two major host rocks for the ores. Table 14 summarizes the trace-element data for the three vein systems; correlation coefficients for selected elements are presented in table 15.

Fluorine, fluorite, and fluorspar in central Colorado

“…Fluorite is present in only small amounts, and it was deposited very late in the paragenetic sequence from fluids that had a predominantly meteoric composition (Wallace and Whelan, 1986). The Schwartzwalder vein materials contain an average of 277 ppm fluorine, as compared to the fresh hornblende-and biotite-rich host rocks that have an average fluorine content of 1,083 ppm (Wallace, 1983).…”