Brookins and Rautman reported in 1978 on the uranium, REE, and thorium abundances, whole rock chemistry and trace element geochemistry, in the Zuni and Florida Mountains, New Mexico. The uranium and thorium abundances in the Precambrian silicic, core rocks of the Zuni Mountains have been determined by delayed neutron activation analysis. The average uranium content of 57 surface and drill core samples is 3.75 ppm and the average thorium content of 25 samples is 17.53 ppm.
The Th/U ratio for 25 samples is 4.55 which is slightly higher than the range of 3.7 - 4.2 reported for many granitic rocks. The uranium in the Zuni Mountains has, in part, been redistributed; and significant amounts may have been removed by weathering or other processes. Combined rare earth element distribution patterns, positive U vs. K 0 correlation, ' and high U content in surface samples suggest local redistribution was significant. No data support the core rocks of the Zuni Mountains as.a large potential in situ resource for uranium.
The data obtained during this study are of interest in that the Zuni Mountains lie to the southwest of and parallel to the Grants Mineral Belt. If the ancestral
Zuni Mountains directly or indirectly served as a source for uranium for the Grants Mineral Belt then this should be reflected in the Th and U abundances and in the Th/U ratio remaining in presently exposed rocks. While it is generally accepted that the presently exposed rocks were covered by Paleozoic rocks during the deposition of the Jurassic Morrison Formation, at least part of the area may have been uplifted in the Cretaceous and certainly in post-Laramide time. Consequently, if the uranium for the Grants Mineral Belt was deposited in Cretaceous or younger time then the Zuni Mountains are a convenient source due to their location at the southwestern edge of the San Juan Basin with the regional drainage presumably to the northeast.
While data for uranium are available for 57 samples only 25 samples have yet been analyzed for Th. The data are shown in' Table 1 and Fig. 2. The Th
samples vary from 4 to 29 ppm with a mean of 17.5 ppm while U varies from 1.8 to 7.4 ppm with a mean of 3.9 ppm. For the Th/U ratios, variation is
noted between 2.27 and 7.82 with a mean of 4.55. The Th, U contents and the Th/U ratios are typical of rocks which have not lost appreciable amounts of U or else very low U contents and proportionately higher Th/U ratios (i.e., 20-50) would be common. Since the REE plots also indicate normal behavior for granitic rocks (Figs. 5-27) then, in support of other evidence, we propose that the presently exposed rocks of the Zuni Mountains were not important contributors for the uranium of the Grants Mineral Belt.
Whole Rock Chemistry for Zuni Mountains
The detailed report on the significance of whole rock chemical study of the Zuni Mountains will be reported separately. This report will address only the aspects of relation (if any) of major element chemical variation with uranium or other trace element data. The data for the Zuni Mountain Precambrian silicic rocks (Table 2) supports their identification as granitic; total Si0 varies from 62.66% (ZM7-7) to 78.16% (ZS-2) but most values fall in the 70 - 76% range. Of interest, no significant variation of Si0 content with depth is noted (with a few exceptions) nor is there widespread difference from drill core to surface samples., Total iron (as Fe 0 ) varies significantly from location to location, but within individual drill holes the variation is slight. This is of importance as significant variation would indicate possible loss or redistribution of iron.
Rare Earth Distribution Patterns for the Zuni Mountains
The rare earth elements were determined by instrumental neutron activation analysis and presented in Table 3 and in Figures 4 to 26. An interesting correlation between high uranium content with negative europium anomalies is indicated for the surface (ZS) and some (ZMl) drill core samples. In Figures 4 to 26 the rare earths are "ratioed" to the North American Shale composite (NAS) in order to properly make comparisons with the nearby sedimentary rocks. That high uranium would be found in zones of a negative europium anomaly does not support widespread uranium loss.
In fact, just the opposite is indicated (note: although it is readily admitted that correlations of this type can be challenged as the Eu is housed in rock forming minerals whereas much of the U is located along grain boundaries, etc.). In theory, a negative europium anomaly is interpreted to indicate oxidizing conditions and thus uranium loss (i.e., low abundance) would be expected. That the opposite is noted supports arguments made elsewhere that widespread uranium loss from the Zuni Precambrian rocks may have been small as opposed to local redistribution.
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