CHEMICAL ALTERATION AND ELEMENTAL MOBILITY AT THE NOPAL I MINE, PEÑA BLANCA DISTRICT, MEXICO

The Nopal I uranium (U) deposit in the Peña Blanca district, Mexico, is a natural analog of some aspects of the proposed repository at Yucca Mountain, Nevada. The U deposit is contained within tuff in a near vertical breccia pipe and is elliptical in horizontal cross section. Two rock sample transects on a horizontal bench which cross cuts the deposit, trending parallel to the short axis of the elliptical ore body and away from the deposit, were used to assess chemical alteration. A meter square grid on the bench was used to locate the samples with respect to the ore body. Whole rock chemical analyses, including major, trace, sulfur, and rare earth elements; petrography; contact gamma measurements; and X-ray diffraction results constrain chemical alteration and elemental mobility. X-ray diffraction results and gamma measurements indicate that the center of the ore body is acid-altered (presence of alunite and jarosite), S rich and relatively low in U. Uranium and other trace elements are enriched at the outer edge of the deposit. Unaltered tuff collected away from the deposit was used to determine element enrichment and depletion ratios (sample/unaltered concentrations). In the center of the deposit, depletion ratios are observed for—in decreasing order of depletion—Rb (0.16), Na, K, Y, Zn, Ca, and Mg (0.6), while enrichment ratios are noted for—in increasing order—Fe (< 2), Cu, Ba, Sc, V, Sr, U, and S (~20). In the outer portion of the deposit Mg and the alkali elements are depleted, with the level of depletion decreasing in the order Mg, (0.19), Na, Rb, and K, while enrichment ratios increase in the order Sc (1.05), Y, Ca, S, Sr, Ba, Zn, V, Cu, Fe, and U (~700). Rare earth elements are marginally enriched or depleted (less than a factor of two) throughout the deposit. These relationships may reflect a sequence of primary uraninite and pyrite mineralization followed by remobilization from the center of the ore body by oxidizing and acidic fluids.

This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.