New Mexico Mineral Symposium — Abstracts

Uranium minerals in the Grants district were first discovered in the late 1940s in the Todilto Limestone, which has yielded about 2% of the total uranium production from the district. The first uranium minerals discovered and mined were brightly colored yellow minerals. Significant quantities of "black ore" containing uraninite (pitchblende) and coffinite were discovered as mining progressed, sometimes associated with blue-black vanadium oxide minerals, and it became evident that the yellow minerals formed by secondary near-surface oxidation of the black ores. Yellow minerals are rare or absent in the deeper mines. Lead-uranium isotopic dating of uraninite indicates that it formed shortly after the limestone was deposited; the yellow minerals formed at various later times.

Fluorite, barite, and pyrite are the most common accessory minerals in and near the uranium ore. Fluorite occurs in small crystals and fine-grained irregular replacements; known occurrences are roughly coextensive with uranium minerals and are probably not related to fluorite vein deposits in the nearby Zuni district. Uraninite occurs in disseminations and replacements along bedding or irregularly throughout mineralized limestone, and occasionally along fractures. Where uraninite is abundant, the limestone may appear red from associated fine-grained hematite. Microscopic galena crystals are associated with high-grade uraninite; much of the lead in the galena may have been derived from decay of uranium. A few deposits extend into the Entrada Sandstone, immediately below the Todilto, with uraninite filling pore spaces between sand grains. The blue-black vanadium minerals are mainly haggite and paramontroseite and often occur in fractures associated with coarse calcite. The most common yellow mineral in oxidized uranium-vanadium ore is tyuyamunite, the calcium analog of carnotite, which forms in the high-calcium limestone environment. It is abundant in thin but conspicuous coatings on fractures and bedding surfaces in the limestone, and occasionally as platy crystals or in pulverulent masses. Tyuyamunite occurs along with the related lower hydrate metatyuyamunite; the minerals differ in their water content and form reversibly from each other depending on humidity conditions. The yellow uranium silicate uranophane may form where the ore is low in vanadium. It occurs as radiating clusters of acicular crystals on fracture or bedding surfaces and occasionally as thicker felted masses or showy acicular crystals in open spaces. Schroeckingerite is rare in oxidized ore, occurring as light-green platy crystals that fluoresce brilliant yellow green in short-wave ultraviolet radiation.

Other scarce minerals in oxidized or partially oxidized deposits are the lead-uranium oxide curite, the calcium vanadates hewettite and metahewettite, and two new minerals from the Grants district: santafeite, a complex manganese vanadate, and grantsite, a sodium calcium vanadate. Goldmanite, another new mineral, is a vanadium-rich garnet related to andradite and grossularite; it occurs in the Laguna area where a Todilto deposit extending into the Entrada sandstone was intruded by a basaltic igneous sill, forming various calc-silicate metamorphic minerals including andradite¬grossularite. Small crystals of goldmanite formed in vanadium-rich parts of the metamorphosed deposit.

Several additional minerals occur in or around the Todilto deposits; their relationship to uranium-vanadium mineralization is not clear. These include small quartz crystals, manganese oxides, and iron oxides resembling limonite. Coarsely crystalline black calcite and paragenetically later white calcite are common in fractures and open spaces. Rare pyrite-coated scalenohedral calcite crystals are also observed. A few other minerals may occur as very minor constituents of the host rocks and possibly of the deposits; some have been reported or tentatively identified in previous work but were not confirmed in this study. The most significant or interesting minerals from the Todilto deposits are: uraninite, UO₂; coffinite, U(SiO₄)_1-x„(OH)_4x; haggite, V₂O₂(OH)₃; paramontroseite, V0₂; fluorite, CaF₂; barite, BaSO₄; pyrite, FeS₂; calcite, CaCO₃; hematite, Fe₂O₃; galena, PbS; tyuyamunite, Ca(UO₂)₂(VO₄)2 • 5-8¹/₂H₂O; metatyuyamu-nite, Ca(UO₂)₂(VO₄)₂ • 3-5H₂O; uranophane, Ca(UO₂)₂(SiO₃OH)₂ • 5H₂O; schroeckingerite, NaCa₃UO₂(CO₃)₃SO₄F 10H₂O; curite, Pb₂U₅O₁₇ • 4H₂0; hewettite, CaV₆O₁₆ 9H₂O; metahewettite, CaV₆O₁₆ • 3H₂O; santafeite, (Mn,Fe,Al,Mg)₈Mn₈(Ca,Sr,Na)₁₂(VO₄)₁₆(OH,O)₂₀ • 8H₂O; grantsite, Na₄CaV₁₂O₃₂ 8H₂O; goldmanite, Ca₃(V,Fe,Al)₂Si₃O₁₂