María Florencia Márquez-Zavalía

Tellurium and precious-metal ore minerals at Mina Capillitas, Northwestern Argentina

Mina Capillitas is an epithermal-type deposit located in Catamarca province of NW Argentina. Six separate stages of mineralization were identified in this complex Cu-Au-S-As-Sb-Pb-Zn association; the native elements (Au and Te) and Te minerals are concentrated in the fourth stage. Gold occurs as small grains with an average fineness of 920, hosted mainly by quartz and coexists with hubnerite and Bi minerals. Tellurium occurs locally in grains up to 10 μm in diameter, and is associated with the Te-bearing minerals of Au, Ag, Bi, Cu, and Ni. These Te minerals include krennerite, calaverite, sylvanite, petzite, hessite, stutzite, goldfieldite, melonite, tetradymite, and possibly volynskite. The grains are generally arranged in larger polycrystalline aggregates. These minerals occur in quartz, generally accompanied by hubnerite, pyrite, chalcopyrite, and Bi- and Sn-bearing minerals. The mineralizing fluids cooled gradually with the typical temperatures ranges characteristic of epithermal systems. Based on the preponderance of the different gangue minerals, we suggest that the pH of the environment became progressively more basic after the third stage of mineralization. This mineral assemblage together with the fluid inclusion homogenization temperature data indicate that the log fS 2 values declined as the fluids deposited the Te-bearing minerals. The absence of tellurium phases in the first three stages suggests that either the source of the tellurium had not yet been tapped or that the log fTe 2 lay below -10 to -17, the minimum activity for the Te phases present in stage IV to form.

Ferrotitanowodginite, Fe (super 2+) TiTa 2 O 8 , a new mineral of the wodginite group from the San Elias pegmatite, San Luis, Argentina

Simmonsite, Na2LiA1F6, a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P21 or P21/m, a = 7.5006(6) Å, b = 7.474(1) Å, c = 7.503(1) Å, β = 90.847(9)o, V = 420.6(1) Å, Z = 4. The four strongest diffraction maxima [d (Å), hkl, I/I100] are (4.33, 111 and 111 _ , 100); (1.877, 400 and 004, 90); (2.25, 131 _ , 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5–3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, Dm is 3.05(2) g/cm, and Dc is 3.06(1) g/cm. The mineral is biaxial, very nearly isotropic, N is 1.359(1) for λ = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is Na1.98Li1.00Al1.00F6. The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.