THE MINERAL ASSEMBLAGE FROM LOOKOUT PASS AREA, SHEEPROCK MOUNTAINS, TOOELE CO., UTAH

The Lookout Pass area in northern Sheeprock Mountains is prospective for Carlin-type gold deposits. It is also well-known to mineral collectors as a source of thallium-bearing mineral specimens. Mineralization occurs in shear zones cutting Paleozoic carbonate strata including silicified, organic-rich and microbrecciated zones. The mineral assemblage available by surface collecting is not well documented, but has been tabulated on the Mindat.org website under “Thallium Prospect (Lookout Pass).” Thallium-bearing species listed include the sulfosalts parapierrotite, pierrotite, vrbaite, and weissbergite, and the oxide avicennite. EDS, XRD, and Raman spectroscopic analyses confirm presence of only parapierrotite, weissbergite, and avicennite. Parapierrotite forms mm-scale dark-gray to black stout lathes and acicular crystals. It exhibits distinct to good cleavage in multiple directions, but neither cleavage nor parting is mentioned in the literature for parapierrotite. Weissbergite forms tiny clusters of acicular crystals, and is present as small intergrowths with parapierrotite, possibly by replacement. Avicennite forms coatings resembling those of manganese oxides. Non-thallium minerals reported include cinnabar, pyrite, realgar, stibnite, sulfur, and various antimony oxides plus minerals forming the host rocks. Neither realgar nor stibnite were identified in this study although stibnite has been frequently attributed to the Lookout Pass locality. Antimony oxides forming radial sprays may be pseudomorphs after stibnite. Minerals newly identified in this study include barite, celestine, graphite, and the Fe-bearing minerals hematite, arseniosiderite, and tripuhyite. The latter three show that Fe is a significant component here of the As+Hg+Sb+Tl geochemical signature found in several other Carlin-type gold deposits.