New Mexico Mineral Symposium — Abstracts

Spectacular wulfenite specimens occur in southwestern North America, particularly in Arizona. The butterscotch-colored, bladed crystals from the Glove mine in the Santa Rita Mountains south of Tucson and the bright red, chunky blades from the Red Cloud mine in the Silver district north of Yuma are prized highlights of many mineral collections. Most of these famous mineral localities are no longer available to collectors, making the historic specimens even more valuable.

Wulfenite, lead molybdate, PbMoO₄, forms in the oxidized zones of lead deposits where the white needle-like crystals of cerussite (PbCO₃) have developed. Surprisingly, the presence of molybdenite is not required. Wulfenite rarely occurs in the same mineral deposits as molybdenite, and then only in the later stages of the deposits. Even there, wulfenite does not occur unless soluble lead minerals, such as cerussite, are present. There had to be enough lead in the system in a relatively soluble mineral to allow the molybdenum in the ground water to combine with lead and oxygen as wulfenite.

Some of the most stunning collectable specimens of wulfenite occur in lead-zinc-silver districts. These mining districts are associated with igneous rocks whose whole rock chemistry plots in the alkali-calcic field in a diagram of whole rock geochemistry of %K₂O versus %SiO₂. Cooling of these types of igneous rocks produced hydrothermal fluids that contained lead, zinc, and silver in solution in the hot water. Other metals were sequestered in the mineral structures of the rock-forming minerals. The Pb-Zn-Ag-rich hydrothermal fluids then intruded into veins, stockworks, and fractures, and in some cases replaced limestone formations in the host rocks. The alkali-calcic districts that contain the most abundant and best specimens of wulfenite were deposited during two time periods: early Laramide (75-65 Ma) and mid-Tertiary (30-20 Ma).

Minerals associated with wulfenite in southwestern North America nearly always include cerussite, and sometimes include vanadinite or mimetite, although these latter minerals are generally found as overgrowths on the wulfenite. Wulfenite primarily occurs in the presence of and later than the lead carbonate, cerussite. The majority of the most collectible wulfenite localities are in lead-zinc-silver mining districts or in the lead-rich zones of other types of deposits. Galena has been oxidized to cerussite by circulating ground water, which may be the most likely source of the molybdenum in the molybdate. None of the mines with good wulfenite specimens contained the molybdenum sulfide, molybdenite (MoS₂). The best guide to good wulfenite localities is the presence of cerussite in lead-zinc¬silver mining districts.