Mimetite after Wulfenite

specimen number:
2133369

location:
San Francisco Mine, Sonora, Mexico

description:
A platy crystal grouping of what was once wulfenite, this now replaced by sparkling yellow crystalline mimetite. Neat pseudomorph specimen.

The San Francisco mine, situated near Cucurpe in north-central Sonora, Mexico, is an underground deposit known for its secondary lead and molybdenum minerals. Geologically, the deposit consists of a fractured, oxidized silver-lead-gold vein system hosted within Cretaceous sedimentary formations and Tertiary volcanic rocks. Supergene weathering and hydrothermal alteration of primary sulfides within the vadose zone concentrated rare elements, yielding tabular crystals of wulfenite and crusts of mimetite inside open rock cavities. While small-scale prospectors worked the area intermittently, formalized mineral specimen mining and structured underground exploitation commenced in the early 1970s. The site operated through various private leases and dedicated mineral-recovery contracts for several decades before active underground extraction slowed in the late twentieth century.

A pseudomorph is a mineral compound that has replaced another mineral while retaining the outward crystal form and habits of the original pre-existing species. In the case of mimetite replacing wulfenite, this geological phenomenon represents an alteration process within the heavily oxidized zones of the deposit. Over time, changing chemical conditions and infiltrating meteoric waters rich in arsenic and lead introduce new elements that attack the original, square-tabular wulfenite crystals. The mimetite precipitates over the wulfenite surface or replaces its internal chemistry, ultimately leaving behind a distinctively shaped wulfenite crystal form composed entirely of orange mimetite.

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