Polybasite after Pyrargyrite

specimen number:
9273028

location:
La Cata Mine, Guanajuato, Mexico

description:

Large crystals of what were once pyrargyrite measuring to 3.0 cm in size across the top of a crystalline white quartz matrix, these replaced by crystallized metallic polybasite. In great condition. Wonderful pseudomorph specimen for this famous locality.

The La Cata mine, situated within the historic northern suburbs of the city of Guanajuato, Mexico, is a legendary underground silver and gold mine celebrated for producing significant wealth and world-class mineral specimens. Geologically, this deposit is localized along the prolific Veta Madre (Mother Lode) fault system, a major regional epithermal vein structure that dips shallowly to the southwest and cuts through the Mesozoic-age Guanajuato Conglomerate and underlying volcanic tuffs. Mineralization occurred during Oligocene magmatic and tectonic activity, where boiling, low-to-intermediate sulfidation hydrothermal fluids ascended through the fault zone, depositing thick bonanza ore shoots rich in silver sulfides, electrum, and base metal sulfides enclosed within a gangue of quartz, adularia, and calcite. Historically, the La Cata workings were initiated by Spanish colonists during the mid-sixteenth century, shortly after the initial 1548 discovery of the district, and the mine achieved immense profitability in the late eighteenth century alongside the neighboring Valenciana mine. Operations survived multiple disruptions, including the Mexican War of Independence, and transitioned through British management, twentieth-century local miner cooperatives, and eventual corporate acquisition. Through 2026, the mine remains a productive unit under modern corporate consolidation, serving as an important geological anchor for studying structural controls and fluid dynamics in high-grade epithermal silver systems.

Mineralogically, the La Cata mine is highly famous among collectors for producing remarkable, sharp pseudomorphs consisting of polybasite replacing pyrargyrite. This specific type of replacement occurs when early-formed crystals of the silver antimony sulfosalt, pyrargyrite, become chemically unstable during late-stage hydrothermal pulses or overprinting events within the epithermal vein system. As the circulating fluids became enriched in copper and silver relative to antimony, a rim-to-core chemical exchange took place, transforming the original pyrargyrite crystal into polybasite while perfectly preserving the original exterior crystal morphology, habit, and sharp faces of the pyrargyrite. These distinctive replacement specimens, showing the hexagonal tabular form of the original mineral but possessing the composition of the latter, provide mineralogists with critical evidence regarding the complex, multi-stage evolution of fluid chemistry and temperature fluctuations during the closing phases of mineralization in the Guanajuato district.

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