Pearceite

specimen number:
2273026

location:
Black Jack Mine, Ward, Boulder Co., Colorado, USA

description:
Bright metallic crystallized pearceite quite rich across the top of matrix. In good condition, a neat locality example for the species. At one time this was part of the collection of Martin Pearce.

The Black Jack mine, also known as the Captain Jack mine, is located in the Ward mining district within Boulder County, Colorado, near the headwaters of the Lefthand Creek watershed. Geologically, the deposit sits within the Colorado Mineral Belt, where Precambrian granites, gneisses, and schists are intruded by Tertiary porphyry dikes, creating complex, high-angle hydrothermal vein systems. These structural veins are heavily mineralized with base metal sulfides, rich silver sulfosalts, and localized gold-bearing pyrite and chalcopyrite. Underground operations at the property began intermittently following the initial regional gold rushes of the 1860s, with the mine gaining formal patent approval in 1917. Underground extraction and seasonal milling continued sporadically through the mid-twentieth century until the primary economic silver and base metal veins were exhausted, eventually leaving a legacy of heavy metal mineralization that led to modern environmental remediation efforts.

Pearceite is a complex silver copper arsenic sulfosalt mineral that crystallizes in the monoclinic system and forms a continuous solid-solution series with its antimony-dominant analogue, polybasite. Geologically, it develops as a primary or secondary mineral within low-to-medium-temperature hydrothermal veins, commonly associated with other silver sulfosalts, acanthite, galena, and sphalerite. The mineral typically occurs as thin, tabular, pseudohexagonal crystals or as compact metallic masses that display an iron-black to dark lead-grey color with a metallic luster and a dark reddish-black streak. Because of its exceptionally high silver content, pearceite serves as an important ore mineral in silver-rich epithermal deposits, and well-crystallized specimens are highly prized by mineral collectors for their sharp, characteristic micaceous habit.

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