Alunite with Limonite, Quartz

specimen number:
2262344

location:
Rosita Hills Mine S, Custer Co., Colorado, USA

description:
Colorless masses of alunite money limonite matrix with minor quartz.

The Rosita Hills mining district, situated in the low western foothills of the Wet Mountains in Custer County, Colorado, is a historic precious-metal and base-metal locality. Geologically, the area is defined by the Oligocene-aged Rosita volcanic center, a deeply eroded igneous complex characterized by thick sequences of andesitic, rhyolitic, and trachytic tuffs, flows, and breccia pipes that unconformably overlie a Precambrian crystalline basement of gneiss and granite. Epithermal mineralization occurred late in the volcanic cycle, depositing rich silver-copper and gold-bearing sulfide veins alongside unique mineralized agglomerate chimneys that carried concentric shells of galena, sphalerite, chalcopyrite, and gold-silver tellurides. Prospectors first discovered silver-lead ore here in 1872, sparking a massive influx of exploration that peaked between 1877 and 1890 with major operations like the Humboldt-Pocahontas vein and the Bassick mine. Although early high-grade reserves were rapidly exhausted, forcing a major decline by 1892, intermittent mining and sporadic reprocessing operations continued through the mid-twentieth century until all organized industrial production ceased around 1958.

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