Shullsburg area, Southwest Wisconsin, USA

The Shullsburg area lies within the Upper Mississippi Valley district of Wisconsin, and its ores belong to the zinc-rich subclass of the Mississippi Valley deposits. Lead-zinc mineralization occurs within dolomites and limestones of Middle Ordovician age, primarily as open void infilling. Sphalerite is the main ore mineral and is noted for its colour banding. Individual colour bands within sphalerite can be traced between deposits, so making it possible to erect a 'sphalerite stratigraphy' for the mining district. The main gangue minerals are quartz, baryte and calcite.

Major Minerals:

Sphalerite, pyrite, marcasite, galena

Minor Minerals:


Trace Minerals:

Include millerite and enargite


The sulphides infill open spaces. Pyrite is disseminated or forms radiating banded intergrowths with marcasite. Sphalerite has delicate colour banding related to its iron content (darker bands are iron-rich) that can be correlated across the deposit. Sphalerite is intergrown with pyrite/ marcasite or with galena. Galena forms euhedral crystals within the centre of vugs. Pyrite and marcasite are replaced by sphalerite and galena


McLimans et al., 1980; Craig et al., 1983; Craig and Vaughan, 1986