Rock of the Week 36: Colloform MVT Zinc Ore

In this week we look at Zn ore from a Mississippi Valley style ore deposits. This is a really nice example of colloform texture. The brown and beige minerals are sphalerite, the lighter coloured sphalerite is richer in Zn than the more brown sphalerite. These deposits from low temperature saline fluids in Karst (limestone) areas. The metal bearing fluids percolate through and dissolve the limestone resulting the collapse breccias as seen here. Dark coloured sphalerite, as shown here is typically more iron rich that the lighter coloured varieties.
Key words: geology, Mississippi Valley, Karst, sphalerite, limestone