<p>Pyritic quartz monzonite from the Cretaceous of Montana, USA. (~5.5 cm across along the base)
</p><p>This rock is from Butte, Montana, known as the “Richest Hill on Earth”.  The mineralized rocks at Butte have produced gold, silver, copper, molybdenum, manganese, and other metals.  Mineralization is hosted in the Butte Quartz Monzonite, a large Late Cretaceous igneous pluton.  These rocks have been intruded &amp; altered by hydrothermal veins containing valuable metallic minerals - principally copper sulfides.  Copper mineralization has been dated to 62-66 million years ago, during the latest Maastrichtian Stage (latest Cretaceous) and Danian Stage (Early Paleocene).
</p><p>This particular specimen is waste rock and not an ore sample.  The quartz monzonite is richly pyritized (click on the photo to zoom in and look around).  Pyrite is iron sulfide (FeS2) - it has a metallic luster and a brassy gold color.
</p><p>Geologic unit: Butte Quartz Monzonite (a.k.a. Butte Pluton), Boulder Batholith, mid-Campanian Stage, late Late Cretaceous, 76.3 Ma
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Locality: loose piece from rubble pile next to a large tailings pond, above the Continental Pit, town of Butte, Silver Bow County, southwestern Montana, USA
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/31040133400/">Pyritic quartz monzonite (Butte Quartz Monzonite, Late Cretaceous; Continental Pit, Butte, Montana, USA)</a>