La Colorada

Geology

The geology of La Colorada consists of Paleozoic to Early Mesozoic metasediments cut by Upper Cretaceous volcanics. All of these units are intruded by Tertiary intrusives that include granitic to dioritic phases and andesitic porphyry. Late-Cretaceous to Tertiary volcanic rocks and associated continental clastic rocks unconformably overlie the Triassic and older rocks. There are two distinct divisions of the volcanics: A Lower Volcanic Complex dated at 100 to 45 m.y. and an Upper Volanic Complex dated at 34 to 27 m.y. The Lower Volcanic Complex is composed mainly of andesite with interstratified rhyolitic ignimbrites and minor interstratified basalt. The overlying Upper Volcanic Complex is composed of extensive rhyolite and rhyodacite ignimbrites with minor interstratified basalt. It constitutes the largest ignimbrite field in the world. The upper sequence unconformably overlies on the older sequence and infills deeply incised paleotopography in the older rocks. Late Cretaceous to Early Tertiary plutonic rocks (diorite, granodiorite to granite) of the Sonoran Batholith outcrop throughout the region and have been dated from 90 to 40 m.y.

The La Colorada Gold District has many of the characteristics of a low sulphidization epithermal-vein type gold-silver deposit. The district underwent a complex hydrothermal history related to Cretaceous plutonic activity, later higher-level plutonic events, and finally a mid-Tertiary vein system which shares characteristics in common with both a deep epithermal environment and a high-level mesothermal system. Alteration can be seen in the older metamorphic and intrusive units mostly as silicification, hematization and argillic alteration. The Tertiary volcanic rocks in the district are clearly post-mineral and are unaltered.