File:Molybdenum ore (molybdenite-quartz veins in alkaline granite, Oligocene, 24-33 Ma; Climax Mine, Fremont Pass, Colorado, USA) (15024574245).jpg

Climax Mine molybdenum ore - multiple molybdenite-quartz veins intruding alkaline granite from the Oligocene of Colorado, USA. (cut & polished slab, CSM # 12334, Colorado School of Mines Geology Museum, Golden, Colorado, USA)

Dark gray = molybdenite (MoS2) Light gray = quartz (SiO2) Mottled yellowish = alkaline granite host rock (Climax Stock, Oligocene)

Central Colorado’s Climax Mine is the largest molybdenum mine on Earth. It is located in the Southern Rocky Mountains, near the northern end of the Rio Grande Rift, a large, Neogene-aged rift valley on the eastern side of the Colorado Plateau.

Molybdenum (Mo - “moly”) mineralization at the Climax Mine occurred as a series of igneous bodies intruded through Proterozoic basement rocks (gneisses and granites). Seven intrusive bodies make up the composite Climax Stock (Oligocene, 24-33 m.y.). Three of them produced significant moly mineralization in the form of molybdenite-quartz veins and disseminated molybdenite (MoS2 - molybdenum sulfide).

The rocks of the Climax Stock are alkaline felsic intrusives. They range from porphyritic alkaline rhyolites to alkaline aplites to porphyritic alkaline granites. In map view, the igneous bodies of the Climax Stock form a roughly circular structure. In cross-section view, each intrusion has an inverted bowl shape.

Published studies on the origin of the Climax Molybdenum Deposit and other Climax-type moly deposits have concluded that they form principally in incipient extensional tectonic settings well inland from shallow-angle subduction zones that have recently ceased.

The rock shown above is a large cut & polished slab of Climax Mine molybdenum ore. Note that the thicker mineralized veins tend to have quartz (light gray) making up most of the vein and molybdenite (dark gray) lining the walls of the vein. Compare that with the thinner veins which are principally composed of ~just light gray quartz or ~just dark gray molybdenite (such molybdenite-dominated thin veins are referred to as “moly paint”).