File:Molybdenum ore (Climax Stock, Oligocene, 24-33 Ma; Climax Mine, Fremont Pass, Colorado, USA) 2 (25796157534).jpg

Molybdenite in alkaline igneous host rock from the Oligocene of Colorado, USA. (field of view ~6.3 cm across)

Dark silvery gray = molybdenite (MoS2). Light colored areas = weathered alkaline igneous host rock

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 host rock for the molybdenite in the sample shown above is mostly obscured by a yellowish-colored weathering film that is probably jarosite (KFe3(SO4)2(OH)6 - potassium iron hydroxy-sulfate). Jarosite is a weathering product ultimately derived from pyrite, which occurs disseminated throughout this rock. The brown-colored material is hematite (Fe2O3 - iron oxide) mixed with jarosite. The hematite is an oxidative weathering product of the disseminated pyrite in the host rock.