The CuMo deposit is located at the south-western end of the Idaho-Montana Porphyry Belt within the Atlanta Lobe of the Idaho Batholith. Igneous complexes in this belt are interpreted to be related to an Eocene, intra-arc rift, and are characterized by alkalic rocks in the northeast, mixed alkalic and calc-alkalic rocks in the middle, and calc-alkaline rocks in the southwest. The CuMo deposit is typical of large, dispersed, low-grade molybdenum ± copper porphyry deposits that are associated with hybrid magmas typified by fluorine-poor, differentiated monzogranite igneous complexes. In terms of potential total contained molybdenum, CuMo ranks first among all current porphyry (Mo) resources.
The local geologic setting is a series of Tertiary igneous rocks ranging in composition from quartz monzonite to rhyolite porphyry that intrude the Idaho Batholith. All phases, with the exception of the rhyolite, appear to be co-magmatic and contain molybdenum mineralization. The deposit appears to be located at the intersection of two regional structural trends: a northeast structural trend, characteristic of the trans-Challis fault system, and an east–west trend that contains a Tertiary dike system. Faults and mineralized structures identified to date dominantly trend to the northeast.
Mineralization on the CuMo property occurs in fractures and veinlets developed within various porphyry units and surrounding country rock of the batholith. The mineralization is associated with quartz monzonite porphyries, but high-grade sections often occur within the older Idaho Batholith quartz monzonite adjacent to or within porphyry bodies. Molybdenite (MoS2) occurs in quartz veins, veinlets and vein stockworks, with individual veins ranging in size from hairline fractures to banded veins up-to-ten centimeters in width. Chalcopyrite occurs in the upper portion of the deposit and is associated with fracture-controlled secondary biotite alteration, and early-stage patches and fracture-controlled dark chlorite-epidote-magnetite +/- pyrite alteration. Molybdenite mineralization generally becomes stronger at depth where the secondary biotite alteration gives way to subtle K-feldspar alteration. Minor tungsten in the form of scheelite is common (40 ppm) and closely parallels the distribution of molybdenum. All phases have been variably overprinted by extensive and deeply penetrating argillic alteration characterized by chlorite, smectite +/- kaolinite and calcite. The deposit as a whole is sulfide-poor and pyrite is generally very minor.
The following lists the major geological stages, identified to date, for the formation of the CUMO Deposit.
| Stage 1 Cretaceous | Idaho Batholith Intrusion, generally quartz monzonite in composition. |
| Stage 2 Early Tertiary | Major deformation event resulting in trans-Challis fault system and intrusions such as the Boise Stock. The older porphyry copper-silver mineralization appears to be related to these older Tertiary rocks. The mineralization results in typical porphyry copper zoned alteration from argillic to propylitic. |
| Stage 3 Tertiary | Intrusion of a series of quartz monzonite to rhyolite porphyry dikes with rhyolite being the youngest molybdenum-copper bearing mineralization accompanies the intrusions along with a second phase of mainly argillic alteration. The dikes and mineralization crosscut the older copper porphyry system. The relationship to the older Tertiary intrusions is unknown at this time. |
| Stage 4 Late Tertiary | The area is crosscut by a series of andesite and lamprophyre dikes and numerous quartz and carbonate veins. The quartz veins carry gold, silver, lead and zinc mineralization. Locally the quartz veins contain coarse remobilized copper and molybdenum. |