NORTHEASTWARD DETACHMENT OF THE LARAMIDE EL CRESTON MINERAL DEPOSIT, CENTRAL SONORA, MEXICO

I propose an exploration model in which the early Eocene El Creston molybdenum ore body identified by AMAX, Inc. in 1981 is displaced northeastward from a pervasively mineralized footwall along a middle Tertiary low-angle normal fault. This interpretation draws on geological mapping, geochemical soil anomalies, and drilling conducted during exploration of proposed footwall rocks of the 6600 hectare “Leon” Property with Colibri Resource Corporation (TSX-CBI). Mo-Cu mineralization is hosted by early Tertiary porphyritic biotite monzogranite and rhyodacite porphyry and wall rocks that include Late Cretaceous granodiorite and various Paleoproterozoic gneisses. Three gold-silver-lead-zinc vein systems that extend ~5 km WSW from the core molybdenum zone are believed to be distal mineralization associated with the same Laramide porphyry system. Two sub-parallel detachment faults separate the mineralized footwall rocks from a hanging wall composed of Paleoproterozoic granite overlain by Mesoproterozoic(?) quartzose sandstone. These faults may represent up dip extensions of the El Creston and Gemini faults (Leon and Miller, 1981), down-dropped to the southwest along a N30W striking antithetic normal fault.

Presented herein is a three-step reconstruction of the mineralized region constrained by outcrops of bedded quartzite and various distinct intrusive bodies exposed in three plates of the detachment system. 4km and 2 km of normal slip are restored on the Creston and Gemini detachment faults, respectively, with lesser displacements on antithetic normal faults. The early Tertiary reconstruction yields a SW-vergent asymmetric anticline defined by the quartzite, likely associated with a Laramide thrust system that guided later nucleation of the Creston and Gemini faults.

The age of primary mineralization is constrained at 54±1 Ma by Re-Os analysis of molybdenite (Barra et al., 2004). This is consistent with the timing of mineralization throughout a regional belt of Cu-Mo porphyry mineral deposits extending from southern Arizona through Sinaloa. Future development of the Leon property and/or consolidation with the El Creston property awaits rebound in molybdenum prices and industry acceptance that the Creston fault in its present manifestation is a low angle normal fault rather than a Laramide thrust.