STRUCTURAL ANALYSIS OF THE NOPAL I URANIUM DEPOSIT, PEÑA BLANCA, MEXICO

There are numerous uranium occurrences and 105 airborne anomalies within the Peña Blanca district; located approximately 50 km north of the city of Chihuahua, Mexico. The Nopal I uranium deposit has been studied extensively by researchers in Mexico, the U.S., and Europe. This deposit is hosted by a brecciated rhyolitic ash-flow tuff, exposed at the ground surface, and extends downward about 90 m; the base of the deposit is about 130 m above the water table. Fieldwork indicates that there are two dominant fault and fracture systems that bound the western and eastern margins of the Nopal I deposit. The western fault is nearly vertical and strikes 305^o. Slickensides on the fault surface dip 20^o. The fracture system that bounds the eastern margin of the deposit strikes ~350^o and shows very little vertical or horizontal movement. A third fault system is exposed along the +10 level (the top) of the deposit. This fault is nearly horizontal with a dip of ~20^o to the west and has slickensides that trend 260^o. The west and east bounding fault and fracture systems occur throughout the Nopal I deposit and appear to have channeled subsequent hydrothermal fluid flow, which resulted in the formation of the breccia pipe that currently hosts the uranium ore. Detailed mapping of the Nopal I deposit shows that brecciation intensifies towards the southwest where the west and east bounding fault and fracture systems intersect. Movement along the west fault appears to have increased brecciation, which in turn enhanced permeability. Therefore, the deposit is zoned, with increasing oxidation towards the southwestern portion. Other minor and younger fault and fracture systems also occur throughout the deposit. These younger fracture systems appear open and permeable to meteoric fluids. The young fractures are lined with secondary U⁶⁺ minerals where they intersect the main ore body.