Geology of the La Colorada mine area is a complex mix of Paleozoic meta-sedimentary units and Mesozoic age coarse grained intrusions into those meta-sediments. Late Tertiary age volcanics and conglomerates lie upon the older rocks with about a 25 degree inclined tilt of the horizontal layering of the volcanics. The large set of La Colorada mine related veins cross all rock types except the youngest layered volcanics and conglomerates.
The veins are epithermal, meaning low temperature and deposited in a near surface environment. Probable depth of deposition ranges from about 300 to 1000 meters for most of the vein material at temperatures ranging from 220 to 300 degrees C. These temperature depositional conditions were determined by fluid inclusion examination from 29 samples of veins. Research of epithermal low sulfide veins has shown that contained precious and base metals are deposited from hydrothermal fluids within areas of an open structure or fissure in which boiling has occurred, as the fluids move upward toward the surface from deeper and higher temperature sources, where boiling does not occur due to higher pressures.
The open fissures are eventually filled to form veins, typically with numerous fine bands of various minerals, including quartz, carbonate, sulfides and precious metals. The veins can be rich with bonanza grades occurring in parts of the veins, particularly where boiling occurs. A vapor rich zone is typically formed immediately above a boiling zone, having only low grade precious metal contents but characteristically anomalous volatile components, including mercury, arsenic and antimony. Boiling zones commonly develop along a generally horizontal plane, at a typical depth of about 200-300 meters below the surface. The entire vertical range of the quartz vein may be up to 800 meters, but usually only 300-400 meters may contain significant precious metals. The upper parts of an epithermal vein typically disperse into broad horsetail or shatter zones which may be more amenable to open pit mining.
La Colorada Structural Model
The "mine trend" vein zones are oriented roughly 070 azimuth (ENE), and contain generally northerly dipping veins. After deposition of the veins, the entire region was tilted to the southwest about 25 degrees, such that the individual veins might dip at more shallow angles than when they were originally emplaced. The prospective boiling zone also now plunges (with the tilting) towards the southwest. Subsequently, by means of east side down block faulting (listric dip-slip), the vein zones were cut into a series of at least six discrete en-echelon blocks along a 10 kilometer trend length. Similar listric faulting lead to the development of the "basin and range" blocks that are characteristic of the southwest USA and northwest Mexico.
At La Colorada, these north-south trending post-mineral cross faults have oriented the veins so that each is exposed to the current erosional surface at different levels relative to the prospective boiling zone horizon. As mentioned above, the boiling zone portions of an epithermal system are where "Bonanza" gold and silver mineralization can be developed. Past mining at La Colorada was conducted in the Gran Central block and the El Creston block, where the boiling zones are exposed to surface. Additional potential for resource expansion is highly probable along extensions of the mineralized trends within these two blocks and between the two pits. There is also exploration potential within at least two other adjacent blocks, where the surface exposures show features typical of a vapor zone above a potentially mineralized boiling zone. Deeper testing at projected Bonanza style boiling zone depths in these 2 adjacent blocks is a priority exploration target. While the complex post-mineral faulting in the area has made tracing the system more difficult, it has also preserved considerable portions of the vein system at relatively shallow depths, offering potential for underground mineable targets.