<<<At the Presidio Mine, the massive limestone at the top of the Permian age Cibolo formation is the most favorable to replacement by solutions, and in the vicinity of the mine is known as the Mina Grande Formation. The erosional surface of the Mina Grande Formation developed karst-topography features providing large open areas that served as a series of channel ways for the mineralizing solutions. Where the conditions were favorable, the deposition of silver and base metal mineralization occurred. The Mina Grande limestone, the product of Permian reef formation, has over two miles of strike length of mineralization zones. The Permian Mina Grande Formation is unconformably overlain by the Cretaceous-age Presidio Formation and the Shafter limestone unit. Fissures and faults are present in all areas of the mine workings, and a large fault that passes through the Mina Grande Mine area interrupts the continuity of the mineralized zones, and possibly served as a conduit for mineralizing solutions. Figure 7-1 is an East-West schematic cross-section through the Presidio mine-Shafter deposit.>>>
This would explain the mineralization at the pit and the viability of same. Limestone easily permeable with numerous faults and fissures acting as conduits. Two miles of strike length. I am guessing the Mina fault allowed mineralization upwards closer to surface.
This would seem to explain Megaw's excitement expressed with the Motley Fool author while driving around looking for the "source". (Peter, feel the source, be the source). Megaw literally wrote the book on this type of carbonate deposit (“carbonate-hosted deposits” because of their irregular but sharp contact with their enclosing carbonate host rocks.) (Megaw, Ruiz, and Titley, 1988).
No wonder. If he could find the "shoot" it could be a very rich area. Lucrative for Aurcana and a fine feather in his cap.
Which leads me to another thought regarding Lenic and Aurcana. I need to mull it over first.