For you geo-geeks,
The “second” Fig. 1 of this paper (somebody made a boo-boo): “Conceptual model…” shows a striking similarity to Fig 1 of AMC’s new power point presentation “Mapping and trenching…”
If you look at the upper right hand corner of Corbett’s figure at the link you’ll see the prototypical low sulfide epithermal system diagram. Note the phreatic breccia in the upper right corner with the 3 parallel “sheeted veins” (labeled “s”).
In AMC’s Fig 1 note the right hand box near the Fortuna Mine. The gray area inside of the yellow and pink is the brecciated area. A bx is an area with enormous past explosivity in which in this case the granodiorite literally exploded into gravel-like angular bits that got glued together usually by molten quartz. Note the width of the yellow “argillic alteration” this is granodiorite that literally got “cooked” by hydrothermal fluids from an underlying intrusive. Imagine the pressure that this system generated to “cook” and blow apart that much granodiorite.
What happens is the magmatic hydrothermal fluids from a magma chamber are released when the roof of the magma chamber blows and it gets hurled towards the surface where it runs into the water table. This induces a huge explosion that shoots the hydrothermal fluids upwards and laterally. A little tiny historical magma chamber and the adjacent area above its historical roof, now called a porphyry, can’t generate forces and pressures like that.
Note the parallelism of the veins on AMC’s fig. 1, they all run pretty much NW to SE except for M 3. This is what “parallel sheeted veins” do. The biggest faults in the area run NE to SW which is 90-degrees different which is the norm for these side faults of smaller size. All of the showings in AMC’s Fig. 1 are just side channels off of the 7 Km long swath of about a dozen intrusives seen on the hyper spectral satellite study of Perez.
Keeping things in context, this Merlin-Fortuna area is just a little tiny sliver of a much bigger entity. It’s nice because it’s hi grade and at surface which makes it an excellent early production opportunity in order to get some cash flowing. The AMC slide deck mentioned that these veins are “preserved” as noted by the “quartz carbonate textures”. Corbett’s diagram shows that green hot air balloon figure labeled “carbonate base metal gold”. In not so well “preserved” epithermal deposits this stuff has been eroded away over the millions of years. The “quartz sulphide gold/copper” is often all that is left but we got lucky here.