RE: Question for Sunshine and other geosFirst off, as I've already stated, I am not a professional geo, I just play one on stockhouse. (Thanks DB for the shout out in your latest piece!)
(1) How is alimestone unit in a part of the Pritchard that came 1.5 to 1.8 billionyears ago, before there were critters alive? - The Pritchard was actually laid down starting about 1.48Ba and at that time there were already "cyanobacteria" (essentially a simple algae). Indeed, there is evidence of cyanobacteria going back 2Ba elsewhere in the world and at least 1.5Ba on the North American craton. What evidence? It is called a "stromatolite" which is basically the equivalent of a coral reef. These stromatolites have been found in a preserved state in Pritchard outcrops outside of the Silver Valley. Indeed, further east around Libby, Montana, which was closer to the edge of the sedimentary basin and away from the most active area of rifting, the Pritchard actually contains some "limestone units". This, however, isn't your grandfather's limestone but more like calcareous mud and carbonate mixed together. Shallower, less disturbed water would have meant more accumulation of algae detritus, which is the probable source of the carbonates in the Pritchard. The Spar Lake redbed stratiform copper-silver deposits such as Troy, Rock Creek and Montanore are all hosted by such detrital carbonates within the Pritchard. So, as I've said before, there is absolutely no reason to be totally surprised that there are some carbonates within the Pritchard in the Silver Valley. The Azteca drills will soon reveal just how much carbonates.
(2) The Silver Valley is apostage stamp in size, compared to the Sierra Madre Trend in Mexicothat is 75 miles long. But Silver Valley has out-produced. - There is little to compare between Sierra Madre and Silver Valley. The former is mostly epithermal deposits in a volcanic terrane whereas the Silver Valley is hosted in one of the world's largest failed intracratonic rift basins. There were literally trillions of cubic yards of sediments laid down in this basin. One of its unique features is the depth of the sediments, up to 20km (12 miles), with the Pritchard accounting for more than half of that. With a number of very large hydrothermal systems being active as a result of intrusives and regional metamorphism, the rich mineralization is easily explained as being leached from the ample sediments. In the Silver Valley, around Libby, and elsewhere, the ore fluids were concentrated and structurally controlled as they formed veins and stratiform bodies often associated with carbonates. This is a different depositional model from the Sullivan deposit in BC, which is a SEDEX deposit formed when huge volumes of hydrothermal fluids were "belched up" in a relatively short period of time and basically precipitated in layers on the sea floor. The structural control at Sullivan, as it were, was the sea floor itself.
(3) Does it make sense that thecarbonate unit failed at 005 and so sulfides strewn upwards there,whereas at 006 the trap held firm and thus no sulfides got through?Could it all be sitting there, super concentrated? - I'm not familiar with every carbonate system out there but the vast majority are replacement or filling. Basically, carbonate rocks dissolve relatively easily, leaving lots of holes and openings, and carbonate rocks also tend to readily react with ore-bearing fluids. So when these ore-bearing fluids move through the carbonates, the minerals will often end up being "trapped". That is, trapped within the carbonates, not below them. Perhaps a better descriptive term would be that the carbonates are "attractive host rocks". Indeed, one of the most effective ore-forming sequences is a carbonate host unit that is partially capped by an impervious layer (such as clay or even a volcanic dike or sill). I can't really think of many analogies where the carbonates themselves have acted as a cap, and where it does happen, it would probably mean that the rocks immediately below the carbonates are an even better host for mineralization than the carbonates. But, what kind of rocks would those be? One big caveat to all of this is the uncertain timing of when the minerals were actually deposited. If it was after the carbonate rocks "firmed up" while the underlying mudstones remained unconsolidated, it would technically be possible (though unusual) for the carbonates to have acted as a "cap" instead of an "attractive host rock". More likely the ore would still be concentrated within the lower layers of the carbonate rock although that would be indicative of a relatively weak, and therefore probably not very well mineralized, system.
(4) Could it all be sitting there, super concentrated? - It is actually quite possible that the massive sulfides in Hole 5A are already hosted in a similar carbonate analogy. The massive sulfides would have essentially bunched up in a stratiform layer as the ore fluids reacted with and replaced the carbonate rocks. Clearly, however, Hole 5A was a localized event at this particular stratigraphy since neither Hole 5B or 6 hit similar massive sulfides on plane. But given that both holes are currently in carbonates at depth, this would at least give some hope that these deeper carbonates may have also acted as "attractive host rocks". As far as "super concentrated", it depends on a number of different factors that are impossible to predict with any level of certainty -- although mineralization (at minimum anomalous) in the upper carbonate sequence would be a good sign. While pending those assays, Azteca is pretty much back at the same place it was a few weeks ago. Ideally that will prove to be a "two steps forward, one step back" outcome but only time will tell.