RE: RE: Sooty black streaks and carbonatesAlso, apparantly the limestone is not organic. I've searched a bit onthe internet and it seems there is a geologic debate about theformation of limestone (organic vs non-organic). If it is indeed notorganic, could it have prevented the spreading of the metals and actedas a trap, as Russell seems to imply?
First, this is not technically "limestone" in the strict sense, it is "carbonate" rock or mud. Limestone is predominantly calcium carbonate (over 50%) and even at the 11173 feet depth where the drilling stopped the combined Ca and CO3 content was "only" 27% over 1 foot. Perhaps there are some thin limestone layers measured in inches or even millimeters interbedded with other rocks to account for the 27% percent assayed content.
Second, the organic or inorganic formation of limestone is not relevant in the way that you appear to be thinking about it, and "debate" is probably not the right word. It is generally recognized that limestone can form under both organic and inorganic processes, although the vast majority of the time there is some organic involvement. In any case, a carbonate rock can act as a "trap" (personally I would use the term "host") regardless of how it was formed, although certain carbonates are better "traps" than others. Generally speaking, biogenic limestones tend to consist of thicker sequences, more porosity and higher purity, therefore offering greater spatial opportunity for minerals to deposit. In addition, organic chemistry can play a part in fixing the magnesium content in carbonate deposits and their immediate surroundings, which in turn can influence the extent of later dolomitization (magnesium partially replacing calcium in the carbonate). Dolomite can have a significant but variable effect on the precipitation of ore-bearing fluids.
Third, an area where "inorganic limestone" could make a difference in comparison to organic is not in serving as a "trap" but rather as a "cap", like a lid on a boiling pot. While unlikely, this is technically possible when a sedimentary layer solidifies sooner than the sediments below it. In the present instance, for example, a sudden change in the basin floor elevation, possibly due to thrust movement along an early incarnation of the Osburn Fault, could have created a temperature gradient causing carbonates to chemically precipitate out of the water in the Two Mile area and admix with the Pritchard sediments. The precipitated carbonates would have filled in the pore spaces between the grains of mud and silt and may have, under the right circumstances, cemented together this particular layer of sediment while the sediments below remained unconsolidated. Subsequent hydrothermal circulation, hemmed in by the cemented layer above and a steep basin topography along the sides, may have effectively scavanged minerals from the loose Pritchard sediments. The circulating fluids could have eventually been concentrated in a tighter and tigher zone below the carbonate-cemented layer as the underlying sediments solidified from bottom up. The hydrothermal system may have busted through in some locations such as in the vicinity of Hole 5A, depositing minerals in a higher stratigraphy. Or more likely, a later intrusive event, or perhaps even regional metamorphism, could have renewed the hydrothermal circulation and remobilized some (but not all, hopes the Azteca shareholder) of the concentrated minerals from below the carbonate "cap" to points above, along fractures or weak points in the now-solidified rock. It's a long shot but one could argue that the current share price already reflects that.