To date, it is highly recognized the association between alkaline/calc-alkaline potassic rocks and gold and copper-gold deposits (Muller & Grooves, 2016; Muller, 2001; Jensen & Barton, 2001). Although not yet perfectly clear in some. I am now focusing on the Roman Magmatic Province (Italy), and I was wondering why, although in some districts (especially Sabatini and Vulsini) the whole rock composition and partially the mineralogy resemble those of some high-k rocks of gold-related deposits (Cripple Creek, for instance, where phonotephrites crop out ), this Magmatic Province didn't develop any precious metal mineralization. Obviously, whole-rock and mineralogical characteristics do not define the chance to develop or not a deposit, but they make comparable these rocks with other ones associated to dep worldwide.
I reach out just to open a discussion about this topic because I think it is very interesting and might lead to distinguishing, in some cases, the limit of the accepted model.
Food for thought! :)
How much potassium must an ultrapotassic rock have?
During exploration for Archean (2.7 b.y.) Abitibi gold-bearing porphyries (probably volcanic in origin) an encouraging sign would be the presence of vein quartz with secondary biotite +orthoclase.
Hi Michael Issigonis, thanks for your interest. Ultrapotassic rocks are defined by Foley, 1987 ( Article The ultrapotassic rocks: Characteristics, classification, an...
) as those having K2O > 3 wt.%, MgO > 3 wt.% and K2O/Na2O > 2.The porphyry you are talking about was hence ultrapotassic rocks-hosted?
Hi Francesco Palozza
In the samples that I have taken the above porphyry does not have all the requirements to be ultrapotassic. Although half the samples had a K/Na ratio more than 2, the MgO was not above 3.
However, its hydrothermal biotite was very prominent:
Let me guess: I think the district might be "unlucky" in somehow not being tapped into the right magmatic process, or at least a complete one that "breached" the surface. I attach below an article that illustrates the situation with regard to porphyry coppers. I think it is the same with extreme alkalic gold systems like Cripple Creek. If you look for the signs of the necessary reactions...then the products...gold-copper should be there...somewhere...among the "trees". Question: is the magmatic system sufficiently oxidized to produce iron oxide "knots" like I show below? I suspect they are necessary. Without such indicators the necessary process might not have happened...or at least not to where you can see it.
The images above are from "sun et al." Mine are not so pretty. Same process I believe.
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