My interest is in recognizing adakite without going through chemical analysis. I believe there are some situations where remnant garnet minerals might be found....particularly in dike breccia clasts or magnatic enclaves within intrusive material. Could the abundant tourmaline in some porphyry coppers be related to the destruction of eclogite/arclogite material? The source of tourmaline in theory and practice is related to breakdown of biotite bearing material, indeed the world's largest copper accumulation shows a downward trend from tourmaline to biotite accessory phases in typical copper sulfide bearing mixes. Importantly, there is virtually no gold in the world's biggest copper accumulatin...the Sur Sur/Los Bronces deposits of Chile. In typical magmatic systematics, copper only associations are mostly associated with the highest Sr/Y ratios...such as at El Teniente, Los Bronces and Pelambres. That suggests depth of origin or evolution of the silicate assemblage is very deep! Sr/Y being a surrogate for depth...according to Massimo Chiaradia. There are many open-access papers discussing this...but none reporting garnets from drill core in mineralized areas. Frankly, the production of Boron suggests the presence of a strong fluxing agent of some kind able to leach metals that are present as oxide or sulfide phases...but perhaps not in silicates...such as in amphiboles...but who can say...yet.
I enclose a USGS open source article by Blatter et al from 2023 for convenience. Note the garnet-in phase disappears about 1GPa in dacites under equilibrium conditions.
I enclose some images from drilling around a small Cu-Mo porphyry target in the high Miocene belt of Chile. Rock units there host tourmaline, mafic magnetic enclaves, apatite, anhydrite and typical hornblende, biotite, plagioclase, quartz associates with chalcopyrite showing exsolution of sphalerite and possible bornite/cubanite. Below I show dacite like material dike material with possible AluminosilicatePhosphate Sulfate alteration, but no obvious garnets. It looks like adakite...but is described as "patchy wormy andesite DH-02". There are few pyroxene minerals. I include one high resolution image of a possible dacite clast in a breccia matrix and one image of a possible garnet from core images.
I believe it might be possible to actually find garnets, or their remnants....such as biotite? in what might be called "Adakite".
This would be a big breakthrough if such issues as tourmaline, garnet and phyllosilicate systematics could be attributed to derivation from a deep/Adakitic source...below 45-50km.
(PS...the rock unit has had advanced geochemical analysis on zircons and has been found...fertile"...and hosts "small" amounts of Cu).
See...PPT discussion on an exposed Andean Porphyry System...on my RG page.
Note: The blue mineral has been identified as "lazulite" (by X-ray?) I am told. Others claim it could be dumortierite...but it is blocky and not "acicular" enough. It is probably not Chalcanthite...as I thought originally. It's color looks like a copper tourmaline...or elbaite, particularly because of its intimate association with other black tourmalines...schorl? and Chalcopyrite etc. but the trigonal prismatic form is not to be found so far...even on small blue crystal bits...which can be seen below. Lazulite...
Maybe this find in a marginal part of a porphyry copper deposit may shed some light on the issue debated herein.
DILL, H.G., GARRIDO, M. M., MELCHER, F., GOMEZ, M. C. and LUNA L.I. (2012) Depth-related variation of tourmaline in the breccia pipe of the San Jorge porphyry copper deposit, Mendoza, Argentina.- Ore Geology Review, 48: 271-277.
Glückauf
HGD
Dear Steve: the last photo shows typical pseudohexagonal basal cross-section of amphibole, I don't think is garnet at all. Regards, Sebastián
I tried to show the situation....in fact it is not even amphibole anymore...it's magnetite and "epidote" with a little something else. My point is that Chemistry is too hard to use...when you have 10,000 meters of core....or more. Hard enough to get core logging into use really. But if anyone can actually see the remains of source material....ecologititc or whatever. I think that is important. In fact, the fertility indicators are on my side in this case and the system has generated a few hundred million tons of say 0.3 inferred. So...one thing...we do have tourmaline...and I am inclined to think it comes from a fertile phyllosilicate source...perhaps serpentine remnants. If the tourmaline is not associated with say...copper or gold...well...you missed the window of generation...like in the oil business...you missed the oil forming system...too hot, too deep, leaky etc. I say...mineralized tourmaline and dikes associated with a prospect should have signs you can see of it's source...just look and look. Sometimes merely bits in a lithic fragment...see below.
There are reports of garnet from what is describe as the Albanico Fm, apparently associated with the proverbial Adakite. Has anyone actually followed the theory to mineralization. The Albanico is adjacent to but faulted off from the famous Rio Blanco tourmaline copper breccia deposits...
Here is a preprint discussing amphibolitizd garnet bearing eclogite...researchgate posting...
Trace element and isotopic zoning of garnetite veins in amphibolitized eclogite, Franciscan Complex, California, USA | Request PDF
My take on adakites is that they may tap into a fertile part of the "wedge" or they may not.
Here are a few examples of garnetiferous adakite...and...with hornblende...but is there any kind of connection to copper mineralization?
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