Ken Snyder in caldera country

Having mapped in detail most of the Midas district for two different mining companies, I have to take exception to the implication in your article that the Ken Snyder mine does not lie in a caldera setting (“Ken Snyder mine nears completion,” T.N.M., Oct. 26-Nov. 1/98). This mine certainly does lie in such a setting.

A small, 2-mile-diameter caldera is centred 11,000 ft. northwest of the main Ken Snyder discovery, and a ring fracture zone lies within 5,000 ft. of the Gold Crown-Colorado Grande veins intersection. I have mapped three volcanic domes which were intruded along this zone and which lie along, and just inboard from, the ring fracture.

Other evidence for the caldera includes a central resurgent mafic dome at North Midas (bimodal volcanism occurred here) and a thick, very coarse clast rhyolitic pyroclastic partial moat filling (referred to as lithic tuff by others), which abruptly thins outside the ring fracture. The coarse clast pyroclastic unit is critical to understanding the geology, as it represents a catastrophic proximal event that transgresses stratigraphy. It is also a horizon that localizes ore.

There may well be an anticline, as has been pointed out by Ken Snyder, but it would have been superimposed upon the caldera and could have been caused by magmatic tumescence. In fact, there are suggestions of other, larger calderas that have not been delineated in the area. That a caldera model did not play a role in the mine’s discovery is not relevant to whether such an environment existed at Midas.

William Fuchs, Ph.D.

Reno, Nev.

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