THE DACITE QUESTION
If volcanic terrain is known to produce rich gold deposits, like Goldfield, Nevada, why do many geologists dismiss out of hand gold deposits in the volcanic terrain of the Superstition Mountains? The answer lies in the Dacite and Tuff Rocks. You see, most of the Superstition Range is formed from just two types of rock; Dacite, which forms from either a pyroclastic flow or as a thick, viscous lava and tuff, which is always formed from pyroclastic flows. Why is this important? Simple, pyroclastic material is not mineralized with metals.
When some volcanoes erupt, they eject a huge amount of ash , and some of this ash may flow down the sides of this volcano in what is called a pyroclastic flow - a mass of very hot (up to 1000 degrees C) ash, gases and rocks traveling up to 100 miles per hour. It was the pyroclastic flow of Mt Vesuvius that first buried the ancient Roman City of Pompeii. After a pyroclastic flow, the material that started out as ash and gas cools into a thick layer of solid rock - layers of non mineralized rock that can be hundreds of feet thick. To get a gold deposit in such country, one of two things must happen. First, the Dacite must overlay a mineralized region, and then, either a vein of material must push up through the dacite from below (post dacite intrusion), or the dacite must be eroded away to expose the pre-dacite material. Since the Western Superstition Range is composed of old volcanic calderas - like Goldfield, Nevada - there is a good chance that the dacite overlays a mineralized region. So, what one needs to find are either a location where erosion has worked down to the pre-dacite underlying rock or where post-dacite intrusions have pushed up through the dacite. However, post-dacite intrusions intrusions present an interesting problem, one that has fooled many a Dutch Hunter.
Just what is a post-dacite intrusion? It means where another, younger rock has pushed its way up through the dacite, such as a vein of quartz. For example, if you drive a nail into a board, the board must be there for the nail to enter it, one might say that in relation to the board, the nail is younger. Let the nail represent the vein and the board represent the dacite, and it would seem obvious that wherever you find nails "veins" in the dacite, you have post-dacite intrusions. And the dacite of the Superstition Mountains is laced with such veins and in places columns of rock that appear to push right through the dacite. Post-dacite intrusions right? Unfortunately, it ain't necessarily so. Dacite is a strange rock in which both the dacite itself and veins or columns in it can form at the same time from the same non-mineralized pyroclastic or lava flow. It is as if the board and the nail in it somehow formed at the same time.
Once the pyroclastic or lava flow has started to cool into dacite, a crust forms and as this crust cools, it shrinks and cracks. These cracks expose the still molten material inside the crust to changes in temperature and pressure. Since the type of rock formed from the flow depends on the temperature and pressure it cools at, veins and columns of non-dacite rock , including quartz, form in the dacite where temperature and pressure are right for their formation. One gets newly formed dacite complete with veins. These different rocks formed at the same time have different hardnesses. After centuries of erosion, the harder rock is left behind, and one has a terrain with what appears to be columns and veins of post-dacite intrusions, but they aren't.
Does this mean that there aren't any post-dacite intrusions? Not necessarily, but it does certainly muddy the waters, making it much harder to identify the true post-dacite intrusions. Just one more nuance to Dutch Hunting.
