In times past, more than a billion years in the past, the familiar mountainous terrain of the southern Sierra Madre of Wyoming, was actually, nothing more than a flat ocean floor in a vast Proterozoic ocean near a subduction zone, where many submarine hydrothermal vents, along with white and black smokers, erupted hot spring fluids, gases, and base and precious metals. These cooled and crystalized into sulfide minerals filling spaces in fragmented rock. Several such deposits formed in this ancient ocean and are now part of Wyoming and Arizona. For example, the great Ferris-Haggarty mine in Wyoming, produced excellent copper-gold-silver rich 'millrock' (volcanoclastic) such as seen in the adjacent photo of a specimen collected from the mine rib of the Ferris-Haggarty mine. And then, there was the world-class copper-lead-zinc-silver-gold massive sulfide at Jerome: talk about an incredible deposit!
What is millrock?
Prior to 1979, the term 'millrock' was essentially unknown except to a few Canadian geologists. So, when a geologist from Conoco Minerals made a discovery of millrock with distinctive colloform massive sulfides and adjacent volcaniclastic breccias at the Itmay mine in the Huston Park area of the southern Sierra Madre, this was the starting gun for a rush to find minable copper, zinc, lead, gold, and silver. Soon, the Sierra Madre Mountains were filled with geologists searching for evidence of ancient black and white smokers, evidence of eruptive paleo-hot springs, and magnetic anomalies over magnetite-rich massive sulfide deposits. It didn't take long, but soon several volcanogenic massive sulfides were found, but a cat and mouse game was played between prospectors and geologists employing their mining rights on public land, and the card-carrying sierra-club forest service employees who did not like to see mining companies exercise these rights expressed in the 1872 Mining Law. So, every time a new discovery was legally made and claimed, the FS withdrew the area as well as access by using wilderness, roadless, etc. designations. Funny, they were never considered roadless or wilderness prior to the discoveries. Where a major base metal district existed, the FS stopped all, legal, access. So, if any politician tells you we are running out of resources - this is nothing more than an admission of their lack of understanding. There are billions (if not trillions) of tonnes of ore, not only in the Sierra Madre and Absaroka Mountains in Wyoming, but also in nearly every known mining district (past and present) in the world.
And this was not the first rush to the Sierra Madre. Rich outcrops of gowan containing copper, silver and gold were found in 1897 along what is now known as Haggarty Creek. This led to the discovery of a rich, massive-sulfide deposit dominated by copper, and development one of the greatest mines in the West as described by a geologist with the US Geological Survey. Similar deposits had already been found (1883) in Arizona at the United Verde mine in the Jerome district.
Being a research geologist at the Wyoming Geological Survey on the UW campus, it was imperative I learn all I could about this new discovery because our office was being invaded by Canadian companies who wanted to also search for VMS (volcanogenic massive sulfides), volcaniclastics, and millrock. Well, it turns out that millrock is nothing more than volcanoclastic breccia associated with submarine hydrothermal vents and is associated with volcanogenic massive sulfides (Edwards and Atkinson, 1986). Just think of millrock as fossilized, submarine volcanic (hydrothermal) vents associated with all kinds of massive copper, zinc, lead, silver and even some gold.
Canadian geologists were well schooled in these types of deposits - they had many up north. This was also true of geologists from Arizona's universities. Such deposits were found on ocean floors where considerable hydrothermal activity occurred. So, massive sulfide metals deposits adjacent to active and prehistoric vents, are referred to as smokers (no, not the cancer-affiliated smokers). And it turns out that the Canadians, who were mining several of these massive sulfides, recognized that the brecciated, volcaniclastic rocks near these smokers and sulfides, were almost always found next to operating mills at operating mines - so, one wise geologist named it 'millrock'.
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| Volcaniclastics at the Itmay mine in the Encampment district, Wyoming. These types of breccia are found adjacent to rich millrock deposits. |
Other things of interest around these massive sulfides include colloform textured massive sulfide deposits that likely congealed in oceanic water to produce rounded blebs of sulfide minerals deposited on the sea floor. Other ore occurs in the volcaniclastics (the millrock). The larger the clasts in the millrock, the closer you are likely to be to the original volcanic vent, and potentially, a Mother Lode.
It turns out that after Conoco Resources found the first volcanogenic massive sulfide deposit at the Itmay mine, other massive sulfides were discovered in the region. So many were found that the Forest Service had to take a deep breath, give up their 8-hour coffee breaks, take some Xanax, and then work an hour or two each week following companies and following geology students who began working on master degrees in the UW geology department under Dr. Robert Houston So, whenever a potentially economic deposit was found, the Forest Service piecemeal withdrew it from mining. And you thought that the US Forest Service was a manager of our public lands.
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| Colloform massive sulfide sample from the Itmay mine, Wyoming. |
Unfortunately, no one told them. The FS office in Laramie wanted to withdraw the entire Sierra Madre and Snowy Range from miners, even though their directive suggested they should be working for the public, rather than for their own biases. It appears that the Laramie office was filled with card-carrying Sierra Club members who were anti-mining, anti-logging, anti-people, etc, etc.
It turns out that Arizona also had Proterozoic-age island arcs with very favorable volcanogenic massive sulfide (VMS) deposits (Hausel, 2019). Not only were some enormous VMS and porphyry copper deposits found in the Patagonia Mountains near the old ghost towns of Harshaw and Taylor in southern Arizona (some of which are classified as major to world-class deposits), there are also famous VMS deposits at the old United Verde and United Verde Extension mines at Jerome in central Arizona. Then there are many more, nearby, VMS deposits.
The United Verde mines were extremely rich massive sulfide deposits - so rich, that mining companies made a fortune mining only the high-grade ore and leaving behind huge tonnages of low-grade ore that likely has considerably more tonnage than the high-grade mined in the past. Some of the high-grade was so enriched in sulfides, that when oxygen reached some of these sulfide ores brought in the mine tunnels, the oxygen began to oxidize ore, giving off so much heat, that some of the rock actually caught fire and burned for years. Just imagine, a rock catching fire in a mine! Within the mine operations, the miners also found excellent black and white, fossilized, smokers that were the source of the ore.
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| Rich, copper-zinc-silver-gold gossaniferous highwall exposed at the United Verde mine, Jerome, Arizona. |
Banded chert was also found in the mines and likely represented siliceous submarine volcanic eruptions that slowly settled by specific gravity in the water, producing color bands based on of slightly differing specific gravity.
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| Black smoker on exhibit at a museum in Jerome |
Such massive sulfides and associated veins occur at a number of localities in the Sierra Madre, such as at the Broadway deposit, Kurtz-Chatterton, and others. But these do not even get close to the number of VMS deposits recognized in Arizona.
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| Steeply dipping, millrock photographed in the Ferris-Haggarty mine by the author. |
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| Massive sulfide from the Kurtz-Chatterton vein Sierra Madre, Wyoming. |
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| Specularite-Chalcopyrite ore from the nearby Charter Oak mine at Puzzler Hill, Wyoming. This ore assay anomalous copper, gold and palladium |
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| Massive sphalerite ore (zinc-sulfide) from the Broadway prospect, Sierra Madre, Wyoming. |
