W. Dan Hausel was the first to recognize that the Rattlesnake Hills was a gold district overlooked by everyone. Thus in 1981 and 1982 he set out to explore this region searching for bulk minable gold deposits and finding numerous gold anomalies. He later mapped the district. Today, we know this district has commerical gold mineralization comparable to the Cripple Creek district in Colorado, as predicted by Hausel 3 decades ago.
At least four types of gold mineralization are recognized in the Rattlesnake Hills:
(1) Stratabound exhalative gold (moderate- to low-grade) in chert and silicified zones parallel to foliation;
(2) Stockworks gold in basement border gneiss;
(3) Secondary veinlet gold crosscutting banded iron formation, and;
(4) Disseminated gold in Tertiary breccia adjacent to alkalic plugs.
The Rattlesnake Hills (RSH) had been overlooked for gold, even though it had all of the earmarks of being a major gold district. In 1981, I decided to take a look as the RSH appeared to have the best possibility for a major gold discovery in Wyoming next to South Pass, where several gold deposits had been found in the past (Hausel, 1989).
The RSH is an old metamorphic terrain (3 to 3.3 billion years old) flanked to the north by younger (Phanerozoic) sedimentary rocks and intruded by a several Tertiary alkalic igneous rocks (40 to 44 million years old). This was an economic geologist’s dream – the geology couldn’t be any better for discovery of major gold deposits, and it was unexplored in modern history. Following my discoveries, other discoveries were made in this district and it is likely that some >1 million ounce gold deposits will be outlined in the near future.
It was as if the entire world had missed this extraordinary terrain. The only geological studies in the area were esoteric research projects at the University of Wyoming that focused on the geochemistry of the igneous (volcanic) rocks. The structure and characteristics of the metamorphic rocks were unevaluated (as was most of metamorphic rocks statewide).
I was amazed that it remaine unevaluated for gold in 1981. It was a greenstone belt intruded by several alkalic igneous rocks – these alone should have attracted attention. The metamorphic terrain was a large fragment of a greenstone belt. Greenstone belts are so important in economic geology that volumes of publications including entire books have been written about these terrains. The terms ‘greenstone belt’, ‘gold belt’, and ‘gold district’ are used interchangeably in many publications because of the common occurrence of economic gold in greenstone belts. Because Wyoming contained a group of greenstone belts, over the years I dedicated myself to specializing in greenstone belts along with my interest in diamonds and colored gemstones.
The RSH were not the only greenstone belt in Wyoming. In the same year (1981), I started another gold rush in the Seminoe Mountains greenstone belt when I found more than a dozen specimens of quartz with visible gold (a rule of thumb is that a hand specimen with a speck of gold visible under a 10x hand lens will assay >1 ounce per ton (opt), and some specimens I found had relatively large specks and rods that were visible without a hand lens). I suspect that anyone with a metal detector would have a field day in that area. I assayed a couple of samples that did not have visible gold and these yielded 2.87 opt Au (gold) for a sample of quartz and 1.15 opt Au for a sample of iron formation. I later identified a few hundred gold anomalies in the South Pass greenstone belt and a few of these are likely associated with major gold deposits.
Greenstone belts almost always have gold deposits. These ancient terrains represent deep basins that were filled with thick piles of sedimentary and volcanic material containing above average gold content. As a result, they have enormous potential for economic gold mineralization. If the gold is leached from the rocks, it could form one heck of a gold deposit. The principal way to concentrate (or focus) gold from these rocks is by tectonic events. Essentially all greenstone belts have been subjected to a number of tectonic events over the past 2 to 3 billion years. These produced considerable folds along with many shear zones (faults) (Figure 1). Some tectonic events were intense enough to heat the gold-bearing rocks and extract a considerable volume of metamorphic fluid with gold in the rocks and minerals. Where fractures developed or where there was intense folding, the fluids were squeezed into these low pressure zones.
The RSH were no exception. In addition to the typical auriferous veins, shear zones, and folds, the RSH were intruded by ‘hot’ volcanic rocks at a later time in geological history. This is what really places the RSH into a category all by itself. So not only does the RSH have gold deposits formed by volcanic eruptions produced during the formation of the original basin (3.3 billion years ago), there are gold deposits that formed during tectonic deformation (possibly 2.6 billion years ago) and gold associated with the hot magmas that erupted around 40 million years ago. Some undeformed jasperoids found along the edge of the greenstone belt suggests a possibility for an even more modern gold event that remains untested (Hausel and Sutherland, 2000).
Setting & Location
The RSH lie west of Casper, along the northern edge of what is referred to as the Granite Mountains, a group of low-lying hills formed of granitic rocks with scattered metamorphic belts (Figure 2). Uranium was mined along the northwestern flank (Gas Hills) of the Granite Mountains and to the south at Crooks Gap. Jade and jasper are also found in the Granite Mountains and the author discovered some ruby deposits in this area: other ruby and sapphire deposits were found by prospectors from Casper. Recently, the author discovered the source for the popular Sweetwater agate and found the first reported fire and precious opals in Wyoming in the Cedar Rim opal field in the Granite Mountains (Hausel, 2008). This region also has gem-quality apatite and diamonds have been reported but are unverified (Hausel, 2009).
DISCOVERY OF A GOLD DISTRICT
In 1981, I headed to the RSH to search for disseminated, replacement, and gold veins. Not knowing much about the area, I decided to drive from the north so I could look for exposed or hidden intrusives in Phanerozoic limestones to search for replacement gold deposits. Access was difficult and I ended up ripping the muffler from my field vehicle. And as I entered the metamorphic terrain, I discovered a quartz vein (exhalite) that I named in honor of my muffler. The first gold discovery in the RSH was named the Lost Muffler Prospect (Hausel and Jones, 1982a).
Since I was employed by the Wyoming Geological Survey, my section (metals and precious stones) had no real budget for assays. I had already used much of my budget to assay a couple of samples from the Seminoe Mountains. My assay budget, as sad as it sounds, was only $100/year. So I had to search for outside funding and applied for a grant from the University of Wyoming Mining and Mineral Resource Research Institute. The grant proposed to search for a bulk minable gold deposit in Wyoming (Hausel and Jones, 1982b). The university was interested as they were looking to expand their institute to conduct metallurgical research on disseminated gold deposits. They provided me with a small grant as much of the money had already been committed to other projects on campus.
At any rate, the Lost Muffler was a significant discovery. A whole new gold district was found - something that rarely happens in modern history. Undiscovered gold deposits are periodically found, but new districts with many gold deposits are almost unheard of, particularly in the lower 48 of the US. A couple of samples were collected at the Lost Muffler that assayed 0.1 to 0.3 opt gold. I traced this vein over an easterly strike length of 2.5 miles!
Following the discovery, I continued reconnaissance in the district when possible. I was spread so thin as I was one geologist with one quarter-time assistant, and I was in charge of all minerals research (gold, copper, lead, zinc, rare earth metals, diamonds, gemstones, platinum group metals, uranium, industrial minerals, ferrous metals, ferroalloy metals, and strategic metals) and was also in charge of mapping all volcanic and metamorphic terrains in the state.
As I continued to investigate the RSH, I found anomalous gold in veins, exhalites, jasperoids, iron formation, stockworks (group of veins) and breccias associated with the Tertiary alkalic volcanic rocks. I was hoping for funds to do a thorough geological, geochemical and geophysical investigation, but priorities were elsewhere. I continued with a $100/year assay budget. If sufficient research funding would have been provided, Wyoming would likely have several operating gold mines today, but as it stands, the state has none.
ACNC, Bald Mountain Mining, and a private consultant from Riverton followed my work in the RSH. ACNC made additional exhalite (vein) discoveries and Bald Mountain and the Riverton consultant staked several claims. Years later, Canyon Resources & Newmont Gold began exploration where I mapped an extensive brittle breccia within a 1 mi2 basin between three Tertiary intrusives referred to as Goat Mountain, Sandy Mountain and Oshihan Hill (the latter two were named by me). These companies explored the breccias & likely made a million ounce gold discovery without realizing it (Hausel, 1996; 1997, Hausel and others, 2000). Presently, Evolving Gold continues to explore this project at Sandy Mountain.
As I mapped this metamorphic belt, it became clear that the metamorphic rocks represented a fragment of an Archean greenstone belt similar to South Pass, the Seminoe Mountains, Elmers Rock and to rich greenstone belt associated gold districts in Australia, Africa and Canada (Hausel, 1994, 1995). It is also important to realize that a very large part of this belt is not exposed! The RSH greenstone belt continues under Tertiary sediments to the south, east & west. And it is likely that a large part of the greenstone belt sits under Tertiary & Paleozoic cover to the north. How much of the belt remains hidden is unknown, but the geology supports that what is hidden is larger than that exposed. Thus it is likely there are several hidden gold deposits!
In addition to the Lost Muffler vein, another 4500-foot zone jasperized breccia vein was mapped that had anomalous gold. Nearby, iron formation also contained anomalous gold. A stockwork along the southwestern margin of the RSH was anomalous. This, the breccias and Tertiary plugs at Sandy Mountain & Oshihan Hill was significant (Hausel, 1995, 1996). Several grab & composite chip samples of brecciated metamorphic rock in the basin south of Sandy Mountain and north of Oshihan Hill contained gold. These were collected over a surface area of one-square-mile suggesting a sizable, disseminated gold deposit occurred in this area! The country rock in this area was brecciated & locally gossaniferous supporting the presence of widespread gold.
Based drilling by Canyon Resources and Newmont in the 1990s, disseminated mineralization averaging 0.042 opt Au was identified (Hausel and others, 2000). This same deposit is under investigation again. Evolving Gold drilled this brittle breccia and intersected significant gold at depth. According to press releases, Evolving Gold intersected a high-grade mineralized zone along the south flank of Sandy Mountain (what they refer to as their North Stock).
Evolving Gold intersected auriferous mineralization in sections 24 and 25, T 32N, R88W. According to Evolving Gold, a 215-foot thick zone was intersected in drill hole RSC-020 along the south flank of Sandy Mountain. This zone averaged 0.315 opt Au.
Within this drill hole, a 120-foot thick zone averaging 0.492 opt Au, a 40-foot zone averaging 1.147 opt Au and a 5-foot zone of 4.526 opt Au were intersected. A composite of mineralized zones includes 610 feet of 0.125 opt Au.
At RSC-007, 464 feet to the northeast, 430 feet of mineralized rock was drilled that yielded an average gold content of 0.08 opt. At RSC-003 670 feet north of RSC-007, 480 feet of mineralization was intersected that averaged 0.085 opt Au. Drilling by Evolving gold has identified a mineralized body that has a 1,440 foot strike length that is 640 feet wide and 1,760 feet deep. The data supports a central high grade mineralized zone surrounded by a low grade gold halo producing a large tonnage gold deposit. Mapping by the author suggests that this mineralized zone will be expanded with additional drilling.
The auriferous ore body was intersected in sections 24 and 25, T 32N, R88W. Targets in this area include: (1) alkalic gold with potential for a multi-million ounce deposit, (2) multiple porphyry targets, including southeast porphyry zone, (3) mineralized porphyry dikes (97.6 feet with 0.066 opt Au, including 48.5 feet at 0.11 opt Au, (4) Deep stockwork mineralization with grades up to 0.287 opt Au.
The following drill results are reported:
North Stock Zone (Sandy Mountain)
RSC-003 (146.3 meters @ 2.92 gpt Au) (468 feet at 0.12 opt Au). RSC-020 (67.1 meters @ 10.80 gpt Au) (214 feet at 0.378 opt Au). RSC-089 158.5 meters @ 2.64 gpt Au (507 feet at 0.09 opt Au)
Antelope Basin (Breccia between Sandy Mountain and Oshihan Hill):
RSC-019 163.1 meters @ 1.25 gpt Au (521.6 feet at 0.044 opt Au). RSC-042 76.2 meters @ 1.70 gpt Au (243.8 feet at 0.06 opt). RSC-078 76.2 meters @ 1.77 gpt Au (243.8 feet at 0.062 opt)
Gold Porphyry Target
RSC-006 14.3 meters @ 2.21 gpt Au (45.8 feet at 0.077 opt). RSC-027 35.1 meters @ 1.74 gpt Au (112.3 feet 0.06 opt). RSC-027 30.5 meters @ 1.89 gpt Au (97.6 feet at 0.066 opt) (from the Evolving Gold website).
So will a gold mine be developed? The RSH are in the middle of nowhere with no nearby active streams and no population to speak of. When I worked in this area it was rare to see anyone – I operated a lonely campsite visited only by coyotes and jack rabbits. So the location is favorable. The ore grades are also very good – as a comparison, many open pit mines in Nevada produce ore that has an average grade of only 0.03 to 0.15 opt Au.
Evolving Gold has a very good property that was considered highly prospective >25 years ago. The very successful drill results achieved by Evolving Gold is extraordinary and they have shown continuation of the mineralization at depth. It takes a rare company to make a mine, and it appears from the results to date, the Evolving Gold may be one of these rare companies. It has been said many times in the industry that "mines are not found - they are made!" This will become Wyoming's first gold mine in more than a century.
If they can make a mine out of this property, it will result in other gold mines in Wyoming. Any mine will result in increased activity in Wyoming. In the past, very significant gold anomalies were also identified at Mineral Hill, Black Butte, Bear Lodge Mountains, South Pass, Seminoe Mountains, Medicine Bow Mountains, the Sierra Madre and elsewhere (Hausel, 1989, 2010). Few of these have been explored in any detail.
Hausel, W.D., 1989, The Geology of Wyoming's Precious Metal Lode & Placer Deposits: Geological Survey of Wyoming Bulletin 68, 248 p.
Hausel, W.D., 1994, Preliminary geological map of the Rattlesnake Hills supracrustal belt, Wyoming: Geological Survey of Wyoming Mineral Report MR 94-1 (scale 1:24,000).
Hausel, W.D., 1995, Preliminary report on the geology and gold mineralization of the Rattlesnake Hills supracrustal belt, Wyoming: Wyoming Geological Association Resources of Southwestern Wyoming Guidebook, p. 361-372.
Hausel, W.D., 1996, Economic Geology of the Rattlesnake Hills Supracrustal Belt, Natrona County, Wyoming: Geological Survey of Wyoming Report of Investigations 52, 28 p.
Hausel, W.D., 1997, The Geology of Wyoming's Copper, Lead, Zinc, Molybdenum and Associated Metal Deposits: Geological Survey of Wyoming Bulletin 70, 224 p.
Hausel, W.D., 2008, Cedar Rim Opal - Discovery of a Giant Opal Field: ICMJs Prospecting & Mining Journal, v. 78, no. 2, p. 18-45.
Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.
Hausel, W. D., 2010, Gold – Geology, Prospecting Methods & Exploration. Gemhunter Publishing, in Press.
Hausel, W.D., and Jones, Suzanne, 1982a, Field notes - Lost Muffler gold prospect, Rattlesnake Hills: Geological Survey of Wyoming unpublished Mineral Report MR82-9, 5 p.
Hausel, W.D., and Jones, S., 1982b, Geological reconnaissance report of metallic deposits for in situ and heap leaching extraction research possibilities: Geological Survey of Wyoming Open File Report 82-4, 51 p.
Hausel, W.D., and Sutherland, W.M., 2000, Gemstones & Other Unique Minerals & Rocks of Wyoming - A Field Guide for Collectors: Wyoming Geological Survey Bulletin 71, 268 p.
Hausel, W.D., Miller, D.R., Sutherland, W.M., 2000, Economic diversification through mineral resources: Wyoming Geological Association Field Conference Guidebook, p. 209-225.