Monday, December 17, 2018

South Pass gold district - prospecting from the air.

Dan Hausel (GemHunter) stands in front of the 
Atlantic City Hilton at South Pass.

I was paid by the Wyoming Geological Survey at the University of Wyoming, along with some federal US Geological Survey grants to live in a tent for 5 summers in the 1980s and map and explore the South Pass greenstone belt. The state supplied me with nourishing instant oatmeal, spam and instant coffee for breakfast, lunch and dinner, while I lived in my personal tent and mapped the 250-square-mile greenstone belt and its gold and iron ore deposits, mines and mineralized trends. Some weekends, I splurged and wandered into the Atlantic City Mercantile to buy a beer and talk to locals to gain insight into their prospects.

Start with a web search on ‘South Pass gold’, then expand the search to ‘South Pass gold prospecting’. Already, you should be finding information on gold deposits, geology, mines, mining districts and towns. Now expand to ‘South Pass gold mines’. As you find mine names, search the web for those mines. It doesn’t take long and you will have a considerable file.

Now examine South Pass on Google Earth. Search “Atlantic City, Wyoming 82520”. This will take you to one of the larger cities in the state at the base of the Wind River Mountains. When I was a ‘snowbird’ resident of Atlantic City living in a tent, the city population sign read “population about 57”. This was probably an exaggeration unless you include pets.

Examine the region at an eye altitude of 50,000 to 10,000 feet on Google Earth and you will see prominent rock lineation that trends roughly E-W to E-NE in hills surrounding Atlantic City. Move your stylus to 42°29’10"N; 108°40’18"W just east of Atlantic City for an excellent aerial view of lineation. The lineation is prominently expressed in the Miners Delight Formation, a thick rock unit formed mostly of meta-graywacke. ‘Meta’ refers to the fact that the graywacke is metamorphosed (recrystallized), and ‘graywacke’ refers to a ‘dirty’ sandstone that has considerable sand, clay and silt that later recrystallized to a mica-cordierite-andalusite quartzite (metagraywacke).

These rocks were abused by tectonic deformation and squeezed into a steeply-dipping and plunging basin 2.8 billion years old such that we now see the edges of the meta-sedimentary (and meta-volcanic) layers. To visualize this, take a book and lay it flat on a table in front of you. Imagine the pages represent layers of rock deposited on top of one another. Now rotate the book 90 degrees and sit it on the binder. As you view each page on end, they will appear similar to lineaments at South Pass, so now you should have some idea of what happened at South Pass. All of that lineation you are looking at on Google Earth is the metamorphosed sedimentary and volcanic rock beds that have been tilted 90 degrees (more or less) and now sitting on end. Try to imagine the force required to do this to this part of the earth’s crust. To make things more complicated, the entire belt was also folded like an accordion and recrystallized.

As you admire some drainages at South Pass on Google Earth such as Big Atlantic Gulch, Rock Creek, Meadow Creek and portions of the Sweetwater River, keep in mind that hundreds of nuggets came from these drainages in the 19th century, and are still being found to this day. Big Atlantic Gulch, Rock Creek and Meadow Creek cut rock foliation, which is good to keep in mind when prospecting for gold placers. Since gold-bearing structures in this region lie parallel to nearly parallel to foliation; thus, any drainage that cuts foliation will sample these gold-bearing structures and the foliation will provide natural riffles at bedrock.

When I mapped South Pass, I came across two prospectors (Buddy Presgrove and Hank Hudspeth) exploring for gold in Smith Gulch (42°31'3"N; 108°42’19"W) with backhoe and trommel. They recovered gold from two pay streaks: one on bed rock, and the other on false bedrock about 4 feet above bedrock. The two recovered about 20 ounces of gold per week in an area that arm chair prospectors claimed had no gold or was all ready mined out.

There are many placers and lodes of potential interest in this area, as there are in most gold districts. Placers that continue to yield nuggets include Rock Creek (42°29'11"N; 108°42’25”W), Spring Gulch (42°31'59"N; 108°40’43"W), Big Atlantic Gulch (42°29'54"N; 108°42’31”W), Meadow Gulch (42°27’49"N; 108°47’9"W) and parts of the Sweetwater River. To obtain information on ownership visit the Fremont County courthouse in Lander and the State BLM office in Cheyenne.