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Sunday, July 14 2019

Alluvial gold refers to tiny gold flakes that come to be through water erosion and movement. In geology, alluvium alluvial goldis loose sediment which has been eroded from a primary source, transported and further eroded by water, and redeposited. Since gold is extremely dense, it is easily trapped alongside other dense alluvial particles. The bits of gold found in these deposits of alluvium are called “alluvial gold.”

The alluvial environment is a very complex one, with many natural forces competing with each other. The forces that lay down gold are a summation of many flood water events of varying intensity that cause the reworking of sands and gravels. Between major flood events, water naturally flows along the already created path and typically the gold present in the gravel or on the surface will not move much. But the larger flooding events can change the drainage routes and even the river’s shape. Some curves can be shut off from the stream and bypassed.  Where the water speed decreases, gravels will drop out of suspension, creating alluvial pay streaks that are typically located along and near the riverbed.

To identify where a pay streak might be located, take the flow of the waterway into consideration. The most productive streaks are formed as a result of major floods that are significant in terms of both water flow and intensity of erosion. Greater amounts of gold are present here as compared to regular gravels. Pay streaks tend to possess a comet-like form. At the “head” or “heart”  is found the richest concentration of gold. At this location, the gravel is coarser and the sandy and silty fraction is much less. This little bit of silt is present only for a few centimeters on the surface, laid down in the last phases of the flood as the silt is dropped. The gold in these gravels is typically small flat flakes, with maybe a small picker or two.

As gold prospectors, our objective is to learn to read a stream and recognize the pay streaks it contains. Think about where you are going to dig before you start and then repeatedly test the gravel you are processing. It is important to consider the presence, form, and depth of the bedrock on which the water and all the alluvial gravel deposits are sitting. In many cases, the gold will naturally concentrate in the lowest part of the riverbed, making a gold-rich path. During high water events, much of the gold is picked up and put back into motion, which leads to forming new pay streaks. Some will be laid back down along the low line of the stream, but may also end up a little farther downstream. The gravels in contact with the bedrock or false bedrock base are often the richest. The same facts apply to the alluvial pay streaks that are formed on gravel bars— the lowest level of the gold-bearing gravel is normally the richest.

The alluvial environment changes over time. Alluvial pay streaks generated 100 years ago could become buried, then subsequently be eroded and exposed again. The erosion might be in part or in total, generating a new series of paystreaks further downstream. Pay particular attention to large boulders and trees. Obstacles like these may partially block the water flow and provide an opportunity for increased gold concentrations, especially behind the obstructions. If there are fissures, holes or natural traps in the stream, be sure to sample these areas, too.

Although high water events are sporadic, when you do find a paystreak caused by one, it can be a very productive spot. Stick with it and keep in mind most are small and narrow and best worked by hand with a sluice and gold pans. 

Nugget of News Blog

Posted by: Denise AT 08:09 am   |  Permalink   |  Email
Wednesday, March 08 2017

You might think that glaciers are important to gold prospecting only in Alaska and the Yukon, but glaciers have also affected placer deposits in the Midwest and some western states, too. 

What is a glacier?

Glaciers are formed when a huge mass of ice is created by accumulated snow. The body of ice grows over the years when more snow falls during colder months than melts during warmer months. Glaciers shrink when the melting is greater than the snow accumulation. Once the weight of the glacier grows large enough, the weight of the snow and ice presses down anglacier goldd compacts. The body of ice will then slowly flow downhill. The power of the solid ice moving downhill rips loose rock and soil. Solid rock underneath the glacier that is firmly attached to the mountain will stay in place, but be polished and smoothed as the glacier slides over it. Scratch marks and gouges appear along the surface of these polished rocks and indicate the direction that the glacier was moving.

Eventually the body of the glacier reaches a warmer elevation where the ice melts more easily. Here, at the foot of the glacier, are gigantic piles of rocks and boulders and debris that have been bulldozed down by the moving massive glacier. These glacial gravels are known as “moraines.” However, finding gold in this unsorted jumble of glacial moraine material produced by a glacier’s bulldozing effect is like finding a needle in a haystack—not worth the time to process…. until the glacier’s natural melt waters or another water source washes through it, and does a bit of natural sorting.

Alluvial gold deposits

Normal alluvial placer deposits form by the power of flowing water washing away lighter materials and gravels and leaving behind heavier material such as gold— very similar to what happens in a sluice box when the flow of water washes away the lighter materials and the gold is deposited in the riffles.  When flowing streams and glacier melt water process and wash away the glacial moraine gravel, the gold works its way downward.  Gold will then be deposited on bedrock or false bedrock (such as clay or other packed material). As the glaciers themselves melt, the huge quantity of water creates an effective sluicing system that can sort the gold out of gravels. These flows can leave behind very profitable pay zones along the channels cut by the ice melt waters on bedrock benches, among packed false bedrock gravels, and on the downstream side of bedrock high spots.

Where to look for glacial placer gold deposits in the West

In California, glaciers still exist in the southern Sierra Nevada, small parts of the Cascade ranges and the Shasta-Trinity Alps. In the northern Sierra Nevada and higher parts of the Klamath and Trinity ranges, that area’s placer deposits were greatly affected by glaciers that existed thousands of years ago. Glacial derived placer gold is important in a number of other western states, too, including Idaho (Boise Basin and Mount Pisgah), Montana (Pioneer District, upper portions of Gold Creek near Deer Lodge) and in Colorado (Fairplay District, Arkansas River Valley in southern Lake County).

Where to look for glacial placer gold deposits in the Midwest

What you might be very surprised to know is that glaciers are the source of nearly all the placer gold found across the Midwest. During the past ice ages (which were repeated events, not just a one-time occurrence), great sheets of glacial ice leveled the middle parts of North America, acting like giant bulldozers by pushing enormous amounts of material southward from Canada. When the ice melted, the area was left with extensive glacial moraine (gravel) deposits. Although it’s spotty and sporadic, some of these glacial deposits have gold, depending on where the glacial gravels came from in Canada and if that region was gold-bearing. Glacial gold is found in several of the northeast states such as New York, Vermont, and Pennsylvania. Glacial gold is also found more extensively in Midwest states such as Indiana, Illinois, Ohio, Michigan, Wisconsin, Iowa, Minnesota, Kansas, Nebraska, Missouri and the Dakotas.

All things considered, while glaciers can destroy a placer deposit, the moraines they leave behind can produce some very worthwhile deposits of gold, and are worth prospecting if you live in or visit these areas.

Posted by: Denise AT 03:03 pm   |  Permalink   |  0 Comments  |  Email
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