How to locate a placer deposit

Author: NICOLE MCCLEAFTuesday, July 22, 2014

How to locate a placer deposit

Categories: Best of the Buzzard, How-To's

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Gold is recoverable from placer deposits chiefly due to two properties: its high density and its inability to join with other meals in chemical combinations. Because of its high density, gold settles out of flowing water and naturally finds its way into the deepest crevices available to it. Because it doesn’t oxidize like iron, silver or other metals, once trapped in such deposits it stays there until liberated by the gold seeker.

Running water carries sediment, including gold. A direct relationship exists between the velocity of the water and the size of the particles of sediment it will move. A given velocity of water will carry much larger particles of quartz sand, for instance, than of dense gold. When water bearing gold particles loses velocity, the gold is dropped before the sand and accumulates in natural traps, where gold has accumulated over the years. Most of the large virgin gold accumulations have been removed from the Northwest during the gold rushes of the 19th century. Many of these areas were reworked during the depression years of the mid-1930s. Gold was found that the early workers had missed and it was also found that more gold had been deposited during the 50-odd years of uninterrupted stream sorting. This same condition exists today; many of the old deposits have not been worked in 50 years and the gold that has accumulated since then can be recovered now.

Some areas, which have small virgin accumulations of gold, were not touched by early workers and are still awaiting discovery by the weekend gold seeker. Most of these are small natural riffles in minor gold-bearing areas. To locate such spots, you must be familiar with stream mechanics.
Figure 1
Flowing water seeks the shortest distance downhill. Since the shortest distance between any two points is a straight line, water attempts to flow in a straight line downhill. Changes in grade, bedrock type, faults, natural obstacles, old channels, etc., all alter the ideal linearity of a stream, so stream flow may be interpreted as a series of straight lines between relatively close points.

 It may easily be seen from Fig. 1, that water at different points of the stream is flowing at different velocities. Where the velocity arrows strike the bank, material is being rapidly removed, sorted and moved downstream. Such areas are called ‘cutoff banks.’ Cutoff banks are on the outside bends of streams, indicated by ///// in Fig. 1. Opposite cutoff banks are areas of relatively low velocity where material is dropped, termed, ‘slip-off banks.’ Slip-off banks are a likely place to look for gold.

Figure 2
Obstructions in a stream provide natural riffles or traps, for gold. Boulders, islands, logs, resistant rock veins, pools, cracks in bedrock, etc., all provide velocity drops in a stream which concentrate gold behind them or in between them. (Fig. 2) In streams were gold is known to be moving today, and where spring floods are a common occurrence, it is wise to look behind obstructions on the floodplain for gold. A favorite panning spot for the author is a stream in northwest Washington where the spring flood causes the water to rise onto rough, moss-covered canyon walls. The moss is stripped from the recessed areas of the cliff, washed and panned to recover the fine gold, which adheres to it. By being careful not to remove more than half the moss from any one area, it is possible to use this technique year after year in the same area! If the moss is found to contain sufficient fine gold, it is worthwhile to burn it in a large can and pan the ashes. This technique is also often used for recovering fine gold from the carpet or corduroy linings of sluice boxes.
When you locate an obstacle in a stream, work as deeply as possible, for gold tends to work its way down inside small fractures and pits. Ideally, you should be able to scrape and whisk broom the bedrock surface to recover all available gold. Cracks should be cleaned with ice picks or dental tools. 

In many areas of the Northwest, bedrock is covered by tens to hundreds of feet of glacial debris (sand and gravel.) In such places, gold may be found concentrated in old stream channels above the present water level on floodplains. Such deposits are called perched placers. A friend and I found a perched placer in 1978, which yielded six ounces of gold in two days. Oddly enough, the work of finding the deposit was done by a less-experienced panner, with us collecting the rewards. The other panner, had sampled the stream, working upstream from the confluence of it and another stream. He told us he had worked three days, finding small amounts of gold all the way, until he reached his present position at which point he was no longer finding any. He had hoped to uncover the source of the gold, but upon ‘running out,’ decided to leave. We simply returned to the last place he indicated that he had found gold, and dug in the flood bank until we found the perched placer from which the gold was being eroded each spring and re-deposited downstream.

Before panning a river with which you are unfamiliar, a quick survey of the geology of the area will help ascertain the likelihood of gold being found there. In the Northwest, gold is usually associated with igneous rocks. Igneous rocks are those that originated in a molten state deep within the earth. Igneous rocks are classified in two divisions; extrusive, those that are extruded onto the surface of the earth in lava flows and intrusive, those that upwell from deep within the Earth but crystallize before they reach the surface like granites. Igneous rocks are easily distinguished from the other major rock types (metamorphic and sedimentary) by the fact that they are made up of crystals. Intrusive igneous rocks have a salt-and-pepper appearance due to the intergrowth of different colored crystals, usually black and white, but altering to browns and grays. Extrusive igneous rocks are usually too find grained for this texture to be observed by the naked eye, but it may be seen with the use of a magnifying glass. Extrusive igneous rocks are light gray to black (sometimes red-brown) in color.

Sedimentary rocks are those derived from old stream and ocean deposits, so it can easily be seen that the weathering of that type of deposit would produce little gold. Sedimentary rocks are usually layered and break more or less easily along those layers. Common sedimentary rocks are sandstone (lithified gravel), shale (lithified mud) and limestone (fine grained calcium carbonate — calcity — precipitated from warm seas).

Metamorphic rocks are those that have been altered by the heat and pressure resulting from burial deep within the crust. They are usually banded more or less conspicuously, often alternating light and dark banks or exhibit a wavy surface texture, with obvious mineral segregation along the surfaces. Common metamorphic rocks are gneiss, schist and quartzite. By looking at the dominant rock type indicated by boulders in the stream, you can ascertain what rock types the stream flows through and gain insight as to whether it is a good place to prospect.

Besides the obvious use in seeking placer gold, panning is used in other geologic work such as taking heavy sediment samples in order to identify other metallic deposits in the area and as a method of locating lode (in place) gold deposits. The technique in both cases is to work upstream from the place where the initial samples are found, in order to locate the source. Looking for lode gold deposits this way is only indicated if the placer gold you are panning is quite fresh looking: ie: sharp-edged nuggets with little flattening, indicating that they have not traveled far. Of course, perched placers as previously discussed can be found by further prospecting of water worn gold. The chances of finding an untouched lode gold deposit are reduced by the amount of prospecting that has been done in the area and by the heavy glacial erosion of the Northwest, which has completely removed many lode deposits, leaving only their placer remains. The vast amounts of sediment moved by the glaciers has also helped the panner by concentrating gold which occurred finely disseminated throughout the rocks of one area (thereby not mine-able as a lode) into relatively small areas of richer concentration in sediments.

— George Massie

George “Buzzard” Massie founded the GPAA in 1968, and later the LDMA, Gold Prospectors magazine and Outdoor Channel. He passed away in 1993, but his words of wisdom still ring true today. To order your copy of Best of the Buzzard, call GPAA Membership Services at 1-800-551-9707.

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