The GOLD HUNTER

The Search for Gold, Silver, Platinum Groups & Gemstones

Prospecting for Gold

Prospectors refer to two types of gold deposits: (1) placer and (2) lode (Hausel, 2001). Some famous placer deposits include the Nome and Flat placers in Alaska. Examples of lode deposits include the Mother Lode in California and Homestake in South Dakota.

Unfortunately, it is not always possible to make a clear distinction between these two types of gold deposits. For instance, the great Witwatersrand gold deposits in South Africa, which have been the most productive in the world, are classified geologically as paleoplacers. Because these are formed of brittle, consolidated rock (mined to depths of greater than 12,000 feet), most prospectors would consider them to be lode deposits. However, geologists classify these as fossil (paleo) placers, since the gold was deposited in streams more than 2.5 billion years ago.

Another not so clear distinction may arise with eluvial deposits. Eluvial deposits are essentially composed of detrital material weathered in place from a nearby (often underlying) source. Gold from an eluvial deposit would show little or no evidence of transportation. Since eluvial deposits are unconsolidated, some prospectors would consider them placers, even though they may directly overlie a lode.

PLACERS

Placer gold deposits consist of detrital gold and other material transported in streams or by wave action and concentrated with other heavy minerals known as black sands. Black sands primarily consist of dark opaque minerals with greater than average specific gravity, which may include magnetite, pyroxene, amphibole, ilmenite, garnet, sphene, chromite, and monazite, as well as some rare light-colored minerals with relatively high specific gravity such as cassiterite and scheelite.

Other minerals of potential economic interest with relatively high specific gravity that may occur in gold placers include ruby, sapphire, scheelite,

Schematic diagram showing eluvial mineralization caused by weathering and erosion of a hidden vein.

diamond, platinum, and palladium.

For example, while prospecting for diamonds in the Laramie Mountains in southeastern Wyoming, we recovered samples with trace amounts of ruby and sapphire (Hausel and others, 1988; Hausel, 1998). These had to have been derived from nearby mica schists and gneisses. So some where up-stream from these positive samples (one should still be able to get copies of the report from the Wyoming Geological Survey) are source beds of ruby and sapphire. Immediately north of these samples is where we discovered the Palmer Canyon iolite-kyanite-ruby-sapphire deposit, so there are other gemstones to be found in the central Laramie Mountains.

I remember another example of a prospector prospecting in the Crows Nest of South Pass. In this area, his riffles to his sluice continued to clog with what he thought was very heavy quartz (there is no such thing).  After finally shinning short wavelength ultraviolet light on the heavy white material, he discovered that his riffles were filling up with scheelite, a tungsten ore.

Upon erosion of bedrock and other parent materials, these heavy minerals are mixed with abundant light-colored, glassy, transparent to opaque minerals with low to average specific gravity such as quartz, apatite, feldspar, and mica. Along with these lighter minerals, minerals with high specific gravity are slowly moved in streams with moderate to high water velocity. The sediment carrying capacity of a stream diminishes with decreased velocity. The heavy minerals concentrate by settling out where diminished velocity occurs; such areas are marked by a distinct increase in black sands. Heavy minerals tend to concentrate at the bottom of a stream along the leading edge of stream meanders, behind obstructions (i.e., rocks, cracks in bedrock) and at waterfalls. Since many streams lack sufficient velocity to carry gold for any great distance, much of the gold in these streams (particularly where it is concentrated in pay streaks) is probably transported during flash flooding events or during heavy spring runoff.

The distances heavy minerals can be transported are not known with any accuracy. Some minerals can be transported great distances. For example, because diamond is 6000 to 8000 times harder than any other mineral and is not very heavy (specific gravity of 3.52 compared to 2.87 for quartz), there are cases where transport distances for diamonds has exceeded 600 miles.

Such great transportation distances for gold are not possible. Gold is too heavy (specific gravity of 15 to 19.3), so when found in streams it is thought to have been derived from a nearby source. In some unusual cases, gold may be transported greater than normal distances while in solution. In Alaska, geologist Paul Graff showed me evidence of gold crystallizing in nuggets downstream but relatively near some lode sources. Maximum transportation distances for gold in solution is unknown, but could be relatively great.

Flash flooding events appear to be important in producing pay streaks. Pay streaks, or lenses of gold-enriched gravel, are often found in zones of coarser-grained pebbles and cobbles. The pay streaks may be scattered over one or more intervals in a vertical column of gravel.

Where meanders occur in streams, gold may concentrate on the inside of the initial curve in the channel, as well as in the bank (point bar) on the upstream part of the inner meander where gold was deposited in the past. As an example, one of my favorite places to take students in my prospecting courses is in a dry campground adjacent to a historical gold placer. Here the bank gravel contains enough gold to keep the interest of the students.

In addition to modern placers, some regions contain paleoplacers. Paleoplacers are simply fossil placers that were deposited by streams or by wave action along prehistoric seas in the geologic past. In most cases, these may not lie anywhere near an active stream or sea today; thus, mining would either require transporting water to the paleoplacer, or transporting material from the paleoplacer to water.

The color change (upper arrows) more than 1 foot above the gold pan (circled) marks the site of a pay streak in the drainage. This pay streak was probably produced during a flash flooding event, or during unusually high spring runoff. A second pay streak was found at the base of the open cut near the standing water (lower arrow). Even though this placer was essentially located in a dry drainage when mined, it was immediately downslope from some lode deposits, and provided a favorable site for gold concentration and both streaks were above bedrock.

Schematic diagram showing historical development of a meander. Where the stream begins to meander, the water velocity decreases, and minerals with higher specific gravity concentrate there (stippled areas). Through time, the meander may mature, leaving deposits on the inside banks as the stream migrates. Material in the stream as well as the adjacent bank material (which may be high and dry after episodes of flooding and high water) will contain heavy minerals and possibly gold.

Where the paleoplacer consists of relatively unconsolidated gravel, it can be mined in a manner similar to a sand and gravel operation. If the operation is located near a road, the sand and gravel by-product can be used in road construction. Conversely, gold can be extracted as a by-product of sand and gravel operations. For example, gold was found in several sand and gravel operations and placers adjacent to Interstate 80 in southern Wyoming (Hausel and others, 1993). Where paleoplacers are extremely old and well consolidated, such as in the Witwatersrand, the gold is typically mined underground.

Left - Dry paleoplacer at Dickie Springs, Wyoming. Note the coase gravel at base of this paleoplacer. This stream-deposited gravel contains gold (see below).

LODES

One might think of lode deposits as veins or other consolidated rocks that contain anomalously high quantities of metal (e.g., gold). Many lodes occur as distinct quartz veins. These may form linear to tabular masses of quartz within country rock. One important characteristic of many productive veins is the presence of sulfides, such as pyrite (fool’s gold) or arsenopyrite (arsenic-pyrite).

Gossan in the Yankee Girl gold mine in the San Juan Mountains, Colorado. Note the red, yellow and bleached rocks found everywhere in this photo - this is one giant gossan and you can bet there is still a lot of gold to be found here!

When pyrite oxidizes, it produces sulfuric acid and rust, resulting in a gossan at the surface and a potential supergene zone (a mineral deposit, or enrichment, formed by descending fluids) a few tens of feet below the surface. Gossans are the oxidized sulfide-rich parts of veins and other mineral deposits that have a distinct, rusty appearance. These gossans offer excellent visual guides in the search for gold and other mineral deposits. In any historic mining district, you will often find dozens, if not hundreds, of old prospect pits dug into the rusty rocks. Prospectors learned to recognize these gossans as important guides to ore deposits.

Gossans are good places to search for high-grade gold in lodes. The recognition of gossans in the field can be very helpful to the prospector. For example, gossans produced from the leaching of pyrite are typically very rusty (reddish-brown) in appearance; gossans produced from arsenopyrite are typically greenish-yellow. Gossans are so important that an entire book was written on their different characteristics (Blanchard, 1968).

Faulted quartz vein in meta-tonalite porphyry exposed in the mine rib at the Mary Ellen gold mine, South Pass.

Large gossans that cover several acres may be situated over giant sulfide-enriched veins or massive sulfide deposits. These may contain gold and/or valuable base metals (copper, zinc, lead, etc). One very large gossan in the Hartville uplift in eastern Wyoming is so distinct that it has been named "Gossan Hill"—it overlies a massive sulfide deposit. One of the better places to look for specimen-grade gold samples is within gossans containing boxworks. Boxworks is a distinct vuggy and rusty rock.

Schematic diagram illustrating the gossan cap overlying a gold-bearing vein.

Some faults and associated breccias may also be mineralized. Breccias are zones of broken rock containing distinct angular rock clasts. When found, gold may occur in the matrix of the strongly limonite-stained gossan surrounding the rock fragments. Other faults, known as shears, may also be mineralized. These shear zones consist of granulated rock. Within many of these shears, gold is often found associated with rust-stained quartz. Many shear zones, particularly those in greenstone belts, have been quite productive for gold. In some gold mining districts in the world, nearly every foot of the exposed shear zone has been prospected at the surface.

This hand specimen of boxworks exhibits pore spaces that formed where sulfide minerals used to be. The sulfides were leached and removed during the past. Gold, which often is found in the sulfide known as pyrite, is inert, and may remain in place within the boxworks.

Ore shoots

Many veins have sporadic gold values with localized ore shoots enriched in gold. Some of these shoots may be enriched 100 to 1000 times the average value of the vein. The challenge given the prospector is how to recognize these shoots.

Ore shoots can be structurally or chemically controlled. Where pressures and/or temperatures dramatically dropped during hydrothermal mineralizing events, structurally controlled ore shoots occur. Chemically controlled ore shoots may occur where there was a chemical reaction between the mineralizing fluids and country rock.

Breccia with angular clasts of country rock in a limonite-rich matrix.  

When searching for structurally controlled ore shoots, it is necessary to look for places where one would expect the pressure to have decreased along vein systems. Some structurally controlled ore shoots are found in folds. Many fold closures in gold-bearing veins will be enriched in gold. Another type of structurally controlled ore shoot includes vein intersections. Some intersections of gold-bearing veins have been dramatically enriched in gold.

Folded rocks at the Carissa mine, Wyoming. Often the crests of folds are enriched in gold and other metals and such folds 'plunge' to unknown depths into the earth. Often, the plunge of an ore shoot is overlooked by geologists and prospectors, even though the fold may continue undergroup for several hundred or several thousand feet. All of these veinlets are part of a broad structure known as the Carissa shear that was locally mapped over a width of 1,000 feet with strike length of 970 feet and dtilled to a depth of 930 feet. It likely continues to much greater depths.

There are many other types of structurally and chemically controlled ore shoots. For example, while prospecting in the Gold Hill district in the Medicine Bow Mountains of Wyoming, I noted that gold was almost exclusively found in veins adjacent to amphibolite. Veins in quartzite were unproductive. Additional information on ore shoots can be found in various books on economic geology and ore deposits (see Earll and others, 1976; Evans, 1980; and Peters, 1978).

Another view of the Carissa shear zone (lode) at South Pass. Most prospectors and geologists would miss this structure if it wasn't for the fact that it is exposed in the glory hole. The lode has very little quartz, but has many close spaced fractures and mylonitic (crushed) country rock. The primary shear zone averages about 0.15 to 0.3 opt Au (ounces per ton gold) and is enclosed by a giant less mylonitized shear envelope that is about 1,000 feet wide.  In the photo, Jon King (geologist) stands on the right side just above the wooden poles. As an example of how rich this deposit is - the Homestake mine averaged about 0.3 opt Au and many currently operating mines in Nevada mine gold ore that averages about 0.02 to 0.09 opt Au.

The search for productive gold deposits requires a good background in prospecting and economic geology as well as some luck. There are still many placer and lode deposits to be found, although the discovery of entirely new mining districts is rare. In all my years as an exploration geologist, I have only been able to find one new gold district. However, I have found many gold deposits within known districts.

Folds are often good places to search for ore shoots. The photo demonstrates an isoclinal fold in amphibolite.

Some of the better areas to search for gold are historical mining districts. In my experience, it is rare that any ore deposit has been completely mined out. Many historical and modern mines still contain workable mineral deposits as well as nearby deposits that have been overlooked. Many well-known giant mining companies of the past were notorious for overlooking significant ore deposits. Thus, one could potentially make a living just following up on the exploration projects of many of these past giants [as well as some projects of present giants].

References cited

Blanchard, R., 1968, Interpretation of leached outcrops: Nevada Bureau of Mines Bulletin 66, 196 p.

Earll, F.N., and others, 1976, Handbook for small mining enterprises: Montana Bureau of Mines and Geology Bulletin 99, 218 p.

Evans, A.M., 1980, An introduction to ore geology: Elsevier, Amsterdam, The Netherlands, 231 p.

Hausel, W.D., 1997, Copper, lead, zinc, molybdenum, and associated metal deposits of Wyoming: Wyoming State Geological Survey Bulletin 70, 229 p.

Hausel, W.D., 1998, Diamonds and mantle source rocks in the Wyoming Craton, with a discussion of other U.S. occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p.

Hausel, W.D., 2001, Placer and lode gold deposits: International California Mining Journal, v. 71, no. 2, p. 7-34.

Hausel, W.D., Marlatt, G.G., Nielsen, E.L., and Gregory, R.W., 1993, Study of metals and precious stones in southern Wyoming: Wyoming State Geological Survey Mineral Report MR 93-1, 54 p.

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream-sediment sample results in search of kimberlite intrusives in southeastern Wyoming: Wyoming State Geological Survey Open File Report 88-11, 11 p. (5 plates) (revised 1993).

Hausel, W.D., and Sutherland, W.M., 2000, Gemstones and other unique minerals and rocks of Wyoming—A field guide for collectors: Wyoming State Geological Survey Bulletin 71, 268 p.

Peters, W.C., 1978, Exploration and mining geology: John Wiley and Sons, New York, 696 p.