Wednesday, March 6, 2013

Gold Discovered!

Want to discover your own gold deposit? Follow our GemHunter website, or Gold Hunter website, or Diamond Prospector website and check out our Gem and Gold Hunter Facebook page

You can also check out our articles at the ICMJ's Prospecting and Mining Journal, or stop by the library to read one of our books.


Looking towards the portal of the Giant King mine, California
Hope to see you in the hills!



 


Monday, January 31, 2011

GOLD - A Prospector's Guide to Finding Precious Metals (Wyoming Examples)

Anyone see 'Gold Rush Alaska?' Now there is an example of wanabe prospectors who should have read a few books about gold mining, or at least hired a consulting geologist or mining engineer. One should never run out and buy a $million in mining equipment and then go find a place to use it. This is the exact opposite of how one prospects and mines for gold.


CLASSIFICATION OF GOLD DEPOSITS
There are many geological designations and classifications of gold deposits. For prospectors and government, a simple classification scheme is used: (1) placer gold, (2) lode gold, and (3) leasable gold (Hausel, 2001). Examples of placers include Nome and Flat, Alaska, the Yuba River, California, Alder Gulch, Montana; Douglas Creek, Big Atlantic Gulch and Rock Creek, Wyoming. Famous lodes include the Mother Lode, California and the Homestake, South Dakota. In Wyoming, some lodes include Miners Delight, Carissa and Mary Ellen in the Wind River Mountains. Leasable deposits are those that are available to lease from State government, private mineral estates, reservations, etc, and these include both placers and lodes. One example would be the Copper King gold-copper deposit near Cheyenne that is located on a school section. Even though this would be considered a lode deposit, it is located on State of Wyoming land and cannot be staked and would have to be leased from state government.

Faulted quartz vein (lode) exposed
in bedrock (metatonalite) on the
120-foot level of the Mary Ellen
gold mine, South Pass

Lodes are deposits found in place in bedrock. They are defined as veins, seams, shear zones, faults, fold closures, breccias and disseminated and massive replacement deposits. It is not always possible to make a clear distinction between a lode and placer. For instance, the great Witwatersrand deposits of South Africa are classified as paleoplacers (paleo meaning fossil or ancient) by geologists. Since the Rand paleoplacers are composed of very hard and brittle consolidated rock in well-defined seams in bedrock, these would be considered as lodes by prospectors and by the 1872 Mining Law. Even so, they are just very old placers, or fossil placers, deposited in a prehistoric river systems >2.5 billion years ago.

Placer Deposits
Placer gold deposits consist of secondary detrital gold that has been transported and reworked by water. This gold could be found in beaches, streams, rivers, alluvial fans, etc. Some placers form as a result of wave action along a beach at the edge of a lake, sea, or ocean, such as the great diamond deposits along the west coast of Africa, or the famous gold deposits of Nome, Alaska.

Gold placers can be wet or dry. If the placer formed in an active stream from a distal source terrain, cobbles and boulders associated with the gold will be rounded and stream worn, as would much of the gold. If the placer is eluvial or proximal, it will contain more angular cobbles and boulders and the deposit could sit high and dry away from any running water. The gold in these placers could be angular as well.

Julian Creek, Alaska nuggets.
The large nugget (> 1 ounce)
is stream worn and rounded
suggestive of a distal source
but the remaining nuggets
provide conflicting evidence.
These have features
suggestive of a more proximal
source for some of the gold.
The gold content of placers will vary along the length of the stream and at depth and it is important to learn about paystreaks in placers. Paystreaks are zones in gravel that are enriched in gold content. These may form during flash flooding events and some placers will have more than one paystreak preserved in layers of coarse gravel at depth or further down the stream. When prospecting placer deposits, a good prospector will also look for additional valuable minerals, such as rubies, sapphires, diamonds, etc., and should always keep in mind that somewhere upstream, the gold originated from a lode deposit. Was it a proximal source, a distal source, or was there more than one source upstream?

In addition to gold, diamonds were found at the Cortez Creek placer in Wyoming by prospector Paul Boden (RIP). At the Poker Flat placer in California, Dan Hausel found gem-quality sapphire and benitoite with gold.

While working in Alaska, our company (WestGold) examined placers at Donlin Creek in the Kuskokwim Mountains because the gold morphology suggested a proximal source. Some gold was angular (like corn flakes) and some nuggets looked so fragile as if they could have been precipitated from water in place. It was apparent that much of this gold originated from the immediate area. This resulted in a multi-million ounce lode gold discovery up stream. But at another placer known as Julian Creek, the stream yielded considerable gold, but some of the gold was rounded suggested of a distal source. We did not spend much time looking for the source of that gold, but somewhere along that drainage is an excellent source (or sources) for placer gold, particularly since much of the gold was coarse and many rounded nuggets weighing more than one ounce were found by prospectors.

A fragile nugget from Snow
Gulch, Donlin Creek, Alaska.
The nugget projects off a
rounded rhyolite pebble.
The pebble is steam worn,
but the nugget is pristine &
suggestive of gold that
precipitated from the water.
Some placers will regenerate
over time due to this
phenomenon whereby organics
& organic acids appear to
mobilize gold in water &
precipitate the metal so that
nuggets will actually grow in
place.
Placers always have heavy minerals known as black sands. These are dark opaque minerals with greater than average specific gravity that can include magnetite, ilmenite, garnet, sphene, chromite and monazite. Panning along the banks of most active streams will result in recovery of black sand concentrates. If you are an accomplished gold panner; you should be able to remove all light-colored material from your pan in a very short time. Sometimes this process can be sped up using a combination grizzly pan with a gold pan particularly where one encounters considerable coarse material with fine grained material.

Minerals with relatively low specific gravity such as quartz, feldspar and mica, will also be found with black sands. Mica may be abundant and comes in varieties such as silver-white sericite, bronze phlogopite, black biotite, green chlorite, pink to lavender lepidolite and silver-gold colored muscovite. Even though mica has low to moderate specific gravity (2.7 to 3.4) it forms two dimensional plates that are difficult to extract from a gold pan, simply because they cut through water like a knife. They reflect light from a mirror-like surface and tend to roll around in the water during panning. Through the years, dozens of prospectors brought in samples for identification that they thought was gold; about 99% were fooled by mica. One such novice even spent an entire winter panning and jumping someone else’s claim on the Middle Fork of the Little Laramie River while recovering barrels of mica thinking it was gold (Hausel, 1999).

Jay Roberts pans for heavy
minerals in the Laramie Range.
In this exercise, he used a
grizzly pan (a pan with 0.25 inch
holes drilled in the bottom) &
sieved the material onto the gold
pan. At this point, he quickly
discarded the coarse material
looking to be sure there are no
nuggets or large diamonds. Next
he quickly panned all of the
material in the gold pan. Gold
being heavy should stay in the
bottom of the pan.
A rule of thumb for differentiating gold from mica is that gold will not reflect light like a mirror, it will not roll or flip over and over in the water while panning; gold is too heavy and will stay on the bottom of the pan. When a prospector finally sees native gold in a pan, mica is no longer mistaken for precious metal. Other minerals of interest with relatively high specific gravity that are periodically found in placers include cassiterite, garnet, scheelite, ruby, sapphire, sphene and diamond as well as metals with specific gravities close to that of gold such as mercury, platinum and palladium.

While working for Western Archon & searching for diamonds in California, we recovered benitoite and sapphire in a placer at Poker Flat in the Sierra Nevada upstream from the gold and diamond placers in Sacramento. While prospecting for diamonds in the Laramie Mountains, panned samples contained trace ruby and sapphire (Hausel and others, 1988; Hausel, 1998b) thought to have been released from nearby mica schist and gneiss during erosion. These provide evidence for undiscovered gem deposits. Two prospectors from Rock Springs also found a few hundred pieces of corundum (including one of 90 carats) in a placer near the Big Sandy River at the southern tip of the Wind River Mountains. The corundum was poor quality, but the source area remains undiscovered. Ruby schist with transparent ruby and pink sapphire was purportedly found a few miles upslope from this area (Ron Frost, personal communication, 2005), but the source of the schist remains unverified and questionable as are similar reports of diamonds in Tourist Creek in the Wind River Mountains.

In streams, heavy minerals (dark and light colored) are intermixed with light colored minerals with low specific gravity. This mix tends to migrate downstream due to gravity and is accelerated with rainfall and flash-flooding. Nearly all placer gold paystreaks in deserts are associated with flash flooding events and many occur in fanglomerates with angular cobbles and boulders. Thus better paystreaks (with few exceptions) are almost always associated with coarse gravel on bedrock, or can be several feet above bedrock.

The carrying capacities of streams will diminish with decreasing water velocity. Greater amounts of heavy minerals concentrate where stream velocities decrease. The heavy minerals concentrate at the bottom and 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, the vast majority of gold (particularly where concentrated in pay streaks) is transported during flash floods or heavy spring runoff. Flash flooding is very important in transporting gold down slope. When searching for placer gold, one should always look for layers of coarse gravel and remember that there usually has been more than one past flash flood. Flash flooding events may be recognized by coarse layers of mixed gravel (possibly with tree limbs, etc), in a stream bed that is separated by layers of fine-grained gravel, sand and clay. The sand and clay would have been transported and deposited in the past during times when the water velocity was relatively low, as compared to cobbles and boulders that would have required considerable water energy to move.

The distance heavy minerals are transported by water is not known with accuracy, but gold nuggets will concentrate near their source because of such high specific gravity. A rule of thumb is the closer one gets to a gold source, the coarser the gold. This is seen in many mining districts including South Pass where gold-bearing shear zones near South Pass City, Atlantic City and Miners Delight provide a good source for placer gold. Nearly all placers in this belt were mined within a few feet to a few hundred feet of the shear structures (lodes). One exception was Rock Creek, which was mined downstream from the principal lodes for a distance of 7 to 8 miles before this operation ceased in 1941. The fact that these people were still mining with a dredge in 1941 indicates that the operation only terminated because of the outbreak of war, rather than running out of gold. At the outbreak of the Second World War, all mining operations unnecessary to the security of the country were closed. Thus Rock Creek below the tailings piles very likely represents an overlooked commercial gold deposit! Particularly at today’s gold price.

Some minerals can transport greater distances. Diamond is an extreme example. Diamond is 6000 to 8000 times harder than any other mineral except lonsdaleite (which is 30 to 58% harder than diamond and much rarer) and carbonado, but diamond has moderate specific gravity of 3.52 compared to 2.87 for quartz. There are examples in Africa where diamond appears to have transported more than 600 miles in rivers (Erlich and Hausel, 2000). Such great transportation distances for gold are not possible.

Where meanders form in streams, if gold is present, it will concentrate on the inside of the initial curve in the meander (where water velocity and carrying capacity of the stream declines significantly). Keep in mind that dry stream banks adjacent to a wet pay streak were also deposited by the same stream in the past, thus if you find gold in the active stream, you will find gold in the bank adjacent to the stream, such as in the bank gravels next to the Bobbie Thompson campground in the historical Douglas Creek gold district in the Medicine Bow Mountains.

Paleoplacers for the most part are poorly explored due to lack of water. Thus, one has to find a source for water to mine these. Where paleoplacers are old and well consolidated, such as the Witwatersrand in South Africa, gold is mined underground. Where paleoplacers consist of relatively unconsolidated gravel, they can be mined similar to sand and gravel operations. If the operation is near a road, the gravel can be sold and used in road bed construction. Conversely, gold can be extracted as a by-product from some sand and gravel operations (Hausel and others, 1993, 1994).

Paleoplacers are common in Wyoming due to considerable erosion in the past. Some of these paleoplacers are gigantic and appear to have considerable gold resources and remain mostly unexplored at best. The Wyoming paleoplacers include ancient, consolidated, brittle, Proterozoic (~2 billion years old) paleoplacers in the Medicine Bow and Sierra Madre Mountains that are the unexplored equivalents of the Witwatersand deposits. These Wyoming deposits have produced several uranium, thorium, gold and diamond anomalies.

Precambrian tretched pebble conglomerate
 from the Medicine Bow Mountains.


Consolidated Tertiary
conglomerate from the
Seminoe Mountains. Even
though both are fossil placers,
most would be considered
as lodes by the 1872
Mining Law since they are
hard, brittle, rock found
in place.


The Medicine Bow and Sierra Madre paleoplacers have yielded scattered gold anomalies and a few diamonds. These are uranium-thorium enriched deposits. One Proterozoic paleoplacer exposed in the Ferris-Haggarty mine in the Sierra Madre forms a rare copper-gold-silver deposit and may be one of the only copper paleoplacers in the world, although others would argue that this deposit is mill rock (volcanoclastic). Based on mapping by Graff (1979) and field observations, this deposit appears to be a paleoplacer with rounded pebbles and cobbles but the presence of copper in a paleoplacers is difficult to support. More research is necessary to determine its origin and it could easily represent a mill rock formed by a submarine black smoker nearly 2 billion years ago.

Other paleoplacers include poorly consolidated to unconsolidated Tertiary age (~50 to 20 million years old) paleoplacers at South Pass, Seminoe Mountains and other localities in Wyoming. Those at South Pass likely have zones with commercial amounts of gold based on their source terrain. For the most part, the Mining Law would classify these younger paleoplacers as placers. Such large paleoplacers suggest that the gold and lithic fragments were derived from a nearby source terrain. 

Fanglomerate near Dickie Springs
has gold interspersed within
sandy & silty matrix surrounding
cobbles & pebbles.The rocks are
characteristic of a proximal source
suggesting that most, if not all
of the gold, was derived from South
Pass greenstone belt now buried
under nearby Tertiary cover. Thus
this fanglomerate likely had an
eluvial to proximal gold source.
Also note at the base of this prospect
 pit, cobbles are comparatively
coarse & angular suggestive of
flash flooding. The Dickie Springs-
Oregon Gulch paleoplacer was
estimated to have 28.5 million
ounces of gold according to the
USGS!

Gold deposits south of South Pass in the Dickie Springs-Oregon Gulch area have pebbles, cobbles and boulders that are for the most part angular and suggestive of a nearby to eluvial source terrain. In places, these paleoplacers have been modified and reworked by narrow and restricted intermittent streams which have upgraded the gold content. Thus higher gold contents are usually found in gravels with rounded pebbles and cobbles in this area. Gold in much of this paleoplacer is often angular suggestive of a proximal or eluvial source.
Gold from eluvial deposits will show little or no evidence of transport. Some of these may be classified as fanglomerates – basically conglomerates within alluvial fans. Most such conglomerates will have angular cobbles and boulders that suggest little wear and tear from water transportation, although portions of fanglomerates and eluvial deposits should show reworking and rounding of pebbles locally. Gold in fanglomerates is usually angular, just like the pebbles and boulders. The gold in reworked deposits typically will have rounded pebbles and the gold itself will be rounded.


Dry placer (fanglomerate) at
the Vulture mine, Arizona. This
deposit is a consolidated
conglomerate that geologists
would classify as a dry placer due
to abundant rounded stream
worn pebbles. Even though it is
Tertiary in age, it is consolidated
enough to drive an adit into the
conglomerate.
Eluvial deposits are essentially detrital material weathered in place from an underlying source rock. These are particularly abundant in arid regions of Arizona and New Mexico and most likely sit on or adjacent to lodes. An example is the Greaterville placers in Arizona. These consist of mixed eluvial and alluvial material with rhyolite and granite lithic fragments that are angular. A few nuggets recovered from the deposit were described to have attached galena. According to the 1872 mining law, the Greaterville placers would likely be classified as placers, but because of close and intricate association with veins, many such deposits likely sit on top of a hidden lode. Eluvial deposits are small but some may overlie a high-grade lode and probably should be staked with both placer and lode claims.

If you spend much time looking for gold, you are likely to come across other valuable minerals, such as platinum, palladium, ruby, sapphire, gem garnet, diamond, scheelite and others. Look in your black sands for these as they will stay in a gold pan with your gold flakes and nuggets.

A classical diamond with distinctive
trigons found in a gold placer.
Many diamonds have been found in gold placers and it is likely that thousands were thrown away by prospectors not realizing that they had something of incredible value.



Gold from the Dickie Springs-Oregon Gulch
paleoplacers. Note that coarse flakes (0.25
 inch across), have rough, angular edges,
similar to corn flakes. This morphology
indicates the gold was likely eluvial & the
source to be within a few feet to a few
hundred yards rather than many miles as
 suggested by past researchers.



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