Monday, August 23, 2010

Prospecting For Gold

Prospecting For Gold

Spring arrives with prospectors combing the hillsides in a hearty search for yellow metal - GOLD!. Some look and find nothing, others find a little gold or another treasure: ruby, sapphire, benitoite, gem garnet or diamond. Others catch a incurable case of 'gold fever' such that they will give up everything - their homes, jobs and common sense just to search for gold. Some are so taken by the fever that they are exposed to scams and con-men who will take whatever worldly possesssions are left. It is like Congress and taxes.

There are many types of gold deposits to a geologist - hydrothermal, mesothermal, epithermal, replacement, etc., but to prospectors, there are only two types: placer and lode (Hausel, 2001, 2010). Famous placer deposits include Nome and Flat, Alaska and Alder Gulch, Montana. Examples of lode deposits are the Mother Lode, California and the great Homestake in South Dakota.

Gold from Douglas Creek, Wyoming. Note the gold pan also has a small pyrope garnet (red) that was recovered by prospector, Paul Allred from Arizona. Pyrope is a indicator of nearby diamond deposits. In this region of the Medicine Bow Mountains, many diamond indicator minerals were identified (particularly around the Middle Fork of the Little Laramie River, and also near South French Creek) as well as two gem-quality diamonds found by prospector, Paul Boden from Wyoming on Cortez Creek. Diamonds were even reportedly found in drill core by Superior Minerals during exploration of some 2.5 billion year old quartz pebble conglomerates many years ago.

Unfortunately, there is not always a clear distinction between lode and placer gold depsoits. For instance, the great Witwatersrand gold deposits in South Africa, the most productive in the world, are classified geologically as paleoplacers. Because they occur in brittle, consolidated rock (mined to depths of greater than 13,000 feet), most prospectors would consider these to be lode deposits. However, geologists classify the great Rand deposits as fossil (paleo) placers, since the gold was deposited in streams and rivers more than 2.5 billion years ago.

Eluvial gold typically sits over a vein or lode. Eluvial deposits are restricted in size but may be enriched in gold. A lode may not be exposed at the surface, but if you are finding quartz in alluvium with gold, a vein is likely hidden under the alluvial (eluvial) cover. Such deposits are common in Arizona, though few of the eluvial-alluvial deposits have been explored in detail in a search for the underlying lodes.

 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. There are many examples of eluvial gold in Arizona. The arid environment  is favorable for eluvial deposits due to the lack of active streams.

Placer deposits
Placers consist of detrital gold and other valuable minerals transported in streams or by wave action to be concentrated with other heavy minerals known as black sands. Black sands 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. If you ever panned near Wilson Bar or Wilson Gulch at South Pass, Wyoming, you may have found all of this heavy, nagging, white quartz that was impossible to pan out. Well, it probably wasn't quartz. With a shortwave ultraviolet light, this heavy quartz likely would fluoresce blue-white simply because it was not quartz, but instead scheelite, a tungsten ore found in some of the gold ore at the Burr and Hidden Hand mines (Hausel, 2009). When found,on public land, placers can often be claimed under the 1872 mining law. But if you want the lode under the placer, you better look at filing a lode claim too.

Other minerals of potential economic interest with relatively high specific gravity may occur in gold placers such as ruby, sapphire, diamond, platinum, and palladium. While prospecting for diamonds in the Laramie Mountains in southeastern Wyoming, several samples with trace amounts of ruby and sapphire were recovered along with heavy minerals (Hausel and others, 1988; Hausel, 1998). These were eroded from nearby, undiscovered, corundum (sapphire, ruby) mica schists and gneisses. How do you tell if you have ruby or sapphire in your gold pan? Look at crytal habit. The habit is the common form of the crystal. Ruby and sapphire form hexagonal crystals that are bounded by two pinacoids (basically flat surfaces).

While prospecting for diamonds in the Sierra Nevada of California, I found sapphires and benitoite near Poker Flat. And one prospector (Paul Boden) found a couple of excellent gem-quality octahedral diamonds while searching for gold on Cortez Creek in the Medicine Bow Mountains, Wyoming, and another prospector (Frank Yassai) found several diamonds in Rabbit Creek, Colorado while prospecting for gold.

During erosion of bedrock, these heavy minerals mix 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, 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. In southern Africa, diamonds are found in kimberlite pipes, in stream and river placers and in extremely rich beach placers along the west coast of the continent.

 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 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.

The color change (upper arrows) more than 1 foot above the gold pan (circled) mark the site of a pay streak in Smith Gulch discovered by prospectors Hank Hudspeth and Buddy Presgrove. This streak was produced during a flash flooding or 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 located in a dry drainage when mined, it was immediately downslope from several lode deposits that provided a favorable site for gold concentration. At this point, the prospectors had not yet reached bedrock, where there is likely another pay streak.

 Flash flooding events appear to be important in forming 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.
Schematic showing development of meander. Where the stream starts to meander, water velocity decreases & minerals with higher specific gravity concentrate (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 & possibly gold and diamond.

 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 the past in my prospecting courses was near Bobbie Thompson adjacent to a historical gold placer in Douglas Creek, Wyoming. Here the bank gravel sits away from the active stream, but contains enough gold to keep the interest of the students.

 In addition to modern placers, some regions contain paleoplacers. Places like Wyoming and the Witwatersrand of South Africa are famous for paleoplacers scattered over large regions. In the Witwatersrand, the paleoplacers are so important, that they have produced about 50% of all of the gold mined in human history. Today, they have the deepest mines on earth. In Wyoming, most paleoplacers have either not been prospected, or only have been cursory examined at best, even though it is a safe bet that economic gold deposits occurs in some of these. 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.
Wayne Sutherland, WSGS geologist, examines paleoplacer at Dickie Springs to the south of South Pass. Note all of the rounded boulders and cobbles typically found in active streams and rivers.

 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.

A little sample of gold recovered from the dry paleoplacer near Dickie Springs. This gold suggests there is a hidden lode somewhere between this site, and the exposed South Pass greenstone belt to the north. Hecla Mining explored this area and identified a likely target - a sulfide-bearing iron formation at depth that likely contains gold - and this too, remains unexplored.

 In the South Pass greenstone belt in western Wyoming, giant paleoplacers surround the region at McGraw Flats to the north and Oregon Buttes-Dickie Springs to the south. And there are smaller ones in between. The southern paleoplacer was reported by Love and others (1978) of the US Geological Survey to contain more than 28.5 million ounces of gold, yet most of this area is unexplored. Along the northern flank of the Seminoe Mountains greenstone belt, the Miracle Mile paleoplacer is unexplored even though myself and field assistants recovered gold from the dry paleoplacers nearly everywhere we sampled. This paleoplacer was discovered by prospectors Charlie and Donna Kortes, also contains dozens of G10 pyrope garnets that indicate somewhere in this region is a very rich diamond deposit or deposits. Keep your eyes out for diamonds when looking in any placer or paleoplacer!  Paleoplacers in the Medicine Bow and Sierra Madre Mountains in southern Wyoming yielded some gold and diamonds, but are rich in uranium and thorium.

Lode deposits
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).

Classic lode deposit. This auriferous quartz vein in metatonalite at the Mary Ellen mine at South Pass was offset along a small, reverse fault. Lodes are considered in situ deposits in hard rock.

 When pyrite oxidizes, it produces sulfuric acid and rust (a massive sulfide deposit of pyrite will smell like rotten eggs, and a massive arsenopyrite deposit will smell like garlic, and both can have considerable gold and silver), 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 guides to ore deposits.

Gossans at Red Mountain in the San Juan Mountains, southern Colorado. Note all of the red to light yellow-colored rock found nearly everywhere in the photo. These are gossans that contain significant amounts of gold and silver.

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).

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 I ended up naming it “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.

This specimen of boxworks exhibits pore spaces formed where sulfide minerals used to be. The sulfides were leached and removed. Gold, which often is found in pyrite, is inert, and may remain in place within the boxworks, while some of the iron from the pyrite stains the rock and much of the sulfur is carried downdip. At Bradley Peak in the Seminoe Mountains, I found nearly a dozen of these samples and started a gold rush in 1981. Even this area remains essentially unexplored to this day!

 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 rock fragments. Other faults, known as shears, may also be mineralized. These shear zones consist of granulated rock. Within many 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.

A breccia (angular fragments) cemented by quartz - a good place to check for gold. Such breccias are formed in faults or by the release of gas under pressure that produces a fault zone or a breccia pipe. Note the difference between the breccia with angular rock fragments (left) and the Tertiary-age (about 30 million years old) paleoplacer with rounded pebbles (below left) and the stretched pebble conglomerate (very old paleoplacer nearly 2 billion years old) (below right) All three can contain gold.

 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. Any where an igneous rock (hot) comes in contact with a reactive rock (such as limestone) is a great place to find gold and other minerals.

 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.
The author (right) leads discussion on field trip at the Carissa mine at South Pass. The shear zone in the background is rich in gold [average grade reported at 0.3 opt Au, much higher than the ore currently recovered from mines in Nevada (0.02 to 0.15 opt Au) (opt Au= ounces per ton of gold)]. Although not visible to the untrained eye, this giant gold-bearing structure lies in a large fold in the shear. The ore zone is 970 feet long, nearly 1,000 feet wide and continues to a minimum depth of 930 feet (and likely continues to a few thousand feet deep). The property was withdrawn by the State of Wyoming even though it likely hosts a few million ounces of gold worth a few $billion. Another example of your taxes at work.

 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 gold was almost exclusively found in veins adjacent to amphibolite. The same 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).

What does gold look like?
Most people have a difficult time identifying gold at first. Gold is very heavy! It is 15 to 19 times heavier than water, it is malleable (it will easily scratch with a pocket knife), and has a distinct gold color that does not tarnish. Most people mistaken mica, pyrite (fool's gold), or chalcopyrite (copper-fool's gold) for real gold.  These latter minerals are brittle and will crush to a fine greenish black powder. But don't be fooled. Some pyrite (fool's gold) may contain up to 30 parts per million gold hidden in the crystal structure (about an ounce per ton). To test for this gold, you will either have to assay, or powder the pyrite and pan it for gold. And chalcopyrite may have as much as 20 parts per million gold hidden in its crystal structure. 

Large specimen of mica (muscovite) shows a mirror-like surface, bronze-color, and will break into tiny pieces by a pocketknife unlike gold. Tiny mica flakes will easily move around in a gold pan while panning. As you pan, if the gold material stays flat on the surface of your pan and is difficult to move, it may be gold. However, if it moves easily, rotates or spins in the water, it is not gold. Mica is hard to pan out of a gold pan simply because it is essentially 2-dimensional and will cut through the water like a knife.  Whereas gold in the pan below is angular, heavy and a brightly yellow-gold color. It does not have mirror-like surfaces and will stay put in the pan. Pyrite will crush to a greenish black powder and the same with chalcopyrite.

The search for productive gold deposits requires a good background in prospecting and economic geology as well as some luck. However, there are litterally hundreds of occurrence and deposits in nearly every state in the West including Alaska. The best way to begin prospecting is to get a book that describes the gold mines and placers and visit these as I have found there are always many deposits near old gold mines that have been overlooked. This is how I found more than a hundred gold deposits and anomalies. An understanding of geology also helps: I found an entirely new gold district (Rattlesnake Hills in the early 1980s) that was missed by everyone else, simply because of the geology. It had very favorable geology and is currently being explored and drilled by several companies even though I discovered this district nearly 30 years ago! I was also on the discovery team of the giant Donlin Creek gold deposit in Alaska. Part of our discovery team (Rob Retherford, Bruck Hikock, Toni Hinderman) had recognized that some place gold at Donlin Creek was like corn flakes, very angular. Paul Graff visited the area with Mark Bronston and Richard Garnett and WestGold decided to explore this region. I was hired to map the deposit - it was a major discovery that includes more than $42 billion in gold! Yet this discovery occurred all the way back in 1988 and the gold deposit, considered one of the largest in the world, still is not being mined (but is under exploration).

So, get hold of books in your area that describe where gold deposits are found. Pick out the exciting areas and look at the deposit described in a book and look around for what the old prospectors missed (they missed a lot!). Search for publications at your local geological survey (usually they have a few good publications). If you are in Wyoming, I published numerous books that are available on the internet, the University of Wyoming bookstore and the Wyoming Geological Survey. In particular, get copies of Bulletin 68 and 70 and Report of Investigations 44. If in Arizona, there are likely hundreds of lode gold deposits that have been missed because of so many eluvial placers with no reported gold source (the gold came from somewhere!). Colorado and California have hundreds of possibilities, but personally, I would look in Arizona, Wyoming, Montana and Alaska. For additional information on gold, gold in Nevada, New Mexico, Utah, Idaho, Washington, Oregon and South Dakota, watch for other blogs and keep track of my GOLD and Consulting websites as I will periodically update these. Myself and my son (Eric) who is also a geologist, are currently writing a couple of books on gold and we will tell you exactly where to look.

 While you are looking for gold deposits, remember, there are probably just as many if not more gemstone and diamond deposits that have been missed by prospectors and geologists. I recently found a major field of cryptovolcanic structures that are likely diamondiferous kimberlites sitting right along Interstate 80 west of the State Capitol of Wyoming. With a good arm, one could probably hit some of these with a rock next to the interstate. These remain unexplored and were just discovered a couple of years ago! 

 Some of these are so obvious, that it makes one wonder what everyone has been doing. Take for instance the Cedar Ridge opal deposit. Probably the largest opal deposit in North America was sitting right on the side of the main highway to Riverton, Wyoming and exposed in numerous road cuts in an oil and gas field and in a pipeline - but totally overlooked. Even after the announcement of this major field in 2003, it still remains pretty much unexplored! This deposit contains opals in road cuts that weigh more than 100,000 carats and has common, fire and precious opal and some spectacular 'Sweetwater' agates. How anyone could have overlooked this, is beyond comprehension. But it sat there for several million years, untouched, other than a few brief mentions of the presence of opalized rock in old USGS reports!

Then there is likely the two largest colored gemstone deposits on earth that I found at Grizzly Creek and Raggedtop Mountain in the Laramie Range. How these can remain essentially untouched is beyond my understanding. At one deposit, I found gem iolite as large as 24,000+ carats with pieces in the outcrop that likely weigh hundreds of thousands (if not millions) of carats. The other deposit may host as much as 2.7 trillion carats based on past geological reports (that missed the fact that these were gemstones). Just imagine how valueable these deposits are even if you mined them, cut the stones, sold them and only made $1 profit! The primary gemstone, iolite, can be cut for $0.5/carat and is sold for $15 to 150/carat. Nice profit! For those of you who wonder - I do not have claims on any of these, it was considered unethical when I was employed at the WGS (Although, today I am a consultant).

 There are 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 and you should be able to do the same armed with a little knowledge.

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 and ignoring others. For example, AMAX explored a large porphyry copper-silver-gold-lead-zinc deposit in the Absaroka Mountains southeast of Yellowstone. They focused on the prophyry and ignored nearby vein deposits that assayed >100 opt silver! 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]. If you would like more information on how to hunt for gold and diamond deposits, catch a field trip.

Examples of pyrite (fool's gold). Note that these are brassy colored (not gold colored). They are also brittle and the upper photo shows crystalline (cubic) pyrite. Upper specimen from the Lost Muffler gold prospect, Rattlesnake Hills and lower specimen from the Pickwick vein, Kirwin district, Wyoming. But don't throw them away: pyrite can contain a few hundred parts per million to potentially 2,000 ppm hidden in its crystal structure!

  • Some 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., 1989, The Geology of Wyoming's Precious Metal Lode and Placer Deposits: Wyoming Geological Survey Bulletin 68, 248 p.

  • Hausel, W.D., 1991, Economic Geology of the South Pass Granite-Greenstone Belt, Wind River Mountains, Western Wyoming.Geological Survey of Wyoming Report of Investigations 44, 129 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., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.
  • Hausel, W.D., 2010, How to find gold: Lost Treasure Magazine, July, p. 56-60.
  • 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.
Specimen of chalcopyrite in quartz (with green malachite and silver-colored specularite) from the Kurtz-Chatterton mine (a great, unexplored, gold prospect) from the Sierra Madre, Wyoming. The chalcopyrite is the brassy-orange material in the specimen. Some chalcopyrite can contain as much as 20 ppm Au (a considerable amount of gold) hidden in the crystal structure along with some silver.

Just hit a rock and smell garlic? No, it was not your Itallian friend (hopefully), it was most likely the smell of arsenic from the arsenopyrite that you just hit with your rock hammer. Arsenic-pyrite, or arsenopyrite, often is found around many gold or silver deposits and can hold up to 1,000 ppm gold hidden in its crystal structure. At South Pass, whenever I found arsenopyrite, I usually had high silver assays. At Donlin Creek, Alaska, we always had high gold assays when we found arsenopyrite (or stibnite). Thus, these were good gold guides. Sample collected from South Pass.

Cuprite (earthy red), malachite (green) and tenorite (black) from the Sunday Morning prospect, Seminoe Mountains, Wyoming. These minerals can all contain some silver and gold in their crystal structure. Malachite will emit CO2 bubbles just like soda pop when sprayed with dilute (10%) hydrochloric acid. Spray cuprite and tenorite with dilute hydrochloric acid and rub a well used rock hammer in the wet mineral and it will replace the worn parts of your hammer with native copper.

Gold from the Carissa Mine, South Pass. Click and look at the visible gold in this specimen. And gold from Rock Creek (below).

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