.

It is also interesting to note that the years of highest production were also the years of lowest price. While this seems to make no sense, this situation occurs because it takes so many years to do exploration, find deposits, plan and permit mines and then finally get them into production.

The vast bulk of this production is from large mines where the deposits consist of microscopic particles principally hosted in sedimentary (or sometimes volcanic) rocks. Many of these deposits lie along a few well known geologic trends, and the two best known are  the Carlin Trend, and the Eureka/Battle Mountain/Cortez trend. The microscopic gold in these deposits is ideal for recovery by cyanide leaching. The small particle size means the precious metals are easier to dissolve. Its tiny size also explains why the old timers never found these deposits as their principal means of exploration was the gold pan.

The normal host rocks for these unusual gold deposits are thin-bedded silty or argillaceous, carbonaceous limestone or dolomites, commonly associated with carbonaceous shale. A lesser percentage of the host rocks are intrusive rocks such as small stocks and felsic dikes. The typical mineralogy of these gigantic mines includes: Native gold (very fine grained), pyrite, realgar, orpiment, arsenopyrite, cinnabar, fluorite, barite, stibnite with quartz, calcite, carbonaceous organic matter. The deposits contain generally less than 1 percent of fine-grained sulfides. They often feature selective replacement of carbonaceous carbonate rocks adjacent to and along high-angle faults, or regional thrust faults or bedding with silica replacing the carbonate rocks with a jasperoid quartz  material.

The Carlin trend continues to produce millions of ounces of gold each year from a number of truly gigantic mines, but virtually all the gold found in these locations is microscopic in size (typically a few microns in diameter). While the ore grade at some locations in the mines is fairly high, no metal detector can detect micron sized gold particles (and who would want to dig them if one did!). Even a gold pan will not capture these tiny particles. However a standard fire assay will capture and reveal them without any difficulty. Most of these ores are processed using cyanide solutions in either vat leach or heap leach configurations. As far as placer deposits, there are a few very small placer deposits near Carlin at Lynn Creek and some other nearby drainages. These creeks were small producers, perhaps 10,000 ounces total for all the placer workings combined. Nuggets up to about an ounce were found. These placers lie on the other side of a major thrust fault from the more productive mines at Carlin. The geologists interpret the placers as being a leakage halo in a less favorable rock type, showing the presence of much richer deposits which lie below them, across the thrust fault in more favorable rock types. A very extensive report with all you would ever want to know about Carlin type hard rock deposits can be downloaded at: Gold Deposits of the Carlin Trend (NBMG Bulletin 111)
http://www.nbmg.unr.edu/dox/b111/b111.htm

There was also some placer gold mined out at Tuscarora, which is another area where there are big open pit mines, but again most of the gold from the big mines in that area is microscopic and not detectable. In any case, most Carlin type deposits in Nevada do not have placers at all and produce only micron sized gold - great for cyanide leaching, but no good for metal detectors. This is the real reason why our state produces very large quantities of gold, but very little placer by comparison. While our state of Nevada may lead the US in gold production, the fact is that most of the state's gold is microscopic. Now even though Nevada does not produce placer in proportion to the production of its huge sediment hosted deposits, the state does still produce some nice placer gold.  Most of the placer gold which is found in Nevada with metal detectors comes from a geologically very different area, which lies outside the Carlin Trend and is quite unlike the sediment hosted deposits. The most extensive placer area in Nevada is located in the northern part of the state in Pershing and Humboldt counties.

While Nevada has a storied history of rich gold and silver discoveries, One of the best example in recent decades comes from the Sleeper Mine.  This property is another example of a significant Nevada mine that produced some very rich ore, but no placer gold is found in this part of Humboldt county. Here is a bit of information about the famous Sleeper Mine, its discovery and mining history:

In the early 1980s, Amax, a mining company famous in the USA for the Climax Molybdenum mine in Colorado, had rights on some old gold mining properties in Northern Nevada, and were drilling them for bulk minable gold deposits. They had found some small amount of low grade ore, perhaps suitable for open pit mining, and were continuing to drill, looking for more ore - an amount sufficient to justify the construction of a mining facility. They sent out a geologist and he completed all the planned drill holes he was supposed to do in that round of work. It takes some weeks to process the assays and get a report of the results, but nothing of great importance was found with the holes he had just drilled (but he just didn’t know that yet). There was some sort of delay, which prevented him from going on to the next drill target property for a week or so. So, for the heck of it while he was waiting, he put a drill hole into the playa, on an old lake bed, near the edge of the hills where they had been drilling. There were no indications of anything of importance on the surface, which was deeply covered in dry lake bed sediments. There was nothing that outcropped to the surface in that area, so it was a totally blind, wildcat hole, based only on the intuition of the geologist. He selected the spot as it seemed to be the likely intersection of two fault zones out on the playa. Luck was with him and the core came out filled with obvious rich visible gold threads suspended through the rock. The high grade ore from this mine was extremely rich, and additional drilling showed more and more super rich ore. The Sleeper became one of the richest open pit mines in Nevada - much of the ore was close to an ounce per ton which is rather rich for a surface mine. Over its life, it produced over one million ounces of gold.

No gold was found at Sleeper with a metal detector before the drilling was done as the deposit was buried beneath about 60 feet of barren lake bed clay and silt. No doubt a good detector could identify some of the ore mined there, but 20 meters is more than just a bit too deep. Some folks have recovered a bit of placer from some nearby mines in the hills to the east which have been known since the 1880s, but the sleeper deposit itself was buried just too deeply. The amounts of placer gold from the adjoining hills were quite small. The Sleeper mine area is still being explored by various mining companies for additional rich deposits, but so far no additional important high grade deposits have been found in the area, despite considerable exploration efforts. Some lower grade material that may be economic at today's high prices has been found in this extensive and ongoing exploration of the area.

Ongoing Mining and Exploration efforts in are covered by the ICMJ Mining Journal magazine.

Flat placer gold flakes from Round Mountain, Nevada

Large Heap leach operations, sprinkle cyanide solution over ore and
then collect the solution as it drains through and remove the metals.