The Geology of Coarse Gold Formation

By Chris Ralph

The goal of metal detecting for gold is finding coarse nuggets. In order to better understand where and how to find them, it might be good to examine how and where coarse gold forms. Nearly all placer gold, both coarse and fine, originates from the erosion of primary hard rock gold deposits. Some placers are re-concentrated from older placers, but at least some time in the past, these came from primary deposits. A small amount of coarse gold has probably resulted from secondary enrichment processes, but the majority grew as a primary deposit from circulating hot water solutions under pressure – these are often called hard rock deposits.

Its hard to imagine things like gold or quartz being dissolved by water solutions, but if the water is hot enough, the pressure high enough, and the chemistry is right (acids, and other elements like sulfur are present), then gold, quartz and other things you don’t expect to see dissolving will go into solution. The solutions move by natural convection (hot things rise), and as they rise the waters cool as they move farther from the heat source in the ground and closer to the surface. The gold is combined with sulfur to form gold-sulfur chemicals that dissolve in the water. I have specifically decided to avoid going into the geochemistry of gold solutions in this whole discussion, because I just don’t think it’s necessary to do that. The important fact is that at heat and temperature, gold will react with sulfur and other elements to form soluble chemicals. These chemical complexes are not all that stable, so that when the waters cool and the pressure drops, the chemicals decompose, releasing the gold to form nuggets. Sulfur is very common in geothermal waters (like hydrogen sulfide - the odor of rotten eggs). Most natural hot springs have that sulfur odor quite strong, and most gold - quartz veins have at least some sulfides like pyrite present.

Also as the waters cool, things like quartz and sulfides become less soluble and they come out of solution to form veins. The most common conduits for these solutions are natural fault zones; this is why most veins are shaped like fault zones, a long and narrow plane. This is the process that forms nearly all gold-quartz veins. Veins also commonly form at the meeting of two different rock types, also for the reason that on the contact where the two rock types meet is a zone where water circulates better than through the solid rock itself. Sometimes a zone of broken or fractured rock develops with no strong single fault shear, and in that case a series of small parallel veins may develop. These are known as stockwork zones, and it is not unusual that coarse gold would form in these situations. Sometimes strong single vein systems will fray out into a stockwork system at the ends of the vein, or where it crosses into a different rock type. In some conditions, pipe or plug shaped deposits may form where geologic conditions favor tube shaped openings.

At times the circulating mineral waters can go though wide zones where a whole area of rock is porous, and if there is enough solution moving and depositing gold, you might get a big disseminated deposit, like those mined by open pit here in Nevada. The most common geochemistry required to dissolve gold involves sulfur, which is why sulfides like pyrite, galena and arsenopyrite are so commonly associated with gold veins. In most gold bearing veins, the gold does not form into well-developed crystals. Unlike quartz which often forms good crystals, gold requires a very specific set of circumstances to grow into good crystal shapes. Few gold deposits produce good crystal specimens, as most gold forms naturally as irregular masses and lumps. Many of the irregular masses have a form suggesting that they might have been melted at one time, but that is virtually never the case. However, where the right conditions are present, gold can form in a variety of crystal shapes, including dendrites, leaf, deformed octahedrons and cubes. The placer field at Rye Patch (Majuba) in Nevada is one of the places where the conditions were right for the growth of crystallized gold. This location has gained a reputation for some nice looking specimens – the term "chevron" is used to describe the typical shape. Not all Rye Patch gold has the chevron shape, but the chevron shaped nuggets are fairly common. The chevrons I think are a combination of the dendrite and the octahedron forms. Some unusual set of chemistry, temperature and pressure conditions present at Rye Patch led the gold to grow into these attractive V shaped crystals.

The prospector using a metal detector is primarily interested in coarser gold as larger nuggets are easier to detect, and of course, more valuable! It turns out that coarse gold most commonly forms in small veins (for the sake of argument, lets say for our discussion that coarse gold is stuff a Penney weight or larger). Big veins, with a width and length large enough to be mined commercially underground, almost always produce small gold - wires, small pieces, etc - even though they may be fairly rich. Of course, there are a number of exceptions to this rule, but generally speaking, it is true. So why, in general,does big gold form in small veins, while mostly only small gold forms in big veins?

Growth of coarse minerals from water solutions requires stable conditions where large crystals or nuggets can continue to grow over an extended period of time. It takes some time to grow big gold, so conditions must be fairly stable to allow this to occur. This is pretty much the same for quartz, tourmaline, mica or gold – and most other minerals that commonly grow from water solutions. Rapidly changing conditions, including rapid drops in pressure or temperature generally lead to the formation of fine-grained crystals - the minerals are forced to come out of solution so fast that large crystals cannot form.

Often, the source of the heat is some sort of cooling magma (lava, still molten or solidified but still hot). This kind of heat source throws off so much heat energy that the rock may well be warm to within 100 feet of the surface, even if the source is over 2000 feet down. The temperature does not drop off suddenly once the solution gets a short distance from the source. The drop in temperature as you move away from the heat source is gradual, and usually fairly linear, at least within a few hundred feet of the surface. So the temperature of the rock (and the water solutions as well) becomes directly related to how far they are from the heat source. The rate of cooling then becomes a simple function of how fast the fluids move upward.

With that in mind, small feeder veins, and splits off the main vein are kind of the quiet backwaters for the solutions forming veins. Smaller openings just don't allow as much flow volume, and the slower flowing solution can take its time to cool and release its mineral load. The small flows of the small veins move upward slowly and have a chance to take their time to cool. This results in fairly stable conditions in the growth zone where decreases in temperature and pressure are such that minerals, including gold, can grow to a good size. These smaller veins are still spotty in their values, with some parts rich in coarse gold, and other parts low in grade and nearly barren. Usually these veins are far too small for any commercial underground mining operations, with widths of up to a few inches and strike lengths of 5 to perhaps 50 feet.

Wide fault zones and other large openings in the rock generally lead to more solution flow, and greater quantities of quartz, sulfides and gold, which means bigger veins, but also it means more rapidly changing conditions, which means rapid deposition. This generally leads to growth of fine grained minerals. In larger openings, the larger flows move quickly upward and the solutions become over-saturated, so the minerals in solution must drop out quickly, forming fine grained minerals of small size. In fact, it is common that, where there are large fault zones which conduct the fluids quickly upward, the waters are still fairly hot when they actually reach the surface. This often results in the formation of a hot springs. Steamboat Hot Springs, just south of Reno, is a classic example of this. Most productive hard rock veins are not uniformly rich, but have rich sections, which are called ore shoots. These are usually oriented down the dip of the vein, and represent places where rapid changes in pressure or temperature caused the metal bearing solutions to drop much of their metals content. For the average underground mining operation, the fact that the mineralization is fine grained is no problem, but the advantage of the greater volumes of gold deposited is important. Often, even on the occasions when coarse gold is formed in a larger vein system, it occurs where the vein pinches down or splits out into smaller veins.

Lets take a look at some field examples. In some districts, small vein openings are all that occurs. The top of Rich Hill in Arizona is an intensely jointed granite. Hot solutions circulating through the joint cracks in the granite were all of the "small vein" type. There were no large faults that allowed larger volumes of fluid to circulate. Erosion over the years concentrated the coarse gold which grew in these small veins, which in turn produced an extremely rich placer deposit. In the same way, the placers at Gila City (Dome) in Yuma County Arizona produced some considerable gold, though no rich veins were found at that location. The Arizona Bureau of mines came to the conclusion that "many pockety or small low grade veins supplied the gold". At Gila City, the accumulation of gold from those many small sources resulted in a rich placer. Still another example would be the placers at Rye Patch (Majuba) in Nevada. The gold is derived from a series of small quartz veins which occur in shale in a 2 mile wide belt on a pediment east of the range front along the Majuba mountains. Many of Nevada's mountain ranges have large fault zones which parallel the mountain ranges at the edge of the range, where the valley meets the range. The displacement on that fault probably created the heat source which formed the veins. This range front fault trends north south and little if any gold is found west of the fault zone in the mountains themselves. The quartz veins in the mineralized zone are not large and have not been mined as veins underground, as they are just too small. Most have a strike length of perhaps 15 to 30 feet.

In some districts, fine gold is all there is - and as a result, there are basically no placers formed. This is the main reason why some rich hard rock gold districts don’t have much in the way of associated placers. The most productive gold vein district in Nevada (not including the Carlin type deposits) was Goldfield. This District was discovered about 1903, and produced nearly five million ounces of gold, much of it from extremely high grade ores. The smallest veins were several feet wide, but the largest were more than 30 feet wide and stretched to thousands of feet in length. In spite of the high grade ores found there, no significant placers were associated with the Goldfield deposits and all the gold in the veins was very fine grained. The hot fluids circulated through pre-existing faults which provided large openings for the waters to circulate quickly through. The Jarbidge District in Elko county, Nevada is another example from Nevada of a district with good gold veins, but only fine gold and little or no placer. So when planning a trip to detect around old hard rock mines, be sure to find out, not just if the gold found was "free gold", but just how coarse the gold from that district was.

In some districts both large and small veins occur. At Randsburg in southern California, rich veins were mined underground yielding over one million ounces of gold. These veins are located in and around the town of Randsburg, though a few of the vein deposits trended off to the south of town. Mineralization in and around two of the largest vein systems (the Yellow Aster and Baltic) have been mined as an open pit deposit in recent years. The placers, which yield gold coarser than what was found in the veins, are for the most part located on the pediment north of town - away from the productive larger veins. The Kofa placers of Yuma County; contain both larger veins which were mined underground and smaller veins which produced coarse gold. USGS Bulletin 620 says the following about the coarse gold found in these placers: "It has evidently been derived from the disintegration of auriferous veins in the metamorphic rocks as it is much coarser than the gold contained in the North Star and King of Arizona veins". The North Star and King of Arizona veins were two larger veins which were mined underground. The Vulture placers of Maricopa County, Arizona also fall into this category. The Arizona Bureau of mines states in Bulletin 168 that "The origin of the placer gold, in Red Top basin at least, appears to have been the small quartz veins in that vicinity. The gold of these veins, like that of the adjacent placers, appears to be coarser than that of the Vulture vein".

The tendency for coarse gold to form in smaller veins is not a hard and fast rule and there are some large veins which have produced very coarse gold (like the 16 to 1 mine in the Allegheny District of California). However, big gold in small veins is by far the most common scenario for the formation of coarse gold. So it is true in general - just allowing for a few exceptions here and there.

It is a fact that many of the well-known placer districts of the western US have as their source of gold small veins which contain sparse pockets of coarse gold. This is the main reason why many rich placer districts have seen little or no hard rock mining, because these small veins were difficult for old timers to find and work profitably. While this was frustrating to old time miners hoping for a few larger lode deposits, it presents a big opportunity to the modern prospector armed with a metal detector. Unlike the old timers, we are able to cover large areas checking for residual placers quickly and when found, work them efficiently. The hills and slopes above the gullies and washes in these districts are excellent targets for the detector, as some of these small veins have formed residual placer deposits which are still sitting on the hill slopes, lying under a thin veneer of soil and clay - just waiting to be found.

Copyright 2003 By Chris Ralph
All Rights Reserved