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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 rapidly 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. For many years geologists have observed that the veins of nearly
every gold district contain gold that is finer than the placer deposits
which may be located in the same general area. |
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To explore these concepts, lets take a look at some field examples of
gold deposits in the Western
US. 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. There are a few commercial veins in the Rich Hill area, but they
are away from the top of Rich Hill and the potato patch placer area. 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. There are
actually many examples of rich plaster areas were few if any significant
gold veins were found (at least none were found that were suitable for
commercial mining). 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. This is exactly the opposite of the placer fields that contain no
important veins. The most productive gold vein district in Nevada (not
including the bulk 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 extremely high grade
gold 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, and if any
placer gold was ever found
there. |
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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, spotty
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 nearby residual placers on the hillsides quickly and when found, work
them efficiently. The hills and slopes above these once rich gullies and
washes in these districts are excellent targets for the prospector armed
with a metal detector. This is because some of the small veins that formed
the stream placers have left valuable residual placer deposits still sitting
on the hill slopes and ridges above the ravines. These residual placers are
lying under a thin veneer of soil and clay - spectacular coarse
gold nuggets, just waiting to
be found.
Copyright 2009 By Chris Ralph
All Rights Reserved
Continue on to:
Coarse Gold: Part I, Nugget Growth
Coarse Gold: Part II, Geologic Examples
Return To:
All About Placer Gold
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