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Favorable Gold Geology


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My favorite indicators are gold nuggets.

Everything you ever want to know about the subject is covered in Chris Ralph's excellent book, "Fists Full of Gold".

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I am very simplistic in my methodology. I map existing gold deposits and look for relationships with certain rock types or structures, like faults. Better yet, this work has almost always been done for me so all I need to do is seek out the appropriate reference material.
 
You will often see certain rock types come up again and again when researching gold. Greenstone, schist, and slate. Gold commonly occurs in veins associated with these rock types. There also is a close relationship of gold with granitic intrusives. In Alaska gold and quartz monzonite (a granitic rock type) intrusives are closely related.
 
In California Mother Lode country you have strong associations with both rock types and certain faults. Faults provide a pathway for mineralized solutions. From http://www.mindat.org/loc-98002.html "The northwest-trending Mother Lode Belt traverses western Tuolumne County and is associated with the Melones Fault Zone. The rocks of this belt are typically metavolcanic, metasedimentary, and ultramafic, some of which have been hydrothermally altered to assemblages as described below. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins and bodies of mineralized country rock adjacent to veins. Mother Lode veins are characteristically enclosed in Mariposa Formation slate with associated greenstone."
 
So on a larger scale you simply want to put yourself on ground that has certain rock types according to your research. Here is a map showing the relationship of gold with greenstones and faults in part of Australia.
 
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Quartz tends to be a sign that mineralized solutions were traveling in the rock system, that may or may not have had gold in solution. Most quartz is barren. Gold also needs a reason to precipitate or drop out of solution, and iron acts as a precipitate or reason for it to stop and deposit. Quartz and iron stones together are a very good indicator.
 
It is not just rock types but faults and regional geology. Gold very often clumps on a map into certain areas, and these often are strung out in a certain direction, known as a trend line or simply trend. The trend follows certain rock types and faults known to be associated with gold and most are very well known, like the California Mother Lode. Here is a map of a portion of the Battle Mountain trend in Nevada.
 
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If you want to get real serious, here is one of the best references available Models and Exploration Methods for Major Gold Deposit Types This stuff can seem like Greek but if you go slowly and use Google a lot to learn words you do not know eventually this stuff makes a lot of sense. It literally is your map to treasure.
 
However, I do have to close with this, that relates very much to what Lunk has said.....

But those who have been engaged in practical mining for long, finding by experience that no two mineral fields are exactly alike in all their characteristics, have come to the conclusion that it is unwise to form theories as to why metals should or should not be found in certain enclosing rocks or matrices. Some of the best reef gold got in Victoria has been obtained in dead white, milky-looking quartz almost destitute of base metal. In South Australia reef gold is almost invariably associated with iron, either an oxide, as " gossan ; " or ferruginous calcite, " limonite; " or granular silica, conglomerated by iron, the " ironstone " which forms the capping or outcrop of many of our reefs, and which is often rich in gold.

But to show that it is unsafe to decide off-hand in what class of matrix metals will or will not be found, I may say that in my own experience I have seen payable gold in the following materials :—

Quartz, dense and milky, also in quartz of nearly every colour and appearance, saccharoidal, crystalline, nay, even in clear glass-like six-sided prismatic crystals, and associated with silver, copper, lead, arsenic, iron as sulphide, oxide, carbonate, and tungstate, antimony, bismuth, nickel, zinc, lead, and other metals in one form or another ; in slate, quartzite, mica schist, granite, diorite, porphyry, felsite, calcite, dolomite, common carbonate of iron, siliceous sinter from a hot spring, as at Mount Morgan ; as alluvial gold in drifts formed of almost all these materials ; and once, perhaps the most curious matrix of all, a small piece of apparently alluvial gold, naturally imbedded in a shaly piece of coal. This specimen, I think, is in the Sydney Museum. One thing, however, the prospector may make sure of : he will always find gold more or less intimately associated with silica (quartz) in one or other of its many forms, just as he will always find cassiterite (oxide of tin) in the neighbourhood of granite containing muscovite (white mica), which so many people will persist in terming talc. It is stated to be a fact that tin has never been found more than about two miles from such granite.

From what has been said of its widely divergent occurrence, it will be admitted that the Cornish miners' saying with regard to metals generally applies with great force to gold : " Where it is, there it is" : and " Cousin Jack" adds, with pathetic emphasis, " and where it is generally, there I ain't."

 
From GETTING GOLD: A PRACTICAL TREATISE FOR PROSPECTORS, MINERS, AND STUDENTS.
BY J. C. F. JOHNSON, F.G.S., 1897

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Check out Part IV, "Arizona Lode Gold Mines and Mining", Bulletin 137, Arizona Bureau of Geology and Mineral Technology, Geological Survey Branch.  It is an overview of geological features where gold may be found.

 

PM me if you would like me to e-mail you a copy of Part IV.  Bulletin 137 was originally compiled in 1934, but Part IV is still applicable today.  Please send me your personnel e-mail address so I can send it to you direct.  I am not sure I can attach a file to a return PM.

 

 

Ken

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Great write up Steve. Thanks for taking the time to put that together.

This is an awesome site and I appreciate your effort.

I will second what Lunk said too.

I learned a lot reading Fists Full of Gold. Lots of information in that book.

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Check out Part IV, "Arizona Lode Gold Mines and Mining", Bulletin 137, Arizona Bureau of Geology and Mineral Technology, Geological Survey Branch.  It is an overview of geological features where gold may be found.

 

PM me if you would like me to e-mail you a copy of Part IV.  Bulletin 137 was originally compiled in 1934, but Part IV is still applicable today.  Please send me your personnel e-mail address so I can send it to you direct.  I am not sure I can attach a file to a return PM.

 

 

Ken

 

You can download Arizona Lode Gold Mines and Mining", Bulletin 137 below in its entirety as a pdf file.

 

http://www.azgs.az.gov/Mineral%20Scans/gold_bull137_ocr.pdf

 

Part IV begins on page 239:

PART IV

SOME HINTS ON PROSPECTING FOR GOLD

By G. M. BUTLER,

Director) Arizona Bureau of Mines

Unfavorable Areas

The saying that "gold is where you find it" is certainly true. There are, nevertheless, certain conditions that are so unfavorable to the occurrence of gold in any considerable quantity that prospectors would do well to avoid areas in which these conditions exist. Among such unfavorable areas are the following:

1. Areas where large masses of granite and related, coarse grained, crystalline igneous (once molten) rocks outcrop, particularly if these outcrops are not cut by dikes or other intrusions of finer grained, usually light colored igneous rocks such as porphyry, rhyolite, or andesite.

2. Areas where large masses of gneisses and the other crystalline schists outcrop unless they are cut by or in the vicinity of dikes or other intrusions of igneous rocks.

3. Areas where large masses of sedimentary rocks such as limestone, sandstone, and shale outcrop unless they are cut by dikes or other intrusions of the relatively fine grained, light colored igneous rocks previously mentioned, and, even where so cut, sedimentary areas rarely contain workable quantities of gold unless the sediments have been metamorphosed (changed in character by pressure and heat) to marble, quartzite, or slate.

4. Areas where large masses of dark colored, relatively heavy igneous rocks, such as peridotite, diabase, and basalt or malpais outcrop.

5. Areas in which nothing but the unconsolidated or loosely consolidated material that fills the valleys between the mountain ranges in southern and western Arizona outcrops.

A study of the geological map of Arizona described in. the back of this bulletin will enable anyone to learn where the great sedimentary and valley-fill areas are situated in Arizona and to avoid them. The great granitic and schistose masses are also shown on that map, but its scale is so small that intrusions of igneous rocks hundreds of feet wide cannot be shown thereon. It is, therefore, necessary to examine the granitic and schistose areas to determine whether the conditions there are unfavorable. It is not true that valuable gold lodes never occur in areas described

as generally unfavorable, such as in a great mass of granite without intrusions of other igneous rocks, for instance, but a prospector will usually save time and money by avoiding such areas.

Favorable Areas

1. Probably the most favorable area in which to prospect for gold is one where the country rock is made up of surface flows, sills, dikes, and other intrusions of relatively fine grained, light colored, Tertiary igneous rocks such as rhyolite, trachyte, latite, phonolite, and andesite.

2. As has already been suggested, prospecting in areas where there are outcrops of granitic or schistose rocks that are cut by dikes or other intrusions of relatively fine grained, light colored igneous rocks may prove profitable.

3. Areas in which the country rock is some type of porphyry, especially if several varieties formed at different time are found there, may contain deposits of gold that can be worked profitably.

4. Gold lodes that may be worked profitably are sometimes formed around the borders of great masses of granitic igneous rocks, both in the granitic and in the surrounding rocks, but more commonly the latter.

5. Areas in which some gold has already been found are naturally more favorable than places that have never produced

any gold. This statement applies particularly to areas where considerable prospecting has been done-any area in continental United States excepting much of Alaska.

Structures that May Contain Gold

The term "lode" as used in the Federal statutes is applied to all deposits of "mineral in place" formed beneath the surface of the earth. When gold is found in sand or gravel, either loose or cemented, the occurrence constitutes a gold placer deposit. The gold found in lodes has arisen from great depths in solution and has been precipitated or deposited from such solutions by relief of pressure, cooling of the solutions, and other causes. There must, then, be some form of opening or zone of weakness through the rocks along which the solutions may rise. Although the mineral-bearing solutions sometimes find their way toward the surface through masses of porous material that have roughly the form of vertical cylinders, lodes are usually long and relatively

narrow.

From what has been said, it should be evident that the existence of openings or lines of weakness in the rocks is the condition that fixes the position of a lode. Most frequently, the solutions rise through a crack or fissure that extends a long way downward or through a series of interlacing cracks or fissures. If the solutions precipitate or deposit ore minerals and gangue (the worthless minerals deposited along with ore minerals) in a single, clean-cut fissure, filling it full of ore minerals arid gangue, the result is a simple fissure vein. If there has been movement of one wall relative to the other, parallel to the fissure, and ore minerals and gangue have been deposited in the fissure, the result is a fault fissure vein. Both simple and fault fissure veins are commonly called by miners "true fissure veins."

Where movement or faulting has occurred at considerable depth and the pressure is great, the walls of the fissure may be polished or "slickened" and clay-like, finely crushed rock called "gouge" may be formed. Subsequent deposition of mineral in the fissure would force the walls apart and the gouge may then be found along both walls. After the vein has formed, the movement or faulting may be resumed and "slickensides" and gouge may then be formed in the vein material itself, or, if the pressure is then relatively light, the vein material may be broken into fragments, thus forming one type of so-called "brecciated vein." If the mineral-bearing solutions rise through and deposit ore minerals and gangue in the zone of interlacing, closely spaced cracks, the result is a shear zone and that is a very common type of lode.

The contact between two different rocks, especially an igneous rock and something else, as porphyry and schist, or two different rocks, is usually a line of weakness. If a mass of igneous rock is involved, shrinkage occurs on cooling and that shrinkage tends to cause the igneous mass to pull away from the rocks with which it is in contact. If solutions pass up through and deposit ore minerals and gangue along such a line of weakness, the result is called a contact fissure vein.

There are a number of other varieties of lodes, but they are rarer than the types mentioned and it would unduly lengthen this chapter to describe them. It should be emphasized that most of the structures mentioned contain so little valuable material that they cannot be mined profitably. Nevertheless, they are the things that must be sought and any such structure 10und should be carefully investigated. Furthermore, this investigation should not be confined to the vein matter itself, but should include the walls since they are sometimes impregnated with material that has been precipitated from solutions that have penetrated them. They should be assayed, especially if they appear to be softened and altered. Moreover, the wall rock itself is sometimes completely replaced with vein material to considerable distances from the vein itself.

Surface Characteristics of Gold Lodes

The actual outcrop of a lode usually consists of a zone or band of material that differs in character from the material on both sides of it, but the discovery of such a zone or band does not prove that a lode has been found. It might consist, for instance, of a bed of limestone between beds of shale, all turned up at a steep angle and eroded. The outcrop of a lode has, fortunately, other characteristics that make it possible to recognize it as follows:

1. It does not consist entirely of any single species of rock. It may be composed almost entirely of some species of rock, but there will be veinlets running through it in that case.

2. Because the solutions that form the lode usually deposit sulphide of iron along with the gold, and sulphide of iron is changed to yellowish brown to dark brown oxide of iron when exposed to the atmosphere, the outcrops of lodes are usually heavily "iron stained." Stains produced by the oxidation of minerals that contain other metals than iron may also be present, such as green or blue (copper), black (manganese), light yellow (molybdenum or lead), lilac (cobalt), etc.

3. Because some of the iron and all of the sulphur, as well as other soluble ore and gangue minerals, may be carried away in solution when the lode is exposed to atmospheric weathering, the outcrop is apt to be decidedly porous. The grade of dense outcrops is usually very low.

4. The gangue mineral most commonly associated with gold is quartz and a gold lode that does not contain some quartz is rare indeed. Low grade gold veins that consist almost entirely of quartz are not uncommon. Other gangue minerals sometimes associated with gold (usually one or two of them are present except in the almost pure quartz or quartz-pyrite veins) are carbonates (particularly calcite), adularia feldspar, sericite (very finely granular white mica), fluorite, etc.

It should not be assumed that a valuable discovery has certainly been made when a lode that consists of heavily ironstained, porous rotten quartz, where it outcrops, has been found. Such material often contains little or no gold. By crushing and panning it, the presence of gold can sometimes be ascertained, but the only safe method to use to learn whether it is valuable is to have it assayed by a reputable assayer. It should be remembered, however, that few lodes contain even approximately the same proportion of gold for any distance along their outcrops. Gold, as well as other ore minerals, is concentrated at certain points in the lode in what are called "shoots," "ore shoots," "ore channels," etc., of relatively limited extent. Moreover, values may be concentrated along one wall or at a certain distance from a wall. If, then, a lode has been discovered, samples at numerous points along its outcrop and from various positions in it should be crushed and panned or assayed before deciding whether the ore is or is not valuable.

Identifying Gold

Sometimes specks, grains, or thin plates of gold are visible in the outcrop of a lode and, in that case, they may be recovered by crushing and panning. It is, therefore, important that a prospector should be able to identify such material and, especially, to distinguish it from other substances with which it may be confused. Ores of nearly all substances excepting the very rare metals like gold, silver, and platinum, usually occur in masses of such size that they may easily be seen, and anyone familiar with these minerals will recognize them by their physical characteristics. It is usually unnecessary for a mineralogist to assay a specimen in order to ascertain whether it contains considerable quantities of copper, zinc, lead, manganese, etc., for instance.

 

Such is not the case with gold, however. In the great majority of instances, it occurs in tiny grains that are distributed through the gangue or included within other minerals and are quite invisible. They may be sometimes recovered by panning, but frequently they are so small or are so firmly locked up in other minerals that only fire assaying will reveal their presence. Rarely, gold is combined with tellurium to form minerals that are called tellurides and they can be recognized, if present in visible grains, by a mineralogist who is familiar with their characteristics, but there is no known deposit of telluride gold ore in Arizona, and it is not likely that one will be found. It is fortunate that telluride ores do not occur commonly here since special methods of assaying and treating them must be used.

In spite of the fact that visible gold is not common in lodes, it is important, as stated, that prospectors should be able to recognize it when it is found. They should be able to do so by the fact that it is the only soft, yellow, metallic lustered substance found in nature that may be easily flattened without breaking and easily cut with a knife blade or indented with a needle or any other small, sharp-pointed instrument. It may be confused with pyrite, chalcopyrite, and several other sulphide minerals, but pyrite is so hard that it cannot be scratched and all sulphide minerals that resemble it crush into a black powder instead of cutting cleanly. Gold is sometimes confused with plates of yellow mica, but they are much softer and yield a white powder when scratched with the point of a needle. Sometimes there is so much silver present as an impurity in the gold that the color becomes almost silver-white, but it is rarely that there is not some yellow tint to the alloy. Chemical tests are necessary to distinguish gold that contains 20 per cent or more silver from metallic silver.

Seeking Gold Lodes

Many gold lodes, but by no means all of them, when exposed to erosion, disintegrate and the gold in them washes down into the beds of streams that drain the area. A common method of prospecting for gold lodes is to pan or use a small dry washer on the material from washes and stream beds and to endeavor to trace the placer gold back to its source. If placer gold is irregular in shape and more or less jagged or if quartz is still embedded in the grains or nuggets, it has not moved far from its source. If, however, it is smoothly rounded or flattened and contains no quartz, its source may be many miles away.

If particles of gold have been found in a stream bed or wash and have been traced up stream to a point where they suddenly become much less numerous or disappear, the gold has probably come from a lode directly uphill from the point where it was last found in the stream bed or wash. By trenching or panning and dry washing the earth on both sides of the wash and up hill therefrom, it may be possible to determine in which direction to seek the source of the gold. If the direction from which it came can be thus ascertained, it then becomes necessary to remember

what has been said about the appearance of the outcrop of a lode and to hunt for one.

It should be said, however, that even if the outcrop of a lode that has been the source of placer gold is found, it may be too small or two low grade to be mined profitably, although the placer deposit formed of the gold released from it is large and rich. It is always possible that this placer gold may have come from a wider or a richer part of the lode, that has been removed by erosion.

It is also true that some valuable gold lodes do not yield much, or any, placer gold, and this statement may apply to all the lodes in an entire district, such as the Oatman district in Arizona. The absence of placer gold should not, then, deter a prospector if other conditions appear not unfavorable and, especially, if gold has been found in the district.

When no placer gold is found in a district, it is necessary to search for a lode or indications of the existence of a lode without the guidance of placer gold particles that have been shed from it. One should, then, search through the gulches, washes, and stream beds for pieces of "float" (fragments of ore minerals and gangue, such as "rusty quartz," broken from a lode and washed down therefrom). Their angularity and smoothness will give some indication of the distance they have travelled, but they are usually too scarce to be followed back to their source in the same way that 'placer gold can be traced.

Having decided to try to find a lode on a given hill or a mountain, it is usually best to walk along the slope, endeavoring to keep on the same level, and search carefully for float or other indications of the existence of a lode. If nothing is found at one level, climb a few hundred feet and again circle the hill or mountain at that level. Continue alternately to climb and circle until the top has been reached. While climbing or circling, if the ground is covered with broken rock or soil, look for the following things that may be caused by the existence of a lode at the point where they are seen:

1. A trench or ditch that does not run directly down the slope of the hill or mountain.

2. A sudden change of slope.

3. A sharp notch that crosses a ridge that has a rather uniform altitude on both sides of the notch.

4. Several springs in a line.

5. A sudden change in the kind or quantity of vegetation (may indicate a contact or, if the change in vegetation is found over a narrow strip of ground, a lode may be beneath).

6. A change in the nature of the rock fragments (indicates a contact).

It is true that there are many possible causes for the existence of all the things just mentioned, and that the presence of a lode is only one of them, and not a very likely one. Nevertheless some trenching should be done when such features occur, especially if float has been found below, and not above. When digging in search for a lode on a hill side that is covered with several feet of loose rock and soil, it should be remembered that float works down hill and a lode should be sought several feet above the point where the highest float is found.

Sampling Lodes

When a lode has been found and it is decided to have one or several assays made, most inexperienced prospectors make the serious mistake of collecting samples that are altogether too small. The majority of the samples sent to the Arizona Bureau of Mines to be assayed are each a single fragment weighing a pound or less or two or three smaller fragments. Such "specimen" assays are a complete waste of time and money. They furnish almost no indication as to whether a lode has been found that can or cannot be mined profitably. Not only is valuable ore concentrated in shoots (Page 249), but the grade often varies greatly at distances of only a few inches in a shoot. A single specimen might easily assay $50 a ton and yet represent only 2 inches of ore. On the other hand, barren specimens can often be found in lodes that will run $50 a ton, for instance, as a whole. The only safe way to do is to cut very large samples clear across a lode so that each truly represents the ore that might be mined at that point, and crush and quarter them down or send the whole sample to an assayer. He will charge no more for assaying a 10-, 20-, or 30-pound sample than for a one-pound sample.

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Thanks Steve. 

 

Infavorable locations #5 isn't necessarily applicable to placer gold as there are numberous areas where gold laden deposits have eroded over time leaving bajada and other type placers and the resulting stream placers in washes.  Of course originally these areas where in favorable structures before eroding

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A great thread.

There are a lot of forums out there about detecting for gold.

There is nothing like this forum.

 

Again, well done Steve and the many contributors. 

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So, if I am poring over aerial map views of Arizona gold country for instance and I come across a rock formation that looks like the Great Wall of China , say 20-50 feet wide, and can be traced in a fairly straight line  across the countryside for hundreds of feet or  even a mile or more, sometimes quartz, sometimes unknown rock, is this what is referred to as a dike ? Is this a good gold sign? Where does one find fault line maps? I've never seen one. Other times I see areas of color, gray and red/orange especially. I know the red /orange are iron gossans. Sometimes the gray splotches are irregular and sometimes can be traced as a band for a half mile or more. I am assuming they are a mineral but don't really know?

 

-Tom

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just follow all the claims, that's where the gold is.

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