Naturally occurring gold metal is called native gold. It is most commonly found in residual, eluvial and alluvial deposits and in the weathered portions of gold-bearing sulfide lodes. It may also be present locally in unoxidized lodes and quartz veins. Free gold may occur in sizes ranging from microscopic particles to nuggets weighing several kilograms.

Gold deposits can, on the basis of their mode of origin and means of transport be placed into three major groups:

(1) primary deposits - which include lode, vein, and disseminated deposits;

(2) secondary deposits - which include residual, eluvial, desert, alluvial, bench, beach, glacial, and eolian;

(3) lateritic deposits - which contain gold-enrichment zones in the weathering profile.

PRIMARY DEPOSITS

Primary deposits occur in hard, crystalline rocks, which have commonly been affected by either metamorphism and igneous activity or both. Primary deposits are commonly called veins (reefs) or lodes. Veins consist mainly of material such as quartz, which has been introduced along with the gold. Lodes are generally zones of sheared and altered country rock that have been impregnated with sulfides, carbonates, and quartz; mineralized sedimentary horizons (e.g. banded iron-formation) are also referred to as lodes. In vein deposits, gold tends to occur in the free state, whereas in lodes it is usually bound up in the sulfide mineral. The location of lodes and veins within the host rock is controlled by fractures, bedding, folds, and/ or changes of rock type.

Gold seldom, if ever, occurs by itself. It is almost always alloyed with silver and there are usually associated sulfides such as pyrite, pyrrhotite, arsenopyrite, chalcopyrite and galena. In some areas, scheelite (a tungsten mineral) and stibnite (an antimony mineral) are also known to occur in association with gold. Primary deposits are the source of all secondary deposits.

Where exposed to weathering processes at or near the Earth’s surface, the sulfide and telluride minerals are broken down by the chemical action of air and water. The gold contained may be shed at the surface to form eluvial and alluvial deposits, or be carried in solution down the lode channel to be redeposited and enriched at the water table. The latter process is called supergene enrichment (see Lateritic deposits) and was of great importance to the early prospectors. Many of the small mines they operated would have been uneconomical without it; and many did become uneconomical once the water table was passed.

SECONDARY DEPOSITS

Secondary deposits are formed where gold shed from an outcropping vein or lode and is concentrated by the action of wind and water to form either: eluvial deposits in soil or slope debris near the outcrop; or alluvial deposits in stream channels. According to Vanderburg, " Gold placer deposits have been derived from the disintegration and weathering of auriferous veins and mineralized rocks. The disintegration of rocks is accomplished slowly by natural agents, namely, the wind, rain, flowing streams, frost, changes in temperature, growth of vegetation, chemical action and movements of the earth’s crust. These agents, working throughout geologic time, reduce the rocks to gravel, sand, silt and clay, and liberate the gold. "

Gold mined from secondary (or placer) deposits is invariably recovered as the native metals; gold-bearing sulfides and tellurides do not withstand the erosional processes that form the deposits. Gold from secondary deposits ranges in grain size from ‘mustard’ or ‘flour’, in which individual particles weigh less than millionth of a gram, to nuggets, which, exceptionally may weigh several kilograms. These nuggets probably formed in the oxidized parts of lodes or veins and have since been weathered out into the surrounding soil. When found in stream gravels they have been rounded and flattened by abrasion during transport. Secondary deposits can be broken down into various classes, listed below is just one proposed classification scheme.

Residual (or seam diggings)

A residual placer is, in effect, a concentration of gold at or near its point of release from the parent rock. In this type of placer the enrichment results from the elimination of valueless material rather than from concentration of values brought in from an outside source. The gold is commonly associated with loose ‘ferruginous’ detritus, and occurs as angular particles, often still attached to iron or quartz. Residual placers may be rich but they are not likely to be large and as a class, they have been relatively unimportant.

Eluvial Placers

Eluvial or ‘hillside’ placers usually represent a transitional stage between a residual placer and a alluvial placer. Where one type merges into another, they cannot be clearly distinguished. They are characteristically found in the form of irregular sheets or surface detritus and soil mantling a hillside below a vein or other source of valuable mineral. It should be noted that the parent vein or lode may or may not outcrop at the actual ground surface. Eluvial placers differ from residual placers in that surface creep slowly moves the gold and weathered detritus down hill, allowing the lighter portions to be removed by rain wash and wind. As the detrital mass gravitates downhill, a rough stratification or concentrate of values may develop but this is rarely perfected to the degree found in alluvial placers.

Desert Placers

Desert placers in the Southwest occur under widely varying conditions but taken as a whole, they are so different from normal stream placers as to deserve a special classification. When dealing with the usual desert placer the mineral examiner must learn to disregard some of the rules of stream deposition, or at least, he must learn to apply them with caution. Desert placers are found in arid regions where erosion and transportation of debris depends largely on fast-rising streams that rush down gullies and dry washes following summer cloudbursts. During intervening periods, varying amounts of sand, gravel or hill-side detritus is carried in from the sides by lighter, intermittent rain wash which is sufficient to move material into the washes but not carry it further. When the next heavy rain comes, a torrential flow may sweep up all of the accumulated detrital fill, or only part of it, depending on intensity and duration of the storm and depth of fill. Under such conditions the movement and concentration of placer gold will be extremely erratic. Moreover, when the entire bedload is not moved, any gold concentration resulting from a sudden water flow will be found at the bottom of the temporary channel existing at that time. This may be well above bedrock.

Desert miners have learned from experience that gold enrichments are sometimes found resting on caliche layers, particularly those near the ground surface, but such surface or near-surface concentrations are commonly small, residual-type accumulations of gold left behind where lighter material has been removed by rain wash and wind action. In other words, such enrichments result from the removal of valueless material rather than from the concentration of gold by normal stream processes.

Alluvial placers

Alluvial placers are the most widespread type in the Western States and, accordingly, are the type most frequently encountered in mineral examinations. Individual deposits vary so much that few general statements can be made concerning them but for the purpose of this review, they can be conveniently divided into:

a. Gulch placers

b. Creek placers

c. River deposits

a. Gulch placers: Gulch placers are characteristically small in area, have steep gradients and are usually confined to minor drainages in which a permanent stream may or may not exist. This type of placer is, as a rule, made up of a mixture of poorly sorted gravel and detritus from adjacent hillsides. Because of steep gradient, the gravel accumulations are often thin and discontinuous. Boulders are commonly found in quantities that preclude all but simple hand mining operations. The gold is likely to be coarse and well-concentrated on bedrock. Gulch placers were usually the first to be found by the early miners and because most can be worked with simple hand tools, unworked remnants of shallow gulch deposits are not likely to contain material that would yield a profit today. The early-day miner was generally well-schooled by experience , and a diligent worker. Any pay gravel that he left was usually cleaned up by the patient Chinese who followed. Nevertheless gulch placers still hold potential for today’s prospector armed with a modern metal detector.

b. Creek placers: In many districts creek placers have been important sources of gold but like the gulch placers most were carefully prospected by the early miners and worked out, where worthwhile to do so. Many of the lower-grade remnants left by the early hand miners have since been exploited by some form of mechanized mining, notably by dragline dredges during the depression years of the 1930’s. Creek placers are currently being mined in Alaska with nonfloating washing plants and moveable sluices utilizing various combinations of hydraulic and mechanical excavation equipment.

c. River deposits: River deposits are represented by the more extensive gravel flats in or adjacent to the beds of present-day rivers and as a class, they have been our most important source of placer gold. They are generally similar to creek placers but the gold is usually finer, the gravel well-rounded and large boulders fewer or absent. Although the over-all deposit may be low-grade, pay streaks and bedrock concentrations capable of supporting dredging or other large-scale mining operations are not uncommon. At many places in California, the early miners diverted rivers through tunnels or bypassed the water in flumes to permit mining the river bed. In this manner, many miles of the middle and upper reaches of the principle gold-bearing rivers were effectively worked. The lower reaches of many of these streams were systematically dredged and, at one time, where conditions were favorable, worked at a profit.

Bench placers

Bench placers are usually remnants of deposits formed during an earlier stage of stream development and left behind as the stream cuts downward. The abandoned segments, particularly those on the hillsides, are commonly referred to as "Bench" gravels. Frequently there are two or more sets of benches in which case the miners refer to them as "high" benches and "low" benches. In California and elsewhere, most bench deposits were quickly found by the early miners who proceeded to work the richer bedrock streaks by primitive forms of underground mining. At the time these were referred to as "hill diggings." Following the development of hydraulic mining in the 1850’s, many of the larger bench deposits were worked by hydraulicking and the smaller ones by ground sluicing. During the depression years, much of the so-called "sniper" mining was carried out on the remnants of bench gravels.

Beach placers

Beach placers may form where gold-bearing material is carried into the oceans by streams, or along the wave-cut base of a gold-bearing coastal plain. With the exception of the highly productive beach placers discovered at the turn of the century at Nome, Alaska, none have been of great importance in the Western States. Typical beach placers along the Pacific coast are found as erratically distributed, somewhat lenticular concentrations or streaks of black sand minerals with varying amounts of finely-divided gold and in some places, with platinum-group minerals. Beach-placer black sands can be expected to consist largely of magnetite and limonite but significant amounts of chromite are found in some Oregon beach sands. In the case of gold-bearing beach placers, the individual black sand concentrations are seldom over 100 feet long or more than a few feet thick. Those found on the active beaches are the result of storm and tidal action, and they come and go with changing conditions of the beach. Some of the most productive placers have been found in ancient, elevated beaches that are now several miles inland.

Glacial placers

The fundamentals of glacial placers have been well set out by Black welder (1932) as follows:

Since it is the habit of a glacier to scrape off loose debris and soil but not to sort it at all, ice is wholly ineffective as an agency of concentration for metals. Gold derived from the outcrops of small veins is thus mixed with large masses of barren earth. Attempts to mine gold in glacial moraines, where bits of rich but widely scattered float have been found, are for that reason foredoomed to failure.

If a glacier advances down a valley which already contains gold-bearing river gravel, it is apt to gouge out the entire mass, mix it with much other debris and deposit it later as useless till. Under some circumstances, however, it merely slides over the gravel and buries it without distributing it.

On the other hand, the streams born of glaciers or slowly consuming their moraines have the power to winnow the particles of rock and mineral matter according to size and heaviness. Such streams may form gold placer deposits in the well-known way by churning the load they carry and allowing the heavy minerals to sink to the bedrock. Placers may therefore be found in the deposits of glacial rivers if there are gold veins exposed in the glaciated area upstream. Nearly all the gravel which has been dredged for gold along the foothills of the Sierra Nevada was deposited by rivers derived in part from glaciers along the crest of the range, but most of the gold was probably picked up in the lower courses of such rivers. Since glacial rivers choke themselves and build up their channels progressively, their deposits are likely to be thicker and not so well concentrated as those of more normal graded rivers which are not associated with glaciers."

Eolian placers

In desert regions the wind may act as an agent of concentration by blowing sand and the lighter rock particles away from a body of low-value material and leaving an enriched surface veneer containing gold or other heavy minerals in a somewhat concentrated state. There have been many cases where wind-caused surface enrichments supported the activities of itinerant miners using hand tools and simple dry washers.

LATERITIC DEPOSITS

Gold, from a primary source, may be remobilized and transported through weathered rock by the action of groundwater. In this process, gold is taken into solution with iron, in a reducing environment, and is reprecipitated at the water table, in an oxidizing environment. The secondary precipitation produces zones of gold enrichment in parts of the lateritic weathering profile. Laterite forms under tropical to subtropical climatic conditions. Lateritic deposits are usually low-grade, large-tonnage deposits suitable for mining by way of large-scale open pit operations.

REFERENCES:

1) Gold in Western Australia, Geological Survey of Western Australia; Department of Minerals and Energy

2) Placer Examination: Principals and Practice, BLM Technical Bulletin 4

3) Advanced Prospecting & Detecting for Hardrock Gold, Jim Straight