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From the November 2000 Issue of ICMJ

Residual, Lateritic, and Gossan "Soils" as Potential Nuggetshooting Sites By Jim Straight

Introduction
Nearly everyone who has drywashed or metal detected for placer gold within hardrock gold-mining areas is aware of “residual placers” (a.k.a. seam diggin’s) that have slowly concentrated by physical and chemical dissolving and removal of worthless vein-matter, leaving valuable minerals, such as gold, to form as “residual placers.” “Laterites” and “gossans” can also form a concentration of resistant minerals, such as free-milling gold, by the downward leaching of soluble minerals within the matrix.

Soils form by the slow weathering of solid rock. In dry mountainous regions, the soil zones may be poorly developed with only a few inches of topsoil directly above bedrock. Climates affect the type of soils formed. As an example, granite weathers to quartz and feldspar. In turn, quartz weathers to sand, and feldspar weathers to clay. Under the topsoil there are soil zones—the “A”-zone consists of humus sand and clay. The “B”-zone consists of roots, more clay, and iron oxides. The lowest “C”-zone consists of partly weathered rock resting above the bedrock. In wet, warm areas, the downward leaching of iron-oxides within the “B”-zone can form lateritic soils.

There are two types of weathering: physical and chemical. The effect of physical weathering is best seen in hilly or mountainous areas where freezing and thawing, forest fires, cloud bursts, tree roots, gravity and other mechanical agents break rocks apart to form smaller pieces. Chemical weathering alters the composition of the minerals within the “B-” and “C”-zones. Water, oxygen and acids seep into rocks beneath the topsoil to dissolve soluble minerals and remove them by a process called leaching.

Residual Soils
Bateman (page 294) identifies a “residual concentration deposit” as “the accumulation of valuable minerals when undesired constituents of rocks or mineral deposits are removed by weathering.” Thus, a residual deposit can consist of nearly any resistant mineral, such as gold, that has been concentrated by the removal of soluble minerals.

Residual soil mantles represent only a small portion of the original, primary, gold-bearing hardrock vein material. They have concentrated in place as “seam diggin’s” on a dry hillside while rainfall, wind, gravity and other agents carry away the lighter soluble minerals such as calcium, potassium, and sodium that are abundantly found in primary hardrock veins, lodes, and stockworks. Streams do not act upon residual placers. As weathering continues, a residual placer slowly creeps down-slope, forming an eluvial hillside dryplacer. If running water captures an eluvial dryplacer, it becomes by definition an alluvial stream placer.

Lateritic Soils
Lateritic soils form in thin shallow sheets, from a few feet to about 20 feet thick. Since laterites are impervious, they cannot drain, something needed for their continued downward growth. Also, the overlying soil that forms above the laterite becomes infertile, as the roots of plants cannot penetrate downward into the laterites hard slag-like crust.

Misused and ambiguous terminology can be confusing for anyone serious about learning prospecting methods. As an example, suppose I was to say that, “In Australia, gold is found within a ‘cap-rock.”’ I know what I “mean” by cap-rock, but suppose I fail to define a cap-rock to your satisfaction? No problem, you grab from a shelf one of your half-dozen or so miscellaneous reference books, such as the fourth edition of Putnam’s Geology; a freshman level college textbook on physical geology.L

By looking up “cap-rock” you determine that it’s an “impermeable rock, commonly shale, that prevents oil and gas from escaping upward from the reservoir rock.” Whoa, something is wrong here! This textbook definition doesn’t fit my off-the-wall definition of a cap-rock.

Now if I were to substitute the term “hardened lateritic soil,” for “cap-rock,” you would see an illustration with the following caption, “Lateritic soil hardened to a rock-like substance, eastern Australia. The soil formed several million years ago in a more humid climate, and probably hardened in later dryer climates.”

Then by reading more about laterites within Putnam’s, or any other college-level textbook on physical geology, you learn that lateritic soils are widespread in tropical climates, and they are found within the southeastern United States and elsewhere. Laterites are residual soils that form during the hydrolysis of iron-containing soils by weathering of a well-drained, highly wet terrain, where oxidation can persist over thousands of years. During hydrolysis, soluble elements such as calcium, sodium, potassium, and sodium leach out. Even silica has been removed, and what remains are mainly iron-oxides such as goethite, and aluminum rich oxides such as bauxite. Any gold present, being nearly insoluble, concentrates within the top foot or so of “capping.”

You now have a working background on anything mentioned regarding prospecting gold-bearing lateritic soils.

Australian detectorists have become proficient in detecting for eluvial gold within the brick-red, hardened laterite soil that formed in earlier geological times when parts of Australia supported a warm and wet summer climate followed by dryer winters. This caused the water-table to fluctuate, and by intense chemical weathering nearly all of the soluble minerals and elements, except for iron and aluminum oxides (and gold), leached downward to form a thin sheet of laterite.

In Australia, these lateritic soils either outcrop onto the earth’s surface, or are exposed by earth moving equipment. Then, they are prospected by metal detectors for the insoluble gold nuggets found within the shallow exposures of laterite.

Australian detectorists usually experience some “ground effect” tuning problems while detecting these iron-rich laterite soils, which are sometimes also known as “ironstones.” However, due to the insolubility of gold, large to small nuggets are removed from the laterites by cold chisels, picks, jack-hammers, rippers, and even explosives.

Gossans
Gossans, like laterite soils, also form an iron-bearing, weathered soil horizon consisting of non-soluble minerals such as gold. However, here is where the similarity ends. While lateritic soils are shallow and form during tropical-like weather conditions, a gossan is a weathered zone above a sulphide deposit. It can be extensive, with depths of hundreds of feet. Gossans form by “secondary enrichment” above the water table by the oxidation and leaching out of sulphur as anhydrous or hydrous sulphates. Below the water table the sulphides form a zone of enrichment, which includes the majority of ore minerals.

Gossans generally indicate the form and size of underlying sulphide deposits. Visible on the surface are insoluble iron compounds, such as goethite, a common pseudomorph of limonite, that forms what is known as a cellular “box-work” crust consisting of sharply angled, thick-to-thin-walled plates and blades of insoluble iron.

Mineralogists (see McKinstry reference) who study the characteristics of the insoluble iron-rich boxwork, are able to predict with considerable accuracy the identity of the original sulphide minerals that were leached, in addition to limonite and other oxide ores within the gossan. Here, native gold and silver, plus various carbonate sulphates, and silicate minerals can be mined with the oxide zone above the water table. In some gossans, nearly insoluble free-milling gold can be found within the blades of the honeycombed surface cavities of limonite left during the solution and downward leaching of auriferous sulphide minerals.

Conclusion
Other miscellaneous soils which can also trap placer gold include “caliche” and “hard pan.” Caliche is mainly found within the desert areas of the southwestern United States. It is a sandy-silt, cemented by calcium carbonate deposited by the evaporation of ground water brought to the surface by capillary action. True hardpan, as defined in a soil mechanics book (see references), is an impervious rock-like soil horizon formed by the accumulation of cementing minerals. Both caliche and hardpan can act as shallow bedrock to trap and hold gold.

Residual, lateritic and gossan “soils,” as well as caliche and hardpan, form within shallow depths. By erosion or bulldozing, they can be found on the surface. All, under certain conditions, can carry free-milling gold of detectable size. With the continuous development of metal detectors able to handle highly mineralized ground, some of these overlooked areas may now be worth detecting. The references listed at the end of this article may be of some help to guide those of you that wish more information.

Good luck. Don’t trespass. Be sure to backfill your holes; don’t dig-up vegetation and peacefully co-exist with all of the critters, even the rattlesnakes, living in the areas you hunt.

References Bateman, Allan, 1950, Economic Mineral Deposits, Prentice-Hall Inc., Second ed., 916 pages. Larson, Edward & Burkland, Peter, 1981, Putnam’s Geology, Oxford University Press, Fourth ed., 789 pages. McKinstry, Hugh, 1948, Mining Geology, John Wiley & Sons, First ed., 680 pages. Sowers, George B. & Sowers, George F., 1961, Introductory Soil Mechanics and Foundations, Macmillan Co., Second ed., 386 pages.