Tropical Gold Geochemistry

7/26/2019 Tropical Gold Geochemistry

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Gold Exploration in Tropical Landscapes

Part 5: Dispersion of Gold in Deposits Exposed to Weathering

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Tropical Gold Geochemistry... | Gold Fineness ... | Supergene Enrichment ... | Oxidized Ore ... |Review #5 ...

Tropical Gold GeochemistrySession Headings: Historical Research in Tropical Gold Geochemistry

You will cover the following points in Part 5: Dispersion of

Gold in Deposits Exposed to Weathering.

historical research in tropical gold

geochemistry

the Bre-X fraud

the future of tropical gold

geochemistry

gold dispersion factors

gold dissolution

gold ligands in the regolith

the amount of sulphides

present in the gold

mineralized system

water table fluctuation

chloride ion ligands in arid

regions

organic ligands in surface

soils

chelating agents in plant

roots

mechanical dispersion

gold fineness

gold grains in lateritized gold systems

gold grain morphology

secondary gold precipitation

the mushroom effect of gold in the

regolith profile

supergene gold enrichment in laterite

regolith

sigmoidal gold curve along the regolith

profile
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lateritized gossans

oxidized ore

oxidation of epithermal gold deposits

Historical Research in Tropical Gold Geochemistry

(See Summaryfor main points)

Considering that laterite regolith covers one third of the earth's land surface, there

is a limited amount of research published on gold dispersion in the regolith for

deeply weathered, primary gold deposits. The general lack of publicly available

data on tropical gold geochemistry concerning large, economic gold deposits is due

to exploration company confidentiality.

There are a several early key papers on gold dispersion, attempting to explain gold

concentration zones that occur within the laterite regolith of primary gold depositsexposed to tropical climates (Penrose (1894); Emmons (1917)). Most of the studies

published on tropical gold geochemistry are from the 1960s to the mid-1990s,

reaching a height from 1990 to 1997, with projects, short courses and workshops

being held globally, including the following.

CRCLEME conference (1998): Regolith '98: New Approaches to an Old

Continent, Kalgoorlie, Australia, Eds. Britt and Bettenay, 75 pp.

MRDU Short Course #21 (1997): Exploration Geochemistry of Tropical

Environments, Vancouver, BC, Canada

17th International Geochemical Exploration Symposium (1995): Exploringthe Tropics, Eds. Kaylene and Camuti, Townsville, Australia, extended

abstracts, 379 pp.

PDAC Short Course (1994): Prospecting in Tropical and Arid Terrains, Ed.

Lynda Bloom, 468 pp.

Third International Symposium on the Geochemistry of the Earth Surface

and Mineral Formation (1993): Chemical Geology Special Issue. Vol. 107

CSIRO/AMIRA Regolith Geochemistry Projects (1987- 1993): Exploration

for concealed gold deposits, Yilgarn Block, Western Australia

Smith et al (1992): Laterite Geochemistry for Detecting ConcealedMineral Deposits, Yilgarn Craton, Western Australia. Summary Report for

CSIRO-AMIRA Project P240 covering period 1987 to 1991. 170 pp.

EUROLAT (European Network on Tropical Laterites and Global

Environment) 1991: Supergene ore deposits and mineral formation. 5th

international meeting. Berlin, Germany; EUROLAT 1997: Weathering

Processes: Mineral deposits and soil formation in tropical environments.

9th international meeting. Strasbourg, Germany

Second International Symposium on the Geochemistry of the Earth
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Surface and Mineral Formation (1990): Chemical Geology Special Issue.

Vol. 84

As well, a number of excellent books were published at this same time on tropical

weathering, including the following.

Atlas of Micromorphology of Mineral Alteration and Weathering by J. E.

Delvigne (1998), Canadian Mineralogist Special Publication 3, 516 pp

Regolith, Soils and Landforms by Ollier and Pain (1997), John Wiley &Sons, 316 pp

Developments in Earth Surface Processes 2 (1993) Weathering, Soils &

Paleosols, edited by I.P. Martini and W. Chesworthin particular, the

following three chapters:

Ch 15: Diversity and Terminology of Laterite Profiles by Y.

Tardy, p. 379401

Ch 16: Geochemistry and Evolution of Lateritic

Landscapes by Y. Tardy and C. Roquin, p. 407437

Ch 17: Metallogeny of Weathering, an Introduction byD.B. Nahon, B. Boulang and F. Colin, p. 445467

Handbook of Exploration Geochemistry (1992), Volume 4: Regolith

Exploration Geochemistry in Tropical and Subtropical Terrains, by C.R.M.

Butt and H. Zeegers, Elsevier. 607 pp

Although valuable locally, conclusions from Western Australian research on gold

dispersion are drawn from studies on regolith environments that have undergone

a different climatic history than the rest of the tropical regions; making the

Australian case studies perhaps less comparable to laterized gold systems in

tropical rainforests, such as in the Guiana and Amazonia Shield in South Americaand the Birimian Shield and Congo Craton of West and Central Africa.

Bre-X Fraud


In 1993, a mineral exploration company based in Calgary, called Bre-X, began

promoting their Busang gold project in Borneo, Indonesia. In July 1995 theNorthern Miner ran an article that Bre-X had proven reserves of 2.4 million ounces

(Moz) of gold in the ground at Busang. Shortly thereafter, a Kilborn engineering

report stated that Busang contained 70 Moz of gold. The vice president of

exploration for Bre-X, geologist John Felderhof, was heard boasting at PDAC in

1996 that they had the world's largest gold deposit with well over 100 Moz of gold

(Goold and Willis (1997)). By May 1996 the stock price for Bre-X had risen to an

incredible $285 (Canadian dollars). In February of 1997, Freeport-McMoRan, one

of the world's largest producers of copper and gold and holding company of the

giant Grasberg Cu-Au porphyry mine, also in Indonesia, outbid other suitors,
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including Placer Dome and Barrick, to do a deal with Bre-X to acquire Busang. Prior

to Freeport's due diligence evaluation of the Busang site, the chief geologist for

Bre-X, Michael de Guzman, fell from a helicopter. A body was recovered in the

jungle over time, but it was mostly eaten by wild pigs and not recognizable.

Freeport drilled twin holes and collected duplicate samples. Investigation of the

gold grains in the original samples showed large, rounded and abraded grains with

silver rims that were found only in the coarse reject samples assayed by Bre-X,

whereas the new drill core samples contained little gold and it was fine-grained.Freeport knew they were looking at sample tampering leading to false assay

results at Busang.

Shortly thereafter, Strathcona Mineral Services presented an

independent report documenting the Busang fraud: the drill core

samples had been carefully and systematically salted with alluvial

gold grains to produce the amazing gold assay results that provided

the basis for the resource calculations. The Bre-X stock price fell

rapidly with this news. At its peak, Bre-X was worth six billion dollars.

By the end of March, 1997, in the stock market crash that followedthe news of the fraud, investors lost an astounding 3 billion dollars.

Following the Bre-X scandal of 1997, confidence in gold exploration at tropical

latitudes dropped around the globe; exploration funds dried up and research on

gold behaviour in tropically weathered gold deposits lost its glitter. It would take

close to a decade to bounce back again.

To protect the Canadian investor from future similar fraud, the Canadian Institute

of Mining (CIM) has created the National Instrument (NI) 43-101, which outlines

the standards required for disclosure (such as Company news releases andannouncements of resource calculations) of mineral projects. The instrument also

requires that a qualified person (QP) sign off on the information. The QP should be

a reputable professional geoscientist or engineer who has knowledge of the

mineral property concerned and who has a minimum of 5 years' experience in the

mineral commodity and is qualified to make the statements in the report.

With the resurgence in tropical gold exploration, driven by high gold price and a

multitude of recent primary gold discoveries in the tropics (as noted in Primary

Gold Discoveries in the Tropics: Table 1), the exploration geologist today is facing

a generational knowledge gapwith regard to the tropical geochemistry of gold.

Most of what is in the literature today dates to at least twenty years ago and there

has been a lack of common terminology amongst these researchers. As well, a

number of unnecessary new terms and categories have been proposed.

Tropical Gold Geochemistry: Looking Ahead

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The revival of tropical gold geochemistry should start with common ground.

Figures of laterite regolith profiles used in gold dispersion

studies of lateritized primary gold deposits should include,

whenever possible, the entire regolith, from the weathering

front at depth to the surface soils, and contain the following.

clearly differentiate each of the individual regolith

horizons present, including the overlying A and B soil

horizons

position of water table (if possible, determine position during wet and dry

season)

position of weathering front (oxidation front) at depth

location of test samples (pore water, soil or regolith) plotted along the

regolith profile

pH conditions (if possible)

use scale bars to indicate thicknesses of zones

Sometimes the researcher has to be resourceful and creative to be able to sample

from as many of the regolith horizons as possible in one project area. This could be

achieved through pitting, sampling roadcuts, open pit bench exposures and mine

adits, and drill core to be able to thoroughly sample along a regolith that may

reach one hundred meters or more in thickness. Drill core data can be useful for

determining depth of oxidation (weathering front), but for the upper horizons, drill

core is often unreliable, as core loss is common in the clay rich horizons of the

laterite regolith.

Gold Dispersion Factors


As is the case in nature, each gold deposit displays a unique set of geochemical

characteristics, both at surface in the soil geochemistry, as well as laterally and

vertically through the regolith. The variety in parent rock material, gold

mineralization style, paleoclimatic history, and regolith landform regimes (residual,

erosional, depositional) of the gold deposit will all differ from project to project.

What is helpful for the gold exploration geologist is that with knowledge of a

certain set of gold dispersion factors for their gold project site, the chemical

processes responsible for gold dispersion can be identified and gold enrichment

and depletion zoneswithin the regolith can often be predicted. This is critical in

geochemical sampling in field surveys (soil sampling, auger, pit and trench) to

know which regolith horizon to sample. Gold dispersion factors include:

gold dissolution,

gold ligands in the regolith,
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the amount of sulphides present in the gold mineralized system,

water table fluctuation,

chloride ion ligands in arid regions,

organic ligands in surface soils,

chelating agents in plant roots, and

mechanical dispersion.

Gold dissolution


Gold dissolves? Yes it can. Gold dissolves by forming complexes with certain

ligands that can temporarily bring a small part of the gold particle into solution,

before dropping the gold back into solid state. Gold-ligand complexes are weak at

first in the lower saprolite, then become increasingly stable higher up in the

saturated part of the regolith, in more oxidizing and acidic waters, and are able to

disperse the newly formed gold particles a little further away from the source.Gold-ligand complexes are also active in the upper A and B soil horizons.

Gold ligands in the regolith


A number of gold complexes, or ligands, are proposed to explain gold dissolution

and mobility in the lateritic weathering crust of a gold mineralized system hosted

in the bedrock. The ligands most likely responsible for gold complexing are:

hydroxide (OH-),

thiosulphate (S2O22-),

cyanide (CN-),

chloride (Cl-), and

fulvic acid (an organic acid).

The presence, strength and actions of these ligands depend on the climatic zone of

the system, organic component, the Eh-pH conditions and water table position and

fluctuation within the regolith. Any given gold deposit exposed in tropical climatic

conditions will have several of these ligands at work at once, in different parts of

the regolith profile.

At the very base of the weathering front, where pH conditions are near neutral,

porosity is lowest and sulphides have not yet begun to oxidize, hydroxyl

complexes are the first to act on exposed gold grains (Bowell et al. (1993); Colin et

al. (1993); Porto and Hale (1995)), forming weak complexes with gold that are

unstable.
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In the lower saprolite (LSAP) horizon, oxidation of the pyrite, arsenopyrite, or

galena and sphalerite begins. The resulting chemical reactions release H+ ions and

sulphates, creating increasingly acidic conditions (lowering pH) and producing

thiosulfate ligands, which begin to bond with gold. At first, hydroxyl complexes

may help to strengthen the thiosulfate Au complex, but pH conditions at the base

of the oxidation front are not acidic enough to retain gold in solution and it is

quickly re-precipitated. At depth in the regolith, the re-precipitated, fine-grained

secondary gold occurs very near to and/or within the primary gold system.

Amount of sulphides in gold system


When exposed to the weathering process, a sulphidic ore body tends to develop a

zonal arrangement of different mineral associations which reflect various degrees

of oxidation (Sato (1960)). The presence of sulphides in the system correlates to an

increase in production of thiosulfate complexes, which in turn dissolve and

mobilize gold faster, dispersing gold further away from the primary source.Therefore, increased sulphides equal higher gold mobility in the regolith.

Oxidation of sulphides, or ferrolysis, continues to create acidic reactions, which

helps to accelerate hydrolysis (breakdown of silicates into clays), while thiosulfate

ligands continue to reduce primary gold particles, moving the gold into solution,

and keeping it there a little longer so that groundwaters may disperse the gold

laterally a few centimetres before it re-precipitates.

In the upper saprolite (USAP) or pallid zone, still within the saturated zone and

directly below the water table, conditions are extremely acidic and oxygenated.

Here, hydrolysis has broken down the silicates into clays, porosity is highest,

ferrolysis is complete and most of the gold is now mobilized and in solution as

gold-thiosulfate complexes. The increase in natural acidity from the sulphides

being oxidized will also ultimately have an effect on the overall thickness of the

regolith forming over the gold deposit: proximal to the mineralized zone, there is

likely to be a thicker regolithdeveloped than further away from the deposit.

Water table fluctuations


The mottled zone (MZ) is positioned above the water table, along with the laterite

and overlying upper soil horizons. The lower mottled zone is a zone of extreme

leaching. Here, acidic rainwater infiltrates the regolith and enters the groundwater

system. With daily and seasonal water table fluctuations, gold complexes in the

upper saprolite are leached, and fine secondary gold particles are dispersed. This

chemical dispersal will only go as far as these acidic conditions exist, which is in

and around the primary gold mineralized system.
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The oxidizing environment at the water table results in a process called

ferruginization, where aqueous iron changes into solid iron oxides and hydroxides

(hematite and goethite) in the upper mottled zone. These mottles harden

(indurated) into concretions of goethite and kaolinite. Gold co-precipitates in the

mottles with the ferruginization process. As the regolith profile lowers over time,

upper mottles become incorporated into the base of the laterite horizon.

Chloride ions as ligands


In arid areas, such as the Yilgarn Block in Western Australia, high concentrations of

chloride ion exist in saline groundwater due to low rainfall and strong evaporation.

Combined with low pH conditions near the top of the water table, chloride ions

form complexes with gold and silver. In arid conditions, the chloride ion is a very

likely candidate for a gold ligand (Mann (1984)).

Organic ligands in surface soils


Contrary to commonly held views, organic matter contents in tropical soils are not

very different from those in the temperate region. The main reason for this is the

absence of a direct relationship between a dark brown colour and organic carbon

content. Many red and yellow soils in the tropical rainforest climate, Af, and

tropical monsoon climate, Am, actually have higher organic carbon content than

black clay-rich soils found in the drier end of tropical forests; the savanna, Aw

(Nahon (1991)).

Tropical forests produce about five times as much biomass and soil organic matter

per year as comparable temperate forests (Nahon (1991)). But decomposition

rates in the tropical soils are much higher than for temperate regions, because

temperatures are so high in the tropics. Thus equilibrium exists between formation

of biomass and decomposition of organic matter that is similar to that found in

temperate forests. For tropical rainforest (Af) and monsoon climates (Am) with

thick vegetation, where decomposition is rapid and associated organic acid activity

is high, organic ligands dominate in the upper soil horizons.

In the upper soil horizons, fulvic acid(FA) is the dominant ligand. FA forms colloidal

particles that are highly mobile. Colloids have exceptionally high surface area,

which means a lot of binding sites for chemical reactions. FA colloids bind easily

with gold in acidic (pH=2 to 6) conditions (Bowell et al. (1993)). The binding

mechanism of the FA colloid involves an initial formation of a gold complex,

possibly another hydroxyl complex. This intermediate complex is then slowly

reduced by fulvic acid to a gold-fulvate colloid.

Humic acidsare often not considered in the dissolution of gold because humic acid
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is only soluble in bases, while most tropical soils tend to be acidic.

Cyanideis another organic ligand associated with near-surface dispersion of gold

particles. Cyanide is a product of the breakdown of organisms, a process that

occurs in the decomposition phases that yield hydrogen cyanide at the base of the

O and in the A soil horizon (Bowell et al. (1993)). Gold is readily dissolved in the

presence of thiocyanate ion, especially in acid solutions. The bacteria Thiobacillus

ferroxidansobtains its energy by oxidation of thiocyanate to sulphate, carbonate

and ammonia. Many bacteria oxidize thiosulfateand are present in most surficialenvironments. Gold thiosulfate is not appreciably absorbed by plants.

Chelating agents in plant roots


In the A soil horizon, some plant roots exude acids to lower soil pH. These acids act

as chelating agents to dissolve metal ions into solution for their uptake as

nutrients for the plant, which takes place in the B horizon beneath it. Chelatingagents can be organic acids such as citric acid, or amino acids or hydroxamate

siderophores (produced by symbiotic micro-organisms). The chelating agent binds

strongly to a metal ion and the resulting chelate is drawn to the plant roots, where

the metal ions are absorbed from soil solution into the root surface, in some cases

aided by fungi living on the roots.

Chelating agents produced by plant roots are likely intended to target and release

a specific group of plant nutrient ions from the soil and transmit them in

complexed form to the root. In the case of essential nutrients, such as Zn and Fe, a

thin zone of depletion develops around the roots, which promotes dissolution and

diffusion of these ions towards the root. Some concentrations become

oversaturated and mineral precipitation can occur, such as calcite, hydrous oxides

of Fe and Mn, amorphous silica and gypsum.

Many non-nutrient elements including Au and Ag can also be complexed by the

same organic chelating agents, and are thus mobilized in the regolith by the same

process (Gilkes (1999)). Those ions (such as Al, As, Au and Ag) are dissolved by the

same chelating agents, yet excluded from plant uptake by ion selective root

membranes, which only accept those metals that are essential to plant nutrients.

Those metals that are transported but not accepted gather in an accumulation

zone around the rootlets in the B Horizon.

In the upper soil horizons, gold complexes are formed in the A soil horizon and

leached by percolating meteoric waters to accumulate in the lower B soil horizon.

The gold is released; either dissociated or desorbed from the organic complexes in

the reducing environments characteristic of the lower B soil horizon. This makes

the B soil horizon the ideal sampling medium in soil surveys.
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Figure 3, at right, shows gold

dispersion of weathered primary

gold system in the laterite

regolith of Af climate. Meteoric

waters infiltrate and percolate

the upper soil horizons of the

laterite regolith, to recharge the

groundwater table with oxygen-rich and slightly acidicwaters. Gold distribution in the regolith is the result of

ligands, chelating agents and physical spreading and

settling of gold particles. This effect of gold dispersion

becomes more pronounced in mineralized systems that

are rich in sulphides, as oxidation of sulphides aids the

release of thiosulfate complexes. Gold systems with low

sulphide content will have a less pronounced dispersal of

gold within the regolith.

Mechanical dispersion


Wide dispersion halos of gold at surface are first and foremost the result of the

deep, chemical, lateritic weathering processes that take place in the saprolite,

mottled zone, laterite, and soils.

Exposure of lateritized gold deposits to nature's elements of wind, water andgravity results in the further lateral and vertical dispersion of gold particles at the

Earth's surface. For example, with the heavy rains characteristic of tropical

latitudes, gold grains may be dispersed across the surface by sheet wash, collecting

in pockets and potentially forming placer deposits. Resistant gold in quartz vein

fragments settle through the upper soil horizons below into the laterite and enter

the stone line.

An example of the mechanical dispersion of gold is the

Posse deposit in Central Brazil (Porto and Hale (1995)).

The climate is tropical savanna Aw, of the cerrado type,

with annual precipitation of 1,800 mm, concentrated in

the months of October to April. The laterite regolith is

about 40 m thick, containing a mottled zone and an iron

crust at the top (as is expected in Aw). The stone line is

found high up in the laterite profile and contains residual

gold-in-quartz vein fragments.
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Tropical Gold Geochemistry - Figure 1
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Figure 1: The story of Bre-X should never be forgotten as its effects are still felt today. Investors remain

wary of gold projects in Indonesia and elsewhere in tropical jungles. Young geologists entering the field

today are not familiar with tropical gold geochemistry due to a lack of research funding following this

deceit (Bre-X book: Goold and Willis (1997)).



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Figure 2: Idealized regolith profile indicating (hypothetical) location of samples collected; note the

difference in regolith thickness over the quartzite and the shale and that the weathering front has a

different contact depending on rock type: straight over quartzite and wavy over the shale (source: D.

Voormeij)



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Figure 3: Gold dispersion of weathered primary gold system in the laterite regolith of Afclimate

(source: D. Voormeij)



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Figure 4: In this cross section of the Posse gold deposit in the tropical savanna "cerrado" of Brazil, theregolith profile on the right shows the stone line is a semicontinuous horizon 0.5 to 1 m thick, above the

ferruginous zone. It is comprised of ferruginized quartz fragments, scattered pisoliths and rare iron

crust fragments, all set in a goethitic-kaolinitic clay matrix (Porto and Hale (1995)).



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Figure 5: Schematic representation of lateritic lowering over time, resulting in a regolith profile that

contains lateritized quartz vein fragments, gold particles, pisoliths, and duricrust fragments (Porto and

Hale (1995))

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Table 1: Nutrient and non-nutrient metals for plants

Biologically essential for plants Non-essential for plants

Si, Mn, Fe, Co, Ni, Cu, Zn Al, As, Ag, Au, Pb, U, Sb

Table 1: Nutrient and non-nutrient metals for plants (Gilkes (1999))

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Tropical Gold Geochemistry