"Rock of the Month # 268, posted for October 2023" ---
NATIVE ARSENIC
Arsenic is one of some 33 elements in the periodic table that
occur as native elements, i.e.,
largely uncombined with other elements (such as oxygen or sulphur)
and occurring in their own particular crystalline form (even if that is invisible to the naked eye).
Some native elements are much better-known and more widely-distributed, such as
gold, silver and copper, or carbon in its very different guises as diamond and graphite (see Wilson in Boyle, 2024, Text Box 1 and Chapter 3C endnotes).
Some are of very restricted occurrence, such as native iron (conditions on Earth are such that
iron usually occurs as oxides, carbonates, silicates and other compounds, in
conjunction with other elements). Still others are vanishingly rare.
Native arsenic is relatively common, though the element is found more often in sulphides and sulphosalts and other compounds.
The MINLIB bibliographic database has some 3,200 records (3.5% of the total) that are flagged for arsenic,
but just 86 that mention native arsenic. The element demands attention around the world, from
China and India to
Cornwall and Nevada.
A myriad assemblage of secondary minerals of arsenic, such as oxygen-bearing arsenates and phosphates, can form in terrestrial weathering (Bowell et al., 2014).
Arsenic is commonly found at lower concentrations, not only in those sulphides and sulphosalts
which contain essential arsenic, but as a minor component of ore minerals
of other elements. Minerals that may incorporate ppm to percent levels of arsenic include oxides (cassiterite),
sulphides (pyrite, sphalerite) and native copper.
Thus arsenic occurs in the ore deposits of other elements, notably gold and silver as well as base metals.
Native arsenic has trigonal symmetry, and also has two rare orthorhombic trimorphs, arsenolamprite and pararsenolamprite.
Apparently white when freshly polished, it tarnishes rapidly to grey or black, with a submetallic to dull lustre.
The dull, dark grey surface is due to oxidation to arsenolite, naturally-occurring
As2O3.
A fresh face in reflected light is white with a creamy tint, not as bright as silver.
It is anisotropic, may show twin lamellae, and
often displays a colloform texture of concentric layers, or radiating spheroids
(Uytenbogaardt and Burke, 1971, pp.84-85).
The element arsenic has atomic number 33, and like aluminium and
gold has just one stable isotope: 75As.
The estimated average crustal abundance of arsenic is 2.5 ppm, which is roughly comparable to tin, say, and about one-tenth as abundant as copper.
The two small samples shown here are both from the Kuse mine, near the gold-mining town
of Bau, southwest of the regional capital Kuching in the westernmost part of Sarawak
(a part of Malaysia, on the northern coast of Borneo).
THE BAU DISTRICT, AND A FEW OTHER ARSENIC LOCALITIES
Wilford (1955) wrote up his surveys of what was then British Borneo, 1949-1955.
The region has a hot and humid tropical climate and
a varied
topography, which is a reflection of diverse rock types. The oldest rocks are schists
of possible Devonian age. Economic assets include gold, arsenic and antimony, as well as diamonds.
Jurassic and Cretaceous limestones at Bau are cut by a line of
Tertiary dacite and microgranodiorite intrusions. The main Au deposits occur in
limestone and immediately overlying shale near the intrusions.
Ore minerals include sphalerite and galena, native arsenic and native antimony.
Percival et al. (1989) noted that the Au deposits of Bau
have yielded 1.2 million
ounces of Au plus significant amounts of Sb and Hg.
There is an inferred association between
calc-alkaline intrusions, vein deposits, and replacement-type disseminated Au
deposits. Hydrothermal alteration and Au mineralization occur in massive
limestone and shale in the core of the Bau anticline, whose crest is cut by
several sets of near-vertical faults. There is a structural control of the Miocene
granodiorite and dacite porphyry intrusives. Silica, native arsenic,
stibnite, pyrite, realgar and local arsenopyrite were all deposited from
fluids into brecciated shale and limestone along faults and limestone-shale
contacts. The resulting jasperoid and replacement bodies account for most of the
local Au production. Some ores carry >20% As.
Mustard (1997) notes evidence of old mining activities
across a 250 km2 area around the township of Bau.
Chinese miners were active in the district as early as 1820.
Mustard cites cumulative
production, by 1997, as >2 million oz from placer Au occurrences and over 50 hard-rock
mines. The Tai Parit mine produced 700,000 oz in 1898-1921 and 1991-1997,
and modern resources of about 1 million oz were delineated in the area, mostly in
the Jugan and Pejiru deposits.
Triassic to Jurassic andesitic
volcanics are the oldest rocks that outcrop within the Bau Au field. This igneous
basement is overlain unconformably by 500 m of late Jurassic massive micritic
limestone, in turn followed by a 1,500-m-thick flysch sequence (mostly shale).
A NNE-striking belt of Miocene intrusives
(granodiorite porphyry stocks and dykes) cuts the field, and all known
deposits lie within 5 km of this. The Bau field is complex
with some seven categories of mineral deposits.
There are some Hg-As-Ba deposits. The dissolution of
limestone and karst / collapse breccia formation is important in formation of
some of the sediment-hosted deposits. Gold occurs in silica and in arsenical
pyrite (according to unpublished data, pyrite at Pejiru contains up to 18%
As, surely a record).
Hutchison (2005) reviewed the whole northern fringe of the great island of Borneo.
He notes the "non-volcanic epithermal deposits" of the Bau
district (ibid., pp.151-156).
High-level granite, granodiorite and diorite plutons provided a heat
source to remobilize metals from older continental crust. Volcanism is
lacking, and there is often an important Hg association. Western Sarawak is marked by important Miocene epizonal
magmatism, age dated at 23 to 8 Ma.
As noted by earlier workers, Hutchison explains that the Bau mining district is related to that line of small,
high-level Tertiary dacitic to granodioritic stocks and dykes.
The complex arsenical ores with native arsenic, stibnite, pyrite and native gold
appear, based on fluid inclusions, to have formed at low temperatures
(140-250°C). The mineralization may be akin to Carlin (Nevada, USA) and
Bingham (Utah, USA). The lower temperatures are appropriate to an epithermal regime where native arsenic, realgar, cinnabar and other
minerals occur.
Concerning the arsenical Au ores of the
Krokong area of Bau, a representative bulk analysis of such ore
is said to grade 12 ppm Au, 1.68 ppm Ag, a remarkable 8.09% As and 1.91% Sb.
Not surprisingly, arsenic and cyanidation
(in old workings) represent an unsolved environmental problem (ibid., p.155).
Europe has a number of localities for native arsenic, which occurs, e.g.,
in brecciated marbles in eastern
Carinthia, in Austria (God and Zemann, 2000). Lengenbach in the Valais of
Switzerland is a famous locality (Roth et al., 2014).
This site is roughly 65 km E.N.E. of Sion near the Swiss-Italian border.
As of December 2013, Lengenbach was the type
locality of a remarkable 31 mineral species.
There are more than 125 known species in this dolomite-hosted deposit, including
many arsenic sulphides and sulphosalts, as well as native arsenic.
Johan (1985) describes the Cerny Dul deposit in the Giant Mountains of the Czech Republic.
This is a different style of deposit, with calcite veins in metamorphic rocks, with a complex paragenesis of
various arsenides and native arsenic.
Hydrothermal fluids with neutral to acid pH may be able to carry significant arsenic,
which can thus occur in significant quantities in tin deposits containing
cassiterite and arsenopyrite, such as: Llallagua in Bolivia;
Panasquiera in Portugal; and Cligga Head and Geevor, Cornwall, S.W.England
(Heinrich and Eadington, 1986). In Cornwall, the Dolcoath mine in Camborne was notable for
being profitable for both copper and tin, with
arsenic as a byproduct (Embrey and Symes, 1987).
Arsenic is found in the hotter vein systems, including those with greisen
alteration, as at Cligga Head.
Canada has many documented occurrences of arsenic mineralization (Hurst, 1927),
from the Yukon to Newfoundland.
Papezik (1966, 1967) described an occurrence of native arsenic
in the Whalesback and Little Bay Cu
mines on the Springdale Peninsula, Newfoundland.
The largest mass filled a fracture in an altered
basic dyke on the 1500 level of the Little Bay mine.
The arsenic displayed a rough colloform texture and contained inclusions of
rammelsbergite (NiAs2).
The arsenic may have formed by decomposition of arsenopyrite -rich sulphides
intruded by the dyke.
ARSENIC in the ENVIRONMENT
Arsenic, like antimony, is one of a number of "indicator elements"
used in geochemical exploration, as a vector towards deposits of
gold especially, where these elements may also be concentrated.
The assay reveals the element, even in the absence of some of the colourful
secondary minerals found in the weathering zone,
such as realgar and orpiment, annabergite and erythrite.
The details of the use of arsenic as an "exploration vector",
and the toxicology of arsenic affecting human and animal health, are beyond the scope of this short(-ish) essay.
Arsenic occurs in the environment, generally in the form of sulphides, and commonly as a minor to trace component (mostly <1 wt.%
down to parts per million or below) of common sulphides such as pyrite (FeS2 - fool's gold).
Arsenian or arsenical pyrite with percent levels of arsenic
is generally associated with ore deposits, especially of gold,
but lower levels occur also in pyrite in
widespread sedimentary strata.
This can cause regional health effects, as in areas of the northern Indian subcontinent, where
wells penetrating pyritic sediments promote oxidation, allowing arsenic to enter the well water
to be ingested by the human and livestock populations on the surface.
The situation in Bangladesh and adjacent West Bengal became critical, and was intensively studied in the 1990s.
Endemic arsenic poisoning occurred via tube well waters in West Bengal, with deep wells emitting
waters with arsenic levels to 3.7 ppm, way over the WHO permissible limit of 0.05 ppm As for drinking water
(Banerjee, 1993).
Detailed findings of a survey in a region of West Bengal (Mandal et al., 1996) identified 560 villages
that were affected, with over 1 million people drinking arsenic-contaminated water and
over 200,000 having arsenic-related diseases.
Analysis of about 20,000 water samples from tube wells indicated
that 45% had arsenic content >0.05 mg/L (50 ppb, 0.05 ppm).
Williams (2001) reviewed arsenic measurements in the mine waters of 34 gold and
base-metal mining localities in seven countries, including in Sarawak, Brazil and Ecuador.
The peak values at these sites
range from 0.005 to 72 mg/l, with the potable water threshold (as noted above) exceeded in 25 cases.
Arsenic-rich materials are a generally-unwanted byproduct
of the processing of ores from a range of mineral deposit types.
As a case study of arsenic as a byproduct, and the problems which that poses,
let's consider the occurrence of arsenic at the old Giant gold mine,
which is located
near Yellowknife, the capital of the Northwest Territories of Canada.
Here gold deposits were hosted in large part along shear zones cutting metavolcanic rocks
("greenstones"), with concentrations of
quartz, carbonates and sulphides, and elevated levels of arsenic, antimony, gold, silver and other elements
(Boyle, 1961).
The Giant mine
lived up to its name, working a world-class gold deposit that produced
more than 7 million ounces (220 tonnes) of gold metal.
Jamieson (2014) studied the situation and noted that, from 1949 to 1999,
ore was roasted as pretreatment for
cyanidation, a common treatment regime
in cases where much of the gold is locked into "refractory" sulphides such as arsenopyrite.
Arsenopyrite (FeAsS) in the ore accounted for an estimated 20,000 T of
As2O3 released via stack emissions, a dramatic point source of arsenic pollution.
Over the mine
life, an estimated 237,000 T As2O3 was stored underground as the mining
proceeded.
This was the residue from working the ores from the closed Giant and Con mines.
The waste is stored in 14 large underground vaults (Braden, 2010).
Arsenical maghemite (dimorph of hematite) and hematite were also produced, and, in mine
tailings, these now leach As into the environment. Biofilms in
the old mine workings include yukonite, an hydrated Ca
Fe arsenate (Jamieson, 2014).
The mine tailings contain fine-grained sulphides such as pyrite and arsenopyrite.
The calcine residue is perhaps 10% of the tailings by weight, but contains 80% of the total As load.
Iron oxide products of the roasting, such as
nanocrystalline maghemite, contain up to 7 wt.% As (Walker et al., 2015).
ARSENIC in the ECONOMY
Arsenic has a range of traditional uses, in pesticides and wood preservatives,
and other applications
in glass manufacture, lasers, and even in medical procedures.
More recently came the realisation that
gallium arsenide (GaAs) might yield superior semiconductors.
However, the uptake seems to have been slow relative to the
standard silicon chips. If widely adopted, this would boost the market for gallium metal,
and provide a valuable use for some of the surplus arsenic!
Electron velocities are faster in GaAs than Si, and so GaAs circuits are faster (at
equal or lower power) than silicon circuits (Brodsky, 1990).
5G technology for wireless communications should require
large amounts of uncommon elements, such as Ga. China contributed 95% of primary Ga production in 2018 (354 out of 372
tonnes), the Ga coming in large part as a byproduct of treatment of bauxite (Al) and Zn ores
(Heckbert, 2019). China is also the dominant producer of arsenic at present.
I wonder if the Yellowknife "arsenic stockpile" is a possible substitute for primary As production,
turning a liability into an asset (?).
REFERENCES
Banerjee,R (1993) Death by slow poisoning. India Today 18 no.13, 141-142, July.
Bowell,RJ, Alpers,CN, Jamieson,HE, Nordstrom,DK and Majzlan,J (editors) (2014) Arsenic: Environmental Geochemistry, Mineralogy, and Microbiology. Mineral.Soc.America Reviews in Mineralogy and Geochemistry 79, xv+635pp.
Boyle,RW (1961) The Geology, Geochemistry, and Origin of the Gold Deposits of the
Yellowknife District. GSC Memoir 310, 193pp.
Boyle,RW (2024) A History of Geochemistry and Cosmochemistry. Volume 1. Prehistory to the end of the Classical Period (A.D. 476). Cambridge Scholars Press, Newcastle upon Tyne, England (Wilson,GC, Butt,CR and Garrett,RG, editors), in press.
Braden,B (2010) Fixing a Giant mess. Can.Min.J. 131 no.2, 10-13, February.
Brodsky,MH (1990) Progress in gallium arsenide semiconductors. Scientific
American 262 no.2, 68-75, February.
Embrey,PG and Symes,RF (1987) Minerals of Cornwall and Devon.
British Museum (Natural History) / Mineralogical Record Inc., 154pp.
God,R and Zemann,J (2000) Native arsenic-realgar mineralization in marbles
from Saualpe, Carinthia, Austria. Mineralogy and Petrology 70, 37-53.
Heckbert,D (2019) 5G technology should boost demand for gallium, cobalt.
Northern Miner 105 no.26, 8, 23 December.
Heinrich,CA and Eadington,PJ (1986) Thermodynamic predictions of the
hydrothermal chemistry of arsenic, and their significance for the paragenetic
sequence of some cassiterite- arsenopyrite-base metal sulfide deposits.
Econ.Geol. 81, 511-529.
Hurst,ME (1927) Arsenic-bearing Deposits in Canada. GSC Econ.Geol.Ser. 4,
181pp.
Hutchison,CS (2005) Geology of North-West Borneo: Sarawak, Brunei and Sabah.
Elsevier, 421pp.
Jamieson,HE (2014) The legacy of arsenic contamination from mining and
processing refractory gold ore at Giant mine, Yellowknife, Northwest
Territories, Canada. In `Arsenic: Environmental Geochemistry, Mineralogy, and
Microbiology' (Bowell,RJ, Alpers,CN, Jamieson,HE, Nordstrom,DK and Majzlan,J,
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xv+635pp., 533-551.
Johan,Z (1985) The Cerny Dul deposit (Czechoslovakia): an example of Ni-,
Fe-, Ag-, Cu-arsenide mineralization with extremely high activity of arsenic:
new data on paxite, novakite and kutinaite. TMPM 34, 167-182.
Mandal,BK plus 12 (1996) Arsenic in groundwater in seven districts of West Bengal, India - the
biggest arsenic calamity in the world. Current Science 70 no.11, 976-986, 10 June.
Mustard,H (1997) The Bau gold district - east Malaysia. In `World Gold '97',
Australasian Institute of Mining and Metallurgy Publ. 2/97, 294pp., 67-77.
Papezik,VS (1966) Native arsenic in Newfoundland. Can.Mineral. 8, 670-671.
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Percival,TJ, Radtke,AS, Bagby,WC, Gibson,PC and Noble,DC (1989) Bau, East
Malaysia: arsenic-rich sedimentary-rock hosted gold deposits spatially and
genetically associated with epizonal magmatism. GSA Abs.w.Progs. 21 no.6,
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Roth,P, Raber,T, Drechsler,E and Cannon,R (2014) The Lengenbach quarry,
Binn valley, Switzerland. Mineralogical Record 45, 157-196.
Uytenbogaardt,W and Burke,EAJ (1971) Tables for Microscopic Identification of Ore Minerals.
Elsevier, 2nd revised edition, 430pp.
Walker,SR, Jamieson,HE, Lanzirotti,A, Hall,GEM and Peterson,RC (2015) The
effect of ore roasting on arsenic oxidation state and solid phase speciation
in gold mine tailings. Geochemistry: Exploration, Environment, Analysis 15,
273-291.
Wilford,GE (1955) The Geology and Mineral Resources of the Kuching-Lundu
area, West Sarawak, including the Bau Mining District. Geological Survey
Department, British Territories in Borneo, Memoir 3, 254pp. plus 2 maps.
Williams,M (2001) Arsenic in mine waters: an international study.
Environmental Geology 40, 267-278.
![arsenic from Sarawak [280 kb]](http://www.turnstone.ca/As-Bau-3.jpg)
![arsenic from Sarawak [284 kb]](http://www.turnstone.ca/As-Bau-4.jpg)