Artisanal Miners of the Amazon: Part 2, The Effects of Mercury

A worker identifies a gold-rich layer and breaks it up for later washing. 

Why Use Mercury?

• Bonds with Au forming an amalgam 
• Process is quick relative to other recovery methods
• Cost is lower than other  recovery methods

The simple low cost design of sluice boxes make them easy to maintain. Slurry is introduced through the barrel at the top controlling the flow and distribution of material over the sluice box surface.

Mercury Loss

• Between 10 to 30% of mercury used in artisanal mining is lost to the environment. 
• It is estimated that 196,000 tonnes of Hg was lost in Central and South America from 1570 - 1900 (Nriagu, 1993)

The Effects Include:

Exposure by ingestion, inhalation or contact with skin can lead to: 

• damage of the digestive, nervous and immune systems 
• kidney and / or lung failure
• complications with fetus development

Artisanal miners are known for exploiting near surface gold deposits. In some cases, to increase the recovery of fine gold these miners will use mercury in their process. Mercury loss is common and can lead to hazardous effects to the local environment.  

The sluice box, a trademark tool for extracting gold from alluvial deposits.

Mercury in the Environment

• Bacteria can form organic methylmercury
• Both mercury and methylmercury enter the food chain
• methylmercury is able to migrate through cell walls

The sluice box surface. Expanded steel creates turbulence cells when water flows over, trapping heavy minerals. A layer of prospector's matting lines the bottom of the box, this layer is removed for cleaning after a 2-3 day cycle.

The chart shows the biomagnification of 1 ppm of mercury in water. Mercury is not easily removed from an organism causing it to bioaccumulate, concentrations then increase as the heavy metal is passed through the food chain.                (Environment Canada website)

There are gravity recovery methods that allow for the extraction of fine gold without the use of mercury. However these methods have higher start-up and operation costs versus traditional methods.  Government regulation and incentives towards the use of gravity recover methods is required to reduce the unnecessary use of mercury in the future.

A worker washes a gold-rich clay layer into a sump that is later pumped over a sluice box. Mercury is often introduced into the environment at this stage making management and recovery difficult. 

Reference

Nriagu, J.O., 1993, Legacy of Mercury Pollution: Nature, v. 363, p. 589.

Iron Oxide Copper-Gold Deposits: Andean Overview

Mantoverde Open Pit

Intro

This post was inspired by a SEG (Society of Economic Geologists) field trip to northern Chile. A week was spent visiting numerous IOCG type mineralization occurrences between Antofagasta and Copiapo.  

The Coastal Cordillera of northern Chile and Peru is home to the youngest known IOCG belt (Jurassic to Early Cretaceous; ca 180 to 100 Million years ago), characterized by an arrangement of deposit styles including: polymetallic veins, breccia pipes, mantos and skarns. The Andean IOCG belt is a major contributor to Chile’s copper production. Deposits range in size from 0.5 to 500 Million tonnes with grades varying from 0.4 to 10 percent copper and gold values generally below 0.5 grams per tonne (Sillitoe, 2003).

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Locations of Andean IOCG type deposits and their respective metallogenic belts. (modified from Sillitoe, 2003)

Andean type IOCG deposits generally display the following characteristics:

• Structurally Controlled: Hosted within fault or shear zone systems parallel to main orogenic zones (e.g. Atacama Fault Zone) that have undergone regional extension.

• Associated with magmatic intrusions of gabbrodiorite to diorite composition.

• Copper mineralization hosted within veins (e.g. Julia), breccia pipes (e.g. Manto Verde), mantos (e.g. Atacama Kozan) and skarns (e.g. Las Pintadas).

• Volcanic Host: Host lithology is within a suite of volcanic assemblages including basalt to andesite, tuffs and minor sedimentary successions. (e.g. La Negra Formation and the Punta del Cobre Group)

Deposit Formation

Regional extension along orogen parallel fault or shear zones allows for the uprising and emplacement of mantle derived intrusions mainly of gabbrodiorite to diorite composition. The intrusive bodies create hydrothermal convection cells allowing for mineralized fluids to precipitate copper sulphides and iron oxides along structures and permeable lithologies. Regional potassic and/or sodic-calcic alteration zones are commonly associated with large IOCG deposits.

Mineralization Style

Iron oxide mineralization commonly occurs as hematite (specularite) and/ or magnetite; with a transition of hematite to magnetite with increased depth.

Copper mineralization is characterized by chalcopyrite, bornite and chalcocite; with a transition towards increased chalcopyrite at deeper paleodepths. Secondary copper oxide mineralization such as malachite, azurite, chrysocolla and atacamite are common at or near surface.

To keep this post short, that is all I will cover for now. So with what we know about these types of IOCG deposits, how do we explore for more? Where will the next IOCG belt be discovered?

Brodie

 

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Schematic diagram of Andean IOCG mineralization styles (modified from Sillitoe, 2003).

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Breccia

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Quartz vein with bornite

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Atacamite

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Azurite

Reference 

Sillitoe, R.H., 2003, Iron oxide-copper-gold deposits: An Andean view: Mineralium Deposita, v. 38, p. 787–812.

For a great review on IOCG deposits check out:

Groves, D.I., Bierlein, F.P., Meinert, L.D., and Hitzman, M.W., 2010, Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits: Economic Geology, v. 105, pp. 641-654.

Artisanal Miners of the Amazon: Part 1

To start, I would like to share some photos taken across the Amazon Basin, primarily within the Guiana Shield. Artisanal miners exploit alluvial gold shedding off of gold-rich regions (likely eroded orogenic gold deposits) of the amazon and in some cases mine in-situ gold deposits in chemically weathered environments. Their methods are simple yet effective. Often pumping a few days worth of slurry over a sluice box, panning the trapped concentrate and repeating until the area is depleted of accessible gold. 

Fine gold in a pan concentrate. Additional work is required to separate the gold from iron-rich minerals or "black sands". 

Hard work and skill can pay off big. These workers are part of a modern day gold rush, prospecting in remote areas, pushing the boundaries of known gold occurrences.  

An artisanal miner shows off his spoils. He explains,  four miners working a single stream for a month produced the amount shown. Others are not as fortunate, junior explorers included. 

Quicksilver. Despite the health hazard, mercury is still used in the recovery of fine gold. Here excess mercury in a pan is poured into a submerged cup for later use.

The current dependence on mercury to recover gold has caused environmental concerns. If used irresponsibly mercury can contaminate soil and drainages with devastating long-term effects. Development and promotion of efficient gravity recovery methods, such as centrifugal concentrators, are important to help eliminate the risks of mercury contamination. 

A gold amalgam is produced by mixing a gold-rich concentrate with mercury. The mercury is later removed by heating in a retort. 

Governments are pushing for the replacement of mercury recovery techniques with the use of gravity recovery methods. I plan to expand on these methods in future posts.

For more information on mercury and its effects check out the Mineralogical Association of Canada Short course series Volume 34, Mercury: Sources, Measurements, Cycles and Effects.

Brodie