Gold that ends up as a recognised Good Delivery bar begins its journey deep underground or in alluvial deposits. The path from raw ore to a refined, tradeable bar involves several distinct stages: mining, ore processing, smelting, and final refining. Each step is designed to concentrate the gold and remove impurities until the metal meets the stringent standards required for international bullion markets.
Mining Methods
Gold is extracted using two principal methods, chosen according to the nature of the deposit.
- Hard-rock (lode) mining – When gold is locked within solid rock, miners dig tunnels or create open pits to access the ore. The rock is blasted, drilled, and hauled to the surface for processing. This method accounts for the majority of newly mined gold.
- Placer mining – In riverbeds and ancient streambeds, gold particles have been freed by weathering and concentrated by water. Miners use pans, sluices, or large dredges to separate the dense gold from lighter sand and gravel. Small-scale artisanal operations often use this method, as do some industrial-scale dredging projects.
From Ore to Concentrate
Once ore reaches the surface, the gold must be separated from the surrounding rock. The process varies depending on the ore type, but common steps include:
- Crushing and grinding – The ore is broken down into a fine powder to liberate gold particles from the mineral matrix.
- Gravity concentration – Dense gold particles are separated using centrifugal force or shaking tables. This method works well for coarse gold and is often used early in the process.
- Flotation – Chemicals are added to a slurry of ground ore, causing gold-bearing minerals to attach to air bubbles and rise to the surface as a froth, which is skimmed off.
- Leaching (cyanidation) – For fine gold that is not easily recovered by gravity or flotation, a dilute cyanide solution is percolated through the ore. The cyanide dissolves the gold, forming a gold-cyanide complex in solution. The gold is later recovered from the solution, typically using activated carbon or zinc precipitation.
These processes produce a concentrate or a gold-bearing carbon product that still contains significant impurities.
Smelting and Dore Production
The concentrate is then smelted at high temperatures in a furnace. Fluxes are added to bind with impurities and form a slag that floats on top of the molten metal. The heavier gold and silver mixture sinks to the bottom and is poured into moulds to solidify. This intermediate product is called dore bullion and typically contains 60–90% gold, with the remainder being silver and other base metals.
Refining to Good Delivery Standard
Dore bars are sent to specialised refineries where they undergo further purification to meet the minimum fineness of 99.5% gold required for Good Delivery bars. Two main refining techniques are used:
- Miller process – Chlorine gas is bubbled through molten dore. The chlorine reacts with silver and base metals to form chlorides, which rise to the surface as a salt layer and are removed. This process can achieve a purity of about 99.6%.
- Wohlwill electrolytic process – For higher purity (99.99% or more), the Miller-process gold is cast into anodes and placed in an electrolytic cell. An electric current passes through a gold-chloride solution, causing pure gold to deposit on a cathode while impurities remain in solution or settle as sludge. This method is more expensive but can produce gold of exceptional purity.
After refining, the molten gold is cast into bars weighing approximately 400 troy ounces (about 12.4 kilograms). Each bar is stamped with a serial number, the refiner’s hallmark, the fineness, and the year of manufacture. Bars that meet the specifications set by the London Bullion Market Association (LBMA) are known as Good Delivery bars and are accepted for settlement in the global wholesale gold market.