Gold

 

Crystal Forms and Aggregates

Octahedral, dodecahedral, and cubic crystals occur, as well as combinations of these forms, but they are uncommon and are often distorted. Dendrites, wires, nuggets, encrustations, and small flakes are the more common forms. Crystals are often stacked into elongated groups, and may form in lines or patterns, especially in herringbone formation. Spinel twinning in groups of small crystals is well-known habit. Crystals may form in hopper growths.

Specific Gravity	15.5 - 19.3
Luster	Metallic
Cleavage	None
Fracture	Hackly
Tenacity	Ductile and malleable
Other ID Marks	Excellent conductor of electricity
Complex Tests	Soluble only in aqua regia

Environment	In Quartz veins and high temperature hydrothermal deposits, as well as placer deposits.
Rock Type	Sedimentary, Metamorphic

COMMON MINERAL ASSOCIATIONS

Quartz, Pyrite, Arsenopyrite, Silver, Sphalerite, Galena, Limonite

Gold may occur as deposits called lodes, or veins, in fractured rock. It may also be dispersed within Earth's crust. Most lode deposits form when heated fluids circulate through gold-bearing rocks, picking up gold and concentrating it in new locations in the crust. Chemical differences in the fluids and the rocks, as well as physical differences in the rocks, create many different types of lode deposits.

Over millions of years, gold flakes and nuggets worn away from veins are swept into bodies of water. The heavy gold settles in stream-, lake- and riverbeds, and on the sea floor, forming placer deposits.

1. Superheated waters emerge from "smokers," spring-like vents in the seafloor. This occurs where tectonic activity forces the spreading of the oceanic crust. Metal-rich minerals, including small amounts of gold , are deposited as the heated gold-laden water mixes with the cold seawater. 

2. Gold-laden water heated by magma-molten rock-in Earth's shallow crust forms a variety of lode gold deposits. Hydrothermal-hot water-fluids rich in sulfur form gold ores in rocks of active volcanoes. The deposits of Summitville, Colorado, are an example.

3. Gold minerals form in hot rocks in and around volcanoes. Low sulfur, gold-bearing hydrothermal fluids form when hot rocks heat ground water. An example of these low-sulfur fluids are hot springs like those at Yellowstone National Park. The ores of Round Mountain, Nevada, are typical low-sulfur deposits.

4. Chemical interactions between hot fluids and sedimentary rocks form deposits of tiny, even invisible, gold particles. The Meikle Mine of Nevada contains such "invisible" gold. 

5. Fractures form in Earth's crust as mountains rise. Hydrothermal fluids flow into these spaces and form gold-bearing quartz veins. These fluids are created by hot, deeply buried metamorphic rocks. The Mother Lode and other deposits of the Sierra Nevada gold belt in California are examples.

6. Ancient gold deposits are found in greenstone belts: volcanic belts more than 2.3 billion years old. Although gold deposits continue to form in active volcanic areas, greenstone belts and their gold deposits no longer form on Earth today. Examples include parts of Canada, Zimbabwe and Australia. 

7. Placer deposits form at Earth's surface when weathering action exposes gold from other, older lode deposits. The gold is swept into, and settles in, streams, lakes, rivers or the sea floor. Many placer deposits are of recent geologic age, but some are billions of years old.

Sensor-based ore sorting can be used to significantly upgrade giving them to the concentrator. With less barren material being treated, there is a knock-on effect which produces further savings in a range of ancillary mining activities from haulage costs through to a reduced tailings footprint. All these factors contribute to a lower mining cost per ton. The primary aim is to increase mine recovery by reducing the cut-off grade and processing the extra material without compromising current mine production. This creates new mill capacity by rejecting waste material without significant capital expenditure.

High grade gold-silver ore from Hishikari mine , this specimen consists of translucent quartz which is typical in low temperature type gold vein. Two streams of the black band at the center of photo are gold-silver poor pyrite rich part where no gold-silver mineral visible. Close to these pyrite rich black bands there is clay rich pale greenish banded part. Many tiny gold-silver minerals are visible there. Upper part of photo, Some gold-silver minerals are visible sparsely at the grey banded quartz.

A high grade ore zone, in sheared pyrite-quartz rocks, with some oxidation it .

Goldschmidt classification:	Siderophile

Cyanidation of gold ore is used in ore which is carbonaceous or copper-bearing minerals.

Sulphide ores of gold use autoclave pressure oxidation it .

Gold Ore

Ores from which gold can be extracted easily, without resorting to alternative processes such as pressure leaching or other intense chemical treatment, are usually referred to as free milling ores. Free milling ore bodies, by definition, can be milled easily. You can also extract the gold easily using standard gravity and CIL/CIP processes using cyanide as lixiviant (gold extractions > 90%) .

Refractory generally means that you are getting gold extractions of less than 90%. However, refractory has also been used as a term to describe gold enclosed or locked in other minerals and uneconomic to extract. Many high grade, gold bearing ore bodies have been known for some time but have been considered too difficult or uneconomical to extract due to their properties. They have been passed around from junior to major – and back again. They require a realistic, economical process (or price) to extract the gold.