# Classifications of gold-bearing materials 1



## alshangiti (15 أبريل 2010)

According to their mineralogical and historical characteristics the gold- bearing material can be classified as: 
· Placers. 
· Free milling ores. 
· Oxidized ores. 
· Silver-rich ores. 
· Iron sulphides. 
· Arsenic sulphides. 
· Copper sulphides. 
· Antimony sulphides. 
· Tellurides. 
· Carbonaceous. 
Each of these classes of gold bearing materials has especial mineralogical characteristics which affect their processing and are considered in the next sections.
Gold Step-by-Step
The amount of gold present in placer ores is usually low compared with the associated primary hard rock deposit from which they were formed. However, due the easy of operation and low costs, placers are often commercially significant and may be the forerunner to further underground mining. The capital and operating cost of placer operations can be very low, allowing economic mining of ores containing as little as 0.2 gr/t Au. Nevertheless, the contribution of placer gold (excluding palaleo placers) to annual gold production is small. 
In several notable cases the proportion of gold present in placers exceeds that in parent deposit. These deposits are called giant placers and have many millions of ounces of gold. These deposits were particularly significant in the 19th century gold rushes.
The gold mineralization in placer differs from all the other classes because the ore is in a particulate or loosely consolidated form and the gold has been liberated to a large extent by natural processes. Consequently, the savings in grinding costs compared to other types allows very low grade ores to be treated economically.
Gold grains of several centimeters in diameter occasionally occur, although sizes of below 50-100 µm are more normal. There is usually an inverse relationship between the gold particle size and the distance from the parent deposit. In the case of Snake River (Montana, USA), very fine gold has been mined up to 400 km from the source, following periods of flooding. 
The evaluation of ore grade in placer is difficult because of the low grade and unusually coarse gold grain size. Extremely large samples of several hundreds of tonnes must be treated in a mineral concentration sampling plant for the evaluation of a placer ore grade and for flowsheet design. Commonly, this is a gravity concentration plant comprised of spirals, jigs and centrifugal concentrators. Estimates of gold grade by this method may differ greatly from the eventual average gold concentration recovered from operations with ratios of recovered to expect gold varying from 32 to 149% in one study.
The fineness of gold in young placers depends on the original source and varies from 600 to 900. Placer gold grains have been found to have an outer rim which has a higher fineness. This has been attributed to silver dissolution, as silver sulphate or silver carbonate, and is supported by evidence that fineness increases with distance downstream. The lower surface silver ******* gives placer gold a deeper/orange appearance than gold in hard rock ores.
The degree of gold liberation and the surface chemical properties of placer gold are important in the effectiveness of gravity concentration and amalgamation. As most gangue minerals are lighter than gold, unliberated gold grains are recovered less efficiently by gravity concentration. Detailed surface chemical data on gold grains in placer deposits are not abundant, however, it is known that sulphur and hydrocarbon adsorption can occur and the presence of impurities in the gold significantly affects amalgamation.
Free Milling Gold Ores


Free milling ores are defined as those from which Cyanidation can extract approximately 95% of the gold when the ore is ground to size 80% < 75 µm, as c ommonly applied in industrial practice, without incurring prohibitively high reagent consumptions. Frequently some of the gold is recovered by gravity concentration and amalgamation and gangue minerals composition does not significantly affect the processing requirements. The two main classes of free milling ores are palaeo-placers and quartz vein gold ores. Some epithermal deposits may be free milling but more commonly contain significant concentrations of sulphides and are considered in other classes.
Palaeo-placers are literally fossilized placers, the most famous being the Witwatersrand lake bed reefs in South Africa. Other includes Jacobina (Brazil), Blind River Elliot Lake (Canada) and Tarkwa (Ghana). This kind of deposits consists of lithified (the formation of massive rock from loose sediment) conglomerates which contain small rounded pebbles of quartz in a matrix of pyrite, fine quartz, micaceous material and small quantities of heavy resistant materials such as magnetite, uraninite, platinum group metals, titanium minerals and gold.
From mineral processing point of view palaeo-placers differ from young alluvial placers as the gold is unliberated and the ore is consolidated. Crushing and grinding is required to liberate the gold to an extent which allows efficient gold extraction. Palaeo-placer gold deposits have been mined at depths of up to 3 km and therefore both mining and mineral processing costs are generally more than an order of magnitude greater than those for young placer deposits.
Various non-placer gold ore types can be classified as free milling. These are usually formed as a result of deposition from hydrothermal solutions. Epithermal deposits may fall in this category, but quite often have some refractory components. Quartz-gold veins or lode comprise a variety of deposits which are essentially hydrothermal veins of quartz and gold that either replace wall rock or fill open spaces along fracture ones. Most are pre-Cambrian or tertiary in age and can occur to depth in excess of 1 km. the main categories are as follows:
· · Auriferous vein, lodes, sheeted zones and saddle reefs in faulted or folded sedimentary rocks. 
· · Gold/silver veins, lodes or stock works and irregular silicified bodies in fractures, faults, shear, breccia or sheeted zones in volcanic rocks. 
· · Gold/ silver occurrence as above, though in a complex geological environment comprising sedimentary, volcanic and various igneous intrusive and granitized rocks
Oxide ore 
· An oxidized ore is defined as one in which the ore material has been oxidized or weathered, possibly in a zone that is atypical of the primary sulphide deposit, and for which some especial processing may be required. 
· Oxidation and other hydrothermal alteration process lead to the breakdown of rock structure, resulting in increase permeability. This often allows high leaching extractions to be achieved by heap leaching of run-of-mine ore, even though the ore particle size is very coarse.
· A detrimental feature of rock oxidation and alteration is the formation of considerable amounts of hydrated, amorphous and poorly crystalline silica, sulphate, oxide and hydroxide gangue phases. These phases have relatively high solubilities in comminution and cyanide leaching, and may act as strong cyanicides, due to the generation of extremely large and fresh surface areas with high sorption potential.
· Gold usually occurs either liberated or in the alteration products of pyrite and other sulphidic minerals. The most common of these are the iron oxides, such as hematite, magnetite, goethite and limonite, although gold can be associated with manganese oxides/hydroxides. Generally the degree of gold liberation is increased by oxidation; however, in some cases protective coatings of secondary and hydrated oxides on gold are commonly encountered.
· Oxidized ores differ from primary ores as a large proportion of fines are often generated on grinding, or during heap leaching and minerals such as clays are more abundant. The presence of clays, such as pyrophyllite, talc, kaolinite and montmorillonite can have important process implications, for example: decreased heap leaching pad permeability, increased slurry viscosities in processing and hence increased energy required for slurry mixing, blinding of activated carbon in CIL or CIP processes.
· Carbonate minerals such as calcite, dolomite and siderite are more common in oxidized ores and affect pH control, especially in oxidative pretreatment processes. In the oxidized zone of vein deposits, native gold generally has a lower silver *******, due to the great solubility of silver. The distribution of gold in supergene enriched and lateritized deposits is shown in Fig. 2.3. Gold is present as unaltered liberated or partially liberated grains and as redistributed secondary gold at depth, with some material mobilization. A zone of supergene enrichment may exist near the primary ore body and below a depleted region. 
· The deposits of Yilgarn region (Australia) have been subjected to prolonged and extensive oxidation to depths of 50-100 m. these ores respond well to heap leaching or agitated cyanide leaching and overly primary sulphidic deposits. Gold occurs as residual primary gold and redeposited supergene mineralization in iron oxide/silicate gangue. This secondary gold occurs as fine grains or with sponge texture, has low silver ******* and is closely associated with iron oxides. The proposed deposition mechanism is by dissolution of gold in saline groundwater and redeposition under reducing conditions. 
· Oxidized ores may also contain various oxide copper minerals, many of which dissolve in cyanide and may interfere with gold leaching and recovery processes. 


· 


Silver rich ore 
Although gold is almost always associated with silver, when the silver grade is high (> 10gr/t) and/or is present as electrum, the processing may need to be modified. The greater reactivity of silver particularly influences the behavior of gold in flotation or other process. Silver has a value of about one-hundredth that gold, but tends to occur at higher grades and therefore may be a significant source of revenue to a gold operation. The drawback is that the larger volume or mass recovered product may cause reduced gold recovery. 
Native silver can occur at more than 95% purity, though this is rare and more commonly silver is associated with Au, Cu, Pb. It has similar conductive, malleable and ductile properties to gold, though has a higher reflectivity. The density of silver is 10-11 and the melting point is 960.5 oC.
Some of the world's richest silver deposits are epithermal, containing hydrothermal veins of quartz, carbonates and fluorite, often in altered tertiary rocks. The fineness of gold in epithermal deposits is low about 500-800


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