* high-sulphidation epithermal deposits:
High-sulphidation deposits result from fluids (dominantly gases such as SO2, HF, HCl) channeled directly from a hot magma. The fluids interact with groundwater and form strong acids. These acids rot and dissolve the surrounding rock leaving only silica behind, often in a sponge-like formation known as vuggy silica. Brines containing gold and sometimes copper that also ascend from the magma then precipitate their metals within the spongy vuggy silica bodies. The shape of these mineral deposits is generally determined by the distribution of vuggy silica. Sometimes the vuggy silica can be widespread if the acid fluids encountered a broad permeable geologic unit. In this case it is common to find large bulk-tonnage mines with lower grades.
The acidic fluids are progressively neutralized by the rock the further they move away from the fault. The rocks in turn are altered by the fluids into progressively more neutral-stable minerals the further away from the fault. As a result, definable zones of alteration minerals are almost always are formed in shell-like layers around the fault zone. Typically the sequence is to move from vuggy silica (the centre of the fault) progressing through quartz-alunite, kaolinite-dickite, illite rich rock, and finally chlorite rich rock at the outer reaches of alteration. Alunite (a sulphate mineral) and kalonite, dickite, illite and chlorite (clay minerals) are generally whitish to yellowish in colour. The clay and sulphate alteration (referred to as acid-sulphate alteration) in high-sulphidation systems can leave huge areas (sometimes up to 100 square kilometers) of visually impressive coloured rocks.
* low-sulphidation epithermal deposits
low-sulphidation veins are formed when the fluids interact with greater amounts of groundwater as they rise from the hot magma. The protracted boiling of the fluids in low-sulphidation systems produces high grade gold (greater than one ounce gold per ton) and silver deposits. The fluids interact with the surrounding rock for a much longer period of time than the quickly channeled high-sulphidation fluids. As a result, the fluids become dilute and neutralized and the silica dissolves. The silica is later precipitated in the veins as quartz, often sealing the fissure closed. When this occurs, the pressure of the gases underneath the sealed fault builds until the seal is ruptured, which provokes catastrophic boiling and the precipitation of gold. After this explosive boiling event, passive conditions return, and quartz precipitates once again. This cyclical process results in the well-known banded texture of the quartz-adularia veins typical of low-sulphidation vein systems. Quartz-adularia veins can contain high-grade gold and silver deposits over vertical intervals of generally 300 to 600 metres. Within this vertical dimension, high gold grades can make for a large amount of easy to mine gold in a narrow compact area.
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CARBONATE REPLACEMENT DEPOSITS
Carbonate replacement lead zinc silver (“CRD”) deposits are high-grade sulphide ore deposits that form in carbonate sedimentary rocks, particularly limestones.
Examination and study of CRD deposits has resulted in the consensus that their lead, zinc and silver rich ores are deposited by high-temperature fluids which transport the metals away from a molten rock (intrusive such as a granite) and into carbonate rocks (limestone) which the molten rock intrudes. These fluids move away from the molten intrusive along faults and, upon encountering layered limestone, replace the limestone with lead, zinc silver and iron sulphide complexes. As a result the deposits are typically lense-shaped or elongate tabular bodies that are referred to as mantos or chimneys.
As Megaw (1998) pointed out the size of these deposits (averaging 10-13 million tonnes and upto over 50 million tonnes) and the high grades (ranging from 2 to 12% lead, 2 to 18% zinc, 60 to 600 g/t silver, up to 2% copper and up to 6 g/t gold) make them extremely desireable targets to explore for.
CDR deposits often form in close association with skarn deposits. Skarn deposits are areas where high-temperature stable minerals have replaced and recrystallised limestone that form immediately at the intrusive-limestone contact. Large areas can be affected by an intrusive and the fluids that the molten rock can introduce. As a result, in general, skarn deposits are associated with large areas of altered rock, within which the ore deposits are located. In contrast CRD deposits, which form at greater distances from the intrusive generally have little to no alteration associated with them, often resulting in razor sharp edges between unaffected or unaltered limestone and high-grade sulphide lead-zinc-silver ore.
As pointed out by Titley (2001), CRD deposits were first recognized in Mexico and Peru in the 18th century and in the United States in the 19th century. In an arid environment, such as exists in Mexico, the rich silver lead zinc ores were oxidised over time, leaving high-grade silver ores at surface. Ensuing 20th century development below the weathering enriched, high silver surface zones identified the primary lead zinc silver sulphide ores at depth.
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Mesothermal gold deposits are formed from hot water that precipitates gold under high temperatures and pressure, generally at great depths in the earth's crust (around 10km). In these deposits, high gold grades tend to be continuous over large vertical ranges.
This process has formed throughout geologic history but younger examples include the Mother Lode District in California and the Bralorne-Pioneer District in B.C. Nearly 80% of B.C.'s gold production has been mined from mesothermal veins, which includes Almaden's Elk Gold Mine.
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Porphyry deposits are one of the most important classes of mineral deposits for the production of copper, molybdenum, and gold. These deposits are formed at the uppermost portion of a rising, finger-like column of molten rock. This column solidifies (and thereby forms rocks) generally about 1 to 8km beneath the surface of the earth. Rocks that form in this way are known as intrusive rocks or intrusions - mainly because of their shape. The terms porphyry and porphyritic refer to a texture found in intrusive rocks which are very finely-grained but studded with coarse crystals. This texture - much like chocolate chips in a cookie - is formed when the molten rock cools at different speeds. As the liquid molten rock cools slowly, large crystal begin growing. As the rock subsequently cools rapidly, fine-grained rock is formed (known as matrix or groundmass) around the larger crystals. This porphritic texture is typically formed when magma partially cools deep within the earth and then quickly rises towards the earth's surface in a finger-like intrusive and cools very rapidly.
Porphyry deposits form around the apex of an intrusive. Mineral-rich fluids which have separated from the cooling molten intrusive seep into the surrounding rock (known as country rocks) and precipitate copper, gold, and other metals in a hardened formation known as a carapace. The products of the reaction between the country rock and the molten intrusive are more extensive if the country rock is reactive, if it were limestone for example.
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