March 11, 2017 | Author: Anonymous | Category: Documents
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Sometimes the vuggy silica can be widespread if the acid fluids encountered a broad permeable geologic unit. HF. low-sul...


EPITHERMAL SYSTEMS The association of gold mineralization with volcanic and geothermal hot spring activity has long been recognized by prospectors and geologists. We now know that this association is a consequence of the hot magmas which not only produce volcanic eruptions and volcanic rocks but also are the source of  the hot fluids that transport gold and other metals and may in fact be the source of gold itself. Fluids emanating from a molten magma are extremely hot and under high pressure deep below the surface. As these fluids rise they mix with surface waters and change the composition of the rocks with which they come into contact. This process is known as alteration. !ventually the fluids breach the surface and form either acidic lakes known as fumaroles common in the craters of volcanoes or dilute neutral hot springs like those at "ellowstone or the #eysers in $alifornia. These two different surface manifestations % acidic lakes or neutral hot springs % reflect two different fluid types that each result from the two different paths taken by the magma as it rises to the surface. &oth form gold deposits and are known respectively as low' and high'sulphidation gold deposits. (n both subtypes gold will largely be precipitated from ).* kilometers depth to surface.

+ecognizing that gold precipitates near the surface in these systems the great American geologist Waldemar ,indgren coined the term epithermal in -// epi meaning shallow and thermal referring to the heated fluid. The chemist Werner #iggenbach further subdivided epithermal gold deposits into low and high sulphidation types 0illustrated right -1. ,ow and high do not refer to each type2s relative amount of sulphide minerals 0metal complexes of sulfur with metals1. +ather the distinction is based on the different sulfur to metal ratio within the sulphide minerals of each subtype. While this discussion deals with high'sulphidation epithermal systems it is worth mentioning that low' sulphidation systems also form economic gold deposits although they develop under vastly different chemical conditions.

HIGH-SULPHIDATION DEPOSITS 3igh'sulphidation deposits result from fluids 0dominantly gases such as 45 ) 3F 3$l1 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. #old and sometimes copper'rich brines 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. 4ometimes the vuggy silica can be widespread if the acid fluids encountered a broad permeable geologic unit. (n 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 0the centre of the fault1 progressing through quartz'alunite to kaolinite'dickite illite rich rock to chlorite rich rock at the outer reaches of alteration. Alunite 0a sulphate mineral1 and kalonite dickite illite and chlorite 0clay minerals1 are generally whitish to yellowish in colour. The clay and sulphate alteration 0referred to as acid'sulphate alteration1 in high'sulphidation systems can leave huge areas sometimes up to -66 square kilometers of visually impressive coloured rocks. ALTERATION IN A HIGH-SULPHIDATION SYSTEM:

(n contrast 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 0greater than one ounce gold per ton1 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. 7uartz'adularia veins can contain high'grade gold 0greater than one ounce gold per ton1 and silver deposits over  vertical intervals of generally /66 to 866 metres. Within this vertical dimension high gold grades can make for a large amount of easy to mine gold in a narrow compact area. ALTERATION IN A HIGH-SULPHIDATION SYSTEM:




Yanacocha, Peru The "anacocha deposit is operated by 9ewmont #old $orp. (t is one of if not the largest high' sulphidation deposit in the world and one of the largest gold deposits of any geologic type. +eserves stand at -): ;illion Tonnes of -.6 g
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