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. Gold 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. 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 to kaolinite-dickite, illite rich rock, to 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.
ALTERATION IN A HIGH-SULPHIDATION SYSTEM:
In 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 (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 (greater than one ounce gold per ton) 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.
ALTERATION IN A HIGH-SULPHIDATION SYSTEM:
Epithermal Systems
Posted by Marvel Labels: ephithermal system, gold, gold deposits, high-sulphidation
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. Eventually 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 Yellowstone or the Geysers in California. 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. Both form gold deposits and are known respectively as low- and high-sulphidation gold deposits. In both subtypes gold will largely be precipitated from 2.5 kilometers depth to surface.
Recognizing that gold precipitates near the surface in these systems, the great American geologist Waldemar Lindgren coined the term epithermal in 1933, epi meaning shallow and thermal referring to the heated fluid. The chemist Werner Giggenbach further subdivided epithermal gold deposits into low and high sulphidation types (illustrated right1). Low and high do not refer to each type’s relative amount of sulphide minerals (metal complexes of sulfur with metals). Rather 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.
