Low sulphidation epithermal deposits occur in the form of polymetallic veins that form up to 1.5 kilometers deep and at a temperature between 200°C to 280°C, in addition, they occur related to volcanic centers on the continent and are considered important for the extraction of gold (Au) and silver (Ag).
Also several of these deposits usually have significant concentrations of lead (Pb), zinc (Zn) and sometimes copper (Cu) as sulfide minerals.
General properties and characteristics
Veining and mineralization can be continuous along fault lines of up to a couple of kilometres, but although veining can occur deeper than 1 km, cheap mineral only develops in restricted vertical intervals of approximately 500m. , and in some cases only up to 200 m.
However, depths of mineralization are estimated to be between approximately 200 and 700 meters below the water table, or rarely up to 1.5 km depth.
Mineralization can also occur in breccia pipes, with ore minerals in the clasts or in the breccia matrix.
- Economic metals: Au, Ag and to a lesser extent Pb, Zn and Cu.
- Depth: Typically 1 km and up to 1.5 km.
- Formation temperature: 200°C to 280°C (ore minerals).
- Associated rocks: Lava flows of intermediate to felsic composition (andesites, dacites, rhyolites, rhyodacites) and pyroclastic rocks.
- Geological environment: Subduction zones, volcanic arcs (mostly continental), near centers or volcanic structures (on the periphery).
- Structural control: Very marked, associated with geological fault lines and regional tectonics.
- Hydrothermal alterations: Argillic, silicification, propylitic and advanced argillic (steam heated)
- Gangue minerals: quartz, chalcedony, carbonates (calcite and rhodochrosite), adularia, illite, and sometimes barite.
- Base metallic minerals: Pyrite, sphalerite, marcasite, galena, electrum, gold (Ag sulfosalts, arsenopyrite, argentite, chalcopyrite, tetrahedrite) and tellurides are sometimes abundant.
- Geochemical association: Au, Ag, As, Sb, Hg, Zn, Pb, Se, K, Ag/Au
- Form of the deposits: Veins that fill spaces and stockworks.
Geological setting and associated rocks
Low sulphidation epithermal deposits occur associated with convergent plate tectonic boundaries, in continental and oceanic volcanic arcs generally included in epithermal deposits.
They are often considered as the upper part of a much larger and more complex mineralization system that begins with the formation of porphyry type copper deposits.
That is to say that this type of deposits occurs associated with large intrusions in the crust and volcanic centers.
Low sulphidation epithermal deposits are hosted in a variety of rock types and are generally found at greater distances (2–10 km) from volcanic centers as opposed to high sulphidation deposits which are very close.
These deposits are hosted by sub-aerial volcanic rocks that occur around the volcanic structures (within about 10km), and the typical sequences of box rocks are sequences of intermediate to felsic composition lava flows and pyroclastic rocks.
Structural control
In the deposits of low sulphidation epithermal structural control is well marked, such that mineralized veins occur in interconnected networks or swarms of small to large veins (up to 10 m thick), where mineralization is in the veins and adjacent walls, hydrothermally altered from the box rock.
The veins often have subparallel orientations and, for example, are hosted along a set of faults that are more extensive than the related volcanic centers.
Geological model and hydrothermal alterations.
Let’s delve this topic for each category:
Ore minerals and metals
The veins with the ore minerals are dominated by quartz and adularia (low temperature potassium feldspar) accompanied by calcite, chlorite and other variable gangue minerals.
Ore minerals are disseminated in veins and also occur disseminated in strongly altered box rocks adjacent to veins.
The sulfide mineral assemblage is highly variable from one deposit to another and from one vein to another, due to the sulfidation state.
Thus, pyrite or marcasite are the dominant sulfides in intermediate sulfidation epithermals.
While pyrrhotite with arsenopyrite (FeAsS) are dominant in low sulphidation epithermal.
Base sulfides such as galena, sphalerite, and chalcopyrite are common in conjunction with manganese minerals, including rhodochrosite.
Stibnite (Sb2S3) is often important in some deposits.
Native gold or electrum (Au – Ag alloy) is the main host for Au.
Instead, silver is hosted in electrum, in acanthite silver sulfide (Ag2S), in silver sulfosalts such as proustite (Ag3AsS3) and pyrarargyrite (Ag3SbS3).
Native silver is a common secondary mineral formed after these Au sulfide ores in incipiently weathered deposit levels.
Gold and silver telluride minerals are important hosts for precious metals in some deposits.
Zones of extremely high grade bonanza ore (greater than 30 ppm Au and locally greater than 1000 ppm) occur locally along and within the depth range of some veins.
Texture
Many veins have symmetrically developed bands with different mineral assemblages on a scale of the order of 1 to 10 centimeters.
The bands are indicative of evolving or oscillating physicochemical conditions during the progressive filling of the veins from the wall to the center.
The vughs and euhedral growth in space are indicative of a “fissure” open space in the vein at the time of mineral growth.
Bands of crustiform and coloform growth at fine scale are indicative of rapid mineral precipitation.
When chaoloform bands involve a silica mineral, we infer that the silica initially precipitated from the fluid as fine-grained cryptocrystalline polymorphs such as chalcedony or opal.
Pseudomorphic quartz replacements in lamellar calcite are common and likewise indicate changing physical and chemical conditions in the vein over the time of mineral precipitation such that the calcite grew rapidly out of solution as laminated grains and the solution then became saturated. with respect to calcite and was replaced by quartz.
Hot spring deposits
Hot spring deposits occur in sub-horizontal masses (terraces) of siliceous sinter in the form of porous silica or recrystallized banded silica.
The ore minerals in these deposits include very fine-grained As, Sb and Hg sulfides of a few micrometers, for example, cinnabar, realgar and orpiment, which disseminate in the silica sinter and give the silica a gray coloration. observable characteristic in hand sample.
Associated hydrothermal alterations
the veins low sulphidation epithermal they have halos up to a few meters wide of argillic alteration facies with illite and silicification, and are hosted in volumes of altered rock that extend up to several kilometers laterally.
Propylitic alteration is common in host rock as a more extensive or regional alteration, and is associated with clay minerals, especially smectite.
Unlike the propylitic alteration that occurs in porphyry copper where chlorite is common, in copper porphyry low sulphidation epithermal occurs at a lower temperature between 200°C to 275°C.
In addition, subtle lateral and vertical zoning of regional propylitic alteration intensity and mineral content may be evident.
Regarding steam heated alteration, it is characterized by presenting advanced argillic alteration with the presence of cryptocrystalline polymorphs of silica that are typically preserved at low temperatures of the order of 100°C, this differentiates it from advanced argillic alteration of high sulphidation epithermals.
Zoning
Many alteration minerals are stable in restricted temperature or pH ranges, therefore they provide significant information for reconstructing the thermal and geochemical structure of the hydrothermal system.
The associated alteration in low-sulfidation deposits is produced by near-neutral pH hot springs, with temperature decreasing with both decreasing depth and increasing distance from the hydrothermal vent.
Thus, the accumulation of ore minerals must occur in areas of hydrothermal vents.
Therefore, the ore and gangue minerals associated with low sulphidation epithermal it typically occurs between 180°C to 280°C, which is equivalent to an approximate depth of 100 m to 800-1500 m (Hedenquist and Henley, 1985).
The graph shows that the stable minerals associated with ore minerals in this type of deposits are quartz, calcite, adularia and even illite, so if you find this association, you would be at the core of the system.
While minerals such as biotite and amphibole are stable at temperatures higher than 280°C, it could indicate that the epithermal system has eroded, that is to say that we are at the base at a greater depth of the system where it is unlikely to find ore minerals.
On the contrary, minerals such as smectite and illite/smectite, chlorite/smectite, laumontite associations could indicate that you are very close to or at the top of the mineralization system.