Epithermal Vein-Type Gold Deposits November 9, 2011

Review to previous posts about Epithermal Deposits, in this case I would like to tell you about spesific of Epithermal Deposits especially epithermal vein type.
1. Condition
An epithermal gold deposit is one in which the gold mineralization occurs within 1 to 2 km of surface and is deposited from hot fluids. The fluids are estimated to range in temperature from less than 100°C to about 300°C and, during the formation of a deposit, can appear at the surface as hot springs.The deposits are most often formed in areas of active volcanism around the margins of continents.

2. Two type of fluid LS and HS
Epithermal gold mineralization can be formed from two types of chemically distinct fluids Ä “low sulphidation” (LS) fluids, which are reduced and have a near-neutral pH (the measure of the concentration of hydrogen ions) and “high sulphidation” (HS) fluids, which are more oxidized and acidic.
Low Sulphidation Fluid type : LS fluids are a mixture of rainwater that has percolated into the subsurface and magmatic water (derived from a molten rock source deeper in the earth) that has risen toward the surface. Gold is carried in solution and, for LS waters, is deposited when the water approaches the surface and boils
High Sulphidation Fluid type: HS fluids are mainly derived from a magmatic source and deposit gold near the surface when the solution cools or is diluted by mixing with rainwater.The gold in solution may come either directly from the magma source or it may be leached out of the host volcanic rocks as the fluids travel through them

3. Mode of Deposition
In both LS and HS models, fluids travel toward the surface via fractures in the rock, and mineralization often occurs within these conduits. LS fluids usually forth large cavity filling veins, or a series of finer veins, called stockworks, that host the gold. The hotter. more acidic HS fluids penetrate farther into the host rock, creating mineralization that may include veins but which is mostly scattered throughout the rock.
LS deposits can also contain economic quantities of silver, and minor amounts of lead, zinc and copper, whereas HS systems often produce economic quantities of copper and some silver.
Other minerals associated with LS systems are quartz (including chalcedony), carbonate, pyrite, sphalerite and galena, whereas an HS system contains quartz, alunite, pyrite and copper sulphides such as enargite.

4. Geochemical Exploration – Indicator Elements
Geochemical exploration for these deposits can result in different chemical anomalies, depending on the type of mineralization involved. LS systems tend to be higher in zinc and lead, and lower in copper, with a high silver-to-gold ratio. HS systems can be higher in arsenic and copper with a lower silver-to-gold ratio.

5. Location and Grade
Many countries have epithermal gold deposits, including Japan, Indonesia, Chile and the western U.S., each of which occupies a portion of the “Rim of Fire,” the area of volcanism that rings the Pacific Ocean from Southeast Asia to western South America. Epithermal gold is also found in British Columbia at the Baker mine, in the Toodoggone district, and near the Taseko River.
Epithermal gold deposits, which contribute significantly to the world’s gold supply, are an important exploration target which must be evaluated carefully based on the amount of metal they might provide, and at what cost.
The amount of gold in any type of deposit is calculated based on the ore’s grade (the amount of gold per tonne of rock) and tonnage (total number of tonnes) available at that grade. The higher the grade of the material, the lower the tonnage required to make recovery economical.

A high-grade deposit could have gold values ranging from 10 to more than 150 grams per tonne, whereas a low-grade deposit grades in the range of 1 to 5 grams. Low-grade deposits may have up to, and possibly more than, 200 million tonnes of rock, whereas a high-grade deposit is frequently smaller.

6.  Distribution of Gold within a deposits
Assay results acquired through drilling are important indicators of a deposit’s grade and tonnage. High grades over short distances can be as significant as low grades over longer distances, and both types of deposit can be mined profitably.
Drill results, however, offer only a limited view of a deposit and may be difficult to reproduce. For instance, a single drill hole may intersect a high-grade zone in an otherwise low-grade (high sulphidation type epithermal) deposit, giving the appearance of a higher grade than actually exists.

7. Other Metals as by products
Factors other than tonnage and grade come into play in calculating the economic significance of an epithermal deposit. For instance, the presence of other metals in the ore can increase the value of a deposit, and many epithermal deposits contain a significant silver and/or copper content.