Summary:
The Golden Chest deposit is recognized as an “intrusive related” orogenic gold-quartz vein system by IDR geologists. However, recently there has been the suggestion that the Golden Chest deposit type could be a Reduced Intrusion-Related Gold System (RIRGS). The RIRGS deposit classification is already mired in confusion over nomenclature. It should be remembered that each ore deposit has its own specific and unique characteristics and not all deposits exhibit all characteristics of any particular classification. Deposit models are sets of guidelines, not absolute parameters. No one model will describe multiple deposits, one model describes one deposit. That being said, there is considerable overlap between the two deposit model types and the Golden Chest deposit has characteristics of both orogenic and RIRGS model types.
Before 1998, the term mesothermal was used for orogenic gold-quartz vein systems. Other synonyms for this type of vein are shear-hosted lode gold, low-sulfide goldquartz veins and California Mother Lode veins. These “mesothermal” or orogenic deposits are associated with regionally metamorphosed terranes of all ages. Some of the largest gold deposits in the world are associated with orogenic vein systems. World class orebodies are generally 2 to 10 kilometers (km) long, 1 km wide, and are mined down-dip to depths of 2 to 3 km. This deposit type is responsible for a significant portion of the world’s gold production.
Mineralization and Alteration
The primary geological control to mineralization at the Golden Chest is the Idaho Fault. The Idaho Fault separates the hanging wall (Prichard Formation-Member H) from its footwall (Prichard Formation-Member G). Another geologic control is the Timber King Fault, which in many respects, is similar to the Idaho Fault in how it influenced mineralization.
The Idaho Fault acted as the conduit for the mineralizing fluids that produced most of the veins at the Golden Chest Mine. The veins can appear to be stratiform to Prichard bedding and conformable to the Idaho Fault and are centered along the Idaho Fault to form a sub-parallel, stacked vein set. The H-Vein is conformable with the Timber King Fault, occurring on both sides of the fault plane. The alteration is generally weak and mostly occurs immediately adjacent to the veins. Silica, chlorite, pyrite, carbonates, and minor sericite are the primary alteration components seen. The Northwest Striking Minor Faults have been seen to influence grade and tonnage in individual veins as they have been mined. The exact mechanism of control has yet to be determined. The East-West Striking Minor Faults seem to be associated with numerous small quartz veins and igneous sills and dikes.
Vein Types
There are three main types of quartz-gold veins found at the mine; banded, brecciated and massive. Most of the gold production and best grades come from the banded quartz veins. The banded veins consist of thin, sub-parallel shear surfaces that result from compression. The bands are composed of quartz, fine sulfides and phyllosilicate septa (derived from wall rock). A banded vein example is shown below with visible gold grains circled in red.
Many of the veins at the mine are brecciated. Both the breccia clasts and matrix can be mineralized in this vein type. Brecciation events have modified or destroyed the textures of both the banded and massive vein types.
Massive quartz veins are characterized by a lack of banding or brecciation. They are also distinguished by a general lack of sulfides. The massive veins can have good gold values, but usually not as rich as the banded or brecciated veins.
Vein Names and Locations
Most of the veins recognized at the mine are along or near the Idaho Fault. Both the veins and fault dip moderately to the west at approximately 45 degrees, except for the H-Vein, which typically dips 65-75 degrees west.
Veins are found in both the hanging wall and the footwall of the Idaho Fault, and they are concentrated within 100 meters of the fault. Vein density increases with proximity to the Idaho Fault with the most abundant veining occurring within 50 meters of the fault. The historical workings at the mine appear to target veins that are generally within 25 meters of the Idaho Fault.
The “Idaho Vein” is the name given to the quartz vein found in the immediate footwall of the Idaho Fault. The “H-Vein” is the name given to the quartz vein found immediately next to the Timber King Fault.
There have been discrepancies in identifying the different veins throughout the years, because the veins pinch, swell, and split along strike. Because of this, the correlating of veins between mining levels or even along strike is difficult.
Lithologic Control of Veins
The two main lithological controls for veining at the Golden Chest are:
- Rheological: brittle-ductile contrast;
- Permeability and porosity.
Most of the veins at the mine are preferentially found in areas of rheological contrast. Vein development is at its strongest when brittle units, like quartzite and blocky siltite, are in contact with the more ductile laminated siltite-argillite beds.
The permeability and porosity of the lithologic units at the Golden Chest is largely related to grain size. The fine-grained units act as aquitards to hydrothermal fluids while the coarse, more permeable units accommodate and hold the hydrothermal fluids. The quartzite units of the Prichard Formation are more permeable and porous than the surrounding siltite-argillites and are more susceptible to hosting widespread silica-flooding and silicification.
At the mine, the hanging wall to the Idaho Fault is mainly composed of thinly laminated siltite-argillite, with almost no quartzite beds, and these rocks are not as susceptible to silicification. However, the footwall lithologies of the Idaho Fault are primarily light grey, fine-grained quartzites and blocky siltites and these units are very susceptible to pervasive silicification.
Vein Mineralization
Gold mineralization at the Golden Chest is mostly associated with the sulfide minerals pyrite, galena, sphalerite, and chalcopyrite. Less commonly, gold can be found adjacent to arsenopyrite or scheelite. Both the mineralogy of ore and gangue are:
· Ore mineralogy: pyrite, galena, chalcopyrite, sphalerite, arsenopyrite, gold, scheelite, minor covellite, tetrahedrite;
· Gangue mineralogy: quartz, chlorite, carbonates, sericite, muscovite, minor feldspar.
Commonly, native gold is seen as intergrowths associated with pyrite, galena, sphalerite, and chalcopyrite. Gold mineralization occurs along the grain boundaries or inside clusters of these sulfides.