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. This report will continue to use the orogenic deposit classification for the Golden Chest.
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).
The Idaho Fault acted as the conduit for the mineralizing fluids that produced most of the veins at the Golden Chest Mine. The veins are generally stratiform and conformable to the Idaho Fault and are centered along the Idaho Fault to form a sub-parallel, stacked vein set.
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 Minor Faults (Northwest Striking) 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 Minor Faults (West Striking) seem to be associated with numerous small quartz veins and igneous sills and dikes. However, the nature of this association has not been determined.
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).
Many of the veins at the mine are brecciated. Both the breccia clasts and matrix can be mineralized in this vein type.
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.
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.
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.
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.
Gold mineralization occurs in veins associated with multiple faulting and folding events in the Coeur d’Alene Mining District. The mineralization occurs as gold-quartz veins associated with an orogenic deposit type. The orogenic system at the Golden Chest appears to have an association with igneous rock activity. Hence, the vein deposits may be described as intrusion-related orogenic gold. The principal vein being exploited at the Golden Chest Mine is associated with the Idaho Fault, which juxtaposes the quartzites of the upper Prichard Formation against finer-grained argillites, also of the upper Prichard Formation.
Veins occur adjacent to the Idaho Fault both in its footwall, and in its hanging wall. The mineralization occurs in two types of quartz veins, banded and massive. These veins are generally conformable to bedding in the Proterozoic age Prichard Formation. The banded veins, which occur primarily in argillite, contain, pyrite, arsenopyrite, galena, sphalerite, and visible gold. Thicker, massive veins occur in quartzite and contain pyrite, galena, chalcopyrite, sphalerite, scheelite and visible gold.