|Andrew Watson reports that the hot springs are related to emplacement of a Magma chamber at depth, which also is the driving source for the mineralization. The regional geology is a young 1,000,000 year volcanic sequence that overlies a Cretaceous shale sequence, with the gap between the two known as an unconformity. This unconformity plays a critical role in the formation of the deposit.|
The deposit is unusual in that it is composed of 125 separate veins clustered in distinct systems aligned an intersection of SW-NE structure and N-S structures. The veins which average 1-3 meters wide are hosted in a corridor of alteration that is 500 to 1000 meters wide and about 3 kilometers long. The veins are banded quartz-adularia - a low temperature pink feldspar mineral, and quartz-clay with extremely high grade, greater than 1,000 g per t gold mineralization in what are termed Ginguro or Silver-Black bands which are thin bands composed of electrum, chalcopyrite, galena and sphalerite. The mineralization does note outcrop at surface but starts 100 meters down, with the bulk of the mineralization occurring at the contact between the Pleistocene volcanics and the Cretaceous shales.
The deposit formed at contact between the underlying shales and volcanics, because as the older shales were deformed by the magma and domed, they fractured forming the space for the veins to form, while the more recent volcanics deformed plastically, meaning they did not fracture, but leaving space for the veins to form, so mineralization was concentrated at the contact between the two, which led to the extraordinarily high grades within a 100 meter interval.
While not the biggest or the best gold ore deposit, Hishikari is a world-class deposit as it reliably has an average grade of about 40 g / t Gold and has produced a spectacular 6.2 Million ounces of gold from 3.9 Million tonnes of ore, and has a reserve of 4.8 Million ounces in 3.5 Million tonnes.