The Cyanide Tanks Traditional Cache

There is not a sea nearby, all this sand is what is left when the crushed ore is treated to remove the gold.
This was performed in the two tanks you can see here. The process was done with Cyanide.

Cyanide Process
Cyanide, in the form of a very dilute sodium cyanide solution, is used to dissolve and separate gold from ore. The process used to extract gold using cyanide was developed in Scotland in 1887, and was first used in large scale commercial mining by the New Zealand Crown Mines Company at Karangahake in 1889.

The concentration of cyanide used in this process is normally in the range of 0.01% and 0.05% sodium cyanide (100 to 500 parts per million).   Cyanide leaching is usually done along with a physical process like milling, crushing, or gravity separation. The pH of the resulting slurry is raised by adding lime or another alkali to ensure that cyanide ions do not change into toxic cyanide gas (HCN).  The gold is then further concentrated and reduced, before being smelted into gold bullion.