The Big Picture
Auriferous quartz deposits mined on Great Barrier are part of the structure of Te Ahumata Mountain. It has three reef systems in which gold and silver are found - Sunbeam, Barrier and Iona Reefs. There had been a copper mine at Miner's Head 1841-67 and traces of gold were found in the ore. The Sunbeam Reef was discovered by the Sanderson brothers in 1892. Their farms were divided into six leases in 1893. By 1898 ten companies were vigorously but unsystematically working their claims on the Ahumata Field.
Golden Adventure
Benjamin and William Sanderson Jnr left Great Barrier Island to join the gold rush in the Coromandel that had started in August 1867. While there, they learned the Barrier is an extension of the Coromandel Peninsula, formed by the same processes in a similar geological era. So, they thought that the same gold had to be found on the island too. The Sanderson brothers got digging - and yes, gold and silver bearing quartz reefs were found here too.
Iona Mine
The Iona Gold and Silver Mining Company had a claim of 100 acres, registered on 30 November 1896. The mine operated 1902-06, through five adits (horizontal shafts like this one). The adit at GZ is the lowest of the adits at 190m above sea level. It runs 35m into the rock.
No Easy Money Here
The hardness, variability and fineness of the Iona Reef quartz created problems in mining, crushing and treatment. Also, the richest pockets of ore were found right near the surface. The deeper you went, the greater the cost and the less the returns.
Some say large-scale open-cast mining is the only economically viable mining method here. A French company investigated, but the Second World War stopped that.
Flying Rocks
The stampers were 1500m away from the top adits. An aerial tramway carried rock to the stampers, with the weight of a full bucket on the downhill run, pulling an empty bucket back up. The Iona stamper battery was just downhill from here.
Explosives were stored at the northern end of Okupu foreshore. A horse-drawn wagon carried the explosives up to the mine adits.
Once the ore was crushed, it had to be transported to Oreville, to the cyanide treatment plant, which could extract the gold.
No Gold, But Plenty of Cave Weta
The mine shaft is a home for cave weta. Look up on the ceiling and you'll see them. The 50 species of cave weta have extra long antennae, long, slender legs and are quite docile. Although they have no hearing organs on their front legs like other weta, they are sensitive to vibrations through pads on their feet. They eat lichens, fungi and dead organic material like seabird carcasses.
clarkie369