Rhyolitic rocks are volumetrically significant in the eastern part of the
Coromandel gold province, yet deposits hosted by rhyolite (Kapowai, Broken
Hills, Neavesville, Wharekirauponga and Waihi Beach) are generally low grade and
subeconomic, especially compared with andesite-hosted deposits. In contrast,
rhyolitic volcanic centres in Nevada host a number of economically significant
epithermal gold deposits such as Round Mountain, Sleeper and Bullfrog.
The Nevada deposits are associated with 27-20 Ma continental arc volcanics
dominated by ignimbrites (eg Round Mountain), or younger 17-9 Ma bimodal
rhyolite-basalt volcanism related to extensional basin-and-range tectonics (eg
Sleeper). The Round Mountain deposit is localised on a caldera boundary fault
and the mineralisation was deposited soon after eruption of the host rhyodacitic
ignimbrite. In other Nevada deposits, hydrothermal systems are linked to the
closing stages of flow-dome complexes and mineralisation is localised along
normal faults associated with extensional basin-and-range tectonism. The style
and composition of the mineralisation is similar in both the Coromandel and
Nevada deposits and consists of sheeted and stockwork quartz veins, containing
electrum as the main ore mineral and only minor sulphides (mainly pyrite)
Likewise, the vein minerals were deposited from low salinity hydrothermal fluids
in the temperature range of 200-270C. Both the Neavesville and Round Mountain
deposits are geochemically anomalous in molybdenum. The deep oxidation of the
Nevada gold deposits, dry climate and low level of competing land uses combine
to make it economic to mine and process very low grade ore (0.2 - 2.0 g/t Au) by
heap-leaching with cyanide solutions in most areas of Nevada. Although extensive
low grade gold mineralisation is known at deposits such as Wharekirauponga and
Neavesville, it is sub-economic because of shallow oxidation and land use
conditions that are unfavourable for open pit mining and heap
leaching.