The history of Otago’s goldfields stretches nearly 150 million years into the past, when the region was mostly mountainous and New Zealand was still part of Gondwanaland. The pressure of colliding plates along the eastern side of that ancient continent resulted in the formation of New Zealand’s oldest rocks (mid-Cambrian rocks of approximately 540 million years ago on the west coast of South Island). New Zealand split apart from Gondwanaland 85 million years ago. Throughout that time, it has undergone constant geological change, unlike its nearest neighbor Australia. The islands of New Zealand repeatedly sank beneath the ocean, only to rise once more and continue its eastward march. The cycle of submergence and uplifting, combined with volcanic activity, has resulted in both a unique and geologically young environment in the New Zealand of today. New Zealand’s current uplift began 20 million years ago.
Illustration
created by Stephen Read for
the website New
Zealand Gold
Gold occurs in both hard-rock and alluvial settings, the later resulting from the process of erosion upon the first. The past 150 million years (and the past 85 even more so) have been an especially active time for New Zealand, and geologic processes (mainly seismic and geothermal) below its surface have formed large gold deposits. The foliated structure of schist is easily split apart when earthquakes occur. The cracks and splits in schist rock are then filled with veins of quartz, and it is within this quartz that gold is often found. These veins are formed when hot water (200-400o C) rises through fractures in the schist up to 15 kilometers below the surface.
Quartz and gold precipitate out of the heated solution as it cools. This hard-rock gold is the original source for the second setting of gold, placer deposits. Placer deposits refer to gold found in sands and gravels of streams. The geological processes that result in placer deposits are complex, but can be simply understood as resulting through basic weathering. Once gold is freed from the hard-rock lodes (either through chemical or physical means), it is transported by various methods to other locations, often settling in a stratum between other layers that contain gravels, sand and bedrock. Gold, along with quartz, is deposited in waterways as the surrounding bedrock is eroded away near the surface of the Earth (approximately 15 kilometers of rock has been removed from the Otago Region). Many alluvial and eluvial deposits are the result of multiple successions of erosion, resulting in a concentration of gold in many deposits. Since gold is 19 times more dense than water, it does not move far in streams except as small particles (less than half a millimeter), and it is the denseness of gold that allows it to settle and rest in gravel beds while other detrital materials continue to wash away.