The Manuherikia River valley is a broad depression in the schist bedrock that is flanked by the Dunstan Range and the Raggedy Range. These ranges are broad up-folds (antiforms) of schist that are separated by the down-fold (synform) of the Manuherikia valley. Hence the topographic features that dominate the area have been mainly formed by the geological folding, with only minor modification by river erosion. The gentle western slopes of the Raggedy Range, in particular, reflect the wide but shallow fold structure. The fold structures have been forming slowly for the past 1 million years, and are still actively, although imperceptibly, deforming. Uplift rates are typically between 0.1 and 0.5 mm/year. The ranges have also been getting wider at similar rates, so that exposed schist bedrock extends progressively into the valley margins. The schist bedrock was covered by soft 5-20 million year old river and lake sediments over the whole area. These sediments have been eroded from the ranges, but are still preserved in the valley. The lake sediments are prominent along the rail trail between Oturehua and Chatto Creek.
The southern portion of the Raggedy Range has been made more complicated by the presence of a set of faults at the margin of the valley. The faults are geologically ancient, but some of these faults have acted as weak zones during the present folding of the region. These faults have become reactivated, leading to steeper hillsides on the valley margin between Ophir and Alexandra, and between Alexandra and Earnscleugh on the western side of the Clutha River. The most prominent feature of the faulted zone is Tucker Hill, between Galloway and Alexandra. This relatively rugged valley wall contrasts strongly with the gentle slopes of the northeastern and central portions of the Raggedy Range.
The Manuherikia River flows mostly on the southeastern side of the valley, adjacent to the Raggedy Range, for most of its length. It has been forced to remain on that side of the valley by the large gravel fans that have emanated from the Dunstan Range and partially filled the central and northwestern parts of the valley. These fans have buried the 5-20 million year old sediments over much of the Manuherikia valley.The combination of these fans, the increasing width of the Raggedy Range, and uplift along the reactivated faults along the southeastern side of the valley, has resulted in the Manuherikia River being forced to cut its way through a narrow schist gorge between Ophir and Chatto Creek.
The Manuhrikia faults were formed during uplift of the schist bedrock from the middle of the Earth’s crust (about 10-15 km). This occurred about 100 million years ago, in what was mountainous topography across Otago. Those mountains have since been eroded away, leaving the generally low relief landscape that is currently being deformed. At the time of formation of these faults, the South Island was being pulled apart, and the faults formed during this regional extensional activity. Reactivation of the faults is currently occurring because of crustal-scale compressional processes associated with the Alpine Fault(*) along the western side of the Southern Alps. This has resulted in reversal of the movement along the faults, causing local steps in the topography, such as at Tucker Hill.
The fault zones include abundant crushed rock, some of which has been transformed to clays. Small amounts of glassy material formed where the ancient earthquake movement events were so rapid that the adjacent rocks have melted and then solidified almost instantaneously. Many of the fault surfaces are relatively soft and planar, and are easily reactivated, whereas others have been cemented with quartz, and have remained intact, as shown in the adjacent photographs. Other parts of the faults have been partially cemented by the carbonate mineral ankerite, which contains iron; weathering of that mineral causes prominent orange-brown staining on outcrops.
The quartz cement in some faults was emplaced by hot water moving through the schist bedrock from below, channelled along the crushed rock fault zone. This process also introduced gold into the fault zones, and the gold was focussed in quartz-rich rocks (quartz veins). Quartz-rich rocks with minor gold occur along the northeastern trend of many of the Manuherikia faults between Ophir and Earnscleugh. The small amount of gold in these rocks has been insufficient to be mined in the past. However, similar gold-bearing fault rocks occur along a northwesterly trend at Ophir, and these have been mined for gold historically. Both sets of gold-bearing faults formed at approximately the same time, about 100 million years ago. These gold-bearing deposits are broadly similar to those at the gold mine at Macraes (*), although the Macraes gold was emplaced at an earlier time, about 140 million years ago, before the schist bedrock was uplifted from the middle crust (10-15 km deep).
Minor uplift and erosion of the schist bedrock occurred about 20 million years ago, and this led to development of a pile of quartz-rich river sediments resting on the schist. Some of the quartz in these sediments was derived from the immediately underlying schist, and some was derived from pre-existing quartz-rich sediments. Gold from both these sources accumulated in 20 million year old quartz gravels, especially in the zone immediately on top of the schist bedrock. The quartz vein fragments with gold, in the photographs above, were extracted from some of these quartz gravels. However, most of the gold occurs as flakes between 0.1 and 1 mm across. The gold became concentrated near the floor of ancient river channels during transport because it is extremely dense (19 times denser than water). These concentrations of gold are called placer deposits, as distinct from the quartz vein-bearing gold in the schist bedrock.
Groundwater slowly moving through the quartz gravels has dissolved some of the quartz from the surfaces of the quartz pebbles, and then deposited that quartz nearby as a cement in between the quartz pebbles. In some portions of the quartz gravels, this process has resulted in formation of very hard fully-cemented layers, called silcrete (similar to concrete, but held together with quartz rather than man-made cement). These layers can form large hard slabs on hillsides when the sediments around them have been eroded. The slabs eventually break up to form smaller boulders, that are commonly called “Sarsen stones”. Most of these silcrete boulders have smooth surfaces coated with yellow-brown rusty iron oxide. The shapes of the original quartz pebbles that formed the framework for these silcretes are commonly obscured by the quartz cement deposited over them. However, some silcretes retain their original quartz pebble texture through the subsequent erosional processes.
The slow rise of the Raggedy Range over the past million years has resulted in erosion of the overlying quartz-rich sediments, and some of the underlying schist bedrock. Erosion of the bedrock formed small steep-sided gullies that characterise the lower slopes of the range. These gullies are more prominent on Tucker Hill at the southwestern end of the Raggedy Range, where the faults along the valley margin have caused steeper topography. Gravel fans have formed at the foot of these gullies, and gravels partially fill the floors of the gullies along the range margins. The gravels contain abundant schist debris, much of it slabby and angular. In addition, the gravels contain some round quartz pebbles that originated in the 20 million year old quartz gravels. Gold from the quartz veins in faults in the schist bedrock, and gold from the quartz gravels, ends up in the young gravels in the gravel fans at the base of the rising slopes. Gold is particularly concentrated at or near the base of the young gravels, often right at the underlying schist bedrock surface in channel floors. In this basal zone, large bedrock boulders (up to metre scale) have accumulated, accompanied by larger silcrete boulders from the 20 million year old sediments.
The Manuherikia River flows over schist bedrock for most of its course. However, the source of the river is in greywacke mountains to the northeast of St Bathans (*). Consequently, the present Manuherikia River is currently transporting abundant greywacke gravel from those source mountains. Greywacke forms hard rounded boulders, quite unlike the flaky, slabby schist. Further, most schist debris that reaches the Manuherikia River is rapidly broken up during river transport. Hence, even at Alexandra, where the Manuherikia River meets the Clutha River, the gravels in the river bed are dominated by greywacke, not schist. The schist fragments leave only pebbles of the quartz vein material that pervades most schist outcrops.
Few, if any, of the pebbles of schist quartz veins contain gold, although some rare gold-bearing quartz pebbles undoubtedly occur in places. Some gold flakes from the 20 million year old quartz gravel deposits make their way from those deposits, through the schist debris fans on the valley walls, into the main Manuherikia River channel. Hence, placer gold concentrations can build up on the bed of the Manuherikia River, and this gold was a target for historic dredging activity near Alexandra, over 100 years ago. Small accumulations of this gold build up and migrate during modern floods.
As the Manuherikia has been forced to flow along the southeastern margin of its valley, it has to continue to erode downwards as the schist ridges rise alongside it. This continuing uplift and downcutting has resulted in the river abandoning some of its older channels, with their gold-bearing, greywacke-rich, gravels. These old channels are being progressively uplifted higher above the present river level. The now-dry abandoned channels have been mined historically in numerous places between Ophir and Alexandra. The higher, older abandoned channels have also been eroded, with the schist bedrock and the 20 million year old quartz gravels, into the schist-rich gravel fans in gullies on the rising hillsides. Gold from the old Manuherikia River channels has been reconcentrated into some of these gullies as well.