Geology of the Coromandel region with emphasis on some economic aspects

Abstract

An account is given of the early geological exploration and mining history of the Coromandel Goldfield. The writer's interpretation of the basement and igneous geology, and of the metallogenesis of gold-silver and base metal ores has resulted in radical changes. The original threefold sub-division of the Jurassic sedimentary basement rocks (Manaia Hill Group) has been condensed to two formations (Tokatea Hill and Manaia Hill) on the basis of their detrital content and structure; standard sections are described. The older (Manaia Hill Formation) is characterised by lithic volcanic greywacke- and subgreywacke-type sedimentary rocks, that were derived from a landmass comprising an assemblage of calc-alkaline volcanic and plutonic rocks, with minor sedimentary rocks. The deposit was formed by turbidity currents on a continental slope bordering a geosyncline. Secondary minerals diagnostic of low grade metamorphism of the Zeolite and Prehnite-Pumpellyite Facies occur locally in the Manaia Hill Formation. These not only include normal prehnite, laumontite-leonhardite, analcime, and actinolite, but also prehnite with anomalously low refractive indices. In contrast, the rocks of the younger formation (Tokatea Hill) are feldspathic greywackes almost devoid of volcanic detritus except near the formation boundary with Manaia Hill Formation. They have a sericiterich, authigenic matrix, but no zeolites or prehnite. The sediments were derived from a landmass of low relief composed of granodiorite-diorite plutonic rocks and minor sedimentary rocks. The lower parts of the formation are slope turbidites while the upper parts are the product of stable deposition in deeper water. The structure of the Manaia Hill Group is relatively simple over most of the Coromandel area. The beds have a north-west to north-north-west strike and are folded about axes with a similar trend; joint directions corroborate this conclusion. A north-east trend has been imposed on the rocks of the Moehau region by the intrusion of Paritu Quartz-diorite. Conformity between the two formations is shown by gradational detrital content and by structural continuity. In older descriptions, the volcanic rocks of Coromandel were subdivided into an older, gold-bearing, and a younger, barren series of andesites with minor dacites and rhyolites. By ignoring post-eruptional properties (i.e. hydrothermal alteration) a different view of the volcanic stratigraphy is demonstrated. The extrusive rocks are now subdivided into nine formations with no major regional break but with disconformity or unconformity between them caused by local quiescence. With the addition of two intrusive formations with volcanic affinities (Castle Rock Dacite and Kai-iti Porphyrites) all the volcanic rocks are included together as the Coromandel Group of Miocene age. The new effusive formations from oldest youngest are: Port Charles Andesite; Omoho Formation (rhyolitic); Kokumata Dacite; Te Karaka Andesite; Cousin Jack Andesite; Parakete Formation (dacite-andesite); Rauporoa Basaltic-andesite; Tuateawa Andesite; and Beesons Island Volcanics (andesite-dacite). The last formation has been considerably redefined since it is the sole surviving name from earlier published descriptions of the volcanic geology. Two zeolites (phillipsite and levyne) are described. Hydrothermal alteration and mineralisation cut across formation boundaries and are controlled by fault and shear zones. The writer, unlike earlier workers, recognises faults as important in the development of the geology. The fracture pattern in the basement rocks not only controls the regional fault pattern but also the fracture pattern in the volcanic rocks immediately overlying the basement. Stress patterns have been synthesised from structural analyses of faults, shears, veins, and gold-bearing lodes, which suggest that since the Mesozoic, a major compressional stress has been directed approximately north-east/south-west. A local secondary stress (north-north-west/south-south/east) is indicated during the mid-Tertiary. Major late Tertiary normal fault movement is explained by postulating horizontal tension relief by gravity following differential vertical movement along pre-existing faults, as a result of doming induced by intrusion and eruption of magma. A regional gravity survey has been tied to the known geology and structure. Models of possible mass distribution are computed for individual Bouguer anomalies. Major geological features are confirmed but stations are too spread to relate mineralisation with the gravity pattern. The results of a stream sediment reconnaissance geochemical survey of cold extractable and total copper, lead, and arsenic, are appraised in relation to known geological structural and historical features. The effects of pH, flocculants, and contamination are considered and the analytical methods reviewed. The standards of anomalous concentration levels are statistically calculated, and related to different possible sources of metal within a given terrain. Recommendations for further exploration and analytical work are made for those areas where possible economic mineralisation has been detected. In particular, the Moehau region near Paritu Quartz-diorite shows strong signs of copper enrichment. No examples of ore from any of the old mines were available; the study of the sulphide mineralisation has been confined to outcrops of veins, and samples from tip-heads. Although many of the samples show indefinite mineral relationships, the similarity of paragenetic sequence for all areas suggests that the deductions for the whole field are valid. Several minerals have not been previously recorded from Coromandel, notably wolframite, molybdenite, pyrrhotite and tetrahedrite. The structural control of mineralisation by the fracture pattern is emphasised, and also the absence of any direct genetic relationship to the volcanic rocks of the Coromandel Group. Instead, there is a zonal genetic relationship of the ore minerals of the Moehau region about Paritu Quartz-diorite. The similarity of the paragenetic sequence of other deposits to that of the Moehau region suggests similar sources, i.e. hydrothermal exhalations from consolidating subjacent granodiorite-type plutons. Whilst future economic prospects for Coromandel itself my depend on deeper workings approaching such bodies, the Paritu Quartz-diorite pluton could contain a more accessible porphyry copper deposit, with associated tungsten and molybdenum minerals.