Skinner, D.N.B. 1974 Aggregate resources of the Hauraki Region (Coromandel and Kaimai Ranges). Lower Hutt: New Zealand Geological Survey. Report NZGS 66. 48 p.
Abstract: Most of the rock quarried in the Hauraki Region ie the Coromandel and Kaimai Ranges, is andesite, although rhyolite is also used, particularly near Tauranga; 'greywacke' is used very little as it occurs only in the northern part of the peninsula where as yet there has been little demand. However, the reserves of 'greywacke' are large and generally accessible, and their extent is better known than most other rocks. Andesite will remain the major source and may have to be transported from the western side of the peninsula, via the Kaimai rail tunnel to Tauranga, a region of large demand but with very few high quality rock resources. Prospecting for andesite aggregate will need to be concentrated in the vicinity of areas of demand, as the bush cover, deep weathering and erosion obscure most of the available rock except in stream valleys and thus make a regional assessment difficult to complete. Rhyolite will continue to have an important use for less stringent civil engineering works near Tauranga, when used under controlled conditions. Ignimbrite and pumice will have limited use and then only if there is a lack of better material. Prospecting possible sites should, as a matter of economic necessity, involve core drilling across the area and petrographic examination of the cores to test overburden, fracture, weathering, mineralisation and alteration. All clay and deeply weathered rock should be rejected; hydrothermally altered rock should be well tested before quarrying commences; aggregate should be rejected if 'argillite' (mudstone) forms more than a few per cent of the available rock; zones of intense veining should be rejected if the predominant mineral is calcite or zeolite, but not necessarily if prehnite or quartz is the main vein mineral, unless the rock is so altered as to have become subject to deep weathering. As many, if not most, of the volcanic aggregates of the Hauraki Region have been hydrothermally altered and contain clays or opaline silica, they should be tested before using them with high alkali cements.