Mineral resource assessment of the West Coast region, New Zealand

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Christie, A.B.; Barker, R.G.; Brathwaite, R.L. 2010 Mineral resource assessment of the West Coast region, New Zealand Lower Hutt, N.Z.: GNS Science. GNS Science report 2010/61 238 p. + 1 CD

Abstract: The West Coast region of the South Island is endowed with a wide variety of geological resources as a result of a complex geological history from early Paleozoic to Recent. Gold, coal, aggregate, limestone, and rip rap are the most important resources currently extracted. Additional commodities mined at present or in the past include antimony, clay, dimension stone, mica, greenstone (pounamu), silica sand, and silver. Other commodities of potential interest include feldspar, fluorite, garnet, molybdenum, monazite, serpentinite, titanium, tungsten, and zirconium. There are also occurrences of asbestos, barite, beryllium, chromium, copper, lead, nickel, talc-magnesite, tin, uranium, and zinc. Gold production began with the gold rushes of the 1860s. Approximately 240,000 kg of gold have been mined from fluvioglacial and beach placer deposits, predominantly of Pleistocene age, and about 100,000 kg have been produced from auriferous quartz veins in Ordovician greywacke. Current mining of the placer deposits produces more than 300 kg (possibly >1000 kg) of gold annually from c. 35 small operations and the Grey River dredge, and hard rock mining at Globe-Progress near Reefton produces c. 2,700 kg of gold annually. The West Coast coal region comprises the Greymouth, Buller, Pike River, Reefton and 10 other smaller coalfields that have produced more than 120 Mt to date. Coal mining began in 1864 (Greymouth Coalfield) and production has been continuous to the present, amounting to more than 55 Mt from Paparoa (Late Cretaceous-Paleocene) and Brunner (Late Eocene) coal measures. Buller and Greymouth coalfields produce all of New Zealand's bituminous coal, whereas production from other areas within the region is of sub-bituminous coal. Current annual production is around 2.5 Mt and estimated recoverable coal resources are more than 200 Mt. Aggregate sources are abundant and most of the 0.2 Mt annual production is from pits working greywacke and granite gravels in Quaternary fluvioglacial deposits and modern river channels. A few hard rock quarries work greywacke, granite or limestone. Large deposits of Oligocene limestone are present. Current annual production is about 0.93 Mt in 2006, the latest year for which detailed statistics are available: 840,000 t for cement making, 90,000 t for agricultural lime and 3000 t for industry and roading. Extensive deposits of Holocene ilmenite-rich sand are present along the coast with resources of 175 Mt estimated for the Barrytown-Twelve Mile, Nine Mile, and Carters deposits. Accessory minerals include garnet, gold, zircon, monazite, rutile, and cassiterite. Barrytown is the most intensively explored deposit and has resources of 50 Mt of sand at an average grade of 13.8% ilmenite (equivalent to 6.9 Mt of ilmenite), 0.2% zircon, 100 mg/m3 gold, and less than 0.1% each of monazite and rutile. Large deposits of ilmenite are also present at Westport. Kaolinitic clay, for producti on of fire bricks and building bricks, has been worked in the past from deposits in coal measures, deeply weathered basement rocks, and Tertiary mudstone units (e.g. Blue Bottom Group). Sandstone with a high silica content (ganister) is present in coal measures of the Buller, Charleston, and Greymouth coalfields, and silica sand occurs at Little Totara River and Ross. Granitoid intrusive rocks and their contact zones in the metasedimentary country rock contain mineral occurrences of fluorite, scheelite, barite, chalcopyrite, galena, molybdenite, cassiterite, and sphalerite; pegmatite dikes at Charleston contain beryl, feldspar and mica; and ultramafic rocks have asbestos, chromite, greenstone, nickel, serpentinite, and talc. Resources of rock aggregate, sand and limestone have not been quantified, but are estimated to be large and sufficient to meet foreseeable local demand. Previous estimates have been used for in-ground resources of sub-bituminous and bituminous coal (818.65 Mt) and for ilmenite beach sands (approximately 30 Mt of ilmenite). Other potential mineral resources have been estimated using a three step process involving mineral deposit models, a geographic information system (GIS) of spatial data sets, and a counting method of assessment. In total, estimates have been made for 28 metallic mineral deposit models and for 10 non-metallic mineral deposit models. There has been detailed geological mapping over the region, but no detailed airborne geophysical surveys or regional geochemical surveys have been undertaken. These types of surveys would provide new data that may lead to the discovery of new mineral deposits. Scenarios are proposed whereby the value of the West Coast's annual mineral and coal production could increase from the current estimated NZ$602 million to NZ$866 million, by increasing production of individual commodities to past annual maximums (between 2000 and 2009, except 1995 for placer gold). (auth/DG)