ECONOMIC GEOLOGY 441 geological environments, the oil and gas sector being a prime example Views on the factors that have controlled or generated mineral deposits (e.g., where in the world to look for kimberlite pipes?) are of paramount importance to cost-effective exploration Many small specialized groups undertake exploration on this basis Their successes may be sold to large companies, which have the policy options of either buying their future mineral resources or organizing their own exploration teams on a long-term basis Exploration programmes generally follow phases of target definition, reconnaissance, selective followup, and detailed exploration of prospects A great and growing number of exploration technologies, including heavy mineral, geochemical and geophysical surveys, are available to supplement the essential art and science of sound geological mapping based on wide-awake observation At the detailed stage of exploration, physical methods are used, such as pitting, trenching and especially the various types of drillholes now available Mineral exploration is a high-risk enterprise justified by potentially high reward, but a historically low success rate overall Organizations with a relatively high success rate tend to be well managed, exploring in carefully researched and targeted areas, employing teams of well-qualified and motivated people to use the most cost-effective sequence of search technologies, with financial backing stable enough to ensure long-term effort The constant application of sound geological judgement to ensure that exploration data at all stages make geological sense, and especially when deciding whether to abandon a prospect or press on at higher cost and effort, makes disciplined mineral exploration a professionally rewarding activity (Figure 8) Feasibility Studies and Mine Development At some point enough is known about a prospect through physical exploration to either abandon it or proceed to further detailed exploration work A decision must then be made whether the prospect is worth the high cost risk of development into an operating mine This is the period of feasibility study, when a multidisciplinary team will rigorously test the prospect against all the criteria for full economic status The professional contribution from economic geology is the foundation stone upon which the entire edifice will be built, and confidence in the reliability of all geological data is essential The main geological information will include the following: Knowledge of the general characteristics of the type of deposit involved Complete mineralogical and chemical information, including grain size and distribution, and useful by-products or deleterious constituents, that affect amenability of the deposit to processing The three-dimensional geometry and depth of the deposit, the physical condition of the ore and its wall rocks, and other factors affecting amenability to mining methods A set of clear records including reports, geological and other survey maps, drillhole data and logs, and assay data Mineral resource and reserve estimates, including estimates of the degree of error attached to such basic data as sampling methods, sample preparation and assays, bulk density of ore and gangue types, the range and distribution of values within the deposit, and the continuity of mineralization between drillholes or other exposures The process is illustrated by some geological aspects of the case history of the Lihir gold mine, situated in a volcanic caldera on the island of Lihir, Papua New Guinea (Figure 9) After years of encouraging physical exploration, in 1985, the epithermal pyritic gold ore failed the test of amenability to conventional processing Tests on an alternative advanced technology worked well, but meant that Figure McArthur River, Queensland, Australia A large stratiform pyritic Zn Pb sulphide deposit (A) The gossanous discovery outcrop, broken open to reveal white secondary Pb and Zn minerals (B) Polished surface (about 20 mm across) of very fine grained stratiform sulphide ore The micron grain size of the base metal sulphides in this large and easily mined deposit made it too difficult and expensive to process by conventional technology Photographs: GR Davis