Giant Mesothermal Gold Deposits

The majority of world metal and fuel resources are contained in a very small proportion of the total number of world deposits that represent the upper end of the size spectrum of deposits. To survive in the increasingly competitive resource industry, mining companies must focus on finding or acquiring deposits of this type. The purpose of this workshop is to attempt to identify geological features that are characteristic or diagnostic of these very large deposits and their regional and local geological setting. One possibility is that very large deposits are just abnormally large "normal" deposits, being generated by essentially the same ore-forming process. Alternatively, the ore-forming process which generates very large deposits may be significantly different. The concern here is not with the well-known size differences between deposits of different geological type that contain the same metal (i.e., the difference between porphyry copper deposits and volcanogenic massive sulfide deposits as copper resources).A number of different deposit types are being considered because there is very little useful quantitative data on the specific problem that we are addressing for any one deposit type. We hope that our lack of data on individual deposit types can be overcome by putting together parts of the puzzle from different deposit types. The paucity of data on the relationship between size and geological characteristics is unfortunate, but not surprising. Rarely is research concerned with widely accepted assumptions, no matter how important the implications of those assumptions. Few assumptions are more deeply ingrained in geology than the idea that ore-forming processes are independent of scale.In this introduction, four general issues will be considered that are pertinent to the topic that is the focus of this volume: 1. ) what is a mineral deposit? 2.) how is size to be defmed so that it approximates as closely as possible a purely geological variable (as opposed to a mixed geologic-economic variable)? 3.) how are polymetallic deposits, and deposits of different geological type to be compared? 4.) what are the geological factors, and the nature of their interaction, that control the size of ore deposits?A mineral deposit is defined as a single mineralized body, or a group of spatially associated mineralized bodies. In most cases, ore zones forming part of a mineral deposit are close enough to be developed and mined from one set of underground workings. In the case of deposits mined by open pit, however,