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Three types of copper deposits porphyry, strata-bound, and massive sulfide are described, and the distributions of tonnages and grades for 267 deposits are compared with normal and lognormal frequency distributions. The relationships between grades and tonnages are analyzed by examining the correlation coefficients of these variables. Conclusions reached include the following: (1) Geologic factors influencing tonnage of a particular deposit type are probably distinct from those influencing grade; (2) frequency distributions of tonnages and grades approximate lognormality, making it possible to predict probability of various tonnage-grade classes and to test correlation between variables; (3) no significant correlation was found between tonnage and grade for porphyry or strata-bound deposits; (4) significant negative correlation between tonnage and grade was found for the massive sulfide subset, probably reflecting a mixture of high-grade low-tonnage massive ores, low-grade high-tonnage stockwork, and disseminated ores characteristic of some massive sulfide deposits; (5) significant negative correlation was found between tonnage and grade for the mixture of deposit types in the whole sample. Extrapolation on the basis of the negative grade-tonnage correlation shown for the mixed population seems to imply the occurrence of large-tonnage very low grade deposits, which could be important in supplying future copper needs. This extrapolation is misleading, however, for both statistical and geological reasons. Furthermore, large-tonnage very low grade deposits in the porphyry class of the population are found to be very rare occurrences. ACKNOWLEDGMENTS This study was greatly helped by discussions with and data supplied by G. H. Goudarzi of the U.S. Geological Survey and various industry geologists. PORPHYRY COPPER DEPOSITS Disseminated copper deposits, genetically related to felsic or intermediate intrusive igneous rocks, are called porphyry copper deposits. Copper minerals Al

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______________________________ 1 General geologic and economic considerations concerning Introduction ____________________________ 1 controls on deposit grade and tonnage ____________ Acknowledgments _________________________ 2 Grade and tonnage analysis ____________________ Sulfide deposits __________________________ 2 Nickel sulfides associated with komatiitic rocks ______ Deposits in stable tectonic settings _____________ 2 Nickel sulfides associated with small intrusions ______ Deposits in areas that have been tectonically active ____ 2 Comparison of nickel sulfide deposits in small intrusions Komatiites _________________________ 2 with those associated with komatiitic rocks ______ Small intrusions _____________________ 3 Nickel-laterite deposits ____________________ 11 General characteristics ____________________ 3 Summary and conclusions _____________________ Nickel-laterite deposits _____________________ 3 References cited _________________________