In some cases, mineral resource data were available only for total district resources, but not for individual mines within the district. For these districts, the resource data are presented in appendix table A2 of the report. There is significant variability in the resource data for these deposit types, and ore controls vary from one region to another. Therefore, grade-tonnage estimations are best evaluated as subsets of the data in appendix table A1 where local mineralization styles and ore controls characterize the region being evaluated for grade-tonnage relations. Furthermore, consideration should also be given to the tendency for MVT resources to occur in large mineralized regions.
Criteria used to classify the deposits and districts as MVT versus CD in appendixes A and B were based on the classifications assigned to the deposits in the literature and the opinions of the authors that relied on personal observations of the deposits or, in many cases, on descriptions of the geological setting and lithology of the ore-hosting sedimentary rock sequences. Six deposits are included in appendix A as “Unclassified” because the descriptions of the tectonic setting and host rock sequences were insufficient to allow confident discrimination between the two major types of Pb-Zn deposits.
The resource information for the deposits is limited to publicly accessible resource information from sources cited in appendix tables A1 and A2. Some deposits and districts are not presented in the compilation (for example, Central Missouri and Northern Arkansas districts, U.S.A.), because publicly accessible resource information was not available for a variety of reasons. It should be noted that many factors (for example, metal prices, location, corporate policies, national politics, and so forth) influence the determination of the resource data in appendix tables A1 and A2. Furthermore, publicly available data (on which table A1 is based) are not necessarily the most recent. Therefore, the data in table A1, although considered to be the best currently available, do not necessarily reflect the true nature of mineralization in the ground.
Care must be taken with the usage of this data compilation because there are limitations to the data. Some resource data are old and have not been recently updated. Different deposits listed will be characterized by different metal cut-off grades in their definition of ore tonnages. Some deposits are still in the exploration phase and in the future are likely to have more accurate mineral resource estimates.
Many of the deposits do not have absolute mineralization ages listed, because of the difficulty of directly dating the ore minerals. Numerous papers have been published presenting dates of ore deposition, and careful consideration went into determining if the methods shown accurately reflect the age of ore formation, or something else. Dates deemed unreliable by the authors of this report have been excluded from this data compilation. Some deposits also have ambiguous or conflicting classifications reported in the literature. Caution was exercised in determining the correct deposit-type classification. Because this is a global compilation, aspects such as location, metal prices at the time of resource estimation, and regional politics all play roles in the resource estimates. Lastly, reporting of resource estimates is not as strictly controlled in some countries relative to others; therefore, overestimation of metal tonnages may characterize some deposits hosted in certain countries.
In this report, we do not include sandstone-Pb, sandstone-hosted Pb, or Pb-Zn vein districts such as those in Freiberg, Germany, or Coeur d’Alene, Idaho, because these deposits probably represent different deposit types. We do not include fracture-controlled deposits in which fluorite is dominant and barite typically abundant (for example, Central Kentucky; Hansonburg, N. Mex.) or the stratabound fluorite-rich, but also lead- and zinc-bearing deposits, such as those in southern Illinois, which are considered a genetic variant of carbonate-hosted Pb-Zn deposits. Many of the deposits do not have absolute mineralization ages listed, because of the difficulty of directly dating the ore minerals. Some deposits and districts are not presented in the compilation (for example, Central Missouri and Northern Arkansas districts, U.S.A.), because publicly accessible resource information was not available for a variety of reasons.