Compilation of mineral resource data for Mississippi Valley-type and clastic-dominated sediment-hosted lead-zinc deposits (original) (raw)

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. Clastic-Dominated and Mississippi Valley-Type Lead-Zinc Deposits Classifications of the SH Pb-Zn ores in Leach and others (2005) were organized around traditional subgroups-MVT and SEDEX deposits-and were further subdivided based on classifications in the literature. A fundamental concern with the genetic-model-based classification of "SEDEX" is that it imparts an inherent "exhalative" genetic component to deposits. Most deposits classified as SEDEX lack unequivocal evidence of an exhalite in the ore or alteration component. Consequently, the presence of laminated sulfides parallel to bedding is commonly accepted to be permissive evidence for exhalative ore. However, some deposits traditionally classified as SEDEX did not form from sulfide exhalites. In this report, we avoid process-related, interpretive-and model-driven features to classify the deposits. Deposits are instead characterized by the nature of the sedimentary sequences and their interpreted tectonic environment within which the ores formed. This approach uses the relation that ores classified as SEDEX in Leach and others (2005) are hosted in clastic-dominated sedimentary rock sequences in mainly passive margin, continental rifts and sag basins. We use the term "clastic-dominated lead-zinc" (CD Pb-Zn) for these deposits and avoid genetic and temporal (for example, syngenetic, diagenetic, syn-diagenetic) attributes to the deposits. The ores can be hosted in shale, sandstone, siltstone, mixed clastic units, or as carbonate replacement ores within a clastic dominated sedimentary rock sequence. The CD deposits may be further subdivided based upon specific tectonic or geologic settings in which the deposit formed, which include passive margins (PM), continental rifts (RF), continental sag basins (CS), and back-arc basins (BA). An alternative classification of BHT (Broken Hill-type) is listed for some deposits, a subtype with unique characteristics similar to the Broken Hill, Australia, deposit (Leach and others, 2005). We retain the traditional term of MVT Pb-Zn for sediment-hosted Pb-Zn deposits in carbonatedominated platform sequences because this terminology does not include a genetic component. Although the traditional use of the term MVT does imply a broad time component of simply being epigenetic with respect to its host rocks, we recognize that some MVT ores may have a syngenetic, diagenetic, or burial metamorphic temporal component to deposit or ore district formation. The most important characteristic of MVT deposits is their location, mainly hosted in dolostone and limestone in platform carbonate sequences and typically located at flanks of basins, orogenic forelands, or foreland thrust belts inboard of the clastic rock-dominated passive margin sequences. They have no spatial or temporal relations to igneous processes, which sets them apart from skarn or other magmatic Pb-Zn ores. Many subtypes or alternative classifications have been applied to MVT deposits since their inception as a distinct ore type by Bastin (1939). These alternative classifications reflect geographic and/or specific geological features that some workers believe set them apart as unique (for example, Appalachian-, Alpine-, Reocin-, Irish-, and Viburnum Trend-types). However, we do not consider these alternative classifications or subtypes to be sufficiently distinct to warrant using them in this report. Limitations of the Data Compilation 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 3 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 cutoff 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. Data Fields The attributes within the tables are defined below.