Sediment-Hosted Lead-Zinc Deposits : A Global Perspective

Abstract

Sediment-hosted Pb-Zn depoSits contain the world's greatest lead and zinc resources and dominate world production of these metals . They are a diverse group of ore depoSits hosted by a wide variety of carbonate and siliciclastic rocks that have no obvious genetic association with igneous activity. A range of ore-forming processes in a variety of geologiC and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphiCenvironments. The depoSits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subtypes: Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX). Despite the "exhalative" component inherent in the term "sEDEX,"in this manuscript, direct evi- dence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The distinction between some sEDEX and MVT deposits can be quite subjective because some SEDEX ores replaced carbonate, whereas some Mvr deposits formed in an early diagenetic environment and display lam- inated ore textures. Geologic and resource information are presented for 248 deposits that provide a framework to describe and compare these depoSits. Nine of the 10 largest sediment-hosted Pb-Zn deposits are SEDEX. Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) depoSits and 15 MVT (excluding Irish-type) deposits. Despite the skewed distribution of the deposit size. the two de- posits types have an excellent correlation between total tonnage and tonnage of contained metal (Ph + Zn), with a fairly consistent ratio of about 1011, regardless of the size of the deposit or district. Zinc grades are ap- proximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the mediao Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVf deposits compared to a subset of SEDEX depoSits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geolOgiC time shows that they are mainly a Phanerowic phenom- enon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerowic. MVT deposits dominantly formed in platform carbonate sequences typically located withinextensional wnes inboard oforogenic belts, whereas SEDEX deposits fonned in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions ofyears younger than their host rocks; however, a few are close « - 5 m.y. ) to the age o f their host rocks. In the absence o f direct dates for SEDEX deposits. their age of [annatian is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis. whereas some deposits fonned at least 20 m.y. after sedimentation. Fluid inclusion, isotopic stumes, and deposit modeling suggest that MVf and SEDEX deposits fonned from basin brines with similar temperatures of mainly 90° to 200°C and 10 to 30 wt percent NaCI equiv. Lead iso- tope compositions for MVf and SEDEX deposits show that Ph was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide criteria that distinguish MVT from SEDEX subtypes. How· ever, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVT suI· fur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVT ores extend over a wider range .md include more data with negative isotopiC values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deJXlsits is their de· positional environment, which is detennined by their respective tectonic settings. The contrasting tectonic set- ting also dictates the fundamental deposit attributes that generally set them apart, such as host·rock lithology, deposit morphology, and ore textures. Brief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVf group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no Significant differences in grade tonnage values between MVT deposits and the s·ubset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the Irish ores commonly ascribed as uniquely Irish type is in fact no different from several MVf deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain significantly higher contents of Ag and Ph relative to SEDEX deposits. In tenns of meWan values, Broken Hill-type de- poSits are almost three times more enriched in Ag and one and a half times more ennched in Pb compared to other SEDEX deposits. Metamorphism is a characteristic feature but not a prerequisite for inclusion in the Broken HiIl·type category. and known Broken HilI·type examples appear to occur in Paleo- to Mesoprotero- zoic terranes. Broken Hill.type depoSits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category o f SEDEX deposits .