Iron-rich and iron-poor Mississippi Valley-type mineralization, Metaline district, Washington

Two texturally and mineralogically distinct types of Mississippi Valley-typ e mineralization are hosted by the upper Cambrian Metaline Formation in nor theastern Washington. Josephine-type mineralization is iron poor and coarse grained, whereas Yellowhead-type mineralization is iron rich, fine grained , and commonly botryoidal. Josephine mineralization is found largely in the Josephine lithofacies, a fragmental unit near the contact between dolomite s of the Metaline Formation and shales of the overlying Ledbetter Formation . Yellowhead mineralization forms several planar zones several hundred mete rs below, largely in the light-gray bedded dolo-stone lithofacies of the Me taline Formation. About 12.7 million tonnes (Mt) of Josephine-type ore (1.3 % Pb, 3.0% Zn) and 730,000 t of Yellowhead-type ore (4.5% Zn, 0.5% Ph) were produced from the district before 1997, and about 5.5 Mt of ore (7.2% Zn, 1.3% Pb) remain, largely in the Yellowhead zone. Ore textures in both types of mineralization indicate a complete lack of eq uilibrium among adjacent miner als, and this is confirmed by geochemical me asurements. Although fluid inclusion homogenization and freezing temperatur es are similar for both types of ore, fluid inclusion leachate and gas anal yses differ for each mineral. Compositions for Josephine minerals appear to form a cluster that differs from compositions for Yellowhead minerals, sug gesting that ore minerals were deposited from two parent brines that change d composition slightly to deposit each mineral. Isotopic analyses indicate that sulfur in both types of mineralization was probably derived from the s ame source whereas lead definitely came from two distinct sources. These ob servations indicate that ore formed when two parent metal-rich brines mixed with the same source of sulfur. Abundant pyrite that retains the form of m arcasite suggests that the original brine in at least the Yellowhead ores w as moderately acid. Geochemical speciation calculations indicate that relat ively high temperatures are needed to dissolve enough iron to make iron-ric h Mississippi Valley-type deposits. Furthermore, moderately acid brines wit h relatively low sulfur contents are enriched in iron whereas those with hi gh sulfur contents lack iron, suggesting that the variations in iron conten t of these deposits reflect variations in the dissolved sulfur content of t he parent brines.