Mississippi Valley Type Mineral Deposits

1. Mississippi Valley TypeMineralDeposits

3. Why Study Mineral Deposits? • History & Economics. Mining was a vital part of early Midwest • economy: • 1. Lead mining began in the Upper Mississippi Valley about 1820. • 2. Mineral Point was founded in 1827. • 3. Territory of Wisconsin was created in Mineral Point in 1836. • 4. First Territorial Governor (later US Senator) was Henry Dodge, • a mine owner from Mineral Point. • B. Science. • 1. Minerals are chemicals. • 2. Mineral deposit formation involves chemical and • physical processes that concentrate minerals. • 3. Studying ore deposits is like working in an outdoor chemistry • laboratory

4. What are Mississippi Valley Type Ore Deposits? A. Deposits of Lead (Galena (PbS)), Zinc (Sphalerite (ZnS)), and Copper (chalcopyrite (iron/copper sulfide)) in sedimentary rocks (commonly carbonates). B. Host rocks are often significantly older than ore minerals. C. Generally there is no nearby igneous source of heat or fluids. Why is this important? 1. Mineral deposit components must be transported to a site, then precipitated. 2. Most substances are more soluble (more easily transported) in hot fluids than in cold ones. D. Various mineral “thermometers” indicate that deposition took place at relatively low temperature (a little over 100°C) and at only a km or so depth.

5. In detail, the chemistry of Mississippi Valley Type (MVT) is fairly complicated. A. Did the metal ions and the sulfur travel together? 1. Objection: PbS and ZnS are very insoluble. 2. However, perhaps the sulfur was transported as sulfate ion (SO4-2) and reduced by organic matter near the deposits. 3. Or, maybe lead and sulfide formed as a soluble complex so that they could travel together. B. Perhaps a sulfide-rich solution mixed with a metal-rich solution at the site of deposition. Problem: There are matching precipitation bands over long distances. C. No model fits all the observations comfortably. D. But, the deposits EXIST, therefore there must be an explanation!

6. How the Mineral Components Got There Mineral Deposit Source Region

7. basinal brine hypothesis According to the basinal brine hypothesis of ore formation, hot saline fluids similar to oilfield brines migrated out of sedimentary basins and along aquifers, eventually forming ore deposits in sedimentary host rocks at distances of the order of 100 km from the basins. This hypothesis explains why the major element compositions, high salinities, D/H and 18O/16O ratios and temperatures of Mississippi Valley-type ore-forming fluids are remarkably similar to those of oil-field brines found in present-day sedimentary basins

8. MVT ConceptText Fig. 3.33

9. GENESIS OF MISSISSIPPI VALLEY-TYPE LEAD-ZINC DEPOSITSDirnitri A. Sverjensky The most important characteristics of Mississippi Valley-type deposits are the following: 1. They occur principally in limestone or dolostone that forms a thin cover over an igneous or highly metamorphosed Precambrian basement. 2. They consist of bedded replacements, vuggy ores, and; veins, but the ore is strongly controlled by individual strata. 3. They contain galena, sphalerite; pyrite, marcasite, fluorite, barite, chalcopyrite, dolomite, calcite, and quartz. 4. They are not associated with igneous rocks, except in the case of the Kentucky-Illinois district. 5. They always occur in areas of mild deformation, expressed in brittle fracture, broad domes and basins, and gentle folds. 6. The ore is never in the basement rocks, but its distribution is often spatially related to basement highs, with the ore located within sandbanks, ridges, and reef structures that surround the basement highs. 7. The ore is at shallow depths, generally less than 600 m relative to the present surface, and was probably never at depths greater than about 1500 m. 8. There is always evidence of dissolution of the carbonate host rock, expressed by slumping, collapse, brecciation, or thinning of the host rock, that provides clear proof that the ores are epigenetic.

10. More! (Just note that there are some pretty “cool” ways of studying these rocks) 9. The carbon and oxygen isotopic compositionsof the host rocks are normal for such rocks but are lowered adjacent to ore, which suggests that the host rocks were recrystallized in the presence of a fluid. 10. Fluid inclusions in sphalerite, fluorite, barite, and calcite always contain dense, saline, aqueous fluids and often oil and/or methane. The total dissolved salts range from 10 to 30 wtg/o and are predominantly chloride, sodium, and calcium, with much smaller amounts of potassium and magnesium. Homogenization temperatures are generally in the range 50-200C. 11. Reconstruction of the total sediment thickness over the ore, together with normal geothermal gradients, suggests temperatures much lower than the fluid inclusion homgenization temperatures. 12. The hydrogen and oxygen isotopic compositions of the water in the fluid inclusions are similar to those of the pore fluids in sedimentary basins. 13. The ranges of sulfur isotopic values and the degree of approach to isotopic equilibrium between sulfides are different for each district. In some districts, the source of the sulfur could not have been magmatic and thus must have been sedimentary. 14. The isotopic composition of the lead in galena is extremely radiogenic and thus yields future model ages, which suggest sources in the upper crust. The lead isotopic values are often zoned across whole districts, within individual deposits, and even within single crystals of galena; such zoning suggests multiple sources of lead, a long period of mineralization, or both.

11. Tectonic setting and other factors (Ref: Introduction to Ore-Forming Processes, Laurence Robb, Blackwell, 2004) I. “The majority of MVT deposits worldwide formed in the Phanerozoic Eon, but more specifically, in Devonian to Permian times [related to formation of Pangea]….; some deposits… also formed during the Cretaceous-Paleogene period [related to the Alpine and Laramide orogenies]… associated with compressional tectonic regimes.” II. Other factors A. Carbonate host rocks in hydrologic contact with orogenic belts B. Low latitude (at time of formation) 1. High rainfall (at time of formation) 2. Association with sabkhas producing high salinity solutions C. Transport problematic (It is difficult to pinpoint where the MVT materials came from and how they travelled.)

12. Additional material

13. Geochemical models for transportation and precipitation of metals and sulfur

14. Questions to resolve 1. What were the mechanisms of fluid flow and the pathways during migration? 2. How long did the flow systems persist, and how much fluid passed through the site of ore deposition? 3. Did the brines become ore-forming fluids before, during, or after migration? 4. What were the sources of the ore-forming constituents, their mechanisms of transport, and their concentrations in the brines? 5. What chemical reactions were responsible for the precipitation of the sulfide ore minerals? McLimans study of Upper Mississippi Valley deposits: Relatively high fluid inclusion temperatures (to 220C) Distinctive color bands in sphalerite traceable for distances of km in some cases C. Repeated deposition and dissolution of sphalerite McLimans et al used B. and C. to argue for a solution that carried both metals and sulfur (rather than mixing at the site). [Models II. or III. of Table]

15. Relationships between aquifer lithologies, states of saturation of migrating fluids, and metal abundances in resultant ores

16. Fletcher Mine, Viburnum TrendText Fig. 2, p.209

17. Summary of the characteristics of three Mississippi Valley-type districts

18. Age and duration of Mississippi Valley–type ore-mineralizing events M. T. Lewchuk & D. T. A. Symons The ore-magnetization ages for the six districts span >150 m.y., and these ores are emplaced in sediments that are as much as 200 m.y. older, but within each district the host-ore [magnetization] pairs differ by only a few million to a few tens of millions of years. This suggests a genetic link between host-rock remagnetization and ore precipitation. Fluid-inclusion data and conodont alteration indices (Sangster et al., 1994) for the host rocks indicate mineralization or later temperatures that are far too low to thermally reset an existing remanence (Pullaiah et al., 1975). Thus, any remagnetization of the host rocks must have been the product of chemical interaction involving fluids. The distinct host-rock and ore characteristic remanent magnetization directions indicate that the fluid responsible for the host-rock remagnetization did not simultaneously precipitate the ore minerals. Either the fluid and/or its environment changed or a second fluid passed through each district.

19. Mean pole positions, plotted on Paleozoic apparent polar wander path 1990), for ores (circles) and host rocks (diamonds) from six North American MVT ore districts. Stars indicate published mean poles for those districts where new data differ. Su, Dl, Dm, Du, Cl, Cu, Pl, Pu, and Trl refer to Late Silurian through Early Triassic periods.

20. St. Peter Sandstone unconformity. ‘‘St. L.’’ represents the St. Lawrence group and ‘‘T.C.’’ represents the Tunnel City Formation. The vertical channel depicts a paleo-river valley later filled in by the St. Peter Sandstone (after Heyl et al., 1959; Ostrom, 1967; Arnold et al., 1996). This area was never buried more than 1 km. The project, to test the formation of quartz overgrowths, indicates that these were unrelated to MVT ore deposition; however, the article summarizes MVT literature for UMV.

21. The model of MVT fluids envisioned by Arnold et al. (1996) predicts that overgrowths increase in d18O by about 9‰ if fluid composition is approximately constant and temperatures decrease from 110C in the south to 50C in the north. Bottom line: Overgrowths Are low T Meteoric.

22. SULFUR ISOTOPES FROM MISSISSIPPI VALLEY-TYPE MINERALIZATION IN EASTERN WISCONSIN John Lucjaz

23. INTRODUCTION TO GEOENVIRONMENTAL MODELS OF MINERAL DEPOSITS R. R. Seal II, Nora K. Foley, and R. B. Wanty

26. Naturally occurring isotopes of lead • Isotope Atomic mass Natural abundance (ma/u) (atom %) • 204Pb 203.973020 1.4 • 206Pb 205.974440 24.1 • 207Pb 206.975872 22.1 • 208Pb 207.976627 52.4

27. SULFUR ISOTOPES FROM MISSISSIPPI VALLEY-TYPE MINERALIZATION IN EASTERN WISCONSIN John Lucjaz

28. Mineral Stability vs pH, fO2Text Fig. 3.34