320 



UNITED STATES MINERAL RESOURCES 



tures, or by bioherms and associated sedimentary 

 and organic features. Typically, ore does not occur 

 on faults of large displacement, although it common- 

 ly occurs adjacent to them. The prevalent occurrence 

 of the ore bodies in carbonate rocks is probably re- 

 lated to the tendency of these rocks to fracture 

 readily and to produce breccias, to develop zones of 

 secondary porosity and even karst environments dur- 

 ing periods of preore ground-water movement, and 

 to be highly soluble and reactive. Many individual 

 ore bodies form runs of ore that are several hundred 

 feet wide and 100-200 feet high and have been fol- 

 lowed for a mile or more, each containing several 

 million tons of ore. In the Southeast Missouri lead 

 district the average grade of ore in the epigenetic 

 sti*atiform deposits is 3-8 percent lead, 0.5-1 percent 

 zinc, and 0.10 percent copper; nickel, cobalt, cadmi- 

 um, silver, germanium, and indium are present in 

 minor amounts. In northeastern Morocco, similar 

 deposits contain 5-7 percent lead in the form of 

 galena and also contain sphalerite, chalcopyrite, and 

 secondary dolomite. 



A few of the larger and better known districts con- 

 taining strata-bound deposits of epigenetic origin 

 include the Southeast Missouri (Snyder and Gerde- 

 mann, 1968), Tri-State (McKnight and Fischer, 

 1970; Brockie and others, 1968), Upper Mississippi 

 Valley (Heyl, 1968), and Metaline (Dings and White- 

 bread, 1965) districts in the United States, the Pine 

 Point deposit in Canada (Campbell, 1967), Laisvall 

 in Sweden (Grip, 1950, p. 367-369), Mechemich- 

 Maubach and other districts in West Germany (Beh- 

 rend, 1950), Silesia-Cracow in Poland (Galkiewicz, 

 1967 ; Gruszczyk, 1967) , Touissit-Bou Beker and oth- 

 er areas in Morocco (Agard, 1967; Jouravsky and 

 others, 1950), and Mirgalimsai, Kazakhstan (Shcher- 

 ba, 1971, p. 173). Other countries that contain epi- 

 genetic strata-bound lead deposits include Spain, 

 France, Belgium, Austria, Italy, and Tunisia. Similar 

 districts here believed to have emerging and potential 

 importance as lead-producing areas in the United 

 States include the Kentucky-Illinois and central Ken- 

 tucky districts and the recently discovered central 

 Tennessee district. Older districts of the same gen- 

 eral type that may continue to be important sources 

 of lead include the Alpine lead-zinc deposits of 

 Europe and the lead and zinc deposits of the Appa- 

 lachian Valley of the Eastern United States. 



Continuing detailed studies of the epigenetic 

 stratiform deposits throughout the world indicate 

 that although these deposits are similar in size and 

 geological character, they differ in important geo- 

 chemical details. In general, they may be divided into 

 three types: (1) the Upper Mississippi Valley type. 



(2) the Alpine type, and (3) the stratiform or strata- 

 bound deposits of normal replacement origin. The 

 Mississippi Valley type deposits are named from the 

 extensive lead and zinc ore bodies in the Tri-State, 

 Southeast Missouri, Upper Mississippi Valley, and 

 other districts in the Central United States but are 

 also recognized in Europe and elsewhere. As stressed 

 by Brown (1970, p. 116-117), they are characterized 

 by isotopically anomalous lead of the J-type, which 

 contains an excess of the three radiogenic compo- 

 nents Fb-'"^, Pb-", and Pb-°^ indicating a "future" 

 age. In ore bodies from which an appreciable number 

 of analyses are available, the J-type lead has a fairly 

 wide range in its isotopic character, indicating that 

 the solutions from which it was deposited were not 

 magmatic in origin. Isotopic analyses of the sulfur in 

 the Mississippi Valley type deposits show that it is 

 within the range of sulfur of biogenic origin, the sul- 

 fur of connate brines, and normal depositional sul- 

 fides in sedimentary rocks. 



Studies of fluid inclusions in the sulfide and gangue 

 minerals of the Mississippi Valley type ore bodies 

 suggest that they were deposited from metal-rich 

 brines of moderate salinity at temperatures of 70°- 

 150°C. These brines are of the Na-Ca-Cl type — 

 about five to 10 times as concentrated as sea water — 

 and closely resemble the connate waters found in oil 

 fields in adjacent sedimentary basins. The source of 

 the metals is unknown ; the sources favored by most 

 geologists who have worked in the Mississippi Valley 

 districts are the rocks and saline deposits of the 

 sedimentary basins or, in particular, the igneous and 

 metamorphic rocks of the crystalline basement be- 

 neath the ore-bearing strata. The tendency for many 

 of the deposits to be localized in stratigraphic traps 

 on the flanks of broad domes suggests that during a 

 period of minor tectonic deformation, connate brines 

 containing metals and other components of the ore 

 bodies that had been leached from adjacent rocks 

 migrated upward along new and preexisting con- 

 duits. Ore deposition then followed as a result of 

 chemical and physical changes in the brines when 

 they reached the sites of ore deposition. 



The Alpine lead deposits are similar in many re- 

 spects to the Mississippi Valley type deposits but 

 differ from them in containing B-type lead, whose 

 isotope composition indicates an age older than that 

 of the rocks and structures that enclose the ore 

 bodies. As noted by Brown (1965, p. 65), the isotope 

 composition of the lead in a large number of strati- 

 form deposits in central Europe indicates an essen- 

 tially uniform Devonian age, although the ore bodies 

 occur in strata ranging in age from Devonian to Tri- 

 assic. Such leads may have been carried by connate 



