Abstract Single zircons from several porphyry dykes bracketing the time of formation of the Elatsite porphyry Cu–Au deposit (Bulgaria) were dated by high-precision U–Pb isotope analysis, using thermal ionization mass spectrometry (TIMS). On the basis of cross-cutting relationships, and the mineralogy and geochemistry of igneous and altered rocks, five dyke units are distinguished. The earliest porphyry dyke is associated with, and overprinted by, the main stage of ore-related veining and potassic alteration. U–Pb analyses of zircons yield a mean 206 Pb/ 238 U age of 92.1 ± 0.3 Ma, interpreted to reflect the time of intrusion. Zircons of the latest ore forming dyke, crosscutting the main stage veins but still associated with minor potassic alteration and veining, give an intrusion age of 91.84 ± 0.3 Ma. Thus, ore mineralization is confined by individually dated igneous events, indicating that the entire time span for the ore-forming magmatism and high temperature hydrothermal activity extended over a maximum duration of 1.1 Ma, but probably much less. Zircon analyses of a late ore dyke cutting all ore veins and hosting pyrite as the only sulphide mineral give a concordant 206 Pb/ 238 U age of 91.42 ± 0.15 Ma. Based on a spatial relationships of the magnetite–bornite–chalcopyrite assemblage with coarse-grained hydrothermal biotite and K-feldspar, a Rb–Sr age of 90.55 ± 0.8 Ma is calculated using the two K-rich minerals. This age is interpreted as a closing date for the Rb–Sr system at T ≈ 300 °C consistent with published K–Ar data. Therefore the entire lifespan of the magmatic–hydrothermal system is estimated to have lasted about 1.2 Ma. Soon after, the Cretaceous complex was exposed by erosion, as shown by palaeontologically dated (Turonian; 91–88.5 Ma) sandstones containing fragments of porphyry dykes. Geochemical discrimination ratios suggest a mixed mantle and crustal source of the Cretaceous magma. Isotope analyses of Sr, Nd and Hf confirm the conclusion that all porphyry rocks within and around the Elatsite deposit originate from an enriched mantle source at Cretaceous times, with crustal contamination indicated by moderately radiogenic Pb.

Abstract At the Cerro Verde district in southern Peru, granodiorite porphyry stocks formed two adjacent porphyry copper-molybdenum deposits that collectively form one of the largest copper districts in the world, with current resources of ~17 million metric tonnes (Mt) of copper. The district is located within the Coastal batholith of western Peru. The Coastal batholith in the Cerro Verde area consists of the Cretaceous Tiabaya and the Paleocene Yarabamba granodioritic plutons. Granodiorite porphyry stocks associated with the porphyry coppermolybdenum deposits were emplaced into the Yarabamba plutons. The granodiorite porphyry stocks are composite, steep-walled cylinders. Breccia bodies of diverse textures are localized in the apical parts of both stocks. The breccia fragments are predominantly of stock composition proximal to the intrusion and grade outward to heterolithic fragments of stock and Yarabamba wall rocks. Hydrothermal matrix breccia bodies are widespread, situated in the outer areas of the breccia column, and contain tourmaline, chalcopyrite, and molybdenite within the matrix. New zircon U-Pb dating confirms and refines earlier work, indicating that magmatism and mineralization at Cerro Verde occurred about 61 Ma. The hypogene mineralization is bracketed by the Yarabamba batholith host (~62 Ma), well-mineralized stocks at Cerro Verde and Santa Rosa (~61 Ma), and postmineral plugs (~60–59 Ma). The U-Pb ages are consistent with all crosscutting relationships. Each granodiorite stock is associated with similar sequences of alteration and mineralization. Biotite veinlets and halos containing chalcopyrite formed in the deeper areas of both deposits and are cut locally by later quartz veins and quartz-K-silicate veinlets containing chalcopyrite and molybdenite. Tourmaline-bearing sulfide veins with K-feldspar and chlorite envelopes form an inverted cup-shaped shell that overlaps the medial and upper parts of quartz-K-silicate veinlets. In distal positions, tourmaline veins contain sericite and are bordered by sericitic alteration. Quartz-sericite-pyrite veins and envelopes form an expansive stockwork in the upper part of the deposits and are transitional to and overprint K-silicate alteration. The Cerro Verde and Santa Rosa stocks formed individual copper and molybdenum ore shells within K-silicate alteration that forms thick-walled cylinders in the medial and upper parts of the deposit. The shells merge at depth to form one NW-SE–oriented mass that is 4.6 × 1.6 km in size using a >0.2% Cu value. Copper grades of 0.7 to 0.4% result from chalcopyrite-dominant veinlets occurring with chlorite in quartz-K-silicate alteration and are localized proximal to the stocks. The copper grades into a zone of 0.4 to 0.2% Cu within the biotitealtered zone at depth. Two discrete molybdenum ore shells are contained within the copper ore shell and are located proximal to the granodiorite stocks. The highest abundance of molybdenite is inward toward the stock and is zoned outward to lower grades. Breccia pipes contain abundant chalcopyrite and molybdenite within the matrix and are the source for the highest-grade ores in the district. The pipes truncate the majority of the veins containing copper, molybdenum, and tourmaline veins and bottom within quartz-rich K-silicate veinlets. Supergene mineralization consists of zones of leached capping, oxide copper mineralization, and an enriched chalcocite blanket developed above the copper ore shells within sericitic alteration. The oxide copper deposits contain isolated brochantite with chrysocolla in tourmaline breccia bodies situated above a laterally continuous and deposit-wide chalcocite enrichment blanket. Whereas molybdenum contents are little affected by supergene processes, copper and silver are generally leached from sericitically altered rocks and concentrated downward in the sulfide enrichment blanket by a factor of 1.5 to 2 compared to the subjacent protore. The oxide ores reflect in situ oxidation of a mature enrichment blanket hosted within rocks lacking abundant pyrite. The proximal location and approximately synchronous formation of the Cerro Verde and Santa Rosa porphyry copper-molybdenum deposits formed an expansive K-silicate alteration system and related ore shells that encompass both intrusive centers. In detail, multiple episodes of hydrothermal alteration and metal introduction can be inferred spanning no more than about 1 m.y. Although hydrothermal activity began with the emplacement of the Yarabamba granodiorite, most metals were introduced with the composite Cerro Verde-Santa Rosa stocks, and activity waned with formation of late breccias and ceased in late barren porphyries that truncate ore. These patterns are quite similar to those of other giant Andean porphyry systems, notably El Salvador, Los Pelambres, and Toquepala.

Abstract A review of representative metallogentic provinces brings out present trends in the interpretation of ore deposits and touches upon many major problems of ore deposition. The individual provinces vary in size from single mining districts to regions several hundred miles in extent; the limits usually are difficult to place. The great molybdenite deposits of Climax, Colorado, may be classed as a single molybdenite province or it may be considered as a part of the larger, more varied Front Range mineral belt. The ore deposits of the Peruvian Andes contrast with those of the Eastern Cordillera of Bolivia, yet gradations exist that suggest a certain unity to the mineralization. Some geologists describe the copper deposits of Northern Rhodesia and Katanga as a well-defined metallogenetic province, whereas others look upon these deposits as a part of the larger mineralized region that includes the uranium-copper-cobalt ores of Shinkolobwe and the copper-zinc ores of Kipushi. The association of the ores of Sudbury, Cobalt and Keweenaw Point with basic igneous rocks of late Precambrian age suggests a logical grouping of the deposits as a unit, although the processes of ore deposition may be quite distinct. Among the larger metallogenetic provinces that seem to be well established are those of the Coast Range of British Columbia and Alaska, the Sierra Nevada, and the copper belt of Chile and southern Peru. Of somewhat smaller size, the gold province of South Africa is more sharply defined. The age of ore deposition constitutes a major problem in many of the metallogenetic provinces under review. Some districts may permit a close dating of the ores and may give evidence of more than one metallogenetic epoch. In many districts, however, the age of the mineralization is fixed only within broad limits and is based largely on indirect evidence. This may lead to a difference of opinion as to the age of the mineralization and the number of epochs that are represented. In the San Juan region of Colorado, two distinct metallogenetic epochs are clearly established. In the Arizona-New Mexico province the deposition of copper took place during more than one metallogenetic epoch, although some question remains as to the exact age of certain deposits. The varied ore deposits of Idaho, grouped as a single metallogenetic province, illustrate the difficulty in the dating of ores. The principal deposits may be of Nevadan or of Laramide age; the ores of shallow type may represent one or two subsequent epochs of metallization. Studies elsewhere in the West suggest that the metallogenetic epochs are not so sharply defined as previously supposed. The close connection between basic magmas and the chromite, nickel-copper and titaniferous iron Ores is well established, although relatively few deposits are of strictly magmatic type. The possible connection between ore deposits and nearby granitic intrusions has been the subject of closer analysis. In the Precambrian districts of Canada, for example, the distribution of the massive sulfide deposits suggests that they are more closely related to the Keweenawan basic intrusions than to the Algoman granites. Ore deposits occurring in regions that show little evidence of intrusive igneous activity are difficult to explain. To account for these deposits, several theories have been stressed as alternatives to the hydrothermal theory and have been applied to the uranium ores of the Colorado Plateau, the copper deposits of Northern Rhodesia, the gold deposits of South Africa, and the iron ores of metamorphic terranes. Metallogenetic provinces outlined by deposits of sedimentary origin are well illustrated by the iron-formations of Precambrian age; there is substantial agreement that these formations represent chemical sediments. The possible correlation between epigenetic ore types and tectonic environments is being given increased attention. Characteristic of the highly metamorphic Precambrian terrane are deposits of high to moderate intensity, including gold-quartz veins, massive sulfide deposits, and various types of iron ores. Some of these can be duplicated in the more deeply eroded orogenic belts of later age, such as the Sierra Nevadas and Appalachians. In less deformed Precambrian rocks, ores of lower intensity prevail, as illustrated by the uranium, copper-cobalt and native copper deposits. In the younger mountain ranges, the ores are more varied and represent a wide temperature range, with many examples of lateral zoning. The ores correlated with the post-tectonic period of volcanism are perhaps the most distinctive: the gold-silver ores of the bonanza type, mixed base-metal ores, and uncommon metal combinations which reflect a high degree of telescoping. At the low-temperature end of the sequence are deposits characteristic of the stable regions: the lead-zinc, copper-cobalt, uranium, barite and fluorite deposits, many of which lie far from exposed igneous rocks. In many districts, careful study has revealed highly significant stratigraphic and structural controls in ore deposition that are not readily apparent. This is true not only of the complex metamorphic areas of the Precambrian and the highly deformed belts but also of areas of mild deformation where details of stratigraphy may be of utmost importance. The continued application of the principles of geochemistry, geophysics and geology, combining field and laboratory study, should be of value not only in determining the origin of the ores but also in explaining the metallogenetic provinces.

Many structural domes and anticlines rise from undeformed surroundings; the general crust of the earth is neither shortened nor extended by these local swells or blisters, but the area of the plate upfolded is increased. With plastic material the increase is effected largely by flow; with brittle material, largely by fracture. Many mining districts are associated with upfolds of this type. Districts here described fall into two categories: (1) those whose structural frame is a dome, and (2) those whose structural frame is an anticline. Domes may show fractures which radiate from the apex, or concentric fractures which are segments of circles, of varying diameters but with a common center, the apex of the dome. Both types may appear on the same dome; the fractures of each type aid enlargement of the plate during doming. Sunlight and Kirwin, Wyoming, are minor districts with dominant radial fracture patterns. Vein matter was deposited while the walls of the vein fractures were being pulled apart. With a radial vein system the only way in which all the vein walls could be simultaneously pulled apart is by stretching of the fabric of an expanding dome. The structural setting of the Ophir, Utah, lead-silver district, and of the Matehuala, Mexico, copper district is that of a half dome truncated by a normal fault with downthrow away from the domical apex. Displacement on the fault is greatest opposite the apex and decreases progressively in both directions, becoming zero at the spring line. No part of the dome ever existed on the down-thrown side of the fault, which was a fracture before doming. Maximum uplift was centered on one side of the fault and there produced the half dome; but the pre-existing fracture offered locally an easier mode of uplift by rise of the foot-wall block. At Ophir ore shoots followed intersections on the half dome of radial fractures with limestone beds. At Matehuala stretching during doming was effected largely by flow of limestone, but a monzonite stock intruded in the half dome was too brittle to flow. The limestone pulled away from the unyielding stock; fractures concentrated around the periphery of the stock localized the Dolores copper ore bodies. In the Silverton-Telluride district, Colorado, fractures radiate from a relatively large, roughly circular graben, along whose margin monzonitic stocks were intruded. Evidence suggests that this fracture pattern resulted from domical uplift, with the graben at the apex. Transfer of volcanic material from depth to the surface in the central area produced a sag which has eliminated the upward bulge of the dome. Mineralization advanced outward from the graben step by step with the outward growth of the radial fractures. Copper-silver pipes within the fault zone bounding the graben were formed first, followed successively outward by base-metal deposits as veins, which were reopened to admit gold and silver, and by precious-metal vein deposits in the outermost zone. The structural setting of La Plata, Colorado, is that of a dome, truncated, south of its apex, by a zone of high-angle faults striking eastward. Displacements on the faults are greatest opposite the domical apex. The doming is accentuated by a horseshoe-shaped hinge fold, open on the south. Along the fold dips of the strata steepen sharply; outside it, dips are gentle, whereas inside it, they are nearly flat. Several stocks were intruded along the fold, and others inside it. The steep flexural fold and abundant intrusions suggest upward shove of a flat-topped piston, perhaps a magma column congealed in its upper part, but fluid and under pressure below. Fractures are abundant along the horseshoe fold, and, with respect to the dome, fall into two classes, radial and concentric. Gold-silver deposits were concentrated chiefly within or near the horseshoe fold, and in the eastward-trending fault zone. Doming, which began during the intrusive epoch, persisted through the period of metallization, because at that time older fractures were reopened, new radial and concentric fractures were created, and fractures of both generations became loci for ore bodies. The structure at Rico, Colorado, is that of a dome with eastern elongation. Superimposed upon the major dome, toward its eastern end, is a doubly plunging anticline, also with easterly trend, cut by fractures which parallel its axis, and by fractures normal to the axis. Rich ore bodies were localized at a stratigraphic horizon originally occupied by a bed of gypsum which was dissolved, leaving silty material which the ores replaced. Ribbon-shaped mantos lay directly above fractures, both of the longitudinal and transverse sets. Ore solutions ascended these fractures to form the mantos during late stages of the doming. The Goldfield, Nevada, district lies on the southwest flank of a dome encircled by a belt of intense alteration, and of complex fracturing, which probably coalesces at depth into one or more persistent faults concentric with respect to the domical apex, and which formed the channel for altering and metallizing solutions. The volcanic rocks were brittle when first fractured by doming, but solutions rising along the fractures of the circular belt softened the rock by alunitization and kaolinization. Silica-bearing solutions then created the irregular silica “ledges” at horizons close to the then surface. The soft rock encasing the ledges flowed, as doming persisted, but the brittle ledges fractured. Ledges which had no “keel” below them were inaccessible to gold-bearing solutions; those with keels extending down to the main ore channel received the rich ore bodies. Many mining districts are associated with doubly plunging anticlines, which with brittle rock are broken by fractures which strike parallel or normal to the axis. At Creede, Colorado, older extrusive rocks were flexed into a north-trending anticline. Younger extrusive rocks do not share in the folding, but faults which strike parallel to the anticlinal axis and dip toward it cut and displace both older and younger rocks; they form a graben along the crest of the buried anticline. These faults originated as tension fissures, the result of arching of the older volcanic rocks. After extrusion of the younger volcanic rocks, renewed uplift was concentrated along the abutments of the arch, in the footwalls of the graben faults. These faults were propagated upward through the younger volcanic rocks. The eastern graben fault, the Amethyst, fingers out at its southern end. Most of the silver ore of the district came from the southern segment of the Amethyst vein. Here intense local uplift in the footwall, unable to utilize the split-up fault as a lubricated plane of movement, tore apart the walls to permit entry of the silver-bearing solutions. The structure at Bodie, California, is that of an irregular anticline upon which are superimposed several domes. The country rock is volcanic. Most of the faults and veins strike parallel to the anticlinal axis and dip toward it; but the Fortuna fracture, which carried the richest ore body, lies in anomolous relation to the anticline, for it is neither a longitudinal nor a cross fracture. It seems to have resulted from an earlier deformation, but to have been utilized by the uplift which formed the anticline in such a way that its flat segment gaped open to admit rich silver- and gold-bearing solutions. Guanajuato, Mexico, lies on the northeast flank of a major anticline which plunges southeast. The anticline carries a crestal graben. The graben fault on the northeast flank is the Veta Madre, with maximum displacement on the northwest; displacement decreases progressively southeastward, in the direction of plunge of the anticline. Like the Amethyst fault at Creede, the Veta Madre originated as a tension fissure, but became an antithetic fault when the arch broke into segments under continued uplift. Major silver-ore bodies on the Veta Madre were localized where differential movement of the walls brought shallow cups in the footwall surface opposite planar areas in the hanging-wall surface. At El Oro, Mexico, the attitudes of remnants of an andesite flow overlying shale, together with the fracture pattern, indicate deformation to form a broad anticline trending north-northwestward. The San Rafael vein lies along a normal fault striking parallel to the anticlinal axis, with downthrow on the west, toward the axis. Faulting had been completed by the time of mineralization. Early, low-grade vein matter welded the fault, but arching continued and with it an urge toward resumption of faulting, prevented by the welding. The resulting strong shearing strain produced a number of vertical feather-joint branches in the hanging wall of the fault. These were mineralized by solutions rich in gold and silver. The Mogollon, New Mexico, district lies on the west flank of a large anticline trending and plunging north-northeastward. The Pacific-Great Western and Queen faults strike parallel to the anticlinal axis and dip eastward toward the axial plane. The block between these faults contains a local bulge truncated on the east by the Queen fault. Displacement on the fault is greatest opposite the crest of the bulge and decreases progressively in either direction. The local uplift in the footwall increased the displacement on the Queen fault, but it took place in the hanging wall of the Pacific-Great Western fault. The original displacement was reversed in the segment affected by the bulge. The bulge has the form of a doubly plunging anticline trending northward, parallel to the Queen and Pacific-Great Western faults. The chief productive veins of the district occupy cross fractures normal to the anticlinal axis. Most of those north of the highest point on the up-bowed axis dip southward, whereas most of those south of that point dip northward. These fractures gaped open, under continued bulging, in time to receive the richest surge of silver-gold solutions. The following generalizations appear valid. Uplift in these districts was accompanied by development of tension fissures. Uplift and consequent stretching of the arching plate persisted through the period of mineralization, but by this time stretching in many areas had reached a stage at which Assuring could no longer facilitate it; Assuring was succeeded by graben and antithetic faulting. Ore deposition sometimes preceded this faulting but more often followed it. The fracture pattern on these domes and anticlines developed as uplift progressed. Solutions deposited vein matter in those fractures which were permeable at the time and accessible from the main solution channel. Mesothermal deposits associated with domes and anticlines fall into groups defined by age of mineralization and by metallographic provinces, but epithermal deposits are scattered from one end of the Cordilleran region to the other. They show, however, a preference for major uplifts. Silverton, Rico, La Plata, and Creede lie on a tectonic element marked by recurrent uplift from the close of the Paleozoic to the Pleistocene. Epithermal deposits in Mexico are concentrated on the site of the persistently positive Occidental geanticline. The crystalline basement lies deep throughout much of Nevada, but Goldfield, Tonopah, and other epithermal districts lie above or close to relative highs in the basement which are much larger than the local uplifts with which these districts are associated. These major uplifts were developing while epithermal metallization was taking place. The whole Cordilleran region was fast assuming its present shape. The local phenomena of uplift, Assuring, intrusion, and metallization were satellitic features superimposed on the uplift of the Cordilleran region as a whole. Because of this fact, a deep-seated origin for epithermal ores is suggested.

Abstract Copper-gold skarns peripheral to the porphyry copper deposit in the Bingham mining district, Utah, contain a geologic resource of over 7 million tons of copper and 9 million ounces of gold. The skarn deposits, hosted by Pennsylvanian limestone beds within the contact metamorphic aureole of the Tertiary Bingham stock, have been mined intermittently since 1896. In the late 1960s, two large deposits were discovered at depth on the north side of the district by The Anaconda Company and Kennecott Copper Corporation: the Carr Fork mine, operated by Anaconda from 1979 to 1981, and the North Ore Shoot deposit, which has not entered production. Recent drilling by Kennecott on the eastern flank of the North Ore Shoot has resulted in discovery of the Fortuna deposit. These three areas are all part of the same, continuous skarn system. A remarkable horizontal and vertical extent of skarn is displayed within the Bingham district. Mineralized skarn occurs in a zone roughly 2,000 feet wide around the quartz monzonite porphyry phase of the Bingham stock and extends over a mile vertically from surface exposures to the deepest drill-hole intersection. Skarn ore, delineated by the +1 percent copper grade contour, lies outside the porphyry copper ore shell and extends to a lower elevation. The Carr Fork orebodies consist predominantly of chalcopyrite in retrograde-altered garnetite skarn. In the North Ore Shoot and Fortuna deposits, massive sulfide replacement bodies are the principal ore type, although mineralized garnetite is also present. Pyrite-gold mineralization that crosscuts copper-gold skarn has been found in the Bingham skarn deposits, but its relationship to other gold mineralization in the district is unclear. Beyond the zone of copper-iron-gold mineralization, lead-zinc-silver deposits occur in limestone and other host rocks within a broad halo nearly one mile wide.

Abstract Sediment-hosted stratiform deposits of copper, lead, and zinc are distinguished from volcanogenic massive sulfide deposits by a lack of associated volcanism. They are distinguished from Mississippi Valley-type deposits by an early timing of mineralization relative to deposition of host sediments, by greater conformity with their hosts, and by higher iron sulfide and silver contents. These two types, however, are probably part of a continuum in terms of characteristics and mechanism of formation. Characteristics of the diverse sediment-hosted stratiform deposit type are illustrated by 28 selected examples, which include several of the world s giant deposits. Metal ratios of sediment-hosted stratiform copper and lead-zinc deposits are distinctly separate from one another and unlike the range of ratios in volcanogenic deposits. Most sediment-hosted stratiform lead-zinc deposits contain abundant iron sulfide, in contrast to the copper deposits which are characterized by low sulfide iron contents. Barite is abundant and massive in several of the lead-zinc deposits but is sparse in stratiform copper deposits. Sediment-hosted stratiform deposits of both copper and lead-zinc range in age from about 2,000 m.y. to recent and occur in tectonically active intracratonic settings, commonly in fault-controlled sedimentary basins. They do not appear to be related to orogenic events or to events at margins of tectonic plates. Local tectonic settings and relations to basin margins and faulting are diverse. We interpret their unifying features to be that each provides a situation in which basinal ground water could be moved to a shallow site of sulfide deposition. Regional stratigraphie settings include (1) first marine transgressions over red terrestrial sections (e.g., Zambian deposits, Kupferschiefer, White Pine, Boleo), (2) within marine (and lacustrine) sections containing red beds (e.g., Dzhezkazgan, Udokan, McArthur River, Redstone River, Spar Lake, Irish deposits), (3) terrestrial sections containing red beds (e.g., Corocoro, Nacimiento, Largentière), and (4) marine sections without red beds (e.g., Sullivan, Meggen, Rammelsberg). Evaporites (bedded or interstitial gypsum, anhydrite, or carbonate after sulfate) or other evidence of aridity (salt casts, desiccation features) are closely associated with deposits in all settings but the last. Although they lack any close association with volcanism, most deposits occur in basins with some regionally contemporaneous volcanic activity or with a significant quantity of volcanic rocks in underlying stratigraphie sections. Sedimentary facies are important local controls on sulfide distribution, but host lithology varies widely from district to district and commonly within districts and is among the least definitive features of these deposits. Most sediment-hosted copper deposits (except in Shaba) are associated with bedded host or footwall units that retained high permeability after early diagenesis. The copper mineralization is invariably “disseminated” (including stockwork and breccia matrix). Most of the ore is in reduced sediments that contain (or can be inferred to have originally contained) abundant organic matter. Ore occurs at a redox interface with footwall or hanging-wall or laterally equivalent units that are hematitic (red now or before metamorphism) and barren of sulfide and organic matter. In contrast, most lead-zinc deposits are in sediments that became rather impermeable during early diagenesis. The ores are mostly either massive or banded (i.e., interbedded with sedimentary rock), although some are disseminated. Many deposits are also associated with evaporites and red beds, though less directly than the copper deposits, and the ore is not obviously controlled by the interface between oxidized and reduced conditions. Zonation in copper deposits typically follows the sequence: barren (hematite but no sulfide) chalcocite → bornite → chalcopyrite → pyrite, expressed both outward and upward. In many deposits this zonation does not reflect the paleogeography directly. Zonation is commonly continuous across alternation of depositional environments and probably reflects post-depositional hydrologie regimes and chemical gradients. In deposits with lead-zinc as well as copper, the general zonal sequence Cu-Pb-Zn-Fe is observed. In several deposits this sequence evidently marks increasing distance along an ore solution pathway. Sedimentary iron-formation, both sulfide and oxide facies, appears to have formed in distal portions of some lead-zinc mineralizing systems. Zoning patterns are varied. The timing of metal fixation relative to deposition of host sediments and their diagenetic modification is crucial but rarely constrained by unambiguous evidence. Concretions and authigenic minerals provide some of the best evidence, indicating fixation of base metal sulfides after early diagenesis at McArthur River and Shaba, but much earlier at Rammelsberg. The lack of definitive detailed documentation of evidence for timing and of subtle nonsulfide zoning patterns in most deposits seriously limits our ability to interpret their origins. The lack of information on temperatures of mineralization in most deposits is also critical. Unlike volcanogenic massive sulfide deposits, sulfur isotopes in sediment-hosted stratiform deposits do not reflect secular variations in the isotopic composition of seawater and appear to be dominated by processes at the site of ore deposition rather than the source of the sulfur. Narrow clustering of δ 34 S values in lead-zinc deposits appears to reflect systems dominated by mineralizing fluids, in contrast to scattered δ 34 S values in copper deposits which reflect sulfate reduction reactions at lower temperature and in systems dominated by the depositional environment. The data are ambiguous as to mechanisms and to definition of the ore-forming systems. At McArthur River, carbon and oxygen isotopes support an interpretation of mineralization dominated by a hydrothermal ore-forming fluid. We speculate that most sediment-hosted stratiform deposits of both copper and lead-zinc formed early in the diagenetic history of their enclosing sediments from brines derived from the sedimentary basin itself. They are thus diagenetic in the broad sense, even though the metals may have been introduced epigenetically into the system defined by the present physical limits of mineralization. The wide and gradational range of deposit characteristics derives from more or less independent variation of factors linked in a source-transport-trap chain of coincidence. The stratiform copper deposits are probably formed from cool sulfate-rich brines, derived at an early stage of basin evolution and migrating updip to reducing sites of deposition. The lead-zinc deposits are probably formed from similar but more evolved brines that have been heated and probably chemically reduced deeper within the basin. Some of the latter may have been formed by exhalation of dense basinal brines on the sea floor.