Sequential measurements of molluscan radiocarbon are demonstrated to be an effective proxy of seasonal and El Niño–related upwelling variation in coastal Peru. A Trachycardium procerum valve from southern Peru was measured through ontogeny for radiocarbon via accelerator mass spectrometry (AMS) as well as δ 13 C and δ 18 O. A specimen collected in 1984 near Casma, Peru (∼9.30°S) grew before and during the 1982–1983 El Niño/southern oscillation warm event. Shell morphology recorded El Niño warming as a shallow growth break with subsequent realignment of aragonite crystal microstructure. The presence of this growth pattern indicates that shell material was precipitated during the warm event and that each AMS sample could be independently identified to represent a defined period in the El Niño/southern oscillation cycle. Samples taken from portions of the shell precipitated prior to the El Niño warm event (before the diagnostic growth break) had a mean value of 99.8 percent modern carbon (pMC), with a maximum seasonal range of 2.1 pMC. During warming, as indicated by a negative excursion in δ 18 O and the growth break, there was an abrupt increase to 107.9 pMC. Aragonite precipitated near the margin of the valve, after the El Niño/southern oscillation event concluded, had radiocarbon values approaching those present before the growth break. We attribute this radiocarbon distribution to variations in vertical mixing of surface and deeper upwelled water of greater ventilation age. As an El Niño event begins and the thermocline deepens, less deep water reaches the surface. Thus, radiocarbon values in shell precipitated during El Niño appear younger (more positive) relative to non–El Niño periods, which represent periods of more intense upwelling. The results from the modern specimen validate the use of molluscan radiocarbon as a proxy of upwelling conditions related to El Niño/southern oscillation and suggest the utility of similar analysis of more ancient valves in both oceanography and paleoclimatology.

Abstract Phanerozoic lode gold deposits are invariably associated with convergent plate margins and occur within close proximity to major translithospheric structures or compressional to transpressional-transtensional shear zones. The deposits are almost entirely structurally controlled and the nature of the immediate host rock does not generally play an integral part in ore formation. Nonetheless and unlike the majority of their Archean and Proterozoic analogues, Phanerozoic lode gold deposits are primarily hosted in several kilometer-thick sequences of marine sedimentary rocks which accumulated on pre-collision continental margins and/or in prograding arc-trench complexes. The sedimentary successions are commonly under lain by, and interspersed with, bimodal volcanogenic rocks which formed as a result of magmatic processes related to spreading, arc formation, plate collision, and subduction. The largest Phanerozoic lode gold systems are found in sub- to medium-grade greenschist metamorphosed terranes which have been caught up in the accretion of one or more allochthonous microplates and associated oceanic crust to an active continental margin. Mineralization in these collisional settings closely follows peak meta-morphism of the immediate host rocks and is temporally associated with exhumation of the orogen and addition of heat into the thickened crust via lithospheric delamination processes. Generation of CO 2 -rich aqueous ore-forming fluids involves metamorphic devolatilization of subcreted hydrated crust and the devel op ment of laterally and vertically extensive hydrothermal plumbing systems. Rich Phanerozoic lode gold deposits display a very close spatial and temporal relationship with syn- to post-tectonic felsic intrusive rocks but generally predate the emplacement of the granitoids. The deposits typically consist of quartz gold lodes in fault and shear systems at or above the brittle-ductile transition and form at P-T conditions of 1 to 3kbars and 250° to 400°C: they are characterized by relatively straightforward parageneses and a lack of pronounced vertical mineral or ore zonation. Episodic brittle reactivation in response to short-lived tectonic pulses is common and can result in remobilization of pre-existing mineralization and the formation of secondary lode systems. Alteration halos around Phanerozoic lode gold systems vary from a few centimeters to several tens of meters and reflect variations in the host-rock lithology and reactivity, permeability and porosity, orientation of bedding in metasedimentary rocks relative to auriferous veins, and fluid composition. On the deposit scale, lithogeochemical information obtained from wall-rock alteration assemblages represents by far the most valuable exploration tool. Broad bleached zones characterized by carbonate, sulfide, and sericite altera tion surrounding mineralized zones provide an exploration target of increased magnitude. Geochemical traverses generally indicate depletion of Na 2 O and increased values of CO 2 , H 2 O, K 2 O, S, As, Au, and possibly Sb, within five to several tens of meters from the auriferous lodes.

Abstract Since the first studies of sulfur isotope variations in natural materials (Thode, 1949), it has been apparent that there are large and dramatic variations of 34 S/ 32 S ratios and that sulfur isotope studies are a powerful tool for interpreting the origins of sulfur-bearing minerals. However, sulfur is such a common element in the Earth's crust (sixteenth most abundant, averaging 0.03 wt %; Mason, 1966), and is involved in so many igneous, hydrothermal, biological, and surficial processes that a simple measurement of δ 34 S, without constraining geological, biological, and geochemical data, is often unenlightening. In many sedimentary and hydrothermal systems, geologists are confronted with multiple sulfur sources, large fractionations of sulfur isotopes during oxidation-reduction reactions that sometimes produce disequilibrium effects, and strong chemical and physical gradients at the site of mineral deposition. Despite significant advances in the understanding and utilization of sulfur isotopes to characterize ore-forming processes (Ohmoto, 1972; Ohmoto and Rye, 1979; Shanks et al., 1981; Janecky and Shanks, 1988), interpretations may be ambiguous and, in ancient ore deposits, difficult to test. Part of this difficulty has been due to an inability to resolve fine-scale spatial variations in isotopic fractionation between successive zones or between coexisting minerals. The development of laser and ion microprobes for high spatial-resolution stable isotopic analyses has opened new research frontiers

Abstract Salt domes, their cap rocks, and the adjacent sedimentary strata represent a major economic resource in the Gulf Coast and elsewhere. The resources and utilization of the salt dome setting are remarkably diverse. Major economic products of the salt dome environment are salt, cap rock-hosted native sulfur deposits, and oil and gas that occur on the dome flanks and in the cap rock. Some cap rocks are sources of limestone, gypsum, and anhydrite, and some host commercial concentrations of Zn and Pb. Uranium is concentrated in strata above or adjacent to some diapirs. Caverns, excavated within the salt, serve as product storage as diverse as crude oil, including the Strategic Petroleum Reserve, liquified petroleum gas, and hazardous waste. In addition, the Gulf Coast is a geologically young basin with many components that commonly are regarded as critical for the development of sediment-hosted mineral deposits. Metalliferous formation waters are locally present in the Gulf Coast Basin and have been proposed as modern analogs for the ore-forming fluids for ore deposits in older sedimentary terranes. Synsedimentary growth faults are important features that control local depositional facies and subsequent fluid movement in the Gulf Coast. Zn-Pb-Ag sulfide concentrations in two principal host settings (salt dome cap rocks and shelf carbonates) have been identified in the Gulf Coast. These sulfides have the most direct genetic affinity with Mississippi Valley type ore deposits that typically occur in older sedimentary terranes, although the cap rock occurrences have similarities to SEDEX-style mineralization. Recently discovered barite mounds on

Abstract The Winnfield salt dome is located within the North Louisiana Basin, about four miles west of the town of Winnfield in Winn Parish. The WinnRock quarry, in the Winnfield cap rock, provides an unequalled setting to study detailed features related to the complex evolution of a salt dome. Studies of the WinnRock quarry exposures have revealed that salt dome cap rocks record fluid events within the local sedimentary basin that may span many tens of millions of years. This article is modified from an earlier guidebook (Kyle and Ulrich, 1993) to which the reader is referred for more background information about Gulf Coast geology and halokinesis. This field trip will examine the Winnfield salt dome cap rock zones for evidence of their formational mechanisms, with particular emphasis on the development of mineral concentrations within the cap rock. Cap rocks are the result of complex evolutionary processes associated with dissolution of the rising halite diapir, the accumulation of the less soluble components, and the alteration of these materials, largely by bacterially controlled diagenetic processes that involve hydrocarbon oxidation. The cap rock contains the “fossil” components resulting from fluid interaction, including petroleum degradation, at or just below the contemporaneous sea floor. These processes are similar to active systems associated with petroleum seeps overlying shallow salt diapirs in the offshore Gulf. The WinnRock quarry also exposes concentrations of metallic sulfide minerals within the calcite and anhydrite zones. These sulfide concentrations were formed from the intermittent supply of relatively hot, metal-bearing brines to the

Abstract The salt domes of the Texas and Louisiana Gulf Coast have produced an incredible amount of wealth in the form of oil and gas, as well as native sulphur. This belt of domes is distinct from the domes in the interior salt basins (Fig. 1) which have not produced sulphur despite the common presence of cap rock that is similar to the host rock for the commercial sulphur concentrations. Sulphur (the spelling used by the sulphur industry) is also known as brimstone from the Anglo Saxon byrn stone for burning stone and is referred to as the Queen of the industrial minerals. Sulphur is an essential requirement for the fertilizer and galvanizing industries and the amount used mirrors a country’s GNP, reflecting that a growing and robust economy consumes a lot of sulphur. A current summary of the sulphur industry is provided by Wessel (1994). The Gulf Coast sulphur industry is a relatively young one and was pioneered by Herman Frasch who perfected his technique of Frasch sulphur production in 1902 using superheated water pumped into the sulphur-bearing formation through a nested casing string to melt sulphur and then recover it through an inner casing string (Fig. 2). The Frasch sulphur industry is young enough that the author has worked with geologists who personally knew Frasch. From the outset of sulphur production, geologists attempted to explain the its origin and its distinctive occurrence in a salt dome cap rock section that consists of anhydrite and gypsum overlain by a vuggy

Abstract This paper discusses the geology of the Main Pass 299 sulphur and oil deposit as a part of a field conference examining the mineral resources of the Gulf Coast salt dome province. A complementary core workshop provides an opportunity to examine cap rock core from the Main Pass deposit. Interpretations of the geology of the Main Pass deposit presented in this paper are based on data from twenty exploration wells plus the data acquired from sulphur production wells and from oil and gas development wells. Main Pass 299 is located in the federal offshore Gulf of Mexico about 90 miles southeast of New Orleans and about 30 miles east of Venice, Louisiana (Fig. 1). Water depths average 210 ft. Oil and gas rights on the dome were acquired by Chevron in 1962, who produced in excess of 57 million barrels of oil and 38 billion cubic feet of gas from sandstone reservoirs on the flanks of the salt dome. At the time sulphur was discovered, Chevron had not drilled on top of the dome. The Main Pass 299 salt dome sulphur deposit was discovered by a joint venture partnership composed of Freeport-McMoRan Resource Partners - FMRP (58.33%); IMC Fertilizer, Inc. (25%); and Homestake Mining Company (16.67%). Main Pass 299 is one of six salt domes drilled in 1988-1989 by FMRP from a total of eleven offshore salt domes acquired in the February 1988 Federal Offshore Sulphur Lease Sale by FMRP and joint venture partners. In late 1988, FMRP as operator

Abstract Selected samples from a representative core (Hole SW-1 -27A) through the Main Pass 299 cap rock-hosted sulfur deposit were studied petrographically, and complementary geochemical and stable isotope analysis were conducted. The intervals selected include core from the oil-rich, sulfur-barren cap rock above the oil-water contact through the sulfur-bearing zone to the anhydrite cap rock zone (from 1,670 ft to 1,961 ft subsea). Descriptions of texture and morphology and whole-rock composition, including any hydrocarbon and sulfur occurrence, are provided. The various lithologies examined in this petrographic analysis included cores and thin sections taken from the sulfur barren, oil-rich cap rock, oil/water contact interval, and the sulfur-rich and sulfur-barren anhydrite horizons. The core in this well is consistent in composition and texture with a bacterially produced salt dome calcite cap rock. Much of the upper, sulfur barren cap rock is composed of numerous micro-faulted and calcite-cemented, imbricate structures and breccias. Most of the rock in these zones consists of a micritic matrix with variable amounts of rhombic dolomite (probably representing a residual concentration from the diapiric salt); variable amounts of secondary calcite and celestite occur in late pores and fractures. The sulfur-bearing horizon typically occurs in areas of relatively high porosity (10-30%) and is associated with sparry calcite, in addition to micritic limestone. Hydrocarbon (oil) horizons typically occur in the uppermost part of the well, from ∼1,670 ft to ∼1,722 ft subsea. Hydrocarbon stringers, from late-stage migrations, were also observed occurring in a deeper zone, from ∼1,879 ft to 1,922 ft subsea,

Abstract The second day of the trip consists of a core workshop focusing on the worldclass Main Pass 299 cap rock-hosted sulfur-oil deposit which is offshore of the Mississippi delta, but also including cores from selected Texas salt domes including Boling, Hockley, Davis Hill, and Butler. The features observed in these cores may be compared with features seen in the unparalleled WinnRock quarry exposures of a salt dome cap rock. Main Pass 299 and Boling represent the largest cap rock-hosted sulfur concentrations in the Gulf Coast. Both have a significant amount of associated petroleum resources, including co-production of sulfur and oil at Main Pass. This workshop affords the first public viewing of the Main Pass 299 cores, and the Danielson et al. (1995) and Spera and Kyle (1995) articles in this guidebook represent the first published articles on the geology of the Main Pass 299 sulfur-oil deposit. Hockley is the best documented example of a Gulf Coast metal-rich salt dome cap rock with established resources of Zn-Pb-Ag sulfides hosted by both calcite and anhydrite zones, along with major barite concentrations. The Hockley cap rock was a metals exploration target for several companies from 1977 to 1982 which resulted in the drilling of over 60 cores, most of which penetrate the entire cap rock sequence; Agee (1990) included a summary of the distribution and general character of these cores. These cores have provided the basis for numerous studies at The University of Texas at Austin which have focused on the origin of

Abstract The gold deposits hosted in the Carolina Slate Belt (CSB) have been studied in great detail by university, government, and industry workers. Appendix A contains a brief compilation of work in which readers can find detailed descriptions of the structure, geochemistry, and geology of the deposits, as well as some idea of the overall architecture of the CSB. This brief introduction is summarized from several sources, including Feiss (1985) and Scheetz (1991). This guidebook contains chapters on each deposit to be visited, as well as a chapter discussing exploration techniques used in the southeastern U.S., where lack of exposure and deep weathering preclude the use of many traditional exploration methods. The CSB can be traced regionally along strike from northern Georgia to southern Virginia (Fig. 1). The CSB in a general sense is composed of two major elements. The first is an older sequence of dominantly felsic volcanic rocks, including flows and tuffs. Age dating by Wright and Seiders (1980), Carpenter et al. (1982) and Butler and Fullagar (1975) yielded ages of 586 + 10 Ma (U/Pb on zircon), 566 ± 15 Ma (U/Pb) and 554 ± Ma (Rb/Sr), and 522 + 24 Ma (Rb/Sr whole rock), respectively. The second element is a younger sequence of predominantly epiclastic units including mudstones, quartz siltstones and sandstones, shales, slates and phyllites. Biostratigraphic age dates range from late Proterozoic (on the basis of Pteridium fragments, see Gibson el al., 1984; Gibson and Teeter, 1984) to middle Cambrian (on the basis of sponge spicules,

Abstract Renewed gold exploration in the Carolina slate belt of North and South Carolina that led to the discovery of the Ridgeway gold mine (#5 on Figure 1) by Amselco (now Kennecott) began at about the same time as the last Geological Society of America field trip to focus on southeastern metal deposits. That trip occurred in the fall of 1980 as part of the national GSA meeting in Atlanta, Georgia (Bell, Carpenter, and Feiss, 1980). On that trip at stop 8, Henry Bell described abundant hydrothermal alteration similar to the Haile and Brewer mines in an area with no known gold mines. He noted that auger cuttings from a hole drilled by the South Carolina Geological Survey in the 1960’s near stop 8 at Mount Rehovah church contained 0.06 ppm gold and 220 ppm copper in the upper 9 feet of rock and 10 ppm molybdenum from 30-40 feet. Because the values decreased with depth, the gold was attributed to surface enrichment (Bell, 1976). Samples collected by Bell (1976) of small drainage basins in the region contained up to 1000 ppm tin in heavy mineral concentrates that were also anomalous in gold, boron, lanthanum, niobium, beryllium, and copper (Figure 16 of Bell, Carpenter, and Feiss, 1980). Bell et al. (1980) stated “Geochemical data suggest that this region is on the periphery of an area having many of the ore-bearing characteristics of the Haile and Brewer mines” Stop 8 was only hundred’s of feet west of the present site of the

Abstract Kennecott Minerals Company (KMC) operates a 15,000-ton-per-day open-pit gold mine located approximately 5 miles east of the town of Ridgeway, and 25 miles north of Columbia, South Carolina (Figure 1). The Ridgeway gold mine is one of four gold mines which were put in production in South Carolina in the 1980’s. It is currently the only operating gold mine in the Eastern United States. The Ridgeway mine produces gold bullion from two bulk-mineable, open-pit deposits located one mile apart. Low-grade oxide and sulfide ore produced from the siliceous deposits is blasted and hauled to a central mill complex. Ore is milled to minus 200 mesh, and gold is extracted by a carbon-in-pulp, electrowinning process. The mine operates 24 hours a day, and employs over 100 people. The mine began producing gold in December 1988 and total developed in order to achieve an ultimate production goal of 1.5 million ounces of gold bullion by the year 2000. The North Pit is 2,000 feet long, 1,400 feet wide, and 320 feet deep. The South Pit is 2,600 feet long, 1,500 feet wide, and 360 feet deep. The South Pit is being mined at full scale, and is expected to be mined out by June 1996. Activity in the North Pit is currently focused on the stripping of oxide ore and waste at the 420-440 level along the northern half of the pit. Future development plans for the North Pit include deepening the pit from the 120 elevation to -40 elevation. This paper

Abstract The Brewer Gold Mine is situated within the Carolina Slate Belt of the Piedmont physiographic province approximately 1 km west of Jefferson, SC and 13 km northeast of the Haile Gold Mine, Kershaw, SC (Figure 1). Regionally, the mine is located about 80 km southeast of Charlotte, NC and 80 km northeast of Columbia, SC. The mine was discovered in 1828 and began as a placer operation. Butler (1985) provides a chronological history of production. Historic production was from four main pits that are, from largest to smallest, Brewer, Hartman, Hilford Cut, and Topaz. Minor underground workings were developed from these pits. Gold production occurred in various intervals with the last time period from 1934 to 1940. Pre-modern gold production is estimated at a minimum of 22,000 ounces. A number of companies have prospected and drilled at the Brewer including the USBM. Exploration drilling on the property (core and rotary) totals 44,026 ms. Approximately three-quarters of the drilling was performed by Nicor/Westmont/Brewer both in pre-production exploration drilling and development drilling while in production. In 1987, Westmont Mining Inc. formed the Brewer Gold Company and production began in mid-1987 as an open-pit, heap leach operation. The reserve base at the onset of mining was 5.1 million tons @ 0.042 opt Au (4.6 Mt @ 1.4 g/t). Mining continued from three different pits; Brewer, B6, and Northwest Trend (Figure 2) and ceased January 1993. A total of 5.66 million tons of ore was mined with 177,674 ounces of gold being recovered. Table