Abstract Data from seismic-reflection profiles, sidescan-sonar images, sediment cores, and surface samples outline the shallow stratigraphy and surficial sedimentary environments of the Boston Harbor estuary. Bedrock and till of pre-Wisconsinan age form an irregular acoustic basement surface that variably crops out or is buried as much as 35 m below the harbor floor. Where buried, this surface is overlain by discontinuous ice-proximal glacial deposits and by glacio-marine muds. These accumulated in late Wisconsinan time when ice retreat and marine submergence occurred simultaneously owing to crustal depression. During the immediate postglacial period, sea-level regressed due to crustal rebound, and texturally diverse fluvial and estuarine sediments were deposited in small channels that were cut into the subaerially exposed upper Wisconsinan drift. As the harbor was resubmerged in Holocene time in response to the eustatic rise of sea level, waves beveled the substrata, and localized deposits of marine sands and muds accumulated atop the trans-gressive unconformity. The modern wave and current regime in the harbor area has produced three surficial sedimentary environments characterized by erosion, deposition, and sediment reworking, respectively. Areas of erosion are characterized by bedrock outcrops and coarse lag deposits that have been scoured by tidal currents and waves; they are common along mainland and insular shorelines, within large channels having strong tidal currents, and across much of the harbor entrance. Areas of deposition consist of sandy and clayey silts and are found on shallow subtidal flats and in broad bathymetric lows where tidal currents are relatively weak. Environments of sediment reworking are characterized by intermittent erosion and deposition and have diverse grain sizes; they are associated with many different bathymetric features and are indicative of highly variable bottom currents.

Abstract The Ladolam gold deposit on Lihir Island, Papua New Guinea, is one of the world's largest epithermal gold deposits with a gold resource of 37 million oz. Open-pit mining commenced at Ladolam in 1997. Epithermal gold mineralization formed at <0.5 Ma and is telescoped upon a slightly earlier porphyry environment. A remnant geothermal system is still active. Lihir Island is located within the Tabar-Lihir-Tanga-Feni volcanic island chain, a group of alkaline silica-undersaturated volcanic rocks of Plio-Pleistocene age (<3.5 Ma) in the Province of New Ireland. Tabar-Lihir-Tanga-Feni magmas were generated in a tensional tectonic regime from a mantle that was preenriched with alkali elements following subduction reversal in the late Miocene. Host rocks to the mineralization are a breccia complex consisting mostly of clasts from preexisting mafic volcanic rocks and altered alkaline intrusions. The setting of the Ladolam breccia complex is a volcanic-hydrothermal diatreme that formed during explosive decompression following sector collapse of the volcanic edifice. Host rocks to the shallow ores are polymictic hydrothermal vent breccias and breccia pipes that overprint the porphyry centers. Three stages of mineralization are observed. Stage I: Biotite-orthoclase-anhydrite ± magnetite with minor copper-gold-molybdenum porphyry mineralization as veinlets and disseminations; stage II: Refractory sulfide gold mineralization associated with pervasive adularia-pyrite (-leucoxene-illite) alteration in near-surface ores, grading downward into barren anhydrite-adularia-pyrite-vermiculite alteration; and stage III: Quartz-calcite-adularia-pyrite-marcasite ± electrum stockwork veins that overprint and replace earlier anhydrite veins. Stage II refractory sulfide mineralization comprises the bulk-mineable shallow ores in the Minifie area. Stage III veins are mostly subeconomic, although a tabular zone of silicic breccia is an important ore type in the Lienetz area. The latter is capped by advanced argillic alteration of steam-heated origin. Propylitic alteration occurs on the margin of the monzonite intrusion centers, overprinting brecciated volcanic rocks. Fluid inclusion and stable isotope data are used in conjunction with geologic attributes to interpret a four-stage evolution for the Ladolam hydrothermal system. Stage I porphyry: Pervasive biotite alteration of the monzonites predated sector collapse and extensive brecciation. No fluid inclusions suitable for microthermometric study were found. The δ 34 S values of pyrite (−1.4 to +2.2‰) and δ 18 O value of hydrothermal phlogopite (+6.2‰) are consistent with mineralization from a magmatic fluid. Stage II transitional epithermal: Deep brines of magmatic origin (5–10 wt % NaCl equiv) at 200° to 300°C contained dissolved monovalent-divalent ions and significant gases (CO 2 + H 2 S). Wide-ranging salinities in fluid inclusions from deep anhydrite veins (5–>32 wt % NaCl equiv) is explained by open-system boiling and vapor loss. Fine-grained auriferous pyrite and adularia precipitated in breccias at ∼200°C, immediately above an anhydrite-K feldspar alteration zone as fluids vented to the surface. Pyritic ores were deposited by rapid cooling associated with mixing of the magmatic ore fluid (δ 18 O ∼6‰) with cool ground water (δ 18 O ≤0‰), and boiling. The δ 34 S values of pyrite (−7 to +2‰) and deep anhydrite (13 ± 2‰) suggest a magmatic sulfur source. Stage III epithermal: Quartz and calcite veins formed under hydrostatic boiling point for depth conditions. Upwelling stage III fluids at 230°C were moderately saline (5 ± 0.5 wt % NaCl equiv) and contained low abundances of dissolved gases (up to 3.5 wt % CO 2 + H 2 S). This brine mixed with dilute meteoric ground water (near 0 wt % NaCl equiv at 170° ± 20°C), depositing quartz as it cooled. Isotopic compositions of deep carbonates (δ 13 C = −4‰, δ 18 O = 14‰) indicate a magmatic origin for the brine (δ 13 C(CO 2 ) ∼ −3‰, δ 18 O(H 2 O) = +6‰). At Lienetz, an advanced argillic cap formed due to tectonic uplift of this area relative to Minifie. Silicic breccia ores in the Lienetz area contain pyrite-marcasite with lighter δ 34 S values (−13 to −2‰) compared to pyrite from Minifie (δ 34 S, −3 to −1‰). Lienetz ores probably formed at temperatures below 200°C as reduced fluids mixed with descending, oxidized acid-sulfate waters beneath the advanced argillic cap. Gold from the earlier transitional stage II event may have been remobilized by the acid-sulfate waters and reprecipitated as electrum with fine-grained quartz in this mixing zone. Geothermal fluids in the modern system are neutral-pH chloride-sulfate brines that have a salinity similar to stage III (∼5.5 wt % NaCl equiv) but contain few if any magmatic volatiles. The modern system is interpreted to be a remnant of the ore-forming one. The deep brines are of magmatic origin (δ 18 O = +6‰, δD = −25‰) and are diluted by meteoric waters (δ 18 O = −6‰, δD = −40‰) within the top few hundred meters. These waters discharge into the Kapit-Coastal and Luise harbor areas, while acid-sulfate hot springs of steam-heated origin are associated with areas of steaming ground to the west and north of the major orebodies.

Abstract Land subsidence due to fluid withdrawal has been reported from many parts of the world. It has de-veloped most commonly in overdrawn groundwater basins, but subsidence of serious proportions also has occurred in several oil and gas fields. Subsidence due to groundwater overdraft occurs in many places in Japan, where it has caused dangerous environmental conditions in several heavily populated areas. For example, in Tokyo, 2 million people in an area of 80 sq km now live below mean high-tide level; subsidence is only partially controlled, and the difficulties of achieving full control are great. The San Joaquin Valley in California is the area of the most intensive land subsidence in the United States. Subsidence, which affects 4,200 sq mi (10,875 sq km), reached 28 ft (8 m) in 1969. The total volume of subsidence to 1970 was about 15.5 million acre-ft. Surface-water imports to subsiding areas have reduced groundwater extractions and raised the artesian head, causing subsidence rates to decrease. In the Santa Clara Valley at the south end of San Francisco Bay, excessive pumping of groundwater between 1917 and 1967 caused as much as 180 ft (50 m) of artesian-head decline and maximum land subsidence of 13 ft (4 m). A fourfold increase in surface-water imports in 5 years has achieved a dramatic rise of artesian head—70 ft (20 m) in 4 years. Subsidence rates have decreased from as much as 1 ft (0.3 m) per year in 1961 to a few hundredths of a foot in 1970. Wilmington oil field, in the harbor area of Los Angeles and Long Beach, California, is not only the oil field of maximum subsidence (29 ft or 9 m) in the United States, but also the outstanding example of subsidence control by injection and repressuring. Large-scale repres-suring was begun in 1958 by use of injection water obtained from shallow wells. Subsidence of some bench marks was stopped by 1960. By 1969, when 1.1 million bbl of water per day was being injected into the oil zones, the subsiding area had been reduced from 20 to 3 sq mi (52 to 8 sq km) and parts of the area had rebounded by as much as 1 ft (0.3 m). Methods employed to measure the change in thickness of sediments compacting or expanding in response to change in effective stress include (1) depth-benchmark and counterweighted-cable or “free”-pipe extenso-meters with amplifying and recording equipment; (2) casing-collar logs run periodically in a cased well; and (3) radioactive bullets emplaced in the formation behind the casings at known depths and resurveyed by radioactive detector systems at a later time. In evaluation of potential land subsidence due to fluid withdrawal, an essential parameter is the compressibility of compactible beds. When effective (grain-to-grain) stress exceeds maximum prior (preconsolidation) stress, the compaction is primarily inelastic and nonrecoverable, and the virgin compressibility may be 50–100 times as large as the elastic compressibility in the stress range less than preconsolidation stress. If fluid pressures in a compacting, confined system are increased sufficiently to eliminate excess pore pressures in the fine-grained sediments, subsidence will stop. If fluid pressures continue to increase, the system will expand elastically and the land surface will rise.