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Abstract: This paper presents results of a laser ablation - inductively coupled plasma - quadrapole mass spectrometer (LA-ICP-QMS) U-Pb dating study of small in situ zircon grains from samples collected [...]

The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina's stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma tU-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that.s host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllie alteration composed of cuartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllie alteration. This zoning included a low-temperature (<110°C) halloysite + kaolinite that extends from 800 to 250 m below present-day surface and is deeper than higher temperature (>120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite. sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.14 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40Ar/39Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite elesure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillicargillic alteration. Sulfur isotope results of sulfides (-2.5 to +4.9%c; mean +0.7%e; n = 20) and a sulfate (barite; + 10.5%o; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high COa contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fe mine and Campamento deposit represent parts of a porphyry copper system--specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment. [ABSTRACT FROM AUTHOR]

The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics. [ABSTRACT FROM AUTHOR]

SuSu Knolls comprises three steep-sided conical volcanic peaks, standing on a N-NW-trending ridge in the eastern Manus basin, a complex zone of convergence between the major Indo-Australian and Pacific plates. The knolls consist of three porphyritic andesite-to-dacite domes, each 1.0 to 1.5 km in diameter, with crests ranging from 1,150 to 1,520 m below sea level. An intense hydrothermal plume, with peak transmission anomalies in excess of 40%, originates from SuSu Knolls, and associated hydrothermal venting, with fluid temperatures exceeding 300°C and sulfide mineralization, has been detected on the crests of all three edifices. The most important of these is the 2.47 million metric ton (Mt) Solwara 1 copper-gold deposit on Suzette Knoll, slated for mining by Nautilus Minerals. The porphyritic dacite is commonly strongly altered by reaction with acidic fluids. Fragments of sulfide mineralization, collected as part of the seafloor talus surrounding the crest of North Su and South Su, are characterized by an assemblage of pyrite-(fukuchilite)-enargite ± covellite-chalcopyrite. By contrast, there are actively venting chimneys on the crest of Suzette that are typically zoned, with chalcopyritepyrite- tennantite inner zones and barite-dominated outer zones. The knolls are covered by black sulfidic sediments that contain up to 2.4 wt % Cu and 3.2 ppm Au. Primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized by natroalunite, alunite (North Su only), cristobalite, tridymite, and rare quartz and kaolinite. Most volcanic rocks exhibit a patchy surficial coating of native sulfur, which may also fill vesicles. No altered rocks were dredged from Suzette because of the thick sulfidic sediment cover. Mass-balance calculations for altered and unaltered rock from North and South Su show that major and trace lithophile elements are depleted in the altered rocks, except for Ti, Al, Si, and Zr (all near-immobile), in concert with the destruction of the primary minerals and removal of primary components by acidic fluids. Sulfur and chalcophile trace elements (i.e., As, Au, Ba, Cd, Cu, Mo, Pb, and Se), typically associated with magmatic Cu-Au mineralization, are enriched by orders-of-magnitude in both leached and mineralized rock. Sulfide and native sulfur <534S values from all three domes range from -7.4 to +0.4%o, indicating a magmatic component for the sulfur. A small group of ore elements (i.e., Ag, Bi, In, Sb, and Zn) are strongly enriched in mineralized breccias and depleted in tire altered dacites, suggesting redistribution during alteration. The presence of advanced argillic alteration, the paragenesis of the Fe-Cu-As sulfide assemblage and presence of native sulfur, together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at SuSu Knolls, are consistent with a submarine high sulfidation magmatic Cu-Au hydrothermal system. Furthermore, the sulfide assemblage pyrite-enargite (±covellite-chalcopyrite) observed at North and South Su, and orders-of-magnitude enrichment of chalcophile elements in both altered and mineralized volcanic rocks, relative to unaltered volcanic rock, are also characteristic of subaerial high sulfidation epithermal mineralization. The SuSu Knolls hydrothermal field is a good example of a modern, high sulfidation, Cu-Au submarine hydrothermal system. [ABSTRACT FROM AUTHOR]

A subseafloor replacement-style barite and sulfide occurrence was drilled in shallow waters at the Palinuro volcanic complex, the northernmost Aeolian arc volcano in the Tyrrhenian Sea, Italy. Using a lander-type drilling device, 11 successful drill holes yielded a total of 13.5 m of core from a sediment-filled depression located at a water depth of 630 to 650 m. The longest continuous drill core recovered consists of 4.84 m of massive to semimassive barite and sulfides with abundant late, native sulfur overprint. Seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active, although black smoker activity does not occur on the seafloor. The recovered drill core shows that the subseafloor deposit is zoned with depth. The top of the mineralized zone is comprised of a variably silicified vuggy barite-sulfide facies that shows notable polymetallic metal enrichment, while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades. Metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time. The barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis. During the main stage of the mineralization, the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite, trace famatinite, and rare cinnabar. Lower temperature precipitates formed during the main stage of mineralization include sphalerite, galena, pyrite, opal-A, and barite, which are associated with traces of Pb-Sb-As sulfosalts such as bournonite-seligmannite, or semseyite. A distinct mineral assemblage of fine-grained anhedral enargite, hypogene covellite, chalcopyrite, and galena is commonly associated with colloform sphalerite, galena, and pyrite as a late phase of this main stage. Colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence. The deposit is interpreted to have formed from fluids having an intermediate-sulfidation state, although excursions to high- and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite. Laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition (δ34S = -39 to + 3‰), whereas a more narrow range is observed in the massive pyrite at depth (δ34S = -10 to 0‰). Similar variations were also documented for the late native sulfur overprint. Overall, the negative sulfur isotope ratios at depth, the intermediate- to very high sulfidation conditions during mineralization, and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth. Biological processes are inteipreted to have played a major role during late stages of ore formation. The combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite, the intermediate- to high-sulfidation characteristics, and the strong biological influence on the late stages of mineralization are distinct from other modem seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modem volcanic arc environment. The barite and sulfide occurrence at Palinuro shares many characteristics with porphyry-related base metal veins and intermediate-sulfidation epithermal deposits, suggesting that metallogenic processes associated with arcrelated magmatic-hydrothermal systems are not restricted to the subaerial environment. [ABSTRACT FROM AUTHOR]

The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of ~25 km², including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki and Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y, from ~3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks and volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, -2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including the -1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the ~0.02 Ma Lapangan tuff. Extensive advanced argillic alteration crops out for -7 km along the unconformity between the basement rocks and the Imbanguila dacite formation and consists of quartz-alunite ± pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite ± kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is -4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the -1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches -600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore. This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion: The alunite absorption peak at -1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spec-trometry (LA-ICP-MS) indicates that die Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Pb, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu <0.1wt % and Au <0.1 ppm) and alunite-bearing samples, show that Pb and Ag/Au, plus Hg and Ag, decrease toward the intrusive center, and Sr/Pb and La/Pb ratios increase. Normalizing whole-rock Pb to the (Na + K) molal content produces a proxy for the alunite mineral composition, and this ratio provides the same indications as the LA-ICP-MS analyses of alunite. The concealed Victoria epithermal veins consist of intermediate sulfidation mineralization on the southwest flank of the porphyry. The veins are not exposed, but their presence at depth is indicated by subtle alteration (illite or interstratified illite and/or smectite or smectite + pyrite) and geochemical (As, Se) anomalies at the surface. The anomalies are strongly dependent on erosion level; no anomalies were found where the surface is >~350 m above the upper extent of the veins. An airborne geophysics survey indicates that the Far Southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite-destructive phyllic alteration. Such low magnetic anomalies on the margin of a large lithocap elsewhere may deserve attention. The directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry-epithermal districts. [ABSTRACT FROM AUTHOR]

Mineralized hydrothermal breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal brecciation. Explosive phreatic brecciation was followed by in situ hydraulic brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia-and/ or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 mad 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (~0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ~4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal breccia bodies. [ABSTRACT FROM AUTHOR]

Early Miocene volcanism associated with a maar-diatreme breccia complex preceded main-stage epithermal gold mineralization at the Kelian gold mine, Kalimantan, Indonesia. Prior to brecciation, andesite intrusions (19.7 ± 0.06 Ma) were emplaced into a package of felsic volcaniclastic rocks and overlying carbonaceous sandstones and mudstones, and a weakly mineralized geothermal system was established. Intrusion of quartz-phyric (19.8 ± 0.1 Ma) and quartz-feldspar-phyric rhyolite (19.5 ± 0.1 Ma) into the active geothermal system triggered widespread fragmentation and formation of the maar-diatreme complex. Subsurface phreatomagmatic and phreatic explosions disrupted the preexisting hydrothermal system, producing three composite diatreme breccia bodies (the Tepu, Runcing, and Burung Breccias). The diatremes consist of polymict breccias and sandstones that contain abundant carbonaceous matrix. A distinctive facies association comprising coherent rhyolite, jigsaw-fit rhyolite breccia, and matrix-rich breccias that contain wispy to blocky juvenile rhyolite clasts define the root zones of the diatremes. The surficial products of maar-diatreme volcanic activity at Kelian are preserved as large blocks of well-stratified breccias. They contain accretionary lapilli and were deposited by a combination of wet, pyroclastic base-surge, fallout, and cosurge fallout processes. Evidence for syneruptive resedimentation of the pyroclastic deposits is preserved in poorly stratified breccia beds. Megablocks of phreatomagmatic base-surge deposits were dropped down several hundred meters from the maar environment into the underlying diatremes. Volcanism in the Kelian maar-diatreme complex was dominated by a combination of phreatomagmatic and phreatic processes, with subordinate hydraulic, tectonic, and dry magmatic fragmentation. The carbonaceous matrix-rich diatreme breccias acted as aquicludes during subsequent hydrothermal activity, focusing fluid flow into the wall rocks adjacent to the diatremes, where epithermal gold mineralization and hydrothermal brecciation occurred. [ABSTRACT FROM AUTHOR]

Massive sulfides at the Myra Falls volcanic-hosted massive sulfide (VHMS) camp, Vancouver Island, British Columbia, Canada, are overlain by white chert, black chert, argillite, and siltstone. White chert is best developed above the Battle orebody, where it forms a siliceous caprock (3-5 m thick) above the massive sulfides. There is a gradational lateral change from white chert above massive sulfides to black chert and unaltered argillite, 100 to 150 m south of the Battle orebody. Chert horizons are 'also located above the Ridge and Extension ore zones, but only minor chert lies above the HW orebody, which is instead overlain by a thick argillite sequence. The chert and argillite share similar sedimentologic and petrologic features, including abundant parallel laminations, interbedded turbidites, radiolarian-rich layers, soft-sediment deformation, scours, flame structures, and small phosphatic concretions. These features indicate that white and black chert formed as a replacement of mudstone rather than as exhalative or biogenic deposits. Silicification occurred early in the depositional history of the fine-grained sediments and was contemporaneous with some ore formation. Early syndepositional features are still visible in the chert, with primary pore spaces such as radiolarian tests filled by silica, rutile, apatite, and minor sulfides displaying open-space crystal growth. The presence of minor ore-clast breccias above the orebody indicates that at least parts of the Battle orebody were exposed on the sea floor. Metal zoning is observed in the cap-rock horizon above the Battle orebody, with higher Cu, Zn, and Cd contentrations in chert directly above massive sulfides, higher Pb, Sb, and Ag contentrations in black chert at the edge of the siliceous cap rocks and lower metal concentrations in the distal argillite. Primary fluid inclusions in spherical quartz patches in chert above the Battle orebody indicate that hydrothermal fluids passing through sediment were between 135 to 250°C and had salinities ranging from 3.0 to 12.1 wt percent NaCl equiv. These data are similar to those for fluid inclusions measured in quartz interstitial to sulfides in the underlying Battle orebody, which have temperatures of homogenization ranging from 140° to 250°C and salinities from 3.0 to 12.4 wt percent NaCl equiv. Fluid inclusions in the Battle orebody display a slight increase in salinity and homogenization temperature with depth, which may reflect the overprinting of earlier high temperature stages by cooler fluids as the hydrothermal system waned or varying degrees of mixing between hydrothermal fluids and seawater. A minimum depth for deposition of the cap rocks (>200 m) is estimated, based on sedimentologic features such as fine parallel laminations and interbedded sandstone turbidites, which indicate deposition below storm wave base. Greater water depths (1,000-1,500 m) are suggested by the lack of evidence of boiling in fluid inclusions. Low O2 concentrations in the bottom water of the Battle basin are suggested by the absence of bioturbation, lack of fossils of benthic fauna, degree of pyritization v',dues >0.90, elevated Zn, Pb, Cu, Cd, As, Sb, Ag, Ba, and V, low Fe and Mn, and W(V + Ni) > 0.8 in the unaltered argillite. Paleosea-floor reconstructions indicate that the Battle and HW orebodies formed in small basins along a northwest-trending ridge. The finegrained sediments were deposited in depocenters within paleotopographic lows.… [ABSTRACT FROM AUTHOR]

The Waterloo massive sulfide deposit occurs in the Cambro-Ordovician Seventy Mile Range Group in the Charters Towers district in northern Queensland, one of the principal submarine successions in eastern Australia that contains abundant volcanic-hosted base metal deposits. The deposit consists of several small dismembered, stratiform, blanketlike, pyrite-chalcopyrite-sphalerite-galena massive sulfide lenses hosted by intensely altered volcanic rocks. The deposit and its host rocks have been deformed during regional deformation of the Middle to Late Ordovcian and were affected by two subsequent events of brittle deformation. The Waterloo deposit formed on top of a lava-dominated andesite succession comprising coherent volcanic units and related juvenile volcaniclastic rocks. The hydrothermal activity leading to the deposition of massive sulfides commenced during the waning stages or after the cessation of the andesite volcanism. The blanketlike mineralization is interpreted to have formed largely at or very close to the sea floor in a moderately deep to deep marine environment. Formation of the massive sulfides was accompanied by the mass-flow deposition of coarse quartz-feldspar crystal-rich volcanic debris that records broadly contemporaneous, probably explosive, rhyolitic volcanism occurring outside the immediate study area. This resedimented volcaniclastic unit retains a record of hanging-wall alteration, indicating that the Waterloo hydrothermal system was operative during and after burial by the coarse volcaniclastic material. The deposition of the coarse volcaniclastic rocks was followed by the intrusion of a feldspar-porphyritic dacite cryptodome that was also subject to hydrothermal alteration associated with the massive s ulfide formation. Facies relationships suggest that the cryptodome locally breached the wet and unconsolidated coarse volcaniclastic material to emerge on the sea floor. The emplacement of the partially extrusive dacite cryptodome coincided with the wani ng of the Waterloo hydrothermal system. The subsequent period of relatively quiet sedimentation was occasionally interrupted by the emplacement of synsedimentary basalt and andesite sills as volcanism at Waterloo shifted back to more mafic compositions. This period was followed by the mass-flow deposition of coarse feldspar-quartz-vitric volcanic debris and the onset of another period of intense basalt- to andesite-dominated volcanism. Geochemical analyses of volcanic rocks from the host-rock succession of the Waterloo deposit and correlation with regional data suggest that the massive sulfide deposits in the Charters Towers district formed during a specific period of Cambro-Ordovician back-arc basin development dominated by mantle-derived volcanism. Comparison with the volcanic architecture of other known massive sulfide deposits in the district indicates that massive sulfide mineralization at Charters Towers is mainly associated with silicic shallow, intrusion-dominated volcanic centers and occurrences of this volcanic facies should be considered prospective for other deposits in the belt. [ABSTRACT FROM AUTHOR]

Conical seamount is a submarine alkali basalt volcano situated 10 km south of Lihir Island in the Tabar-Feni island chain, Papua New Guinea. Polymetallic (Zn-Pb-Ag-Au-As-Sb) veins, pyritic stockwork, and hydrothermally altered rocks discovered on the summit of Conical seamount represent a unique example of submarine gold mineralization with mineralogical, chemical, and textural characteristics common to some subaerial epithermal systems. The giant Ladolam epithermal gold deposit (42 Moz Au) is situated in the crater of an alkali stratovolcano on Lihir Island. Portions of the epithermal mineralization and alteration assemblages at Ladolam are similar to those observed on Conical seamount. Conical seamount has the lightest, hydrothermal (i.e., nondiagenetic) sulfide δ34S values (-17.5 to +6.1&Permil;) measured to date from modern sea-floor hydrothermal systems. A previous model for the Conical seamount hydrothermal system suggested a contribution of magmatic volatiles in the earliest stages of mineralization (stage 1), followed by a combination of sulfur leached from the underlying volcanic rocks and reduced seawater sulfate during the main base and precious metal precipitating event (stage 2). New sulfur isotope data allow a reinterpretation of this model that suggests a greater input of magmatic volatiles, plus boiling, throughout both stage 1 and 2 mineralization. Sulfides from the Ladolam gold deposit have a range of sulfur isotope values (-12.9 to +3.6&Permil;) similar to that of Conical seamount. These sulfur isotope data have previously been interpreted as the result of magmatic volatiles escaping from a crystallizing magma and mixing with dilute meteoric ground water. Anhydrite associated with the gold mineralization has two populations of δ34S values (8-14 and 20-22&Permil;), which indicate the interaction of magmatic volatiles with seawater that has previously been discounted. The range of sulfide and sulfate... [ABSTRACT FROM AUTHOR]

A preface to the August 2007 issue of "Economic Geology" is presented.