Duration Of Hydrothermal Activity Research Articles

Mineralogy and sulfur isotopic compositions of sulfides from Yunzang (25.3°S) hydrothermal field, South Mid-Atlantic Ridge: Implications for formation mechanism and maturation of sulfide chimneys

A newly discovered hydrothermal field, named Yunzang and predominantly hosted by mafic rocks, was recently identified at 25.3°S on the South Mid-Atlantic Ridge (SMAR). This is the first report on the sulfide mineralogy and sulfur isotopic composition of sulfide chimney and massive sulfide ores collected from Yunzang field, SMAR. Based on mineralogical morphology, texture, crystallinity, assemblage and zonation, five specific stages of mineralization can be divided for the sulfide chimney at Yunzang. These stages range from low- to high-temperature hydrothermal conditions, along with an additional stage of seafloor weathering. The observed variations in morphology and crystallinity from low- and medium-temperature stages to high- and high- to medium-temperature stages suggest an evolution in the ore-forming conditions. Initially, the environment was characterized by instability and low-temperature conditions, with significant seawater influx. Subsequently, there was a shift towards a more stable regime dominated by high-temperature ore-forming fluid condition. The positive δ34S values of all the sulfides from Yunzang hydrothermal field suggest 79–92 % and 69–79 % of sulfur were generated from the leaching of basalt for sulfide chimney and massive sulfide, respectively, the rest sulfur were from reduced seawater sulfate. The δ34S values of massive sulfide (3.5–6.8 ‰, average = 5.1 ± 0.9 ‰) are mostly higher than that of sulfide chimney (1.1–4.5 ‰, average = 2.8 ± 0.8 ‰) are mainly caused by sub-surface reactions between preexisting sulfate and re-circulating fluid. An incremental increase of δ34S values from the external edge to the interior orifice indicating the Yunzang chimney maturation is relatively lower and the duration of hydrothermal activity was short. These results enrich our comprehension of mineralization in the seafloor hydrothermal system.

Evidence for Remobilization of Barite Affecting Radiometric Dating Using 228Ra, 228Th, and 226Ra/Ba Values: Implications for the Evolution of Sea-Floor Volcanogenic Massive Sulfides

Samples of chimneys (black smokers) and other volcanogenic massive sulfide (VMS) mineralization were collected from hydrothermal vent fields associated with submarine arc and back-arc volcanoes in the western Pacific. Each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site, information that is vital when establishing the time elapsed to amass an ore deposit. In each case, the radiometric dating relied on the radium isotopes 228 Ra and 226 Ra with half-lives of 5.75 and 1,600 years, respectively, which coprecipitate with barite and then decay. We began dating VMS mineralization 15 years ago, knowing that the 226 Ra/Ba decreases with time, with the Ba and Ra assumed to be from a common source such that the initial 226 Ra/Ba value, i.e., the value at the onset of mineralization, was constant. Thus, the age of an “old” chimney (up to 15,000 yr) was calculated by comparing its 226 Ra/Ba value with the initial value established from nearby “young” chimneys ( 228 Ra. Since then, we have discovered that chimneys and other forms of sea-floor VMS mineralization can contain barite from two or more hydrothermal events. That is, older barite can be dissolved and reprecipitated (remobilized) from earlier mineralization deposited subseafloor, or earlier sea-floor mineralization overprinted (crosscut) by new. In many cases, the 226 Ra/Ba dating did not provide an absolute age for the older material but, rather, a difference in age between two samples that had incorporated varying proportions of older barite that was low or “dead” (i.e., >15,000 yr) with respect to 226 Ra, due to radioactive decay. Remobilizing of older barite is consistent with zone refining of metals in large VMS deposits, but we have only considered this recently. However, our investigations show that this can affect the initial 226 Ra/Ba values for VMS mineralization from arc and back-arc volcanoes, and also from mid-ocean ridges. Where sufficient samples contained 228 Ra, we estimated an age for the remobilized barite by plotting a 226 Ra/Ba versus 228 Ra/Ba mixing line for the VMS at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for >15,000 years. Such long histories of hydrothermal activity indicate the propensity for producing large metal-rich deposits on the seafloor.

Timing and duration of hydrothermal activity at the Los Bronces porphyry cluster: an update

New geochronological data from the Los Bronces cluster of the Rio Blanco-Los Bronces mega-porphyry Cu-Mo district establish a wide range of magmatism, hydrothermal alteration, and mineralization ages, both in terms of areal extent and time. The northern El Plomo and southernmost Los Piches exploration areas contain the oldest barren porphyritic intrusions with U-Pb ages of 10.8 ± 0.1 Ma and 13.4 ± 0.1 Ma, respectively. A hypabyssal barren intrusion adjacent northwesterly to the main pit area yields a slightly younger age of 10.2 ± 0.3 Ma (San Manuel sector, U-Pb), whereas in the Los Bronces (LB) open-pit area, the present day mineral extraction zone, porphyries range from 8.49 to 6.02 Ma (U-Pb). Hydrothermal biotite and sericite ages are up to 0.5 Ma younger but consistent with the cooling of the corresponding intrusion events of each area. Two quartz-molybdenite B-type veins from the LB open pit have Re-Os molybdenite ages of 5.65 ± 0.03 Ma and 5.35 ± 0.03 Ma consistent with published data for the contiguous Rio Blanco cluster. The San Manuel exploration area within the Los Bronces cluster, located about 1.5–2 km southeast of the open-pit extraction zone, shows both the oldest hydrothermal biotite (7.70 ± 0.07 Ma; 40Ar/39Ar) and breccia cement molybdenite ages (8.36 ± 0.06 Ma; Re-Os) registered in the entire Rio Blanco-Los Bronces district. These are also older than those reported from the El Teniente porphyry Cu(-Mo) deposit, suggesting that mineralization in the late Miocene to early Pliocene porphyry belt of Central Chile commenced 2 Ma before the previously accepted age of 6.3 Ma.

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province, China

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126Ma Guojialing granite. These deposits collectively formed at ca. 120±5Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (>120t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New 40Ar/39Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130±4Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1km/m.y. from depths of 25–30km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.

Constraints on fluid evolution during metamorphism from U–Th–Pb systematics in Alpine hydrothermal monazite

The timing and duration of hydrothermal activity during orogenesis are difficult to constrain, because such systems are open and multistage. Using mm-sized monazite crystals from two Alpine clefts from the Central Alps (Switzerland), we demonstrate that combined U–Th–Pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization. Our data highlight four major characteristics of the cleft monazites: (i) extreme Th/U ratios (ii) significant common Pb in the U system (up to 39% 206Pb), (iii) excess 206Pb (up to 54%), and (iv) precise and reliable Th–Pb ages.Comparison of our results with literature data indicates that Th/U in monazite is a remarkable discriminant of geological conditions, capable of distinguishing metasedimentary, magmatic and hydrothermal origin. Hydrothermal monazite crystals are characterized by Th/U ratios up to 629, among the highest values ever reported. Extreme Th/U ratios in monazite enhance incorporation of 230Th, a short-lived intermediate product in the 238U–206Pb decay chain, generating 206Pbexcess which, if not accounted for, results in 206Pb/238U ages that are too old, for example in the samples investigated here by as much as 300%.For the two zoned monazite crystals studied in detail, 208Pb/232Th ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. 232Th/208Pb ages for the older domains (15.2±0.3Ma and 14.1±0.3Ma, respectively) are consistent with the structural relationships of the hydrothermal veins, indicating early retrograde genesis. The youngest rim age at 13.5±0.4Ma for the Blauberg crystal marks termination of monazite precipitation. The resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity.In both crystals, the outer rims have the highest 206Pbexcess, confirming a two-stage crystallization. Two scenarios are envisaged to account for the 206Pbexcess evolution. In the first, increasing 206Pbexcess is caused by a modification of the Th/U fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent U into the fluid. Alternatively, it is possible that 230Th increased with time in the fluid. In this second case, monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238U–206Pb decay chain and progressively equilibrated (t<0.5Ma).

Exothermic and endothermic soils of Iceland

This paper is based on the results of the study of soils and soil-forming rocks on five key sites in Iceland, including the coastal plain (the Hvammur key site), piedmont plain (the Hveragerði key site), the Holocene lava plateau (the Reykjanes key site), and the zone of modern hydrothermal activity in the north of the rift zone of Iceland (the Theistareykir and Namafjall key sites). The studied soils are subdivided by us into the groups of exothermic and endothermic soils. Exothermic soils are the soils that develop from the congealed volcanic deposits. These soils are specified by the homogeneous dark gray color and sandy texture. Their mineral components are weakly transformed. Exothermic soils developed under good drainage conditions have the low content of organic matter (about 1.5% Corg). The soil reaction is neutral within the entire profile. Under impeded drainage conditions, the organic carbon content in the exothermic soils reaches its maximum of about 7%, and the soil reaction varies from acid to neutral values. Endothermic soils are the soils that are subjected at present or were subjected in the past to the high-temperature hydrothermal metamorphism. They have mottled color patterns with sharp color contrast in the soil profile; their texture is loamy or clayey. These soils are rich in secondary minerals with a predominance of smectite; kaolinite, pyrite, anatase, gypsum, and other secondary minerals can also be found in them. The properties of endothermic soils are largely controlled by the provincial features of the hydrothermal activity, including the chemical composition of hydrothermal water, the soil temperature, the duration of hydrothermal activity, etc.

The morphology of intrusion-related vent structures and their implications for constraining the timing of intrusive events along the NE Atlantic margin

Intrusion-related vent structures found within Palaeocene sediments and near the Palaeocene–Eocene transition in basins along the NE Atlantic margin have been studied in detail using 3D seismic data. The vents formed at the palaeo-seabed as a result of sill intrusion and the vent fill ranges in composition from entirely remobilized sedimentary to dominantly magmatic. The vents are commonly developed above sill edges and crests in sills to which they are linked by pipe- and diatreme-like structures. The vents also develop above the upper tips of deep-seated faults and at fault intersections. The vents can be used to constrain the approximate timing of igneous sill intrusion and duration of hydrothermal activity. It is estimated that using the seismically constrained stratigraphic context of these vents to establish the timing of sill emplacement is associated with an uncertainty of &gt;100 ka and up to as much as 800 ka. Interpretation of the vents indicates that sill intrusion took place during a number of discrete phases within the Palaeocene and earliest Eocene. These structures and their timing of formation have important implications for climate change studies, basin-scale processes including fluid flow and diagenesis, as well as hydrocarbon exploration.

Evaluation of Radioisotope Dating of Carlin-Type Deposits in the Great Basin,Western North America, and Implications for Deposit Genesis

A variety of techniques have been used in attempts to date the mineralization in Carlin-type deposits in the Great Basin, with highly variable results. These techniques and results are reviewed in this paper, along with presentation of two new dates, for the Rodeo deposit on the Carlin trend and for the Barneys Canyon deposit in Utah. Complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form Carlin-type deposits is considered highly unlikely. Therefore, dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which Au was deposited. For the deposits that have been investigated to date, sericite dates rarely, if ever, record the age of Au mineralization. However, sericite ages do appear to record pre-Au events in some districts. Such events may have contributed to ground preparation and, to a much lesser extent, to the tenor of the ore. Fission-track and U/Th-He techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with Au. Rb-Sr dating of galkhaite, an Hg sulfosalt, provides the only direct age of mineralization, but galkhaite has been recognized in only a few locations (and dated in only two deposits). In a similar manner, adularia is contemporaneous with Au at Twin Creeks, but Twin Creeks is the only Carlin-type deposit where adularia has been reported. Several other techniques (U-Pb, ReOs, Sm-Nd) have been used in attempts to date the deposits but with limited success. The best available ages are those from preore and postore igneous events and those from galkhaite. Galkhaite from Getchell indicates a mineralization age of ~39 Ma and that from Rodeo (Carlin trend) indicates a mineralization age of 40 Ma. A new U/Th-He age (34 Ma) for apatite at Barneys Canyon suggests contemporaneity with the nearby Bingham porphyry Cu system. The most reliable data from several deposits and districts in the Great Basin indicate that Carlin-type mineralization occurred in a relatively narrow time interval, between 33 and 42 Ma. This interval coincides with the transition from a compressional to an extensional tectonic environment in the region. Although similar tectonic environments have existed at different times and places in western North America, no other Carlin-type mineralization is known to exist in the region. Therefore, although the age and tectonic environment of these deposits is now reasonably well constrained, it is clear that tectonic environment alone is insufficient to explain the genesis of Carlin-type deposits. Additional, perhaps unique, factors such as host rock geochemistry must have contributed to the development of these exceptional deposits.

Duration of hydrothermal activity at an oceanic spreading center, Mid-Atlantic Ridge (lat 26°N)

Hydrothermal manganese oxide coats talus on the Mid-Atlantic Ridge at lat 26/sup 0/N until spreading moves the rock past the thermally and structurally active rift-valley wall. Hydrothermal activity is replaced by hydrogenous ferromanganese oxide precipitation on ocean crust older than 0.7 m.y. on the ridge-crest highlands.