Sarah A. Gleeson

Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield

We have determined the chemical composition of ∼350-m.y.-old solutions extracted from fluid inclusions, and strontium isotopic compositions of hydrothermal minerals from the Irish zinc-lead orefield. These data show that ore-forming fluids were derived from evaporated seawater and acquired metals by deep circulation within fractures in continental crust. Mineralization occurred in the near-seafloor environment when these solutions returned to the surface via thermohaline convection and mixed with brines rich in H 2 S produced by bacterial reduction of seawater sulfate. The results indicate that deep penetration of seawater or evaporated seawater into the continental crust can occur in rift zones or extending passive margins and that this process can generate large volumes of base metal ore-forming solutions. Our results are inconsistent with topographic flow models for mineralization in the district, and support deep convection models for ore formation. The widespread development of evaporitic brines on the Laurussian continental margin under late Paleozoic greenhouse conditions is likely to have been critical for generating numerous accumulations of base metals in sedimentary basins at this time.

Origin of retrograde fluids in metamorphic rocks

Retrograde alteration of crystalline basement rocks is often the result of infiltration of water from sedimentary basins or the surface. At high crustal levels, fluid advects at near-hydrostatic pressure, and large fluxes can lead to mineralisation as well as retrogression. In the mid-crustOT . 3008CU surface-derived fluids preserved in fluid inclusions show evidence of overpressuring. It is proposed that fluid is pumped down into dry basement during faulting and causes a ductile response with recrystallisation that leads to them being sealed in and compressed. The distribution of fluid inclusion homogenisation temperatures is quite different for the two cases, and more extensive wall rock reaction at elevated temperatures can increase salinity. q 2000 Elsevier Science B.V. All rights reserved.

The high-temperature behavior of defect hydrogen species in quartz: Implications for hydrogen isotope studies

Abstract A micro-infrared spectroscopic study of hydrothermal vein quartz known to have anomalous δD signatures has identified two hydrogen reservoirs. In samples that generate an isotopic signature in accordance with that anticipated under the accepted model of quartz crystallization, submicroscopic aggregates of liquid water are the dominant hydrous species. Samples which generate an anomalous δD signature contain, in addition to liquid water, structurally incorporated hydrous species associated with impurity cations. Infrared spectra obtained during in situ stepped heating experiments, coupled with infrared analysis at 25 ℃, demonstrate that hydrogen liberated between 300 and 500 ℃ is chiefly molecular, liquid water. Hydrogen liberated at temperatures greater than 500 ℃ is dominantly that associated with specific structurally incorporated cation defects. Since both defect hydrogen and molecular water are contemporaneously incorporated from the precipitating medium during crystallization, we propose that irregular δD signatures, released following decrepitation at temperatures greater than 500 ℃, are due to isotopically fractionated hydrogen released from interstitial OH defect sites in the quartz structure. δD signatures obtained from stoichiometrically anhydrous minerals are generally interpreted under the assumption that the hydrogen measured comes uniquely from decrepitated fluid inclusions. Instead, we suggest that δD ratios obtained from hydrothermal quartz reflect a sum of the contributions made by individual hydrogen reservoirs, each with a potentially distinctive δD signature. Thus, if the overall δD signature is attributed entirely to fluid inclusion phases, the nature of the precipitating fluid may be misinterpreted. Hydrogen extracted as molecular water at between 300 and 500 ℃ provides a true reflection of the hydrothermal solution associated with crystal precipitation.

A basement-interacted fluid in the N81 deposit, Pine Point Pb-Zn District, Canada: Sr isotopic analyses of single dolomite crystals

The Mississippi Valley-type Pb-Zn deposits of the Pine Point district (Northwest Territories, Canada) are located close to the eastern edge of the present day Western Canadian Sedimentary Basin. The deposits are thought to have formed as the result of basin-wide fluid flow in the Presqu’ile Barrier, the host to the ore deposits. A laser ablation multi-collector inductively coupled plasma mass spectrometric study of 87Sr/86Sr ratios of ore-related dolomites from the N81 deposit at Pine Point indicates that at least two sources of Sr were present in the mineralizing system. One fluid has a range in Sr isotopic values from 0.07073 to 0.71200 and is interpreted to be derived from Middle Devonian seawater that interacted with clastic units in the basin. The second fluid has higher Sr isotopic values (up to 0.71520), similar to those found in some Canadian Shield brines, and is interpreted to represent an evaporated seawater-derived brine which has interacted with crystalline basement rocks. Reactivation of old structures in the basement may have provided a pathway for cross-formation fluid flow to the site of mineralization. The data suggest that the stratigraphic location of the Pine Point District, near the interface between the Western Canadian Sedimentary Basin and its basement, may have exerted a fundamental control on the formation of these deposits.

Determination of the origin of salinity in granite-related fluids: evidence from chlorine isotopes in fluid inclusions

Thermal ionisation mass spectrometry of the Cs2Cl+ ion has been used to determine the δ37Cl value of two sets of magmatic fluids trapped in fluid inclusions. Samples were studied from the Capitan pluton in New Mexico and the SW-England batholith, both of which have high temperature fluids whose δD and δ18O values are distinctly magmatic. Relative to standard mean ocean chloride, the δ37Cl values of the Capitan fluid inclusions cluster around 0‰, while those from SW-England cluster around +1.9‰. We conclude that Cl in the Capitan magmatic fluids was derived from local evaporite sequences whereas in SW-England the Cl appears to be more representative of a deep magmatic source. In SW-England fluid inclusions of lower temperature and salinity, in E–W veins, have δ37Cl values that also appear to be magmatic. The younger N–S veins, that contain high salinity fluids, have δ37Cl values that indicate a seawater origin for the Cl.

Re-Os dating of pyrite confirms an early diagenetic onset and extended duration of mineralization in the Irish Zn-Pb ore field

The Irish Midlands region contains one of the world’s largest hydrothermal Zn-Pb ore districts, but uncertainty exists in the timing of mineralization relative to host rock ages. Consequently, genetic models for ore formation are poorly constrained and remain controversial. Here we use Re-Os geochronology to show that ore-stage pyrite from the Lisheen deposit formed at 346.6 ± 3.0 Ma, shortly after host rock deposition. Pyrite from the Silvermines deposit returns an age of 334.0 ± 6.1 Ma, indicating that at least some mineralization occurred during later burial. These age determinations show that the much younger paleomagnetic ages reported for the Irish Zn-Pb deposits reflect remagnetization during the Variscan orogeny, a process that we suggest affects paleomagnetic dating more widely. The Re-Os ages overlap with the ages of lower Carboniferous volcanic rocks in the Midlands, which are the product of magmatism that has been invoked as the driving force for hydrothermal activity. The relatively low initial Os ratios for both Lisheen (0.253 ± 0.045) and Silvermines (0.453 ± 0.006) are compatible with derivation of Os from these magmas, or from the Caledonian basement that underlies the ore deposits.

More than a trace of oxygen: Ichnological constraints on the formation of the giant Zn-Pb-Ag ± Ba deposits, Red Dog district, Alaska

Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag ± Ba district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. We present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. The burrow linings and infill are replaced by barite, hydrothermal quartz, and sulfide minerals, and the lack of compaction suggests that mineralization was diagenetically early. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, we propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.

Improved detection limits for transient signal analysis of fluid inclusions by inductively coupled plasma atomic emission spectrometry using correlated background correction

The detection limits for ICP-AES bulk analysis of fluid inclusions by thermal decrepitation are limited by fluctuations in background emissions from the plasma. The transient nature of the analyte signal prevents sequential off-peak estimation of background levels. The established technique of time-interpolation of background signals measured between the analyses of samples gives somewhat unrealistic estimates because the background is not measured during the decrepitation event. A new procedure, correlated background correction (CBC), uses the high degree of correlation between the relative fluctuations in the spectral background at different wavelengths to effectively predict the background emission under transient analyte peaks. An estimate of the spectral background is made at a wavelength with no analyte signal or spectral interference, in this case 371.030 nm which is otherwise used to measure yttrium. The relative fluctuation in this intensity is used to predict the background emission intensity at all analyte wavelengths during the decrepitation event. The detection limits achieved using CBC are improved typically by a factor of five over those estimated without any correction of background fluctuation, thus increasing the range of sample material amenable to analysis.

Constraints on the source and evolution of mineralising fluids in the Norrbotten Fe oxide-Cu-Au province, Sweden

Norrbotten County, Sweden, is well known for the iron oxide-apatite deposits of the Kiruna and Malmberget areas. These are spatially associated with iron oxide Cu (± Au) deposits and a genetic link between the two has been suggested as a part of the IOCG spectrum of deposits. Here, we report the results of a fluid inclusion microthermometry and bulk crush leach halogen and chlorine stable isotope study, carried out to test that hypothesis. Quartz veins from late stage veins from Fe oxide-apatite deposits and Cu-(Au) deposits contain hypersaline brine inclusions, with a salinity range of 32 to 38wt.% NaCl eq. in Fe-oxide bodies, and of 38 to 47wt% Nacl eq. in Cu-Au deposits. In Cu-(Au) deposits these are sometimes accompanied by CO2-rich inclusions.

Acceptance of the SEG Waldemar Lindgren Award for 2007

Mr. President, colleagues, and friends: I would like to thank the SEG for this prestigious award. I would also like to thank Richard Herrington and Jamie Wilkinson, who nominated me and, particularly, Richard, who traveled to chilly South Africa to participate in the awards dinner. I have to say I was very surprised to receive this award. I feel I do not fit the usual profile of a Lindgren awardee in that my research does not focus on “a deposit type”; instead, I use a broad range of field and analytical techniques to study a whole spectrum of hydrothermal ore deposits; in reality I (proudly!) describe myself as a generalist. I note that I am just the second woman to win this award since its inception. Several years ago it seemed that every conference I went to mentioned the personnel crisis facing our industry. Half the undergraduate and graduate students at the University of Alberta are women. Then, where do they go? It is a real shame that so many bright, talented people feel that they have to choose between a professional career in the earth sciences and reproducing the human race. If we are serious about needing more skilled people in the work force then it is time that we start being more creative about how we do business. I hope that it won’t be another 20 years before we see another female Lindgren awardee because, to quote one of my undergraduate students, “Geogirls rock”! This “Geogirl” is Irish and grew up in Nenagh, close to the Silvermines Pb-Zn deposit. I decided I wanted to be a geologist at the age of 14, partially due to the influence of my excellent geography teacher (Mr. McLoughlin) …