Abstract Many ore deposits are hosted by metamorphic rocks, and metamorphic fluids have been invoked as a source for various deposits, especially gold deposits. Metamorphic fluid compositions reflect original sedimentary environment: continental shelf sequences yield saline metamorphic fluids with little dissolved gas while metasediments from accretionary and oceanic settings host less saline fluids with significant CO 2 contents. The principal difficulty in reconciling ore deposits with a metamorphic origin is that many form quickly ( c. 1 Ma), whereas metamorphic heating is slow ( c. 10–20 °/Ma). Gravitational instability means that fluid cannot be retained. Metamorphic ores may nevertheless form by: (a) segregation leading to enrichment of pre-existing concentrations; (b) infiltration of water-rich fluids from schists into marbles at high temperature overstepping decarbonation reactions and allowing fast reaction that locally draws down temperature; and (c) rapid uplift driving dehydration reactions owing to pressure drop. Some orogenic lode gold deposits fit well with a purely metamorphic origin during rapid uplift, but others are problematic. At Sunrise Dam, Western Australia, anomalies in Sr-isotope ratios and in apatite compositions indicate a partial mantle/magmatic source. Low salinity, H 2 O–CO 2 fluids commonly associated with hydrothermal gold reflect the effect of salt on gas solubility, not the origin of the fluid.
Book Reviews Atlas of Metamorphic-Metasomatic Textures and Processes, by S. S. Augustithis. Elsevier, Amsterdam, 1990. 238 pp. US$177.25. B. W. D. Yardley B. W. D. Yardley Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 32, Issue 6, December 1991, Page 1337, https://doi.org/10.1093/petrology/32.6.1337 Published: 01 December 1991
Laser ablation.inductively coupled plasma.mass spectrometry (LA-ICP-MS) has become recognized as a sensitive, efficient, and cost-effective approach to measuring the major-, minor-, and trace-solute compositions of individual fluid inclusions in minerals. As a prerequisite for the routine analysis of natural inclusions in our laboratory, the precision and accuracy of the technique was assessed using sets of multi-element synthetic fluid inclusions. Five multi-element standard solutions were prepared, and incorporated as fluid inclusions in quartz crystals at 750 °C and 7 kbar. Fluid inclusions were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole reaction cell for the removal of Ar-based interferences. The internal standard used in all cases was Na. Analytical precision for K, Rb, and Cs is typically better than 15% RSD, whereas Li, Mg, Ca, Sr, Ba, Mn, Fe, Cu, Zn, and Cl analyses are typically reproducible within 30% RSD. Measured concentrations approximate a Gaussian distribution, suggesting that analytical errors are random. Analyses for most elements are accurate within 15%. Limits of detection vary widely according to inclusion volume, but are 1 to 100 µg/g for most elements. These figures of merit are in excellent agreement with previous studies. We also demonstrate that, over the range investigated, precision and accuracy are insensitive to inclusion size and depth. Finally, the combination of our LAICP- MS analyses with microthermometric data shows that charge-balancing to NaCl-H2O equivalent chloride molality is the most valid approach to LA-ICP-MS data reduction, where chloride-dominated fluid inclusions are concerned.
Fractured Crystalline Rocks (FCR) are being considered in several countries as hosts for radioactive waste repositories. In FCR, radionuclides may be transported relatively rapidly by bulk groundwater flow through open fractures, but much more slowly by diffusion through porewater in the rock matrices. Rock matrix diffusion (RMD) is the diffusion of radionuclides in the aqueous phase, between open fractures and rock matrices. Sorption or co-precipitation on the fracture surfaces and walls of the matrix pores causes further radionuclide retardation. RMD may be important in a repository's safety case and has been investigated by many published short-term (to a few years) laboratory and in-situ experiments. To improve understanding over longer timescales, we investigated evidence for RMD of several natural radioelements, and radioelement analogues, in five exemplar fractured crystalline rock (FCR) samples aged between c. 70 Ma and c. 455 Ma. The sample suite consisted of two samples of Borrowdale Volcanic Group (BVG) meta-tuff from northwest England, a sample of Carnmenellis Granite from southwest England and two samples of Toki Granite from central Japan. Uptake or loss of the studied elements is limited to an altered damage zone in each sample, coupled to mineral alteration processes. These zones are most extensive (a few tens of millimetres) in the Toki Granite samples. We also found unstable primary igneous minerals to persist in the immediate wallrocks of fractures in studied granite samples, suggesting that pores were not permanently water saturated in these samples. Although only a small sample suite was studied, the results show that while RMD may be important in some kinds of FCR, in others it may be negligible. Site-specific information is therefore needed to determine how much reliance can be placed on RMD when developing a safety case.
As noted above, crustal fluids are generally composed predominantly of H2O, with C-species, various salts and variable amounts of rock-forming components. Both nearly pure CO2 fluids and high salinity brines ([Fig. 1.1][1]) occur in some environments. As such, fluids have distinctly lower densities
Research Article| March 01, 1988 Immiscible fluids in metamorphism: Implications of two-phase flow for reaction history B.W.D. Yardley; B.W.D. Yardley 1Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England Search for other works by this author on: GSW Google Scholar S. H. Bottrell S. H. Bottrell 1Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England Search for other works by this author on: GSW Google Scholar Author and Article Information B.W.D. Yardley 1Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England S. H. Bottrell 1Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (3): 199–202. https://doi.org/10.1130/0091-7613(1988)016<0199:IFIMIO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B.W.D. Yardley, S. H. Bottrell; Immiscible fluids in metamorphism: Implications of two-phase flow for reaction history. Geology 1988;; 16 (3): 199–202. doi: https://doi.org/10.1130/0091-7613(1988)016<0199:IFIMIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Fluid immiscibility between a CO2-rich fluid and brine has been described from several medium- to high-grade marbles and calcsilicates. Here we also report evidence for CH4-H2O immiscibility in low-grade metamorphic rocks from Wales and suggest that the flow of such immiscible fluids may be comparable to that of water-oil or water-gas mixtures in petroleum reservoirs. This analogy implies that where one fluid is being produced by reaction it will flow out of the rock while the other remains immobile, except insofar as it is soluble in the more abundant fluid phase. Hence, extensive reaction may take place in the presence of coexisting immiscible fluids, each having high activities. It is proposed that this type of flow behavior not only accounts for the development of salt-saturated fluids in marbles but also may be responsible for the elimination of carbonate and extensive loss of graphite from pelites in the lower part of the greenschist facies. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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