Skarns and Skarn Deposits
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Skarn deposits occur throughout the world and have been mined for a variety of elements. This paper describes the basic stages of skarn formation and the main causes of variation from the general evolutionary model. Seven major classes of skarn deposits (Fe, W, Au, Cu, Zn, Mo and Sn) are briefly described, and relevant geological and geochemical features of important examples are summarized in a comprehensive table. The important geochemical and geophysical parameters of skarn deposits are discussed, followed by a summary of important petrologic and tectonic constraints on skarn formation. Finally, exploration models are presented for several major skarn types, with a plea for field mapping as a fundamental basis for future studies.Keywords:
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Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb−Y−Nb and Rb−Yb−Ta space, particularly on projections of Y−Nb, Yb−Ta, Rb−(Y + Nb) and Rb−(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in the elucidation of post-Archaean tectonic settings.
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Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type ( eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.
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Research Article| February 01, 1993 Zonation patterns of skarn garnets: Records of hydrothermal system evolution Bjørn Jamtveit; Bjørn Jamtveit 1Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, England Search for other works by this author on: GSW Google Scholar Roy A. Wogelius; Roy A. Wogelius 2Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, England Search for other works by this author on: GSW Google Scholar Donald G. Fraser Donald G. Fraser 2Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, England Search for other works by this author on: GSW Google Scholar Author and Article Information Bjørn Jamtveit 1Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, England Roy A. Wogelius 2Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, England Donald G. Fraser 2Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (2): 113–116. https://doi.org/10.1130/0091-7613(1993)021<0113:ZPOSGR>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 Bjørn Jamtveit, Roy A. Wogelius, Donald G. Fraser; Zonation patterns of skarn garnets: Records of hydrothermal system evolution. Geology 1993;; 21 (2): 113–116. doi: https://doi.org/10.1130/0091-7613(1993)021<0113:ZPOSGR>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 Chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower Paleozoic sedimentary rocks within the contact aureole around the Drammen granite in the Oslo rift, southern Norway. Major and trace element zonation profiles, the latter obtained using a scanning high-resolution proton microprobe, reveal early infiltration-controlled growth of relatively grossular rich garnets, the major and trace elements compositions being buffered by local mineral dissolution. Subsequent rapid, epitaxial growth of andradite-rich garnet on grossular-rich cores marks the onset of vigorous and focused fluid flow along high-permeability zones. During this later stage, the hydrothermal fluid composition was to a large extent externally controlled, and the andradite precipitating from these fluids was characterized by high As and W contents. The zonation patterns of the andradite-rich garnets record at least five intermittent growth periods, with rapid andradite precipitation from fluid batches with high fO2, and progressively decreasing As and W contents. Thin layers, poor in Fe, As, and W, but relatively high in Al and Mn, represent periods of slow growth rates between the major pulses of hydrothermal fluids. The marked rimward decrease in the As and W contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling-controlled distillation processes at depth. 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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Phase relations in the systems PbS-Sb 2 S 3 -Bi 2 S 3 and PbS-FeS-Sb 2 S 3 -Bi 2 S 3 were studied using evacuated capsule technique, X-ray powder diffraction, reflected light microscopy, and electron microprobe analysis. Seven new phases found in the systems are designated as phases C, K-K Fe , X, Y-Y Fe , Z, W, and R for convenience in their descriptions. Phases C, K-K Fe , and R have compositions and crystal structures comparable to cosalite, kobellite, and bismuthian robinsonite, respectively. Others have no distinct natural counterparts.The cosalite-like phase (C) when synthesized has a composition of 2PbS.(Bi (sub 0.80) Sb (sub 0.20) ) 2 S 3 and is stable to 715 degrees + or - 5 degrees C. No Sb-free cosalite was observed in the system. Kobellite, on the other hand, was synthesized from a series of compositions from 6PbS.FeS.Sb 2 S 3 .2Bi 2 S 3 (K Fe ) to 3PbS.Sb 2 S 3 .Bi 2 S 3 (K).Phase X, 9PbS.11 (Bi (sub 0.64) Sb (sub 0.36) ) 2 S 3 , is the only ternary phase synthesized in the PbS-poor half of the system PbS-Sb 2 S 3 -Bi 2 S 3 . Phase Y ranges in compositions from 2PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 (Y 1 ) to 2PbS.(Sb (sub 0.76) Bi (sub 0.24) ) 2 S 3 (Y 2 ). The Sb-rich end members form equilibrium assemblages with sulfantimonides, whereas the Bi-rich end members form equilibrium assemblages with sulfbismuthinides. Members of intermediate compositions exist in equilibrium with galena. Phase Z, a low-temperature form of phase Y, is stable below 407 degrees + or - 8 degrees C.Phase W has a composition of 3PbS.(Sb (sub 0.80) Bi (sub 0.20) ) 2 S 3 and exists in a very limited association with other phases in the system PbS-Sb 2 S 3 -Bi 2 S 3 . Phase R has a composition of PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 and a range of solid solution toward robinsonite.The system PbS-FeS-Sb 2 S 3 -Bi 2 S 3 is characterized by the formation of a single-phase plane defined by Y Fe , 6PbS.FeS.2Sb 2 S 3 .Bi 2 S 3 , Y 1 , 2PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 , and Y 2 , 2PbS.(Sb (sub 0.76) Bi (sub 0.24) ) 2 S 3 . In the region between 66.66 and 60.00 mole percent PbS, stable binary assemblages consisting of (K-K Fe ) and (Y-Y Fe ) solid solutions are: K Fe -K-Y 1 , K Fe -K-Y Fe , K Fe -K-C, K Fe -K-lillianite solid solution, Y 1 -Y Fe -K, Y 1 -Y Fe -K Fe , Y 1 -Y 2 -K, Y 2 -Y Fe -K, Y Fe -Y 2 -boulangerite, and Y 2 -Y Fe -2PbS.Sb 2 S 3 .
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