Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand
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Abstract Lead isotopic compositions were determined for sulphides from Pliocene-Pleistocene gold-bearing veins in the Alpine Schist and from Miocene gold-bearing veins and vein breccias from the Shotover-Macetown area in the northwest Otago Schist belt. The lead isotopic signatures are consistent with derivation of Pb in the vein minerals predominantly from metasedimentary rocks that underlie the region. Differences in Pb isotopic signatures between deposits are interpreted to result from lateral and vertical lithological variability within the source rock mass. The host rocks also contain metabasic rocks with N-MORB, E-MORB or within-plate basalt chemistry. However, the observed Pb isotopic signatures in the gold-bearing veins preclude incorporation of significant amounts of Pb from the metabasites. The Pb isotopic signatures of lamprophyre dikes that were intruded into the Otago Schist coeval with Miocene gold mineralisation are distinctly more radiogenic than those of the hydrothermal veins. Thus, although the lamprophyre dikes were emplaced into similar extensional structural sites to the gold-bearing veins, there was no genetic relationship between lamprophyre dikes and gold mineralisation.Keywords:
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The eastern Ferris Mountains and Bear Mountain area of south-central Wyoming contain a complex assemblage of Archean and Proterozoic rock units, including a metasedimentary and metavolcanic supracrustal sequence named the Spanish Mine metamorphic suite, three granitic plutons (Turkey Creek, Ferris Mountains, Bear Mountain), and at least three sets of mafic dikes. The Spanish Mine metamorphic suite was deposited, intruded by mafic sills and (or) dikes, and underwent amphibolite-grade metamorphism and folding just prior to, or synkinematic with, the intrusion of the Turkey Creek metaplutonic suite, U–Pb dated at 2733.5 ± 2 Ma. A second set of mafic dikes intruded the Turkey Creek metaplutonic suite prior to mylonitic shearing and late-stage folding along the Miners Canyon shear zone. These events were followed by intrusion of the Ferris Mountains plutonic suite at ~2717 Ma. The emplacement of the yet undated granite of Bear Mountain records the last phase of Neoarchean magmatism. The timing of magmatism and deformation support a model that the Ferris Mountains basement rocks formed in an arc terrane distinct from the older Wyoming craton and were accreted to the province in the Neoarchean. Intrusion of a third mafic dike set is dated at 2113 ± 15 Ma and may date the silicified, epidote-rich zones that crosscut all of the Archean rock units. Lead isotopic compositions of galena within the Spanish Mine metamorphic suite indicate additional Proterozoic activity and mineralization.
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Abundant gold deposits are distributed in the Jiao-Liao-Ji Belt of the North China Craton. Gold deposits, characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock, of the Jiaodong Peninsula and Liaodong Peninsula are underlain by rocks of similar geological settings characterized by uplifted blocks of metamorphosed Archean and Proterozoic rocks surrounded by relatively unmetamorphosed Mesoproterozoic rocks and Mesozoic granitoid plutons. The huge order-of-magnitude difference in reserves between the Liaodong gold province (500 t) and the Jiaodong gold province (5000 t) attract the attention of geologists. Different C–H-O-S-Pb isotopes and isotopic dating have sparked controversy over the source of the gold deposits. In this study, we conducted integrated analyses of zircon U-Pb dating, bulk-rock compositions, mineralogical compositions and high-precision gold analyses on a series of meta-clastic sedimentary rocks from the Paleoproterozoic Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton, of which the Au analyses reveal Au mobilization during prograde metamorphism. In the Gaizhou Barrovian-type metamorphic zone, the bulk-rock Au contents of samples within various metamorphic zones show a monotonic decrease up-temperature, from 2.09 ppb in the chlorite zone to 0.07 ppb in the sillimanite zone. The evolution in Au content during prograde metamorphism of other formations are identical to that of the Gaixian Formation. The Au concentrations of biotite in different metamorphic zones show little variation ranging from 0.66 to 0.35 ppb, whereas the Au concentrations of muscovite in different metamorphic zones mainly decrease from 2.60 ppb in the chlorite zone to 0.57 ppb in the sillimanite zone with increasing metamorphic grade in samples without metallic sulfides. Coincidently, there is a dechlorination process in biotites during regional metamorphism. Chlorine, released by metamorphism, may have played a vital role in the migration of Au, which could absorb onto muscovite and other minerals, during prograde metamorphism. Given an Au content of 4.67 t/km3 in the chlorite zone rock approximately equivalent to the protolith, metamorphism up to upper amphibolite grade would force the rocks to release 52% of the Au in the Gaizhou Barrovian-type metamorphic zone. Taking into account previous studies in the interpretation of our results, we propose that Precambrian rocks, such as the meta-clastic sedimentary rocks of the Gaixian Formation, may have contributed considerable amounts of Au to the Liaodong gold province. The Liaohe Group releasing large amounts of Au during Paleoproterozoic metamorphism, may thus provide the primary Au enrichment for the Mesozoic gold deposits.
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Within the Lake of the Woods greenstone terrain, in the Wabigoon Subprovince of northwest Ontario, gold is in faults, shears, and tension veins that developed in response to a late Archean northwest-directed contraction and emplacement of contemporaneous plutons. Nine rocks in this area have been dated using U-Pb analyses of zircon in order to obtain precise time constraints on gold mineralization and to compare its timing with the age of major crustal events in the region. A mineralized dike at the Duport mine, Shoal Lake, is old, whereas a later dike that cuts mineralized rocks has an age of . Gold mineralization is therefore distinctly younger than calc-alkaline and tholeiitic rocks dated at in the Shoal Lake area and in the High Lake area to the north. Hydrothermal activity associated with mineralization at the Duport mine overprints the amphibolite-facies contact metamorphic aureole of the Snowshoe Bay batholith (). Gold is hosted by the Canoe Lake stock () and within structures adjacent to the stock that were formed during its emplacement, and it is part of a magmatic-hyrothermal alteration pattern surrounding, but not within, a late phase of the High Lake stock (). These data indicate that gold mineralization was closely linked in time to the emplacement of late intrusions and was likely a short-lived event that occurred at about 2709 Ma. It is contemporaneous with regional amphibolite-facies metamorphism in the Winnipeg River Subprovince to the north and with the earliest evidence for tectonic contact between the Wabigoon and Winnipeg River Subprovinces. Zircon from the Snowshoe Bay batholith shows inheritance from a source at least 2800 Ma in age. Rocks this old are unknown in the greenstone belt but are found in the adjacent Winnipeg River Subprovince, implying that sialic material derived from, or correlative with, the Winnipeg River Subprovince may have underlain greenstone units in the Shoal Lake area at 2709 Ma. These data, combined with other geochronological studies in the region, are compatible with collision of oceanic arc-derived volcanic rocks of the Wabigoon Subprovince with a continent or continental fragment now represented by the Winnipeg River Subprovince. Thrusting and imbrication of greenstone sequences over the edge of the sialic mass occurred at about 2709 Ma. Contemporaneous late magmatism and mobilization of auriferous fluids may have resulted from the interaction of slab and mantle-derived magmas with overridden sialic material.
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Regionally metamorphosed rocks of Triassic and Jurassic age together with granitic plutons and metamorphosed ultramafic-mafic rocks form an elongate belt of approximately 300 sq mi in the northern Klamath Mountains of Oregon. The crystalline complex is named the May Creek Schist belt. Detailed mapping and petrographic studies in the northernmost part have established the pattern of isograds; metamorphism is progressive, and grade increases toward an axis which includes the locus of granitic plutons and ultramafic-mafic rocks. The pattern of isograds indicates steep thermal gradients and suggests that crystallizing granitic plutons supplied the energy for metamorphic reactions. Based on assemblages in pelitic rocks, metamorphism is interpreted as intermediate between low-pressure, regional contact metamorphism and higher pressure Barrovian-like metamorphism.
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Research Article| October 01, 1961 Granitic Formations in the East-Central Sierra Nevada Near Bishop, California PAUL C BATEMAN PAUL C BATEMAN U. S. Geological Survey, Menlo Park, Calif Search for other works by this author on: GSW Google Scholar Author and Article Information PAUL C BATEMAN U. S. Geological Survey, Menlo Park, Calif Publisher: Geological Society of America Received: 20 Jun 1960 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1961, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1961) 72 (10): 1521–1537. https://doi.org/10.1130/0016-7606(1961)72[1521:GFITES]2.0.CO;2 Article history Received: 20 Jun 1960 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation PAUL C BATEMAN; Granitic Formations in the East-Central Sierra Nevada Near Bishop, California. GSA Bulletin 1961;; 72 (10): 1521–1537. doi: https://doi.org/10.1130/0016-7606(1961)72[1521:GFITES]2.0.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 SocietyGSA Bulletin Search Advanced Search Abstract This report establishes lithologic units among the granitic rocks of the east-central Sierra Nevada near Bishop, California. In this area the Sierra Nevada batholith is composed chiefly of quartz-bearing plutonic rocks ranging in composition from quartz diorite to alaskite but includes scattered small masses of darker and older plutonic rocks and remnants of metamorphosed sedimentary and volcanic rocks. The granitic rocks are in discrete plutons, either in sharp contact with one another or separated by thin septa of metamorphic or mafic igneous rock or by late aplitic dikes. The granitic rocks are grouped into lithologic units on the basis of composition, texture, and intrusive relations. The units include six new formations, three informal units made up of the rocks in several plutons, and four informal units that include the rocks in single plutons. The new formations are the Inconsolable Granodiorite, Tinemaha Granodiorite, Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite, Lamarck Granodiorite, and Tungsten Hills Quartz Monzonite. 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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Abstract Folded Mesozoic quartzo-feldspathic sandstones and siltstones have been intruded and hornfelsed by small Miocene? stocks of diorite, quartz diorite, and granodiorite, and by numerous dikes of quartz andesite and dacite. The dikes intrude both the metasedimentary and plutonic rocks and radiate from an area now occupied by an explosion breccia pipe. Pyrrhotite-pyrite-chalcopyrite mineralisation occurs in two environments, (a) along closely spaced joints in a fine grained pyroxene diorite, where it is associated with secondary amphibole and biotite alteration, and (b) between fragments in the breccia pipe where it is associated with chlorite, adularia, sericite, calcite, epidote, and quartz alteration. In both environments permeability of host rocks was important in localising the sulphide mineralisation and hydrothermal alteration, both of which post-date the injection of the dikes. Hydrothermal fluids are thought to have originated from the deeper seated equivalents of the hypabyssal rocks. Preliminary sampling and geological mapping suggest that the copper ore reserves on Coppermine Island are, (a) certain—500,000 tons, (b) probable—8,000,000 tons, and (c) maximum possible—30,000,000 tons. The grade of the mineralisation is estimated to be approximately 0–5% Cu.
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