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Flat REE Pattern (flat + ree_pattern)
Selected AbstractsThe Ashele VMS-type Cu-Zn Deposit in Xinjiang, NW China Formed in a Rifted Arc SettingRESOURCE GEOLOGY, Issue 2 2010Bo Wan Abstract The Ashele Cu-Zn deposit is a typical VMS deposit in Chinese Altay located in the southern margin of the Altaid orogen. The deposit occurred in the polyphase fold system, and the main orebody is located at the hinge of the syncline. All orebodies show lenticular form, and are stratabound by a suite of early to middle Devonian bimodal volcanic rocks. The hosting basalt is low K tholeiite and characterized with high Mg, Fe, Ca and low K, Ti. These basalts show flat REE pattern with Ce negative anomaly (Ce/Ce* 0.73,0.76). Niobium, Ta, Zr, Hf are depleted and Rb, Ba, Th, U, Sr, Pb are enriched with respect to the N-MORB. Both the Sr and Nd isotopes show depleted properties, while the (87Sr/86Sr)i and the ,Nd(t) range from 0.70469 to 0.70488 and 4.6 to 5.3, respectively. All geochemical and isotopic data from the hosting basalt show that it originates from an island arc source. We also report the S isotope data from the massive orebody, and ,34S, change from 1.8, to 5.6,. The S isotope data provide evidence that the sulfur originates from a mixing source between magma and seawater sulfate. We propose that the mafic magma provides the ore-forming metal and some percentage of sulfur, while it also acts as a heat engine which makes the fluids leach the metal from the underlying volcanic rocks. Combining the geological characteristics of the Ashele and geochemical data, and comparing with other Cu-Zn VMS deposits in the world, we propose that Ashele formed in a rifted arc setting. [source] REE, Mn, Fe, Mg and C, O Isotopic Geochemistry of Calcites from Furong Tin Deposit, South China: Evidence for the Genesis of the Hydrothermal Ore-forming FluidsRESOURCE GEOLOGY, Issue 1 2010Yan Shuang Abstract The Furong tin deposit in the central Nanling region, South China, consists of three main types of mineralization ores, i.e. skarn-, altered granite- and greisen-type ores, hosted in Carboniferous and Permian strata and Mesozoic granitic intrusions. Calcite is the dominant gangue mineral intergrown with ore bodies in the orefield. We have carried out REE, Mn, Fe, and Mg geochemical and C, and O isotopic studies on calcites to constrain the source and evolution of the ore-forming fluids. The calcites from the Furong deposit exhibit middle negative Eu anomaly (Eu/Eu*= 0.311,0.921), except for one which has an Eu/Eu* of 1.10, with the total REE content of 5.49,133 ppm. The results show that the calcites are characterized by two types of REE distribution patterns: a LREE-enriched pattern and a flat REE pattern. The LREE-enriched pattern of calcites accompanying greisen-type ore and skarn-type ore are similar to those of Qitianling granite. The REE, Mn, Fe, and Mg abundances of calcites exhibit a decreasing tendency from granite rock mass to wall rock, i.e. these abundances of calcites associated with altered granite-type and greisen-type ores are higher than those associated with skarn-type ores. The calcites from primary ores in the Furong deposit show large variation in carbon and oxygen isotopic compositions. The ,13C and ,18O of calcites are ,0.4 to ,12.7, and 2.8 to 16.4,, respectively, and mainly fall within the range between mantle or magmatic carbon and marine carbonate. The calcites from greisen and altered granite ores in the Furong deposit display a negative correlation in the diagram of ,13C versus ,18O, probably owing to the CO2 -degassing of the ore-forming fluids. From the intrusion to wall-rock, the calcites display an increasing tendency with respect to ,13C values. This implies that the carbon isotopic compositions of the ore-bearing fluids have progressively changed from domination by magmatic carbon to sedimentary carbonate carbon. In combination with other geological and geochemical data, we suggest that the ore-forming fluids represent magmatic origin. We believe that the fluids exsolved from fractionation of the granitic magma, accompanying magmatism of the Qitianling granite complex, were involved in the mineralization of the Furong tin polymetallic deposit. [source] Magmatic evolution of the Mantos Blancos copper deposit, Coastal Range of northern Chile: insight from Sr,Nd isotope, geochemical data and silicate melt inclusionsRESOURCE GEOLOGY, Issue 2 2008Luis E. Ramírez Abstract The Mantos Blancos copper deposit (500 Mt at 1.0% Cu) was affected by two superimposed hydrothermal events: (i) phyllic alteration related to a rhyolitic dome emplacement and brecciation at ca 155 Ma; and (ii) potassic, sodic and propylitic alteration at ca 142 Ma, coeval with stocks and sills emplacement of dioritic and granodioritic porphyries, that locally grade upwards into polymictic magmatic hydrothermal breccias. Major hypogene copper sulfide mineralization is related to the second event. A late-ore mafic dike swarm cross-cuts all rocks in the deposit. Two types of granodioritic porphyries can be distinguished from petrographic observations and geochemical data: granodiorite porphyry I (GP I) and granodiorite porphyry II (GP II), which resulted from two different trends of magmatic evolution. The concave shape of the rare earth element (REE) distribution pattern together with the weak or absence of negative Eu anomalies in mafic dikes, dioritic and GP I porphyries, suggest hornblende-dominated fractionation for this magmatic suite. In contrast, distinct negative Eu anomalies and the flat REE patterns suggest plagioclase-dominated fractionation, at low oxygen fugacity, for the GP II porphyry suite. But shallow mixing and mingling between silicic and dioritic melts are also likely for the formation of the GP II and polymictic breccias, respectively. Sr-Nd isotopic compositions suggest that the rhyolitic dome rocks were generated from a dominantly crustal source, while the GP I has mantle affinity. The composition of melt inclusions (MI) in quartz crystals from the rhyolitic dome is similar to the bulk composition of their host rock. The MI analyzed in quartz from GP II and in the polymictic magmatic hydrothermal breccia of the deposit are compositionally more evolved than their host rocks. Field, geochemical and petrographic data provided here point to dioritic and siliceous melt interaction as an inducing mechanism for the release of hydrothermal fluids to form the Cu mineralization. [source] Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei: Implications for Crustal AccretionACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006LIU Shuwen Abstract: The middle segment of the northern margin of the North China Craton (NCC) consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The ,2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg#less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantle-normalized spider diagrams, ,Nd(t)=+0.65 to ,0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with ,Nd(t)= ,1.64 may not be an identical magmatic evolution series with granitoids that have ,Nd(t) values of +3.19 to +1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the ,311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with ,Nd(t)= +8.42, implying that the mafic magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low ,Nd(t)= ,13.37 to ,14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between mafic magma that came from ,311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions (,2.49, ,2.44 and 311 Ma), except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block. [source] | ||||||||||