Coupling Mineralogy, Textures, Stable and Radiogenic Isotopes in Identifying Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn–Pb Deposits
Lola YesaresDrew DrummondSteven P. HollisAileen L. DoranJulian F. MenugeAdrian J. BoyceRobert J. BlakemanJohn H. Ashton
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Abstract:
Irish-type deposits comprise carbonate-hosted sphalerite- and galena-rich lenses concentrated near normal faults. We present new data from the Tara Deep resource and overlying mineralization, at Navan, and the Island Pod deposit and associated Main zone orebodies, at Lisheen. Tara Deep mineralization predominantly replaces Tournasian micrites and subordinate Visean sedimentary breccias. The mineralization is mainly composed of sphalerite, galena, marcasite and pyrite. A range of Cu- and Sb-bearing minerals occur as minor phases. At Tara Deep, paragenetically early sulfides exhibit negative δ34S values, with later phases displaying positive δ34S values, indicating both bacterial sulfate reduction (BSR) and hydrothermal sulfur sources, respectively. However, maximum δ34S values are heavier (25‰) than in the Main Navan orebody (17‰). These mineralogical and isotopic features suggest that Tara Deep represents near-feeder mineralization relative to the Navan Main orebody. The subeconomic mineralization hosted in the overlying Thin Bedded Unit (TBU) comprises sphalerite replacing framboidal pyrite, both exhibiting negative δ34S values (−37.4 to −8.3‰). These features indicate a BSR source of sulfur for TBU mineralization, which may represent seafloor exhalation of mineralizing fluids that formed the Tara Deep orebody. The Island Pod orebody, at Lisheen, shows a mineralogical paragenetic sequence and δ34S values broadly similar to other Lisheen orebodies. However, the lack of minor Cu, Ni, and Sb minerals suggests a setting more distal to hydrothermal metal feeder zones than the other Lisheen orebodies. Pb isotope data indicate a very homogeneous Lower Palaeozoic Pb source for all Navan orebodies. Lower Palaeozoic metal sources are also inferred for Lisheen, but with variations both within and between orebodies. Carbon and oxygen isotopic variations at Navan and Lisheen appear to result from fluid-carbonate rock buffering. The emerging spectrum of mineralogical and isotopic variations define proximal to distal characteristics of Irish-type systems and will assist in developing geochemical vectoring tools for exploration.Keywords:
δ34S
Uraninite
Marcasite
The lead-zinc mineralizations from Western Cantabria are studied in this paper, mainly from a petrological and geochemical point of view. The host rocks are mesozoic dolomites which present some outstanding evidences: cavernous aspect, irregular distribution, coarsegrained, relict texture with ghosts and cloudy centers, and low geochemical content in Sr (50 ppm) and high in Mn and Fe (1200 ppm and 4%. respectively), These characteristics aIlow classify them as originated by late diagenetic dolomitization processes. The primary ores are sphalerite, galena, pyrite, marcasite , and sometimes barite. These minerals fill cavities and other open spaces, being the most important texture composed by altemating bands of sphalerite of different colour (colloform texture) with scattered crystals of galena, pyrite, and marcasite, The general morphology is tipically stratabound and not stratiform. The secondary minerals, product of weathering, inelude smithsonite from sphalerite, cerussite from galena, and goethite from pyrite and marcasite. The characteristics of the host rocks and mineralizations show they were formed epigenetically, with existence of a source rock, migration of mineralizating solutions and deposition of them in a host rock of dolomitic character.
Marcasite
Dolomitization
Texture (cosmology)
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The Jinshachang lead–zinc deposit is mainly hosted in the Upper Neoproterozoic carbonate rocks of the Dengying Group and located in the Sichuan–Yunnan–Guizhou(SYG) Pb–Zn–Ag multimetal mineralization area in China.Sulfides minerals including sphalerite,galena and pyrite postdate or coprecipitate with gangue mainly consisting of fluorite,quartz,and barite,making this deposit distinct from most lead–zinc deposits in the SYG.This deposit is controlled by tectonic structures,and most mineralization is located along or near faults zones.Emeishan basalts near the ore district might have contributed to the formation of orebodies.The δ34S values of sphalerite,galena,pyrite and barite were estimated to be 3.6‰–13.4‰,3.7‰–9.0‰,6.4‰ to 29.2‰ and 32.1‰–34.7‰,respectively.In view of the similar δ34S values of barite and sulfates being from the Cambrian strata,the sulfur of barite was likely derived from the Cambrian strata.The homogenization temperatures(T ≈ 134–383°C) of fluid inclusions were not suitable for reducing bacteria,therefore,the bacterial sulfate reduction could not have been an efficient path to generate reduced sulfur in this district.Although thermochemical sulfate reduction process had contributed to the production of reduced sulfur,it was not the main mechanism.Considering other aspects,it can be suggested that sulfur of sulfides should have been derived from magmatic activities.The δ34S values of sphalerite were found to be higher than those of coexisting galena.The equilibrium temperatures calculated by using the sulfur isotopic composition of mineral pairs matched well with the homogenization temperature of fluid inclusions,suggesting that the sulfur isotopic composition in ore-forming fluids had reached a partial equilibrium.
δ34S
Gangue
Tetrahedrite
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The isotopic composition of five samples of lead from the radium mines of Great Bear Lake indicates that the lead within the pitchblende veins is all of radioactive origin, even including that which now exists as galena, though a galena occurring close to a pitchblende vein is composed of ordinary lead contaminated with radioderived lead. The data raise unusual problems.
Uraninite
Lead (geology)
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δ34S
Isotope Analysis
Isotope Geochemistry
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Mineral succession of Zn-Pb ores in the Pomorzany deposit and preliminary geochemical characteristics of the selected sulphide generations
Detailed study of sulphides in Zn-Pb deposit of Pomorzany resulted in recognizing several generations of sphalerite, marcasite-pyrite and galena. These sulphides form two paragenetic assemblages. The older paragenetic association occurs in metasomatic ore deposits while the younger one - within brecciated deposits. Sphalerites of these two associations differ in trace elements aboundances. No such differences were found in galena except its cubic variety of the youngest generation showing specific geochemical features.
Przeprowadzono badania siarczkow w zlozu Zn-Pb w Pomorzanach. Stwierdzono obecnośc szeregu generacji sfalerytu, markasytu-pirytu i galeny, ktore tworzą dwa zespoly paragenetyczne. Starszy zespol paragenetyczny wystepuje w zlozu metasomatycznym, mlodszy zaś wchodzi w sklad zloza brekcjowego. Stwierdzono ilościowe zroznicowanie pierwiastkow śladowych w sfalerytach reprezentujących oba typy zloz. Zroznicowania takiego nie wykazuje galena, odrebny sklad wykazuje jedynie galena kostkowa najmlodszej generacji.
Marcasite
Metasomatism
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Uraninite
Radiogenic nuclide
Uranium ore
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The Maria Cristina Zn-Pb skarn deposit, hosted by carbonate rocks of the Lower Cretaceous Chanarcillo Group in northern Chile, is described to provide a basis of comparison to adjacent (~2 km) barite deposits and regional Pb-Zn and Ba deposits with Mississippi Valley-type affinities. Strongly retrograded garnet (andradite to Ad30Gr70), diopsidic pyroxene, and epidote skarn, and barite-bearing massive sulfides (sphalerite-pyrite-galena-marcasite-magnetite) occur at the contact of potassically altered diorite porphyry of mid-Cretaceous age. The skarn mineral compositions, similar to those of Cu skarns, may reflect emplacement in a high f O2 context. Sulfur isotope values of sulfides (–8.8 to +7.1‰) and barite (14.0–26.1‰) indicate disequilibrium conditions and different sulfur sources, including magmatic sulfur. Lead isotope ratios of galena and strontium isotope ratios of barite indicate a similar metal reservoir for the different ore deposit types, constituted by the intercalated Lower Cretaceous volcanic rocks or mid-Cretaceous intrusive rocks. 40Ar/39Ar dating of hydrothermal K feldspar suggests an age between 94 and 90 Ma for the mineralization.
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