Mississippi Valley-type Deposits Research Articles

Pre-Variscan tectonic events and Late Ordovician magmatism in the Central Pyrenees: U–Pb age and Hf in zircon isotopic signature from subvolcanic sills in the Pallaresa massif

Late Ordovician magmatism is well-represented in all massifs of the Eastern Pyrenees and the Catalan Coastal Ranges but has not been reported in the central parts of the Pyrenean Axial Zone. This work shows the first geochronological evidence for Late Ordovician magmatism in the Pallaresa massif. This massif is a large E–W trending antiformal structure cored by Paleozoic rocks with a Variscan tectonothermal overprint. The lower Paleozoic sequence shows an Upper Ordovician succession lying unconformably on older Cambrian–Ordovician beds. Despite Variscan overprint, the Cambrian–Ordovician rocks record evidence of a pre-Variscan penetrative fabric which is not present in the rest of the overlying Paleozoic succession. In the eastern part of the Pallaresa massif, rhyodacitic to dacitic subvolcanic rock forming sills intercalated within the Cambrian–Ordovician succession close to the base of the Upper Ordovician rocks have been identified and sampled. These volcanic rocks have a Variscan penetrative foliation and lack the additional pre-Variscan deformation. U–Pb zircon dating of one these subvolcanic levels by U–Pb CA-ID-TIMS, has provided an age of 453.6 ± 1.5 Ma (Sandbian) and the zircon has given ɛHf450 values between − 2.4 and − 5.9, suggesting a mixed source. These new data strengthen the Paleozoic correlation of the Central and Eastern Pyrenees indicating that the Late Ordovician magmatism in the Pyrenees was associated with a pre-Variscan event responsible for the Upper Ordovician angular unconformity and the presence of a penetrative pre-Variscan deformation affecting the underlying Cambrian–Ordovician sequence. This new data, coupled with the occurrence of Zn–Pb SEDEX o Mississippi Valley type deposits associated with Upper Ordovician rocks, suggest the evidence of extensional tectonic activity during Late Ordovician times.

The genesis of the Hehuashan Pb–Zn deposit and implications for the Pb–Zn prospectivity of the Tongling district, Middle–Lower Yangtze River Metallogenic Belt, Anhui Province, China

The Hehuashan lead–zinc deposit is located in the central Tongling Cu–Au–Mo mining district of eastern China and consists of stratabound ore bodies that are generally hosted by dolostone and limestone units of the Lower Triassic Lower Nanlinghu Formation. The deposit contains two types of sphalerite, namely tan to brown fine-grained (∼40 μm) type I sphalerite that is hosted by a dolomite, calcite, quartz and sericite gangue assemblage and colorless to brown coarse-grained (∼2 mm) type II sphalerite hosted within clasts in calcite-cemented breccias or within dolostone-hosted veins. Plotting these sphalerites in a Fe vs. Cd diagram indicates they have a similar composition as those within typical Mississippi Valley-type (MVT) deposits, suggesting that the Hehuashan deposit may have an MVT affinity. In addition, in-situ sulfur stable isotopic analysis of fine-grained (−7.8‰ to +3.0‰) and coarse-grained (+3.4‰ to +9.7‰) sphalerite indicates that the sulfur in the hydrothermal fluids that formed the deposit was derived from the country rocks within the Hehuashan area. Fluid inclusions within calcite and fluorite from the Hehuashan deposit have low homogenization temperatures (mean of 126 °C) and moderate salinities (mean of 7.1 wt% NaCl equivalent) that demonstrate the deposit formed from low temperature and moderate salinity hydrothermal fluids. This suggests that the Hehuashan deposit was almost certainly not formed from magmatic-hydrothermal fluids but instead was formed as a result of the mixing of basinal brines and meteoric water, again consistent with an MVT origin for this deposit. In summary, the Hehuashan lead–zinc deposit is an MVT deposit, suggesting that the Tongling district of Anhui Province, China is prospective for MVT mineralization and that the Triassic dolostones and limestones in this area represent high priority exploration targets for this style of mineralization.

Textural characteristics and trace element distribution in carbonate-hosted Zn-Pb-Ag ores at the Paleoproterozoic Black Angel deposit, central West Greenland

The Black Angel deposit represents the most important base metal deposit in Greenland, having produced 11.2 million tons of Pb-Zn-Ag ore from 1973 to 1990. The deposit is hosted by a greenschist facies calcitic marble of the Mârmorilik Formation of the Paleoproterozoic Karrat Group. The ore consists of sphalerite, pyrite, and galena, with minor amounts of chalcopyrite, arsenopyrite, tetrahedrite, freibergite, tennantite, stannite, briartite, rutile, and graphite. Pyrite occurs as porphyroclasts and as fine-grained euhedral grains and is commonly surrounded by sphalerite, galena, and chalcopyrite. Deformation textures such as pyrite annealing, fracture healing by sulfides as well as “Durchbewegung” textures are common. Electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry trace element analysis show that tetrahedrite-freibergite and galena are enriched in Ag and represent the main Ag carriers. Galena also hosts substantial amounts of Sb, Sn, and Bi. The presence of Ge-rich chalcopyrite and briartite (Cu2(Zn,Fe)GeS4) inclusions in sphalerite indicates that the Black Angel deposit may be host to an hitherto unrecognized Ge endowment. It is suggested that sphalerite and briartite co-precipitated from a hydrothermal fluid having an intermediate sulfidation state. The origin of the deposit remains ambiguous. The calcitic host rock, epigenetic style of mineralization, presence of anhydrite, and occurrence of hydrothermal breccias are consistent with an origin as Mississippi Valley-type deposit. However, much of the mineralization is syn- to late-tectonic and syn-metamorphic, and the sulfide textures are consistent with a metamorphic overprint. An origin as carbonate-hosted polymetallic Kipushi-type deposit is thus more likely, since these deposits constitute the primary deposit type from which briartite has been previously documented.

The genesis of the Ali Ou Daoud Jurassic carbonate Zn-Pb Mississippi Valley-type deposit, Moroccan central High Atlas: Constraints from bulk stable C-O-S, in situ radiogenic Pb isotopes, and fluid inclusion studies

The proximity of the Ali Ou Daoud Zn-Pb ore deposit (AOD) to the Ikkou Ou Ali diapiric and magmatic paleohigh raises the question of the role of halokinesis and magmatism in the emplacement of the ore. The AOD ore deposit is hosted in hydrothermally dolomitized Bajocian carbonates. The mineralogical paragenetic sequence is simple and consists of dolomite-pyrite-galena-sphalerite-calcite. The mineralization is tectonically and unconformity controlled. Microthermometry of fluid inclusions in sphalerite indicates that ore precipitated from a NaCl-KCl-CaCl2-MgCl2 basin-derived hot (Th = 94–170 °C) and saline (Salinity = 13.8–27.3 wt% NaCl eq.) mixed ore-forming fluid. Post-ore calcite also formed from a similar brine (Th = 176–206 °C, Salinity = 17.6–20.5 wt% NaCl eq.). The δ34S values for sulfides range from 7.8 to 23.4‰ V-CDT with most values in the range of 16–23.4‰ V-CDT. Sulfur was mainly derived from the Triassic-Jurassic sulfates through thermochemical-sulfate reduction. The carbon–oxygen isotope values of calcite are overall similar to that of the barren Bajocian limestone and dolostone indicating that the calcite-forming fluid is in equilibrium with the carbonate host rock. Galena samples have a homogeneous Pb composition with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.358 to 18.376, 15.636 to 15.663 and 38.486 to 38.606 respectively. This Pb isotopic composition points to the Paleozoic siliciclastic rocks as the main source of lead and presumably other metals with a possible contribution of Precambrian rocks. The long-distance fluid flow through a thick sequence of Paleozoic-Jurassic rocks ensured the homogeneity of Pb isotopes of the ore-forming fluids before they reached the loci of deposition. The emplacement of the ore occurred during Late Middle Jurassic-Early Cretaceous extensional tectonic activity in relation to thermal doming when a deep-seated, Paleozoic-derived metal-bearing fluid migrated upward along E-W- and NE-SW-trending faults. At the site of deposition, these hot, metal-carrying fluids mixed with a warm, shallow, metal-poor, and sulfur-rich fluid as revealed by a wide range in salinity and almost similar temperatures. The proposed fluid-mixing led to the formation of hydrothermal metric-sized karsts and to the precipitation of sulfides in the Bajocian carbonates.

Genesis of fahlore in the Tianbaoshan lead–zinc deposit, Sichuan Province, China: a scanning electron microscopy–energy dispersive spectroscopy study

The Tianbaoshan deposit, located in the southwestern part of the Yangtze Block, is a representative Pb–Zn deposit in the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province. The Pb–Zn orebodies are hosted in the upper Sinian Dengying Formation dolostone. The predominant minerals are sphalerite, galena, pyrite, chalcopyrite, quartz, and calcite with minor arsenopyrite, fahlore, and dolomite. The deposit is characterized by relatively strong Cu mineralization. However, the relationship between Pb–Zn and Cu mineralization is unknown. We analyzed the mineralogy and composition of fahlore, chalcopyrite, arsenopyrite, sphalerite, and galena using scanning electron microscopy–energy dispersive spectroscopy, with the aim of providing new evidence for the genesis of the Pb–Zn–(Cu) ore. The results show that the Cu ore in the deposit is dominated by chalcopyrite and fahlore, both of which formed before or during the Pb–Zn ore-forming stage. The fahlore showed dramatic compositional variation and was characterized by negative correlations between Ag and Cu, and between As and Sb, suggesting substitution of Ag for Cu, and that As and Sb substitute in the same site in the fahlore lattice. Based on backscattered electron images and composition, the fahlore was divided into two types. Type I fahlore crystallized early and is characterized by enrichment of Cu and depletion in Ag and Sb. Type II fahlore formed after Type I, and is rich in Ag and poor in Cu and As. Moreover, galena and fahlore are the host minerals of Ag. The variation of valence state with As host mineral—from fahlore to arsenopyrite—indicates the metallogenic environment changed from relatively oxidizing to reducing with a high pH. In the light of Gibbs energies of reciprocal reactions and isotherms for cation exchange, the composition of the fahlore implies its ore-forming temperature was lower than 220 °C, corresponding with typical Mississippi Valley-type (MVT) deposits. Based on the geologic character and geochemical data of this deposit, we suggest that the Tianbaoshan deposit belongs to the MVT deposit category.

Geophysical Investigation of the Pb–Zn Deposit of Lontzen–Poppelsberg, Belgium

The drillhole information from the Lontzen–Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface. The localisation of the lodes makes them excellent targets for further exploration with geophysics. This deposit is classified as a Mississippi Valley Type (MVT) deposit. It consists mainly of Pb–Zn–Fe sulphides that display contrasting values in resistivity, chargeability, density, and magnetic susceptibility, with regards to the sedimentary host rocks. The dipole–dipole direct current (DC) resistivity and induce polarization (IP) profiles have been collected and inverted to successfully delineate the Pb–Zn mineralization and the geological structures. Short-spacing EM34 electromagnetic conductivity data were collected mainly on the top of Poppelsberg East lode and have revealed a conductive body matching with the geologically modelled mineralization. Gravity profiles have been carried out perpendicularly to the lode orientation; they show a strong structural anomaly. High resolution ground magnetic data were collected over the study area, but they showed no anomaly over the ore deposits. The geophysical inversion results are complementary to the model based on drill information, and allow us to refine the delineation of the mineralization. The identification of the geophysical signatures of this deposit permits targeting new possible mineralization in the area.

In situ geochemical characterization of pyrite crystals in burial dolomites of St. George Group carbonates

Secondary ion mass spectrometry (SIMS) was used to measure the δ34S of pyrite disseminated in burial dolomite matrix of Boat Harbour formation at Main Brook and Daniel’s Harbour (about 130 km apart). At Main Brook, δ34S values for the pyrite grains show wide variation (–15 to +20‰ (n = 20)), but are mostly negative. Combined with a paucity of two-phase fluid inclusions in the host burial dolomite, the depleted δ34S values suggest that the pyrite is a direct product of bacterial sulfate reduction (BSR). Predominantly negative δ34S values were also obtained for sampled pyrite at Daniel’s Harbour and Port au Choix; however, relatively high homogenization temperatures (>100 °C) of two-phase fluid inclusions in the host dolomite is incompatible with a BSR process for pyrite formation. More so, Daniel’s Harbour is a site for main stage sulfide mineralization (Mississippi Valley-type (MVT) system), hosted in similarly burial dolomite and that has been previously constrained to be associated with thermochemical sulfate reduction (TSR). A relative proximity of the currently studied pyrite samples to this MVT system deposit is thus inconsistent with an in-situ BSR for these pyrites. The analyzed pyrite grains are commonly encased in bitumen, and they postdate their host burial dolomite and predate deep burial saddle dolomite. Taken together, the depleted δ34S signature in the pyrite was likely inherited from migrated hydrocarbons in the reservoir. Incursion of an initial pulse of hot sulfate-rich brine into the formation can cause thermal cracking of hydrocarbons, thereby releasing its low δ34S. Thus the inherited low δ34S signature was likely a product of an earlier BSR that occurred in the kerogen or source organic materials in source rock. Subsequently, the main stage sulfide mineralization (MVT deposit) occurred via in-situ TSR. These findings have an implication for the paragenetic history of sulfide minerals precipitated during MVT mineralization episode.

Variations in Zn and S isotope chemistry of sedimentary sphalerite, Wusihe Zn-Pb deposit, Sichuan Province, China

The Sichuan–Yunnan–Guizhou (SYG) metallogenic province, southwestern China, is one of the largest Zn-Pb producers in the world. The Zn-Pb deposits in this area have been studied over many years and are commonly classified as Mississippi Valley Type (MVT) deposits. We investigated geological and mineralogical features of the Wusihe deposit and found that ores formed at the sedimentary metallogenic stage exhibit banded and normal grading textures and have different mineral assemblages from other deposits in this area. These features have not been reported previously, suggesting special formation mechanisms for these sedimentary ores. In this study, we firstly investigate the isotope geochemistry of Zn and S in syngenetic sphalerite from the Wusihe deposit. We elucidate spatial and temporal variations of Zn and S isotopes to provide a better understanding for Zn and S isotope fractionation during the hydrothermal processes. The δ34SCDT values of microdrilled sphalerite range from 9.4‰ to 20.9‰ (mean = 14.3‰), which are ca.15‰ lower than that of sulfate from the Dengying Formation, suggesting that reduced sulfur in sphalerite formed through thermochemical sulfate reduction (TSR) but not microbial sulfate reduction (MSR). At the hand specimen scale, no significant Zn isotopic fractionation was observed from bottom to top of the banded sphalerite ore and we propose a model that Zn isotopes are homogeneous in Zn-bearing hydrothermal fluid, and Zn2+ was rapidly and completely precipitated to form sphalerite. This process results in minor Zn isotopic variations in sphalerite (Δ66Zn = 0.08‰) and initial hydrothermal fluid composition was calculated to be δ66Zn values = 0.37 ± 0.03‰. By contrast, positive linear relationships were observed in the sphalerite ore with normal grading between δ66Zn and δ34SCDT values and between δ34SCDT values and Zn/Cd ratios, which are not reported previously. Combined with previous studies, it is proposed that the variations in fluid temperatures may be the key factor that results in such positive correlations.In addition, this study gives some new insights into geochemical and isotopic behavior of Zn and S in hydrothermal systems, and provides an initial glimpse into the utilization of Zn isotopes as an environmental tracer for reconstruction of Zn isotope compositions in the Ediacaran Ocean.

Prediction of ore fluid metal concentrations from solid solution concentrations in ore-stage calcite: Application to the Illinois-Kentucky and Central Tennessee Mississippi Valley-type districts

Knowledge of the concentrations of Zn and Pb in Mississippi Valley-type (MVT) ore fluids is fundamental to understanding MVT deposit origin. Most previous attempts to quantify the concentrations of Zn and Pb in MVT ore fluids have focused on the analysis of fluid inclusions. However, these attempts have yielded ambiguous results due to possible contamination from secondary fluid inclusions, interferences from Zn and Pb in the host mineral matrix, and uncertainties about whether the measured Zn and Pb signals represent aqueous solute or accidental solid inclusions entrained within the fluid inclusions. The purpose of the present study, therefore, was to try to determine Zn and Pb concentrations in MVT ore fluids using an alternate method that avoids these ambiguities by calculating Zn and Pb concentrations in MVT ore fluids theoretically based on their solid solution concentrations in calcite. This method was applied to the Illinois-Kentucky and Central Tennessee districts, which both contain ore-stage calcite.Experimental partition coefficient (D) values from Rimstidt et al. (1998) and Tsusue and Holland (1966), and theoretical thermodynamic distribution coefficient (KD) values were employed in the present study. Ore fluid concentrations of Zn were likely most accurately predicted by Rimstidt et al. (1998) D values, based on their success in predicting known fluid inclusion concentrations of Mg and Mn, and likely also most accurately predicted ore fluid concentrations of Fe. All four of these elements have a divalent ionic radius smaller than that of Ca2+ and form carbonate minerals with the calcite structure. For both the Illinois-Kentucky and the Central Tennessee district, predicted ore fluid Zn and Fe concentrations were on the order of up to 10’s of ppm. Ore fluid concentrations of Pb could only be predicted using Rimstidt et al. (1998) D values. However, these concentrations are unlikely to be reliable, as predicted ore fluid concentrations of Sr and Ba, which like Pb also have a divalent ionic radius larger than that of Ca2+ and form carbonate minerals with the aragonite structure, did not consistently agree well with known concentrations of Sr and Ba in fluid inclusions.The ore fluid Zn concentrations predicted in the present study lie within the range of Zn concentrations typical of modern sedimentary brines and are high enough to allow deposition of the observed amounts of Zn in the Illinois-Kentucky and Central Tennessee districts within ranges of geologically reasonable times and ore fluid flow velocities. If the pH of the Illinois-Kentucky and Central pH ore fluids was as low as current evidence suggests to be possible, then these ore fluids could simultaneously have transported enough sulfide with their Zn to account for the observed amounts of sphalerite in the districts.

The Cristal Zinc prospect (Amazonas region, northern Peru). Part I: New insights on the sulfide mineralization in the Bongará province

The Cristal Zn prospect consists of a mixed sulfide and nonsulfide mineralization located in the Bongará province (Amazonas region, northern Peru). The mineralization is hosted by carbonate rocks of the Pucará Group, deposited in a Mesozoic extensional basin on the western margin of the Brazilian-Guyana shield. Zinc sulfides at Cristal occur in the roots of the nonsulfide concentrations, and are locally present also nearer to surface. The sulfide mineralization postdates two hydrothermal dolomitization phases (Dol1 and Dol2) and the sulfides occur mainly in veins, cavity fillings or as disseminated mineralization, within sparry to saddle dolomite. They mostly consist of dark-brown sphalerite, intergrown with smaller amounts of pyrite. Sphalerite shows a distinct Fe-zonation, with average ca. 7 wt% Fe (max. ca. 12 wt% Fe), and is Ge-rich (mean concentration of 142 ppm). Galena is rare. The Cristal sphalerite has sulfur isotopic compositions of δ34S = 14 to 15 ‰ VCDT. Oxygen isotopic compositions of dolomites are: δ18O = 24.4 to 24.7 ‰ VSMOW for Dol1 and 18.4 to 22 ‰ for Dol2. δ34S and δ18O values of Cristal sulfides and dolomites are similar to those observed in two Mississippi Valley-type (MVT) deposits located ca. 20–30 km south of the Cristal prospect, namely the Florida Canyon and Florcita deposits. This could be consistent with one or more events within a same MVT mineralizing system, acting at the district scale. The Pb isotope compositions of sphalerite from two different areas of the prospect (named Cristal s.s. and Charlita North) define two distinct data-point clusters (centered around averages of 206Pb/204Pb = 18.850 ± 0.002, 207Pb/204Pb = 15.685 ± 0.002, 208Pb/204Pb = 38.752 ± 0.004, and 206Pb/204Pb = 19.042 ± 0.002, 207Pb/204Pb = 15.712 ± 0.002, 208Pb/204Pb = 39.080 ± 0.004, respectively). This difference requires distinct metal-bearing hydrothermal pulses in the mineralized area and/or distinct Pb sources. The Pb isotopic compositions of the Cristal s.s. and Charlita North sulfides are intermediate between the compositions of galena from the San Vicente and Shalipayco MVT deposits, and record a contribution from an old crustal component. The Paleozoic basement, which has Pb isotopic ratios roughly matching those of dolomites and sulfides from Cristal and Charlita North areas, represents the most reliable candidate to be an end-member source of the metals of the Cristal sulfide mineralization. The second end-member could be an igneous source, isotopically identical to the Late Paleozoic to Early Mesozoic intrusives of the Peruvian Eastern Cordillera, or the Triassic volcanic rocks occurring within the Mitu Group.

Germanium enrichment in supergene settings: evidence from the Cristal nonsulfide Zn prospect, Bongar\xe1 district, northern Peru

Supergene nonsulfide ores form from the weathering of sulfide mineralization. Given the geochemical affinity of Ge to Si4+ and Fe3+, weathering of Ge-bearing sulfides could potentially lead to Ge enrichments in silicate and Fe-oxy-hydroxide minerals, although bulk rock Ge concentrations in supergene nonsulfide deposits are rarely reported. Here, we present the results of an investigation into Ge concentrations and deportment in the Cristal supergene Zn nonsulfide prospect (Bongará, northern Peru), which formed from the weathering of a preexisting Mississippi Valley-type (MVT) sulfide deposit. Material examined in this study originates from drillcore recovered from oxidized Zn-rich bodies ~ 15–20 m thick, containing ~ 5–45 wt% Zn and Ge concentrations ~ 100 ppm. Microanalysis and laser ablation-ICP-MS show that precursor sphalerite is rich in both Fe (mean Fe = 8.19 wt%) and Ge (mean Ge = 142 ppm). Using the mineral geothermometer GGIMFis—geothermometer for Ga, Ge, In, Mn, and Fe in sphalerite—proposed by Frenzel et al. (Ore Geol Rev 76:52–78, 2016), sphalerite trace element data from the Cristal prospect suggest a possible formation temperature (TGGIMFis) of 225 ± 50 °C, anomalously high for a MVT deposit. Germanium concentrations measured in both goethite (mean values 100 to 229 ppm, max 511 ppm) and hemimorphite (mean values 39 to 137 ppm, max 258 ppm) are similar to concentrations measured in hypogene sphalerite. Additionally, the Ge concentrations recorded in bulk rock analyses of sphalerite-bearing and oxidized samples are also similar. A persistent warm-humid climate is interpreted for the region, resulting in the development of an oxidation zone favoring the formation of abundant Zn hydrosilicates and Fe hydroxides, both able to incorporate Ge in their crystal structure. In this scenario, Ge has been prevented from dispersion during the weathering of the Ge-bearing sulfide bodies and remains in the resultant nonsulfide ore.

Ore genesis of the Wusihe carbonate-hosted Zn-Pb deposit in the Dadu River Valley district, Yangtze Block, SW China: evidence from ore geology, S-Pb isotopes, and sphalerite Rb-Sr dating

The Wusihe carbonate-hosted Zn-Pb deposit (3.7 Mt. Zn + Pb at a grade of 8.6% Zn and 2.0% Pb) is the largest deposit in the Dadu River Valley district of the Sichuan-Yunnan-Guizhou metallogenic province of southwest China. Three types of orebodies occur: (1) stratiform, banded and lamellar, within dolomite of the Neoproterozoic Dengying Formation; (2) vein type; and (3) breccia type. Four stages of mineralization are distinguished: (i) pyrite stage, (ii) pyrite-pyrrhotite-galena-sphalerite-bitumen stage, (iii) sphalerite-galena stage, and (iv) bitumen-calcite stage. Sphalerite and galena from stages II and III show δ34S ranges from +7.1 to +9.7‰ and +9.1 to +13.1‰, respectively. High-precision in situ lead isotope analyses of sulfides show 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of 37.938 to 38.336, 15.579 to 15.682, and 17.951 to 18.195, respectively, which suggest a mixing of lead from the basement and the host rocks. Rb-Sr isotope analyses for six sphalerite samples of stage II yielded an isochron age of 411 ± 10 Ma (MSWD = 1.4). Combining all available ore geology and geochemical data, together with fluid inclusion data reported previously, we suggest that the Wusihe deposit is a Mississippi Valley-type (MVT) deposit.

The giant Upper Yangtze Pb–Zn province in SW China: Reviews, new advances and a new genetic model

In the western margin of the Yangtze Block, SW China, the Emeishan large igneous province (ELIP) is spatially associated with >400 carbonate-hosted epigenetic Pb–Zn deposits. These deposits form the giant Upper Yangtze Pb–Zn metallogenic province with >20 Mt base metals. In the southeastern part of this province, the important Pb–Zn deposits include those of the Yinchangpo, Yunluhe, Maozhachang, Tianqiao, Banbanqiao, Mangdong, Shaojiwan, Liangyan, Qingshan, Shanshulin, Nayongzhi and Guanziyao deposits. Sulfide ore bodies in these deposits are (i) hosted in late Ediacaran to middle Permian limestone, dolomitic limestone and dolostone; (ii) structurally controlled by reverse fault-anticline tectonic systems; and (iii) spatially associated with the ELIP flood basalts and mafic dikes, and early Permian, early Carboniferous and early Cambrian organic matter-rich black shales. C–O isotopic compositions suggest that dolostone and limestone, mantle-derived rocks of the ELIP, and sedimentary organic matters supplied C–O to the hydrothermal systems through water/rock (W/R) interaction. New and existing S isotopic compositions of sulfides imply multiple sources of S and the reduction of sulfate through both abiotic thermochemical (TSR) and bacterially mediated (BSR) processes. Zn isotopes indicate that the sources of Zn were most likely related to the ELIP with various contributions from sediments and basements locally. Pb isotope signatures are suggestive of derivation of Pb from basements and sedimentary rocks with variable influences from the ELIP. Sr isotopes support that mantle-derived rocks, sediments and basements were involved in Pb–Zn mineralization, and they have various contributions in different deposits. We consider that the Pb–Zn deposits in the Upper Yangtze province are the mixed products of multiple S species-bearing solutions and metal-rich fluids, both of which were derived from, flowed through or interacted with multiple lithostratigraphic units in the western Yangtze Block. The change of tectonic regimes from extension to compression after eruption of basalts of the ELIP, and then to extension during Early Mesozoic, facilitated extraction, migration, and excretion of ore-forming metals and associated fluids. Mixing of fluids and reduction geochemical barrier activated TSR, causing cyclical carbonate dissolution, CO2 degassing and recrystallization (namely carbonate buffer). All these processes triggered continuous precipitation of huge amounts of hydrothermal minerals. Underplating and eruption of ELIP basalts provided heat flow, fluids and volatiles, whereas the basalts acted as an impermeable and protective layer, and even as ore-hosting rocks. These Pb–Zn deposits have spatial and genetic association with igneous activities of the ELIP, and are characterized by high ore grades (>10 wt% Pb + Zn), high concentrations of associated metals (e.g. Cu, Ag, Ge, and Cd), and medium-low temperatures (usually < 300 °C) and salinities (commonly < 20 wt% NaCl equiv.), all of which are significantly different from those of typical Mississippi Valley-type (MVT) deposits. Hence, the carbonate-hosted epigenetic Pb–Zn deposits in the Upper Yangtze metallogenic province representing to a new type of Pb–Zn deposits that are hosted in platform carbonate sequences and formed within compressional zones of passive margin tectonic settings.

Sulfur and lead isotope systematics: Implications for the genesis of the Riópar Zn-(Fe-Pb) carbonate-hosted deposit (Prebetic Zone, SE Spain)

The Zn-(Fe-Pb) deposits of the Riópar area (Prebetic Zone, SE Spain) are hosted by dolostones that replace Berriasian to Valanginian (Upper Jurassic-Lower Cretaceous) limestones. Mineralization consists of hypogene sphalerite, marcasite and galena, and supergene calamine zones. The hypogene ores are associated with a saddle dolomite gangue. The ore bodies occur as discordant and stratiform lenses, ore-cemented breccias, cm- to mm-wide veins and veinlets, disseminations and stylolite porosity filling within the host dolomites. The main ore controls include stratigraphy and/or lithology, tectonics (faults, fractures and breccias) and availability of metals and sulfur. The morphologies and epigenetic character of the hypogene ore bodies are consistent with the classification of this mineralization as a Mississippi Valley-type (MVT) deposit. The Ga/Ge geothermometer in sphalerite yielded a temperature range of 194–252°C, which represents the temperature of the source region of the ore solution. This value is comparable to the temperature obtained in the ore deposition site, 159±15°C from the Δ34S geothermometer in sphalerite galena pairs. This similitude points to a hydrothermal fluid that did not cool down significantly during flow from the fluid reservoir area to the precipitation site. δ34S values of base-metal sulfides (−7.5 to +3.5‰) are consistent with thermochemical reduction of Triassic sulfate (seawater and/or derived from dissolution of evaporites) by interaction with organic compounds (e.g., hydrocarbons, methane), which reduced sulfate to sulfide in the deposition site. The lead isotope ratios (206Pb/204Pb=18.736–18.762; 207Pb/204Pb=15.629–15.660; 208Pb/204Pb=38.496–38.595) of galena suggest that Pb, and probably other metals as Zn, is derived from continental crustal rocks. On the other hand, these relations points to a unique metal source probably derived from the Paleozoic basement rocks. The relationship between bedding-parallel stylolites, dolomitization, sulfide precipitation and Alpine tectonic affecting the MVT ore, suggests a relative timing range for the mineralization in the Riópar area of 95–20Ma (Upper Cretaceous-Tertiary). The sulfide mineralization and the associated dolomitization are thus explained by the contribution of two fluids that mixed in different proportions during dolomitization and mineralization: i) a fluid probably derived from Cretaceous seawater saturating Mesozoic sediments (Fluid A), characterized by being dilute and initially low temperature, which should have contained organic rich compounds in the ore deposition site (e.g., hydrocarbons and CH4 dissolved gas); and ii) a high salinity hydrothermal brine (Fluid B) rich in both metals and sulfate, circulated through the Paleozoic basement. During the pre-ore dolomitizing stage the fluid phase was dominated by the diluted fluid (Fluid A>Fluid B), whereas in a later fluid pulse, the proportion of the high salinity fluid increased (Fluid A<Fluid B) which allowed sulfide precipitation. MVT exploration in the Prebetic Zone should focus towards the SW of the Riópar mines, in the vicinity of the Alto Guadalquivir-San Jorge fault.

Metal source of giant Huize Zn-Pb deposit in SW China: New constraints from in situ Pb isotopic compositions of galena

The Huize Zn-Pb deposit in SW China is a large Mississippi Valley-type (MVT) deposit containing more than 7 Mt Pb and Zn reserves at an ore grade of ∼25–35% Pb+Zn, and it also contains economically important resources of Ag, Cd, In, Ge and Ga. To provide better constraints on the major source of the ore metals in the deposit in situ Pb isotope composition analyses were carried out by using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). The results indicate that the galena grains overall have restricted Pb isotopic compositions with 206Pb/204Pb of 18.486–18.526, 207Pb/204Pb of 15.738–15.806, and 208Pb/204Pb of 38.914–39.076, which are distinctively different from those for the host dolostones and Permian Emeishan flood basalts. However, the clastic sequence composed of sandstone, siltstone and slate of Proterozoic Kunyang Group has 206Pb/204Pb of 18.789–23.274, 207Pb/204Pb of 15.712–16.005, and 208Pb/204Pb of 38.169–38.933, slightly radiogenic than that for galena. It is thus likely that the clastic sequence of the Proterozoic Kunyang Group is the source of ore metals in the Huize deposit. Leaching experimental results indicate that Pb and Zn in the clastic rocks are easily accessible, and the leaching by 10% HCl resulted in the preferential release of common Pb in leachates which have 206Pb/204Pb of 18.305–18.785, 207Pb/204Pb of 15.629–15.714, and 208Pb/204Pb of 37.695–38.593, similar to the ratios for galena. The thick clastic sequence of Proterozoic Kunyang Group is therefore the predominant source of ore metals in the Huize deposit. Precipitation of sulfides from ore-forming fluids resulted from secular interaction of deep-circulated fluid and the Kunyang Group in a relatively short ore-forming process may be the major reason for the nearly constant Pb isotope signatures of the Huize Zn-Pb deposit and insignificant contribution of the host rocks.

The Karst-Hosted Mina Grande Nonsulfide Zinc Deposit, Bongará District (Amazonas Region, Peru)

The Mina Grande zinc nonsulfide deposit (Amazonas region, Peru) consists of several accumulations in karst cavities of Zn oxide minerals, derived from weathering of a Mississippi Valley-type deposit hosted in Mesozoic carbonate rocks of the Condorsinga Formation (Pucara Group). The karst cavities hosting the nonsulfide ores were developed in the Miocene along northwest-southeast faults (avg 230° dip, 70° dip angle) associated with regional structures, and locally along stratification joints (avg 8° strike, 278° dip, and 26° dip angle). Thus far, the Mina Grande Zn accumulations have been partially mined in two pits (named “Fase A” and “Fase B”) and explored in a third area, named “Fase C.” In 2008, the resources were as follows: Fase A = 160,000 metric tons (t) @ 21.2% Zn, Fase B = 36,400 t @ 28.9% Zn, and Fase C = 116,700 t @ 22.5% Zn. The nonsulfide mineral assemblage, consisting mostly of hydrozincite, smithsonite, and hemimorphite, is associated with few remnants of the hypogene ore (pyrite and sphalerite). Mineralogical and petrographic studies revealed several texturally distinct smithsonite and hydrozincite generations, which are characterized by extremely negative δ 13 C compositions, pointing to the prevailing contribution of an isotopically light carbon component from the oxidation of organic matter-derived carbon, and different δ 18 O isotope compositions: 26.9 to 27.2 and 26.0 to 26.3‰ δ 18 O VSMOW for smithsonite, and averages 24.6 and 23.7‰ δ 18 O VSMOW for hydrozincite. These distinct δ 18 O compositions indicate that smithsonite and hydrozincite precipitated at least after two depositional stages. These stages were probably related to several periods of uplift that occurred in the Bongara district since at least ~10 Ma, when the transition from the Pebas to Acre systems affected the Amazonas foreland basin in the Miocene. The karst activity, to which the supergene mineralization is related, was restricted to the late Miocene and early Pliocene periods. Climatic conditions reconstructed from the ecosystems persisting in the region from Late Tertiary to Recent time suggest that the Mina Grande supergene mineralization was associated with several weathering episodes that occurred under a climate resembling the present-day climatic conditions.

Hydrothermal Fluid Origins of Carbonate-Hosted Pb-Zn Deposits of the Sanjiang Thrust Belt, Tibet：Indications from Noble Gases and Halogens

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization. Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 × 10 −3 ) and (2.86 ± 0.04) × 10 −3 , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) × 10 −3 . In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of ~0.8 × 10 −6 and extend from (2.1 ± 1.1) × 10 −6 to (506 ± 12) × 10 −6 . This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios. The fluid inclusions have 40 Ar/ 36 Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric 36 Ar and variable 129 Xe/ 36 Ar and 84 Kr/ 36 Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric ( 36 Ar, 84 Kr, 129 Xe) and radiogenic (e.g., 40 Ar ✼ ) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The 3 He/ 4 He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 ± 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived 3 He. The fluids with the highest 3 He/ 4 He also have 4 He/ 40 Ar ✼ close to the mantle value, suggesting the 3 He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( 40 Ar ✼ = radiogenic 40 Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H 2 S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.

Characterization of Germanium Speciation in Sphalerite (ZnS) from Central and Eastern Tennessee, USA, by X-ray Absorption Spectroscopy

X-ray absorption near edge structure (XANES) spectroscopy was used on zoned sphalerites (ZnS) from two world-class Mississippi Valley Type deposits, the Central and Eastern Tennessee Mining district, USA, in order to investigate germanium oxidation states. Due to the low germanium concentrations of these samples, it was necessary to perform the X-ray absorption spectroscopy (XAS) in fluorescence mode. The overlapping of the Zn Kβ and Ge Kα emission lines meant that a high energy-resolution was required. This was achieved using crystal analysers and allowed a bandwidth of 1.3 eV to be obtained. Experimental spectra were compared to XANES calculations and three configurations of germanium incorporation into sphalerite were identified. The first two, the most prevalent, show germanium (II) and (IV) surrounded by sulphur atoms in tetrahedral coordination, suggesting the replacement of Zn by Ge. In the third configuration, germanium (IV) is surrounded by oxygen atoms. This third configuration is unexpected for a zinc sulphide mineral and it resembles that of argutite (GeO2).

Calcareous sediments of the Muwaqqar Chalk Marl Formation, Jordan: Mineralogical and geochemical evidences for Zn and Cd enrichment

Immature organic-rich siliceous chalk (‘oil shale’) and organic-poor limestones of the Maastrichtian-Paleocene Muwaqqar Chalk Marl Formation (MCM) (central Jordan) deposited on the southern Neo-Tethys epicontinental shelf provide a perfect example of carbonate sedimentation in a bioproductive upwelling environment. The MCM sediments have been studied by XRD, SEM, EMPA, sequential extraction, ICP-MS, GC–MS, and FTIR to gain insights into causes of their unusual composition. The sediments are remarkable by exceptionally high enrichment in phosphorus and redox sensitive elements (RSE), mainly Cd (up to 225ppm), Zn (1500ppm), and Mo (up to 180ppm), as well as in Ni, V, Cr, and U, with a total RSE budget reaching 3200ppm, coupled with up to 23wt% organic matter and 4.3wt% sulphur. The bulk organic matter consists of type I/II kerogens sulphurised during sulphate reduction. Redox sensitive metals were brought to sediments mainly by biogenic shuttle, while the terrestrial input was minor, and hydrothermal fluids apparently did not contribute to total RSE. The metals can reside in sulphide (Zn-Cd-(Cu)) in sphalerite or/and würtzite; Fe-Ni-V-Cu-(Mo) in pyrite, carbonate (Zn-Cd-(Mo-Ni-V)), and organic (Ni-V-Cu) phases. Authigenic Cd-rich sphalerite and würtzite are much more abundant than pyrite in immature ‘oil shales’, for three main reasons: (i) S-bearing ligands coordinating Cd and Zn in primary organic matter; (ii) high sulphur in organic matter; and (iii) low concentrations of reactive iron in bottom sediments. Limestones redeposited under oxic environments lose all sulphides, but high Zn (up to 337ppm) and Cd (up to 29ppm) become redistributed into the newly formed carbonates. Thus, shelf carbonates of different ages deposited under anoxic/sulfidic conditions in zones of high bioproductivity, as well as their derivative limestones and dolomites, can be the primary Zn and Cd storage for Mississippi Valley-type deposits with high Zn/Pb and Cd/Zn ratios.

Cadmium isotope fractionation in the Fule Mississippi Valley-type deposit, Southwest China

High-precision cadmium (Cd) isotope compositions are reported for sphalerite, galena, and smithsonite from the Fule Zn–Pb–Cd deposit, a typical Mississippi Valley-type deposit in Southwest China. Dark sphalerite has lighter δ114/110Cd values (0.06 to 0.46 ‰) than light sphalerite (0.43 to 0.70 ‰), and the Cd in galena is primarily in the form of sphalerite micro-inclusions with δ114/110Cd of −0.35 to 0.39 ‰. From early to late stages, δ114/110Cd values of smithsonite regularly increase from 0.19 to 0.42 ‰, whereas Cd/Zn ratios decrease from 252 to 136; the δ114/110Cd variation pattern of supergene smithsonite reflects kinetic Rayleigh fractionation during low-temperature processes. From the bottom to the top of the orebody, the dark sphalerite has different patterns in δ114/110Cd values, Cd/Zn ratios, δ34S values, and Fe concentrations compared to the light sphalerite, indicating that dark and light sphalerite formed by different processes. The varying patterns of δ144/110Cd values and Cd/Zn ratios within light sphalerite are similar to those of layered smithsonite, and the δ144/110Cd values have a positive correlation with δ34S values, indicating that Cd isotope fractionation in the light sphalerite was due to kinetic Rayleigh fractionation. Instead, in dark sphalerite, the δ144/110Cd values have a negative correlation with δ34S values and a positive correlation with the Cd/Zn ratio. Thus, it can be concluded that dark sphalerite could be modeled in terms of two-component mixing (basement fluid and host-rock fluid), which is in agreement with previous explanations for the negative correlation between δ66Zn and δ34S in some typical Zn–Pb deposits. We propose that the significant variation in Cd isotope composition observed in the Fule Zn–Pb–Cd deposit confirms that Cd isotopes can be used for tracing fluid evolution and ore formation.