Types Of Hydrothermal Deposits Research Articles

Using titanite to constrain geochronology and physicochemical conditions of hydrothermal mineralization events: A case study of the Duotoushan Fe–Cu deposit, Eastern Tianshan, NW China

As a common Ca‐ and Ti‐ bearing silicate mineral in many types of hydrothermal deposits, titanite usually contains high concentrations of trace elements. Its mineral chemistry and U–Pb isotope can reveal the physicochemical conditions and geochronology of the ore‐forming systems. In this contribution, we present a detailed study on titanite which precipitated during the Fe mineralization in the Duotoushan deposit. Due to coexistence with hydrothermal minerals, low Th/U ratios (0.02–0.3), and depletion in rare earth elements (REE), all the studied titanites are classified into a hydrothermal group. The negative correlation between [REE3+ + (Al, Fe)3+] and [Ca2+ + Ti4+] indicates that REE mainly entered the lattice of titanite via the mechanism of substitution. In addition, titanite grains are characterized by HREE, Zr, and Nb enrichments with LREE depletion, suggesting that the complexation of F− at neutral to alkaline pH conditions may have caused the fractionation of REE. At Duotoushan, the titanite grains coexist with quartz, epidote, amphibole, and magnetite, and exhibit positive Eu anomalies (δEu = 1.04–1.31) but lack Ce anomalies, indicating that ore‐forming fluids may have been derived from a relatively low oxygen fugacity and high fH2O environment. Titanite yielded an in‐situ U–Pb lower‐intercept age of 307.2 ± 4.8 Ma (MSWD = 0.97), consistent with the syn‐ore amphibole 40Ar‐39Ar plateau age (305 ± 6 Ma). Since the mineralization ages are obviously younger than country rocks, the previous syn‐sedimentary ore‐forming model for Duotoushan Fe–Cu mineralization can be excluded. Integrating the characteristics from ore deposit geology, periods of mineralization events, and spatial–temporal distribution of magmatism, we proposed that the Duotoushan Fe–Cu mineralization event may be linked with a hidden granite in its orefield.

Silver enrichment and trace element deportment in hydrothermal replacement reactions: Perspective from the Nageng Ag-polymetallic deposit, East Kunlun Orogen, NW China

Mineral replacement reactions (dissolution-reprecipitation and solid-state diffusion) have been reported in various types of hydrothermal deposits. However, the metal transfer and partitioning between the parent and daughter minerals and the role of different replacement reactions in silver enrichment remains poorly understood. Here we present the petrographic textures, major and trace elements, and sulfur isotopes of different generations of pyrite and marcasite associated with replacement reactions from the Nageng epithermal Ag-polymetallic deposit, East Kunlun Orogen (EKO, NW China). The mineralization includes the pyrite-quartz (I), quartz-siderite-sulfides (II), quartz-fluorite-rhodochrosite-sulfides-sulfosalts (III), and quartz-calcite (IV) stage. This deposit is characterized by a metal zonation from deep-level Cu-Pb-Zn mineralization to shallow-level Ag-Pb-Zn mineralization, which correspond to main ore stages II and III, respectively. Stage II ores contain the colloform pyrite (Py1) and porous pyrite aggregate (Py2), both of which have higher Co, Ni, and W contents than stage III pyrite (Py3 and Py4) and marcasite (Mc1 and Mc2). In contrast, stage III pyrite/marcasite are enriched in Ag and other ore metals (e.g., Pb, Zn, Sb, Mn, and Sn). The sharp contact between Py3 and Py4/Mc1, together with the porosity in Py4/Mc1, suggests that Py3 and Py4/Mc1 represent the parent and daughter phase of dissolution and reprecipitation. The crystallographically-oriented and metallic inclusion-rich Mc1 has higher Ag concentrations (median 246 ppm) than Py3 (median 180 ppm), and commonly occurs in breccia-type ores related to hydraulic fracturing. This suggests that (i) the dissolution-reprecipitation reaction may have remobilized and concentrated silver and associated ore metals, and (ii) the fluid oxidation related to hydraulic fracturing may contribute to dissolution-reprecipitation reaction. However, the randomly-oriented Mc2 is porous and has homogeneous compositions. This, together with the prevailing occurrence with rhodochrosite in vein-type ores, indicates that Mc2 was transformed from Mc1 via Mn-rich fluid-driven solid-state diffusion under relatively mild physicochemical changes. Mc2 has the most homogeneous and highest Ag concentrations (avg. 1142 ppm) and other ore meals among all the studied Fe disulfides, suggesting that the marcasite has higher trace element compatibility than pyrite, and that the solid-state diffusion reactions promoted Ag enrichment more efficiently than dissolution-reprecipitation reactions. The mostly positive δ34S values (−1.09 to 8.50‰) of the Nageng pyrite and marcasite are consistent with a magmatic origin. The δ34S variations among different pyrite and marcasite generations are attributed to the fluctuations between episodic replenishment of magmatic fluids and subsequent oxidation in respond to hydraulic fracturing and mixing with meteoric water. Our findings reveal higher Ag compatibility of marcasite over pyrite, and that replacement reactions (especially solid-state diffusion) greatly promote ore metal enrichment.

Incorporation of geological constraints and semivariogram scaling law into geostatistical modeling of metal contents in hydrothermal deposits for improved accuracy

Detailed metal distributions in a deposit can be used to determine the generation mechanism and process of the deposit type and improve mining development efficiency. Two geostatistical methods were developed that incorporate geologic information and a semivariogram scaling law to improve the spatial estimation accuracy of metal contents at the fine scale. In the first method, a binary dataset is prepared by assigning 1 to the location of the targeted geologic category in each borehole column and 0 elsewhere. Principal component analysis is implemented to generate a geologic model using principal values and ordinary kriging (OK). By overlapping with a 3D geologic model, OK of the main metal contents is then performed by limiting the content data with the same geologic category as the calculation point. This method is termed kriging with a geological constraint (KGC) and focuses on the correlation between the geologic category and metal content. The next step downscales the KGC model using a semivariogram scaling law to obtain finer spatial resolution (KGC-DS). This law is experimentally derived by repeatedly thinning out the original sample data and approximating the relationship between the mean distance of the data pair and semivariogram parameters (range, nugget effect, and sill) using a power function. This approximation can produce semivariograms in a virtual scenario with more sample data than the original. These two methods were applied to two types of hydrothermal deposits: one of the largest kuroko (volcanogenic massive sulfide) deposits in northern Japan using 77 vertical or sub-vertical drilling data and one porphyry copper deposit in northern Sulawesi, Indonesia using 58 drilling data. In the first case study, the accuracy in the high-content zones increases and the metal content distribution becomes considerably more detailed than the OK result by reducing the smoothing effect. The superposition of the geologic model and high-content zones can be used to trace the ore solution flows and interpret the deposit formation process. For the second case study, oxidation, supergene, and hypogene mineralization zones with high copper contents are clearly detected and the relationship between high-content zones and hydrothermal alteration type is identified. The results of these two case studies therefore demonstrate the effectiveness of KGC and KGC-DS.

Fluid Evolution and Scheelite Precipitation Mechanism of the Large-Scale Shangfang Quartz-Vein-Type Tungsten Deposit, South China: Constraints from Rare Earth Element (REE) Behaviour during Fluid/Rock Interaction

Unlike classic skarn-type scheelite deposits directly acquiring sufficient Ca2+ from surrounding limestones, all of the scheelite orebodies of the Shangfang tungsten (W) deposit occur mainly in amphibolite, and this provides a new perspective on the mineralization mechanism of W deposits. The ability of hydrothermal scheelite (CaWO4) to bind REE3+ in their Ca2+ crystal lattices makes it a useful mineral for tracing fluid-rock interactions in hydrothermal mineralization systems. In this study, the REE compositions of scheelite and some silicate minerals were measured systematically in-situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to assess the extent of fluid-rock interactions for the Late Mesozoic quartz-vein-type Shangfang W deposits. According to the variations in CaO and REE among scheelite and silicate minerals, the amphibole and actinolite in amphibolite may be able to release large amounts of Ca2+ and REE3+ into the ore-forming fluids during chlorite alteration, which is critical for scheelite precipitation. Furthermore, an improved batch crystallization model was adopted for simulating the process of scheelite precipitation and fluid evolution. The results of both the in-situ measurements and model calculations demonstrate that the precipitation of early-stage scheelite with medium rare-earth elements (MREE)-rich and [Eu/Eu*]N 1. Even though the partition coefficient of REE is constant, the later-stage scheelite will also inherit a certain degree of MREE-depletion and [Eu/Eu*]N future from the residual fluids. As a common mineral, sheelite forms in various types of hydrothermal ore deposits (e.g., tungsten and gold deposits). Hence, the improved batch crystallization model is also possible for obtaining detailed information regarding fluid evolution for other types of hydrothermal deposits. The results from model calculations also illustrate that the Eu anomalies of scheelite are not an effective index correlated to oxygen fugacity of fluids but rather are dominantly controlled by the continuous precipitation of scheelite.

Rare earth elements and carbon-oxygen isotopes of calcite from the Tongjiachong Cu deposit, South China: Implications for fluid source and mineral precipitation

Calcite is a major gangue mineral in many types of hydrothermal deposits, and its chemical and carbon-oxygen isotopic signatures can shed light on the metallogenic evolution. The Tongjiachong Cu deposit, located in the northern margin of the Yangtze Block in South China, is hosted by dolostone of the Ediacaran Doushantuo Formation. Empirical geothermometric calculation indicates that the pre-ore chlorite was formed at 265–305 °C. Copper mineralization occurs mainly as veinlets in a calcite-chalcopyrite-bornite ± ankerite assemblage. Exsolution lamellae of chalcopyrite from bornite indicate that both minerals were formed coevally at 200–250 °C. Mineral chemical analysis on the syn-ore hydrothermal calcite from Tongjiachong suggests that all the calcite samples are enriched in rare earth elements and yttrium (REYs) (ΣREY = 53.7–119.9 ppm). They have similar Y/Ho (38.3–42.3) and varying La/Ho (21.8–86.7), and display consistent convex PAAS-normalized REY patterns. The distinct positive Eu anomalies (2.38–3.45) and lack of Yb anomalies (0.92–1.05) suggest initial fluid temperatures of 250 to 420 °C and calcites deposition temperatures of <250 °C. The lack of negative Ce anomalies reflects suboxic to anoxic conditions of the ore-forming fluids.Dolomite samples from the dolostone ore host show a marine carbonate C-O isotopic affinity (13CPDB = −4.0 to −2.7‰; δ18OSMOW = +19.8 to +20.2‰). Comparatively, the hydrothermal calcite contains lower 13CPDB (−7.4 to −6.1‰) and δ18OSMOW (+14.1 to +14.8‰), and display a positive 13CPDB vs. δ18OSMOW correlation. Simulated calculations of water-rock (W/R) interactions and ore geological constraints suggest that the initial fluids were likely originated from organic-rich formation water. We proposed that W/R reactions may have been the main drive for calcite precipitation at Tongjiachong, whilst decreasing temperature and W/R reactions may have led to the chalcopyrite precipitation.

Structural Analysis of the Luowei Orefield in Xidamingshan, Guangxi, China

AbstractMany types of hydrothermal deposits (e.g. W, Bi, Pb, Zn, Ag) are confined by faults and hidden granodiorite in the Luowei Orefield in Xidamingshan, Guangxi, China. The orebodies in the Luowei W–Bi deposit are predominantly layered and distributed along bedding in sandstones of the Cambrian Xiaoneichong Formation. The orebodies in the Lujing Pb–Zn deposit are controlled mainly by west‐south‐west (WSW)‐trending faults, and those in the Fenghuangshan Ag deposit are controlled mainly by west‐north‐west (WNW)‐trending faults, which were reverse faults during mineralization and were later reactivated as sinistral strike‐slip faults. The Luowei fault was formed postmineralization and resulted in sinistral displacement of the subsurface granodiorite and the Cambrian strata. A tectonomagmatic mineralization model of the Luowei Orefield is proposed, and the following conclusions were made. (i) Under a regional N–S compressive stress regime, WSW‐ and WNW‐trending reverse faults and N–S‐trending tensional fractures were formed. (ii) Magma intruded along the tensional fractures. Under the force of magmatic thermodynamics, mineralizing fluid migrated along bedding planes in sandstones and formed W–Bi orebodies at favorable sites. Some fluid migrated along WSW‐ and WNW‐trending faults to sites farther from the magma source, forming vein‐type Pb–Zn and Ag orebodies. (iii) After mineralization, under ~E–W compression, a NW‐trending left‐lateral slip fault was formed, cutting the subsurface granodiorite and orebodies. Concurrently, sinistral shear slip occurred on WNW‐trending ore‐controlling faults. However, the small displacement on these faults did not change the overall distributions of the rock mass and orebodies.

Discriminating hydrothermal fluid sources using tourmaline boron isotopes: Example from Bailingshan Fe deposit in the Eastern Tianshan, NW China

Tourmaline is an important B-bearing mineral in many types of hydrothermal deposits, and its chemical and boron isotopic signatures shed light on the nature and evolution of the metallogenic processes. Two generations of hydrothermal tourmaline in potassic-altered rocks (tourmaline I) and subsequent veinlets (tourmaline II) have been identified at the volcanic-hosted Bailingshan Fe deposit in the Chinese Eastern Tianshan, NW China. Both tourmaline I and II have low Ca contents and X-site occupancy, and belong to the alkali group. Tourmaline I grains plot in the schorl field or between the dravite and schorl fields, whereas tourmaline II grains are largely Mg-rich dravitic. Mineral textural and geochemical features suggest that the compositions of tourmaline I and II were mainly controlled by the host rock and hydrothermal fluid compositions, respectively.In-situ B-isotope analyses of tourmaline I yielded δ11B values of +1.7‰ to +4.1‰, whereas tourmaline II yielded slightly higher values of +3.5‰ to +6.6‰. Calculated B-isotope compositions (δ11BH2O) of the hydrothermal fluids in equilibrium with tourmaline I are +3.3‰ to +5.7‰ (at 550 °C), suggesting a mixed magmatic fluid (major) and basinal brine (minor) origin. Meanwhile, tourmaline II-forming fluids yielded δ11BH2O values of +6.2‰ to +9.3‰ (at 400 °C). Based on geological constraints and modal calculations, we suggest that the observed B-isotope variations from tourmaline I to II are mainly controlled by Rayleigh fractionation process, and indicate that they may have crystallized from similar fluids.The presence of basinal brine incursion into the K-rich hydrothermal fluids and geological constraints lead us to suggest that the Bailingshan deposit was likely formed in a Paleozoic Aqishan-Yamansu arc-related basin in the Eastern Tianshan.

Multistage Skarn-Related Tourmaline From the Galinge Deposit, Qiman Tagh, Western China: A Fluid Evolution Perspective

Abstract The Galinge skarn deposit, the largest iron polymetallic skarn deposit in the Qiman Tagh metallogenic belt (western China), was formed via multi-stage fluid-rock interactions. It is divided into six ore domains from east to the west. Skarn-related tourmaline is ubiquitous in the V ore domain of the Galinge deposit, occurring both in the altered basaltic andesite (Tour-I) and in the sandstone (Tour-II). The tourmaline composition in both rock types is within the dravite–uvite solid solution. Some Tour-I crystals show compositional growth zoning in which the early stage uvite cores (Gen-1) are overgrown by second-stage dravite rims (Gen-2). Some Tour-I crystals also show overgrowth rims and fracture-infilled textures (Gen-3). Some other Tour-1 tourmalines without clear growth zoning (Others) show an intermediate composition between Gen-2 and Gen-3. The varying composition of the zoned tourmalines records important information about the evolving hydrothermal fluids and host rocks. Gen-1 and Gen-2, displaying a narrow and high range of Fe 2+ /(Fe 2+ + Mg) ratios, are much more equilibrated with mafic host rocks. The alkaline (K + Na) content of tourmalines is associated with the salinities of the ore-forming fluids. The lowest Na + K content of Gen-3 indicates that it may have been equilibrated with a low-salinity fluid environment in which the concentration of metal-chlorite complexes decreased. The Gen-3 stage is considered to be the main ore-forming event. Tour-II have similar Ca/(K + Na + Ca) ratios with Gen-1 and Gen-2 ratios, which indicates that they are contemporarily formed by the same fluid as Tour-I. Through compositional comparison of the tourmalines with those from other hydrothermal deposit types, the Galinge skarn-related tourmalines are overwhelmingly controlled by the MgFe –1 substitution mechanism. This is different from the compositions of tourmalines in porphyry, VMS, and vein-greisen type deposits, which are, respectively, controlled by the Fe 3+ Al –1 , (CaMg)(NaAl) –1 and (NaMg)(□Al) –1 , and (Fe 2+ Fe 3+ )(MgAl) –1 substitution mechanisms. Different tourmaline compositions and substitution mechanisms could be used as guides for mineral exploration.

MECHANISMS OF CRYSTAL GROWTH AND ZONING OF HYDROTHERMAL PYRITE FROM Sb-Au DEPOSITS IN NIZKE TATRY, SLOVAKIA

Pyrite is one of the most common ore minerals in hydrothermal ores and a hallmark of many types of hydrothermal deposits, e.g. , mesothermal quartz veins with pyrite, arsenopyrite, and visible or invisible gold. In addition to any other mineralogical investigation, it is possible to examine and interpret the crystal size distribution (CSD) of the mineral which is under investigation. The conditions of mineral-forming processes can be diverse in terms of temperature, pressure, and compositional variables, but the CSD shapes are usually convergent and by far the most common CSD type is the lognormal distribution. In this study, we determined the CSD for pyrite crystals from mesothermal quartz-pyrite-arsenopyrite-stibnite-gold ores from the Nizke Tatry Mountains (Western Carpathians, Slovakia). A large number of crystals were measured manually from ore sections or from stereographic images. All samples show a crystal size spectrum from ≈2 μm up to ≈550 μm. We discovered that all CSDs show a lognormal or pseudo-lognormal shape with low logarithmic size variances (β2). In order to understand the history of nucleation and growth of pyrite formed in hydrothermal milieus, we compared our measurements to simulations with different growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program (after [Eberl et al. 1998][1]). From the shape of the distributions, the logarithmic mean (α) and β2, we inferred that the crystals grew in an open system with its associated growth mechanisms (surface-controlled growth and supply-controlled growth), since no evidence for ripening processes in the data or in the numerical simulations of our data could be found. Since ripening as a growth mechanism can be excluded, we have to assume that the growth of the pyrite crystals and so the formation of the whole ore body occurred on a relatively short geological time scale. This preliminary result opens more questions related to the emplacement, circulation, and supersaturation of hydrothermal fluids and to the formation of large ore bodies over short time scales. [1]: #ref-10

Carbon and oxygen isotope zoning around Carlin-type gold deposits: a reconnaissance survey at Twin Creeks, Nevada

This study was undertaken to determine whether wallrocks around the Twin Creeks Carlin-type gold deposits exhibit oxygen isotope haloes similar to those found around other types of hydrothermal deposits. Mineralization at Twin Creeks is hosted by Ordovician Sequence shales containing some carbonate minerals and by Pennsylvanian–Permian Etchart Formation limestone. Analysis of orthophosphate-soluble carbonate from these rocks shows that oxygen isotope haloes are detectable in Ordovician Sequence shales but not in Etchart Formation limestone. The soluble fraction of Ordovician Sequence shales at Twin Creeks has δ 18O values of 12 to 24‰ and δ 13C values of 0 to −10‰. Most samples fall along a poorly defined trend that extends from δ 18O of about 24‰ and δ 13C values of about 0, which are typical of unaltered limestones, toward lower values for both isotope systems, which are typical of rocks that have undergone alteration by hydrothermal fluids. Plots of these values along two sections through the ore body show that δ 18O values of wallrocks are lowest in the ore zone and increase outward, forming a halo several hundred meters in size. In the same plots, δ 13C values of the wallrocks do not show systematic spatial variations. The soluble fraction of Etchart Formation limestones at Twin Creeks have δ 18O values of 25 to 5‰ and δ 13C values of 4 to −10‰, but do not show any systematic spatial variation relative to mineralization at the scale of our samples. Failure of the Etchart Formation samples to show detectable haloes is probably related to deposition of post-ore carbonate minerals or lower ore fluid : rock ratios. Material balance calculations used to model the isotopic composition of average Ordovician Sequence shales indicate that changes in temperature and water : rock ratio were probably not sufficient to account for the wide range of isotope compositions observed in these rocks. The most likely additional factor contributing to this range of values was a change in the composition of the altering fluid, probably by mixing of the ore fluid with surrounding meteoric water. These results suggest that Carlin-type gold deposits are surrounded by haloes of low δ 18O values, but that detection of these haloes could be complicated by local compositional variations and post-ore modification of the wallrocks.

High Temperature Polymetallic Sulfide Deposits in Back Arc Environment: Lau Basin SW Pacific: ABSTRACT

During the French-German diving cruise Nautilau, black smokers were observed for the first time in a back-arc environment. Twenty-two dives have been completed to investigate the southern Lau basin. The objectives were to understand the genesis of sulfide ores, the volcanic and tectonic activity in a back arc close to an island arc. The four diving sites in a water depth of about 2,000 m are located between 21{degree}25'S and 22{degree}40'S. Three types of hydrothermal deposits were discovered during the cruise: (1) Low temperature (40{degree}) deposits that are related to discharge through highly vesicular andesite and dacite. They are characterized by extensive deposits of Fe-Mn oxides underlaid by sulfides. (2) Medium- to high-temperature barite/sulfides mineralization was observed in many places along the ridge. The most important field, a few hundred meters in diameter and 20 m high, consists of barite chimneys and massive barite boulders mixed with massive sulfides. (3) Very high temperature black and white smokers were discovered at the central Valu Fa Ridge. The chemistry of the fluid and the plume is described elsewhere. The temperature measured at the vents (342{degree}C) and the general anomalies of the bottom seawater (up to 25{degree}C) indicate that the area is onemore » of the most active known in the oceans at the present time. A complete cross section was sampled through a massive sulfide deposit including the stockwork.« less