Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.     

Water quality impacts from mining in the Black Hills,South Dakota,USA

The focus of this research was to determine if abandoned mines constitute a major environmental hazard in the Black Hills. Many abandoned gold mines in the Black Hills contribute acid and heavy metals to streams. In some areas of sulfide mineralization local impacts are severe, but in most areas the impacts are small because most ore deposits consist of small quartz veins with few sulfides. Pegmatite mines appear to have negligible effects on water due to the insoluble nature of pegmatite minerals. Uranium mines in the southern Black Hills contribute some radioactivity to surface water, but the impact is limited because of the dry climate and lack of runoff in that area.     

Rb-Sr provenance ages from weathered and stream transported quartz grains from the Harney Peak Granite,Black Hills,South Dakota

Weathered quartz grus and stream transported quartz of the Harney Peak Granite, Black Hills, South Dakota, contain low concentrations of Rb (generally 0.3–6.8 ppm) and Sr (0.2–2.0 ppm) and variable Sr isotopic ratios (0.759–1.070).Six of seven single grains of large composite quartz grus which recently entered the weathering environment define an apparent isochron age (about 1800 Myr) and initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio (0.7066) that approximate the whole-rock isochron age (1707 Myr) and initial ratio (0.7143) of the Harney Peak Granite. Apparently the Rb-Sr systematics of these grains were not significantly altered during initial weathering. Leached fluid inclusion material from a ca. 2 g aggregate of composite quartz grains contains very little Rb or Sr (0.019 and 0.17 μg, respectively) and has a very low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio (0.739). The Rb and Sr content of the quartz grains appears to be concentrated in minute, heterogeneously-distributed mineral inclusions.Five aggregates of more completely weathered, small non-composite quartz grains produce a widely scattered pattern on an isochron diagram with all samples plotting below the 1707 Myr isochron. Examination by SEM of these grains shows solution and precipitation features on their relatively large effective surface areas. The differential precipitation of Rb is believed to have been the major perturbating chemical process during weathering.Three aggregates of stream quartz grains define an apparent isochron age of 1777 Myr and an initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.720 that suggest the initial ‘igneous’ Rb-Sr characteristics of the stream quartz were re-attained during their transportation, probably as a result of removal of the outer weathered surface by abrasion. The apparent resistance to chemical weathering of stream quartz and quartz which has just entered the weathering environment suggests that this mineral may be extremely useful in studies of provenance and the geochronology of strongly weathered terranes.     

Sediment pore-water interactions associated with arsenic and uranium transport from the North Cave Hills mining region,South Dakota,USA

The extent of historical U mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota, USA. While previous studies reported watershed sediment and surface water As and U concentrations up to 90× established background concentrations, it was unclear whether or how localized changes in sediment redox behavior may influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid–liquid As and U distributions as a function of sediment depth. Pore-water and solid phase As and U concentrations, Fe speciation, Eh and pH were measured to ascertain specific geochemical conditions responsible for As and U remobilization and transport behavior. At a mine overburden sedimentation pond adjacent to the mine sites, high total aqueous As and U concentrations (4920 and 674 μg/L, respectively) were found within surface water during summer sampling; however pond dredging prior to autumn sampling resulted in significantly lower aqueous As and U concentrations (579 and 108 μg/L, respectively); however, both As and U still exceeded regional background concentrations (20 and 18 μg/L, respectively). At a wetlands-dominated deposition zone approximately 2 km downstream of the sedimentation pond, pore-water geochemical conditions varied seasonally. Summer conditions promoted reducing conditions in pore water, resulting in active release of As(III) to the water column. Autumn conditions promoted oxidizing conditions, decreasing pore-water As (As_pw) 5× and increasing U_pw 10×. Peak U pore-water concentrations (781 μg/L) were 3.5× greater than determined for the surface water (226 μg/L), and approximately 40× background concentrations. At the Bowman–Haley reservoir backwaters 45 km downstream from the mine sites, As and U pore-water concentrations increased significantly between the summer and autumn deployments, attributed to increased Fe reduction processes. Geochemical modeling suggests solid-phase Fe reduction promotes the liberation of pore-water As and U via suppressing the formation of thioarsenite. Intermittent hydrological processes facilitate As and U transport and deposition throughout the watershed, while biogeochemical-influenced redox changes cycle As and U between pore and surface water within localized environments.     

Dilution and removal of dissolved metals from acid mine drainage along Imgok Creek,Korea

The dilution factors (D_i) and removal fractions (R_i) of pollutants from acid mine drainage (AMD) were quantitatively estimated using two different methods, the conservative component and mass balance method, along Imgok Creek in Korea. The conservative component method assumes that SO₄ is a perfectly conservative component and calculates D_i and R_i from the concentration ratios of SO₄. The mass balance method solves the simultaneous equations relating the concentrations of dissolved components to their precipitation stoichiometries to obtain D_i and R_i. The results from both methods are little different, indicating that SO₄ concentration is a good indicator of dilution for Imgok creek. The calculated D_i's of pollutants quickly decrease from the site of AMD input to the site a few km downstream, but then remain more or less constant over the reaches farther downstream. This is because D_i loses its sensitivity in the reaches where difference in SO₄ concentration between the main stream and combining tributaries significantly diminishes. The calculated R_i's show that approximately 90, 95, and 75% of the original Fe input were removed from the streamwater in October 1996, April 1997, and October 1997, respectively. Aluminum was almost completely removed in April 1997, but only 50% of the original Al was removed in October 1997. The removal of Fe was due to the precipitation of schwertmannite or ferrihydrite and Al due to amorphous Al₄(OH)₁₀SO₄. The maximum removal fraction of dissolved SO₄ was only 5%. The other metals from AMD were significantly removed from the stream water only in April 1997. These metals were removed not by precipitation but by adsorption on and/or coprecipitation with Fe/Al-compounds. The relatively abundant freshwater supply in April 1997 might raise stream pH higher than the adsorption edge and consequently, contribute to rapid metal attenuation by forcing not only more precipitation but also more adsorption of the dissolved metals.     

Chemistry of potassium feldspars from three zoned pegmatites,Black Hills,South Dakota: Implications concerning pegmatite evolution

An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The $\text{K}\text{Rb}$ and $\text{Rb}\text{Sr}$ ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, are consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs) have been modified by subsolidus reequilibration of the feldspars with late-stage residual fluid.$\text{K}\text{Rb}$ ratios of the potassium feldspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between $\text{K}\text{Rb}$ and mineralogy is as follows: Harney Peak Granite (barren pegmatites) > 180; Li-Fe-Mn phosphate-bearing pegmatites = 90?50; spodumene-bearing pegmatites = 60?40; pollucitebearing pegmatites < 30. Although the $\text{K}\text{Rb}$ ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Harney Peak Granite, overlapping $\text{Rb}\text{Sr}$ ratios and the general increase in Sr and Ba with decreasing $\text{K}\text{Rb}$ indicate the genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting.     

Transport and sediment–water partitioning of trace metals in acid mine drainage: an example from the abandoned Kwangyang Au–Ag mine area, South Korea

Transport and sediment–water partitioning of trace metals (Cr, Co, Fe, Pb, Cu, Ni, Zn, Cd) in acid mine drainage were studied in two creeks in the Kwangyang Au–Ag mine area, southern part of Korea. Chemical analysis of stream waters and the weak acid (0.1 N HCl) extraction, strong acid (HF–HNO₃–HClO₄) extraction, and sequential extraction of stream sediments were performed. Heavy metal pollution of sediments was higher in Chonam-ri creek than in Sagok-ri creek, because there is a larger source of base metal sulfides in the ores and waste dump upstream of Chonam-ri creek. The sediment–water distribution coefficients (K _d) for metals in both creeks were dependent on the water pH and decreased in the order Pb ≈ Al > Cu > Mn > Zn > Co > Ni ≈ Cd. K _d values for Al, Cu and Zn were very sensitive to changes in pH. The results of sequential extraction indicated that among non-residual fractions, Fe–Mn oxides are most important for retaining trace metals in the sediments. Therefore, the precipitation of Fe(–Mn) oxides due to pH increase in downstream sites plays an important role in regulating the concentrations of dissolved trace metals in both creeks. For Al, Co, Cu, Mn, Pb and Zn, the metal concentrations determined by 0.1 N HCl extraction (Korean Standard Method for Soil Pollution) were almost identical to the cumulative concentrations determined for the first three weakly-bound fractions (exchangeable + bound to carbonates + bound to Fe–Mn oxides) in the sequential extraction procedure. This suggests that 0.1 N HCl extraction can be effectively used to assess the environmentally available and/or bioavailable forms of trace metals in natural stream sediments.     

A natural attenuation of arsenic in drainage from an abandoned arsenic mine dump

At the abandoned As mine in Nishinomaki, Japan, discharged water from the mining and waste dump area is acidic and rich in As. However, the As concentration in the drainage has been decreased to below the maximum contaminant level (0.01 mg/l for drinking water, Japan) without any artificial treatments before mixing with a tributary to populated areas. This implies that the As concentration in water from the waste dump area has been naturally attenuated. To elucidate the reaction mechanisms of the natural attenuation, analysis of water quality and characterization of the precipitates from the stream floor were performed by measuring pH, ORP and electric conductivity on-site, as well as X-ray diffraction, ICP-mass spectrometry and ion-chromatography. Selective extractions and mineral alteration experiments were also conducted to estimate the distribution of As in constituent phases of the precipitates and to understand the stability of As-bearing phases, respectively. The water contamination resulted from oxidation of sulfide minerals in the waste rocks, i.e., the oxidation of pyrite and realgar and subsequent release of Fe, SO₄, As(V) and proton. The released Fe(II) transformed to Fe(III) by bacterial oxidation; schwertmannite then formed immediately. While the As concentrations in the stream were lowered nearly to background level downstream, those in the ochreous precipitates were up to several tens of mg/g. The As(V) was effectively removed by the formed schwertmannite and had been naturally attenuated. Although schwertmannite is metastable with respect to goethite, the experiments show that the transformation of schwertmannite to goethite may be retarded by the presence of absorbed As(V) in the structure. Therefore, the attenuation of As in the drainage and the retention of As by schwertmannite are expected to be maintained for the long term.     

REE-Depleted Leucogranites, Black Hills, South Dakota: a Consequence of Disequilibrium Melting of Monazite-Bearing Schists

Two isotopically distinct but otherwise chemically similar leucogranitesuites in the Proterozoic Horney Peak Granite, Black Hills,South Dakota, have contrasting light rare earth element (LREE)concentrations. Most samples of a relatively ¹⁸O-depleted suitehave LREE- enriched, chondrite-normalized patterns, typicalof melts derived from metasedimentary protoliths, whereas allsamples of the regionally significant, relatively ¹⁸O-enrichedsuite have LREE-depleted patterns. The latter patterns are interpretedto have resulted from disequilibrium melting of schists. Monaziteand perhaps other accessory minerals remained armored by biotiteand garnet which did not partake in the muscovite dehydration-meltingreaction that produced LREE-depleted melts. The REE concentrationsin the LREE-depleted samples are below saturation levels formonazite at reasonable melting temperatures and melt water contents,whereas the REE concentrations in the LREE-enriched samplesyield 700–800C monazite saturation temperatures, reasonablefor biotite dehydration-melting reactions. LREE depletions,analogous to those in the LREE-depleted granites, are also foundin leucosomes of partially molten schists, thought to be theprotolith for the granite. In contrast, the melanosomes holdthe accessory minerals and bulk of the LREEs. KEY WORDS: accessory minerals; leucogranites; Black Hills; monazite; partial melting ^*Corresponding author at Department of Geological Sciences, University of Missouri. Telephone: 314-884-6463. Fax: 314-882-5458. e-mail: geolpin{at}showme.missouri.edu.     

Nd, O and Sr isotopic constraints on the origin of Precambrian rocks, Southern Black Hills, South Dakota

The Nd, O and Sr isotopic characteristics of Precambrian metasedimentary, metavolcanic and granitic rocks from the Black Hills of South Dakota are examined. Two late-Archean granites (2.5-2.6 Ga) have T_dm ages of 3.05 and 3.30 Ga, suggesting that at least one of the granites was derived through the melting of significantly older crust. Early-Proterozoic metasedimentary rocks have T_dm ages that range from 2.32 to 2.45 Ga. These model ages, in conjunction with probable stratigraphic ages ranging from 1.9 to 2.2 Ga, indicate that mantle-derived material was added to the continental crust of this region during the early-Proterozoic. Previous studies of the Harney Peak Granite complex have reported U-Pb and Rb-Sr ages of about 1.71 Ga and most granite samples examined in this study have Sr isotopic compositions consistent with that age. Two granite samples taken from the same sill, however, give two-point Rb-Sr and Sm-Nd ages of 2.08 ±0.08 and 2.20 ±0.20 Ga (∑²²⁰⁰_Nd = −15.5), respectively. In addition, whole-rock and apatite samples of the spatially associated Tin Mountain pegmatite give a Sm-Nd isochron age of 2000 ±100 Ma (∑²²⁰⁰_Nd = −5.8 ±1.8).

The Sm-Nd, O and Rb-Sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post-crystallization processes, and the age of the pegmatite and parts of the Harney Peak Granite complex remain uncertain. Processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material, assimilation, mixing of REE between granite and country rock during crystallization via a fluid phase and post-crystallization mobility of Sr. The Nd isotopic compositions of the pegmatite and the Harney Peak Granite indicate that they were not derived primarily from the exposed metasedimentary rocks.     

U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.     

Chemical availability of arsenic and heavy metals in sediments from abandoned cinnabar mine tailings

The understanding of the solid-phase speciation of arsenic in soils and sediments is important in the evaluations of its potential mobility and availability in the environment. The spoil heaps of abandoned mercury mines contain waste materials with high arsenic and heavy metals concentrations. The weathering of these tailings can cause their mobilization to the surroundings. In this work, the mobility and availability of arsenic and some heavy metals were evaluated in sediments from two heavily polluted mercury mining districts in Asturias (NW Spain). For this purpose, a slightly modified version of the Bureau Community of Reference sequential extraction scheme was applied to sediments. The total contents in the operationally defined fractions were analysed by inductively coupled plasma-atomic emission spectrometry. Extremely high total arsenic concentrations were found in all sediments ranging from 4,000 to 24,800 mg kg^?1. High easily mobilizable arsenic contents were found in the first mining area, related to the solubilization of Ca-bearing phases, supported by extracts analyses, X-ray diffraction results and the positive correlation found among the As and Ca fractionations. Ni and Zn were the most mobile among heavy metals, being Cr the least mobile, suggesting an anthropogenic origin due to the metallurgical processes, transport or dispersion generated from neighbour spoil heaps. In the second mining area, the bulk of As was concentrated in the residual phase, as well as Cr, Cu, Fe, Pb, Ti, indicating a mineralogical origin and the low availability of these elements. The strong correlations established between As and Fe and Ti distributions support the hypothesis that As is mainly associated to structural mineral phases in these sediments.     

高岭土/菱铁矿杂化材料制备及除砷性能研究

采用KOH活化后的高岭土和菱铁矿为主要原材料,磷酸酸化过的花生壳为造孔剂,采用高温活化的方法,通过正交静态吸附试验,研究其在常温常压下对水中砷的吸附去除效果。结果表明,在高岭土、菱铁矿、花生壳用量之比为2∶2∶1,KOH浓度为0.3mol/L,浸泡时间为6h,活化温度为900℃时,制备得到的粉末状材料具有最好的除砷效果,可达到95%。利用直径40mm,长450mm的有机玻璃柱进行动态连续吸附试验,结果表明此材料可以连续有效除砷20h左右,除砷效果平均可达到84%。此材料吸附除砷符合Langmuir等温吸附模型。     

The generation and crystallization conditions of the Proterozoic Harney Peak Leucogranite,Black Hills,South Dakota,USA: Petrologic and geochemical constraints

The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO₂, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6 for the biotite granites and 13.2 ± 0.8 for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO₂ and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with =1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at 6 kbar, <1 and temperatures 800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, <1, the relatively high TiO₂ and Ba concentrations and the relatively low values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low- biotite granites generated at a deeper, hotter region and the high- tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high- melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.     

Hydrogeology of the vicinity of Homestake mine, South Dakota, USA

The former Homestake mine in South Dakota (USA) cuts fractured metamorphic rock over a region several km² in plan, and plunges to the SE to a depth of 2.4 km. Numerical simulations of the development and dewatering of the mine workings are based on idealizing the mine-workings system as two overlapping continua, one representing the open drifts and the other representing the host rock with hydrologic properties that vary with effective stress. Equating macroscopic hydrologic properties with characteristics of deformable fractures allows the number of parameters to be reduced, and it provides a physically based justification for changes in properties with depth. The simulations explain important observations, including the co-existence of shallow and deep flow systems, the total dewatering flow rate, the spatial distribution of in-flow, and the magnitude of porosity in the mine workings. The analysis indicates that a deep flow system induced by ~125 years of mining is contained within a surface-truncated ellipsoid roughly 8 km by 4 km in plan view and 5.5 km deep with its long-axis aligned to the strike of the workings. Groundwater flow into the southern side of the workings is characterized by short travel times from the ground surface, whereas flow into the northern side and at depth consists of old water removed from storage.     

和尚桥露天铁矿地下水疏干方法研究

和尚桥铁矿是一个大型露天磁铁矿,由于矿区的特殊位置,建设生态矿山、最大限度地减少矿山生产对环境的扰动势在必行。矿山工业布局、独特的水文地质环境和安全生产需要,均给矿山地下水疏干提出新要求。根据该铁矿露天采矿场不同地段的地质、水文地质和工程地质条件,提出了地表和地下联合疏干的方法。利用所建立的地下水疏干预测模型,模拟该方案疏干矿坑地下水的可能,为矿山安全生产、地下水综合利用提供决策依据。