porphyry-Cu Deposits Research Articles

Data-Driven Index Overlay and Boolean Logic Mineral Prospectivity Modeling in Greenfields Exploration

Index overlay and Boolean logic are two techniques customarily applied for knowledge-driven modeling of prospectivity for mineral deposits, whereby weights of values in evidential maps and weights of every evidence map are assigned based on expert opinion. In the Boolean logic technique for mineral prospectivity modeling (MPM), threshold evidential values for creating binary maps are defined based on expert opinion as well. This practice of assigning weights based on expert opinion involves trial-and-error and introduces bias in evaluating relative importance of both evidential values and individual evidential maps. In this paper, we propose a data-driven index overlay MPM technique whereby weights of individual evidential maps are derived from data. We also propose a data-driven Boolean logic MPM technique, whereby thresholds for creating binary maps are defined based on data. For assigning weights and defining thresholds in these proposed data-driven MPM techniques, we applied a prediction-area plot from which we can estimate the predictive ability of each evidential map with respect to known mineral occurrences, and we use that predictive ability estimate to assign weights to evidential map and to select thresholds for generating binary predictor maps. To demonstrate these procedures, we applied them to an area in the Kerman province in southeast Iran as a MPM case study for porphyry-Cu deposits.

A mixture of mantle and crustal derived He–Ar–C–S ore-forming fluids at the Baogutu reduced porphyry Cu deposit, western Junggar

Most large to huge porphyry Cu deposits (PCDs) are oxidized, making the Baogutu reduced porphyry Cu deposit (RPCD) a relative rarity. CH4-bearing ore-forming fluids formed at several hydrothermal stages, however, their source is still unclear. To address this issue, isotopic investigations of sulfide He–Ar–S and calcite C were conducted. Fluid inclusions hosted in sulfides (arsenopyrite, chalcopyrite and pyrite) showed 3He/4He ratios of 0.06–0.30Ra (Ra is the 3He/4He ratio of air=1.39×10−6), 40Ar/36Ar of 311–405, 40Ar∗/4He of 0.06–1.01, and F4He ratios of 902–11,074 (sample BGT-Py 2 yielded a ratio of 100), indicating a predominantly crustal source for the fluids with minor mantle input (less than 5%). The δ13C values of carbonate yielded a value of −7.8‰ (n=3), implying that CO2 was probably sourced from mantle or juvenile lower crust. According to the restricted sulfide δ34S values, the total S isotopic composition of the hydrothermal system was estimated to be 0.0–0.5‰, suggesting that the sulfur was derived from mantle or lower crust magmatic source. According to the published granitoids Nd isotopic compositions at the Baogutu RPCD, fairly young TDM model ages (450–650Ma) suggest that the granitoids were derived from partial melting of a juvenile basaltic lower crust. Thus, we propose that small proportion of mantle-derived fluids (less than 5%), probably rise up and then mix with the fluids of juvenile lower crust under an extensional tectonic setting, forming the mantle-derived Sr–Nd–Pb–S–C but crustal He–Ar isotopic compositions.

Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.

Fast inversion of magnetic data using Lanczos bidiagonalization method

Abstract This paper describes application of a fast inversion method to recover a 3D susceptibility model from magnetic anomalies. For this purpose, the survey area is divided into a large number of rectangular prisms in a mesh with unknown susceptibilities. Solving the full set of equations is substantially time consuming, and applying an algorithm to solve it approximately can reduce the time significantly. It is shown that the Lanczos bidiagonalization method can be an appropriate algorithm to solve a Tikhonov cost function for this purpose. Running time of the inverse modeling significantly decreases by replacing the forward operator matrix with a matrix of lower dimension. A weighted generalized cross validation method is implemented to choose an optimum value of a regularization parameter. To avoid the natural tendency of magnetic structures to concentrate at shallow depth, a depth weighting is applied. This study assumes that there is no remanent magnetization. The method is applied on a noise-corrupted synthetic data to demonstrate its suitability for 3D inversion. A case study including ground based measurement of magnetic anomalies over a porphyry-Cu deposit located in Kerman providence of Iran, Now Chun deposit, is provided to show the performance of the new algorithm on real data. 3D distribution of Cu concentration is used to evaluate the obtained results. The intermediate susceptibility values in the constructed model coincide with the known location of copper mineralization.

Mapping of district-scale potential targets using fractal models

The Pulang porphyry-Cu deposit is one of the most important copper deposits discovered in China in the last five years. In this deposit, the irregular distributions of high concentrations of metals are associated with four intrusive complexes and fault structures. In the present study, fractal models including box-counting dimension (Bd), power-law frequency and Hurst exponent are applied to characterize the vertical distributions of Cu values along boreholes in the deposit, and to delineate target areas in the Pulang copper district. The resulting box-counting model shows that the vertical distributions of Cu in both mineralized and non-mineralized boreholes exhibit self-similarity, with values of Bd ranging from 1.01 to 1.43, and mineralized boreholes have values of Bd higher than those of non-mineralized boreholes. The resulting power-law frequency model shows that the vertical distributions of Cu in mineralized boreholes are bifractal whereas they are monofractal in non-mineralized boreholes. The bifractal vertical distributions of Cu values are generally associated with multiple ore-forming stages/periods or complicated ore-forming functions of geological background. The Hurst exponents of Cu data from all continuously mineralized boreholes are >0.5, indicating that the Pulang porphyry-Cu deposit has good vertical continuity of mineralization, or that the development of orebodies was relatively stable in this geological setting. High Hurst exponents (>0.85) can be utilized to identify subsurface mineralized targets, whereas lower Hurst exponents (<0.85) represent discontinuous mineralized rocks along the margins of the porphyry-Cu deposit near country rocks. Based on calculated values of average Cu grade, coefficient of variation, Bd and Hurst exponent for the Cu data along boreholes, interpolated values of these variables are excellent for mapping of potential targets. The results show that the Complex II (intrusive body) is a potential copper target in the north of the Pulang district, because it has a similar Bd as the Complex I that hosts the Pulang porphyry-Cu orebody in the south of the Pulang district. In addition, the Complex I has a potential porphyry-Cu target at depth between explorations lines 7 and 15 based on high Hurst exponents and favorable geological setting. The application of the fractal models discussed in this study is convenient, simple, rapid and direct for outlining potential exploration targets without processing multiple geological, geophysical, and geochemical datasets from disparate sources.

Integration of various geophysical data with geological and geochemical data to determine additional drilling for copper exploration

Abstract In this paper, three data-driven methods (i.e., Bayesian, k-nearest neighbour (k-nn) and neural network classifiers) are used to generate a prospectivity map for porphyry-Cu deposits. Different data layers of geological, geophysical and geochemical themes are integrated to evaluate the Now Chun porphyry-Cu deposit, located in the Kerman province of Iran, and to prepare a prospectivity map for mineral exploration. Both the Bayesian and k-nn methods showed correct classification rates (CCR) of 52.38% for 21 boreholes divided into five classes. Three types of the neural networks including multi-layer perceptron, radial based function (RBF) and probabilistic neural network are applied to evaluate the result. Among neural networks used, the RBF neural network generated the highest CCR equal to 80.95%. Multi-classification of the prospect for detailed study could increase the resolution of the prospectivity map and decrease the drilling risk.

Support vector machine for multi-classification of mineral prospectivity areas

In this paper on mineral prospectivity mapping, a supervised classification method called Support Vector Machine (SVM) is used to explore porphyry-Cu deposits. Different data layers of geological, geophysical and geochemical themes are integrated to evaluate the Now Chun porphyry-Cu deposit, located in the Kerman province of Iran, and to prepare a prospectivity map for mineral exploration. The SVM method, a data-driven approach to pattern recognition, had a correct-classification rate of 52.38% for twenty-one boreholes divided into five classes. The results of the study indicated the capability of SVM as a supervised learning algorithm tool for the predictive mapping of mineral prospects. Multi-classification of the prospect for detailed study could increase the resolution of the prospectivity map and decrease the drilling risk.

Ferric-iron-rich tourmaline from the Darasun gold deposit, Transbaikalia, Russia

We have studied tourmaline of the schorl – dravite – povondraite series in a sample from the Darasun porphyry-style gold deposit, Chita region, Transbaikalia, Russia. A combination of analytical procedures has been used to determine composition in terms of major elements (electron microprobe), Fe 2+ and Fe 3+ (Mossbauer spectroscopy), the probable position of Fe 3+ in the Fe-rich core of a separated grain (FTIR), the Li content (ICP–MS), and conditions of formation (fluid inclusions in coeval quartz). A significant amount of Fe 3+ has been directly determined by Mossbauer spectroscopy (83% of iron is Fe 3+ over the averaged sample). The evolution of chemical composition of the tourmalines studied here can be generalized in the following model. The first generation of low-Al and high-Fe “oxy-dravite” – povondraite (Al~3, Fe~4, Ca~0.2, □ apfu ) is followed by a second generation of higher-Al and lower-Fe “oxy-dravite” (Al~6, Fe~1.5, Ca~0.1, □~0.2 apfu ), and the third generation consists of dravite having the highest Al and the lowest Fe contents (Al > 6, Fe apfu ). Depletion of the second and third generations in Fe is caused by deposition of early sulfides. The evolution in chemical composition of the tourmalines studied here is different from that of redeposited tourmalinites in Western Carpathians, where tourmaline evolves from schorl–dravite to povondraite. The chemical composition of the Darasun tourmalines is different from Fe 3+ -rich tourmalines from evaporite formations in low K, and is similar in Mg content. At the same time, the major type of homovalent substitution (AlFe −1 ) in the tourmalines examined is similar to that in tourmalines from Sn- and Cu-porphyry deposits, and may be accepted as a prospecting guide for the porphyry-style deposits. This finding of povondraite is the third in the world and the first in hydrothermal gold deposit. The fluid inclusions in the associated quartz indicate that that first-stage tourmaline precipitated from boiling fluid at low pressure. Boiling favored the increasing oxygen activity, and resulted in the increase of Fe 3+ in that tourmaline.

Metallogeny of the Northwestern and Central Dinarides and Southern Tisia

The Dinaridic metallogenic province is a part of the Alpine–Himalayan orogenic system, developed as a result of opening and closure of the Tethys Ocean by convergence of the African and Eurasian plates. The northern boundary of the Dinarides is related to the northern African margin (Adria–Apulia). The Tisia mega-unit, a small continental block, positioned between the Dinarides and the Carpathians, is genetically related to the South Eurasian edge. The geology of the Dinarides is constrained by the Alpine Wilson cycle. The major stages of the cycle are: (a) Permian early intra-continental rifting; (b) Triassic advanced rifting; (c) Jurassic oceanization; (d) Cretaceous subduction; (e) Paleogene collision; and (f) Neogene post-collision and extension followed by orogenic collapse. Each stage creates characteristic ore deposits related to the specific geological environments. Stage (a) bears hydrothermal siderite–barite–polysulphide deposits, epigenetic sedimentary uranium deposits, red bed-type, sabkha-type copper and barite deposits and evaporites. Stage (b) favored SEDEX and hydrothermal iron–polysulphide–barite–mercury and MVT deposits. Stage (c) developed chromites, asbestos, talc and magnesite deposits. The spatial position of stage (d) remains poorly constrained. The Southern Tisia unit might be a possible candidate for the Tethyan active continental margin with the Cretaceous subduction zone positioned beneath. Absence of voluminous subduction-related magmatism and mineral deposits, however, favors subduction within the Vardar zone (the easternmost Dinarides), adjoined to the Serbomacedonian ensialic terrain with its large Cu-porphyry deposits. Stage (e) was a prelude to the prolific phase (f) with its numerous hydrothermal Pb, Zn and Sb deposits that mostly occur in the western Vardar zone. The geology and metallogeny of Southern Tisia, with medium/high grade metamorphics, I-type, S-type granites, resembles the Middle Austro-Alpine unit, formed during the main Carboniferous collisional stage. This contribution provides a review of the metallogenic characteristics of the Northwestern and Central Dinarides and Southern Tisia mega-units, based on recently-gained knowledge on the regional geology, petrology and genesis of mineral deposits. Establishment of the plate tectonic model several decades ago greatly contributed to an integrated interpretation of ore deposit genesis. In turn, basic research in the field of ore genesis generated new data that can be used to improve the plate tectonic model.

Trace elements in hydrothermal quartz: Relationships to cathodoluminescent textures and insights into vein formation

High-resolution electron microprobe maps show the distribution of Ti, Al, Ca, K, and Fe among quartz growth zones revealed by scanning electron microscope-cathodoluminescence (SEM-CL) from 12 hydrothermal ore deposits formed between ~100 and ~750 °C. The maps clearly show the relationships between trace elements and CL intensity in quartz. Among all samples, no single trace element consistently correlates with variations in CL intensity. However in vein quartz from five porphyry-Cu (Mo-Au) deposits, CL intensity always correlates positively with Ti concentrations, suggesting that Ti is a CL activator in quartz formed at >400 °C. Ti concentrations in most rutile-bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of <750 °C using the TitaniQ geothermometer. Titanium concentrations of <10 ppm in all veins that formed at temperatures <350 °C suggest a broad correlation between Ti concentrations and temperature of quartz precipitation. In quartz from most deposits formed at 2000 ppm, but in high-temperature quartz, Al concentrations are consistently in the range of several hundred ppm. Aluminum concentrations in quartz reflect the Al solubility in hydrothermal fluids, which is strongly dependent on pH. Aluminum concentrations in quartz therefore reflect fluctuations in pH that may drive metal-sulfide precipitation in hydrothermal systems.

The solubility of copper in hydrous rhyolitic melts: The effects of oxygen, chlorine and sulfur

Porphyry-Cu deposits are unquestionably linked to subduction zone (arc) magmas. Yet the source magmas for these deposits remain unclear, despite recognition that arc-magmas are water-bearing and relatively oxidised. This study focuses on understanding some of the chemical parameters that may affect the solubility of Cu in hydrous melts using experimental petrology. In particular, the effect of oxygen fugacity and Cl and S content of the melt was investigated.The experiments were conducted in an internally heated pressure vessel where the oxygen fugacity is controlled by the addition of H2 to the Ar pressure medium. The sample charges consisted of powdered synthetic glass of rhyolitic composition containing water (7 wt%) and various amounts of NaCl or a S-bearing phase (FeS or CaSO4 depending on fO2). The samples were sealed in welded AuCu capsules and run at 4 kbars and 800 °C and 900 °C for up to 10 days. Twelve samples can be run simultaneously in the same vessel enabling the P, T and fO2 conditions to be identical for each sample. The fO2 was checked using solid Ni–Pd and Co–Pd sensors. The fO2 ranged from NNO−1 to approx. NNO+3; the Cl contents varied from O to 9000 ppm at 900 °C, and O to 6500 ppm at 800 °C; the S content in the melt was very low for all runs, typically <80 ppm. Results show that although there is a very slight positive correlation between Cl and Cu, Cl does not appear to play a dominant role in the solubility of Cu. The main control on Cu solubility is oxygen fugacity, which correlate linearly, Cu increasing with fO2. The oxidation state of Cu in these experiments can be determined using the reaction Log aCuOglassn = n/2 x log ∞O2 + C, where 2n is the valence of dissolved Cu. A slope of 0.25–0.30 suggests dissolution as a Cu2O-like compound where Cu is present as Cu+. This explains the strong correlation between oxidised magmas and porphyry-Cu deposits. The solubility of S was too low to be varied significantly within detection limits and excess sulfur was found to form a discrete sulfide phase. At lower oxygen fugacities (approx. NNO−1) it consists of a Cu–Fe–S phase, whereas at higher oxygen fugacities (up to NNO+3) a Cu–S phase precipitates. This is contrary to the expectation that at high oxygen fugactities sulfide is no longer stable and suggests that Cu greatly expands the stability field of sulphide liquids.

Evolution of a Porphyry-Cu Mineralized Magma System at Santa Rita, New Mexico (USA)

The magmatic processes leading to porphyry-Cu mineralization at Santa Rita are reconstructed on the basis of petrographic studies, thermobarometry, and laser-ablation inductively-coupled-plasma mass-spectrometry analyses of silicate melt and sulfide inclusions from dikes ranging from basaltic andesite to rhyodacite. Combined results suggest that magma evolution at Santa Rita is similar to that of sulfur-rich volcanoes situated above subduction zones, being characterized by repeated injection of hot, mafic magma into an anhydrite-bearing magma chamber of rhyodacitic composition. The most mafic end-member identified at Santa Rita is a shoshonitic basaltic andesite that crystallized at 1000–1050°C, 1–3 kbar and log fO2 = NNO + 0·7 to NNO + 1·0, whereas the rhyodacite crystallized at 730–760°C and log fO2 = NNO + 1·3 to NNO + 1·9. Mixing between the two magmas caused precipitation of 0·1–0·2 wt % magmatic sulfides and an associated decrease in the Cu content of the silicate melt from 300–500 ppm to less than 20 ppm. Quantitative modeling suggests that temporal storage of ore-metals in magmatic sulfides does not significantly enhance the amount of copper ultimately available to ore-forming hydrothermal fluids. Magmatic sulfides are therefore not vital to the formation of porphyry-Cu deposits, unless a mechanism is required that holds back ore-forming metals until late in the evolution of the volcanic–plutonic system.

Composition of groundwaters associated with porphyry-Cu deposits, Atacama Desert, Chile: Elemental and isotopic constraints on water sources and water–rock reactions

Groundwaters were collected around the Spence porphyry copper deposit, Atacama Desert, northern Chile, to study water-porphyry copper ore bodies interaction and test hypotheses regarding transport of metals through thick overburden leading to the formation of soil geochemical anomalies. The deposit contains 400 Mt of 1% Cu and is completely buried by piedmont gravels of Miocene age. Groundwaters were recovered from the eastern up hydraulic gradient (upflow) margin of the Spence deposit, from within the deposit, and for two kilometers down flow from the deposit. Water table depths decrease from 90 m at the upflow margin to 30 m 1.5 km down flow. Groundwaters at the Spence deposit are compositionally variable with those upflow of the deposit characterized by relatively low salinities (900–7000 mg/L) and Na +–SO 4 2−-type compositions. These waters have compositions and stable isotope values similar to regional groundwaters recovered elsewhere in the Atacama Desert of Northern Chile. In contrast, groundwaters recovered within and down flow of the deposit range in salinity from 10,000 to 55,000 mg/L (one groundwater at 145,000 mg/L) and are dominantly Na +–Cl −-type waters. Dissolved sulfate values are, however, elevated compared to upflow waters, and δ 34S CDT decreases into the deposit (from >4‰ to 2‰), consistent with increasing influence of sulfur derived from oxidation of sulfide minerals within the deposit. The increase in salinity and conservative tracers (Cl −, Br −, Li +, and Na +) and the relationship between oxygen and hydrogen isotopes suggests that in addition to water–rock reactions within the deposit, most of the compositional variation can be explained by groundwater mixing (with perhaps a minor role for evaporation). A groundwater-mixing scenario implies a deeper, more saline groundwater source mixing with the less saline regional groundwater-flow system. Flow of deeper, more saline groundwater along pre-existing structures has important implications for geochemical exploration and metal-transport models.

Magmatic anhydrite and calcite in the ore-forming quartz-monzodiorite magma at Santa Rita, New Mexico (USA): genetic constraints on porphyry-Cu mineralization

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to constrain physico-chemical factors ( P, T, f O 2 , and volatile content) responsible for mineralization. The dike contains a low-variance mineral assemblage of amphibole, plagioclase (An 30–50), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1–2 vol.%), but later was replaced by either quartz or calcite. Hornblende–plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-monzodiorite, with one magma having ascended directly from ≥14 km depth. Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts. The assemblage quartz+sphene+magnetite+Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen fugacity at log f O 2 >NNO+1, in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides. The same reasoning generally applies for rocks hosting porphyry-Cu deposits, seemingly speaking against a major role of magmatic sulfides in the formation of such mineralizations. There is increasing evidence, however, that magmatic sulfides play an important role in earlier stages of porphyry-Cu evolution, the record of which is often obliterated by later processes.

Fluid inclusions in quartz from oceanic hydrothermal specimens and the Bingham, Utah porphyry-Cu deposit: a study with PIXE and SXRF

Individual natural fluid inclusions in quartz were selected for non-destructive microprobe analysis by synchrotron X-ray fluorescence (SXRF) and proton-induced X-ray emission (PIXE) in order to compare and contrast the compositions of porphyry-type ore forming brines and two types of ocean crustal brines. The inclusions contained brines with high salt concentrations (typically 20–40 wt.% total), and one or more daughter crystals. The X-ray and proton beams produced detectable characteristic X-rays for many elements and allowed the quantification of concentrations for many elements with atomic number Z>14. Fluid inclusions associated with the core of the Bingham, UT, porphyry-Cu(Mo) deposit contain NaCl brines that also contain (element, typical range in ppm): K, 12,000–100,000; Ca, ≤17,000; Mn, ≤8000; Fe, ≤120,000; Cu, ≤8000; Zn, ≤5000; Br, ≤2000. These values are in good agreement with published estimates, and results from SXRF and PIXE experiments are generally consistent. The Bingham fluid compositions closely resemble brines from the porphyry-Mo(Cu) deposit at Questa, NM. Inclusions from two suites of oceanic plutonic rock samples gave PIXE results consistent with published SXRF analyses from other inclusions in the same samples. New inclusion analyses from the Mathematician Ridge locality indicate the NaCl brines also contain, on average (element, ppm): K, 3500; Ca, 35,000; Mn, 4900; Cu, 400; Zn, 200; Br, 1000. New inclusion analyses from the Oceanographer Transform locality indicate NaCl brines that also contain, on average: K, 37,000; Ca, 35,000; Mn, 10,000; Cu, ≤400; Zn, 2200; Br, 1000. These values are calculated on the assumption that Fe in the inclusions is 50,000 ppm. Compared to continental ore forming brines, the oceanic brines have higher Ca and lower K and Cu concentrations.

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of <2 cm width within massive quartz–tourmaline rocks at Roche. Brecciated tourmaline grains have overgrowths of <400 μm width [Fe/(Fe + Mg) = 0·31–0·99] with four chemically distinct zones (1–4, towards the margins). Variations in overgrowth composition were caused by episodic mixing between Mg-, Al-rich magmatic hydrothermal fluids (dominant in zone 1), with an increasing component of more oxidizing, Fe-rich formation waters (zones 2 and 4). More oxidizing conditions are supported by high Sn contents in zone 2 (<0·35 wt %), with Sn probably present as Sn4+ rather than Sn2+, the usual form in hydrothermal fluids. From X-ray maps, zones 1 and 3 occur exclusively as overgrowths on pre-existing grains, indicating that overgrowth formation was kinetically favoured over tourmaline nucleation. In zones 2 and 4, nucleation and growth occurred, possibly as a result of supersaturation with respect to tourmaline during increased mixing with formation waters. Tourmaline is associated with the main episode of mineralization in many important mineral deposits, often unaffected by alteration. This method of studying hydrothermal fluid evolution may therefore have uses in exploration, particularly for tourmaline-breccia-hosted ores in Cu-porphyry deposits.

Controls on pit lake water quality at sixteen open-pit mines in Nevada

Thirty-five mines in Nevada currently have, or will likely have, a pit lake. The large bulk mineable deposits in Nevada mined below the water table are of several types, including Carlin-type Au, quartz-adularia precious metal, quartz-alunite precious metal and porphyry-Cu (-Mo) deposits. Of the 16 past or existing pit lakes at 12 different Nevada mines, most had near neutral pH and low metal concentrations, yet most had at least one constituent (e.g., SO 4) which exceeded drinking water standards for at least one sampling event. Water quality data indicate that, in general, poor water quality will not develop in Carlin-type Au deposits. Wall rocks in the geologic environment typical of these deposits, and in the specific pits sampled, contain substantial amounts of carbonate, which buffers the pH at slightly basic conditions and thereby limits the solubility of most metals. Similarly, the quartz-adularia precious metal deposits generally have geologic conditions that buffer pH and naturally prevent the development of poor water quality. In both of these deposit types, certain elements such as As and Se that are mobile in neutral to basic waters may accumulate to levels near or exceeding drinking water standards. Pit lakes forming in quartz-alunite precious metal deposits hosted in volcanic rocks or in porphyry-Cu (-Mo) deposits in plutonic rocks are of greatest environmental concern in Nevada, as both deposit types have relatively high acid-generating potential and low buffering capacity. However, the sampled Nevada pits in these deposit types indicate that the water may not be of poor quality. In addition, water quality in some pits may actually improve with time due to the increased water-rock ratio as the pit fills with water, as suggested by pit waters at one mine in a Carlin-type deposit (Getchell) that improved between 1968 and 1982. Although water quality in pits in each deposit type is generally good, local, site specific conditions (e.g., surface water inflow) and variations (e.g., evaporation rates) result in some pit lakes (e.g., Boss) in the quartz-adularia deposit type being of substantially poorer water quality than other lakes (e.g., Tuscarora) in the same deposit type. Despite underlying geologic controls based on deposit type, site specific variations in hydrogeologic conditions and surface geologic features can result in differing water quality in pit lakes in the same deposit types, and these factors may, in some cases, provide an overriding control on the geochemical evolution of specific pit lakes.

Helium and Argon in Ore Fluids: So What?

The isotopic composition of He and Ar trapped in crustal fluids and fluid inclusions carries a signature of the source of the fluids and the processes which have affected them. There are three distinct sources of the noble gases; atmosphere, crust and mantle. Atmospheric gases carried in groundwaters are characterised by the presence of 36Ar and 4~ and minor amounts of helium (3He/4He = 1.39 x 10 _6 = Ra). The crust is a source of radiogenic 4~ and 4He, very low levels of3He (-0.01-0.05 Ra) and no 36Ar. The prime signature of the mantle is helium with a high 3He content (e.g. 3He/4HeMoRB ~ 8 Ra), together with 4~ and, in most situations, negligible 36Ar (4~ ~ 40,000). Fluids trapped in ore minerals frequently carry all three components (Stuart et al., 1995). We have investigated He and Ar isotopes, sometimes in combination with halogens, trapped in ore fluids from a variety of areas and types of mineralisation. Our aims have been to see what qualitative aspects of the mineralisation processes are reflected in noble gas patterns and to what extent these can be used to resolve conflicting interpretations, what aspects can be understood in a quantitative way, and whether these can be used, for example, to model the large scale features of specific ore deposits. In this report we contrast the He and Ar isotope observations from two widely separated porphyry-Cu deposits (PCD) with those from an MVT deposit. The Machangqing PCD deposit in West Yunnan province, China, is part of the Ailaoshan-Jinshajiang alkali-porphyry intrusive zone of Eocene age formed in an extensional setting. The mantle derived magmas intrude lower Ordovician microelastic rocks and Devonian limestone and appear to have assimilated some crustal material. The Sungun PCD is located in NW Iran and is associated with a series of calc-alkaline hypabyssal igneous intrusions of Oligo-Miocene age. The magma was emplaced in a continental margin tectonic setting related to the subduction and assimilation of the Persian plate during the major Alpine orogenic phase. The Sungun porphyries occur as stocks and dykes intruding a series of Oligocene dacitic breccias, tufts and trachyandesitic lavas, Eocene arenaceous-argiUaceous rocks, and UpperCretaceous limestones. The He and Ar compositions of gases released by crushing sulphides from the two deposits are shown in Figs. 1 and 2. 4~ is the only isotope present in high abundance in all components and consequently this is the only method of representing all the data, end members and processes in a single figure. Mixtures of three end members define a mixing plane, while elemental fractionation of He and Ar drive compositions radially from, and perpendicular to, the ASW axis. In both figures the data lie on 3component mixing planes. The three end members are fluids enriched respectively in atmosphere (ASW), mantle helium and argon ( label led EARLY) and crustal helium (labelled LATE). The 3He/4He ratios, 1.2 Ra (Machangqing) and 2.25 Ra

PIXE and SEM analyses of fluid inclusions in quartz crystals from the K-alteration zone of the Rosia Poieni porphyry-Cu deposit, Apuseni Mountains, Rumania

PIXE and SEM analyses of fluid inclusions in quartz crystals from the K-alteration zone of the Rosia Poieni porphyry-Cu deposit, Apuseni Mountains, Rumania

Petrography and mineral chemistry of the composite Deboullie pluton, northern Maine, U.S.A.: Implications for the genesis of CuMo mineralization

Biotite and apatite mineral chemistry, particularly halogen abundances and ratios, are used to investigate the relation of the two contrasting parts of the Deboullie composite pluton (syenite-granodiorite) located in northern Maine. Biotite mineral chemistry helps to classify the weakly developed porphyry-style mineralization (CuMo) associated with syenitic rocks of the Deboullie pluton. Unmineralized syenite consists of K-feldspar+plagioclase+quartz+biotite+hornblende±clinopyroxene and minor amounts of apatite, titanite, magnetite, zircon and allanite. Biotite and apatite occur within the matrix of the rocks and within small multiphase inclusions hosted by clinopyroxene. The inclusions are interpreted to be crystallized melt inclusions rather than solid inclusions, that were trapped by clinopyroxene during growth. The multiphase inclusions consist of K-feldspar+quartz+biotite+apatite+magnetite. The F contents of biotite display a strong positive correlation with the phlogopite component, or X phl [ X phl=Mg/(sum octahedral cations)]. Biotites from the multiphase inclusions have higher F contents and higher values of F ( F+ Cl+ OH) and X phl than biotites in the matrix that presumably formed later during crystallization and/or re-equilibrated with late-stage fluids. Contents of TiO 2 largely overlap for the inclusion and matrix biotite. Apatite compositions plot within the fluorapatite field [ F ( F+ Cl+ OH) = 0.57−0.99] . Apatites in the multiphase inclusions have higher Cl contents than apatites in the matrix or those enclosed in biotite. Biotite-apatite geothermometry for the multiphase inclusions yields magmatic or near-magmatic temperatures (725–850°C); biotite-apatite pairs outside the inclusions yield temperatures that range from near-magmatic (600–775°C) to much lower (< 450°C), again indicating low-temperature re-equilibration or disequilibrium alteration. The relation between ( X F X Cl ) and X phl was used to infer HF/HCl fugacity ratios in associated hydrothermal fluids, and indicates that biotite from the Deboullie pluton is strikingly similar to biotite from the porphyry-Cu deposit at Santa Rita, New Mexico, but distinctly different from biotite from the porphyry-Mo deposit at Henderson, Colorado. The assemblage quartz+magnetite+titanite implies an oxidation state for the Deboullie pluton well above the quartz-magnetite-fayalite buffer, which is also consistent with magmas associated with porphyry-Cu deposits. On a regional scale, biotite compositions from granitic plutons in Maine do not vary in a systematic manner. This implies that biotite compositions no longer directly reflect source compositions. Finally, biotite and apatite compositions of the syenite and granodiorite are different; compositional ranges in the granodiorite are more restricted than those in the syenite. This provides evidence that the two parts of this composite pluton had different origins and are not related by crystal fractionation.