Региональные и локальные геофизические исследования западного пояса надвигов на северо-западе Хуануко, Северное Перу

The complicated tectonic history of Northern Huanuco poses great difficulties for geophysical studies in the area. A terrain-corrected, regional Bouguer gravity map of the area has been produced which reflects this complexity. This map is dominated by a northwest grade, which is apparentlythe result of a Paleozoic tectonic boundary. This boundary seems to have at least locally exerted control on younger features leading to considerable superposition of structures in the area. The Andean chain straddles the western coast of the South American continent, parallel to a subduction zone where the Nazca plate descends beneath the South American continental plate. The area of our research was located in the northern part of the Huanuco province. Four sites were surveyed. The gravity values were calculated dataset and Bouguer cor-rections were applied. Finally, a Bouguer anomaly map of Peru and Bouguer anomaly maps of sites was produced based on these data.Сложная тектоническая история северной части провинции Хуануко создает большие трудности для геофизических исследований в этом районе. Составлена региональная карта гравитации Буге с поправкой на рельеф местности, которая отражает данную сложность. На карте преобладают северо-западные ориентировки, которые, судя по всему, являются результатом влияния палеозойской тектонической границы. Выдвигается версия, что эта граница, по крайней мере локально, контролировала позицию более молодых объектов и тем самым повлияла на значительное усложнение структур в этом районе. Андская горная цепь тянется вдоль западного побережья Южноамериканского континента параллельно зоне субдукции, где океаническая плита Наска погружается под южноамериканскую континентальную плиту. Район проведения исследований располагался в северной части провинции Хуануко. Были обследованы четыре участка. Значения гравитационного поля рассчитаны по набору данных с применением поправки Буге. На основе этих данных составлена карта аномалий Буге территории Перу и карты аномалий Буге на участках исследований

A Rare Au-Sb Telluride Pampaloite from the Svetlinsk Gold-Telluride Deposit, South Urals, Russia

Pampaloite AuSbTe, a rare gold-antimony telluride that was first described in 2019 from the Pampalo gold mine, Finland, was found in samples from the large Svetlinsk gold-telluride deposit, South Urals, Russia. Optical microscopy, scanning electron microscopy, electron microprobe analysis, reflectance measurements, electron backscatter diffraction and Raman spectroscopy were used to study eight grains of pampaloite. Pampaloite forms inclusions (5–30 μm) in quartz together with other tellurides (typically petzite), native gold and, less often, sulfides. In reflected light, pampaloite is white or creamy white in color with weak anisotropism and without internal reflections. The empirical formula calculated on the basis of 3 apfu is Au0.97–1.07Ag0–0.02Sb0.96–1.04Te0.96–1.04 (n = 18). The holotype pampaloite structure was used as a reference and provided the perfect match for an experimental EBSD pattern (12 bands out of 12, mean angle deviation 0.19°). Raman spectra are reported for the first time for this mineral. All studied pampaloite grains exhibit vibrational modes in the range 60–180 cm−1. Average peak positions are 71, 108, 125, 147 and 159 cm−1. According to experimental data for the Au-Sb-Te system, we estimate the upper temperature range of pampaloite crystallization at the Svetlinsk deposit to be 350–430 °C

Обработка данных дистанционного зондирования ASTER для картирования зон гидротермальных изменений в Восточном Казахстане

Porphyry copper deposits are accompanied by extensive aureoles of hydrothermally altered rocks which make it possible to detect them on satellite images in the absence of vegetation. The studyis devoted to using the Earth’s remote sensing data, particularly, satellite images from the Japanese sensor ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), which are used to identify areas that are promisingfor the discovery of porphyry copper deposits and ore occurrences within the copperbelt of Kazakhstan. The analysis of numerous publications that offer various methods for processing ASTER images for the interpretation of hydrothermally altered rocks accompanying porphyry copper occurrences showed that the most effective method for this region is the Crosta technique. The Crosta technique, unlike other methods, does not use primary bands, but their combinations are obtained by the principal components analysis method. Thus, the combination of the results of the principal components analysis with the use of index images and analysis of the geological map made it possible to identify areas of hydrothermally altered rocks in the study area. The described technique helps to predict promising areasfor porphyry copper mineralization of varying degrees of reliability, associated with their hydrothermal processingМедно-порфировые месторождения сопровождаются обширными ореолами гидротермально измененных пород, значительно превосходящих их по площади, которые позволяют обнаруживать их на космических снимках в условиях отсутствия растительности. Исследуется использование данных дистанционного зондирования Земли, в частности космических снимков японского сенсора ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), для выделения участков, перспективных на обнаружение медно-порфировых месторождений и рудопроявлений в пределах медного пояса Казахстана. Анализ многочисленных публикаций, в которых предлагаются различные методы обработки снимков ASTER для дешефрирования гидротермально измененных пород, сопровождающих медно-порфировые рудопроявления, показал, что наиболее эффективным из них для данного района является метод Crosta. В отличие от других методов он использует не первичные полосы (band), а их комбинации, полученные методом главных компонент. Таким образом, сочетание результатов метода главных компонент с применением индексных изображений и анализа геологической карты позволило выделить области гидротермально измененных пород в районе исследований. На основании описанной методики определены прогнозные участки, перспективные на медно-порфировое оруденение различной степени достоверности, связанные с их гидротермальной переработкой

Montbrayite from the Svetlinsk Gold–Telluride Deposit (South Urals, Russia): Composition Variability and Decomposition

A rare gold–telluride montbrayite from the large Svetlinsk gold–telluride deposit (South Urals, Russia) was comprehensively studied using optical microscopy, scanning electron microscopy, electron microprobe analysis, reflectance measurements, electron backscatter diffraction, and Raman spectroscopy. Significant variations in the composition of the mineral were revealed (in wt%): Au 36.98–48.66, Te 43.35–56.53, Sb 2.49–8.10, Ag up to 4.56, Pb up to 2.04, Bi up to 0.33, Cu up to 1.42. There are two distinct groups with much more-limited variation within the observed compositional interval (in wt%): (1) Au 36.98–41.22, Te 49.35–56.53, Sb 2.49–5.57; (2) Au 47.86–48.66, Te 43.35–44.92, Sb 7.15–8.10. The empirical formula calculated on the basis of 61 apfu is Au16.43–23.28Sb1.79–6.09Te32.01–38.89Ag0–3.69Bi0–0.14Pb0–0.90Cu0–1.96. Two substitution mechanisms for antimony are proposed in the studied montbrayite grains: Sb→Au (2.5–5.6 wt% Sb) and Sb→Te (7–8 wt% Sb). The dependence of the reflection spectra and Raman spectra on the antimony content and its substitution mechanism, respectively, was found in the mineral. The slope of the reflectance spectra decreases and the curve in the blue–green region of the spectrum disappears with increasing Sb content in montbrayite. Raman spectra are reported for the first time for this mineral. The average positions of the peak with high-intensity are ~64 cm−1 and ~90 cm−1 for montbrayite with Sb→Te and Sb→Au, respectively. Two grains of montbrayite demonstrate decomposition according to two schemes: (1) montbrayite (7 wt% Sb) → native gold + calaverite ± altaite, and (2) montbrayite (5 wt% Sb) → native gold + tellurantimony ± altaite. A combination of melting and dissolution–precipitation processes may be responsible for the formation of these decomposition textures

Gold and Arsenic in Pyrite and Marcasite: Hydrothermal Experiment and Implications to Natural Ore-Stage Sulfides

Hydrothermal synthesis experiments were performed in order to quantify the states of Au and As in pyrite and marcasite. The experiments were performed at 350 °C/500 bar and 490 °C/1000 bar (pyrite–pyrrhotite buffer, C(NaCl) = 15 and 35 wt.%). The synthesis products were studied by EPMA, LA-ICP-MS, and EBSD. The EPMA was applied for simultaneous determinations of Au, As, Fe, and S, with a Au detection limit of 45–48 ppm (3σ). The analyses were performed along profiles across zonal grains. The concentrations of As and Au up to 5 wt.% and 8000 ppm, respectively, were determined in pyrite and up to 6 wt.% and 1300 ppm in marcasite. In pyrite, the Au concentration decreases with fluid salinity and temperature increases. Strong positive Au–As correlation and strong negative Au–Fe and As–S correlation were identified in pyrite. Comparison of the correlations with theoretical lines implies Au–As clustering. The cluster stoichiometry is inferred to be [AuAs10]. Most probably, As in pyrite presents in the form of clusters and in the As→S solid solution. Incorporation of Au in As-rich pyrite can be controlled by the reductive deposition mechanism. In marcasite, the concentrations of Au are not correlated with the As content. The [AuAs10] clusters enrich the {210}, {113}, and {111} pyrite faces, where the former exhibits the highest affinity to Au and As. The affinity of {110} and {100} forms to Au and As is lower. Implication of the experimental results to data for natural auriferous pyrite shows that the increase of Au content at C(As) > 0.5–1 wt.% is caused by the incorporation of the Au-As clusters, but not because of the formation of Au→Fe solid solution. Therefore, the concentration of “invisible” gold in pyrite is dictated solely by the hydrothermal fluid chemistry and subsequent ore transformations