Late Cenozoic evolution of epithermal gold metallogenic provinces in Kyushu, Japan

Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate. This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben.     

Fluid inclusion and stable isotope study of the Cobre–Babilonia polymetallic epithermal vein system, Taxco district, Guerrero, Mexico

The Cobre–Babilonia vein system formed during a single major hydrothermal stage and is part of the Taxco district in Guerrero, southern Mexico. Homogenization and ice melting temperatures range from 160 to 290 °C and from − 11.6 to − 0.5 °C, respectively. We determined an approximate thermal gradient of 17 to 20 °C per 100 m using fluid inclusions. A thermal peak marked by the 290 °C isotherm is interpreted as a major feeder channel to the veins. The highest content of Zn + Pb in ore coincides with the 220 and 240 °C isotherms. Salinities of mineralizing fluids range from 0.8 to 15.6 wt.% NaCl equiv, and are distributed in two populations that can be related with barren or ore-bearing vein sections, with 0.8 to 6 wt.% NaCl equiv and 7 to 15.6 wt.% NaCl equiv, respectively. δ¹³C and δ¹⁸O water values from calcite from the Cobre–Babilonia vein system and the Esperanza Vieja and Guadalupe mantos range − 5.4‰ to − 10.4‰ and 9.9‰ to 13.4‰, respectively. δ³⁴S values range from 0‰ to 3.2‰ and − 0.7‰ to − 4.3‰ in sphalerite, − 4‰ to 0.9‰ in pyrite, and − 1.4‰ to − 5.5‰ in galena. Both fluid inclusion and stable isotope data are compatible with magmatic and meteoric sources for mineralizing fluids. Also, sulfur isotope compositions suggest both magmatic and sedimentary sources for sulfur.     

Fluid inclusion study of the Plomositas–Los Arcos polymetallic epithermal vein tract, Plomosas district, Sinaloa, Mexico

The most important deposit in the Plomosas–Rosario district, Sinaloa, is the vein tract named Plomositas–Plomosas–Los Arcos. These are NNW–SSE striking veins hosted in rocks of the Lower Volcanic Supergroup (LVS), and also in rocks at the bottom of the Upper Volcanic Supergroup (UVS). Both supergroups belong to the Sierra Madre Occidental. These veins evolved from an early intermediate sulfidation stage (1), rich in base metal sulfides, to a low sulfidation stage (2), rich in silver sulfides and sulfosalts. There is also a 45 m-wide stockwork with native silver and gold. Stage 1 is found in the deeper portion of the veins whereas stage 2 is found in the most shallow portion of the deposit. These stages record fluid inclusion salinities ranging from 7 to 12 wt.% NaCl equiv., and from 0.2 to 3.5 wt.% NaCl equiv., respectively. Homogenization temperatures range from 120 °C for surface samples to 200 °C at a depth of 320 m. The low homogenization temperatures recorded, and the dispersion of veins within host rocks as veinlets, suggest that this deposit formed at shallow depths and was probably blind.     

Constraints on the ore fluids in the Sando Alcalde AuAg epithermal deposit,southwestern Peru: fluid inclusions and stable isotope data

The Sando Alcalde ore deposit (southwestern Peru) has been studied in order to characterize the physicochemical parameters of the ore fluids and to determine the fluid process (mixing or boiling) which involves the precious metal mineralization. Mineralogy, ${\delta }^{18}{\mathrm{O}}_{\mathrm{quartz}}$ isotopic values and fluid inclusion data give arguments in favour of a boiling phenomenon. This conclusion corroborates fluid inclusion studies previously performed in this area on the low-sulphidation epithermal deposits of Arcata, Orcopampa and Apacheta, where boiling has been described as the main factor for ore deposition. To cite this article: A.-S. André-Mayer et al., C. R. Geoscience 337 (2005).     

Evolution of the Peak Hill high-sulfidation epithermal Au–Cu deposit,eastern Australia

The early Palaeozoic Macquarie Arc, southeastern Australia, hosts a variety of major late Ordovician to earliest Silurian subduction-related deposits (e.g., Cadia East, Ridgeway, Cadia Hill, Cowal and Northparkes). However, there is uncertainty about whether coeval high-sulfidation epithermal deposits, which occur in intra-oceanic metallogenic belts elsewhere in the West Pacific, (e.g., Lepanto and Chinkuashih), are also present in the Macquarie Arc. This has led to suggestions that their absence may be due to the poor preservation potential of deposits that form at relatively shallow crustal levels in ancient rocks. We present here an interpretation for evolution of the Peak Hill Au–Cu deposit based on the distribution of alteration facies, sulfur isotope data from several textural forms of pyrite and barite, and an assessment of the regional volcanic and sedimentary facies architecture. These data show that the Peak Hill deposit displays a distinct sub-vertical zoning with a pyrophyllite and vuggy-quartz core, that today extends about 350 m east–west and at least 550 m north–south, which grades out through paragonite+muscovite, kaolinite to a chlorite+epidote alteration zone at the margin. The alteration zoning reflects both lower temperatures and neutralisation of acid fluids with increasing distance from the core, which represents the conduit along which hot acidic hydrothermal fluids were channelled. Several temporally overlapping events of silicification, bladed-quartz-pyrite veining, brecciation and pyrite veining occurred during the last stages of hydrothermal alteration, although most appear to predate mineralisation. Au–Cu mineralisation was associated with late quartz-pyrite-barite veins, and the highest gold grades occur mainly in microcrystalline-quartz-altered rocks in the paragonite+muscovite alteration zone, generally within 50 m outward from the boundary of the pyrophyllite and vuggy-quartz core. Sulfur- and lead-isotope data, and the characteristic zoning of ore minerals and alteration assemblages support a magmatic source for the hydrothermal fluids. Similarities in many of the isotopic signatures between Peak Hill and deposits such as Northparkes support generation of the high-sulfidation mineralisation during the Late Ordovician to earliest Silurian (possibly ca. 440 Ma) metallogenic event. The Late Ordovician to Early Silurian volcanic and sedimentary facies associations at Peak Hill are consistent with alteration and mineralisation occurring in rocks deposited in a submarine setting.     

The first discovery of Xinlong epithermal gold deposit in southern margin of the Bangonghu-Nujiang metallogenic belt: A new expansion of gold prospecting in Northern Tibet

The Xinlong gold deposit is located in Niyma County, Naqu area of Tibet and was discovered by the Institute of Mineral Resources, Chinese Academy of Geological Sciences through the 1∶50000 mineral geological survey. The ore bodies occur in the Zenong Group volcanic rocks in the middle section of the central Lhasa subterrane and are structurally controlled by the NNW-striking faults. Four ore bodies have been found, exhibiting cloddy, dense-sparse, disseminated, and breccia structures. The ore minerals are mainly tetrahedrite group minerals, and other ore minerals include pyrite, chalcopyrite, nevskite, bornite, anglesite, native gold, and silver-gold bearing selenide, etc. The types of alteration are dominated by silicification, as well as middle- and high-graded argillization. The alteration mineral assemblages contain quzrtz, pyrophyllite, and kaolinite. The Zaliela Formation volcanic rocks of Zenong Group are silicified by later hydrothermal fluid with vuggy quartz in some fractured zones. The middle- and high-graded argillization are characterized by pyrophyllitization and kaolinization. The Xinlong gold deposit shows great metallogenetic potentiality and has been revealed by 1∶10000 geological mapping, IP sounding, and trial trenching in the mining area. Combined with the regional metallogenic geological setting, we suppose that a potential epithermal gold belt probably exists in the middle of the Lhasa terrane. The discovery of the Xinlong gold deposit opens a new chapter for the gold prospecting in Northern Tibet.©2023 China Geology Editorial Office.     

Late Paleozoic to Jurassic chronostratigraphy of coastal southern Peru: Temporal evolution of sedimentation along an active margin

We present an integrated geochronological and sedimentological study that significantly revises the basin and magmatic history associated with lithospheric thinning in southern coastal Peru (15–18°S) since the onset of subduction at ∼530 Ma. Until now, estimating the age of the sedimentary and volcanic rocks has heavily relied on paleontologic determinations. Our new geochronological data, combined with numerous field observations, provide the first robust constraints on their chronostratigraphy, which is discussed in the light of biostratigraphical attributions. A detailed review of the existing local units simplifies the current stratigraphic nomenclature and clarifies its absolute chronology using zircon U–Pb ages. We observe that the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru consists of two first-order units, namely (1) the Yamayo Group, a sedimentary succession of variable (0–2 km) thickness, with apparently no nearby volcanic lateral equivalent, and (2) the overlying Yura Group, consisting of a lower, 1–6 km-thick volcanic and volcaniclastic unit, the Chocolate Formation, and an upper, 1–2 km-thick sedimentary succession that are in markedly diachronous contact across the coeval arc and back-arc. We date the local base of the Chocolate Formation, and thus of the Yura Group, to 216 Ma, and show that the underlying Yamayo Group spans a >110 Myr-long time interval, from at least the Late Visean to the Late Triassic, and is apparently devoid of significant internal discontinuities. The age of the top of the Chocolate Formation, i.e. of the volcanic arc pile, varies from ∼194 Ma to less than ∼135 Ma across the study area. We suggest that this simplified and updated stratigraphic framework can be reliably used as a reference for future studies.     

Darriwilian (Middle Ordovician) graptolite faunas of the Sandia Region,southern Peru

Ordovician graptolite faunas of Peru are restricted to a short interval in the Middle to basal Upper Ordovician, found in three regions of the country. All Peruvian graptolite faunas are strongly dominated by shallow water elements of the Atlantic Faunal Realm, represented largely by Didymograptus s. str. and Aulograptus, but a number of faunal elements of the pandemic isograptid biofacies have recently been discovered in the Sandia Region of SE Peru. Peruvian graptolite faunas are reviewed and the new records from the Sandia Region are discussed in detail. The faunas from the Purumpata and Iparo members of the San José Formation range in age approximately from the Undulograptus austrodentatus Biozone to the Holmograptus lentus Biozone (early to middle Darriwilian). The faunas provide a better understanding of faunal composition and diversity in this region and help to correlate shallow water and deeper water graptolite faunas from this time interval. Biserial graptolites are rare in most samples and usually indeterminable, but a single identifiable specimen of Undulograptus austrodentatus was found, indicating a level close to the base of the Darriwilian. A number of specimens of the genera Isograptus and Arienigraptus from the Sandia Region represent pandemic graptolite faunas of the isograptid biofacies, described for the first time from this region. Copyright © 2009 John Wiley & Sons, Ltd.     

Sedimentology, palaeontology, biostratigraphy and correlation of the Late Cretaceous Vilquechico Group of southern Peru

Detailed analysis of the Late Cretaceous Vilquechico Group (formerly Vilquechico Formation) of the Southern Andes allows the recognition of three major sedimentary sequences, defining Lower, Middle and Upper Vilquechico lithologic formations (LVF, MVF and UVF respectively). Some of them (MVF and UVF) include in turn minor sedimentary sequences. In addition to dinosaur trackways, they contain a marine fauna (selachians, actinopterygians, molluscs) in their transgressive basal parts, and lacustrine fossils (charophytes, ostracods, gastropods) in their regressive continental upper parts. Two charophyte biozones characterize the MVF and the UVF respectively. The lithologic and sedimentary features of the major sequences, as well as their palaeontological contents allow large-scale correlations with other Andean series. Such correlations permit us to tentatively ascribe the unfossiliferous LVF to the Coniacian-early Santonian (?) time-span, and the MVF to the Santonian late Campanian interval. The UVF is of latest Campanian-late Maastrichtian age. As a consequence, the assumed correlations between the Vilquechico Group and some of the vertebrate-bearing Andean localities are revised.     

新疆准噶尔地区富硫型与贫硫型浅成低温热液金矿床成矿流体与碳、硫、铅同位素

对新疆准噶尔地区浅成低温热液型金矿床中富硫型的阔尔真阔腊金矿、贫硫型的石英滩金矿进行了流依包裹体的均一温度、爆裂温度、包裹体气液相成分、H、O 同位素、矿体围岩及脉石英包裹体 C 同位素、矿体中黄铁矿等 S、Ph 同位素等系统地进行了研究,综合研究表明,本区该类型金矿成矿流体一般温度低、盐度低,来源主要为循环的大气水、矿石中黄铁矿的 S、Pb 同位素均为深源,暗示金的深部来源:矿体石英包裹体中 CO_2的δ~(13)C 为低于-10‰的有机碳,反映了本区年轻的富含有机质的沉积地层参与了金的成矿。此外,本文首次提出了富硫型阔尔真阔腊金矿床成矿流体中有侵入岩浆热液参与,深部有多金属成矿远景;贫硫型石英滩金矿没有侵入岩浆热液的参与,成矿仅与火山古热液活动有关,其成矿较单一。此外,阔尔真阔腊金矿中低温流体活动较强,金矿化也较强:石英滩金矿低温流体活动相对较弱,金矿化也较弱,也体现了该类型金矿床低温流体活动的越强烈,金矿化越强的规律。     

Palaeomagnetic results from the Early Permian Copacabana Group, southern Peru: Implication for Pangaea palaeogeography

Samples collected from folded carbonate rocks of the Early Permian Copacabana Group exposed in the Peruvian Subandean Zone have been subjected to detailed palaeomagnetic analysis. Thermal demagnetisation of most samples yield stable high unblocking temperature directions dominantly carried by titanomagnetite minerals. This remanence, identified in 32 samples (43 specimens), is exclusively of reverse polarity consistent with the Permian–Carboniferous Reversal Superchron (PCRS). The overall directions pass the fold test at the 99% confidence level and are considered as being a pre-folding remanence acquired in Early Permian times. The Copacabana Group yields an overall mean direction of D = 166°, I = +49° (α₉₅ = 4.5°, k = 131.5, N = 9 sites) in stratigraphic coordinates and a corresponding palaeosouth pole position situated at λ = 68°S,  = 321°E (A₉₅ = 5.2°, K = 100). Combining this pole with the coeval high quality data from South America, Africa and Australia results in a mean pole for Gondwana situated at λ = 34.4°S,  = 065.6°E (A₉₅ = 4.9°, K = 73.6, N = 13 studies) in African coordinates. This pole position supports a Pangaea B palaeogeography in Early Permian times. In contrast, the combined pole for Gondwana diverges from the coeval Laurasian mean pole when assuming the Pangaea A-type configuration. Poor quality of the Gondwana dataset and inclination shallowing in sediments seem to play no role in the misfit between the Permian–Triassic poles from Gondwana and Laurasia in Pangaea A reconstruction.     

Mineralizing fluids of the shallow epithermal Au–Ag deposits of the El Barqueño district, Jalisco, Mexico

La composición de elementos trazas y de isótopos de las secuencias magmáticas del terciario inferior en dos localidades de la Sierra Madre Occidental al norte de México, muestran una variación que manifiesta la composición y la edad del basamento por el cual fueron eyectadas. La corteza subyaciente en San Buenaventura corresponde a la parte asociada al basamento Norte Americano, en cambio en El Divisadero, la corteza subyaciente correspondería a los terranos alocthonos acrecionados durante el Mesozoico.En estas localidades las rocas volcánicas aparecen fuertemente diferenciadas, variando en su composición de basáltica a riolítica (SiO₂=50–76%). Las secuencias erupcionadas en los terranos de acreción presentan la más baja dispersión en su cociente isotópico, con un cociente mínimo inicial de Sr (>0.7044, corregido por la edad), y máximo de Nd (<0.5126) y de Pb (²⁰⁶Pb/²⁰⁴Pb 18.9).Los cocientes isotópicos correspondientes a la serie continental presentan una dispersión variable y una distribución fuera de la serie de los terranos hacia la composición más típica de la corteza antigua (⁸⁷Sr/⁸⁶Sr 0.710 y ¹⁴³Nd/¹⁴⁴Nd 0.5123). Las rocas volcánicas de la zona continental, muestran valores relativamente altos en elementos incompatibles en comparación con las rocas de los terranos acrecionados (Ce/Yb=25–45 vs. 13–33, respectivamente), y al mismo tiempo están empobrecidos en algunos elementos incompatibles como U y Rb (p.e. Th/U=3.8–7.5 vs. 2.5–4.0, respectivamente), lo cual indica una posición estratigráfica superior, con cocientes de ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, y Th/U más altos, y ¹⁴³Nd/¹⁴⁴Nd más bajos.Los resultados tienen implicaciones sobre la composición original y la petrogénesis de las rocas volcánicas. Los cocientes isotópicos de los dos lugares revelan una composición que se sitúa entre las rocas de composición máfica de la Sierra Madre Occidental y los xenolithos intermediarios y félsicos característicos del norte de México o del sud-oeste de los Estados Unidos de América. La relación entre los cocientes isotópicos de las secuencias y la edad del basamento, así como el hecho de que la totalidad de los resultados obtenidos presenten una disposición bien definida, demuestran la fuerte contribución de la corteza en la química de los magmas silicatados. En la serie continental los cocientes de isótopos covarían con los cocientes de Th/Pb y U/Pb, aproximándose a la composición observada en los xenolithos intermediarios y en las rocas de composicion félsica. Esto indica claramente que no se trata de una fusión anatéxica de la corteza inferior, sino más bien de una interacción del magma basáltico del manto y la corteza. La contribución de la corteza es del orden de 20–70%. La estrecha variación del cociente isotópico en la zona de la secuencia de terranos acrecionados refleja la interacción del magma basáltico con una corteza relativamente joven, cuya composición isotópica es similar a los magmas derivados del manto. Los altos cocientes de Th/U y de Th/Rb indican que la contaminación de la corteza tiene lugar en su parte inferior. Además, los cocientes menos radiogénicos de ²⁰⁶Pb/²⁰⁴Pb y ²⁰⁷Pb/²⁰⁴Pb en la serie continental indican que el empobrecimiento de elementos fuertemente incompatibles en la corteza inferior corresponde a un rasgo antiguo. Los cambios seculares de los cocientes isotópicos de la sección estratigráfica indica que la contribución de la corteza superior aumenta con el tiempo, predominando al principio una corteza inferior máfica y, más tarde, una corteza intermediaria de rocas félsicas. Analizando rocas volcánicas externas a las dos secciones fuertemente muestreadas, las diferencias obtenidas en su composición isotópica de las rocas volcánicas, los terranos acrecionados y el basamento antiguo permiten deducir la localización del límite corteza continental-corteza oceánica.

Soil liquefaction during the Arequipa Mw 8.4, June 23, 2001 earthquake, southern coastal Peru

The Arequipa June 23, 2001, earthquake with a moment magnitude of Mw 8.4 struck southern Peru, northern Chile and western Bolivia. This shallow (29 km deep) interplate event, occurring in the coupled zone of the Nazca subduction next to the southeast of the subducting Nazca ridge, triggered very localized but widely outspread soil liquefaction. Although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction (nearing the town of Tacna, close to the Chile border), liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area. This was strongly controlled by the aridity along the coastal strip of Southern Peru. From the sand blow distribution along the coastal area, a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed. The most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main June 23, 2001, event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area. The sand blows are larger to the southeast of the epicenter than its northwestern equivalents. This can be stated in other words as well. The area affected by liquefaction to the northwest is less spread out than to the southeast. Implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important. In cases of highly asymmetrical distribution of liquefaction features such as this one, where rupture propagation tends to be mono-directional, it can be reliably determined an epicentral distance (between earthquake and liquefaction evidence) and an earthquake magnitude only if the largest sand blow is found. Therefore, magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied, which can eventually be the only part exhibiting liquefaction evidence, depending on the earthquake location and the distribution of liquefaction-prone environments.     

埃达克岩与Cu-Au成矿作用：有待深入研究的岩浆成矿关系

研究表明,环太平洋地区新生代斑岩铜矿和浅成热液金矿与同期的埃达克质岩浆活动存在密切的时空与成因联系。埃达克岩是许多世界级的斑岩铜矿的容矿岩,也是许多浅成热液矿化系统的成矿母岩浆。根据目前的研究,二者成因联系可能在于埃达克质岩浆的富流体、高氧逸度和基性源岩等固有属性,有利于Cu、Au等深源金属元素的萃取与富集成矿一因此,这可能是一种潜在的岩浆成矿专属性关系,但对产生这种关系的原因与机制仍然不十分清楚。这有待于今后深入开展成矿与无矿的埃达克岩、成矿的埃达克岩与非埃达克岩、无矿的埃达克岩与非埃达克岩等方面的对比研究,以揭示这种成矿专属性的本质。     

Laumontitization as an Exploration Indicator of Epithermal Gold Deposits： A Case Study of the Axi and Other Epithermal Systems in West Tianshan, China

In the light of field investigation, microscopic study, X-ray phase analysis and mineral infrared spectral analysis, it is considered that laumontitization is of extensive occurrence in the Axi gold orefield. The development of laumontitization and its relationship to mineralization show that the laumontitization appeared mainly at the top of and in the periphery of orebodies, and occurred at the edge of the epithermal system or at the late stage of epithermal system evolution. Therefore, laumontitization can be used as an exploration indicator of epithermal gold deposits. The fluids responsible for laumontitization in the Axi gold orefield are similar to those producing hot spring-type gold deposits or those from modem geothermal fields. Epithermal mineralization of the Axi gold deposit was dated at Carboniferous, indicating that the West Tianshan of China is a region favorable to epithermal-type gold mineralization and preservation. Hence the West Tianshan of China is a target area for exploring epithermal gold deposits.     

Reappraisal of great historical earthquakes in the northern Chile and southern Peru seismic gaps

A critical reappraisal of great historical interplate earthquakes in the occidental margin of South America, including southern Peru and northern Chile, is carried out.A spacetime distribution of the earthquakes associated to the seismotectonics regions defined by the rupture zones of the greatest events (1868, Mw = 8.8 and 1877, Mw = 8.8) is obtained. Both regions are seismic gaps that are in the maturity state of their respective earthquake cycles. The region associated to the 1868 earthquake presents a notable seismic quiescence in the present century.     

Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

The Au-Ag (-Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6-3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H₂S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H₂S enrichment in the Vw-c inclusions observed at -200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.     

Shape and intrusion style of the Coastal Batholith, Peru

The Coastal Batholith of Peru extends over 1600 km parallel to the coast along the Andean trend. Gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2.0–3.2 km, and a thick root 4–10 km wide to the west. Granitic material does not extend to depths greater than 3 km below sea level datum.This study supports recent gravity work which indicates plutons are commonly thin, 5 km or less in thickness. Detailed mapping in the Lima segment of the Coastal Batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression. However, early roof uplift also created some space. Stoping occurs but is not a major space maker. Floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites, tuffisites, microbreccia, faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the Coastal Batholith some space was probably made by a cantilever mechanism. In brief, space making processes involved early roof uplift and regional doming, then floor depression mainly by piston and probably subsidiary cantilever mechanisms and, finally, local stoping producing the cut-out rectilinear nature of the batholith.The Coastal Batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km. Floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt. This is an efficient space making process and is limited here to shallow levels of the upper crust only. The melts ascended to within 2 or 3 km of the surface, up dyke-like conduits then spread horizontally to form tabular plutons.     

新疆阿尔泰南缘托库孜巴依金矿成矿演化：石英脉系、同位素地球化学及其Ar-Ar年代学证据

托库孜巴依金矿区赋存于玛尔卡库里大型韧性剪切带及其次级构造中,含矿构造演化经历了5个主要变形阶段:D1推覆挤压变形与紧闭褶皱,D2左行走滑韧性剪切带,D3造山带的抬升与脆-韧性变形,D4伸展体制下的张性构造,伴生有多金属硫化物-石英脉充填与闪长岩脉贯入;D5晚期近东西向及北西向等多组压扭或张扭性次级断裂。相应地,在构造变形过程中形成一系列形态和矿物组合各异的石英脉,初步划分出8种脉系、4个主要成矿阶段,识别出两个主要成矿事件:①与脆-韧性剪切作用有关的含金黄铁矿-石英脉,②与走滑-伸展转换体制有关的含金多金属-石英脉。氢、氧同位素研究结果显示从早阶段向晚阶段,成矿流体由以变质热液为主向以大气降水为主的方向演化,硫同位素主要为深源硫特征,铅同位素分析结果显示来源于造山带和上地幔之间,并向造山带演化,且与阿舍勒铜矿的围岩和矿石有一定的成因联系,说明早期的火山作用对成矿有一定的贡献。对构造变形形成的黑云母进行Ar-Ar法定年,获得黑云母的坪年龄为(269.94±2.54)Ma,反等时线年龄为(269.38±2.63)Ma,结合已发表的年龄数据,该矿床可能由两个成矿事件形成,分别为290Ma左右和270Ma左右,对应于额尔齐斯构造带的两幕变形,早期主要与其大规模左行走滑有关,而第二成矿事件可能与后碰撞走滑-伸展转换体制有关。