Breccia Pipes Research Articles

The genesis and gold mineralization of the crypto‐explosive breccia pipe in the Yixingzhai gold region, central North China Craton

The Yixingzhai gold deposit is a quartz‐vein–type deposit accompanied by skarn iron and porphyry molybdenum mineralization. There are four crypto‐explosive breccia pipes which provide an important ore‐controlling structure. Here we investigate the composition and compositional zoning of the native gold, garnet, and epidote in the Tietangdong skarn breccia pipe through detailed petrography, electron probe microanalysis, and elemental mapping. The native gold compositions show that they were formed in a relative higher temperature condition. Based on their composition and optical characteristics, the zoning in garnet can be divided into three types. The garnets of Generations 1 and 3 show high contents of spessartine (2.91–10.17 mol%) and grossular (20.94–54.71 mol%) and anomalous birefringence, indicating that they formed under relatively higher pressure and temperature and lower oxygen fugacity. The garnets of Generation 2 are mainly composed of andradite (75.70–99.98 mol%) and show compositional homogeneity, with formation under relative lower pressure and temperature and higher oxygen fugacity conditions. Magmatic‐hydrothermal pulses and multiple crypto‐explosions could be the main geological processes that caused the formation of the zoning textures. Zoning in epidote is more complex than that in garnet, and from core to the rim, the iron content shows an abrupt increase and aluminium content shows decrease. There is also decrease in iron and increase in aluminium in the inner part. These features might record multiple crypto‐explosion processes. Combining the complex replacement features of iron oxides and sulphides in the breccia pipe, we suggest that the Tietangdong breccia pipe formed through several crypto‐explosions. The fluid system in the breccia pipe activated during multiple crypto‐explosions, and oxygen fugacity varied greatly, which was disadvantageous to the accumulation and precipitation of ore‐forming elements. Crypto‐explosions destroyed the ore bodies formed during the early stage, resulting in their scattered distribution throughout the breccia pipe. Cone fractures formed eventually with the shrinking of the cooling breccia pipe after the magmatism ceased. Those fractures provided important ore‐hosting structures for the gold mineralization during the late stage. Our research indicates that there is high prospecting potential for porphyry Mo, Cu, and Au deposits in the deeper domains of the breccia pipes.

Geomorphic and geophysical evidence of a collapse origin for large sandstone depressions atop the western Cumberland Plateau in Tennessee

For over 70 years, closed topographic depressions developed in siliciclastic rocks of the Cumberland Plateau have been hypothesized to form via collapse of rock into caverns formed in the carbonate-rich rocks below. We made observations of surface characteristics and applied geophysical methods across two of these depressions on the western part of the plateau to collect subsurface information and test the cave collapse hypothesis. Electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) were conducted at both Sites 1 and 2 and two-dimensional (2D) multi-channel analysis of surface waves (MASW) was conducted at Site 2. Three zones of material are interpreted at both sites based on field observations, and variations in density (velocity) and resistivity characteristics; a relatively thin cover of organic matter and colluvium, sandstone bedrock primarily observed at the rims and upper slopes of the depression, and a possible boulder/rubble zone identified towards the centers of the depressions. Details derived from LiDAR data, field observations and collected geophysical data for Site 2 allow for the projection of a circular breccia pipe in the subsurface. A 3-stage model of development is proposed for these features: A Youthful Stage occurs after initial collapse of surface material into a subsurface cavity in which the depression has a cylindrical form. In the Mature Stage, the slopes have shallowed, and slope adjustments are made as the breccia pipe fills and settles. Bedrock cliffs are mechanically weathered and influenced by gravity, forming talus that covers slopes and the floor with colluvium mixed with organic matter. The Degraded Stage is entered when the breccia pipe has settled, the bedrock exposures are covered, and the depression begins to fill. The depressions studied for this research are classified as being in the Mature stage. Future geophysical surveys and additional geologic mapping and field work at these and other depressions on the plateau will add to our understanding of their development and evolution over time.

Geochronology and geochemistry of the Dianfang gold deposit, western Henan Province, central China: Implications for mineral exploration

The Dianfang gold deposit, located in the Xiong’ershan–Waifangshan region within the southern margin of the North China Craton, consists of eastern and western ore blocks, which are hosted in the Dianfang breccia pipe and Paleoproterozoic volcanic rocks, respectively. The eastern ore block, which occurs within the southern and hanging wall margins of the Dianfang breccia pipe and in fractures in the rhyolites of the Jidanping Formation, consists of rhyolite-hosted and breccia-hosted disseminated gold mineralization structurally controlled by ENE-striking faults. The western ore block contains gold-bearing quartz-sulfide veins and minor disseminated rhyolite-hosted gold mineralization that occurs in the annular fractures around the Hougou breccia pipe. The two ore blocks have long been researched and prospected as two different genetic types. Detailed fieldwork and microscopic observations reveal that the eastern and western ore blocks contain the same hydrothermal alteration and mineral assemblages, which include K-feldspar, quartz, sericite, chlorite, epidote, carbonate, pyrite, galena, sphalerite, marcasite and chalcopyrite, as well as minor magnetite, anatase, cervelleite, matildite, and bismuthinite. Gold occurs as electrum inclusions in pyrite or along microfractures in sulfides and quartz. Zircon U–Pb geochronology of the granite porphyry around the Hougou breccia pipe constrains the timing of breccia pipe formation to 145.9 Ma which is consistent with the formation age of the Dianfang breccia pipe (145.8 Ma), indicating that the two breccia pipes formed coevally at the end of the late Jurassic, rather than during the Paleoproterozoic, as was suggested by previous studies. The hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 119.5 ± 1.8 Ma, suggesting that the eastern and western ore blocks formed coevally at the end of the early Cretaceous. In situ mineral trace element data, as well as sulfur and lead isotope data suggest that the eastern and western ore blocks were formed by a common ore-forming fluid that evolved from a magmatic-hydrothermal system that was genetically related to a hidden late Mesozoic granite. Pre-existing fractures and breccia then served as ideal conduits for fluid flow, accumulation, and gold deposition. Consequently, we propose that the ENE-striking faults developed in and around the breccia pipes and the annular brittle faults around the breccia pipes are good targets for future gold exploration at the Dianfang gold deposit. Based on these results and data from other gold deposits of similar ages around the margins of the North China Craton, we argue that the gold deposits in Xiong’ershan region formed under conditions of lithospheric extension from the late Jurassic to the early Cretaceous, which resulted from lithospheric thinning due to the westward subduction of the Pacific Plate.

Ore-formation mechanism of the Weilasituo tin–polymetallic deposit, NE China: Constraints from bulk-rock and mica chemistry, He–Ar isotopes, and Re–Os dating

China’s W–Sn resources are dominant on a global scale. A series of Sn–polymetallic deposits has been found in recent years at depth on the flanks of known volcanic–subvolcanic Cu–Pb–Zn–Ag deposits in the Southern Great Xing’an Range, NE China. The large-scale Weilasituo tin–polymetallic deposit, formed at the depth of the Weilasituo Cu–Zn deposit, is most typical of these types of deposit. Tin–polymetallic orebodies are hosted in the early Paleozoic Xilingol Complex, and their mineralization is closely related to Early Cretaceous quartz porphyry. In this contribution, bulk-rock and mica chemical compositions, Re–Os geochronology, and He–Ar isotopic analyses are described in addressing the ore-formation mechanism of the deposit. Geochemical characteristics indicate that the ore-bearing quartz porphyry corresponds to the I-type granite serie. Five types of mica have been identified in the study area based on detailed petrography and EPMA and LA–ICP–MS analyses: (i) early magmatic Fe–Li mica in albitized quartz porphyry; (ii) late magmatic Fe–Li mica in mineralized quartz porphyry; (iii) hydrothermal Fe–Li mica in a cryptoexplosive breccia pipe (distributed around breccia fragments); (iv) Fe–Li mica in quartz veins; and (v) Fe–Li mica in plagioclase gneiss (wall rock). Magmatic zinnwaldite from the upper mineralized part of the quartz porphyry is enriched in Nb (100–160 ppm) and Ta (100–150 ppm). Hydrothermal zinnwaldite from the explosive breccia, quartz veins, and surrounding gneiss is unusually rich in Mg (up to 3.4 wt% MgO) and Sn (up to 200 ppm). 3He/4He ratios of the Weilasituo tin–polymetallic and Cu–Zn deposits are in the ranges 3.38–4.91 Ra and 4.8–4.9 Ra, respectively, indicating ore-forming fluids sourced from mixed crustal and mantle materials. Molybdenite Re–Os dating of W mineralization yielded an isochron age of 129.0 ± 4.6 Ma, similar to the porphyry crystallization and mineralization ages of the Weilasituo tin–polymetallic deposit, and also consistent with the mineralization age of the Cu–Zn deposit. We propose a new metallogenic model, with all subtypes of W–Sn–Cu–Pb–Zn–Ag mineralization in the Weilasituo area belonging to a single complex metallogenic system. Compared with other ore districts, ore-related granitoids in the Southern Great Xing’an Range display relatively uniform emplacement ages and were derived from partial melting of juvenile lower crust rather than old basement. Their ore-forming fluid source was greater influenced by mantle fluids than which found in South China.

Biota Dose Assessment of Small Rodents Sampled Near Breccia Pipe Uranium Mines in the Grand Canyon Watershed.

The biotic exposure and uptake of radionuclides and potential health effects due to breccia pipe uranium mining in the Grand Canyon watershed are largely unknown. This paper describes the use of the RESRAD-BIOTA dose model to assess exposure of small rodents (n = 11) sampled at three uranium mine sites in different stages of ore production (active and postproduction). Rodent tissue and soil concentrations of naturally occurring uranium (U, U, and U), thorium (Th, Th, and Th), and radium (Ra) radioisotopes were used in the dose model. The dose assessment results indicated that the potential internal, external, and total doses to rodents were below the US Department of Energy's biota dose standard of 1 mGy d. As expected, tissue concentrations of U, U, and Th were in approximate equilibrium; however, Ra results in tissue were 1.25 to 5.75 times greater than U, U, and Th tissue results for 10 out of 11 samples. Soil at the three sites also displayed Ra enrichment, so it is likely that the Ra enrichment in the rodents was from soil via typical activities (i.e., burrowing, incidental ingestion, bathing, etc.) or by dietary uptake of translocated Ra. The results suggest that Ra is more mobile in this environment and bioaccumulates in these rodent species (e.g., in bones via the bloodstream). Internal dose accounting suggests that Ra is the radionuclide of most concern for rodent exposure and health.

Improved enrichment factor calculations through principal component analysis: Examples from soils near breccia pipe uranium mines, Arizona, USA

The enrichment factor (EF) is a widely used metric for determining how much the presence of an element in a sampling media has increased relative to average natural abundance because of human activity. Calculation of an EF requires the selection of both a background composition and a reference element, choices that can strongly influence the result of the calculation. Here, it is shown how carefully applied, classical principal component analysis (PCA) examined via biplots can guide the selections of background compositions and reference elements. Elemental data were treated using the centered log ratio (CLR) transformation, and multiple subsets of major and trace elements were examined to gain different perspectives. The methodology was applied to a dataset of elemental soil concentrations from around breccia pipe uranium mines in Arizona, U.S.A., with most samples collected via incremental sampling methodology. Storage of ore at the surface creates the potential for wind dispersal of ore-derived material. Uranium was found to be the best individual tracer of dispersal of ore-derived material to nearby soils, with EF values up to 75. Sulfur, As, Mo, and Cu were also enriched but to lesser degrees. The results demonstrate several practical benefits of a PCA in these situations: (1) the ability to identify one or more elements best suited to distinguish a specific source of enrichment from background composition; (2) understanding how background compositions vary within and between sites; (3) identification of samples containing enriched or anthropogenic materials based upon their integrated, multi-element composition. Calculating the most representative EF values is useful for numerical assessment of enrichment, whether anthropogenic or natural. As shown here, however, the PCA and biplot method provide a visual approach that integrates information from all elements for a given subset of data in a manner that yields geochemical insights beyond the power of the EF.

Geology, geochronology, and geochemistry of the siruyidie’er prospect, Taxkorgan: A possible Miocene porphyry Mo ± Cu deposit in the Central Pamir

The Pamir and Tibetan plateaus, which have a similar terrane structure and evolutionary history since the Phanerozoic, are both host abundant Eocene to Miocene intrusions, however, unlike the Tibetan plateau, coeval porphyry deposits appear to be devoid in the Pamir plateau. In this study, we first report a potentially economic Cu-bearing sulfide mineralization at Siruyidie’er prospect in the Central Pamir. The Siruyidie’er prospect is characterized by the development of 14 breccia pipes in the north block and of Cu-bearing sulfide veins in the south block. Field evidence indicates that the brecciation and mineralization are intimately related to the intrusion of granite porphyry. Zircon U-Pb dating of the granite porphyry yields an age of 13.74 ± 0.21 Ma. This age overlaps with the hydrothermal muscovite 40Ar/39Ar data of 13.68 ± 0.17 Ma, which confirms the hydrothermal activity and mineralization at Siruyidie’er prospect are induced by the granite porphyry. The granite porphyries are shoshonitic in composition and have high K2O (6.03–6.46 wt%; K2O/Na2O = 1.70–1.92), Ba (2983–3316 ppm), LREE (317–363 ppm) and Sr (331–378 ppm), and low MgO (0.54–0.63 wt%), Cr (6.73–8.28 ppm) and Y (7.88–9.91 ppm) contents. The rocks are LREE-enriched ((La/Yb)N = 53–66) and display weakly negative Eu (Eu/Eu* = 0.79–0.87), positive LILES (Ba, Rb, and K) and negative HFSEs (Th, U, Nb, Ta, Ti, and Y) anomalies. They have enriched Hf isotopic compositions with ɛHf(t) values ranging from –8.78 to −5.01. The combined major and trace elements and Hf isotopic characteristics of the granite porphyries suggest that the primitive magmas were produced by high extent partial melting of an ancient crust source, possibly a thickened lower crust in the garnet stability field. A comparison of the granite porphyries at Siruyidie’er prospect with Eocene to Miocene ore-forming granitic porphyries of the Tibetan plateau reveals that the granite porphyries have lower water contents and originated from a continental crust rather than a depleted mantle source. Based on geological, compositional, and age data, we propose that the Siruyidie’er prospect has a potential to form porphyry Mo ± Cu deposit. This study provides the first direct evidence for the possible presence of porphyry Mo ± Cu deposit in the Pamir plateau.

Gold distribution and source of the J4 gold-bearing breccia pipe in the Qiyugou district, North China Craton: Constraints from ore mineralogy and in situ analysis of trace elements and S-Pb isotopes

The Qiyugou district is one of the most important gold deposit distribution areas in the Xiong’ershan region, North China Craton. These deposits are hosted in Archean Taihua Supergroup rocks as gold-bearing breccia pipes. Here, we report a detailed in situ analysis of trace elements and S-Pb isotopes in sulfides from different ore stages, in combination with electron probe microanalysis of ore minerals from the gold-bearing J4 breccia pipe in the Qiyugou district, to unravel the gold distribution and their textural relationships to pyrite, the metal sources and S-Pb isotopic composition variations during the ore-forming processes. The Qiyugou breccia pipes hosted gold ore (over 70 t Au, average grade of 2–5 g/t) that formed in four mineralization stages, i.e., an early K-feldspar-biotite-magnetite-quartz-pyrite (Py-1) stage (I) that can be further divided into two substages, namely the Ia K-feldspar-biotite-magnetite stage and the Ib quartz-pyrite stage; a gold-pyrite (Py-2)-quartz stage (II); a gold-quartz-polymetallic sulfide (Py-3) stage (III); and a late quartz-carbonate stage (IV). The visible native gold and electrum grains occur as small inclusions and disseminations in the fractures within Py-2 and Py-3. Tellurides and Bi-Pb-minerals are widespread in the stage II stockworks and veins, as is intergrowth with visible native gold and Py-2. In situ LA-ICP-MS trace element analysis reveals the presence of invisible gold hosted in three types of pyrite with extremely low Au content (Py-1 ≤ 0.05 ppm; Py-2 ≤ 0.03 ppm; Py-3 ≤ 0.02 ppm). Concentrations of Te, Bi, and Pb in pyrite are high and correlate well with Au concentrations, but As content is low. Therefore, the Te, Bi and Pb played a significant role in gold mineralization in As-deficient ore fluids in the J4 gold-bearing breccia pipe from the Qiyugou district.High-precision in situ laser ablation multiple collector inductively coupled mass spectrometry (LA-MC-ICP-MS) lead isotope analyses indicate that the sulfides have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios from 17.250 to 17.629, from 15.401 to 15.621, and from 37.649 to 38.290, respectively, which are similar to the granite porphyry in the district and indicative of their similar lead sources. High-precision in situ LA-MC-ICP-MS sulfur isotope analyses of pyrite show a large δ34S variation (e.g., from −18.1 to −11.1 per mil for Py-1, from −12.2 to − 6.6 per mil for Py-2, and from −4.9 to −1.6 per mil for Py-3) with a remarkable increasing trend from stage I, stage IIto stage III. This trend may record a decreasing oxygen fugacity of the ore-forming fluids during mineralization.

Re–Os Pyrite Geochronological Evidence of Three Mineralization Styles within the Jinchang Gold Deposit, Yanji–Dongning Metallogenic Belt, Northeast China

The Jinchang gold deposit is located in the eastern Yanji–Dongning Metallogenic Belt in Northeast China. The orebodies of the deposit are hosted within granite, diorite, and granodiorite, and are associated with gold-mineralized breccia pipes, disseminated gold in ores, and fault-controlled gold-bearing veins. Three paragenetic stages were identified: (1) early quartz–pyrite–arsenopyrite (stage 1); (2) quartz–pyrite–chalcopyrite (stage 2); and (3) late quartz–pyrite–galena–sphalerite (stage 3). Gold is hosted predominantly within pyrite. Pyrite separated from quartz–pyrite–arsenopyrite cement within the breccia-hosted ores (Py1) yield a Re–Os isochron age of 102.9 ± 2.7 Ma (MSWD = 0.17). Pyrite crystals from the quartz–pyrite–chalcopyrite veinlets (Py2) yield a Re–Os isochron age of 102.0 ± 3.4 Ma (MSWD = 0.2). Pyrite separated from quartz–pyrite–galena–sphalerite veins (Py3) yield a Re–Os isochron age of 100.9 ± 3.1 Ma (MSWD = 0.019). Re–Os isotopic analyses of the three types of auriferous pyrite suggest that gold mineralization in the Jinchang Deposit occurred at 105.6–97.8 Ma (includes uncertainty). The initial 187Os/188Os values of the pyrites range between 0.04 and 0.60, suggesting that Os in the pyrite crystals was derived from both crust and mantle sources.

Unconfined hypogene evaporite karst: West Texas and southeastern New Mexico, USA

Diverse karst phenomena occur throughout the Gypsum Plain where the Castile Formation crops out over ~1800 km2 in West Texas and southeastern New Mexico. Hypergene karst is extensive and widespread, while traditional hypogene karst manifestations (both caves and intrastratal dissolution) occur in high frequency in the western outcrop region where surface denudation has been the greatest so as to induce surficial breaching. Unconfined hypogene karst occurrences have been recently identified, including two general variations: 1) artesian-like discharge features; and 2) venting structures. Artesian-like discharge features arise at surficially-breached hypogene caves and through high permeability regions on the margins of breccia pipes; these occur when aquifer pressures within the underlying Bell Canyon siliciclastics are sufficiently elevated subsequent to anomalously high precipitation events. Venting structures associated with condensation corrosion-like processes coupled with ascending moisture-rich vapor occur as fracture vents and hydration buckles; fracture vents develop along near-vertical joint sets and hydration buckles form at intermittent zones of high permeability within the core regions of breccia pipes. All venting structures form highly porous, low density, secondary sulfate mineralization at the land surface, creating local topographic highs decimeters to a meter in scale that preclude interception of meteoric waters into these unconfined hypogene karst features. The Gypsum Plain hosts complex karst phenomena that present unique engineering challenges as variable geohazard occurrences ranging from shallow, hypergene caves to deep, complex, hypogene features both formed in semi-confined and unconfined speleogenetic conditions.

Stable isotope constraints on the fluid source of hydrothermal breccia pipes in the Tankwa Karoo depocentre, South Africa: Breakdown of authigenic minerals during sill intrusion

AbstractThe Karoo Basin covers much of South Africa and is an area of prospective shale gas exploration, with the Whitehill Formation the target shale unit. However, the sedimentary succession, including the Whitehill, has been intruded by a series of sills and dykes associated with the Karoo Large Igneous Province (~183 Ma), which are expected to have modified the thermal history of the basin dramatically. Here, we investigate a secondary effect of these intrusions: a series of hydrothermal vent complexes, or breccia pipes, focusing on using O, H, and C isotopes to constrain the origin and evolution of fluids produced during the intrusion of basaltic sills. A cluster of breccia pipes have been eroded down to the level of the Ecca Group at Luiperdskop on the western edge of the Karoo basin; a small isolated pipe of similar appearance crops out 13 km to the east. The Luiperdskop pipes are underlain by a Karoo dolerite sill that is assumed to provide the heat driving fluidization. The pipes consist of fine‐grained matrix and about 8% clasts, on average, of mostly sedimentary material; occasional large rafts of quartzite and dolerite are also present. The presence of clasts apparently from the Dwyka Group is consistent with the depth of formation of the pipes being at, or near, the base of the Karoo Supergroup, between 400 and 850 m below present surface. The presence of chlorite as the dominant hydrous mineral is consistent with an emplacement temperature between 300 and 350°C. The major and trace element, and O‐ and H‐isotope composition of the Tankwa breccias is homogenous, consistent with them being derived from the same source. The δ18O values (vsVSMOW) of the breccias are relatively uniform (7.1‰–8.7‰), and are similar to that of the country rock shale, and both are lower than expected for shale. The water content of the breccia is between 2.7 and 3.1 wt.% and the δD values range from −109‰ to −144‰. Calcite in vesicles has δ13C and δ18O (VSMOW) values of −4.2‰ and 24.0‰, respectively. The low δD value of the breccia rocks does not appear to be due to the presence of methane in the fluid. Instead, it is proposed that low δD and δ18O values are the result of the fluid being derived from the breakdown of clay minerals that formed and were deposited at a time of cold climate at ~290 Ma.

Rb–Sr dating of sphalerite and S–Pb isotopic studies of the Xinxing crypto-explosive breccia Pb–Zn–(Ag) deposit in the southeastern segment of the Lesser Xing’an–Zhangguangcai metallogenic belt, NE China

The Xinxing Pb–Zn–(Ag) deposit is located in the southeastern segment of the Lesser Xing’an–Zhangguangcai metallogenic belt, NE China. It is a small-scale deposit (Pb + Zn < 50,000 tonnes) that occurs in a crypto-explosive breccia pipe. Ore bodies are hosted in Permian granodiorite, structurally controlled by three sets of faults. This paper reports results of Rb–Sr isotopic dating of sphalerite, which yields an isochron age of 182.8 ± 3.1 Ma (MSWD = 1.02). The concentrations of Rb and Sr in sphalerite range from 0.0839 × 10−6 to 0.7124 × 10−6 and 0.4775 × 10−6 to 7.4810 × 10−6, respectively. The initial 87Sr/86Sr ratio of sphalerite is 0.710954 ± 0.000061, indicating that the ore-forming components were derived from a mixed crust–mantle source. Sulfur isotopic values of the sulfides vary in a narrow range from −4.4% to −2.0% (av. of −3.28%), indicating a magmatic source for the sulfur. Lead isotopic compositions of the sulfide minerals (206Pb/204Pb = 18.277–18.323, 207Pb/204Pb = 15.514–15.571, 208Pb/204Pb = 38.016–38.199) are relatively consistent and cluster together between the mantle and orogenic belt lines, but are closer to the latter, reflecting the mixing of mantle-derived and minor crust-derived materials. These results, combined with the region geology, indicate that the Xinxing Pb–Zn–(Ag) deposit resulted from regional tectono-magmatic metallogenic processes that were related to the westward subduction of the Pacific plate underneath Eurasia in the Early Jurassic.

Chemistry and Sr-Nd isotope signature of amphiboles of the magnesio-hastingsite–pargasite–kaersutite series in Cenozoic volcanic rocks: Insight into lithospheric mantle beneath the Bohemian Massif

Amphibole phenocrysts, xenocrysts and cumulate xenoliths from Cenozoic volcanic rocks of the Bohemian Massif (BM) belong to the magnesio-hastingsite–pargasite–kaersutite series. Their host rocks are mostly basaltic lavas, stocks, dykes and breccia pipe fills, less commonly also felsic rocks. Felsic rocks with amphibole cumulate xenoliths represent differentiated magmas which have undergone polybaric fractionation of the mafic minerals. The calculated p–T conditions suggest that almost all amphiboles crystallized in a relatively narrow temperature range (1020–1100 °C) at depths of ~20–45 km (0.7–1.2 GPa) during the magma ascent. These p–T estimates are compatible with the published experimental data on the stability of kaersutite. We therefore suggest the presence of a deep magma chamber situated close to the crust–mantle boundary where amphibole xenoliths to megacrysts could have formed. Nevertheless, crystallization of rare amphibole rims during the magma ascent was observed in a hornblendite cumulate in sodalite syenite from “Giegelberg”. The lowest concentration of incompatible elements in the amphiboles was found in xenocrysts in alkaline basaltic rocks and mantle xenoliths and megacrysts, followed by phenocrysts/xenocrysts in lamprophyric rocks, xenocrysts of cumulates in felsic rocks, and phenocrysts in subvolcanic rocks. Amphibole compositional and Sr-Nd isotope characteristics resemble those of amphiboles from metasomatic clinopyroxene/amphibole veins in mantle peridotites. The initial 143Nd/144Nd and 87Sr/86Sr ratios of amphiboles (0.51266–0.51281 and 0.70328–0.70407, respectively) are similar to those of their whole rocks (0.51266–0.51288 and 0.70341–0.70462, respectively). Amphiboles of the magnesio-hastingsite–pargasite–kaersutite series of the BM are mostly chemically homogeneous, with no pronounced Mg-Fe fractionation and zoning. The amphiboles are characterized by relatively homogeneous εNd(i) = +1.4 to +3.8 values: only a single sample from the České Středohoří Volcanic Complex (CSVC) yielded a negative εNd(i) (−0.6). This testifies to locally elevated proportions of recycled Variscan crustal material during melting of mantle peridotites rich in clinopyroxene–amphibole veins. These veins were formed by metasomatic fluids enriched in High Field Strength Elements (HFSE) and are isotopically similar to Enriched Mantle1 (EM1)-type mantle. Amphibole host rocks occur in areas with a significant concentration of basaltic magmas in rift zones along lithospheric block boundaries of the BM. Lithospheric mantle beneath such zones was probably strongly influenced by metasomatic fluids during the formation of clinopyroxene–amphibole veins in mantle peridotite that facilitated the generation of basaltic magma with amphibole.

Geochronology of the Tumpangpitu Porphyry Au-Cu-Mo and High-Sulfidation Epithermal Au-Ag-Cu Deposit: Evidence for Pre- and Postmineralization Diatremes in the Tujuh Bukit District, Southeast Java, Indonesia

The Tumpangpitu porphyry and high-intermediate-sulfidation epithermal deposit is the largest deposit in the Tujuh Bukit district, southeast Java, Indonesia. The porphyry resource contains 1.9 billion tonnes @ 0.45% Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion lbs of Cu. There are an additional 2.1 Moz Au and 72.9 Moz of Ag in oxidized high-sulfidation epithermal mineralization. Tumpangpitu is located along a NW-striking structural corridor covering an area of 12 × 5 km that hosts several Cu-Au-Mo mineralized tonalitic porphyries, each with varying degrees of metal enrichment. At least eight discrete intrusions spanning the alteration-mineralization sequence have been identified at Tumpangpitu. What is unusual, however, is the presence of both a premineralization, relatively dry volcanic breccia pipe (Tanjung Jahe) and a late-mineralization diatreme complex associated with a significant, large magmatic-hydrothermal system (Tumpangpitu) in the same district. Magmatism, mineralization, and alteration at Tumpangpitu occurred in response to north-directed subduction of the Indo-Australian plate beneath the Asian continental plate margin. The Tujuh Bukit district is floored by early to late Miocene sedimentary and andesitic volcanic rocks. Volcanic-hydrothermal activity at Tujuh Bukit began with the formation of the weakly altered Tanjung Jahe diatreme complex (U-Pbzircon ages of 8.78 ± 0.22-8.52 ± 0.21 Ma). Mineralization at Tumpangpitu was preceded by the intrusion of a large, equigranular, dioritic batholith (5.81 ± 0.20-5.18 ± 0.27 Ma). Hydrothermal activity associated with mineralization has been constrained by U-Pb age determinations from syn- to late-mineralization porphyries that were emplaced in the early Pliocene from 5.40 ± 0.46 to 3.94 ± 0.69 Ma. High- and intermediate-sulfidation Au-Ag ± Cu mineralization and associated advanced argillic alteration (part of a district-scale lithocap) has overprinted and significantly upgraded the top of the porphyry orebody. Ar/Ar dating of alunite (4.385 ± 0.049 Ma) and Re-Os dating of molybdenite (4.303 ± 0.018 Ma) have defined a short time period between the high-sulfidation epithermal and porphyry mineralization events. This suggests extreme rates of uplift, exhumation, and erosion in the vicinity of the Sunda-Banda magmatic arc. Volcanic-hydrothermal activity associated with the Tumpangpitu diatreme occurred during epithermal mineralization (breccia matrix zircon age of 2.7 ± 1.0 Ma with systematic errors). Clasts of high-sulfidation state mineralized rocks are a minor but significant component of the diatreme, and late-stage epithermal veins cutting the diatreme demonstrate an intermineralization timing with respect to epithermal activity in the district, implying that epithermal mineralization continued intermittently for 1 to 1.5 m.y. after porphyry mineralization ceased at Tumpangpitu.

A low-δ18O intrusive breccia from Koegel Fontein, South Africa: Remobilisation of basement that was hydrothermally altered during global glaciation?

The Cretaceous Koegel Fontein igneous complex is situated on the west coast of South Africa, and has a high proportion of rocks with abnormally low δ18O values. The rocks with the lowest δ18O values (−5.2‰) belong to intrusive matrix-supported breccia pipes and dykes, containing a variety of clast types. The breccia rocks range in SiO2 from 44 to 68wt% and their whole-rock δ18O values vary between −5.2‰ and +1.8‰. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of Cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts. Based on the nature of the clasts contained in the breccia, it was emplaced just prior to intrusion of the main Rietpoort Granite at 134Ma. All components of the breccia have low δ18O value and, at least in the case of the gneiss clasts, this predates incorporation in the fluidized material.Although the early Cretaceous appears to have been a period of cold climate, it is unlikely that the δ18O values of ambient precipitation (~−10‰) would have been low enough to have generated the required 18O-depletion. The basement gneiss was probably ~2–3km below the Cretaceous surface, minimizing the possibility of interaction with isotopically unmodified meteoric water, and there is no evidence for foundered blocks of cover rocks in the breccia. There is, therefore, no evidence for downwards movement of material. We favour a model where basement gneiss interacted with extremely 18O-depleted fluid during crustal reworking at ~547Ma, a time of global glaciation. Low-δ18O metamorphic fluids produced by dehydration melting of 18O-depleted gneiss became trapped and, as the fluid pressure increased, failure of the seal resulted in explosive upwards movement of fluidized breccia. Migration was along pre-existing dykes, incorporating fragments of these dykes, as well as the country rock gneiss.

Application of photogrammetry for mapping of solution collapse breccia pipes on the Colorado Plateau, USA

AbstractSolution collapse breccia pipes (SCBPs), that form under the influence of gravity, and sinkholes were studied on the Colorado Plateau in Arizona, USA to identify preserved evidence of their formation mechanisms. Close range photogrammetry was employed for data acquisition and reconstruction of the three‐dimensional (3D) scene of the outcrops. Key geological features were accurately mapped within their true 3D space using photogrammetry. Interpretations of the results suggest that the mechanisms controlling the collapse, and its debris distribution within the pipe, are influenced by in situ discontinuities in the host rocks at every scale from the regional context to the size and shape of the collapsed blocks.

Fluid origin and evolution of Cu-Pb-Zn mineralization in rhyolite breccias in the Lón area, southeastern Iceland

Iceland, the landward extension of the Mid-Atlantic Ridge, hosts dilute, predominantly meteoric hydrothermal systems that rarely form base metal (Cu-Pb-Zn) mineralization. One occurrence of Cu-Pb-Zn mineralization in intrusive rhyolitic breccias is in the Lón area of southeastern Iceland. Petrographic, electron-probe, fluid inclusion, stable isotope, and U-Pb zircon dating analyses on samples from Lón constrain the conditions and timing of sulfide mineral formation.Observations of outcrops and hand samples suggest that hydrothermal fluids precipitated chalcopyrite, sphalerite, galena, quartz, epidote, chlorite and calcite in rhyolitic breccia pipes and adjacent basalt flows. The mean salinity of liquid-dominated, multi-phase fluid inclusions in quartz coeval with chalcopyrite inclusions in quartz is 4.2wt% NaCl, and the mean trapping temperature is 332°C, which is consistent with the results of δ34S geothermometry of coexisting galena and chalcopyrite. Calculated δ18O and δD values of fluids in equilibrium with epidote coeval with chalcopyrite range from −5.2 to −2.7±0.1‰ and from −35.9 to −27.7±0.1‰, respectively. The δ18O values of fluids in equilibrium with quartz coeval with chalcopyrite are up to 5‰ larger than those of fluids in equilibrium with late stage quartz precipitated after chalcopyrite. The U-Pb crystallization age of magmatic zircons in the rhyolite breccia is 2.6±0.1Ma, significantly younger than the proximal 3.7 to 7.3Ma silicic intrusions of southeastern Iceland.Our results indicate that early-stage, mineralizing fluids derived from a mixture of meteoric water, seawater, and a minor magmatic water component exsolved from an evolved anatexis-produced melt. Late-stage fluids were derived exclusively from meteoric waters. Although anatectic dehydration melting of altered basalt produced millions of years of felsic magmatism in southeastern Iceland, only hydrothermal fluids that derived from a mixture of meteoric water, seawater, and brine exsolved from a highly evolved melt concentrated base metals in significant quantity to produce base metal sulfide mineralization.

Assessment of magmatic vs. metasomatic processes in rare-metal granites: A case study of the Cínovec/Zinnwald Sn–W–Li deposit, Central Europe

The Cínovec rare-metal granite in the eastern segment of the Krušné Hory/Erzgebirge (Czech Republic/Germany) formed in the final stage of the magmatic evolution of the late Variscan volcano-plutonic system known as the Teplice caldera. The granite is slightly peraluminous; enriched in F, Li, Rb, Cs, Nb, Ta, Sn, W, Sc and U; and poor in P, Mg, Ti, Sr and Ba. The uppermost part of the granite cupola hosts a greisen-type Sn–W–Li deposit. Borehole CS-1 permits to study vertical evolution of the pluton to a depth of 1597m. A combination of textural and chemical methods was applied to whole-rock and mineral samples to identify the extent of magmatic and metasomatic processes during the differentiation of the pluton and formation of the deposit. As indicated by textural and chemical data, the Cínovec pluton consists of two cogenetic intrusive bodies: a relatively homogeneous biotite granite at depths greater than 735m, and a strongly differentiated zinnwaldite granite above this level. The pronounced differentiation of the zinnwaldite granite magma resulted in further increases in F, Li, Rb, Nb and Ta. A high degree of magmatic fractionation is documented by decreases in the K/Rb ratio from 25 to 15 and in the Zr/Hf ratio from 10 to 5. The increasing influence of the fluid is highlighted by a decrease in the Y/Ho ratio from 29 to 17.The following genetic scenario is proposed: the intrusion of the zinnwaldite granite magma reached subvolcanic conditions and a hem of fine-grained porphyritic granite crystallized along the upper contact. Separation of the first portion of fluid from the oversaturated melt promoted explosive degassing and the origin of breccia pipes. Subsequently, the zinnwaldite granite magma crystallized simultaneously from the upper contact and the footwall inwards. The residual melt between the two crystallizing fronts became enriched in water and volatiles to reach second saturation (“second boiling”). Segregated fluids escaped upwards, causing hydrofracturing of the overlying granite, while the water-poor residuum crystallized in situ in the form of mica-free granite. F- and Li-rich fluids invoked greisenization and created quartz–zinnwaldite veins. Alkalis liberated from feldspars destroyed during the greisenization induced local albitization in the uppermost part of the cupola and K-feldspathisation in its deeper part. The distribution of Sn and W was controlled by fluid processes, while Nb and Ta mainly crystallized from the melt. The results from Cínovec are universally applicable to shallow-intruded rare-metal granites regardless of A- or S-types of the primary magma.

Genesis of the Dianfang breccia-hosted gold deposit, western Henan Province, China: Constraints from geology, geochronology and geochemistry

The Dianfang gold deposit, located in Songxian, western Henan province, China, is one of two breccia-hosted gold deposits in the Xiong’ershan–Waifangshan region. Previous studies suggest that the breccia and host volcanic rocks of the Xiong’er Group formed coevally. The deposit consists of seven vein-type orebodies, occurring within the southern and hanging wall margins of the breccia pipe, as well as in fractures in the rhyolites of the Jidanping Formation, and are structurally controlled by ENE-trending faults. Their mineral assemblage includes K-feldspar, quartz, sericite, chlorite, epidote, carbonate, adularia, and sulfides formed from hydrothermal alteration related to mineralization. Pyrite is the dominant ore mineral, and is associated with minor galena, sphalerite, and chalcopyrite. The trace minerals include magnetite, rutile, cervelleite, matildite, and bismuthinite. Gold occurs as native gold, and electrum inclusions in pyrite, or along microfractures in sulfides and quartz. Four stages of hydrothermal mineralization have been identified: quartz–K-feldspar (I), quartz–pyrite (II), polymetallic sulfide (III), and carbonate–adularia (IV). Zircon U–Pb geochronology of the granitic matrix of the breccia constrains the timing of breccia pipe formation to 145.8Ma, indicating it formed at the end of the Late Jurassic, rather than the Paleoproterozoic, as suggested by previous studies. Hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 121.5±1.7Ma, placing gold mineralization at the end of the Early Cretaceous. The sulfur and lead isotope data suggest that the ore-forming materials have a close connection with magmatic fluid. The initial 87Sr/86Sr value of the sulfides is 0.71056±0.00012, suggesting an ancient crustal source, likely the Archean Taihua Group, mixed with a juvenile mantle component. The trace element characteristics of pyrite indicate the mineralizing fluid was magmatic in origin, sourced from a magma derived from the partial melting of the Taihua Complex, and may have been affected by water–rock interaction at high temperatures under reducing conditions. Geological, geochronological, and geochemical data suggest the Dianfang ore deposit is genetically related to a hidden late Mesozoic granite, and forms part of an intrusion-related auriferous system. Gold deposits of similar ages are widespread at the southern margin of the North China Craton, and are coeval with the emplacement of felsic and mafic intrusions, exhumation of a metamorphic core complex, and formation of extensively faulted basins. This suggests that the Dianfang gold deposit formed under conditions of lithospheric extension caused by crustal thinning, and the destruction of the lithosphere mantle beneath the craton during the Late Jurassic to Early Cretaceous.

Palaeokarst Deposits in Caves: Examples from Eastern Australia and Central Europe

Palaeokarst deposits are most commonly found in excavations, drill holes and naturally exposed at the Earth’s surface. Some caves however intersect palaeokarst deposits. This occurs in large hypogene caves in the USA, thermal caves in Hungary and in many caves in eastern Australia. Palaeokarst deposits in caves respond to cave forming processes in the same way as hostrock: the palaeokarst deposits form cave walls. A range of palaeokarst deposits is exposed in caves including; filled tubes, walls composed of flowstone, large-scale bodies, breccia pipes, dykes, volcaniclastic palaeokarst and crystalline palaeokarst. As well as being exposed in cave walls, palaeokarst deposits can wholly or partly form speleogens (speleogens made from palaeokarst). Records of geological events not preserved elsewhere can occur in palaeokarst deposits in caves. These can be difficult to correlate with conventional geological histories. It is important to be able to distinguish between palaeokarst deposits, relict sediments and phantom rock (in-situ weathered rock, also called ghost rock; Vergari & Quinif 1997). Relict sediments can be distinguished from palaeokarst deposits because the cave walls bound relict sediments while palaeokarst deposits form the cave walls. Palaeokarst can be distinguished from phantom rock, as palaeokarst is unconformable with the hostrock, with structures in the hostrock not continuing across the boundary into the palaeokarst. Hostrock structures and textures do continue across the boundary between unaltered hostrock and phantom rock. Similarly, cave sediments are unconformable or disconformable with the hostrock while phantom rock is conformable with hostrock containing hostrock structures and textures. It has been difficult to explain why palaeokarst occurs in some caves and not others. One explanation worth considering is that palaeokarst deposits are not intersected by caves or sections of caves that contain large perennial streams and/or have undergone large-scale vadose fluvial development capable of escaping from the bounds of structural guidance, such as the caves in the Classical Karst. Key words: palaeokarst, caves, Australia, relict sediments, phantom rock.