Subvertical Pipes Research Articles

Basin and depositional control on the initiation and development of fluid-escape pipes in the Canterbury Basin, New Zealand

The influence of basin and depositional context on the formation and growth of fluid escape pipes in sedimentary basins is not fully understood. While seismic reflection data is commonly used to study these structures, direct observation and sampling are scarce. In this study, we investigate the evolution of hundreds of meter-tall fluid escape pipes (100–800 m) in the Canterbury Basin, New Zealand, using open source seismic and borehole data. Our analysis includes seismic interpretation of 19 unconformities within the Neogene-Quaternary successions, characterization of pipes and fluid-related anomalies in the subsurface, modeling of the shale volume (Vsh), lithology characterization, 3D static modeling, and seismic inversion. We identify thirty-one vertical to sub-vertical pipes with three parts consisting of top, main conduit/stem, and root zone on seismic profiles. Our results suggest that lithology plays a crucial role in the initiation and modulation of pipe structures in the Canterbury Basin, with pipes mostly formed post Tarantian (0.113 Ma) and truncated at the U19 unconformity. Furthermore, vertical fluid migration from overpressured units, such as the tops of contourite mounds, was observed in the pipes. Our findings provide new insights into the subsurface fluid flow and sedimentary architecture of the Canterbury Basin and have wider implications for other passive continental margins with similar characteristics.

Application of Geophysics for Mineral Exploration in the Mount Lyell Region of Tasmania

The Mount Lyell region contains more than 20 significant mineral deposits with 150 MT @ 1.23% Cu 5 g/t Ag and 0.35 g/t Au having been extracted. Many of the ore bodies occur along or near the Great Lyell fault, which is west-dipping and shows a complex history of movement. Significant reverse movement has occurred on the fault juxtaposing older volcanics against younger sediments to the east. Copper deposits are mainly disseminated pyrite-chalcopyrite and occur as sub-vertical pipes. Although historic ground EM surveys conducted in the 80’s were not successful more recent CSAMT surveys did indicate that some of the known orebodies do exhibit a good conductivity contrast to the generally restive host. Transient EM DHEM utilising a large transmitting loop at surface did lead to the discovery of a new deposit. Here we provide a case study of recent exploration work in the Mount Lyell region. We describe the geologic history around Mount Lyell and exploration activities. Recently an airborne electromagnetic survey was performed, using CGG’s Helitem35C system with the MultiPulse waveform (both halfsine and square wave pulses are generated in a single waveform). We describe the Helitem35C results, along with 2D/3D inversion modelling, and how they fit with the known geology. A number of undeveloped mines were detected by the system and the data will be used to target possible extensions of the known ore bodies.

Alteration and mineral zonation at the Mt Lyell copper–gold deposit, Tasmania

ABSTRACTThe Mount Lyell copper deposits are located in the middle Cambrian Mount Read volcanic belt of western Tasmania and consist of more than 24 separate copper–gold–silver orebodies. The dominant copper mineralisation style is disseminated pyrite–chalcopyrite subvertical pipes with subordinate chalcopyrite–bornite ± other copper phases, massive pyrite and base metal sulfides. A zonation in mineralisation style within the pipes is defined from chalcopyrite–magnetite at depth to chalcopyrite–pyrite at intermediate levels, to chalcopyrite–bornite at the shallowest level. Alteration is developed broadly symmetrically around the ore zones and zoned from quartz–chlorite–phengite ± biotite at depth to quartz–muscovite at intermediate levels, and a quartz–muscovite–pyrophyllite–zunyite assemblage at the shallowest levels. This is interpreted to be a result of a fluid that evolved from hot, reduced and neutral conditions at depth to cool, oxidised and acidic conditions at the shallowest level. The chalcopyrite–bornite deposits occur at the top of the hydrothermal system and are associated with intensely silicified rock and muscovite/pyrophyllite alteration. The close relationship of these deposits with the top of the pipes suggests they are part of a single mineralising event. Where the chalcopyrite–bornite deposits are juxtaposed with the Owen Group, rather than a simple chalcopyrite–bornite mineralogy, there are numerous other copper phases, which represent higher oxidation states and collectively suggest variable and fluctuating fluid conditions during deposition. It is proposed that these deposits are formed by an interaction of the reduced hydrothermal fluid with an oxidised fluid generated at very shallow levels within and during deposition of the Owen Group. Mineralisation within the middle Owen Group sandstones and clasts of altered rock within the middle and upper Owen Group sediments marks the end of the hydrothermal system. Around the entire edge of the Mt Lyell field, there is a variation in the white mica composition from proximal muscovite to distal phengite that represents the neutralisation of the hydrothermal fluid by fluid–wall rock interaction.

Discovery of Lower Cretaceous hydrothermal vent complexes in a late rifting setting, southern North Sea: insights from 3D imaging

We present data for 31 kilometre-scale hydrothermal vent complexes and associated sand remobilization that have formed as a consequence of magmatic intrusions within salt strata (southern North Sea, Dutch Sector). This study uses 3D seismic data tied to three wells and artificial neural networks to describe conical and cylindrical vent structures located in the NE part of the Broad Fourteens Basin. The structures are composites of 58 subvertical pipes that form complexes, and are 951 ± 364 m in height and 487 ± 426 m in diameter. Interactions of magma and fluids at depth triggered successive fluid pressure build-ups in Triassic sandstones, resulting in upward-propagated ruptures, followed by the collapse of overlying deposits and the injection and extrusion of fluidized sand. The relationship between vent tops and their overlying deposits indicates a Mid-Hauterivian age, unusual for volcanic activity in the North Sea area. This event could have a significant impact on hydrocarbon exploration, as the hydrothermal vent complexes bypass a regional source-rock and seal, offering new migration routes and connectivity between potential reservoirs. The intrusions also raise questions about the influence of transform tectonics on the focusing of volcanic activity during the late rifting phase of the North Sea. Supplementary material: Tables of seismic attributes and the geometry of the vent complexes as well as a 3D visualisation of the vents are available at https://doi.org/10.6084/m9.figshare.c.3571593

The influence of microbial mats on the formation of sand volcanoes and mounds in the Red Sea coastal plain, south Jeddah, Saudi Arabia

Extensive areas covered by microbial mats have been found in the upper intertidal flats and supratidal pools in the Red Sea coastal plain of south Jeddah, Saudi Arabia. Numerous microbially controlled sediment-surface morphologies are evident, such as flat cohesive mats that commonly pass into mats with wrinkles, reticulates, and tufts, together with erosion pockets and mat chips. These microbial mats form cohesive surface layers that lead to biostabilization of the sediment surface. Fluidization of the underlying sediments is due to tidal influences and pressurized gas escape from decay and photosynthesis of microbial mats and causes deformation and rupture of the cohesive surface mat layer via vertical and sub-vertical pipes. Extrusion of fluidized sediments and water through these pipes leads to redeposition of sediment grains above the surface mat layer to form sand volcanoes and mounds. Most of the sand volcanoes present in the intertidal flats and supratidal pools show a symmetrical morphology, whereas in tidal channels asymmetrical forms are more common. Extrusion of underlying sediments through several adjacent vents leads to coalescence of sand volcanoes to form sand mounds. In this study sand volcanoes are also compared with other cone-like features from the Red Sea, such as gas domes and crab mounds. This comparison should help in differentiating similar cone-like features associated with microbial mats in the rock record.

Geology, Geochemistry, and Mineralogy of the Worthington Offset Dike: A Genetic Model for Offset Dike Mineralization in the Sudbury Igneous Complex

The Worthington offset dike extends for approximately 15 km away from the southwestern margin of the 1.85 Ga Igneous Complex. The dike is zoned with respect to inclusion and sulfide contents. Marginal chilled quartz diorite is transitional into medium-grained quartz diorite. These rocks are sulfide undersaturated, contain small inclusions from the wall rocks, and are preserved along much of the dike. Locally, the dike contains a core of inclusion-rich quartz diorite, which can be choked with inclusions surrounded by semimassive to massive sulfide. The more heavily mineralized inclusion-rich quartz diorite contains 10 to 75 percent amphibolite inclusions, which are petrologically and geochemically similar to the immediately adjacent country-rock amphibolites, locally termed Sudbury gabbros. The semimassive to massive sulfide zones form subvertical pipes, much like the deposits of the Copper Cliff offset dike, and these are associated with locations where the Worthington dike widens from 20 to 30 m to 50 to 80 m. The average metal tenors of the sulfide with > = 5% sulfur are calculated to be 7 percent Ni and 13 percent Cu. Thus the dike ores have a much higher Cu/Ni ratio than orebodies within the contact sublayer (Cu/Ni ~ 1). The medium-grained inclusion-poor quartz diorite and inclusion-rich quartz diorite differ in Ni, Cu, Pt, and Pd abundances, but they have similar major and lithophile trace element abundance levels despite having different inclusion and sulfide contents. Assimilation of inclusions has therefore not significantly changed the composition of the silicate matrix of the inclusion-rich quartz diorite. The composition of the inclusion-poor quartz diorite is a close approximation to average crust and also to the bulk composition of the Igneous Complex rocks. Regional differences in offset geochemistry exist between the North and South Range offset dikes, but we believe that the formation of the inclusion-poor quartz diorite and inclusion-rich quartz diorite silicate magmas predated significant silicate differentiation or silicate gravitational segregation of the melt sheet and that the differences may record primary silicate heterogeneity of the melt sheet. These differences may have developed in response to the different proportions of Archean granitoids relative to Proterozoic sediments and volcanics that contributed to the melt sheet on the North and South Ranges of the Igneous Complex. The first quartz diorite melt was sulfide undersaturated and devoid of amphibolite inclusions. The introduction of this melt took place during or shortly after the generation of the melt sheet and represented a very rapid introduction of quartz diorite magma into radial and concentric dikes with local incorporation of metasedimentary inclusions. The second phase of activity involved the introduction of a sulfide-bearing melt into portions of the quartz diorite melt that were not completely crystallized at the center of the offset dike; this produced the inclusion-rich quartz diorite. The main control on the location of injection of the sulfide-bearing melt appears to be widened domains of the partially crystallized quartz diorite dike that often correspond to contacts between different country-rock types and sulfides, especially where there are gabbro country rocks and/or local development of breccia. The injection of sulfide-enriched melt is interpreted to have taken place along steeply oriented pipes through these heavily brecciated country rocks; this appears to explain why there is a contact relationship between inclusion-rich quartz diorite and inclusion-poor quartz diorite, and also explains the presence of fragments of inclusion-poor quartz diorite and country-rock amphibolites in the inclusion-rich quartz diorite. The introduction of sulfides into these pipes is interpreted to have taken place from the overlying melt sheet and it is suggested that the high Cu/Ni ratios and platinum-group element tenors of these sulfides, compared to contact-style sulfides, indicate that these were some of the earliest sulfides to segregate from the overlying melt sheet. The type and style of brecciation developed in the country rocks along different offset dikes controlled the continuity of the quartz diorite and the location of the orebodies. Intersections of the quartz diorite with domains of breccia appear to have been important in the Copper Cliff offset dike and the development of mineralization may be a response to the presence of favorable channels through which magmatic sulfides were introduced into the dike from the overlying melt sheet. We propose that the quantity of sulfide in a dike is related to the thickness of the overlying melt sheet. The melt sheet thickness was presumably greatest at the Copper Cliff and Frood-Stobie offset dikes, the Worthington offset dike is located below a thinner melt sheet, and the Manchester and Foy offset dikes developed beneath very thin portions of the melt sheet.

Impact‐induced hydrothermal activity within the Haughton impact structure, arctic Canada: Generation of a transient, warm, wet oasis

Abstract— Field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot, impact‐generated rocks following formation of the 24 km diameter, 23 Ma Haughton impact structure. Hydrothermal alteration is recognized in two settings: within polymict impact breccias overlying the central portion of the structure, and within localized pipes in impact‐generated concentric fault systems. The intra‐breccia alteration comprises three varieties of cavity and fracture filling: (a) sulfide with carbonate, (b) sulfate, and (c) carbonate. These are accompanied by subordinate celestite, barite, fluorite, quartz and marcasite. Selenite is also developed, particularly in the lower levels of the impact breccia sheet. The fault‐related hydrothermal alteration occurs in 1–7 m diameter subvertical pipes that are exposed for lengths of up 20 m. The pipes are defined by a monomict quartz‐carbonate breccia showing pronounced Fe‐hydroxide alteration. Associated sulfides include marcasite, pyrite and chalcopyrite. We propose three distinct stages in the evolution of the hydrothermal system: (1) Early Stage (>200 °C), with the precipitation of quartz (vapor phase dominated); (2) Main Stage (200‐100 °C), with the development of a two‐phase (vapor plus liquid) zone, leading to calcite, celestite, barite, marcasite and fluorite precipitation; and (3) Late Stage (<100 °C), with selenite and fibroferrite development through liquid phase‐dominated precipitation. We estimate that it took several tens of thousands of years to cool below 50 °C following impact. During this time, Haughton supported a 14 km diameter crater lake and subsurface water system, providing a warmer, wetter niche relative to the surrounding terrain. The results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems.