Emplacing a cooling-limited rhyolite lava flow: similarities with basaltic lava flows

Accurate forecasts of lava flow length rely on estimates of eruption andmagma properties and, potentially more challengingly, on an understanding of the relative influence of characteristics such as the apparent viscosity, the yield strength of the flow core, or the strength of the lava’s surface crust. For basaltic lavas, the relatively high frequency of eruptions has resulted in numerous opportunities to test emplacement models on such low silica lava flows. However, the flow of high silica lava is much less well understood due to the paucity of contemporary events and, if observations of flow length change are used to constrain straightforward models of lava advance, remaining uncertainties can limit the insight gained. Here, for the first time, we incorporatemorphological observations from during and after flow field evolution to improve model constraints and reduce uncertainties. After demonstrating the approach on a basaltic lava flow (Mt. Etna 2001), we apply it to the 2011–2012 Cordón Caulle rhyolite lava flow, where unprecedented observations and syn-emplacement satellite imagery of an advancing silica-rich lava flow have indicated an important influence from the lava flow’s crust on flow emplacement. Our results show that an initial phase of viscosity-controlled advance at Cordón Caulle was followed by later crustal control, accompanied by formation of flow surface folds and large-scale crustal fractures. Where the lava was unconstrained by topography, the cooled crust ultimately halted advance of the main flow and led to the formation of breakouts from the flow front and margins, influencing the footprint of the lava, its advance rate, and the duration of flow advance. Highly similar behavior occurred in the 2001 Etna basaltic lava flow. In our comparison of these two cases, we find close similarities between the processes controlling the advance of a crystal-poor rhyolite and a basaltic lava flow, suggesting common controlling mechanisms that transcend the profound rheological and compositional differences of the lavas

Commonalities in the emplacement of cooling-limited lavas:insights from the 2011-2012 Cordón Caulle rhyolitic eruption

Lava flows pose a risk to infrastructure and communities near many volcanoes. Although the emplacement processes of low-viscosity basaltic lava flows are relatively well studied, the infrequency of silicic eruptions has limited our understanding of the emplacement of high-viscosity rhyolitic lavas. The 2011-2012 eruption of Puyehue-Cordón Caulle in southern Chile provided a unique opportunity to make scientific observations of an active rhyolitic lava flow. The thesis utilises a multiscale approach to draw comparisons between the emplacement of mafic and silicic lavas by building on the established understanding of basaltic flows. The thesis demonstrates the similarities and differences in the emplacement of cooling-limited lavas at contrasting ends of the compositional spectrum. Observations and quantitative models of lava flow lengthening demonstrate that advance of the Cordón Caulle lava flow was controlled in its latter phases by a cooled surface crust, which suggests similarities to the rheological control of cooling-limited basaltic lava flows. Cessation in flow advance was followed by breakout formation, the first such observations in a rhyolitic lava flow. Breakout formation was triggered by a pressure build-up at the flow margins, due to continued supply of lava along preferential thermal pathways, as well as late stage vesiculation of the flow core. The breakouts developed a morphological range classified into domed, petaloid, rubbly and cleft-split, which reflect the results of advance and inflation processes. Many silicic lavas show pumice diapirs and crease structures at their surface, but such features were rarely observed at Cordón Caulle. The inferred strong surface crust of the Cordón Caulle lava flow, as well as higher viscosity than other rhyolite lava flows, may have locally impeded upwelling to the lava surface, so favouring breakout formation. Insights gained from Cordón Caulle will aid the interpretation of ancient silicic lava flows, and help anticipate the hazards posed by future eruptions of rhyolitic lava

Massive sulfide Zn deposits in the Proterozoic did not require euxinia

Our most important Zn resources occur within clastic-dominated (CD-type) deposits, which are located in a small number of Proterozoic and Phanerozoic sedimentary basins. The most common model for CD-type mineralisation involves sedimentary exhalative (SEDEX) processes, i.e. the venting of metal bearing fluids into a restricted, anoxic H2S-bearing (euxinic) water column. In the Carpentaria Zn Province (Australia), multiple world class deposits are hosted in Proterozoic (1.6 Ga) stratigraphy, where models of the ancient sulfur cycle have also been developed. Focusing on the most recent discovery – the Teena deposit – we report bulk rock and isotopic data (δ34Spyrite values) that provide information on the sulfur cycle during the diagenetic and hydrothermal evolution of the Teena sub-basin. In contrast to the SEDEX model, intervals containing abundant pyrite with highly positive δ34S values (>25 ‰) correspond with euxinic conditions that developed due to high organic loading (i.e. productivity) and not basin restriction. This basin wide feature, which can also be mistaken as a hydrothermal pyrite halo, is genetically unrelated to the subsequent hydrothermal mineralisation that formed beneath the palaeo-seafloor. The formation of CD-type deposits in the Proterozoic does not, therefore, require euxinic conditions

Pyrite chemistry records a multistage ore forming system at the Proterozoic George Fisher massive sulfide Zn-Pb-Ag deposit, Mount Isa, Australia

Trace element (TE) analysis of pyrite via LA-ICP-MS can be used to reconstruct the conditions of pyrite formation in complex mineral systems. The Carpentaria province in northern Australia is host to some of the world’s highest value Zn-Pb (+Ag, Cu) deposits. The genesis of many of these deposits is controversial, with competing models of single-vs. multi-stage ore formation. In this study, LA-ICP-MS data of paragenetically constrained pyrite from the George Fisher Zn-Pb-Ag deposit has been analysed to investigate the chemistry of different stages of ore formation. Pyrite from correlative unmineralized host rocks has also been analysed to investigate evidence of distal hydrothermal anomalism. All LA-ICP-MS data have been statistically evaluated (principal component analysis) and interpreted together with whole rock lithogeochemical data of the same samples. Pre-ore diagenetic pyrite is compositionally similar to other Proterozoic diagenetic pyrite, with some evidence of minor hydrothermal anomalism that with further analysis could help define distal alteration. Pyrite from the different ore stages are compositionally distinct, consistent with a multi-stage system. Ore stage 1 pyrite exceeds background contents of Co, Cu, Zn, As, Ag, Sb, Tl, and Pb and has elevated Co/Ni ratios, whereas only Ni and Co are above background abundances in ore stage 2 and 3 pyrite, of which only ore stage 3 pyrite has high Co/Ni ratios. Ore stage 1 pyrite has a similar composition to hydrothermal pyrite in the undeformed northern Carpentaria CD-type deposits and was likely syn-diagenesis. Ore stage 2 was syn-deformation, and resulted in replacement and recrystallization of pre-existing pyrite, and the expulsion of incompatible TEs. Ore stage 3 formed via a later Cu-Zn-Pb mineralizing event that resulted in a new geochemically distinct generation of Co-rich pyrite. Overall, this study demonstrates the value of paragenetically-constrained pyrite TE data for refining genetic models in complex sediment hosted mineral systems

Pyrite chemistry records a multistage ore forming system at the Proterozoic George Fisher massive sulfide Zn-Pb-Ag deposit, Mount Isa, Australia

Trace element (TE) analysis of pyrite via LA-ICP-MS can be used to reconstruct the conditions of pyrite formation in complex mineral systems. The Carpentaria province in northern Australia is host to some of the world’s highest value Zn-Pb (+Ag, Cu) deposits. The genesis of many of these deposits is controversial, with competing models of single-vs. multi-stage ore formation. In this study, LA-ICP-MS data of paragenetically constrained pyrite from the George Fisher Zn-Pb-Ag deposit has been analysed to investigate the chemistry of different stages of ore formation. Pyrite from correlative unmineralized host rocks has also been analysed to investigate evidence of distal hydrothermal anomalism. All LA-ICP-MS data have been statistically evaluated (principal component analysis) and interpreted together with whole rock lithogeochemical data of the same samples. Pre-ore diagenetic pyrite is compositionally similar to other Proterozoic diagenetic pyrite, with some evidence of minor hydrothermal anomalism that with further analysis could help define distal alteration. Pyrite from the different ore stages are compositionally distinct, consistent with a multi-stage system. Ore stage 1 pyrite exceeds background contents of Co, Cu, Zn, As, Ag, Sb, Tl, and Pb and has elevated Co/Ni ratios, whereas only Ni and Co are above background abundances in ore stage 2 and 3 pyrite, of which only ore stage 3 pyrite has high Co/Ni ratios. Ore stage 1 pyrite has a similar composition to hydrothermal pyrite in the undeformed northern Carpentaria CD-type deposits and was likely syn-diagenesis. Ore stage 2 was syn-deformation, and resulted in replacement and recrystallization of pre-existing pyrite, and the expulsion of incompatible TEs. Ore stage 3 formed via a later Cu-Zn-Pb mineralizing event that resulted in a new geochemically distinct generation of Co-rich pyrite. Overall, this study demonstrates the value of paragenetically-constrained pyrite TE data for refining genetic models in complex sediment hosted mineral systems

Using whole rock and in situ pyrite chemistry to evaluate authigenic and hydrothermal controls on trace element variability in a Zn mineralized Proterozoic subbasin

The mid-Proterozoic stratigraphy of the McArthur Basin (Australia) contains some of the most well-preserved sedimentary rocks of Precambrian age, which are also host to giant, clastic dominant (CD-type) massive sulfide Zn deposits. The most recently discovered CD-type deposit (the Teena deposit) is located in the Teena subbasin and hosted by the 1.64 Ga Barney Creek Formation. The Teena subbasin, therefore, provides the perfect natural laboratory for evaluating authigenic and hydrothermal controls on trace element (TE) variability, both of which contribute to paleoenvironmental reconstructions and ore deposit models. As the Teena deposit formed beneath the paleoseafloor, this also provides the opportunity to evaluate TE zonation around a fossilized subseafloor replacement hydrothermal system. In situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) has been used to define compositional end members in diagenetic and hydrothermal pyrite. The overgrowth of hydrothermal sulfides on diagenetic pyrite is associated with TE anomalism (Tl, Pb, As, Zn) that extends > 100 meters above the main high grade sulfide mineralization the Teena subbasin. The vertical zonation in TEs is consistent with the infiltration of hydrothermal fluids into overlying hangingwall sediments that were undergoing diagenesis. Bulk rock lithogeochemical data record covariation between total organic carbon (TOC) and a suite of TEs (Mo, Co, Ni, V). We suggest this was caused by local hydrographic factors during deposition of the Barney Creek Formation. High TOC/P molar ratios, resulting from regeneration of P in a euxinic water column, are associated with an interval overlying the main maximum flooding surface in the subbasin. The relationships between TOC, P and TEs resemble the redox architecture of a silled basin rather than an open marine margin. Sulfidic conditions developed during periods of high productivity, which were linked to nutrient supply that was enhanced by connectivity with surrounding water masses. The evidence of redox bistability, involving a delicate balance between ferruginous (anoxic, non-sulfidic) and euxinic (sulfidic) conditions, is consistent with recent models for other mid-Proterozoic sedimentary units. Nevertheless, there was a strong localised (101 km2) control on the authigenic and hydrothermal TE chemistry of the Barney Creek Formation in the Teena subbasin, which highlights a key challenge when extrapolating from data collected in partially restricted intracontinental marine settings

Geodynamic controls on clastic-dominated base metal deposits

To meet the growing global demand for metal resources, new ore deposit discoveries are required. However, finding new high-grade deposits, particularly those not exposed at the Earth's surface, is very challenging. Therefore, understanding the geodynamic controls on the mineralizing processes can help identify new areas for exploration. Here we focus on clastic-dominated Zn–Pb deposits, the largest global resource of zinc and lead, which formed in sedimentary basins of extensional systems. Using numerical modelling of lithospheric extension coupled with surface erosion and sedimentation, we determine the geodynamic conditions required to generate the rare spatiotemporal window where potential metal source rocks, transport pathways, and host sequences are present. We show that the largest potential metal endowment can be expected in narrow asymmetric rifts, where the mineralization window spans about 1–3 Myr in the upper ∼ 4 km of the sedimentary infill close to shore. The narrow asymmetric rift type is characterized by rift migration, a process that successively generates hyper-extended crust through sequential faulting, resulting in one wide and one narrow conjugate margin. Rift migration also leads to (1) a sufficient life span of the migration-side border fault to accommodate a thick submarine package of sediments, including coarse (permeable) continental sediments that can act as source rock; (2) rising asthenosphere beneath the thinned lithosphere and crust, resulting in elevated temperatures in these overlying sediments that are favourable for leaching metals from the source rock; (3) the deposition of organic-rich sediments that form the host rock at shallower burial depths and lower temperatures; and (4) the generation of smaller faults that cut the major basin created by the border fault and provide additional pathways for focused fluid flow from source to host rock. Wide rifts with rift migration can have similarly favourable configurations, but these occur less frequently and less potential source rock is produced, thereby limiting potential metal endowment. In simulations of narrow symmetric rifts, the conditions to form ore deposits are rarely fulfilled. Based on these insights, exploration programmes should prioritize the narrow margins formed in asymmetric rift systems, in particular regions within several tens of kilometres from the paleo-shoreline, where we predict the highest-value deposits to have formed

Inhomogeneous Cosmology using General Relativistic Smoothed Particle Hydrodynamics coupled to Numerical Relativity

We perform three-dimensional simulations of homogeneous and inhomogeneous cosmologies via the coupling of a numerical relativity code for spacetime evolution and smoothed particle hydrodynamics (SPH) code. Evolution of a flat dust and radiation dominated Friedmann-Lema\^itre-Roberston-Walker (FLRW) spacetime shows an agreement of exact solutions with residuals on the order $10^{-6}$ and $10^{-3}$ respectively, even at low grid resolutions. We demonstrate evolution of linear perturbations of density, velocity and metric quantities to the FLRW with residuals of only $10^{-2}$ compared to exact solutions. Finally, we demonstrate the evolution of non-linear perturbations of the metric past shell-crossing, such that dark matter halo formation is possible. We show that numerical relativistic smoothed particle hydrodynamics is a viable method for understanding non-linear effects in cosmology.Comment: 15 pages, 12 figures, submitted to PR

Stratiform Host-Rock Replacement via Self-Sustaining Reactions in a Clastic-Dominated (CD-type) Zn Deposit

Stratiform to stratabound replacement of a mixed siliciclastic-carbonate host rock is a defining characteristic of many sediment-hosted base metal deposits. Mineralized rocks in clastic-dominated (CD-type) Zn-Pb ore deposits, which represent our highest value base metal resources, are generally thin (101 m), laterally extensive (103 m), and stratiform to stratabound in fine-grained siltstone and mudstone facies. At the recently discovered Teena CD-type Zn-Pb deposit (Proterozoic Carpentaria province, Australia), the host rock was undergoing burial diagenesis when altered and mineralized by hydrothermal fluids that moved up to 2 km lateral to the fluid input conduit (growth fault) through intraformational intervals. In much of the deposit, carbonate dissolution was an important reaction permeability control, although significant amounts of mineralization also occur in carbonate-free siliciclastic beds. In this study, transmission electron microscopy (TEM) data has been generated on a drill core sample that preserves a sharp reaction front between mineralized and unmineralized domains of the fine-grained siliciclastic compositional end member (carbonate free). Petrographic and mineralogical data provide evidence that oxidized hydrothermal fluids moved through the protolith via reaction permeability that developed from feldspar dissolution. The nature of reactive fluid flow was determined by reactions that took place at the fluid-mineral interface. Pyrite formation during the earliest stage of the hydrothermal paragenesis increased the mineral reactive surface area in the protolith. Acidity was then generated in situ via self-sustaining reactions involving pyrite oxidation, transient Fe sulfate formation, and sphalerite precipitation, which provided positive feedbacks to enhance porosity creation and further fluid infiltration and mineralization. In the absence of carbonate, however, ore fluid pH was buffered by K-feldspar dissolution (~4.5), thereby ensuring sphalerite precipitation was not inhibited under more acidic conditions. All CD-type deposits in the Carpentaria province are hosted by a protolith comprising carbonate, K-feldspar, pyrite, and organic matter; these phases set the boundary conditions for the development of self-sustaining reactions during ore formation. Importantly, these self-sustaining reactions represent a Goldilocks zone for ore formation that is applicable to other sediment-hosted deposits that formed via replacement of mixed siliciclastic-carbonate host rocks (e.g., stratiform Cu)

SULFIDE TEXTURES AND ORE GRADES IN MINERALIZED CARBONATES DEPEND ON PÉCLET AND DAMKÖHLER NUMBERS

Clastic-dominated (CD-type) deposits contain a significant proportion of the global resources of Zn, Pb, and Ag, and include some of the largest deposits that formed through subseafloor replacement. Mineralization textures in these deposits can be highly variable, and the physical properties that control these textures are poorly defined. The style of dissolution in carbonate units can be described by dimensionless parameters (Péclet and Damköhler numbers) that represent fundamental properties of reacting flow systems. Using reaction transport modeling of a CD-type deposit, this work investigates the relationships between Péclet and Damköhler numbers with textures and ore grades. In 1-D and 2-D simulations, a metalliferous brine was reacted with a host rock at variable rates of fluid flow and dolomite dissolution, resulting in different mineralization textures depending on the spatial relationship of the inflowing brine to the reaction front and the flow and dolomite dissolution rate. Ahead of the front, disseminated textures developed at low Damköhler numbers. At, or behind, the front where the Damköhler number was higher, massive or interfingered textures formed, depending on the Péclet number. The shift between massive (higher-grade) to interfingering to disseminated (lower-grade) mineralization led to a correlation between Damköhler and Péclet numbers with ore grade. The models presented here demonstrate the association between mineral kinetics and flow rate with mineralization textures. Therefore, understanding the implications of Damköhler and Péclet numbers can help in interpreting textures on a hand-sample to outcrop scale and patterns of grade and ore geometry