Determination of the optimal transition point between a truck and shovel system and a semi-mobile in-pit crushing and conveying system

One of the most challenging aspects in semi-mobile in-pit crushing and conveying (SMIPCC) system design is determining the optimum depth at which to change from a purely truck-based haulage system to a conveyor-based haulage system. We used scenario analysis to determine the optimum transition depth between a truck and shovel (TS) system and a SMIPCC system. Traditional pit-limit algorithms were used to generate the final pit limit on a copper deposit, which was then divided into four pushbacks. The final operating pushbacks (phases) were designed for both TS and SMIPCC. The end depths for each phase are viewed as candidate transition points to switch from the TS to SMIPCC haulage system. Economic calculations were applied for five different scenarios, including adopting SMIPCC from the outset (pure SMIPCC), after the first, second, and third phases, and finally not using the SMIPCC system (pure TS) at all. The analysis indicates that the second scenario, at a depth of 335 m, results in the lowest cumulative discounted cost (CDC). In this case, the CDC is 17.6% lower than that for the pure TS scenario and 10.7% lower than for the pure SMIPCC system scenario.

Effects of the Invasion of Caulerpa cylindracea in a Cymodocea nodosa Meadow in the Northern Adriatic Sea

Graphical AbstractEffects of the presence of the invasive macroalgae C. cylindracea in the seagrass meadow evidenced by substantial loss in below-ground biomass of C. nodosa and lowering of the redox transition depth in the sediment underlying the mixed settlement.

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

<p>We determine the mantle attenuation (1/Q) structure beneath 70 Myr seafloor in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: in the shallow layer, in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric measured at higher frequency (~3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (V<sub>S</sub>) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (<em>f</em><sub>e</sub>) and determine, at each depth, which values of <em>f</em><sub>e</sub> predict and V<sub>S</sub> that can fit the NoMelt and V<sub>S </sub>values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in <em>f</em><sub>e</sub> by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in <em>f</em><sub>e</sub> reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.   </p>

Coda wave simulations across the Tyrrhenian Basin using radiative transfer

<p>Lateral variations in crustal structure may affect the propagation of Lg phases, guided waves that propagate efficiently only in the continental crust. Seismic paths crossing continental-oceanic transitions are characterized by Lg blockage due to the drastic decrease in crustal thickness. Here, we investigate the effects of crustal thinning on wave propagation in the Tyrrhenian basin using radiative transfer theory. We first model regional coda envelopes (600-800km) using the software tool Radiative3D (Sanborn & Cormier 2018, <em>GJI</em>). It allows to synthesize seismograms envelopes produced by earthquakes by propagating energy packets through a deterministic structure, taking into account the crustal layers, including Moho transition depth, and parameters describing the medium heterogeneities.  Then, we approach the complex problem of meshing, including measured Moho depths, for simulations based on spectral elements (Komatitsch D. et al., 2012, SPECFEM3D,<em> Computational Infrastructure for Geodynamics</em>) and finite differences methods (Maeda et al., 2017, OpenSWPC). The results aim at understanding complex wave attenuation and leakage in the mantle, for future implementations into the Multi-Resolution Attenuation Tomography code (MuRAT – De Siena et al. 2014,<em> JVGR</em>)</p>

From Drip To Plate: Did Subduction Start in Archaean?

<p>The evolution from stagnant/episodic lid to modern-day plate tectonics on earth is not well understood. Geochemical and geomorphological findings indicate that Archaean Eon is the most likely candidate for the onset of plate tectonics. In order to have plate tectonics, the oceanic lithosphere has to be denser than the asthenosphere and subducting slabs must be rheologically strong so that it would stay intact/undeformed during subduction. Our study focuses on investigating the initiation of subduction on the margins of an Archaean craton/continent based on the subcretion tectonic model of Bédard (2018). Here, we use 2-D mantle convection models (StagYY) to understand the controlling parameters for possible subduction or lithospheric downwellings. A 230 km thick craton accompanied by a 60 km thick oceanic lithosphere on both sides is introduced into the model setup. The model domain is divided by 64 vertical cells and 512 lateral cells corresponding to 660 km depth and 2000 km length. Both for the upper and lower boundary, free-slip surface conditions are used. Left and right boundaries are periodic. Velocities are forced to be zero until a critical depth of 60 km, after that a sub-lithospheric mantle flow of 4 cm/yr imposed into the model which is a proxy for a disturbance generated within the mantle by the “overturn upwelling zones”. Our results indicate that cratonic keels can be mobilized by the sub-lithospheric mantle winds and what happens afterward is highly dependent on the surface yield stress, eclogite phase transition depth, deformation mechanism and, most importantly, reference mantle viscosity. Lower viscosity (10<sup>19</sup> Pa s) models resulted in a stagnant-lid regime while the others with the increased viscosity (10<sup>20</sup> Pa s – 10<sup>21</sup> Pa s) yielded in a transition from stagnant to plate-like behaviors.</p>

The coupling transition depth in subduction zones: rheologically controlled and not constant

<p>The subduction zone coupling transition depth, CTD, marks the transition from frictional/ductile decoupling between the two plates to viscous coupling between the subducting plate and convecting mantle. This depth plays an important role in the state of stress, earthquake potential, and the location of the volcanic arc. Based on previous studies of heat flow and seismic structure of circum-Pacific subduction zones, the CTD has been inferred to at a constant 70-80 km. The mechanism for this constancy remains elusive, although models have reproduced the sharpness of the CTD as a consequence of the evolving strength contrast between a frictional (damage) type rheology along the interface and temperature and stress dependent viscosity in the plates and mantle . Using kinematically driven subduction models with such rheology, we find a relationship between the CTD, slab age and velocity that predicts that 91 % of Pacific subduction zones should have an CTD between 65 and 80 km depth, consistent with observations. However, some other zones are predicted to have significantly deeper or shallower CTD.  For example, a 120 km CTD recently found in the Lesser Antilles can be explained by our models . Sub-arc slab depth is bound by a similar age-velocity relation to that derived for the CTD, but offset to ~50 km larger depths. Hence rheology exerts the primary control on the CTD, and the coupling transition depth is in fact not constant but varies with plate age and convergence.</p>

Optimal Extraction Methods Selection for Kakosa South Copper Ore Deposit Applying Modified Technique for Order of Preference by Similarity to Idea Solution Model

Kakosa South copper deposit is located about 450km northwest of Lusaka between Chingola and Chililabombwe. A comprehensive study of Kakosa South deposit was carried out. In Kakosa area the footwall aquifer rocks comprising sandstone and conglomerates which are thin and as such are not expected to represent major aquifers. Copper mineralisation is found in the upper quartzite and ore-shale. The inclination of the deposit ranges from 250 up to 350 . The hangingwall formations above the upper quartzite are represented by a sequence of dolomite and shale formations. Based on Kakosa geotechnical analysis and rock mass classification, fuzzy TOPSIS approach was employed for the selection of optimal extraction techniques. FTOPSIS approach has precise and specific quantities which are used in order to establish criteria and option weights. Triangular fuzzy numbers were determined to represent semantic variables. The fuzzy numbers for Kakosa South parameters were used as input data in the decision making model and matched against the criteria required for the mining method. Applying FDM model, extraction techniques were ranked. The results indicated that open pit extraction technique was ranked first with 78.90 scores followed by sublevel stoping with 66.88 scores. It is concluded that the Kakosa South copper ore deposit can optimally be extracted by open pit mining up to transition depth and transit from open pit mining to underground mining employing sublevel stoping.