Development of a Quantitative Assessment Algorithm for Operational Risks in Mining Engineering

Keywords: ore geology, structural geology, prospection, exploration The requirement for Critical Raw Materials (CRMs) of the last years brought the need for systematic and large-scale prospecting in fast and efficient ways. An example is the ISPRA – UniCa project developing the Metallogenic Map of Sardinia, Italy, drawing upon classifying CRMs’ occurrences. In Sardinia, recent studies highlighted the discrete structural controls displayed by two types of CRMs-bearing ore deposits: a) mixed sulphides (Cu, Pb, Zn, Fe) skarn deposits with variable contents of W-Sn-Bi, Ni-Co, and F minerals, set along shear zones associated with the several thrusts marking the emplacement of the Variscan Nappes with a low-angled, top-to-the SW transport direction; b) F-Ba (±Pb±Zn) low-temperature hydrothermal veins hosting REEs and subordinate Ni-Co-Bi sulfides-sulfarsenides filling normal, high-angled, N-S and E-W striking faults with strike-slip components, crosscutting any Variscan structure. We present two case studies in southwestern Sardinia prospected by comparative modelling: the Rosas Shear Zone (RSZ) and the Santa Lucia-Su Zurfuru (SLZ) mining areas. In the RSZ, the skarn’s protolith is the Cambrian limestones embedded within the Ordovician succession by a system of anastomosing NW-SE Variscan thrusts, enhancing the circulation of the mineralizing fluids and the structurally passive skarn’s emplacement. N-S striking F-Ba veins cut the thrusts. In the SLZ, the protolith and host rocks of skarn are analogous to the RSZ: km-sized Cambrian limestone bodies are metasomatised at their peripheries and show a distinct lithological zonation: limestone, marble, and chlorite-skarn. Preliminary field surveys indicate that the Cambrian limestones outcrop at the cores of NW-SE striking anticlines, whose limbs have been reactivated at the stratigraphic contact by later high-angled faults, which hosts the F-Ba (Pb-Zn) hydrothermal veins. Decimetre-thick veinlets striking N-S also crosscut the limestone bodies. The vein system displays a generally good continuity, and is distinctly zoned: a) the northern zone is barite-rich and characterized by breccia textures, with several generations of fluorite cementing brecciated marble/skarn, rare galena and sphalerite; b) the central zone is dominated by banded fluorite and galena; here, the main system is intercepted by a two kilometres E-W quartz vein that ends up in a barite deposit at the contact with the limestones; c) the southern zone is fluorite-rich: it cuts across the skarn ore at depth. The comparison between RSZ and SLZ helps us to investigate the metallogenic processes that led to skarn and F-Ba hydrothermal systems emplacements, providing new guidelines for regional-scale mineral explorations and potential evaluation. The RSZ skarn model gave us a key to understanding the structural setting of SLZ. The SLZ veins model suggests some mineralogical potential left in the area and guides us in prospecting the same type of deposit by verifying and comparing zonation, continuity, and geometries. Globally, skarn and hydrothermal veins are common and great bearers of CRMs, and the structural-mineralogical comparative modelling we leveraged is an extremely efficient, quick, and low-cost way for prospecting and exploring, meeting all the requisites for modern economic needs.

Free Fall Gravity Drainage as an important recovery mechanism was investigated and analyzed experimentally and by numerical (network model) simulation. Sets of glass micromodels with real pattern of porous media and two numerical network model simulators were prepared in forms of fractured and non-fractured models to study and analyze the Free Fall Gravity Drainage recovery mechanism experimentally and by simulation. After validating the network model simulator, analysis of experimental and simulation results leads to some interesting findings and observations as follow: Based on network modeling simulation, direction of flow through fracture network shows that the oil in the middle horizontal fracture usually flows from one end of fracture to the other end, however in relatively small fracture apertures the oil from both vertical fractures at the ends of middle horizontal fracture flows into the horizontal fracture and then sinks downward to the lower matrix block. This means in relatively small fracture apertures, block to block interaction coefficient (α) reaches to 100%. Another point is that, it was observed from network model simulation data that this critical fracture aperture is a function of pore and throat sizes. Also, experimental and simulation results confirm that there is no break through of gas in free fall gravity drainage process. In addition a new mathematical approach (by means of flow potential and control volume concepts) to block to block interaction effects (Capillary Continuity and Reinfiltration phenomena) is presented to explain and interpret the mechanism of occurrence of these phenomena. This approach proves mathematically that decreasing the fracture aperture leads to increasing the fracture capillary pressure which in turn intensifies the feeding rate of liquid bridge (in the middle horizontal fracture) from upper block and lowers the discharging rate of liquid bridge from lower matrix block. Therefore, the liquid bridge can be more stable between the matrix blocks. As a result, fracture capillary pressure has positive effect on oil recovery from matrix blocks. Finally, a new approach for determination of the matrix block threshold heights at the end of free fall gravity drainage is presented and used to prove that matrix block threshold height inreases with increasing the matrix block height. Also, it will be proved that matrix block recovery factor increases with increasing the matrix block height although matrix block threshold height increases too.

Molybdenum (Mo) is a metal with a role in our daily lives and has many essential industrial uses. It is used as a lubricant and a steel alloying agent, which improves its strength and resistance to corrosion and heat. The behavior of molybdenum in low temperature environments has been extensively studied, however little work has focused on its behavior in high temperature environments like magmas and their associated systems. Outstanding questions include: What is the source of molybdenum in ore deposits, and how is it transported in geologic fluids? This research will analyze and compare Mo and sulfur (S) isotope ratios in molybdenite samples from a variety of ore deposit types including porphyry, epithermal, skarn, and metamorphic systems. Since each type of deposit is formed by the combination of specific geologic processes and conditions (e.g., temperature, redox), we can analyze Mo and S isotopes in molybdenite to characterize variations between ore deposit types. Preliminary samples come from seven locales and represent all four targeted deposit types. The pairing of Mo and S stable isotopes will determine if Mo isotopes and/or Mo-S isotope pairs is a useful geochemical tool to illuminate the geological origin (fluid versus melt; conditions) of molybdenite ore. Such a detailed dataset will provide a framework for this novel geochemical approach to understanding Mo-rich ore deposits.

The Tongling ore district of the Middle-Lower Yangtze metallogenic belt is a famous Cu-Au-Fe-S polymetal region, and its tectonic deformation, magmatic evolution, and metallogenic processes have been studied for decades. In this article, we propose a comprehensive tectonic-magmatic-metallogenic model of the ore-forming mechanism constrained by magmatism and regional deformation. In the Tongling district, the tectonic regime underwent two transitions. (1) In the Middle Triassic, the tectonic regime transitioned from quiescence to intense compression. During contraction, the lithosphere thickened and a series of NE-trending folds developed in the cover sequence; because of the multi-layered structure of this caprock, bedding faults, typically cut by steeply dipping faults, developed widely. (2) From 134 to 150 Ma, the tectonic regime changed from compression to extension. During this transition, mantle–crust interaction was prominent; ore-bearing magma was generated by the mixing of crust-derived and mantle-derived melts triggered by delamination of the thickened lithosphere. Meanwhile, detachment faults developed along the interfaces, for example between the lower and upper crust, serving as emplacement sites for several magma chambers. Ore-bearing magma dikes containing large amounts of volatiles derived from a shallow chamber at about –10 km depth migrated into the cover sequence along the pre-existing steeply dipping faults. Melt injection reworked the structural framework, facilitating further development of steeply dipping faults, as well as the vertical transport of ore-bearing fluids. Hydrothermal fluids derived from the emplaced magmas not only formed a range of deposits, including skarns, porphyries, and cryptobreccias around the intrusions but also widely replaced carbonates along bedding-parallel faults and formed so-called stratabound skarn ore bodies, as well as superimposing synsedimentary orebodies developed in the quiescence stage to form several large polymetallic hydrothermal ore deposits. Various types of ore deposits at different depths are clustered in a single orefield, composing a multi-layered mineralization network. In the network, skarn deposits dominate and are characterized by fluid immiscibility processes and diverse element enrichments. The intense mineralization in the Tongling region was caused by the abundance of metals derived from the mantle, favourable ore-controlling structures, and widespread fluid boiling of magmatic hydrothermal fluids, which facilitated metal deposition during the Mesozoic, as well as the superposition of Mesozoic hydrothermal reworking of earlier Palaeozoic sedimentary ore bodies.

The effect has been appreciated of the geotectonic settings of formation of volcano-plutonic belts on the ore-formation zonality of their related complex porphyry copper ore-magmatic systems (equivalent to ore regions and clusters). It is shown that combinations of different types of ore deposits, spatially and genetically associated with porphyry copper deposits within the volume of the unified systems, depend on the earth’s crust structure, on the position and nature of the ore-bearing magmatic chambers, as well as on the metallogeny of the belt basements. The identified specific characteristics of the complex multiformational metallogeny of the ore-magmatic systems should be taken into account in the forecast and prospecting for both porphyry copper deposits and their associated ore deposits of other ore-formation types.

High thallium content in Kuroko-type ore

Metallogenic events and tectonic setting of the Duobaoshan ore field in Heilongjiang Province, NE China

Intrusive hydrothermal breccias have long attracted interest (e.g., Bastin and Hill, 1917; Farmin, 1934; Bryner, 1961; Mayo, 1976) due, in part, to their widespread occurrence in association with many types of ore deposits. In porphyry copper centers such breccias may host a substantial proportion of the mineralization but, even where barren, they provide critical information on the physical evolution of the (magmatic) hydrothermal environment. Most salient aspects of ore-related breccias are addressed by Sawkins and Sillitoe (1985). Burnham (1985) emphasized the enormous release of energy concomitant with the near-surface intrusion of water-bearing magmas and its role in breccia development, and since the proposals of Reynolds (1954), most workers have accepted an involvement of fiuidization processes in the emplacement of the fragmental materials. Among the more intriguing features of hydrothermal breccias is the occurrence of rounded clasts, particularly in the so-called pebble breccias. These have generally been attributed to shearing, rotation, and attrition of originally angular rock fragments during transport in a particle-charged gaseous tream (see Boothroyd, 1971). Thus, in the Patch breccia pipe, Central City, Colorado, Bastin and Hill (1917) record a correlation between fragment rounding and the juxtaposition of diverse rock types. Extensive downward (e.g., at Leadville, Colorado; Emmons et al., 1927) or upward (e.g., at Tintic, Utah; Farmin, 1934) movement of clasts in breccia pipes has been well documented in several mining districts. In an allied context, Williams (1936) established a correlation between the degree of rounding of fragments and the extent of their upward translation in Pliocene diatremes in Nevada.

Magmatic fluids were degassed before and during the eruption of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The dredged samples of fresh lavas, ranging in composition from basalt to rhyolite, define a calc-alkalic trend that is thought to have resulted from fractionation in a common magma chamber. These rocks have variable vesicularity and vesicle size distributions that record the degassing history of the magma. The highly vesicular basalt and basaltic andesite are the least fractionated melt and experienced preeruptive, syneruptive, and posteruptive degassing. The weakly vesicular felsic rocks formed from an evolved magma that was largely degassed before its eruption. Vesicularity tends to decrease with Si, K, Ba, and Zr and to increase with Ca, Mg, Fe, and Sc in bulk samples, suggesting that the degassing of volatiles was linked to crystal fractionation of the magma. A volatile-rich magma is indicated by high concentrations of H 2 O (0.9–2.5%) and Cl (to 0.45%) in mafic melt inclusions in phenocrysts of the basaltic andesite. A fluid phase in the melt inclusions indicates that the magma was saturated with volatiles in the magma chamber. Volatiles exsolve as an immiscible fluid with increasing crystal fractionation, and the composition of the degassed magmatic fluid changes with the evolving magma. The fluid is CO 2 -dominated during the degassing of weakly fractionated mafic magma and becomes a mixture of CO 2 and H 2 O as H 2 O is increasingly exsolved from the highly fractionated felsic magma. The ore metals in the degassed fluid, as inferred from the compositions of the metallic precipitates found in the vesicles of melt inclusions and matrix glass, progressively change from Ni + Cu + Zn + Fe in basalt and basaltic andesite, to Cu + Zn + Fe in andesite, to Cu + Fe in dacite, to Fe in rhyodacite, and to Fe + Zn (+ Pb?) in rhyolite. This trend suggests that magmatic fluids, released from a fractionating magma, could be a source of metal for various types of ore deposits and mineral occurrences. Significant amounts of magmatic fluid can be degassed during the fractionation of a shallow magma chamber. At least 1.7 percent H 2 O is estimated to have exsolved from the magma at PACMANUS. If a fluid is concentrated in a shallow magma chamber and is discharged along a favorable structural zone that extends to the sea floor, it will contribute large quantities of volatiles and ore metals directly to a sea-floor hydrothermal system, a process demonstrated for the geothermal systems of subareal volcanoes. The focused discharge of a magmatic fluid as a result of preeruptive degassing, particularly in a fractionated felsic magma, could be responsible for the Fe, Cu, Zn, and Pb metals in the sulfide chimneys at PACMANUS. By analogy, a magmatic fluid can provide a major source of ore metals for large or super-large volcanogenic massive sulfide deposits in the geologic record.

Microgram quantities of copper, manganese, and zinc, corresponding to concentrations greater than 100 parts per million, were found in milligram quantities of primary inclusion fluid extracted from samples of quartz and fluorite from two types of ore deposits. The results indicate that neutron activation is a useful analytical method for studying the content of heavy metal in fluid inclusions.

Carbonaceous substances (CS) such as kerogen and derivative bitumens are closely linked to the origin of various types of ore deposits. Examples are metal accumulations in living organisms, black shales, crude oils, solid bitumens, and coals. Although presently a marginal economic proposition, the uranium ores of Oklo provide a classic case. At Oklo, solid bitumen occurs closely associated with the several main uranium ore types and also as discrete veinlets in the 2.1 Ga old country rock shales. These bitumen occurrences have been distinguished on the basis of field evidence, petrography, carbon isotopic signatures, and degree of thermal maturation. Chemical analysis by laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) now allows detailed chemical characterization of CS such as these solid bitumens. For the first time, the technique has been used to produce quantitative in situ trace element and isotopic analysis of bitumen. There is a potential here to make major contributions to the understanding of Oklo and other ore deposits associated with carbonaceous substances.

Economic geologists have sought for most of this century to achieve a unified set of models for epithermal Au-Ag deposits to serve as springboards for exploration and new avenues of research. Prior to the 1860s, form and appearance served as the basis for ore deposit classification, with genetic concepts modifying these early theories in the latter part of the century. In 1913, Lindgren classified vein deposits into separate categories on the basis of pressure and temperature criteria gleaned from documented attributes of many different types of ore deposits and study of metamorphic rock assemblages. In 1922, he coined the term “epithermal” for the shallower, volcanic rock-related banded veins. Elie de Beaumont (1847), LeConte (1883), Posepny (1893), Lindgren (1901), and others recognized the possible link of active thermal systems to the formation of vein deposits. It was noticed that H2S- and CO2-bearing thermal springs deposit gold and silver. The respective roles of magmatic and meteoric fluids in vein formation were hotly debated. However, it was not until the 1950s and the advent of new analytical technologies, including mass spectrometry, and the further study of active geothermal systems by such workers as D. E. White that quantitative evidence of the sources of fluids in veins could be obtained. Between Lindgren’s (1913) classification of ore deposits and World War II was, therefore, a time of expanding the empirical data base on actual deposits and increasing the theoretical data base for understanding wall-rock alteration reactions and silicate mineral stabilities. The discovery in the 1970s of economic epithermal deposits with surficial features that unequivocally tie them to geothermal systems has led to the final proof that they are formed in the deposition-discharge regime of convective, meteoric water-dominated geothermal systems. Metalliferous systems are not randomly distributed, but are related to large, crustal-scale fault systems where there is an ample supply of readily available water. Also, we now recognize that there are two major types of epithermal systems—quartz + adularia + sericite and kaolinite + alunite ± pyrophyllite. Each type is due to differing geochemical inputs into the source regime of geothermal systems: reduced sulfur gases in the case of the adularia-sericite systems and oxidized sulfur gases in the kaolinite-alunite ± pyrophyllite systems. Thermochemical data acquired in the past 30 yrs in combination with high-speed computers have led to numerical simulations of mineral-fluid reaction paths and fluid flow paths. This has put us in a position to unravel finally the causes, meaning, and significance of mineralogical zoning in epithermal systems. This knowledge will aid exploration efforts in establishing drilling targets. Some understanding of paleohydrology will be part of the explorationist’s bag of tools in the near future. In a sense, it can be said that our understanding today is largely a honed perspective based on concepts developed in the early part of the twentieth century. It behooves us to use this ample body of knowledge and experience to set our sights on the most important research needs yet to be addressed.

Use and implications of paleoweathering surfaces in mineral exploration in Australia

The chlorine evolution of arc magmas and the crustal water filter

A rapid, partial leach and organic separation for the sensitive determination of Ag, Bi, Cd, Cu, Mo, Pb, Sb, and Zn in surface geologic materials by flame atomic absorption