ALBERT

Description: Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.

Description: Magmatic sulphides are a widespread component in mafic and ultramafic rocks and contain variable concentrations of nickel, copper and platinum-group elements. Previous literature has been concerned with the whole-rock geochemistry of magmatic sulphide ores and their host-rocks and relatively little attention has been paid to the physical nature of magmatic sulphide transport and accumulation. Our high-resolution X-ray computed tomography study quantifies for the first time the 2D and 3D size, shape and textural relationships, and distribution of disseminated magmatic sulphides and olivine in adcumulates from komatiites. These new data are combined with analysis of trace-element concentrations within sulphides to provide important information about the mechanisms of transport, deposition and post-accumulation migration of sulphide liquid in dynamic magmatic systems. Olivine shows evidence of textural maturation, with larger crystals growing at the expense of small ones to different degrees depending on the sulphide content of the rock. The olivine texture and the presence of poikilitic chromite provide evidence of in situ nucleation of olivine and chromite at the interface between a flowing magma and a basal pile of crystals. Disseminated to strongly interconnected base-metal sulphides are located at contacts between olivine crystals or in some cases can be entirely or partially enclosed within chromite. Based on their 3D morphologies, their size distribution and their Pd concentrations, the sulphides are divided into four main categories: finely disseminated sulphides; disseminated to slightly interconnected sulphides; disseminated to globular sulphides; disseminated to strongly interconnected sulphides. All samples contain a population of sub-spherical sulphide blebs (〈1000 µm equivalent sphere diameter; ESD), which are observed in the olivine–sulphide cotectic proportion and which contain the lowest Pd concentrations. These small droplets are interpreted to have formed by segregation of immiscible sulphide liquid upon cooling of a komatiitic magma flowing in a magma conduit or channel. These newly formed droplets were trapped in situ by the crystallizing framework of olivine and/or chromite. Larger sulphide blebs (up to 10 mm ESD) are present where the sulphide abundance is 〉3 wt % and the sulphide bleb size population is multi-modal. The Pd content of the sulphide blebs is variable and positively correlated with the sulphide bleb size. The overall sulphide abundance, sulphide bleb size and Pd concentrations indicate that these sulphides have been transported in a flowing sulphur-saturated magma over some distance and accumulated at their present site by mechanical processes. Strongly interconnected network to matrix sulphides are observed in samples containing more than 5 wt % sulphide with small variability in Pd concentrations within and between blebs. These sulphides are interpreted to reflect the accumulation and coalescence (by film drainage) of small sulphide blebs. Overall our results show that komatiite-hosted disseminated sulphides form by a mechanical accumulation process that takes place against a background of steady-state in situ nucleation of small blebs along the olivine–sulphide liquid cotectic.

Description: Equilibration between sulfide liquid and olivine is expressed in terms of the exchange coefficient for Fe and Ni, K D = (X NiS /X FeS ) sulfide_liquid /(X NiO /X FeO ) olivine . The positive dependence of K D on Ni + Cu content of sulfide liquid, as well as on f O 2 , has been demonstrated experimentally and gives rise to a critical nonlinearity in the relationship between sulfide and olivine-saturated silicate liquid compositions. Measured K D values for olivine-sulfide pairs from disseminated magmatic sulfide ores at Betheno (Western Australia) and Mirabela (Brazil) are consistent with independent estimates of f O 2 , and fall within the range where the composition dependence of K D is strong. This effect has been modeled quantitatively, using an empirical best fit to available experimental data as a parameterization of the K D variability, and calculating the equilibrium distribution of Fe, Ni, and Mg between coexisting olivine, silicate melt, and sulfide liquid as a function of the silicate/sulfide mass ratio R. It is shown that the composition dependence of K D is a key factor in giving rise to extremely Ni rich sulfides as exemplified by Betheno and Mirabela, where coexisting olivine is also Ni rich. Highly Ni enriched sulfides with anomalously high Ni/Cu ratios may be more common in nature than is commonly recognized and do not need to be explained by hydrothermal alteration processes.

Description: High-resolution X-ray computed tomography (HRXCT) is a nondestructive technique that allows exploration of the three-dimensional distribution of minerals and pore space in natural and synthetic rock samples. Recent technological developments allow the characterization of samples (up to few centimeters in diameter) at a resolution down to 0.7 μ m. This, combined with the development of dedicated workflows, algorithms, and softwares, is opening new doors for the quantitative three-dimensional characterization of ores at the sample scale. When combined with three-dimensional image analysis, HRXCT provides quantitative three-dimensional mineralogical and textural measurements (volume, shape, three-dimensional relationship) that are impossible to assess accurately by any other method. This manuscript describes the principles of HRXCT and presents dedicated methodologies from data acquisition throught to quantitative three-dimensional measurements. These methodologies are applied to Ni-Cu-PGE ore samples but are suitable for a range of geologic materials. The combination of HRXCT with quantitative three-dimensional image analysis applied to geologic materials hosting ore deposits provides a better understanding of ore mineralogy, ore-forming processes, and parameters required for mineral or metallurgical processing, with benefit for both academic studies and mineral industry applications.

Description: The Monts de Cristal Complex of Gabon consists of several igneous bodies interpreted to be remnants of a tectonically dismembered, 〉100 km long and 1–3 km wide, ultramafic–mafic intrusion emplaced at 2765–2775 Ma. It is the most significant mafic–ultramafic layered complex yet identified on the Congo Craton. The complex consists largely of orthopyroxenite cumulates, with less abundant olivine-orthopyroxenite and norite, and rare harzburgite and dunite. Mineral compositions (Fo ol 84, Mg# Opx 85, An plag 60–68, Cr/Fe chromite 1–1·45) and whole-rock data suggest that the parent magma was a low-Ti basalt containing approximately 10% MgO and 0·5% TiO 2 . Trace element and Rb–Sr and Sm–Nd isotope data indicate the presence of an enriched component, possibly derived from crustal contamination of a magma generated in the sub-lithospheric mantle. Most rocks show a highly unusual pattern of strong Pt enrichment (10–150 ppb) at low concentrations of Pd (1–15 ppb), Au (1–2 ppb), Cu (1–20 ppm), and S (〈500 ppm), suggesting that unlike in most other PGE-rich intrusions globally, platinum in the Monts de Cristal Complex is not hosted in magmatic sulfides. Synchrotron X-ray fluorescence mapping has revealed the location of buried small Pt particles, most of which are associated with As. We propose that this constitutes some of the strongest evidence yet in support of magmatic crystallization of a Pt–As phase from S-undersaturated magma.

Description: A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide ores between ores with up to about five modal percent sulfides, called "disseminated ores," and "net-textured" (or "matrix") ores containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated ores in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of "patchy net-textured ores" containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.

Description: The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1 Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2 Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2 Ob , parallel with the plane of a pervasive NE-striking subvertical foliation (S2 Ob ). Finally, a N-S shortening event (D3 Ob ) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3 Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2 Ob , but folded by S3 Ob , indicates that the sulfides were formed during D2 Ob . Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2 Ob ), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3 Ob , where the S3 Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D 1 felsic dikes, (2) fault intersections, and (3) F3 Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.