F. Michael Meyer

The geometry and anatomy of the Los Pijiguaos bauxite deposit, Venezuela

Abstract The Los Pijiguaos bauxite mine in the western part of the State of Bolivar, Venezuela, represents the only currently exploited bauxite deposit in the country. Since mining commenced in 1987, total crude bauxite production has amounted to more than 43 million tonnes (Mt), with a present annual production close to 5 Mt. Proven and probable reserves are around 570 Mt, at an economic cut-off grade of about 44 wt.% alumina (Al 2 O 3 ) and a total silica (SiO 2 ) content of less than 20 wt.%. The orebody has an irregular, dismembered geometry as a result of strong and prolonged fluvial erosion during and after uplift of the region. The isolated minable blocks are located on plateau remnants separated by deep valleys. As a result, only half of the entire lease area is accessible for mining. Los Pijiguaos represents a typical laterite bauxite deposit that formed on a flat topped plateau from weathering of the underlying mid-Proterozoic Rapakivi-type Parguaza granite. The complete laterite profile is characterized by a vertical zonation comprising a concretionary zone (the bauxite horizon), and a mottled zone (the saprolite horizon) on granitic bed rock. The economic bauxite zone attains an average thickness of 7.6 m. On a mine scale, best ore grades and greatest thickness of the ore horizon are at topographic highs. Laterally, grade distribution is also not isotropic but follows distinct WNW–SSE trends. The spatial control on the mineralization is a result of the westerly inclination of the planation surface and the formation of major NW-striking fault systems in the region. Gibbsite is the most abundant mineral in the economic-grade bauxite horizon. The occurrence of minor nordstrandite is also restricted to that zone. Kaolinite is not present in the bauxite zone, but in the lower saprolite zone kaolinite can be more than 5 wt.%. An inverse correlation exists between gibbsite and quartz. Abundance ratios for the two minerals decrease from around 45 in the bauxite horizon to around 2 in the lower saprolite zone. Hematite is commonly more abundant than goethite. In the bauxite horizon, however, the goethite/hematite ratio is greater than 1. Anatase occurs as an accessory phase mainly in the bauxite horizon. Mass balance considerations demonstrate that bauxitization resulted in progressive desilication, hydration, and Al 2 O 3 and Fe 2 O 3 concentration in the residual weathering blanket. Silica as well as alkali and alkaline earth metals were almost completely leached. Mass and volume losses in the order of 61% and 77%, respectively, attest to severe mineralogical, textural, and chemical changes. From an economic point of view, the Los Pijiguaos bauxite represents a high-quality almost pure gibbsitic ore that allows processing to alumina by the low-temperature Bayer process around 140 °C. In terms of material consumption, 2.3 tonnes of dry bauxite are used for every tonne of alumina produced. The accompanying mass of red mud amounts to 550 kg per tonne alumina. The relatively high abundance of hematite in the bauxite has a positive effect on the red mud settling rate and, thus, favors a high extraction yield of the alumina plant. Los Pijiguaos is based on a huge reserve that will provide bauxite for more than 100 years at the current rate of mining. The entire region may contain as much as 6 billion tonnes of probable bauxite resources.

Hydrologic segmentation of high-temperature shear zones: structural, geochemical and isotopic evidence from auriferous mylonites of the Renco mine, Zimbabwe

Abstract Combined structural, mineralogical and geochemical observations in auriferous mylonites of the Renco mine, hosted by late-Archaean, high-grade metamorphic granitoids in southern Zimbabwe, are used to describe the spatially heterogeneous fluid flow and metasomatism that occurred synchronous with deformation at mid- to upper-amphibolite facies metamorphic conditions. Significantly, the narrow (on average 1m-wide) mylonitic shear zones are internally zoned reflecting a pronounced hydrologic segmentation during deformation. Shear zones typically consist of two distinct domains: (i) anastomosing, quartz–feldspar–biotite–hornblende mylonites and/or quartz mylonites, and (ii) tabular-shaped pods, referred to as lithons, that are enveloped by mylonites and that exhibit evidence of transient episodes of brittle fracturing and ductile creep. Whole-rock geochemistry and mass balance calculations indicate dramatic element, volume (up to ≥ 100%) and associated mass gains for the brittle–ductile lithons that are mineralogically reflected in a volumetrically abundant sulphide mineralisation and the formation of a pervasively developed silicate alteration paragenesis. In contrast, mylonites have experienced only minor element and volumetric changes and minor alteration. δ 18 O values for whole rocks and quartz are enriched in lithons compared to wall rocks and enveloping mylonites, which implies the influence of externally derived fluids. These results indicate a strongly domainal fluid flow and mass transfer. Fluid advection was dominated by microscopic and macroscopic fracture permeabilities related to periods of coseismic dilatancy in lithons. The patchy, but partly interconnected distribution of lithons enveloped by mylonites indicates that deformation in the high-temperature shear zones was characterised by transient periods of seismic slip rather than continuous aseismic creep. Based on the well-preserved internal mineralogical and textural development of the Renco shear zones a more general model applicable for fluid advection coupled with the rheological behaviour of mid-crustal shear zones is presented.

Chapter 5.8 Tectono-Metamorphic Controls on Archean Gold Mineralization in the Barberton Greenstone Belt, South Africa: An Example from the New Consort Gold Mine

Publisher Summary This chapter discusses the tectono-metamorphic controls on archean gold mineralization in the Barberton Greenstone belt, South Africa using an example of consort gold mine. The Barberton Greenstone Belt, South Africa, forms part of the oldest nucleus of the Kaapvaal Craton. It contains a well-preserved ca. 3570–3220 Ma volcano-sedimentary sequence, which is surrounded by various generations of tonalite-trondhjemite-granodiorite gneiss domes and sheet-like potassic granites emplaced between ca. 3500–3100 Ma. It is found that although more than 350 gold deposits have been recorded in the Barberton Greenstone Belt, the bulk of the ca. 320 t of gold has been produced from the Sheba–Fairview, New Consort, and Agnes–Princeton mining complexes in the northern part of the belt. The majority of these and associated smaller gold deposits are clustered in a crescent-shaped zone in greenschist facies supracrustal rocks along the eastern margin of the ca. 3230 Ma, tonalitic Kaap Valley Pluton. The New Consort gold mine is situated in the eastern part of the Jamestown schist belt. The systematic variations of ore and alteration assemblages and metamorphic conditions at the New Consort gold mine suggest that the timing of gold mineralization was synchronous with the peak of metamorphism in Fig Tree Group rocks, whereas the main phase of mineralization in rocks of the Onverwacht Group clearly postdates the peak of metamorphism there.

REE Database Management System: Evaluation of REE Deposits and Occurrences

The future availability of Rare Earth Elements (REE) is of concern due to monopolistic market situation, environmentally unsustainable mining, and insecure supply. The inequality between supply and demand as well as the decreased Chinese export quota has boosted exploration activities. There is a lot of information available on REE prospects, dispersed in information instruments such as company reports, scientific literature, and feasibility studies. But there is a lack of a comprehensive system that contains all pertinent data on REE deposits and occurrences. Therefore, a comprehensive REE Database Management System (DBMS) is developed that holds all relevant information concerning REE deposits/occurrences. The system comprises the economic dataset of REE (potential)-producing companies. Moreover, tools are offered for defining parameters that allow parallel comparison of up to five REE prospects. The DBMS consists of evaluation systems that rate deposits as well as occurrences by an attributive defined Rating System. Besides, it contains a Ranking System that ranks deposits according to specific attributes such as Material Grade, deleterious elements, tonnage, and prices. Further, it comprises a Cluster-building tool that allows creating individual REE Cluster groups and enables the implementation of decision rules like a Use Value Analysis. The DBMS is a powerful tool for industrial companies, particularly concerning the unsteady supply and demand situation of REE. Therefore, uncertainties of REE supply could be reduced. Thus, the DBMS might help occurrences to develop better forecasts and minimize risks for better planning the strategy of future purchase of REE.

Geological setting of the Guelb Moghrein Fe oxide-Cu-Au-Co mineralization, Akjoujt area, Mauritania

Abstract The Guelb Moghrein Fe oxide–Cu–Au–Co deposit is located at the western boundary of the West African craton in NW Mauritania. The wall rocks to the mineralization represent a meta-volcanosedimentary succession typical of Archaean greenstone belts. Two types of meta-volcanic rocks are distinguished: (1) volcanoclastic rocks of rhyodacite–dacite composition (Sainte Barbe volcanic unit), which form the stratigraphic base; (2) tholeiitic andesites–basalts (Akjoujt meta-basalt unit). The trace element signature of both types is characteristic of a volcanic arc setting. A small meta-pelitic division belongs to the Sainte Barbe volcanic unit. A meta-carbonate body, which contains the mineralization, forms a tectonic lens in the Akjoujt meta-basalt unit. It can be defined by the high XMg (=36) of Fe–Mg carbonate, the REE pattern and the δ13C values of −18 to −17‰ as a marine precipitate similar to Archaean banded iron formation (BIF). Additionally, small slices of Fe–Mg clinoamphibole–chlorite schist in the meta-carbonate show characteristics of marine shale. This assemblage, therefore, does not represent an alteration product, but represents an iron formation unit deposited on a continental shelf, which probably belongs to the Lembeitih Formation. The hydrothermal mineralization at 2492 Ma was contemporaneous with regional D2 thrusting of the Sainte Barbe volcanic unit and imbrications of the meta-carbonate in the upper greenschist facies. This resulted in the formation of an ore breccia in the meta-carbonate, which is enriched in Fe, Ni, Co, Cu, Bi, Mo, As and Au. Massive sulphide ore breccia contains up to 20 wt% Cu. The ore fluid was aqueous–carbonic in nature and either changed its composition from a Mg-rich oxidizing to an Fe-rich reducing fluid or the two fluid types mixed at the trap site. All lithologies at Guelb Moghrein were deformed by D3 thrusting to the east in the lower greenschist facies. The mobility of REE in the retrogressed rocks explains the formation of a second generation of hydrothermal monazite, which was dated at c. 1742 Ma. Archaean rocks of the West African craton extend to the west to Guelb Moghrein. The active continental margin was deformed and mineralized in the Late Archaean–Early Proterozoic and again reactivated in the Mid-Proterozoic and Westphalian, showing that the western boundary of the craton was reactivated several times.