The Sierra Mojada district consists of multiple types of mineral concentrations ranging from polymetallic sulfide deposits, "non-sulfide Zn" (NSZ) deposits, and a Pb carbonate deposit hosted by Upper Jurassic to Lower Cretaceous carbonates. This study focuses on the two non-sulfide Zn deposits, the Smithsonite Manto and the Iron Oxide Manto, that occur south of the San Marcos fault. The Smithsonite Manto shows karst features, including internal sediments interbanded with smithsonite (ZnCO₃). The Iron Oxide Manto consists of strata-bound zones dominantly of hemimorphite (Zn₄Si₂O₇ (OH)₂·H₂O) that fills pores in Fe-oxides. The mineralogy of the NSZ mineralization consists of smithsonite, hemimorphite and Zn clays (sauconite) associated mainly with calcite and Mn-Fe-oxides. Zn clays are abundant in the Smithsonite Manto, but no Zn clays have been found in the Iron Oxide Manto. This project attempts to constrain the origin of the NSZ concentrations through petrographic and mineralogical study of major Zn-bearing minerals, and their carbon and oxygen stable isotopes and Pb isotope geochemistry. Smithsonite in the Smithsonite Manto occurs as botryoidal aggregates consisting of scalenohedral or rhombohedral microcrystals and banded colloform or massive smithsonite in open spaces, whereas smithsonite in the Iron Oxide Manto occurs as rhombic microcrystals grown in pore spaces or finely intergrown with Fe-oxides. Both Fe-poor and Fe-rich smithsonite are found in the Iron Oxide Manto. Under optical-CL, smithsonite displays complex growth zoning that can be related to variable trace element content. Trace elements semiquantitatively analyzed using LA-ICP-MS show that most blue luminescent smithsonite has lower Mn contents than pink to bright red luminescent zones in smithsonite. Preliminary fluid inclusion petrography in hemimorphite and calcite suggests that fluid composition can be related to precipitation of NSZ minerals from freshwater to slightly saline waters. Calculated salinities for two phase (liquid +vapor) and single phase (liquid) inclusions in hemimorphite range between 0.0 and 1.6 wt. % NaCl equivalent, and salinities of inclusions in calcite were between 0.0 and 1.1 wt. % NaCl equivalent. The oxygen isotope values for smithsonite are relatively constant (avg. [delta]¹⁸O[subscriptVSMOW] = 21.9 ± 0.5[per mille]), whereas [delta]¹³C[subscriptVPDB] values range from -8.4 to -1.1 [per mille]. The oxygen isotope values in late calcite are within the same range of smithsonite, whereas the average values of the carbon isotope are lower by 5 [per mille]. Formational temperature of smithsonite is calculated to be between 26 ~ 40 °C using the modern groundwater composition at Cuatro Ciénegas. Similar Pb isotopic compositions of smithsonite and cerussite to galena suggest the source of metals in the NSZ deposits presumably originate from the sulfide deposits. / text

O Grupo Vazante hospeda o maior depósito de Zn não-sulfetado, hipógeno, conhecido mundialmente, Vazante (>60 Mton @ 20% Zn), que ainda possui concentrações de Pb e Ag. A Mina de Vazante (VZT) e sua continuidade, a Mina de Extremo Norte (EN), são subdivididas em cinco domínios geo-mineralógicos, sendo três deles pertencentes à VZT: Lumiadeira 1 (LUM1); Lumiadeira 2 (LUM2); Sucuri (SUC) e dois à EN: Extremo Norte 1 (EN1) e Extremo Norte (EN2). Estudos de campo, amostragem sistemática, análises petrográficas e com uso de microscopia eletrônica de varredura, incluindo mineralogia automatizada por Mineral Liberation Analyzer, e microssonda eletrônica, e litoquímicas foram empregadas objetivando caracterizar detalhadamente cada domínio, relacionar aspectos geológicos e mineralógicos e apontar possíveis fatores responsáveis por mudanças na qualidade do minério frente aos processos de beneficiamento. Os corpos de minério de Zn não-sulfetado nas minas VZT e EN são compostos essencialmente por willemita (Zn2SiO4), possuem formas anastomosadas e são hospedados por brechas dolomíticas e subordidamente por rochas metapelíticas e metabásicas e brechas hematíticas. Corpos restritos ricos em sulfetos ocorrem, principalmente, imbricados em corpos de minério willemítico ou como veios e vênulas. Os cristais de willemita apresentam texturas de preenchimento, como coloforme e fibro-radiada (tipo I), de reequilíbrio, como granoblástica (tipo II), ou são finamente granulares e preenchem fissuras tardias (tipo III). Desse modo, registram complexa sequência de processos e condições para a geração do minério de Zn não-sulfetado. Zinco, Fe, C, Ca, SiO2, e Mg são os principais componentes químicos do minério. Na mina EN, o teor de Fe (<26,7%) no minério é, em média, mais elevado quando comparado à mina VZT, presente principalmente na forma de hematita. Chumbo e Ag, co-produtos da explotação de Zn, são principalmente encontrados na mina VZT, nos domínios LUM1 e SUC onde são concentrados em corpos ricos em sulfetos. Elementos terrígenos, tais como Al, K, Rb, Sc, Co, Th e La são positivamente anômalos no domínio LUM2 indicando maior presença de metapelitos no setor sul da mina VZT. Quando associado à metapelito, o minério willemítico pode conter até 15% de Zn-talco, podendo assim refletir em menores recuperações de Zn. Quando hospedado por rochas metabásicas, o principal mineral de zinco pode ser franklinita (<25%), mineral deletério para o processamento de minério. Caso hospedado por brechas hematíticas, característica do domínio EN2, a willemita apresenta-se comumente cominuída e os conteúdos de Fe e hematita podem ser limitantes para o beneficiamento. Em síntese, os resultados desse estudo mostram que cada domínio geo-mineralógico apresenta características específicas. Mostrou-se que espessura, formato e profundidade dos corpos, variação da natureza das rochas encaixantes e substituições minerais afetam diretamente a mineralogia e a qualidade do minério de Zn. Tais fatores refletem processos geológicos e características do ambiente formacional do depósito que podem ser restritos ou mais frequentes em determinado domínio, refletindo assinaturas específicas para o minério estudado, que podem ser espacializadas e monitoradas integrando um modelo geometalúrgico. / The Vazante Group hosts the major hypogene, nonsulfide Zn deposit known worldwide (e.g., Vazante; >60 Mton @ 20% Zn), which also contains Pb and Ag concentrations. The Vazante Mine (VZT) and its continuity, the North Extension Mine (EN), are subdivided into five geo-mineralogical domains, since three belonging to VZT: Lumiadeira 1 (LUM1); Lumiadeira 2 (LUM2); Sucuri (SUC), and two to EN: North Extension 1 (EN1); North Extension 2 (EN2). Field studies, systematic sampling, petrography, scanning electronic microscopy, automated mineralogy via Mineral Liberation Analyzer, and lithochemical analyses were carried out aiming to characterize each domain in detail, connect their geological and mineralogical aspects, and point to possible features responsible for changes on ore quality during ore beneficiation. The nonsulfide Zn orebodies at VZT and EN are anastomosed, composed of willemite (Zn2SiO4), and hosted by dolomite breccias and subordinately by metapelite, metabasic rocks, and hematite breccias. Minor sulfide-rich orebodies occur mainly imbricated within willemite orebodies or as veins and veinlets. The willemite crystals show infilling textures, such as colloform and fibrous-radiated (type I), reequilibrium texture, such as granoblastic (type II), or fine-grained textures, when controlled by late fissures (type III). Thus, willemite registers a complex sequence of processes and conditions responsible for the nonsulfide Zn ore generation. Zinc, Fe, C, Ca, SiO2, and Mg are the main chemical compounds of the ore. At EN, the Fe (<26.7%) and hematite contents in the ore are, on average, higher than those of VZT. Lead and Ag, co-products of Zn exploitation, are chiefly found at VZT, in the LUM1 and SUC domains, concentrated in sulfide-rich bodies. Terrigenous elements, such as Al, K, Rb, Sc, Co, Th, and La, are positively anomalous in the LUM2 domain indicating the influence of metapelite host in the south sector of VZT. In this case, willemite ore may contain up to 15% Zn-talc, related to lower Zn recoveries. If the willemite ore is hosted by metabasic rocks, franklinite, a deleterious phase into processing, might be the main Zn-bearing phase (<25%). If hosted by hematite breccias, characteristic of the EN2 domain, willemite is commonly comminuted and the Fe and hematite contents might be a limiting for beneficiation. The results herein presented show that each geo-mineralogical domain exhibits their own characteristics. Ore thickness, format, and depth of bodies, the nature of host rocks, and mineral replacements affect the mineralogy and quality of Zn ore directly. Those factors express geological processes and characteristics of the deposit environment that may be restricted or more frequent at a certain domain, thus creating specific signatures for the analyzed ore, which might be spatialized and monitored integrating a geometallurgical model.

The geology of the Sierra Mojada silver-lead-zinc mining district gives new insights into the stratigraphic evolution of the Coahuila Block and the Coahuila Folded Belt and the history of deformation along the basement-rooted San Marcos Fault Zone. Sierra Mojada provides the opportunity for substantial data collection relevant to the interaction of regional tectono-stratigraphic elements in a generally data-poor region of northeastern Mexico. Active mineral exploration has produced an extensive database of closely spaced drill core. Expansive underground workings facilitate subsurface geologic mapping. Sierra Mojada is situated at the northwestern edge of two tectono-stratigraphic provinces, the Coahuila Block, to the south, and the Coahuila Folded Belt, to the north. The San Marcos Fault, a west-northwest-trending regional structure extends through Sierra Mojada and is the informal boundary between these two provinces. Sierra Mojada is situated on uplifted and deformed late Paleozoic Ouachita siliciclastic strata intruded by Triassic diorites. This basement is diagnostic of the Coahuila Block. Basement rocks are overlain by an immature conglomerate that is interpreted to be the updip equivalent of the Jurassic La Casita Formation. The stratigraphy of Sierra Mojada principally consists of a continuous succession of Barremian through Albian carbonates unconformably overlying the basal conglomerate. The Barremian-Aptian Cupido Formation locally records deepening conditions from a clastic-influenced evaporitic interior to high energy, open water conditions. The shale and lime mudstone of the La Pena Formation were deposited during a Gulf-wide transgression that signals the end of the Aptian. The Sierra Mojada region of the Coahuila Block was inundated throughout the Aptian and was affected by the late Aptian transgression. The Albian Aurora Formation constitutes the bulk of the Cretaceous section. Sierra Mojada exposes the Aurora shelf rim, progressing from platform margin to shelf rim and platform interior facies. The structural features of Sierra Mojada affect the entire Cretaceous section. The high angle San Marcos Fault was reactivated with reverse motion during the Paleogene as a result of Laramide shortening. This juxtaposed basement and Jurassic conglomerate against the Cretaceous carbonates consistent with offset observed along the southern trace of the San Marcos Fault. A local colluvial unit suggests a lag in Laramide deformation. The carbonate strata and colluvial unit were overridden by a low angle, northeast-dipping thrust fault that placed a Neocomian through Aptian sequence atop the autochthonous Aptian-Albian carbonates. The allochthonous San Marcos Formation suggests regional-scale tectonic transport of this immature fluvial conglomerate from a downdip depozone within the Sabinas Basin. Kinematic indicators are consistent with the southwest-northeast axis for maximum compression established for Paleogene shortening throughout the Coahuila Folded Belt. The thrust fault bisects the principal ore zone within the Lower Aurora and upper La Pena Formations. This relation constrains the minimum age of ore emplacement to the Paleogene and suggests mineralization was genetically tied to the late stages of the Laramide Orogeny. / text

Exploration companies commonly rely on geochemistry to identify alteration of distinctive geochemical and mineralogical character, surrounding metal sulphide deposits that were precipitated from hydrothermal fluids. However, examination of raw analytical data is prone to error due to closure effects and the difficulty in removing the effects of background variation in unaltered rocks from the variations imposed by later hydrothermal alteration. Closure can be avoided by using ratios, or by utilising mass balance approaches based on fixing volume, mass or concentration changes between samples of parent and daughter lithologies. Using a parent-daughter approach is limiting, because only pairs of samples can be compared at any one time and because an unaltered equivalent must be produced for each sample examined in this way. Pearce Element Ratio analysis and General Element Ratio analysis (PER and GER) are not restricted in this fashion, and are more amenable to interrogation of large data sets. PER and GER are also capable of decoupling background variation from that variation due to hydrothermal alteration. Furthermore, these ratio methods are readily applied to commercially derived lithogeochemical assays. In this study, various analytical methods and interpretive techniques (including PER and GER) have been applied to identify alteration in rocks around the Century and Elura Zn-Pb-Ag deposits, and to assess whether primary ore-related alteration effects can still be identified once altered rocks have been subjected to weathering. Ratios of trace elements over a conserved element have been used to generate a suite of pathfinder elements for each deposit. Elements enriched in host rocks around both deposits include the economic metals Zn, Pb and Ag, along with Rb and Tl. Sodium is ubiquitously depleted in altered rocks. Other elements in the pathfinder suites are distinctive to each deposit type, and include a number of major and trace elements that are added or removed from the rocks around the mineralised zones. For example, Sb and As are enriched in rocks around Elura mineralisation while Ge and Cd are enriched in samples around Century deposit. Iron carbonate development accompanied by potassic alteration, the destruction of albite and the absence of chlorite are the dominant mineral alteration effects at both deposits. PER and GER diagrams have been used to quantify the intensity of this alteration and allow lithogeochemistry to be used to vector towards high intensity alteration, which is adjacent to Century and Elura mineralisation. These ratio methods are applied to both visibly and cryptically altered rocks at both deposits, and have a very high degree of success in classifying alteration in unweathered rocks. The following simple PER ratios indicate proximity to Elura mineralisation: Ca/C, K/Al for shales, K/(Al-Na) for siltstones/sandstones. The following simple PER ratios indicate proximity to Century mineralisation: Mn/Ti, Mg/Ti and Fe/Ti vs C/Ti, K/Ti vs Al/Ti, K/Ti vs (Al-Na)/Ti. Pathfinder elements can be overlain onto PER and GER diagrams to aid in ranking the prospectivity of samples, and to assess mineral hosts for individual pathfinder elements. Weathering destroys most indicators of alteration in the Elura area, while alteration signatures are better preserved in host rocks around the Century deposit.

Abstract At least 30 gold deposits verified by means of one or more notable diamond drill hole results have been discovered in Central Lapland in the last 20 years, and these can be divided spatially into groups, between which the metal composition varies. The deposits contain varying amounts of sulphides and sulpharsenides as well as gold. Pyrite is the most common sulphide mineral in the gold deposits associated with volcanic rocks, and usually pyrrhotite in those associated with sedimentary rocks. The principal sulphide minerals in those connected with banded iron formations are pyrite and arsenopyrite. A separate group of formations consists of the palaeoplacer gold deposits associated with the molasse-like quartzites and conglomerates of Central Lapland. The iron oxide-copper-gold deposits of Central Lapland, which are a significant potential source of copper and gold, are mostly associated with skarn rocks at the eastern contact of the acidic intrusive rocks of Western Lapland and with skarn rocks occurring as interlayers in metavolcanic and metasedimentary rocks. The gold deposits that have led to actual mining activities in Central Lapland are Saattopora in Kittilä and Pahtavaara in Sodankylä. Apart from the Laurinoja iron oxide-copper-gold ore body in Kolari, copper concentrate has been produced from the Saattopora gold ore deposit and the Pahtavuoma copper ore deposit. Only one gold ore in Central Lapland is being actively exploited at present, that of the Pahtavaara mine, which was worked in 1995–2000 and reopened in 2003. The best starting point for successful gold ore exploration in Central Lapland can be achieved through a thorough knowledge of the deformation zones and their structures and alteration processes and the application of geochemical methods. Magnetic surveys can be of help in identifying and locating deformation zones of interest for exploration purposes and the majority of the associated shear zones and faults. Ore-critical zones usually feature graphite-bearing schists and iron sulphide-bearing sequences that can be traced by electrical methods and used as marker zones to verify the results of geological mapping. Geological, geophysical and geochemical techniques have been used in great diversity, and in particular till geochemistry and bedrock drilling have been methods by which the gold and copper deposits in Central Lapland have been discovered. A total of 7.6 million tonnes of gold and copper ores, including the Laurinoja iron oxide-copper-gold ore, were extracted in Central Lapland over the period 1982–2000. The resulting production of gold during this period was 10 800 kg, together with 21 000 tonnes of copper in concentrates and 4500 kg of silver. The gold and copper ores have been concentrated by gravity separation and/or flotation, since the ores so far taken into production has been of the free milling type. However, a substantial proportion of the deposits in the area contain copper, nickel, cobalt and arsenic as well, in the form of sulphides or sulpharsenides, so that the achievement of commercially saleable products calls for the use of different leaching processes. Deposits have also been found in Central Lapland that have consisted partly or entirely of refractory gold ore in which gold is lying in the crystal lattice of pyrite and/or arsenopyrite, the processing of which by the above-mentioned methods is not economic, as it requires pre-treatment by bio-oxidation or pressure oxidation in order to convert the gold to a cyanide-soluble form.