ALBERT

Description: Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow- or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3–0.5 wt % MnO) and a high Mn/Fe ratio (〉1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss (〈0.15% MnO) contemporaneous with depletion in Na, S, Cl, and REE during low pH phyllic alteration in calc-alkalic porphyry deposits. The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials.

Description: The world-class Veladero high-sulfidation epithermal gold deposit is located near the northern end of the El Indio-Pascua metallogenic belt in Argentina. New 40 Ar- 39 Ar alunite and jarosite ages were obtained at four mineralized centers in the Veladero district: the Amable and Filo Federico orebodies that compose the Veladero deposit, and the Argenta and Fabiana satellite centers. These ages indicate that magmatic-hydrothermal alteration and related gold mineralization were episodic, and activity in the Veladero district occurred over a period of approximately 3 m.y. The earliest magmatic-hydrothermal alteration in the Veladero district is recorded by alunite from Argenta that formed at 13.12 ± 0.18 Ma. At the Fabiana prospect, alunite is dated at 10.09 ± 0.08 Ma. At the Veladero deposit, the alteration and mineralization at the Filo Federico and Amable orebodies formed at different times despite their close proximity. Previously available data indicate that alunite at Amable formed at 11.93 ± 0.09 Ma, whereas Filo Federico alunite samples are younger and yield 40 Ar- 39 Ar ages of 11.05 ± 0.12 and 10.34 ± 0.11 Ma. The oldest postmineralization jarosite alteration at Amable is dated at 11.8 ± 0.3 Ma and the youngest is dated at 8.58 ± 0.17 Ma. The age constraints suggest that the spatially overlapping Amable and Filo Federico orebodies formed approximately 1 m.y. apart, at about 11.9 Ma at Amable and after 11.1 Ma at Filo Federico. The results demonstrate that the Veladero district is host to the oldest centers of significantly mineralized magmatic-hydrothermal alteration within the El Indio-Pascua metallogenic belt. Magmatic-hydrothermal alteration and gold mineralization at the Veladero deposit broadly overlapped with isolated centers of middle to late Miocene volcanic activity in the region, dated at 12.7 ± 0.9 to 11.0 ± 0.2 Ma. In contrast to Veladero, the other high-sulfidation deposits in the belt are significantly younger with reported 40 Ar- 39 Ar alunite alteration ages of other major deposits in the belt ranging from 9.4 (the Lama deposit at Pascua Lama) to 6.2 Ma (the El Indio deposit). These deposits formed during volcanic quiescence caused by shallowing of the Nazca plate below South America during the late Miocene, whereas Veladero formed prior to the cessation of volcanism. Fertile magmas of roughly 13 to 10 Ma were previously thought to be restricted to the Maricunga metallogenic belt approximately 200 km north of Veladero, but the present study demonstrates that mineralization in this age range occurred as far south as Veladero. These results are significant for regional exploration, in particular in the area between the Maricunga and El Indio-Pascua metallogenic belts.

Description: Electron transport in Sb-doped SnO 2 (ATO) films is studied to unveil the limited carrier mobility observed in sputtered films as compared to other deposition methods. Transparent and conductive ATO layers are deposited from metallic tin targets alloyed with antimony in oxygen atmosphere optimized for reactive sputtering. The carrier mobility decreases from 24 cm 2 V −1 s −1 to 6 cm 2 V −1 s −1 when increasing the doping level from 0 to 7 at. %, and the lowest resistivity of 1.8 × 10 −3  Ω cm corresponding to the mobility of 12 cm 2 V −1 s −1 which is obtained for the 3 at. % Sb-doped ATO. Temperature-dependent Hall effect measurements and near-infrared reflectance measurements reveal that the carrier mobility in sputtered ATO is limited by ingrain scattering. In contrast, the mobility of unintentionally doped SnO 2 films is determined mostly by the grain boundary scattering. Both limitations should arise from the sputtering process itself, which suffers from the high-energy-ion bombardment and yields polycrystalline films with small grain size.

Description: The Late Triassic Galore Creek porphyry district is the largest accumulation of Cu-Au prospects in British Columbia, Canada, in terms of contained metal. The principal economic resource is the Central zone deposit. It comprises three high-grade centers: the Au-rich North and South gold lenses and the Cu-dominated Central replacement zone. Alteration footprints, ore shells, and porphyry intrusions in these centers dip 45° to 60° to the west in the North gold lens and southwest in the South gold lens and Central replacement zone, indicative of significant postmineral tilting. This inclination created an oblique vertical expression of the Central zone at surface and at depth, providing a +700-m depth profile through the deposit. Galore Creek is considered to be the end-member of the silica-undersaturated class of alkalic porphyry Cu-Au deposits. It displays a unique hydrothermal alteration footprint, generally devoid of quartz veining. Despite the lack of crosscutting veins, a paragenetic sequence of several discrete potassic and calcic alteration events has been established. During main-stage alteration, two mineralization events occurred that formed the economic resource in the Central zone. Highly oxidizing fluids derived from porphyritic syenite to monzonite intrusions produced initial potassic alteration and sulfide mineralization in the North gold lens and South gold lens. Cu-Au ore shells are characterized by a gold-rich core dominated by bornite associated with hematite-dusted orthoclase, specular hematite, and anhydrite. The second period of hydrothermal activity caused brecciation and calcic alteration in the Central replacement zone. The calcic fluids formed hydrothermal cement grading from Ti-rich andraditic to grossularitic garnet in the core of the breccia to a diopside and magnetite-dominated assemblage at the margins. Calcic alteration also formed in the surrounding host rock. This alteration event was followed by potassic alteration and mineralization, characterized by biotite, anhydrite, and chalcopyrite in the Cu-rich ore shell of the Central replacement zone. Most of the Au is hosted in bornite that is spatially associated with oxidized (reddened) rock units dominated by an alteration assemblage of orthoclase ± hematite ± specular hematite ± garnet ± anhydrite. High-grade Cu in the Central zone is related to abundant chalcopyrite that has commonly replaced an assemblage of biotite ± magnetite ± diopside. This distinct spatial separation strongly suggests that the local redox environment within the wall rocks governed sulfide and gold distributions. The switch from early Au-rich mineralization to late-stage Cu-dominated mineralization appears to have been controlled by varying redox conditions within the magma. The change from more oxidized alteration assemblages (Fe 3+ 〉 Fe 2+ ) to more reduced assemblages (Fe 3+ ≤ Fe 2+ ) is indicative of a change in oxidation potential that may have propelled sulfide saturation in the magma and, thus, the subsequent depletion of most of the Au.

Description: 〈span〉〈div〉Abstract〈/div〉At the buried Atlántida deposit (Cu-Au-(Mo)) in the Atacama Desert of Chile, highly saline pockets of fine-grained material 10 cm – 3 m in diameter were identified on the alluvial surface using remote sensing and detailed regolith mapping. The median salinity (NaCl dominant) of the saline pockets is 2.2% compared to background alluvial material with a median salinity of 0.01%. Their distribution along mapped fault structures and the highly saline nature of the material suggest they form as an expression of groundwater forced through fractures to the surface during seismic activity. A targeted geochemical survey, oriented parallel to the orientation of the structures (sample spacing 250 m along structural trend) specifically sampling saline pockets on relatively old surfaces, was performed over the deposit. Deionized water extraction of soluble salts and analysis by ICP-MS revealed strong correlations of increasing salinity and increasing concentrations of porphyry copper pathfinder elements. Elevated responses of Se, Mo, Re, and Te normalized to a groundwater volume proxy are present directly over the Atlántida deposit. This suggests the rate of erosion and sedimentation is slow enough in the Atacama Desert to preserve surficial anomalies as saline pockets, formed by periodic seismically induced surface flooding of groundwater along faults extending to surface. Targeted sampling of saline pockets along structural trends using weak leach geochemistry in terrains dominated by transported cover can serve as a routine exploration method for the potential discovery of buried copper porphyries, and other styles of mineralization in the Atacama Desert of Chile.〈/span〉

Description: La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies. Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitic alteration with epidote, chlorite, calcite, specularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40 Ar/ 39 Ar geochronology on hydrothermal muscovite (i.e., sericite) to ~3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40 Ar/ 39 Ar ages constrain epithermal mineralization to between ~2.6 and ~1.6 Ma. Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor-rich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from ~143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite 34 S ranges from –16.9 to –11.3 at La Mascota and –8.3 to –6.1 at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on 18 O and D data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water.

Description: The Cerro Bayo low sulfidation epithermal district is located in the Aysén region, Chilean Patagonia, at the western tip of the Deseado Massif epithermal silver-gold province. Mineralization is hosted in the mainly rhyolitic fragmental successions of the Jurassic Ibáñez Formation. New biotite 40 Ar/ 39 Ar and zircon U-Pb ages together with published ages constrain the eruption of this unit to between 158 and 144 Ma. Rhyolitic domes and dikes, including Cerro Bayo proper, aligned on the prominent N-S–striking Cerro Bayo fault and intruding rocks of the Ibáñez Formation were dated by U-Pb on zircon at 146.5 ± 0.2 and 146.3 ± 0.2 Ma. Dacitic domes of 83.0 ± 0.2 and 82.6 ± 0.2 Ma (U-Pb on zircon) are present in the western part of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping, dominantly N to NW and subordinate W-striking quartz veins and, to a lesser extent, in breccias, which, on the basis of new adularia 40 Ar/ 39 Ar ages, were emplaced in three main episodes: (1) Mallines: ca. 144 to 142 Ma, (2) Bahía Jara and Brillantes: ca. 137 to 124 Ma, and (3) Laguna Verde: 114 to 111 Ma, overall spanning 33 m.y. Most Ag-Au has been produced from veins emplaced in the latter two episodes. Veins at Mallines and Bahía Jara are spatially related to the Cerro Bayo fault. The oldest episode of mineralization in the district is similar in age to the youngest epithermal deposits in the Deseado Massif, whereas the economically most important veins are age equivalent to skarn and polymetallic vein mineralization in the Patagonian Andes of southern Chile. The protracted history of epithermal processes at Cerro Bayo records the evolution from continental-scale extension related to the Gondwana breakup to the establishment of Andean-type arc and back-arc environments. The largely extensional tectonics throughout this period resulted in similar epithermal mineralization styles emplaced episodically over a large time interval.

Description: 〈span〉〈div〉Abstract〈/div〉This paper describes saline pockets (10 cm – 3 m in diameter) of fine-grained material distributed on alluvial surfaces. These saline pockets are localized along structural trends at the Atlántida buried porphyry-skarn Cu-Au-(Mo) deposit in the Atacama Region of Chile. The distribution and highly saline nature of the material suggests formation by the pooling and evaporation of groundwater forced through fractures to the surface during seismic activity. These saline pockets are a surface expression of the hydrological effects of seismic activity along faults. Saline pockets with similar distribution and characteristics were also identified at three additional alluvium-covered areas, all located in the Antofagasta Region of Chile. Identification and mapping of these saline pockets relies on the ability to identify the continuation of structures through overlying gravels. Regolith mapping using high-resolution drone imagery and digital elevation modelling identified geomorphic markers of faulting which aided in the mapping of the distribution of saline pockets. Saline pockets provide a unique opportunity to sample the direct expression of transported groundwater reaching the surface from depth and provide a prime target medium for mineral exploration through transported gravels.〈/span〉

Description: Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y 〉 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchía district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, garnet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of 〉1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y 〈11 ppm and Sr/Y 〉40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intramineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur.