Zn-bearing phyllosilicates are common minerals in nonsulfide Zn deposits, but they seldom represent the prevailing economic species. However, even though the presence of Zn-bearing clays is considered as a disadvantage in mineral processing, their characteristics can give crucial information on the genesis of the oxidized mineralization. This research has been carried out on the Mina Grande and Cristal Zn-sulfide/nonsulfide deposits, which occur in the Bongará district (Northern Peru). In both of the deposits, Zn-bearing micas and clays occur as an accessory to the ore minerals. The XRD analyses and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) investigations revealed that the Zn-bearing micas that are occurring in both deposits mostly consist of I/S mixed layers of detrital origin, which have been partly altered or overprinted by sauconite during the supergene alteration of sulfides. Sporadic hendricksite was also identified in the Cristal nonsulfide mineral assemblage, whereas at Mina Grande, the fraipontite-zaccagnaite (3R-polytype) association was detected. The identified zaccagnaite polytype suggests that both fraipontite and zaccagnaite are genetically related to weathering processes. The hendricksite detected at Cristal is a product of hydrothermal alteration, which is formed during the emplacement of sulfides. The complex nature of the identified phyllosilicates may be considered as evidence of the multiple processes (hydrothermal and supergene) that occurred in the Bongará district.
The post-Variscan hydrothermal vein system at Silius (Southeastern Sardinia, Italy) is known mostly for its fluorite-barite-galena mineralization, which has been exploited until early 2006 and is now in maintenance. Distinct fluorspar amounts still exist in the mine, and are evaluated at 2 million tonnes of measured resource at 34.5% CaF 2 (727,495 tonnes fluorspar) and 3.2% Pb (67,724 Pb tonnes in galena). Gangue carbonates of the veins, which consist of calcite and ferroan dolomite, contain the REE- minerals synchysite-(Ce) and xenotime. To check the effective amounts of REE in the Silius orebody, representative samples of the carbonate gangue have been collected from several locations in the mine. ICP-MS analyses have been carried out in the samples with the aim to obtain the bulk chemical composition of the carbonates. The concentrations throughout the mine range between 462 and 2,071 ppm (951 ppm on average), mainly consisting of LREE. The average volume of the carbonate gangue still in place, which would be extracted together with fluorite and galena, is currently considered to be around 532,000 tonnes, to which more 750,000 tonnes of carbonates discarded in the old waste dumps may be added. The corresponding total REE amounts currently occurring in the Silius gangue may sum up to 1,220 tonnes pure REE. The discovery of these amounts of REE in the Silius fluorspar mine may open interesting perspectives for the exploration of subeconomic REE concentrations in this type of deposits, where REE could be recovered as by-product of the fluorite exploitation.
Abstract The Capricornio epithermal Au-Ag vein system is located within the Chilean Paleocene–Eocene metallogenic belt, 60 km ENE of Antofagasta in the physiographic Central Depression, between the Coastal Cordillera and the Domeyko Cordillera. Gold-silver mineralization occurs in multiple epithermal quartz-adularia-carbonate mineral veins, which extend from the surface down to ∼400 m depth. The veins are hosted in Cretaceous volcanic rocks belonging to the Paradero del Desierto Formation. The hypogene ore association at Capricornio is represented by Au-Ag alloy (“electrum”, containing on average 60 wt.% Au), Ag-sulfoarsenides, Ag-sulfoselenides, Ag-sulfoantimonides, Ag-sulfosalts, and acanthite, as well as by base metal sulfide minerals including galena, pyrite, chalcopyrite, sphalerite, and bornite. The hypogene mineralization is deeply weathered down to 200 m. The surficial oxidized blanket (0–150 m depth) is underlain by a secondary covellite-chalcocite-bearing zone, which exhibits a variable thickness up to 50 m in each of the veins. In the oxidized zone base metal sulfides have been replaced by several other metallic minerals: (oxy-hydroxy) chlorides (atacamite, pseudoboleite, cumengeite, herbertsmithite), chlorocarbonates (phosgenite), chloroiodates (seeligerite), chlorophosphates (pyromorphite), chloroarsenates (mimetite), and halides (iodargyrite). In this zone Au still occurs as “electrum”, but after the remobilization of Ag during the process of supergene alteration it is slightly enriched in Au (up to a maximum of 78 wt.% Au) and occurs together with Fe-oxy-hydroxides. Gold has also been locally re-mobilized and may occur as veinlets of native Au. The major Ag-bearing mineral in the supergene zone is iodargyrite. The abundance of Ag-halide minerals, together with the higher Au contents of “electrum” in this zone (compared with that in the hypogene zone) suggests a weathering-related Ag remobilization. Silver was not leached from the deposit and was locally re-precipitated as Ag-oxidized compounds. The abundance of Ag-halide minerals in the uppermost part of the supergene zone suggests a large availability of halide ligands during weathering. This is expected, since Cu- and Ag-halide minerals commonly occur in the supergene weathering zones of porphyry Cu deposits in the Atacama region.
"Mineralogy of the supergene alteration in the Capricornio Au–Ag epithermal vein system, Antofagasta region, Chile." Applied Earth Science, 125(2), p. 74
A preliminary C–O stable isotopes geochemical characterization of several nonsulfide Zn-Pb Tunisian deposits has been carried out, in order to evidence the possible differences in their genesis. Nonsulfide ores were sampled from the following deposits: Ain Allegua, Jebel Ben Amara, Jebel Hallouf (Nappe Zone), Djebba, Bou Grine, Bou Jaber, Fedj el Adoum, Slata Fer (Diapir Zone), Jebel Ressas, Jebel Azreg, Mecella (North South Axis Zone), Jebel Trozza, Sekarna (Graben Zone). After mineralogical investigation of selected specimens, the C–O stable isotopic study was carried out on smithsonite, hydrozincite, cerussite and calcite. The data have shown that all the carbonate generations in the oxidized zones of Ain Allegua and Jebel Ben Amara (Nappe Zone), Bou Jaber, Bou Grine and Fedj el Adoum (Diapir Zone), Mecella and Jebel Azreg (North South Zone) have a supergene origin, whereas the carbonates sampled at Sekarna (Graben Zone) (and in limited part also at Bou Jaber) precipitated from thermal waters at moderately high temperature. Most weathering processes that controlled the supergene alteration of the Zn-Pb sulfide deposits in Tunisia had probably started in the middle to late Miocene interval and at the beginning of the Pliocene, both periods corresponding to two distinct tectonic pulses that produced the exhumation of sulfide ores, but the alteration and formation of oxidized minerals could have also continued through the Quaternary. The isotopic characteristics associated with the weathering processes in the sampled localities were controlled by the different locations of the sulfide protores within the tectonic and climatic zones of Tunisia during the late Tertiary and Quaternary.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)