Post-ore Stages Research Articles

Tectonic control on hydrothermal circulation and fluid evolution in the Pietratonda–Poggio Peloso (southern Tuscany, Italy) carbonate-hosted Sb-mineralization

This paper deals with the development of an exhumed Sb-ore deposit associated with Neogene–Quaternary magmatism of southern Tuscany. This epithermal mineralization represents a fossil hydrothermal system that was probably active during the volcanic activity that produced rhyolitic lava flows (~ 2.3 Ma), presently exposed close to the study area (Roccastrada volcanic complex). Volcanism was coeval with strike-/oblique-slip and normal faults that controlled the hydrothermal circulation. Faults were active until the Pleistocene. Our study, based on fluid inclusion and structural data, demonstrates that fluids were channeled in damage zones of the Pliocene–Quaternary strike-/oblique-slip and normal faults and permeated within damage zones of Middle–Late Miocene extensional detachments, consisting of fractured carbonate rock masses sandwiched between successions with very low permeability. Fluids produced a diffuse hydrothermal alteration both in the cataclasite and in carbonate damage zones where jasperoid strata bounds developed. Fluid inclusion investigation allowed tracing the evolution of the hydrothermal fluids that is interpreted in terms of a progressive cooling of the hydrothermal system. Such evolution could be related to large infiltration of shallow waters from the early mineralizing stage (fluids at about 200–225 °C, salinity around 3 wt.% NaCl equiv.) down to the later post-ore stages, characterized by more diluted fluids (salinity of about 2.1 wt.% NaCl equiv) having lower temperatures (T h around 155 °C). The latest hydrothermal activity in the area is indicated by fluid inclusions hosted in late calcite veins that are characterized by T h values of about 125 °C. This fossil hydrothermal system may provide a close analog to the active geothermal systems occurring in southern Tuscany, thus the results are useful for better understanding the relationships between brittle structures and fluids migration.

Silver–base metal epithermal vein and listwaenite types of deposit Crnac, Rogozna Mts., Kosovo. Part I: Ore mineral geochemistry and sulfur isotope study

Pb–Zn–Ag vein and listwaenite types of mineralization in Crnac deposit, Western Vardar zone, were deposited within several stages: (i) the pre-ore stage comprises pyrite, arsenopyrite, pyrrhotite, quartz, kaolinite and is followed by magnetite–pyrite; (ii) the syn-ore stage is composed of galena, sphalerite, tetrahedrite and stefanite; and (iii) the post-ore stage is composed of carbonates, pyrite, arsenopyrite and minor galena. The vein type mineralization is hosted by Jurassic amphibolites and veins terminate within overlying serpentinites. Mineralized listwaenites are developed along the serpentinite–amphibolite interface. The reserves are estimated to 1.7 Mt of ore containing in average 7.6% lead, 2.9% zinc, and 102 g/t silver. Sulfides from the pre- and syn-mineralization assemblage of the vein- and listwaenite-types of mineralization from the Crnac Pb–Zn–Ag deposit have been analyzed using microprobe, crush-leachates and sulfur isotopes. The pre-ore assemblage precipitated under high sulfur fugacities ( f(S 2) = 10 − 8 –10 − 6 bar) from temperatures ranging between 350 °C and 380 °C. Most likely water–rock reactions, boiling and/or increase of pH caused an increase of δ 34S of pyrite toward upper levels within the deposit. The decomposition of pre-ore pyrrhotite to a pyrite-magnetite mixture occurred at a fugacity of sulfur from f(S 2) = 8.7 × 10 − 10 to 9.6 × 10 − 9 bar and fugacity of oxygen from f(O 2) = 2.4 × 10 − 30 to 3.1 × 10 − 28 bars, indicating a contribution of an oxidizing fluid, i.e. meteoric water during pre-ore stages of hydrothermal activity. The crystallization temperatures obtained by the sphalerite-galena isotope geothermometer range from 230 to 310 °C. The δ 34S values of pre- and syn-ore sulfides (pyrite, galena, sphalerite, δ 34S = 0.3–5.9‰) point to magmatic sulfur. Values of δ 34S of galena and sphalerite are decreasing upwards due to precipitation of early formed sulfide minerals. Post-ore assemblage precipitated at temperature below 190 °C. Based on data presented above, we assume two fluid sources: (i) a magmatic source, supported by sulfur isotopic compositions within pre- and syn-ore minerals and a high mol% of fluorine found within pre- and syn-ore leachates, and (ii) a meteoric source, deduced by coincident pyrite–magnetite intergrowth, sulfur isotopic trends within syn-ore minerals and decrease of crystallization temperatures from the pre-ore stage (380–350 °C), towards the syn-ore (310–215 °C) and post-ore stages (< 190 °C). Post-ore fluids are Na–Ca–Mg–K–Li chlorine rich and were modified via water–rock reactions. Simple mineral assemblage and sphalerite composition range from 1.5 to 10.1 mol% of FeS catalog Crnac to a group of intermediate sulfidation epithermal deposit.

Genesis and formation conditions of deposits in the unique Strel’tsovka Molybdenum-Uranium ore field: New mineralogical, geochemical, and physicochemical evidence

The ambiguity of genetic interpretations of uranium ore formation at Mo-U deposits of the Strel’tsovka ore field led us to perform additional geochemical, mineralogical, and thermobarogeochemical studies. As a result, it has been established that closely related U and F were progressively gained in the Late Mesozoic volcanic rocks from the older basic volcanics (170 Ma) to the younger silicic igneous rocks (140 Ma). The Early Cretaceous postmagmatic hydrothermal epoch (140–125 Ma) is subdivided into preore, uranium ore, and first and second postore stages. The primary brannerite-pitchblende ore was formed in association with fluorite. At the first postore stage, this assemblage was replaced by a U-Si metagel, which was previously identified as coffinite. The metagel shows a wide compositional variation; its fine structure has been studied. The preore metasomatic alteration and related veined mineralization were formed under the effect of sodium (bicarbonate)-chloride solution at a temperature of 250–200°C. The uranium ore formation began with albitization and hematitization of rocks affected by supercritical fluid at 530–500°C; brannerite and pitchblende precipitated at 350–300°C. The chondrite-normalized REE patterns of pitchblende hosted in trachybasalt, trachydacite, and granite demonstrate a pronounced Sm-Nd discontinuity and a statistically significant tetrad effect of W type. These attributes were not established in REE patterns of rhyolites derived from the upper crustal magma chamber. This circumstance and a chronological gap of 5 Ma between silicic volcanism and ore formation do not allow us to suggest that uranium was derived from this magma chamber. According to the proposed model, the evolved silicic Li-F magma was a source of uranium. U4+, together with REE, was fractionated into the fluid phase as complex fluoride compounds. The uranium mineralization was deposited at a temperature barrier. It is suggested that hydromica alteration and the formation of molybdenum mineralization were genetically unrelated to the uranium ore formation.

A Review of Fluid Inclusion Temperatures and Salinities in Mississippi Valley-type Zn-Pb Deposits: Identifying Thresholds for Metal Transport

More than 2800 published fluid inclusion data (primary inclusions) from eighteen typical Mississippi Valley-type (MVT) Zn-Pb deposits and districts were examined to study possible relationships between MVT fluid properties and metal tonnages and ore grades. The mean Th and salinity values for fifteen MVT deposits and districts are 122°C ± 21°C and 20.7 ± 2.6 wt% CaCl 2 equivalent, respectively. In general, temperatures increase from pre-ore to ore stages, and salinities decrease from ore to post-ore stages. From post-ore to late calcite stages, salinity and temperature decrease with various trends within specific districts. Several conclusions are drawn: first, results show that Hanor’s (1996) salinity threshold for metal-rich basinal fluids applies to typical MVT ore fluids (~16 wt% CaCl 2 equivalent). Second, the lower limit of mean Th values of measured MVT deposits and districts (n = 15) of ~75°C is similar to Hanor’s (1996) observed temperature threshold for metal-rich basinal brines of ~60°C (n = 224). Third, there is no statistically significant relationship between fluid inclusion homogenization temperatures (Th) and metal tonnages or ore grades. Fourth, there seems to be a tendency for large tonnage districts/deposits to be associated with mean fluid salinities that are relatively low (~16 to 21 wt% CaCl 2 equivalent) within the overall range of salinity means (~16 to 26 wt% CaCl 2 equivalent). There is also a correlation between higher ore grades and higher salinities. Finally, comparison with other sedimentary rock-hosted Zn-Pb deposit types shows a trend of salinity decrease and temperature increase from MVT, sandstone-hosted and diapir-related Pb-Zn deposits, to vein-type and Irish-type Pb-Zn deposits, and then to sedex deposits. However, more data are needed for sedex deposits. The salinity threshold for significant Zn-Pb transport decreases to ≥~10 wt% CaCl 2 equivalent at ~250°C. This study suggests that fluid inclusion analysis during exploration may be useful in assessing ore-forming potential (threshold salinity) and possible ore grades (salinity).