Academic literature on the topic 'Sphalerite geobarometry'

The Velardeña mining district is economically the most important of Durango state. The ore deposits occur in different skarn zones developed within the intrusive contact between Mesozoic limestones and Eocene granitic stocks and dikes. The most important ore deposits are related to the Santa María dike and Reyna de Cobre porphyritic stock (separated from each other by 10 km). They occur as irregularly shaped replacement masses developed near the intrusive contact and have a skarn paragenesis dominated by calc-silicates and sulfides. The mineral assemblages show replacement textures and are dominated by calcic clinopyroxene (Di97-53Hd42-02Jh04-01) and garnet (Ad100-57Grs43-00) in the exoskarn, with wollastonite particularly abundant in the endoskarn. Hydrous silicates are actinolite, epidote, and chlorite, whereas sulfides include pyrite, sphalerite, pyrrhotite, galena, chalcopyrite, and sulfosalts. Scheelite, hematite, quartz, and calcite are also present. According to sphalerite geobarometry, the skarns formed at hypabyssal depths (~3–4 km). They developed by a succession of replacive mineralizing events, including (a) a prograde stage at temperatures from ≥470 to 335 °C in conditions of low f (CO2), followed by (b) a retrograde stage from 335 to 220 °C. There was a general increase in f (O2), accompanying the temperature decline during the formation of the system, which accounts for a process of mixing with cooler, oxidizing, and dilute water. During the retrograde stage, wollastonite, calcic garnet and clinopyroxene formed. On the other hand, hydrous silicates, sulfides, sulfosalts, scheelite, and hematite crystallized during the retrograde stage. Skarn mineralization is crosscut by veins of calcite, fluorite, adularia, and sphalerite. The vein mineralization formed at temperatures below 200 °C. The different ore deposits of Velardeña constitute a telescoped skarn–epithermal mineral system.

AbstractPyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

With a resource of ∼2.8 Mt at 30%–35% wollastonite occurring at a depth of about 75 m, the Olden (formerly Hawley) prospect is the largest of a swarm of skarns hosted by amphibolite facies, dominantly calcitic marble that occurs adjacent to and as inliers within the Mountain Grove pluton. The post-kinematic intrusion comprises units with a wide compositional range from anorthositic gabbro to alkali-feldspar granite and syenite and widely exhibits megascopic fabrics recording magma comingling and mixing. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) dating of zircon separates from a hornblende diorite unit yields a 207Pb–206Pb age of 1153 ± 2 Ma, significantly older than the contiguous 1070 ± 3 Ma McLean granite. Al-in-hornblende geobarometry on several Mountain Grove units indicates that the intrusion crystallized at pressures of 250–470 MPa, equivalent to mesozonal depths of 10–15 km. The main exoskarn body, ∼200 m long and up to 50 m wide, is dominated by wollastonite, clinopyroxene (Di73–94Hd2–19), and calcic garnet (Gr52–83And12–37) and ranges from massive to podiform. The eastern termination exhibits rhythmically alternating wollastonite- and calcite-rich layers, 10 cm wide and locally with chevron-shaped crenulations. These layers bear no relationship to bedding or metamorphic foliation and are interpreted as “wrigglitic,” i.e., they are a record of a metasomatic front that migrated. Veins of garnet–pyroxene–vesuvianite cut the main exoskarn. Retrograde phlogopite-rich skarns, with erratic serpentine and brucite, contain variable sphalerite and pyrite. Restricted pyroxene–garnet (–wollastonite–scapolite) endoskarn is developed in intrusive rocks contiguous with the exoskarn. Skarn development is ascribed to H2O-rich (XCO2 < 0.3) magmatogene brines and high temperatures (T = 500–650 °C), which caused intense Si, Al, and Fe metasomatism of the marbles, hydrothermal activity taking place at considerable depth. The occurrence of wollastonite around the periphery of the small Long Lake sphalerite deposit, restricted to a marble roof pendant in the Mountain Grove pluton 2.1 km east-northeast of the Olden prospect, indicates that this base-metal mineralization may be an exoskarn, rather than metamorphosed Mississippi Valley type. Incremental-heating 40Ar–39Ar dating of hornblende and biotite from the Mountain Grove diorite yields plateau ages of 1058 ± 14 and 1047 ± 4 Ma, respectively, and an exoskarn phlogopite age of 1074 ± 5 Ma. A genetic relationship between hydrothermal activity and the McLean pluton cannot be ruled out, but a parental role for the older Mountain Grove pluton is favoured on the basis of the close areal relationships of skarn bodies and that intrusion.

The Scott Lake volcanogenic massive sulfide deposit lies near the margin of a large, early kinematic granitoid intrusion in the vicinity of Chibougamau, Quebec. The deposit was contact metamorphosed by the intrusion, and subsequently it was metamorphosed to the greenschist facies during the Kenoran Orogeny. Pyrite, magnetite, and sphalerite are the most abundant metallic minerals, and minor amounts of chalcopyrite, pyrrhotite, and loellingite are also present. Both pyrite and magnetite locally occur as porphyroblasts up to several centimetres in diameter. Metamorphic textures developed in the massive sulfide ore appear to have formed during contact metamorphism, and they remained intact through the subsequent regional event. However, silicate minerals (biotite and possibly amphibole) that grew during contact metamorphism were largely retrograded during regional metamorphism. The presence of biotite indicates that contact metamorphism took place at 400°–500 °C. Application of the sphalerite geobarometer gives a pressure of approximately 4.5 kbar (450 MPa), which probably reflects the later regional event.