Fluid inclusion study of greisens associated with the Nicholson Granite Complex, Murphy Inlier, Northern Territory
Titel:
Fluid inclusion study of greisens associated with the Nicholson Granite Complex, Murphy Inlier, Northern Territory
Auteur:
Ahmad, Masood
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 36 (1989) nr. 2 pagina's 207-218
Jaar:
1989-06
Inhoud:
The Early Proterozoic Nicholson Granite Complex contains Sn-W-bearing quartz-muscovite greisens as irregular pods and as tabular veins with both interstitial and vug-filled cassiterite and minor wolframite. Within the greisens, alteration to muscovite varies with the intensity of alteration from iron-poor non-pleochroic through iron-rich pleochroic to iron-bearing pleochroic fibrous varieties. Because of accompanying silicification, a typical greisen contains about 20% muscovite and 75% quartz; other minerals include pyrite, chalcopyrite, hematite, fluorite, topaz and phlogopite. Six kinds of fluid inclusions are present in vein quartz within the greisens: Type A (liquid + <10% vapour); Type B (liquid + 10-30% vapour); Type C (vapour only or liquid + >80% vapour); Type D (liquid + halite + <10% vapour); Type E (liquid + halite) and Type F (liquid only). Early 'primary' inclusions represented by Types B and C have higher temperatures (range: 190-400°C, mean = 300°C) and lower salinities (range: 0.3-12 wt% eq. NaCl, mean=6 wt% eq. NaCl) than the late pseudosecondary inclusions represented by Types A, D and E (temperature range: 90-330°C, average 165°C, salinity range: 0.5-40 wt% eq. NaCl, mean = 24 wt% eq. NaCl). Co-existing vapour and liquid + vapour inclusions in most samples indicate that solutions were boiling, which is substantiated by vapour phase homogenization of many inclusions. The late high salinity-low temperature fluids were probably caused by extensive boiling. In the early high temperature-low salinity fluids, tin was predominantly transported as a stannous hydroxide complex and precipitation caused by an increase in oxidation and decrease in temperature, both of which are attributed to boiling. The late low temperature-high salinity fluids carried tin predominantly as stannic hydroxide and precipitation was essentially due to lowering temperature. Comparisons with other tin-bearing granites suggest a crustal source for the Nicholson Granite Complex and associated tin greisens.
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