Abstract

Greisenization, albitization, and K-feldspathization occur as pervasive (background) alteration and fracture-controlled (selvage-style) alteration in the Go Sam granite in the Herberton-Mount Garnet tin fields of northeast Queensland. Optical, SEM, and microthermometric investigations of fluid inclusions, principally from vug-filling minerals in selvage-style alteration permit interpretation of the nature and evolution of hydrothermal fluids responsible for alteration of the granite.Saline hydrothermal fluids are dominated by Na-K-Ca-Fe chloride solutions, with Mg, Mn, Zn, Si, Al, Th, Ce, and La relatively minor but important components. Salinities of up to 70 wt percent total dissolved salts have been calculated using the phase volume method.Indicated trapping temperatures of 600 degrees to 650 degrees C for many saline inclusions in vug-filling minerals from feldspathic rocks record minimum temperatures for feldspathic alteration. Inferred temperatures for greisenization are similar and slightly lower (450 degrees -525 degrees C). Trapping temperatures, and the composition of fluids in saline inclusions, suggest late magmatic to early postmagmatic alteration of the granite by predominantly magmatic fluids.Inclusions containing relatively dilute fluids of possible meteoric origin homogenize between 100 degrees and 400 degrees C. Salinities as low as 2 equiv wt percent NaCl are indicated and significant CO (super -2) 3 , HCO (super -) 3 , or SO (super -2) 4 may be present. Associated vapor-rich inclusions indicate that the dilute fluids were probably boiling between 300 degrees and 400 degrees C.A continuous range of salinities from dilute fluids to saline fluids has been found in the inclusions investigated. Dilution of very saline, high-temperature fluids could have occurred by separation of dissolved components as the fluid cooled and by mixing with dilute fluids below 400 degrees C.Data from fluid inclusions in magmatic quartz from the Go Sam granite are similar to those obtained from vug-filling minerals in fracture-controlled metasomatic rocks; they suggest that pervasive alteration and fracture-controlled alteration occurred contemporaneously and as a result of interaction with the same fluids.Consistently high temperatures inferred for the formation of all feldspathic rocks and associated greisens do not fit an established Soviet model, which emphasizes falling temperatures, for the origin of comparable rocks. A tentative model which emphasizes high-temperature evolution of hydrothermal fluids due to changes in chemical parameters (pH, salinity, Na/K, f (sub F 2 ) ) and confining pressures is discussed.

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