The anatomy of a Carboniferous epithermal ore shoot at Pajingo, Queensland; setting, zoning, alteration, and fluid conditions

Abstract

The Paleozoic Scott lode gold-silver epithermal deposit, situated at Pajingo, northeast Queensland, is hosted by feldspathic-volcanolithic sandstones, quartzose sandstones, ignimbrites, andesitic voltanits with interbedded block and ash deposits, lapilli tuffs, and andesitic to dioritic intrusive rocks. The Scott lode deposit contains 1.23 million metric tons of 9.9 g/t gold and 38.9 g/t silver. There are six principal types of alteration assemblages which are zoned with respect to the main vein structure: propylitic, potassic (adularia), intermediate argillic, silicic, kaolinitic, and ferroan carbonate. Overprinting textural relationships indicate that the propylitic type is earliest, progressively followed by potassic (adularia), then intermediate argillic, and finally silicic alteration. Auriferous veins are largely coeval with the silicic and intermediate argillic alteration types. Kaolinitc and ferroan carbonate occur as late-stage alteration products, postdating the precious metal mineralization. Principal ore and sulfide minerals include pyrite, sphalerite, galena, chalcopyrite, hessitc, tennantite-tetrahedrite, argentitc, petzite, electrum, and native gold. Gangue minerals in the veins are quartz, illitc, adularia, calcite, and locally, kaolinitc. The main-stage quartz is zoned with respect to mineralogy, mineralization styles, metal distribution patterns, and vein textres. Three major zones can be recognized: zone I--an upper precious metal, moss chalcedonic crustiform-colloform quartz zone, zone II--intermediate precious + base metal, crystalline quartz-coarse-grained sulfide-dominated, crustiform-colloform quartz zone, and zone IIIdeep sparse base metal, weakly banded crystalline comb quartz zone. The ore interval occurs in zones I and II with the transition to zone III effectively defining the limit of mineralization. Economic mineralization at the Scott lode deposit was laid down from dilute fluids (0.7-2.5 wt % NaC1 equiv) with a very low concentration of CO2. Homogenization temperatures of 170 o to 315oC exhibit vertical variation with higher temperatures at greater depth. The presence of coexisting liquid- and gas-rich inclusions in mineralized quartz, together with adularia, vein brecciation, and pronounced crustiform-colloform banding, indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition. Thermodynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300oC indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work.