Abstract
Alkaline magmatic rocks host some of the world's largest epithermal gold deposits, including Cripple Creek (CO, USA), Porgera (PNG), and Ladolam (New Britain, PNG). As well as the high gold tonnage in these systems, alkaline-hosted epithermal deposits have a number of other notable features, including abundant telluride minerals, quartz-poor alteration assemblages, and a paucity of advanced argillic (acid sulphate) alteration. In spite of evidence for direct magmatic-hydrothermal contributions to these systems, there are very few high sulphidation type epithermal deposits recorded. Alkaline-hosted epithermal are often considered a distinct sub-class of the low sulphidation epithermal class (Sillitoe 2002).
Geochemical models using CHILLER (Reed 1998) were prepared to determine the effect of host rock composition on fluids hosted in alkaline rocks under epithermal conditions (300°C, pressure at vapour saturation). An initial acidic volcanic condensate was progressively reacted with increments of various rock types, including an average andesite composition, and alkaline compositions reported from various ore deposits. The method used follows that of Reed (Reed 1997).
Results show that alkaline magmas are no more effective at neutralising initially acidic fluids than sub-alkaline equivalents, kinetic effects notwithstanding. However, models show that at low water: rock conditions (e.g. rock-buffered equilibrium) that fluids hosted in silica-undersaturated alkaline rock-hosted systems lose hydrothermal quartz. When quartz dissolves out of the alteration assemblage, important pH buffering reactions break down, and fluids rise to pH values of 7–8.2. In contrast, sub-alkaline systems produce abundant hydrothermal quartz and pH is capped at a maximum of 5.5–6, with near-neutral chloride fluids.
At high fluid pH (>7) Au and Te are significantly more soluble. Tellurium solubility is at a nadir at pH 5.5–6; thus the models demonstrate that host rock silica saturation and alkalinity can explain the paucity of Te and tellurides in most low sulphidation epithermal deposits, but their relative abundance in alkaline-hosted deposits. The host rock effects also boost the capability of the fluid to transport gold, and provide an explanation for the unusual enhancement of mineralisation in alkaline rocks.