HYDROTHERMAL FLUID EVOLUTION IN PORPHYRY-TYPE ORE DEPOSITS

Porphyry-type deposits from around the world are characterized by consistent patterns of cathodoluminescent (CL) textures and quartz trace elements that, when combined with fluid inclusion and isotopic analysis, demonstrate the progressive changes in the pressure and temperature conditions of the hydrothermal system. Here, we demonstrate typical features of a number of porphyry-type or deposits.

Early quartz-rich veins (+-mb) with potassic alteration are comprised of mosaics of CL-bright quartz that lacks euhedral shapes or growth zones. Such quartz is characteristically fractured and cut by later CL-darker quartz with euhedral growth zones. Where Fe and Cu-Fe sulfides are present, they are typically surrounded by late CL-dark quartz. CL-bright quartz typically contains 50-200 ppm Ti and 50-200 ppm Al. Unlike Fe and Cu-Fe sulfides, molybdenite is commonly in direct contact with CL-bright quartz and calcite or anhydrite. In many systems, these veins are dominated by low salinity (3-8 wt %), CO₂-bearing (0-10 mol%) B40 fluid inclusions. These inclusions homogenize between 300 and 400°C. Unmixed vapor and brine inclusions are also present in these veins in some systems.

Later pyrite-quartz veins with sericitic alteration typically contain less quartz, which is CL-darker with oscillatory growth zones revealed by CL. This quartz cuts early bright quartz and typically lacks intense fracture networks observed in early veins. Quartz in pyrite-quartz veins typically contains <10 ppm Ti and Al concentrations vary from a ~50-400 ppm. Fluid inclusions in these veins vary, but typically include low salinity, CO₂-bearing B60s that homogenize between 350° and 420°C. Unmixed vapor and brines are present, but less common in pyrite-quartz veins in many porphyry deposits.

In many porphyry systems, late sphalerite and galena-bearing veins contain quartz with distinct euhedral growth zones revealed by CL. Quartz in these veins typically contains less than 5 ppm Ti and 10-4000 ppm Al. Fluid inclusions typically contain 10-30% bubble (B20s). These inclusions are low salinity and typically homogenize at temperatures between 180° and 300°C.

Taken together, these data demonstrate the variations in pressure and temperature evolution among various porphyry-type deposits.