CATHODOLUMINESCENCE AND TRACE ELEMENTS IN QUARTZ FROM PORPHYRY-TYPE ORE DEPOSITS

RUSK, BRIAN; MAYDAGÁN LAURA; FRANCHINI, MARTA; MAO, WEI; LI, XIAOFENG

Vancouver

Congreso; 2014 GSA Annual Meeting; 2014

Geological Society of America

Porphyry-tpye ore deposits are characterized by superposition of multiple vein generations, each with different vein and alteration assemblages and different ore minerals. The superposition of multiple mineralization events obscures relationships between ore minerals, vein minerals, and alteration, complicating studies that attempt to decipher the formation of these deposits using isotopic tracers and fluid inclusions. Quartz cathodoluminescent (CL) textures in porphyry-type deposits relate vein minerals, alteration minerals, and fluid inclusions to specific vein forming events. Porphyry-type deposits from around the world are characterized by a consistent pattern of CL textures and quartz trace elements that, when combined with fluid inclusion analysis, demonstrate the progressive changes in the pressure and temperature conditions of the hydrothermal system. Early quartz-rich veins (+-mb) are typically made up of mosaics of CL-bright quartz grains that lack euhedral shapes or internal growth zonations. Such quartz is characteristically fractured and cut by later CL-darker quartz. Where Fe and Cu-Fe sulfides are present in these veins, they are typically surrounded by late CL-dark quartz. CL-bright quartz in early veins typically contains 100 to 200 ppm Ti and 50-200 ppm Al. Unlike Fe and Cu-Fe sulfides, molybdenite is not always surrounded by CL-dark quartz, and is commonly in direct contact with CL-bright quartz as well as calcite or anhydrite in many systems. Later pyrite-quartz veins with sericitic alteration typically contain very little quartz, and are characterized by CL-darker quartz with internal oscillatory growth zones revealed by CL. This quartz consistently cuts early bright quartz and typically lacks the intense fracture networks observed in early veins. Quartz in pyrite-quartz veins typically contains <10 ppm Ti and Al concentrations vary from a ~50 to 400 ppm. In many porphyry systems, late sphalerite- and galena-bearing veins contain euhedral quartz with distinct euhedral oscillatory growth zones revealed by CL. Quartz in these veins typically contains less than 5 ppm Ti and Al concentrations range from a few ppm to a few thousand ppm. Such trace element concentrations are consistent with low pressures and temperatures typical of epithermal deposits.