GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 212-4
Presentation Time: 2:30 PM


MONECKE, Thomas, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 and REYNOLDS, T. James, FLUID INC, 1401 Wewatta St, #PH3, Denver, CO 80202,

Porphyry copper deposits represent the world’s largest source of copper. The low-grade ore in these deposits consists of stockworks of quartz veinlets and disseminated Cu sulfides. Combined optical CL microscopy and fluid inclusion studies show that porphyry veinlets are composed of a distinct sequence of quartz generations, recording the temperature and pressure evolution of these magmatic-hydrothermal systems.

The first quartz generation in porphyry veins forms anhedral grains with irregular grain margins. Recrystallization textures characteristic of subgrain rotation and grain boundary migration are common. Due to extensive recrystallization, the quartz exhibits a uniform blue CL, with primary growth zones being only rarely preserved. The early quartz formed at lithostatic pressure conditions and high temperatures (ca. >450-550°C). Recrystallization of the quartz is thought to be related to repeated vein opening, with the wall rocks exhibiting ductile behavior. Anhedral to euhedral grains of the second quartz generation show distinct blue and brown growth banding or are characterized by a brown CL color. Dissolution and brecciation textures are common. The quartz formed at low temperatures (ca. <450°C) under fluctuating pressure conditions, marking the lithostatic-hydrostatic transition and the change from ductile to brittle behavior. The second quartz generation predates the formation of Cu sulfides. A third quartz generation forms thin overgrowth on earlier formed quartz. This late quartz shows a brown CL and is typically devoid of fluid inclusions. The quartz formed late in the paragenesis and possibly even postdates the formation of Cu sulfides.

The study revealed that porphyry veins typically exhibit complex microscale textures related to the repeated reopening of the veins and successive formation of at least three distinct quartz generations. Due to the complex relationships, the textural setting of fluid inclusion assemblages cannot be easily assessed without CL mapping, especially as there is no simple correlation between fluid inclusion inventory and CL color. The research casts doubts on models for the formation of porphyry deposits that rely on texturally poorly constrained fluid inclusion studies and paragenetic relationships established without the aid of CL microscopy.