INSIGHTS INTO VEIN FORMATION AND ALTERATION FROM EXPERIMENTALLY SYNTHESIZED HYDROTHERMAL QUARTZ VEINS

Quartz-bearing veins with alteration envelopes characterize many hydrothermal ore deposits. Typically these veins form where aqueous fluids flow through fractures in rocks and vein minerals precipitate in response to fluid cooling, fluid heating (where solubility is retrograde), depressurization, and/or fluid-rock reactions. Scanning electron microscope-cathodoluminescence (SEM-CL) of quartz veins from many hydrothermal deposits reveals that veins that appear simple optically actually formed during multiple episodes of precipitation, and in some cases dissolution. Deciphering the formation history of a vein is critical to interpretation of isotope and fluid inclusion data, to relate vein minerals to specific mineralization events, and to infer the physical and chemical evolution of hydrothermal systems.

To infer how hydrothermal processes manifest themselves through vein mineral precipitation and wall rock alteration, we synthesized quartz veins in flow-through autoclave reactor experiments. Preheated distilled water was injected into the flow reactor unit, which was lined with granite, forming an artificial fracture surface. Isobaric experiments were carried out at temperatures of 300 to 450°C and pressures of 300 to 400 bars.

Experiments resulted in the formation of quartz veins that precipitated in response to pre-set temperature gradients. Wall rock alteration was most intense in the flowpath at the fluid entrance to the autoclave. In this region, quartz dissolved leaving pits, and plagioclase altered to clay minerals. In the region where quartz veins precipitated, alteration is less intense. In synthetic veins, euhedral quartz crystals grow inward from the fracture surface at a variety of angles. SEM-CL images of vein quartz show oscillating euhedral growth zones of varying CL intensity. Microprobe and LA-ICP-MS analyses of vein quartz indicate that the variations in CL intensity correspond to variations in Al concentration in the range of a few hundred ppm. As pressure, temperature gradients, and fluid flow rate were held constant throughout the experiments, the fluctuations in CL intensity and trace element concentration are surprising and suggest either local disequilibrium partitioning of Al into quartz or kinetic control on hydrothermal fluid compositions.