CATHODOLUMINESCENCE INVESTIGATIONS ON VEIN QUARTZ FROM THE FAR SOUTHEAST PORPHYRY CU-AU DEPOSIT, PHILIPPINES: HYDROTHERMAL QUARTZ ALTERATION AND INHERITANCE OF EARLIER FLUID INCLUSION ASSEMBLAGES

It is well established that the cathodoluminescence and fluid inclusion properties of quartz can be used to constrain the genetic conditions of quartz growth in a wide range of geological environments. The present study reports on the findings of a combined CL/FI study at the Far Southeast porphyry Cu-Au deposit, Philippines. It is shown that the CL properties of quartz can be modified by processes of hydrothermal alteration and that CL color alone cannot be used to reliably distinguish different quartz generations in this deposit type. The findings have far-reaching consequences; especially as CL is widely used to define the textural setting of fluid inclusion assemblages selected for microthermometric investigations.

Vein quartz at Far Southeast is largely composed of early anhedral quartz (Q1) that exhibits a bright blue luminescence with well-defined oscillatory growth zoning. The quartz contains abundant brine and contemporaneously trapped vapor-rich fluid inclusions. Q1 quartz shows embayed grain boundaries and fractures that are overgrown by later, dark red-brown luminescing quartz (Q2). The red-brown Q2 quartz occurs as small euhedral crystals in open space fractures and vugs. Compared to Q1 quartz, it is inclusion poor, but locally contains low-salinity liquid-rich inclusions with rare coexisting vapor-rich inclusions. Sulfide deposition postdates formation of the euhedral Q2 quartz.

The microtextural relationships observed under CL are complex. Microbreccias occur in several areas where apparent clasts of blue luminescing quartz are surrounded by a red-brown matrix. Locally, blue luminescing Q1 quartz is transected by zones of red-brown CL. Although there is a marked increase in the abundance of the low salinity liquid-rich inclusions within these zones, earlier brine inclusions characteristic of Q1 quartz are also present. The red-brown luminescing quartz containing these brine inclusions is interpreted to represent the product of hydrothermal alteration of Q1 quartz. Alteration of Q1 quartz resulted in the resetting of its CL properties and the formation of secondary low-salinity liquid-rich inclusions. At the same time, the earlier Q1 fluid inclusion inventory was preserved, suggesting that alteration of Q1 quartz did not involve complete recrystallization.