CATHODOLUMINESCENCE TEXTURES OF VERMICULAR QUARTZ PHENOCRYSTS AT THE EMPIRE CU-ZN SKARN, IDAHO, AND IMPLICATIONS FOR THE MAGMATIC-HYDROTHERMAL TRANSITION

The Empire Cu-Zn skarn deposit is associated with the granite porphyry phase of the Mackay Stock, which consists of quartz monzonite, granophyre, granite porphyry, Mackay Granite, and numerous dykes. Quartz phenocrysts in both granite porphyry and Mackay Granite have an unusual, extremely vermicular texture.

SEM-CL analysis reveals cryptic, concentric primary bands in these vermicular quartz phenocrysts, with the central band comprising most of the quartz grain. The primary bands are always crosscut by deep embayments, with the bands curving toward the embayments and pointing toward the interior. The bands next to shallow embayments are continuous but curve around them. A very narrow secondary band is often present directly around the embayments. The spatial relationship between primary CL bands and embayments indicate that the embayments form at the end of quartz growth whereas the secondary band implies new quartz growth after embayment.

Fluid inclusions in quartz phenocrysts contain multiple daughter minerals including fluorite. The Th ranges from 420 to >600 degree C. These suggest a magmatic origin of the fluid, and together with the presence of fluorite in the skarns, indicate high F activity of the fluid. In addition, fluorite as an accessory mineral and high F content in magmatic biotite (1.43-3.80 wt%) and amphibole (1.53-3.03 wt%) indicate high F activity of the magma.

The CL bands and embayments formed by the following steps. Quartz crystallized from magma and the central band formed. When water reached saturation, it began to exolve from the magma. At the beginning, the amount of hydrothermal fluid was small and the fluid nucleated as bubbles that locally attached to phenocrysts. Both the magma and hydrothermal fluid were rich in F. The high-F bubbles corroded the quartz, forming vermicular tubes which cut across the primary CL bands. The quartz is still growing but at locations of the bubbles, the growth is inhibited, and at the melt-bubble boundary the new growth surface curves slightly toward the bubble, which results in the curving of the primary bands. As more and more fluid exsolved, bubbles coalesced and moved toward the top of the magma chamber. The tubes was then filled with melt whose viscosity is lowered by F, and from which a little more quartz crystallized, forming the secondary bands.