Variation in hydrothermal muscovite and chlorite composition in the Highland Valley porphyry Cu-Mo district, British Columbia, Canada

• Main Author: Alva Jimenez, Tatiana Romy
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/38209

Muscovitic mica and chlorite are common alteration components in porphyry Cu-Mo deposits. Their mineral chemistry varies depending on the degree of fluid rock interaction, pressure and temperature and can therefore be used as alteration vector. At the Valley porphyry Cu-Mo deposit of the Highland Valley porphyry district, southern British Columbia, muscovitic mica forms part of the high-temperature K-silicate assemblage. It also forms in the adjacent Bethsaida Zone and the abandoned Alwin mine some 4 km to the west, where it is associated with quartz-sulfide veins. The host rock of these areas is granodiorite of the Bethsaida phase of the Guichon Creek batholith. In the Bethlehem deposits, fine- to medium-grained muscovitic mica and chlorite form parallel to fractures or disseminated in a more mafic host rock. The muscovitic mica forms as several textural varieties, including vein selvages where they replace the rock, replacement of igneous biotite and feldspar whereas chlorite replaces mostly biotite and amphibole. Muscovite in the core of the Valley deposit is sodium-bearing (ca. 0.6wt%Na2O, 0.8wt%MgO), but is more phengitic (ca. 0.2wt%Na2O, 1.29wt%MgO) at the Alwin mine and the Bethlehem deposits. Titanium, Li, Sn, V and Sr are present in higher concentrations in muscovites from the Valley deposit compared to those from other areas. In contrast Tl, Rb, Cs, B, Mn, Co and Zn concentrations are higher in muscovite from the Alwin mine, Bethsaida zone and Bethlehem than those from the Valley deposit. Major and trace element chemistry of muscovite reflects decreasing temperature and increasing pH from the central porphyry zones Valley to the peripheral hydrothermal systems. High concentrations of Cs, Rb and Tl in whole-rocks indicate extensive muscovite alteration, whereas high Li, Zn, Mn and Co relate to abundant hydrothermal chlorite in the whole rock. The variability in the chemistry of muscovite and chlorite is also detected using short-wave infrared spectra (SWIR). An absorption feature between 2200 nm to 2202 nm characterizes the Na-bearing muscovites from Valley, whereas more phengitic-muscovites from Alwin and Bethlehem deposits have an absorption feature between 2205 nm to 2209 nm. Mg-rich chlorites have an absorption feature at around 2341 nm, whereas the same feature shifts to 2350 nm for Fe-rich chlorites. Muscovite and chlorite chemistry varies laterally in porphyry Cu-Mo districts and is also reflected in SWIR spectra and whole rock geochemistry, techniques widely used in exploration.