P-T path fluid evolution in the Gross Spitzkoppe granite stock, Namibia

Abstract

FRINDT, STEPHEN and POUTIAINEN, MATTI 2002. P-T path fluid evolution in the Gross Spitzkoppe granite stock, Namibia. Bulletin of the Geological Society of Finland 74, Parts 1–2, 103–114. The Gross Spitzkoppe granite stock (GSS) is a zoned 30 km2 epizonal intrusion that consists of three main granites: 1) medium-grained biotite granite (marginal), 2) a coarse-grained biotite granite, and 3) a central, porphyritic granite. The stock contains pegmatites as banded marginal stockscheiders and isolated pockets composed of large alkali feldspar and quartz, dark mica, interstitial fluorite, and euhedral topaz and beryl crystals. In the porphyritic granite there are local wolframite-bearing greisens and hydrothermal fluorite and topaz-rich veins. Fluid inclusion studies were conducted on: 1) topaz and quartz crystals from the marginal stockscheider; 2) quartz, topaz, fluorite and beryl crystals from isolated pegmatites; 3) topaz from a miarolitic pegmatite; 4) beryl and quartz veins from greisenized porphyritic granite; and 5) fluorite from a late fluorite vein in the coarse-grained biotite granite. Preliminary data indicate the presence of three compositionally distinct primary and pseudosecondary inclusion types that are of late magmatic-hydrothermal origin. Type 1. Low salinity (0–10 eq. wt% NaCl) H2O (± CO2) inclusions that homogenize to the liquid phase in the temperature range of 300 to 550 C. These inclusions are from quartz, topaz and beryl. Inclusions in fluorite from the fluorite vein homogenize at ~170 C and have a salinity of ca. 1–2 eq. wt% NaCl. Type 2. Saline (25–30 eq. wt% NaCl) halite-bearing H2O (± CO2) inclusions that homogenize to the liquid phase in the temperature range of 300 to 400 C. These inclusions are from quartz. Type 3. Low salinity (0–3 eq. wt% NaCl) H2O-CO2 inclusions that homogenize to vapor phase in the temperature range of 330 to 550 C. These inclusions are from quartz and topaz. Hydrothermal fluids from greisen minerals are represented by type 1 and type 2 H2O inclusions. They are predominantly of low salinity (~8 eq. wt% NaCl) and homogenize to the liquid phase in the temperature range of 300 to 500 C. Isochores for contemporaneous type 2 and type 3 inclusions with homogenization temperature range of 330 to 400 C indicate a maximum trapping pressure of about 900 bar for the marginal stockscheider.