Compositional Evolution and Formation Conditions of Magmas and Fluids Related to Porphyry Mo Mineralization at Climax, Colorado

Abstract

To reconstruct the magmatic–hydrothermal processes leading to porphyry Mo ore formation at the Climax Mo mine, Colorado, four magma units that were emplaced before, during and shortly after the mineralization events were investigated: (1) a pre-mineralization white dike of the Alma district; (2) the syn-mineralization Chalk Mountain Rhyolite; (3) a lateto post-mineralization rhyolite porphyry dyke; (4) a mafic enclave within the productive Bartlett stock. Melt inclusions, mineral inclusions and fluid inclusions in quartz phenocrysts were investigated by means of laser ablation inductively coupled plasma mass spectrometry, electron microprobe and microthermometry. Based on melt inclusion data both the Chalk Mountain Rhyolite and the rhyolite porphyry were 10 times more fractionated than average granite and show geochemical characteristics of topaz rhyolites. They were saturated in magnetite, Mn-rich ilmenite, fluorite, aeschynite, monazite, pyrrhotite and thorite, and crystallized predominantly at 710–730 C, 1 2–2 6 kbar and log fO2 FMQþ 2 2 (where FMQ is fayalite–magnetite–quartz). The silicate melt of the Chalk Mountain Rhyolite contained 3 56 0 4 wt % F, 0 0960 03 wt % Cl, 3 0 wt % H2O, 15–90mg g Cs, 500–1500mg g Rb and 5–7 mg g Mo, whereas that of the rhyolite porphyry contained 1 160 3 wt % F and 4 961 2 wt % H2O, but otherwise had a virtually identical major and trace element composition. The fluid exsolving from the latter melt had a bulk salinity of 106 2 wt % NaClequiv and contained of the order of 100mg g Mo. After emplacement of the Chalk Mountain Rhyolite magma at subvolcanic levels, extremely fractionated silicate melts coexisting with hypersaline brines (salt melts) and low-density vapor percolated at near-solidus conditions through the rock. These silicate melts contained 6 66 0 4 wt % F, 7 56 0 6 wt % H2O, 0 516 0 05 wt % Cl, and up to 0 5 wt % Cs and 100mg g Mo, whereas the hypersaline brines contained 1–2 wt % Cs and 0 3–0 6 wt % Mo. However, owing to their negligible masses these liquids are unlikely to have played a major role in the mineralization process. The majority of Mo in the Climax deposit appears to have been derived from melts containing 5–7 mg g Mo and bulk fluids containing 100mg g Mo. These concentrations are similar to those found in similarly fractionated melts and fluids in barren and sub-economically mineralized intrusions. However, whereas in the latter intrusions fractionated melts occurred in a rather dispersed state, they seem to have been present as large, coherent masses in the apical parts of Climax-type porphyry Mo-forming magma systems. Efficient segregation of fractionated melts and fluids into the top of mineralizing magma chambers appears to have been promoted by high fluorine concentrations in the silicate melt, which was partly a primary feature, and partly an indirect consequence of other characteristics of within-plate magmatism.

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