Titanium ore potential of small mafic intrusions based on two examples in western Finland.

摘要

This study evaluates the Ti potential of relatively small mafic intrusions not associated with anorthosite complexes, based on new studies of two Finnish ilmenite-rich gabbros, Koivusaarenneva and Kauhajärvi. The Ti potential of small mafic intrusions is much larger than heretofore recognized. Significant sized ilmenite deposits (50 to over 100 Mt) can occur in rather small intrusions such as Otanmäki and the newly discovered Koivusaarenneva gabbro.; The Koivusaarenneva gabbro is composed at least of three successive pulses of Ti-rich tholeiitic parent magma. The middle zone contains voluminous Ti-mineralized (4 to 24% TiO₂) layers with the average ratio of ilmenite to magnetite of 4:1. It is interpreted as generated by open system crystallization under relatively low oxygen fugacity. Ti-rich melt droplets or suspended oxides were removed from a Ti-saturated magma. The Ti depleted magma flowed out of the poorly crystallized intrusion and was replaced by new Ti-saturated magma. By this process it was possible to generate the large mass of Ti from a slightly Ti-rich magma in a small-sized intrusion.; The Kauhajärvi gabbro, contains layers with an average 20 wt% combined ilmenite, apatite and ilmenomagnetite. High P in the magma enabled crystallization of individual ilmenite coevally with Ti-bearing magnetite (ilmenomagnetite) during a relatively high f_O2. The lack of dynamic magma flow through the intrusion limited the potential to generate a large mass of ore.; Based on the Finnish examples, including the Otanmäki Fe-Ti-V ore, the geotectonic environment is probably less important in generation of a Ti-rich intrusion than the generation of a Ti-enriched parent magma by fractionation in a deep crustal reservoir. A low f_O2 environment favors ilmenite crystallization relative to Ti-magnetite. Similar to anorthosite-related ilmenite ores, formation of immiscible oxide melts, with or without fractional crystallization, may be important in generating ilmenite during crystallization of the mafic magmas. A magma-through-flow system in the shallow upper crustal environment is the critical process that enables the concentration of sufficient volume of oxides within small intrusions. The critical concentration of Ti is 4% TiO₂, the ratio of TiO₂/FeO₂ should be 0.23 to 0.50, and ilmenite should occur mostly as individual grains.