Formation of an Fe-Ti Deposit Within an Open-System Magmatic Chamber: Insights from the Storgangen Deposit, Rogaland Anorthosite Province, Southwest Norway

The Storgangen deposit is located in the Rogaland anorthosite province, Norway, and forms an E-W–trending, 4-km-long, and up to 60-m-thick sheetlike intrusion. We have combined field and petrographic observations, whole-rock and mineral compositional variations, and Sr isotope composition through a profile across the deposit. The predominant lithology is a medium-grained, magnetite-ilmenite–rich norite, which is massif at the base and becomes gradually more fine layered upward in the stratigraphy. Although ilmenite is the predominant oxide, there is a decrease in ilmenite proportions relative to magnetite upward in the stratigraphy, which leads to lower whole-rock Ti/Fe ratios in the sampled profile. Mineral compositional variations display a decrease of compatible elements in ilmenite and magnetite (e.g., Co, Cr, V) and in Mg# in orthopyroxene upward in the stratigraphy. These compositional trends support a progressive upward fractionation during the formation of the deposit. Based on the composition of oxides, we estimate that the orebody developed following only about 40% crystallization of the parental magma and preferential accumulation of Fe-Ti oxides at the bottom of the magmatic system due to gravitational segregation. The Storgangen deposit represents a staging chamber within the Rogaland anorthosite province that was part of a larger system. In situ Sr isotope ratios in plagioclase display a slight stratigraphic variation, with 87/86Srinitial varying from 0.70558 to 0.70602, which is greater than values from the host anorthosite in the vicinity of the deposit of 0.70540 to 0.70554. Thus, if contamination occurred, it may have taken place within a deeper staging chamber prior to emplacement. Alternatively, the Sr isotope signature may be inherited from variably contaminated parental primitive jotunites previously described in the Rogaland anorthosite province. Our results support that Fe-Ti-P deposits from the Rogaland anorthosite province likely define a compositional trend from high- to low-Ti/Fe systems, with the Storgangen deposit displaying relatively intermediate Ti/Fe ratios. Such a trend has been proposed for the Central Grenville province and may thus be a global feature of Fe-Ti-P deposits associated with massif-type anorthosites.