The Composition of Gahnite In Granitic Pegmatites From the Pampean Pegmatite Province, Argentina: Implications For Pegmatite Fractionation

Abstract

Gahnite from LCT granitic pegmatites of the Comechingones (Blanca Dora, Juan Román, Magdalena, La Ona, and Sin Nombre pegmatites) and Conlara (Nancy pegmatite) pegmatite districts, Argentina, was analyzed to investigate the relationship between its chemistry, pegmatite mineralogy, and pegmatite melt evolution. Based on the molecular proportions of endmembers (gahnite, hercynite, spinel, and galaxite) the composition of gahnite in these pegmatites is defined by the ranges Ghn 77.7-91.0 Hc 7.4-20.0 Spl 0.25-3.37 Glx 0.7-2.4 , which fall within the previously defined field for granitic pegmatites. Gahnite from the Nancy pegmatite has higher Mg and Mn contents than gahnite from the pegmatites of the Comechingones pegmatite district. Chemical variation within gahnite crystals is characterized by higher Zn (∼2.8 wt.% ZnO) and lower Fe (∼2.6 wt.% FeO) contents near the edge compared to inner portions, reflecting the evolution of the pegmatite melt via simple fractional crystallization. Other crystals show a complex zoning pattern of oscillations in Fe and Zn contents within the crystal and higher Zn and lower Fe near the rim compared to the core. A plot of molecular Fe 2+ + Mg versus Zn + Mn yields a strong negative correlation and best displays the substitutions within the gahnite crystal structure and is the best to determine relative pegmatite melt evolution. Without implying a co-magmatic origin of the pegmatites, based on pegmatite mineralogy, Zn contents, Zn/Fe 2+ ratios, and coupled Fe 2+ +Mg and Zn+Mn values in gahnite, the relative degree of evolution of the pegmatites increases in the order: Sin Nombre → Magdalena → Juan Román → Blanca Dora → Nancy → La Ona. Based on the composition of gahnite in worldwide granitic pegmatites reported in the literature and those obtained here, the composition of gahnite in these rocks is defined by the endmember ranges Ghn 50-98 Hc 1-50 Spl 0-7 . This study shows that the major element composition of gahnite in granitic pegmatites can effectively be used to determine the relative degree of evolution of pegmatite-forming melts.

Rare element minerals’ assemblage in El Quemado pegmatites (Argentina): insights for pegmatite melt evolution from gahnite, columbite-group minerals and tourmaline chemistry and implications for minerogenesis

“…7), together with very low Mg and Mn contents, indicating that they formed from highly differentiated melts. These melts were, indeed, more evolved than the Pampean Range granitic pegmatites from central Argentina studied by Heimann et al (2015).…”

“…Gray field corresponds to Nb-Ta oxides from Totoral LCT rare-element pegmatite district, San Luis, Argentina (Galliski et al 2019). Gray arrow suggests the fractionation trend of Ta/(Ta + Nb) vs. Mn/(Mn + Fe) of columbite group minerals in El Quemado pegmatites Heimann et al (2015) concluded that the major element composition of gahnite in granitic pegmatites can effectively be used to determine the relative degree of evolution of pegmatite-forming melts. The composition of gahnite in the Santa Elena pegmatites is characterized by very high Zn content and Zn/Fe 2+ ratios (Fig.…”

“…In particular, the major element composition of these minerals has the potential to reveal the degree of fractionation of pegmatite melts, as well as to track their geochemical evolution up to the late magmatic stages (Batchelor and Kinnaird 1984;Breaks et al 2005;Trumbull et al 2009;Trumbull and Chaussidon 1999;Badanina et al 2015). Consequently, many studies have provided models of pegmatite evolution based on the fractionation of some key elements in these accessory minerals (Černý et al 1986;Novàk et al 2003;Chudík et al 2011;Van Hinsberg et al 2011;Marks et al 2013;Heimann et al 2015;Feng et al 2019;Zhou et al 2019;Fuchsloch et al 2019). The potential for pegmatite rocks to concentrate rare element minerals of economic interest enhances the importance of determining the chemical composition of their mineral phases and of understanding their petrogenetic history.…”

“…Fig.7Binary Zn vs Fe 2+ (apfu) diagram of gahnite from El Quemado pegmatites. Dark gray fields show the composition of gahnite from granitic pegmatites of the Pampean Pegmatite Province, Argentina(Heimann et al 2015); Light grey field shows the composition of gahnite from granitic pegmatites of the Borborema Pegmatitic Province, Brasil(Soares et al 2007) …”

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

“…Gahnite (ZnAl 2 O 4 ), the Zn end-member of the spinel group of minerals ([Zn,Fe,Mg]Al 2 O 4 ), occurs as an accessory mineral in granitic pegmatite dikes (e.g., Nêmec 1973;Batchelor and Kinnaird 1984;Spry and Scott 1986b;Morris et al 1997;Heimann et al 2015). In addition, gahnite is a common accessory mineral in metamorphosed massive sulfide deposits and Zn-rich hydrothermal alteration zones, a rare accessory phase in marbles, metabauxites, skarns, and aluminous granulites, and even rarer in banded iron formations (e.g., Spry and Scott 1986a, b;Heimann et al 2005Heimann et al , 2006.…”