The boron isotope geochemistry of tourmaline-rich alteration in the IOCG systems of northern Chile: implications for a magmatic-hydrothermal origin

Abstract

Hydrothermal tourmaline is common in the iron oxide-copper-gold (IOCG) deposits of the Coastal Cordillera of Chile where it occurs as large crystals in the groundmass of magmatic-hydrothermal breccias, such as in the Silvita or Tropezón ore bodies, or as small grains in replacive bodies or breccia cement in the ore-bearing andesite, as seen at the Candelaria or Carola deposits. Tourmaline shows strong chemical zoning and has a composition of schorl-dravite with significant povondraite and uvite components. The observed boron isotope composition is fairly variable, between −10.4‰ and +6.0‰ with no major differences among the different deposits, suggesting a common genetic mechanism. The δ 11 B values are significantly lower than those of seawater or marine evaporites and very similar to those of younger porphyry copper deposits and volcanic rocks in the region, indicating that the boron has a common, likely magmatic, origin. The predominant boron source was ultimately dewatering of the subducting slab with a significant contribution derived from the overlying continental basement. The range of δ 11 B values is between those of the porphyry copper deposits and the porphyry tin deposits of the Andes, suggesting that the IOCG mineralization might be genetically related to fluids having more crustal contamination than the porphyry copper deposits; such an interpretation is at odds with current models that propose that the Andean IOCG deposits are related to juvenile melts or to the circulation of basinal brines. Furthermore, the obtained δ 11 B data are markedly different from those of the tourmaline in the Carajás IOCG district (Brazil), suggesting that IOCGs do not form by a unique mechanism involving only one type of fluids.

Tourmaline as a Recorder of Ore-Forming Processes in the Xuebaoding W-Sn-Be Deposit, Sichuan Province, China: Evidence from the Chemical Composition of Tourmaline

“…In hydrothermal ore deposits, tourmaline composition is controlled by bulk composition of the host rock, pressure, and temperature conditions of the system, and the composition of the hydrothermal fluid [29][30][31][32][33][34][35]. Tourmaline is also a common gangue mineral in many ore deposits (e.g., tin, tungsten, beryllium, gold, silver, copper, and uranium) and has been used to characterize the ore-forming processes [9,28,31,32,[36][37][38][39][40][41][42][43][44][45][46].…”

Discriminating fluid source regions in orogenic gold deposits using B-isotopes

“…It is for these reasons that the use of stable B-isotopes in tourmaline has become well established in the study of ore deposits (e.g. Palmer and Slack, 1989;Xavier et al, 2008;Garda et al, 2009;Slack and Trumbull, 2011;Tornos et al, 2012;Duncan et al, 2014;Molnár et al, 2016). While boron isotope analysis by thermal ionization mass spectrometry (TIMS) requires complete chemical dissolution of a bulk tourmaline sample, secondary ion mass spectrometry (SIMS) allows for texturally resolved measurement of δ 11 B in tourmaline at levels of precision that allow detailed analysis of fluid source reservoirs in relation to the mineral paragenesis.…”

The Productora Cu-Au-Mo Deposit, Chile: A Mesozoic Magmatic-Hydrothermal Breccia Complex with Both Porphyry and Iron Oxide Cu-Au Affinities

“…Whilst tourmaline is observed in IOCG deposits globally, it typically only occurs as an accessory phase (e.g., Ernest Henry, Contact Lake Belt, Mumin, 2007). Tornos et al (2010Tornos et al ( , 2012 described abundant tourmaline in Andean IOCG deposits; where it occurs as coarse grained breccia cement (e.g., Silivita and Tropezón) or fine grained replacements (e.g.,…”