are earth elements (REE) are becoming increasingly important in modern society due to their numerous
uses in manufacturing of components for green and high-tech energy industries. Studying the mechanisms
of REE mineral formation in geologic systems is vital for understanding where and how these mineral
deposits form. Previous studies of REE mineral deposits have shown that hydrothermal fluids can play a
key role in the mobilization and enrichment of REE (Williams-Jones et al., 2000; Gysi et al., 2016;
Vasyukova and Williams-Jones, 2018). Fluorite is ideal to study the behavior of REE because of their
compatibility in its structure and it is a ubiquitous hydrothermal vein mineral found together with REE
fluorocarbonates (i.e., bastnäsite and parisite). However, the controls on hydrothermal fluid-mineral REE
partitioning in these deposits are not yet fully understood.
In this study, we present petrographic observations of fluorite veins and fluid inclusions from the Gallinas
Mountains REE-bearing fluorite veins/breccia deposit in New Mexico (McLemore, 2010; Williams-Jones et
al. 2000). The Gallinas Mountains deposit notably contains hydrothermal fluorite and bastnäsite, and is
associated with ~30 Ma alkaline igneous rocks intruded into Permian sedimentary rocks (McLemore, 2010).
The goal of this study is to better understand the cause of REE variations in fluorite as a function of
temperature and salinity of the fluids, and to determine how the REE concentrations change in barren and
mineralized veins. Optical microscopy and cold-cathode cathodoluminescence (CL) is used to distinguish
different fluorite generations and fluid inclusion types. Scanning electron microscopy (SEM) is used to
identify REE minerals, zonation in fluorite, and acquire elemental compositions of different vein minerals.
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Lithogeochemical Vectors and Mineral Paragenesis of Hydrothermal REE-Bearing Fluorite Veins and Breccia Deposits in the Gallinas Mountains, New Mexico
The Gallinas Mountains district located in Lincoln and Torrance Counties, New Mexico, is host to
hydrothermal REE-bearing fluorite veins and breccia deposits. The rare earth elements (REE) are
found in bastnäsite-(Ce) ([La,Ce]CO3F) which is also the primary ore mineral mined in several
important carbonatite deposits (e.g. Mountain Pass in California; Bayan Obo in China). Minor
production of REE, fluorite, Cu, Pb, Zn, Ag, and Fe has been recorded in the Gallinas Mountains
district between the early 1900s and the 1950s. The REE-bearing fluorite veins and breccias are
hosted in Permian sedimentary rocks as well as genetically related trachyte/syenite sills and
dikes emplaced between 28-30 Ma. Previous studies have described the REE occurrences in the
Gallinas Mountains but the controls of hydrothermal processes on the transport and deposition of
REE in the district remain unclear. In this study, we combine microtextural observations with
mineral and bulk rock chemistry of hydrothermal REE-bearing fluorite veins and breccias to
determine the vein types, alteration styles and establish a detailed mineral paragenesis. The
goal of this study is to determine lithogeochemical vectors towards REE enriched zones in the
district by linking thin section and deposit scale observations with mineral and bulk rock
geochemistry. This district is an exceptional natural laboratory for studying the role of
hydrothermal processes for transport/deposition of REE in an alkaline F-rich magmatic-
hydrothermal system because very few deposits worldwide have such well-preserved and
exposed geology.
Hand samples of hydrothermal veins and breccias containing fluorite ± calcite ± barite ±
bastnäsite-(Ce) were collected from outcrops, prospect pits, and mine dumps. Optical microscopy
was used to identify minerals and determine the textural features and crosscutting relationships
of the different fluorite veins. The veins were classified into: i) hematite-fluorite veins; ii) barite-
bearing bastnäsite-fluorite veins; iii) barite-bearing (fluorite)-calcite veins. Nearly all of the barite
crystals in the fluorite veins display dissolution textures (skeletonized crystals) with infilling of
mostly fluorite and minor calcite, suggesting that barite is part of an earlier paragenetic mineral
assemblage. Bastnäsite-(Ce) is commonly found in veins containing barite and occurs either as
disseminated crystals in the fluorite veins or together with fluorite infills around large barite
crystals. A few of the barren fluorite-calcite veins display an intergrowth with euhedral barite
crystals indicating that these could be part of an earlier barite paragenesis. These textural
observations suggest a key control of REE mineralization in the Gallinas Mountains district by a
coupled dissolution of barite-bearing (fluorite)-calcite veins and precipitation of later bastnäsite-
fluorite veins. Geochemical bulk rock data collected from the New Mexico Bureau of Geology and
Mineral Resources database were analyzed using the IMDEX ioGASTM program to definegeochemical signatures of rock types, alteration styles, and vein types. Preliminary data analysis
indicates a positive correlation between Ba, F, and total rare earth oxides (TREO). These trends
corroborate with the observed vein microtextures, suggesting that the interaction of a
hydrothermal fluids with the barite-bearing (fluorite)-calcite veins represents a key process for
defining geochemical vectors in the district.
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- Award ID(s):
- 2039674
- PAR ID:
- 10321002
- Date Published:
- Journal Name:
- SEG 100
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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