PRELIMINARY TEXTURAL ANALYSIS TO CONSTRAIN THE FORMATION (OR DEFORMATION?) OF TINTON PEGMATITES, NORTHERN BLACK HILLS, SOUTH DAKOTA

Lithium-cesium-tantalum (LCT) pegmatites are important resources of rare metals including lithium (Li) used for energy storage and tantalum (Ta) as a key component of electronics. By 2050 the world’s demand for lithium is expected to increase by 965%. Therefore, reliable strategies for exploring and locating new LCT pegmatite deposits are critical. The purpose of this petrographic study is to decipher the primary ore-forming processes and subsequent deformation history of poorly studied fine-grain spodumene-quartz bodies from Giant Volney and Rough & Ready mines, Tinton pegmatite district, Lawrence County, in the northern Black Hills, SD. Typical Tinton spodumene crystals are <5 mm long, unlike >1 dm long spodumene from Etta or Tin Mountain pegmatites, in the southern Black Hills.

Deposits of the Tinton Pegmatite District were historically mined for Sn. More than 105,000 lbs of Sn were produced from the cassiterite-rich pegmatite bodies. Modest quantities of Li and feldspar were extracted during World War II. Tinton is now a ghost town, but more recently, an open pit operation produced Ta ± Nb and further prospecting for Li is currently underway. The geologic literature on Tinton pegmatites is very scarce and outdated. Questions remain whether there is spodumene-quartz-intergrowth (SQI) at Tinton, formed from isochemical breakdown of petalite (Norton, 1982), which would imply a high purity Li ore.

We have documented the mineral assemblages, microtextures, and compositional zoning in relatively undeformed and highly sheared samples, using Olympus Streams for microphotographs in transmitted light and cathodoluminescence (CL), as well as Scanning Electron Microscopy. Less-deformed Li-rich samples are comprised of <10 cm long spodumene, quartz, muscovite, and albite. Likewise, <1 m amblygonite lenses and massive microcline core in the field appear to be of magmatic origin. No petalite was found to date. CL imaging reveals that spodumene in samples with gneissic banding is largely overprinted by dissolution/reprecipitation. However, relict oscillatory zoning with orange-purple CL hues resemble spodumene from true SQI (Dibs, Southern Manitoba). Further quantitative image and chemical analyses are necessary to determine the origin of the Tinton spodumene and associated Sn-Ta-Nb mineralization.