PRELIMINARY TEXTURAL ANALYSIS TO CONSTRAIN THE FORMATION (OR DEFORMATION?) OF TINTON PEGMATITES, NORTHERN BLACK HILLS, SOUTH DAKOTA
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 The geologic literature on Tinton pegmatites is very scarce and outdated.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.