Dusty star-forming galaxies (DSFGs) detected at $z > 4$ provide important
examples of the first generations of massive galaxies. However, few examples
with spectroscopic confirmation are currently known, with Hershel struggling to
detect significant numbers of $z > 6$ DSFGs. NGP6_D1 is a bright 850 $\mu m$
source (12.3 $\pm$ 2.5 mJy) with no counterparts at shorter wavelengths (a
SPIRE dropout). Interferometric observations confirm it is a single source,
with no evidence for any optical or NIR emission, or nearby likely foreground
lensing sources. No $>3\sigma$ detected lines are seen in both LMT RSR and IRAM
30m EMIR spectra of NGP6_D1 across 32 $GHz$ of bandwidth despite reaching
detection limits of $\sim 1 mJy/500 km~s^{-1}$, so the redshift remains
unknown. Template fitting suggests that NGP6_D1 is most likely between $z =
5.8$ and 8.3. SED analysis finds that NGP6_D1 is a ULIRG, with a dust mass
$\sim 10^8$ - $10^9$ $M_{\odot}$ and a SFR of $\sim$ 500 $M_{\odot}~yr^{-1}$.
We place upper limits on the gas mass of NGP6_D1 of $M_{H2}$ $ < (1.1~\pm~3.5)
\times 10^{11}$ $M_{\odot}$, consistent with a gas-to-dust ratio of $\sim$ 100
- 1000. We discuss the nature of NGP6_D1 in the context of the broader submm
population, and find that comparable SPIRE dropouts account for $\sim$ 20% of
all SCUBA-2 detected sources, but with a similar flux density distribution to
the general population.