X-ray powder diffraction (XRD) is a well-established tool in the study of hydrothermal systems, as it allows for the identification and quantification of mineral assemblages. Through the quantification of alteration mineral assemblages geologists can characterize the geometry of a deposit or geothermal system and draw inferences regarding the fluid evolution and environmental conditions of deposition (e.g., pH, T). Traditionally, XRD devices have largely been restricted to laboratories; however, advances in XRD sample holders and X-ray sources have allowed for the development of portable XRD (pXRD) devices. This paper assesses the validity of the Olympus Terra pXRD instrument for qualitative and quantitative studies of hydrothermal systems through comparisons with data from laboratory XRD (Empyrean II diffractometer) and XRF techniques for both synthetic mixtures of natural minerals and a variety of samples from the Kulumadau epithermal gold deposit, Woodlark Island, Papua New Guinea. Diffractograms of synthetic mineral mixtures with known concentrations of quartz, kaolinite, muscovite, albite, and pyrite were analyzed quantitatively using the Rietveld-based Siroquant technique and showed good overall agreement for both devices, with the exception of muscovite, which encountered accuracy issues at concentrations <10% and a lower level of detection of around 5% for the pXRD. The Siroquant analyses of hydrothermally altered rocks based on XRD traces from the Empyrean II unit were confirmed by XRF data. Results from the pXRD unit for collection times of 5, 10, 20, and 40 minutes for representative samples spanning a range of rock types and alteration styles revealed that 5 minutes was sufficient for qualitative analysis, even of minor phases. Overall, the 5-minute collection time also yielded excellent quantitative results, but precision for minor mineral phases increased noticeably with increasing collection times. Quantitative mineral estimates for 20- and 40-minute data sets were compared directly to estimates made using the Empyrean II data and showed excellent correlation with R2 values of >0.92 for all major mineral phases (i.e., >5%). With the exceptions of pyrite and magnetite, minor to trace minerals (i.e., <5%) quantified using the Empyrean II XRD showed poor correlation with the pXRD data; however, the presence of these minerals was identified in greater than 50% of all cases. Due to its portability, robustness, minimal sample preparation, relatively fast collection times, and excellent correlation with laboratory-based XRD devices, the pXRD has been shown to be of great use for rapid acquisition of quantitative mineralogical data by the exploration geologist, allowing for more informed decisions during drilling programs.