Supporting Information A. Wideband pulses The bandwidth of standard monochromatic NMR / NQR pulse sequences can be increased by modulating amplitude and phase (time-dependent field strength B1 and phase 0 ). In the following, we summarize the principle characteristics of the WURST (wideband, uniform rate, smooth truncation) pulse as discussed in [O’Dell et al., 2013] in detail. Considering the RF field 1 B1 the adiabatic condition can be quantified by the factor: Q0 (1,max ) 2 k1 1. (A-1) The RF field is modulated by varying the amplitude (with the modulation parameters β and N): 1 (t ) 1,max [1 sin( t ) ] , N (A-2) and incrementing the RF pulse phase with the sweep rate k 2F / according to: 1 2 (t ) 0 kt 2 . (A-3) The pulse bandwidth F is given by: F (1,max ) 2 , 2 Q0 (A-4) where the parameter defines the pulse duration of the pulse. The maximal magnetic field ω1 at the center of the coil is limited by the available RF current I RF and dimension of the coil. The power requirement P of the resonator (e.g. solenoid) is given by [Mispelter and Briguet, 2009]: P 2 I RF , 2Q0C (A-5) where Q is the quality factor of circuit, C the parallel tuning capacitor in the circuit and 0 is the spectrometer frequency. The RF current (a linear scaling factor: I RF B1 / Bˆ1 ) in a solenoid coil using a wideband pulse is: wide I RF Q0 k , Bˆ (A-6) 1 where B̂1 is the RF field at unit current and Q0 is set (for example) to Q0 2 0.25 to fulfil the adiabatic condition. In contrast, the RF current for a monochromatic pulse is: mono I RF , Bˆ1 (A-7) Substitution of Eq. (A-6) into Eq. (A-4) and using k 2F / we obtain the wideband pulse power relation: P F , Bˆ12 (A-8) The monochromatic pulse power relation (by assuming F 1/ which is an approximation for the frequency transform of the pulse function) in contrast becomes: F 2 . (A-9) Bˆ12 The typical excitation bandwidth of conventional monochromatic RF pulses is limited to a range of about 100-200 kHz by currently achievable RF amplitudes and quality factors Q of (small-volume) laboratory NMR/NQR probes. For large-volume sensors B̂1 (RF field at unit current) become in general smaller and peak power requirements increase dramatically for equal values of F . Hence, the bandwidth of the large-volume sensor has to be reduced to several tens of kHz to meet the power limitations. P In our study, wideband pulses can reduce the peak power requirements for large-volume sensors when operated with large F . The power requirement is linearly dependent on F (Eq. (A-8)). The peak power can be reduction by increasing the pulse duration without F reduction. Moreover, the wideband pulse sequence can even be operated in the non-adiabatic regime and excite frequency ranges of up to several hundred kHz, far exceeding the bandwidths of standard, fixed-frequency RF pulses. B. XRD analysis X-ray Diffraction (XRD) is one of the primary techniques used by mineralogists to examine the physical/chemical make-up of minerals in geochemical laboratories. The narrowness of the XRD lines gives qualitative evidence of purity and crystal perfection. We obtained XRD analysis of all samples discussed in the article and data are shown in Fig. S1-S4. Figure S1: XRD data of specimen FeAs2 showing well crystallised material. All the sharp peaks belong to FeAs2 apart from the peak marked * (possibly FeS troilite). Figure S2: XRD data of specimen FeAsS showing well crystallised material and all the sharp peaks belong to FeAsS. Figure S3: XRD data of specimen As4S4 showing well crystallised material. Figure S4: XRD data of specimen As2S3 showing well crystallised material. C. Detection limit The NQR measurements are based on the XRD validated samples (see Section B in the Supporting Information). In this initial study we calculated the 3-σ detection limit (DL) (expressed as the percentage weight of arsenic and assuming a sensor completely filled with ore) over a 60 s acquisition period as follows: DL3 100 (3 M As / SNR V ) , SNR is the measured signal to noise ratio of a test sample obtained in the given acquisition period, ρ is the bulk ore density, V the available coil volume and MAs the known mass of arsenic associated with the target mineral phase in the test sample.