Volcanogenic massive sulphide deposits generally have zoned mineral alteration systems related to the hydrothermal circulation cells that led to their formation. In ideal situations it is possible to use field observations of such alteration to vector toward potential ore horizons. However, in many cases, such reasoning relies on subsequent detailed petrographic and lithogeochemical investigations; but the fine-grained and cryptic nature of much of the alteration may be an obstacle. A relatively new technique that may alleviate such delays is the use of portable reflectance spectrometers that provide visible/infrared spectroscopy (VIRS). This allows field identification of alteration minerals and compositional inferences that may constitute vectors to potential ore horizons.
Visible/infrared spectroscopy methods were applied to two of the volcanogenic massive sulphide deposits in the Tulks Volcanic Belt of central Newfoundland, namely the Daniels Pond and Boomerang deposits. Both deposits have ubiquitous white mica and chlorite alteration, and mineralogical zonation in the field is poorly developed. Identification of such zonation is further hindered by the inherent lack of outcrop in the vicinity of the deposits.
Results from both deposits, and regional rock sample suites, illustrate a distinct shift in the wavelengths of the Al-OH absorption feature in white micas from distal to proximal environments. The Al-OH absorption feature in white micas distal to ore is generally at >2000 nm (i.e., muscovite to phengite composition), whereas the absorption feature shifts to lower wavelengths (<2000 nm) in samples proximal to ore (i.e., paragonite composition). The VIRS data thus record subtle variations in alteration mineral chemistry that would otherwise be difficult to detect, and which may provide a directional indicator (vector) to favourable horizons.