Tracing mineralogy and alteration intensity using the spectral alteration index and depth ratios at the Northwest Zone of the Lemarchant VMS deposit, Newfoundland, Canada

The use of hyperspectral reflectance in mineral exploration has been steadily increasing in recent decades. This study presents a novel approach that integrates geochemical and spectral proxies to delineate ore formation and alteration processes, which provide new spectral-based exploration parameters that can be used in real time. The precious metal-bearing, bimodal-felsic Northwest Zone of the Lemarchant VMS deposits, Newfoundland, Canada is used as a case study.

Alteration associated with the Northwest Zone includes intense and localized sulfide (pyrite, chalcopyrite, sphalerite and galena) and barite enrichment, and quartz, white mica and chlorite alteration. Zones of elevated Zn (> 5000 ppm) are associated with high chlorite carbonate pyrite index (CCPI), Ishikawa alteration index (AI), Ba/Sr, and low Na₂O values and elevated SiO₂ and K₂O, Fe₂O3, Na₂O, and BaO contents, similar to global alteration signatures in VMS deposits. Mineralized areas contain phengitic white micas with 2200 nm absorption features longer than 2215nm and Mg-rich chlorites with 2250 nm absorption features shorter than 2252nm. Together, these data are consistent with the Northwest Zone having experienced intense hydrothermal alteration during the mineralization event.

A new lithology normalized spectral alteration index (SAI) for white mica and chlorite was developed in order to map and characterize the alteration intensity surrounding the deposit. In addition, depth ratio parameters (2200D/2340D vs 2250D/2340D) were used to characterize mineralogical changes and zonation. Together, these features document a paleo-fluid pathway with Mg-chlorite alteration extending to at least 300 m away from the mineralization, outside the study area, within the andesitic and dacitic units.

This study demonstrates that the use of VSWIR spectral reflectance data coupled with geochemical alteration proxies (i.e., AI, CCPI, Ba/Sr, Na₂O) and lithogeochemical mass balance changes can identify and characterize alteration haloes and paleo-fluid pathways in the vicinity of VMS deposits. More specifically, hyperspectral reflectance can identify and quantify areas of intense alteration using spectral alteration indexes (SAI), estimate the relative abundances of white mica and chlorite using depth ratios, and characterize the chemical composition of the mineral phases, and relate them to specific alteration processes, which is not possible using only geochemistry. One of the main advantages of this method is that hyperspectral reflectance can be rapidly achieved on drill core at a high resolution for a relatively low cost, minimal sample preparation and results are available instantly, compared to a longer wait time for geochemical results, greatly enhancing decision making processes during drilling exploration programs, allowing vectoring and rapid decisions making during exploration programs.