Identifying prospective large concentrations of metal deposits, especially in remote areas, is often hindered by limitations and risk factors. They include blind sampling and the risk of over or under drilling due to undefined geological information for the area in question, as well as the delay involved in returning samples to a laboratory for elemental and mineralogic analysis. Additionally, the high costs associated with traditional methods of analysis often place further limitations on the number of samples analyzed. Mapping the occurrence of alteration minerals associated with a potential economic deposit is central to fine tuning the exploration process.

Economic deposits of copper, gold, molybdenum, and tin are often hosted in porphyritic intrusive rocks. Mineralogical alteration produced in conjunction with metal deposition results in alteration zones that extend well beyond the deposit. Thus, mapping these alteration zones forms a useful vector to the deposit. With the narrow seasonal window available for field exploration, the ability to refine and optimize the exploration program in real-time, while still in the field, can make or break a mining enterprise; or at the very least, potentially shave months or years off of realizing a return on investment.

Near infrared (NIR) reflectance spectroscopy based analysis easily and rapidly identifies many of the minerals that are key to classifying alteration. NIR spectroscopy is a consistent, cost-effective, and field-portable research-grade alternative technology that has been used worldwide for many years in the analysis of natural resources. As a rapid and reliable analytical technique it has proven to be the ideal tool for geologists and researchers in earth studies and environmental sciences.

• Lower risks
• Save time
• Save money
• Real-time—results in seconds not hours
• In situ measurements
• Little or no sample preparation required

ASD offers the mining industry’s premium solution for mapping alteration zone mineralogy of porphyry deposits. The TerraSpec® 4 Hi-Res is the ideal tool for identifying and characterizing many of the minerals associated with porphyry deposits. The field-portable TerraSpec works in the laboratory, in the field, or at a drilling site, providing the real-time information necessary to optimize your exploration process. The system can be used to develop a wide range of models for analyzing many sample types, including outcrops, hand specimens, core, RC drill chips, and metallurgical pulps—all with little or no sample preparation.

The identification of target alteration mineral assemblages is just the start. The subtle compositional and crystallinity variations that occur within an alteration zone provide additional information regarding the thermal and chemical environment at the time of alteration. Mapping alteration assemblages and the associated mineralogic variations helps unravel the complex sequence of alteration events associated with a particular deposit. By discovering the key alteration events associated with ore formation, mineral explorers can often define alteration assemblages that provide vectors to economic deposits, even when those assemblages have been over-printed by later alteration events.

Key alteration zones, along with their diagnostic minerals, include:

• Propylitic: chlorites, epidotes, zeolites, montmorillonite, illites, and carboates
• Potassic: biotite, phlogopite, chlorites, vermiculites, anhydrite, and gypsum
• Phyllic: illites, muscovite, and kaolinite
• Argillic: kaolinite, smectite/montmorillonite, illite, smectite, and halloysite
• Advanced argillic: phyrophyllite, dickite, alunite, zunyite, diaspore, and topaz
• Leached cap (oxidized zone): alunite, kaolinite, illite, diaspore, iron oxides, copper suphates, hydroxides, and jarosite

In addition, reflectance spectra of several compositionally variable mineral groups—such as the pyroxenes, amphiboles, chlorites, and illites—show systematic shifts in feature positions that allow identification of specific compositional members of these groups. The TerraSpec is versatile enough with a wide enough spectral range (350 – 2500 nm) to identify all of these mineral groups and compositional varieties.