Lecture 17 - General Procedures of Geochemical Exploration (Part 1) PDF

GLGY3764 Lecture #17 General Procedures of Geochemical Exploration Part 1 Dr. John Carranza Professor of Economic Geology Geochemical samples Geochemical samples can be collected in a non-systemat...

GLGY3764 Lecture #17 General Procedures of Geochemical Exploration Part 1 Dr. John Carranza Professor of Economic Geology Geochemical samples Geochemical samples can be collected in a non-systematic way (= grab sample collection), or in a systematic way such as over a sampling grid or regular interval.  Geochemical surveys can be carried-out in different scales.  Regional scale  Local scale  Detailed scale (mine- or deposit scale)  All solid geochemical samples can be analysed “totally” or “partly” or for specific minerals 2  Lag: coarse particles at the Common types of geochemical samples surface as a result of selective removal by erosion of fine material; hence, the preferred name when used in the Rocks geochemical context is "lag". Soils regular soils, lag material, other regolith material Drainage material stream sediments, lake sediments, heavy mineral concentrates, suspended fine material in water, water Water from surface and subsurface, from fluid inclusions in minerals Organic material dead organic material, in plants or living beings, in soils Gas in soils, in atmosphere, in minerals 3 Geochemical sample types 4 Factors in the selection of geochemical exploration methods 5 Check-list of a geochemical survey Item Check  Field Party  numbers, composition, experience, leader  Training  what, when, where, by whom  Base Maps  appropriate scale, topography, etc  Numbering Schemes  simple unambiguous, avoid complex alpha-numerics  Field Notes  make sure they are collected correctly  Quality Control  collect field duplicates, in the lab replicates + standards  Communications with  must be simple and direct and given only by designated Lab personnel  Shipping Lists  must accompany every consignment sent to the laboratory  Instructions for the Lab  give clear unambiguous instructions to the laboratory 6 Source: Levinson (1974) Check-list of a geochemical survey Item Check  Return of Data  check duplicates, standards, etc. for quality of analytical data, request re-analysis when in doubt  Data Handling  manual or computer aided, what is best for your project  Interpretation Maps  prepared to summarize geochemical data  Integration Field Notes  used to qualify interpretation of geochemical data  Storage of Data  Archive of Samples  need to be able to retrieve for re-interpretation  Integration with other  at laboratory, office or warehouse Exploration Procedures  ensure good communication with management and  Reporting other project personnel  author of report must be familiar with field program 7 Source: Levinson (1974) Steps in a geochemical exploration survey, part 1 Decide at end of each step whether to continue or not 1. General assessment of the area 2. Pilot study or orientation survey 3. Regional reconnaissance 4. Follow-up surveys 5. Systematic detailed surveys 6. Trenching, pitting, augering 7. Drilling 8 Steps in a geochemical exploration survey 1. General assessment stage Things to consider on : Mineral potential of the area Type of mineral deposit(s) Earlier exploration work Mining licenses in the area Type of exploration survey Specific local circumstances such as access, seasonal aspects but also political issues Costs and many more …. 9 Sample forms….... or modern field data collection 10 Steps in a geochemical exploration survey 2. Consider a pilot or orientation survey A pilot or an orientation survey is conducted in an area with known mineral deposit of the type sought. Its objectives are to determine answers to questions relevant to the planned geochemical survey for generating targets of the same deposit type.  How much, what fraction of sediments, etc. will be sampled?  What elements to analyse?  The most optimal / economic methods to analyse the samples?  Required field information?  Etc. etc. etc. A PILOT SURVEY ALLOWS US TO STANDARIZE OUR METHODS, AVOID CONTAMINATION AND THINK OF QUALITY CONTROL !!!!!! 11 Steps in a geochemical exploration survey 2. Consider a pilot or orientation survey Important elements to be analysed in a geochemical survey lithologic indicator elements pathfinder elements ore indicator elements 12 Lithologic indicator elements Example 13 Lithologic indicator elements See published mean concentrations of elements in different rock types 14 Pathfinder elements Required properties Easily dispersed around mineralisation, thus creating a wide dispersion halo  Sufficiently abundant, so that the halo is distinguishable from the surrounding background values  Not too problematic technically and financially to analyse Due to the improved cheap analytical techniques providing good quality multivariable data with very low detection limits, the use of pathfinder elements is nowadays less prominent. 15 Common pathfinder elements in exploration geochemical surveys 16 Relationship between pathfinder and ore-indicator elements Example of As as a pathfinder for Au ore 17 Steps in a geochemical exploration survey Decide at end of each step whether to continue or not 1. General assessment of the area 2. Pilot study or orientation survey 3. Regional reconnaissance 4. Follow-up surveys 5. Systematic detailed surveys 3 4a 4b 6. Trenching, pitting, augering 7. Drilling 18 Steps in a geochemical exploration survey 3. Regional reconnaissance The main objective of this step is to scan a region for districts that may contain mineralisation.  The samples usually collected in this step are steam sediments, as these have been 90 regarded as nature’s closest approximation to a composite sample of the rock and soil 40 upstream from a sample point (Nichol et al., 1969).  Sampling density varies greatly depending on type and size of deposits expected. 19 Stream sediment sampling Wet or dry stream sediment samples are a fast method for locating mineralisation or environmental pollution sources in a catchment 20 Steps in a geochemical exploration survey 3. Regional reconnaissance - example A stream sediment survey with sampling density of about 1 sample / km2 21 Steps in a geochemical exploration survey 3. Regional reconnaissance 22 Steps in a geochemical exploration survey 3. Regional reconnaissance As 23 Steps in a geochemical exploration survey 3. Regional reconnaissance At end of reconnaissance regional scale sampling: An area is selected for follow-up sampling programme to reconfirm the anomaly and for a quick check on the anomalous catchment. 24 Steps in a geochemical exploration survey 4. Follow-up … tomorrow’s lecture 25