Rock Sampling

Rock sampling reveals the true potential of an area for containing a mineral deposit.  An anomaly in a rock sample from bedrock has had no effects of secondary dispersion, so the location of the sample is the location of the source.  A rock sample anomaly will provide much more valuable information about the location of the mineral deposit because its source is within the mineralizing system, ie, it helps delineate the zone of primary dispersion.  However, this applies only to rock samples collected from bedrock.  Rock samples of float (rock material suspended in colluvium with no indication of proximity to the bedrock source), talus, glacial material, etc... give no indication of location of the source, so even if they are highly mineralized, they are of limited value.  Rubble (rock material suspended in colluvium and due to consistency or other information suggests proximity to the bedrock source) in some cases may be worthwhile to sample.

Several different types of rock samples are collected for mineral exploration.  Most importantly, rock samples are collected to determine the concentration of metals, including both the major and trace metals.  This type of sample is most commonly referred to as a “geochem” sample.  Trace metal values are often useful as “pathfinders”, which means they are closely associated with the metal of interest and may occur within a halo surrounding the mineralization of interest. 

Rock geochem samples are collected in different manners depending on the goal of the sampling.  The principle types include:

• Grab Samples:   A grab sample is a sample of rock material from a confined area (< 1 foot across).  It can be a single piece of rock.  These are the most common types of samples collected.  If it is not specified otherwise, one usually assumes that is the sample type.  The sample usually consists of a single piece of rock, or chunks, which are representative of a specific type of rock or mineralization.

• Composite Samples:  A composite sample consists of small chips of uniform rock material collected over a large area (generally > 5 feet across).  These are the ideal “representative” samples.  The procedure is to collect small pieces of rock over a large area (usually at least 10 feet across) and to make the sample as homogenous as possible.  A composite sample might be collected to determine the background values of trace elements in a particular type of rock, or to determine if ore grade mineralization is present over a large area.

• High Grade Samples:   A high grade sample consists of selective pieces of the most highly mineralized material, in which an effort is made to exclude less mineralized material.  Consequently, a high grade sample is generally not representative of the overall mineralization type.   A high grade sample might be collected to get an idea what the best possible values are, or to provide material for certain types of trace element analyses.  If a such a selective sample does not return good results, then it is unlikely that valuable mineralization is present.  When a high grade sample is collected it is important to note that it is a high grade sample so its values will not be misinterpreted as representing the “average” values.

• Chip Channel Samples:  A chip channel sample consists of small chips of rock collected over a specified interval.  The objective is to obtain the most representative sample possible for the specified sample interval.  Most of the time chip channel samples are collected in succession along a sample line which is laid out in advance using a tape.  This provides a great deal of information about the width and other aspects of the geometry of a mineralized zone.  Often the chip channel samples are collected along the floors or walls of trenches or adits.  When chip channel sampling along walls, sometimes a piece of canvas or plastic is laid out for the material to fall on so as to avoid contamination and make the collection easier.  The freshest material possible is sampled, preferably chipping directly from bedrock.  Sample intervals are set at a specified width, usually ranging from 1 to 20 feet.  For example, in a five foot interval, at the end of the first foot, 20 % of the sample bag should be filled, at the end of the second foot the bag should be filled to 40 %, etc...  Due to the method of sampling, chip channel samples tend to be rather large (up to 20 pounds for a five foot interval).

Several other types of rock samples are sometimes collected to help interpret the history of mineralization in an area, to better understand the relationships between different ore minerals, or to determine more detailed geochemistry.  These types of samples are often collected to evaluate the mineralization in a regional context, or to compare the mineralization with models which might apply to a given situation.  Although they can might be costly, the information they provide can be invaluable.  Some of these sample types include:

• Whole Rock Major Oxide Samples:  Whole rock major oxide samples are most often collected to study the whole rock geochemistry of plutonic and volcanic rocks.  The sample must be completely fresh, unweathered, and unoxidized.  If necessary the weathered rind must be removed by chipping or by using a rock saw.  Samples must also be unaltered by hydrothermal alteration (this adds new components and removes others, such that it will no longer represent the parent magma composition).  The sample is analyzed for the principle oxides, including, SiO2, Al2O3, CaO, Fe2O3, FeO, K2O, MgO, MnO, Na2O, P2O5, TiO2.  Usually at least 98 % of the rock is made up of minerals comprised of some combination of these components.  Not uncommonly igneous rocks contain up to 1 % water.  This water is lost when the rock is oxidized in the furnace (referred to as LOI or “loss on ignition”).  Major oxide analyses are used to classify igneous rocks based on their chemical composition.  These can be used to compare intrusions within a district or to use in regional studies by comparing the analyses with those for known models.
  
  
• Age Date Samples:   Age date samples are used to determine the age of the rocks.  There are several methods, including 40Ar/39Ar, U/Pb, K/Ar, Rb/Sr, and Carbon 14.  They are all based on the half life theory, which states that certain isotopes of certain elements decay to radioactive daughter products at a specific rate, called a decay constant.  Knowing the constant, the amount of parent and daughter product material in the sample is measured and then used to calculate the age of the rock.  The 40Ar/39Ar method can provide reliable age dates up to several hundred million years.  Argon gas forms by decay of potassium and gets locked in the crystal lattice.  The U/Pb method is also quite reliable, and can be used to date rocks up to billions of years old.  Older rocks have longer histories, and during those longer histories more events can occur which cause problems.  For example, metamorphism and tectonic activity.  These can cause opening of the crystal lattice of the mineral being dated, and loss of the daughter product material, causing erroneous results.  Typically these effects cause the methods to yield ages which appear to be younger than the actual age of the rock.  Minerals can also obtain overgrowths during remelting events, causing excess parent material to be present, also making the rock appear younger.  Ar-Ar and U-Pb age dates can be obtained can be obtained from very small amounts of material.  The procedure involves separating the grains of one mineral type to be dated.  Ar-Ar age dates are usually obtained on minerals such as mica or hornblende.  U-Pb age dates are usually obtained on zircon or other accessory minerals which are known to contain small amounts of uranium.

• Petrographic Samples:   Petrographic samples are collected to conduct thin section petrographic analysis of the rock, which is the identification and evaluation of the minerals comprising the rock by using a microscope equipped with both plane and polarized light.   A thin section is made of the rock, which is a  paper thin slice of the rock mounted on a glass slide.  Different minerals have different optical properties when the plane light or polarized light is transmitted through the thin section.  Textural relationships also become apparent, which provides information about the order of crystallization (or paragenesis).  The proceedure is to cut a flat side and use special epoxy to glue the piece of rock called a plug, to the glass slide.  Thin a special trim saw cuts off the part opposite the glass.  Then the rock wafer is polished with special grinders to achieve the desired thickness.  The thickness must be very precise to compare the optical properties with known standards. 

• Fluid Inclusion:   Fluid inclusion samples are typically samples of quartz (others include fluorite, sphalerite or tourmaline).  The samples are prepared similar to a thin section, and examined using a special microscope equipped with a heating stage.  The inclusions can contain solid, liquid or gas, or any combination of these.  The inclusions are formed when they are trapped on the surface as a new layer of the mineral crystallizes.  As the mineral cools down, the phases separate.   The sample is heated gradually while being examined under the special microscope to find the temperature at which the gas or solid crystal in the fluid inclusion will goes back into solution.  This provides valuable information about the temperature and pressure of formation of the ore forming fluids.

• Polish Section:   to look at reflected light properties of ore minerals; ie, sulfide and oxide minerals.
  
  
• Microprobe:    highly sophisticated method to determine mineral compositions and textures using electron beams.

Trench/Adit Mapping

Trench or adit mapping is the process of creating a geologic map, which shows the geology of the floor and walls of the trench or adit.  Adit mapping emphasizes mapping of the walls more than the floor because the floor is often poorly exposed due to the presence of a layer of debree which results from blasting and mucking.  Trench mapping emphasizes floor mapping because:  1) the floor is usually scraped as clean as possible with a dozer or backhoe, and 2) because floor mapping shows a “map view”.  Trench or adit mapping always involves setting up a base line using a tape.  Footage or meter marks are then painted or flagged and labeled.  The base line and footage marks are then drawn to scale on the map page to facilitate mapping.  Often the same base line is used to accomplish a chip channel sampling program.

One approach is to first draw the outline of the floor, which will be oriented with respect to true north and drawn to scale.  The geology of the floor is then mapped just as an ordinary geologic map is made.  The corner of the trench or adit matches the edges of the strip showing the geology.  This is the “map view” (looking straight down) of the geology of the floor.  The edges of the “strip map” represent the two bottom corners of the trench.  The walls of the trench or adit are mapped adjacent to the strip map such that the right wall is mapped as if looking at the vertical on the right, and the left wall is mapped as if looking at the vertical wall on the left.  These can be labeled to indicate they represent the geology of the walls, even though it is usually obvious.  This gives a 3-D perspective of the geology, which greatly facilitates the interpretation of the geometry of features.  For example in determining the dip of layers, faults, joints, etc... on the floor of the trench, it is useful to show where the feature trends as it intersects the adjacent walls.  Structural measurements can be put directly on the map, in notation form next to the appropriate footage mark.

Another simpler approach used to make mapping more rapid is to sketch the floor outline at a standard, average width and not worry about the exact width.  The outline is drawn parallel to the edge of the map sheet without regard to actual geographic orientation.  The azimuth of the axis of the trench or adit floor is carefully measured and noted on the map.  If the trench or adit contains bends, then the new orientation is noted at the appropriate footage mark on the map.

The alteration style can be added to one side or the other of the map if desired.  The alteration can be mapped using colors, patterns or other designators, in the same way the rock types are mapped. 

Figure 13 – 1.  Example of Trench 5 map oriented to true north.

Figure 13 – 2.  Example of Trench 5 map with trench axis parallel with map page edges.