Drilling:Three types of drills are commonly used to delineate mineralization.
Early stage exploration for vein gold and sulfide targets can use small portable diamond drill machines called "gophers", "winkies", or "hydrowinks". These machines can be easily lifted into position by a helicopter or carried by truck to the drill site. A small diameter core is obtained from depths up to 200 feet. The core is usually split in half and one half of the core is analyzed by geochemical or assay methods. These drills can provide an inexpensive look into the third dimension of a surface mineralized showing.
Disseminated copper/gold and Nevada-style gold deposits can be drilled using large truck-mounted reverse circulation drill (RCD) machines. These drills produce rock cuttings at the bit which are blown up the hole by compressed air. The cuttings are separated from the air or water by a cyclone or cyclone-splitter combination. Sample bags of cuttings are usually obtained in five-foot intervals, which is why results from Nevada are almost always quoted in five-foot increments. A problem with RCD is the potential for contamination. If gold is encountered in a drill hole in any significant concentration, this gold can be smeared by the bit for a distance down the hole, and can be smeared up the hole if the compressor is not large enough to push the cuttings completely up the pipe. The resulting assays can appear to be from a wider interval than is actually the case. RCD cuttings are usually analyzed by geochemical methods to start with, then followed by assay methods if significant mineralization is found. The RCD method is relatively inexpensive, with drilling costs usually less than $10 per foot.
Diamond drilling using large machines is the preferred method of delineating mineralization in Canada where access is a problem. Most drill machines are designed to break down into pieces small enough to be lifted by helicopter which allows the drills to be moved rapidly from site to site. Diamond drill costs range from $20 per foot for easily accessible targets to $100 per foot for remote areas.
Diamond drills produce core which ranges from approximately one to two inches in diameter. Drill core is usually split in half by either a core splitter which produces samples of uneven size or by a diamond saw which produces halves of equal size. Half of the core is saved for geological examination and half is bagged and sent to the laboratory for analysis. Drill core is usually analyzed by assay methods.
A press release which contains drill core assays should always quote the length of the mineral intersection as a "true width" or a "drilled length". The difference in these two measurements is extremely important when attempting to assess the economic significance of the press release. A "true width" takes into account the angle the drill hole cut the mineralization and corrects for drill lengths which have traveled oblique to the direction of the mineralized bed or vein. A "drilled length" is the measured length of core that contains mineralization. This length can bear no relationship at all with the "true width" of mineralization and must be treated with caution.
Reserves
Advanced drilling projects are designed to define the average grade of mineralization and the amount of economic ore which could be profitably mined. These ore reserves are usually broken down into three main categories: Proven or Measured Ore, Probable or Indicated Ore, and Possible or Inferred Ore. The definitions for the above categories are given in detail in the Canadian Securities Exchange Policy No. 2A, which is the guideline used by the VSE. These definitions should be referred to when examining corporate information regarding tonnage and grade of an ore body.
It is probably sufficient to say here that "Proven Ore" must be drilled or sampled in three dimensions and have a solid assay database to support the quoted grades. "Probable Ore" can include data from projections over reasonable distances and "Possible Ore" is a flat-out arm wave based on a few drill holes and geological projections.
Should press releases not indicate in which category quoted reserves lie, always assume that the mineralization is "Possible" rather than "Proven" or "Probable".
ANALYSIS
Sample Preparation
The analysis of geological samples must start with the reduction and homogenization of the sample into a form which can be easily handled by the analytical personnel. This initial step is called sample preparation and is the source of many of the errors in a chemical analysis.
Soil and stream sediment samples are usually sieved so that particles larger than fine sand are removed. The fine particles are mixed and a portion is removed for chemical analysis.
Rock samples are treated in a multi-step procedure. Rocks, cuttings, or core are first crushed to about pea-size in a jaw crusher, then passed through a secondary crusher to reduce the size further - usually 1/10 inch. This crushed sample is mixed, split in a riffle splitter and reduced to about one-half pound or 250 grams. This 250 grams is placed in a pulverizer where it is reduced further to -150 mesh.
This reduction process for rocks is relatively trouble-free when testing for base metals, but can be fraught with problems when precious metals are involved. Certain types of pulverizers can smear large gold particles causing a loss of gold onto the pulverizer plates and of course a decrease in gold in the sample. The pulverizer can then spread gold to the following samples creating false anomalies.
Large gold particles can create the "nugget effect" in samples. When nuggets are present, repeat or check analysis of the same sample will not reproduce the initial gold concentration, which in turn leads to uncertainty in the complete sampling and analytical procedure. Measurements of this "nugget effect" can be made by performing a metallic gold analysis, which consists of removing the native metals by screening through a fine sieve. The coarse and fine fractions are then assayed separately. Knowing the weights of each fraction allows the calculation of the total amount of gold in the original sample.
Assay Techniques
Assay procedures differ from geochemical analytical techniques in that they more accurately represent the mass of the sample being analyzed. In general, assay techniques use a significantly larger weight in the determination. Assaying for base metals uses a combination of wet chemistry and instrumental techniques. Wet chemistry utilizes a physical measurement, either the color of a solution, the weight or volume of a reagent, or the conductivity of a solution after a specific reaction. Assaying of mineral concentrates or geological samples containing a high concentration of a specific element are relatively accurate and precise. They are the preferred techniques to determine element concentrations in ore samples.
Precious metals in rock or soil should be analyzed using the fire assay technique. The advantage of fire assaying is that a large sample can be used. One assay ton, the usual amount of sample used for fire assaying, weighs approximately 30 grams. This large size for test samples reduces the "nugget effect" and results in better analytical precision.
The usual fire assay technique for gold and silver transfers a one assay ton portion of the sample into a crucible where it is mixed with a variety of chemicals. This mixture is fused at a high temperature. During this fusion, beads of metallic lead are released into the molten mixture. The lead particles scavenge the precious metals and sink to the bottom of the crucible due to differences in density between lead and the siliceous component of the sample, known as slag.
When the fusion is complete, the molten mixture is poured into a mold and left to solidify. After cooling, the slag is removed from the lead and the lead button is transferred into a small crucible known as a cupel and placed back into a furnace. The lead is absorbed by the cupel, thus leaving a bead of the precious metals at the bottom of the cupel. This technique is the source of the Alchemist's claim of "turning lead into gold".
The gold and silver can be measured by weighing the bead on a balance, dissolving the silver in nitric acid, and then weighing the bead again to determine the mass of the undissolved gold. Silver is calculated by difference. Another method of finishing the determination is to dissolve the entire bead in an acid mixture known as aqua regia and measure gold and silver by atomic absorption. Other forms of measurement include neutron activation analysis and flameless atomic absorption.
An experienced fire assayer can recover
nearly all precious metals from almost all matrices. In the past
there have been claims that a proprietary technique must be used
to recover gold. These claims must be treated with the highest
degree of suspicion. In my experience, there has never been a
sample containing gold where the gold could not be detected using
the fire assay technique.
Down-Hole
Mud-Actuated Hammer
Novatek is developing new technology for drilling
in the oil and gas industry: The down-hole mud-actuated hammer
drill.
REVERSE CIRCULATION DRILLING USING ROTARY
TRICONE BIT OR DOWN THE HOLE HAMMER
Drilling techniques can be
distinguished by :
· the type of the tool cutting the
ground : Tricone bit or DTH ;
· the type of the flushing fluid (mud, water, air) which
carries the cuttings from the toe of the hole to the surface
;
· the way the flushing fluid circulates :
* direct circulation, where
the fluid is injected by the drilling rods and removes the cuttings
to the surface through the annulus between the walls of the drill
hole and the rods,
* reverse
circulation,
where the fluid circulates downwards and carries the cuttings
upwards inside the drilling rods.
http://www.foraco.com/ang/circulation.html
Classical drilling techniques are :
· rotary drilling with tricone bit,
using mud and direct circulation (generally the case of oil drilling
techniques) ;
· drilling using air and DTH, with direct circulation
(generally the case of water well drillings in igneous rock,
Brittany, and West-Africa ).
Double
Walled Drilling
Rod and Air Distributor
The Boniface company from Herault, France has developed a drilling
technique using reverse circulation, either for rotary or DTH
hammer tools.
The development comprises a patented new
type of double walled drilling rod and a special air distributor.
The Principle :
Compressed air is injected inside the annulus of the double walled rod (1), arriving inside the special air distributor (2) to feed the drilling device (3) and allow the removal of the cuttings through the internal hole of the rods. A climbing fluid speed of about 1 000 m per minute is maintained in the central hole of the rods. This speed is independent of the ratio between the drill hole diameter and the rod diameter.
Equipment :
The use of the robustly manufactured double walled rods is simple as well as fast. The external tube has a diameter of 7"5/8 and the internal tube has a diameter of 5"1/2, with conical thread API type (5"1/2 IF). Screwing and unscrewing operations are similar to those with conventional rods. The inner tube diameter is 4"3/4, and is sufficiently large to minimize head losses.
The special distributor has several functions :
·it allows the air flow to feed the air lifting for rotary or the DTH, from the double walled rods,
·it allows, with a special crossing joint tool, the removal of the air and the cuttings through the inner tube of the double walled rods,
·it ensures good verticality of the hole (stabilizer),
·it isolates the toe of the drill hole from its upper section, located above the distributor/stabilize, which has the same diameter as the drilling tool and forms a key part of this reverse circulation drilling method.
Drilling depth depending on the drilling tool diameter- A chart that shows the relationship of distance to the size of the drill. ( provided by the Foraco Company)
A better geological investigation tool :
The advantages of this reverse circulation drilling method, using Rotary or DTH, are as follows :
Geological information is accurate and instantaneous. The cuttings arrive at the drilling head at a high speed from the bottom of the hole. Their dimensions are of centimeter order and in addition there is no mixing of these "bottom of the hole" cuttings with others, which can be the result of erosion of the wall of the hole.
The geological information is continuous (100 % recovery). When using a conventional drilling technique (direct circulation), drilling through fissured or fractured ground, or through cavernous rock is hindered by partial or total fluid loss. On the contrary, removing the cuttings by the rods (reverse circulation) cancels possible fluid and cutting losses and their associated risks (plugging, jamming, pollution of water table).
When dealing with water wells or oil wells, ingress of fluids is immediately identified.
Financial Savings :
The air output to remove the cuttings is significantly reduced when using the reverse circulation method. For example, to drill a 17"1/2 (444.5 mm) diameter hole, the air consumption is in the range of 25 m3/minute when using reverse circulation and 5 to 6 times more with direct circulation. In other words, the reverse circulation method allows economical large diameter drilling (smaller air compressors).
Drilling with no turbulence above the distributor/stabilizer allows much greater hole stability, thus increasing the durability of the rods, which do not suffer from external erosion. The flush surface of the internal pipes of the rod leads to less turbulence than when removing the cuttings through the hole.
Drilling with the reverse circulation method is generally linked with the use of Rotary or DTH hammers. The technique leads to cost savings which increase with rock hardness. The savings are due to faster progress, increased durability of the tools and lighter equipment than direct circulation.
All ground conditions
Reverse circulation techniques can be used in all geological formations, from silty grounds to rock. This technique is an important step in solving problems in large diameter drilling in hard rock (Limestone, granite, etc...) and in fractured or karstic rocks, where total drilling fluid losses are encountered with direct circulation.
Some examples of its applications are given below :
· Underground water
- deepening of existing wells in order to increase production ;
- large diameter water wells in igneous rock (Africa).
· Oil drilling techniques :
- large diameter drilling in hard rock conditions. As an example the Boniface company has performed in Saint Hippolyte du Fort (Gard, France) a pre-boring 400 meter deep in 17"1/2 (444.5 mm) diameter in Karstic limestones (Kimmeridgien period) with an average commercial output of 8 m/hour. This work has been done for Total Oil Company.
· Ground investigations for mining with accurate full cutting recovery giving a continuous geological information.
· Civil works : Piling - Bored pile construction
Glossary
Air core drilling - a variation of Reverse Circulation Drilling.
Diamond drilling
- expensive but the information
gathered from the core samples can be worth its weight in gold.
Percussion
drilling uses steel rock bits of
various design to break up the rock while drilling. Drill hole
diameter can vary greatly, but is commonly about 7 centimetres.
The bit is rotated and the hammer action of the drill is transmitted
through a string of drill rods which have a relatively small
center hole through which compressed air (in the case of dry
ground), or water (in the case of wet ground), is passed. The
air, or water, exits through the center of the bit and forces
the drill cuttings up the hole through the space between the
drill rods and the drill hole wall. At surface the drill cuttings
are passed through a sample splitter and a sample consisting
of 1/8 or so of the material is collected for examination and
assay.
This drilling method is relatively fast and cheap. It is suitable
particularly for disseminated mineralization in ore deposits
such as porphyry copper deposits. The method provides relatively
little control over mixing of drill cuttings as they are forced
up the hole to surface and samples are subject to dilution through
erosion of the drill hole wall. Percussion drill holes are efficient
to depths of about 100 meters
Reverse circulation-Where the fluid circulates downwards and carries the cuttings upwards inside the drilling rods.
Direct circulation-Where the fluid is injected by the drilling rods and removes the cuttings to the surface through the annulus between the walls of the drill hole and the rods.
Dual-wall
reverse-circulation rotary system- High
pressure air is forced down the annular space between the inner
and outer pipes and lifts the cuttings up the inner pipe. Top-bead
drive rotates the entire drill string including the tricone roller
bit shown here
A Down-the
Hole Hammer Bit-Used with the dual-wall
rotary system. An interchange sub located above the bit body
allows the hammer mechanism to continuously fire while reversing
the flow of the drill cutting up the inner pipe.
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