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| Syllabus | |
| 1 History of Mining | |
| 2 Using Maps | |
| 3 Geological Exploration | |
| 4 Geophysical Exploration | |
| 5 Geochemical Exploration | |
| 6 Secondary Exploration | |
| 7 Open Pit Mining | |
| 8 *****Mid Term***** | |
| 9 Strip Mining & Placer Mining | |
| 10 Underground Mining | |
| 11 Permits and Laws | |
| 12 Environment Impact | |
| 13 Jobs and Tools | |
| 14 Mineral Economics | |
| 15 Reclamation | |
| 16**** Final Exam***** | |
| Resources | |
| Staff E-mail | |
Secondary Exploration-Sample Collection
Objectives:
1)The student will explain 3 methods to take subsurface samples
and why engineers want more than one method of sampling.
2) The student will explain why the nature of the material to be sampled and the particular objective of the sampling determine types of drilling needed.
3) The student will describe the advantages and disadvantages of the rotary percussion drill and the reverse circulation drill.
4) Student will list four of the major costs of the sampling/drilling process that may restrict junior companies from risky prospecting.
Drilling Methods and Equipment
Drilling Equipment
To test for, examine, and evaluate a mineral showing in the third dimension, various methods of drilling to obtain samples have been developed. These methods fall into two general categories
a) various types of percussion drills which break up the rock as the hole is drilled producing rock chips like coarse sand which are flushed to the surface by circulating fluids or by compressed air; and
b) core drilling methods which recover more or less continuous rock core from the drill hole by grinding, or cutting, an annular ring of rock from around the central core and recovering the core by a system of retaining core barrels or tubes.
Sampling the subsurface usually involves one or more types of drilling. A number of different types will be discussed, although present-day mineral exploration and development is virtually limited to the use of diamond core, rotary, or rotary-percussion methods (Peters, 1978:Waterman and Hazen, 1968:Sainsbury, 1979)
Rotary
Percussion Drilling
One of the simplest, oldest, and least expensive methods, rotary percussion drilling is essentially
an evolutionary outgrowth of the single- jack driller. A pneumatically
or hydraulically driven hammer that transmits its force to a
rotating drill bit has replaced the driller's arm. Air, or sometimes
water is circulated through the drill-steel column to the bit
to cool its surface and clean the hole of the cuttings by forcing
them along the outside of the column. With air, insufficient
volume and pressure will leave heavy particles in the column.
Excessive airflow can cause erosion of the hole walls and contamination
of the samples. If water is used, special arrangements must be
made to allow sample cuttings to settle, minimizing the loss
of fines, and perhaps clarify and reclaim the water. For sample
collecting, a collar is usually fitted with casing and a tee
connection to divert the fluidized stream of cuttings into a
collector. The advantages to this method is that it is fast,
inexpensive and the equipment is fairly mobile.
On the other hand, this method has its disadvantages also. Raveling of hole walls caused by vibration of the drill string can seriously contaminate or dilute the samples. Large amount of water flowing in a drill hole can cause difficulties where clays are present. Selling clay minerals can plug drill bits and adhere to hole walls. This leads to restricting the airflow and rotation of the drill stem. Difficulties or loss of circulation usually leads to abandonment of the hole unless the hole is the desired object rather than a sample of the material it penetrated.
By far the more common rotary-percussion drilling method for sampling uses a down-hole hammer Picture. Improvements in engineering and design have resulted in smaller diameter pneumatic drill hammers and dual wall drill pipe. Innovations in down-hole hammer design now being tested are aimed at the collection of samples directly from the bit face and forcing them through ports in the bit to the center of the drill pipe. These innovations and improvements are aimed at improving sample integrity.
This site has some great pictures and specs
for equipment. You can choose drilling methods or drilling equipment
to view .
http://midnightsundrilling.com/reverse_circulation.html
Sample collection with reverse circulation drilling is relatively simple in theory. Cuttings transported either through the center or the annulus of the drill pipe are retained in a cyclone collector. When the end of the sample is reached, the drill's stem is retracted a short distance from the bottom of the hole and the airflow or water flow is continued until it is judged that cuttings from the interval have been exhausted form the pipe. The cuttings are then dumped into a splitting device to reduce the sample size.
This procedure is not without problems as far as accurate sampling is concerned. The volume and pressure of the air or fluid supply must first be adequate to force all the cutting out of the hole. This is especially important if the mineral sought has a high specific gravity. Too much airflow can cause excess loss of fines from the exhaust on the cyclone stack this can be alleviated if the stack is extended. Another problem may occur when dumping the fines in the splitter. The fines can billow up above the splitter, cause a loss of samples and create dangerous and hazardous conditions around the machinery.
In collecting wet samples, one is often overwhelmed by the volume and weight of the material. There are also the problems of filtering and drying the samples for transport and handling. Fine minerals, such as gold, float and may be lost which affects the integrity of the sampling. Washing equipment thoroughly between samples and the filtering of the water effects the integrity also.
Environmental concerns about air pollution and conspicuous dust plumes from pneumatic drilling have prompted regulations in some areas that essentially prohibit drilling with air conventionally. It is anticipated that this regulatory tend will continue and that drilling with water and collecting wet samples may become mandatory procedures.
Rotary Drilling
Because of inherent elasticity or other unique physical properties,
certain rock types cannot be drilled and sampled effectively
with percussion equipment but are amendable to rotary drilling.
Most commonly, a tricone bit is used, but special situations
may find the use of plug, fishtail, or drag bits more effective.
Extracting and collecting the sample are essentially the same
as with rotary- percussion drilling.
Read article: Drilling for Natural Gas
See a picture of workers lowering a new rotary bit into a well bore. Notice the sharp teeth that will break up rock formations.
Source: NGSA
The rotary drilling method differs from the percussion method in that it relies on a sharp bit to literally drill through earth and rock layers. It is also used to lift waste materials out of the well bore. On the surface, a complex system of cables, engines, support mechanisms, lubricating devices, and pulleys control the rotation of the bit below the surface, as well as keeping the bit lubricated, and bringing debris out of the well bore.
Diamond Core
Drilling-Medium sized drill
with a depth capacity of 500 meters, the drill can be skid, trailer
or track mounted. The drill can also be lifted into areas of
remote access by helicopter.
Diamond Drilling would have to be one of the most accurate methods
for determining ground formations and is possibly the most used
method for Exploration and Mine development in Australia.
The process involves driving a fast rotating annular bit through
the ground to collect a solid core sample. The bits used are
diamond impregnated (as the name suggests), or other cutting
materials may be used such as Tungsten. Water is the most common
circulating fluid used but as some circumstances necessitate,
air may be used. The cuttings are washed up the hole between
the rods and hole wall and the core sample is collected at the
bottom of the hole in a core barrel.
Reverse
Circulation Drilling -possibly the most favored method
of drilling in mineral exploration. It provides a good quality
sample and is less expensive than diamond drilling.
The process involves a rotating drill string with a bit on the
end that chips (roller) or scrapes (blade) the cuttings from
the bottom of the hole. The cuttings are then returned to the
surface via the interior of the rods. It uses a variety
of 'button' or 'cone' bits to break up the rock while drilling.
The system is designed to combine the fast drilling of percussion
drilling with better control of the collection of the drill samples.
To achieve this, the drill rod of the percussion drill is replaced
by a drill stem of concentric pipes which allow the compressed
air (for dry ground) or water (for wet ground) to be forced down
an inner pipe, exiting near the center of the drill bit, flushing
the drill cuttings up the side of the bit assembly and into slots
in the drill stem above the bit. The cuttings are then forced
up the annular space between the inner drill rod and the outer
drill pipe.
At the surface the rapidly moving rock cuttings and drill fluid are passed through a cyclone which vents most of the compressed air and drops the drill cuttings through a splitter which collects a preset fraction of the cuttings as a sample for assay. Generally a duplicate sample is collected at the same time and a small plastic container is filled for geological examination.
The reverse circulation drill system bores holes in the range of seven to fifteen centimeters in diameter. The drill cuttings are in contact with the drill hole wall only between the face of the bit and the slots into the drill stem. This reduces erosion and dilution from the drill hole walls. Drilling can be stopped at the end of each chosen drill sample interval and the air, or drill fluid, stream can flush the hole to give delivery of fairly accurate sample intervals.
In the more refined center hole recovery systems, the drill fluid (or air) is forced down the hole between the inner and outer drill pipes, exits around the outer face of the drill bit, and carries the drill cuttings back into the drill stem through a center hole in the drill bit to be carried to the surface through the inner pipe of the drill stem. This method prevents contact of the sample material with any part of the drill hole wall. Reverse circulation drill systems can drill to depths of 300 meters or more but are more commonly used for holes to about 150 meters.
Diamond drilling provides a 'drill core' of the rock penetrated by the hole. This drill core provides a continuous sample of the rock penetrated, and the geologist can examine that core as he would a surface rock exposure for rock type, alteration, mineralization, and structural features such as bedding and fractures.
The hole is drilled using a hollow drill bit whose annular face is set, or impregnated, with small diamonds. The bit is attached to a string of hollow drill rods which are rotated by a gasoline or diesel powered engine. Drilling fluids, usually just water, are forced down the inner side of the drill rods, out around the drill bit face and returned up the hole between the drill rods and the drill hole wall carrying the drill cuttings (sludge) which have been produced by the face of the drill bit. The drill core consists of the column of rock remaining in the center of the bit as it cuts a circular hole. This drill core, as the hole progresses, stands within the bit and within the following core tube which is usually 10 feet in length. When the core tube is full, the shock breaks off the core near the bottom of the hole. The core is prevented from slipping out of the core tube by retaining springs. In the old style 'standard' drilling method, the drillers would have to pull the whole string of rods, rod by rod, to recover the core tube and empty out the drill core. With modern 'wire line' systems, a retrieving device is dropped down the hole, within the drill rods, using a wire line cable. This device snaps unto the core tube, unlocks it from the core barrel, and pulls the core tube up to surface to be emptied into the core box.
Drill core, after being placed in the core box, is labeled as to drill hole number and depth of each drill 'run'. The geologist 'logs' the core by recording the rock type, alteration, mineralization, etc. and, commonly, the core is photographed. The core is then sampled for assay.
The sampled core is split lengthwise using a core splitter or a diamond saw. One half is bagged for each sample section and the other half is retained for future reference.
Diamond drill holes can be drilled to depths of 3,000 meters or more but holes exceeding 1,000 meters are relatively uncommon.
Drilling Costs
Costs for drill programs vary widely depending on the type of drilling, the depth of holes, the rock types, the accessibility of the property, the size of the drill contract, etc. Short drill contracts, in the range of 300 to 2000 meters, are relatively high cost on a per meter basis, since all the ancillary mobilization and supervision costs remain much the same and are charged to a small footage.
Percussion drilling might cost in the order of $5 to $10 per foot ($15 to $30/meter). Reverse circulation drilling may cost $8 to $20 per foot ($18 to $60/meter); and diamond drilling may cost $12 to $40 per foot ($35 to $120/meter). These are the basic drilling costs. Additional costs are for the mobilization and demobilization of the camp and equipment, camp construction and supply, support vehicles, communications, site preparation and drill moves, supervision and engineering, assaying and compilation of data. These ancillary costs, depending on the contract terms, property location, time of year, etc. might double the cost per meter of drilling.
Recent trends in permitting and regulatory requirements imposed by several levels of government and overlapping agencies within those governments, have added significant costs to any drill program. These costs include administration costs for permitting, posting of reclamation bonds, lack of flexibility in conducting the field program, cost of reclamation and rehabilitation in the field, supervisory costs reporting to various government agencies, and red tape in recovery of bonds posted before commencement of the program. These requirements are a significant drain on the resources of a junior company.
Underground Bulk Sampling
As each stage of mineral exploration is completed, a decision
is required as to whether the results warrant going on to the
next stage. This is true from the time the prospector receives
his first assays. Each stage is more comprehensive and more expensive
than the preceding stage. On completion of the various stages
of surface exploration and of all the practical surface drilling
to outline a mineral deposit, a point is reached where underground
exploration becomes necessary in order to investigate the deposit
in detail.
In flat topography, as in most parts of the Pre-Cambrian Shield, access to the mineral deposit may require driving a 'decline' or sinking a 'shaft' to reach the mineralized zone to be explored. In rugged topography the zone may be reached by driving a 'decline' or an 'adit'. Approximate costs for such development openings are given in John Tully's paper on mining methods accompanying these lectures. Suffice it to say that the cost of these openings is an order of magnitude greater than for any preceding exploration phase and that some type of pre-feasibility report, quite expensive in itself, would be required before proceeding.
Underground exploration is designed to investigate the mineral deposit for continuity of mineralization, for rock stability and detailed structure, for possible water flows, and to provide bulk samples of significant tonnage for metallurgical studies and test milling.
Bulk samples range from one ton to 1000 tons or more. Test milling procedures may be done in laboratories using small samples, in test plants available in certain localities, or in pilot mills erected to mill perhaps 100 tons per day.
At the Valley Copper deposit, then owned by Cominco, now part of Highland Valley Copper, a large decline was driven using trackless equipment and muck from each round was placed in a surface bin for sampling and mill testing
At Lornex, now also part of Highland Valley Copper, an exploration shaft was sunk and a 100 ton per day mill was operated to carry out mill testing. In 1951, at the Eldorado Mining Camp; Refining, Beaverlodge operation, Saskatchewan, an inclined shaft had been sunk, drifts were driven along the strike of the mineralized zones, cross cuts were driven at intervals across the zones, and muck from these crosscuts was sampled car by car as it was mucked out. From that material, two 25 ton, and one 50 ton, bulk samples were prepared, made up of numbered and tagged 75 pound canvas bags, filled and sewn shut underground, taken to the surface, trucked to an airstrip on the ice of Beaverlodge Lake, flown by DC-3 aircraft to Edmonton and shipped to Port Hope, Ontario for testing. Not a cheap exercise.
At the Harrison Lake gold property, owned by Bema Gold Corporation and under option to Pacific Comox, an adit crosscut was driven across the gold bearing stock. Two short drifts were driven at right angles and three raises were driven, nearly vertically, to provide 1000 tons of material which was treated in a portable test mill. This property is still in the exploration phase though a total of about $7 million has been spent to date.
It is generally only after going through
all these procedures that a true 'ore reserve' can be measured
and a feasibility study undertaken to warrant the financing of
a new mine.
References:
**This article was adapted from
Robert A Metz.. SME Mining Engineering Handbook, 2nd
Edition, Volume 1. (Littleton, Colorado: Society
for Mining, Metallurgy, and Exploration, Inc., 1992), pp.
314-326
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 centimeters.
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.
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.
Resources:
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.
http://www.novatekonline.com/mudhamme.html
Reverse Circulation Drill
http://www.foraco.com/ang/circulation.html
Industry related glossary- very good resource
that defines terms/phrases that apply to the mining industry.
http://www.manoriver.com/glossary.html
Great diagrams of recirculating drills
http://www.integritywell.com/integritywell/services/drilling/consolidated.htm
A website with information on rotary drills
created by NGSA
http://www.sunmachinery.com/DRILLINGPAGE.htm
Now move to the task or questions for the lesson.
Delta Mine Training Center
Copyright © 2001 DMTC. All rights reserved.