What is an Exploration Target? Obviously the goal of exploration is to find an economic mineral deposit. The dimensions of such a deposit may be very small when compared with the area to be searched, and the probability is that the ore zone will be covered by ice, lakes, swamp, soil and possibly by considerable depths of barren rock. The methods of search will be discussed in the following pages of this paper, but a tongue in cheek typical scenario would be as follows:
The traditional pick and hammer prospector is looking for an outcrop, or a displaced boulder, which is mineralized and which will give him a clue to the existence of a mineralized zone. A significant gold discovery may be in the form of a vein 2 to 6 feet wide and 500 feet long. The gold will not be visible except in exceptional cases and the vein will be covered by six feet of soil material. Hence, to be successful, the prospector must find a weathered out fragment of the quartz vein, grind it in a mortar and pestle, pan out three tiny gold "colours", stake his claims and have faith that there is a mine there. He may spend several seasons digging trenches looking for the source of the gold- bearing float.
Geophysicists will choose a favourable geological district and will fly air surveys covering many square miles. They will be looking for a particular type of magnetic or electro-magnetic pattern. On completion of the survey, the maps will show dozens, perhaps hundreds, of anomalous patterns. The ore body, if there is one, could be located beneath one of the most inconsequential looking of those anomalies. It may eventually be found by the 169th drill hole, drilled by the fifth company to explore the area, probably looking for a different commodity.
The geochemist will take stream samples on a regional basis covering many square miles of the supposed favourable terrain. That survey will be followed by more detailed sampling of anomalous drainages and by soil sample grids in anomalous areas. Several anomalies will be recommended for drilling. The area chosen may be relatively acidic, producing extensive geochemical anomalies. The ore body will be on the other side of the hill where a bed of limestone neutralizes the metallic ions in the groundwater, and the resulting anomaly is weak, narrow, and easily missed.
The geologist will have worked out the geology and structure of the area from published maps and linear or circular trends showing on expensive coloured satellite images and air photographs. The intersection of several linear structures coincides with air photo colour anomalies and favourable volcanic rocks. Massive claim staking is recommended. The orebody will be in a non-magnetic, non-conductive, non-outcropping area three miles away where a prospector found three gold colours in his pan.
Of course, we're all looking for an ore deposit, but what range of grades should we expect to make a deposit economic? With the exception of the rare bonanza type of mineral discovery such as the rich silver veins in the Cobalt district of Ontario, which produced some 370,000,000 ounces silver between 1904 and 1947 from veins averaging 4 inches wide, or rare giant gold nuggets, the largest recorded having been approximately 2400 troy ounces, then the following are the sorts of things you would be looking for.
I-1. TYPICAL ORE GRADES
Rock containing 15% zinc, or a combination of lead and zinc which together would equal 15% zinc in value. Since some silver, from a fraction of an ounce to many ounces per ton, may accompany such deposits, its presence will effect the grade in an important manner. If the tonnages are large enough, then lower grades, in the order of 8% to 10% combined metals, may be economic. Cominco's Sullivan ore body at Kimberley B.C. is an outstanding example of a 'Sedex', (sedimentary exhalative) deposit of this kind.
Copper grades vary from about 10% copper in some unusually rich massive sulphide deposits or in secondarily enriched zones capping some weathered sulphide deposits, to about 1% when combined with zinc, lead, silver and gold values in massive sulphide deposits. In porphyry copper deposits, which can be mined by cheaper open pit methods, copper grades vary from about 2% (very rich) to about 0.4% (8 pounds copper per ton of ore), such as at the large tonnage Highland Valley mine east of Ashcroft.
In the case of gold, a high grade deposit may yield 1 ounce of gold per ton of ore. During 1992, Fairfield Minerals mined and shipped some 2200 tons of selected ore containing 9000 ounces gold from their Elk deposit east of Merritt, B.C. That vein deposit is estimated to contain a reserve of 135,000 tons grading 1.59 ounces gold per ton. The vein averages one foot in thickness. More commonly, grades will be in the range of 0.30 ounces per ton, as at the Lupin Mine, NWT, ranging down to 0.107 ounces per ton underground at the Pamour mine operated by Royal Oak in Ontario.
Open pit gold mines, particularly those which can economically use heap leach methods to extract the gold, can mine lower grades. High grade mines of this type include the original 11 million ton reserve at Newmont's Carlin Mine in Nevada. Ore grade was 0.32 ounces per ton. After 100,000 feet of exploration drilling, Newmont's Rain deposit, in Nevada, was estimated to have reserves of 8.3 million tons grading 0.083 ounces per ton. In 1991, Round Mountain, Nevada, had proven and probable reserves of 194 million tons grading 0.0345 ounces per ton. When the Pegasus' Florida Canyon Mine, Nevada, started production the grade was 0.024 ounces per ton.
Whereas the grades of gold and silver mines are generally given in terms of Troy ounces per short ton or in terms of grams per metric tonne, and the grades of base metal mines are usually given in percentage, the grades of mineralization in diamond mines are given as carats per tonne (metric ton) or as carats per 100 tonnes (cphT). In the case of alluvial deposits the grades may be given as carats per cubic yard or per cubic metre. The Argyle mine in Australia is reported to have produced 158 million carats at an average grade of 7.3 carats per tonne in 1990. Between 1923 and 1987 alluvial diamond production in Ghana, West Africa ranged in grade from 1.09 to 4.87 carats per cubic yard. During that 64 year period a total of 72,832,400 carats were produced. In all cases the grade of diamond deposits in carats per tonne or per cubic yard is only part of the story since the value is dependent on the proportion of the diamonds which are of gem quality.
I-2. RELATIVE SIZE OF A MINERAL DEPOSIT
The above figures give some idea of the grades, in ounces per ton or in percent, expected from economic ore deposits. As indicated in those figures the size of a deposit may range from about 100,000 tons to several hundred million tons. In the case of the silver veins at Cobalt, the average width was 4 inches, the length a few hundred feet and the depth less than 300 feet. Fairfield Minerals states that the Elk vein is 1 foot in width, has a 650 foot surface length and has been traced down dip by drilling for 1000 feet. In the case of large tonnage deposits, the surface dimensions are generally much larger. Due to the density (specific gravity) of ore minerals and rocks the volume of a ton of ore is not a constant but depends on the rock type and the ore mineral content. Assuming a specific gravity of 3.0, one million tonnes (metric tons) of ore would occupy a volume of approximately 100 metres wide by 100 metres long to a depth of 33 metres. Hence a circular ore deposit 400 metres in diameter extending to a depth of 300 metres with a specific gravity of 3.0 would contain 113,097,600 tonnes and would occupy an area of less than one mineral claim unit.
MAJOR PORPHYRY COPPER DEPOSITS IN BRITISH COLUMBIA
Size in plan view as reported in: "PORPHYRY DEPOSITS OF THE CANADIAN CORDILLERA", 1976.
Deposit name Surface size of deposit (metres) Original ore reserves (Tonnes/grade Cu)
AFTON 200 X 400 31,000,000 @ 1.0%
BRENDA 350 X 700 160,000,000 @ 0.18%
BETHLEHEM (JERSEY) 250 X 250 60,000,000 @ 0.46%
GRANISLE 175 X 300 81,000,000 @ 0.43%
GIBRALTER EAST 350 X 700
BELL 200 X 450 116,000,000 @ 0.48%
INGERBELLE 300 X 400 65,000,000 @ 0.53%
MT. POLLEY #1 150 X 450
#2 200 X 700 25,000,000 @ 0.49%
ISLAND COPPER 200 X 550
It is interesting to note that some of the largest porphyry copper deposits in British Columbia are in remote areas in the northwest part of the province. These contain zones grading 1% copper but have not been developed because of their remote locations.