Acid rock generation and drainage (ARGD) has been perpetual throughout geological time (as long as sulphides have been oxidizing) and will continue into the future. All ARGD is considered a completely natural process with natural impacts and consequences upon the immediate environment. The waste dumps (tailings) generated from mining are appropriately termed anthropogenic and are an additional source of ARGD. Acid rock generation and drainage does imply a natural "perpetual pollution process" with the added man-made (anthropogenic) effects exacerbating the process by breaking up acid generating material so as to increase the ARGD rate in many areas, be it mining or other projects that require removal and disposal of ARGD material.

The impact of acid rock generation/drainage potential is of paramount concern at all sulphide-bearing mineral deposits. Most of these deposits have an expression of ARGD through weathering and erosion by way of gossan development and dispersion halo which are extremely important indicators of potentially economic metal values. Oxide - supergene development is an ARGD process. Exotic deposits may form in conjunction with the supergene oxidation process whereby mineral-rich acidic solutions migrate laterally from this supergene zone by vertical and horizontal percolation within the vadose zone. These mineral-rich solutions may be transported through paleodrainage systems for up to several kilometers from the source resulting in continuous mineral precipitation (Munchmeyer, 1996). These natural acid rock drainage processes result in in-situ changes that may lead a prospector/geologist to investigate sources of mineralization. For example, at a recent silver discovery in Argentina, the Yamana deposit geologist states "he was attracted to the site by a pronounced 'kill zone' where oxidizing sulphides created acidic soil, killing all vegetation" E&MJ, 1999). It was a similar zone, dubbed "the Big Score" that lead to the discovery of Greens Creek mineralization in Alaska, a massive sulphide deposit that produced almost 10 million oz silver in 1997. There are probably hundreds of other similar prospects throughout the world related to ARGD that never develop into a mine and never get reported but are an indicator of the extent of natural acid rock generation and drainage. Laterization as a major process of ore deposit formation with respect to the Amazon Region is discussed by Costa (1997). Laterization does not necessarily imply acid rock drainage processes but where sulphides are present ARGD will occur.

In geological time, the landscape can be subjected to several periods of laterite/supergene processes. An example is the Amazon region which was subjected to at least two important periods of laterite (with supergene) formation, the older (Eocene to Oligiocene) with development of mature laterites and the younger (Pleistocene) leading to development of immature laterites. Each period resulted in development of ore deposit formation with a corresponding impact on the geochemistry of the surrounding drainage. The acid rock drainage environment and landscape geochemistry scenario is shown in Figure 1.

Figure 1: Weathering and Acid Rock Generation/Drainage Formation of Mineral Deposits
and Their Impact Upon the Geochemistry of the Environment.

Oxide - supergene processes which can take place over several millions of years may result in supergene enrichment that form significant tonnages and enriched grade which can be economical to mine. These type of deposits form a dominant copper resource in South America. Some examples of supergene enriched copper porphyry deposits and massive sulphide deposit which have a supergene manto with indicated grade - tonnage and approximate age are shown in Table 1. Unfortunately many public databases do not classify oxide-supergene as a separate ore category when reporting tonnage and grade, and therefore this table is by no means complete. Another type of oxide deposit is the secondary iron deposits in British Columbia which are derived from oxidation of iron-sulphides and total approximately 3.8 million tons.

Generally, ARGD studies are initiated during the exploration stage and continue during mining operations. Understanding the potential for ARGD is recognized as a key component of mine planning and waste rock disposal. The primary objective of this planning is to reduce and control ARGD through proper handling and disposal of the rock most at risk of generating ARGD. The intention of this paper is make the public, regulators and ARGD practitioners aware of the need to interpret ARGD with respect to mineralizing events throughout a deposit's evolution.

This paper will document some of the historical ARGD events which have impacted the evolution of mineral deposits prior to their discovery or any mining. It also describes a classical example of a natural ancient (historic) and modern acid rock generation/drainage scenario which involves the mining of an oxide - supergene zone (historical ARGD) with specific reference at the Kemess South Mine, a recently developed open-pit gold-copper mine located in north-central British Columbia.

Supergene Mineralization

The kinetics of acid rock generation (and drainage) and their impact upon metal concentrations have been discussed by Mills and Downing (1998) and Mills (1998). Supergene processes, kinetics and mineralization have been discussed in several papers, to which the reader is referred (Guilbert et al. (1980), Anderson (1981), Ague and Brimhall (1989) and Brimhall et al. (1985). A discussion on supergene and surficial ore deposits, textures and fabrics is presented by several authors in volume three of an ore deposits series (Wolf, 1976).

A good summation of the supergene processes is presented in a paper by Ney et. al. (1976). SUPERGENE (secondary) mineralization is formed from metals (occurring in the host rock) transported in oxidized vadose (meteoric) water that moves mainly downward through porous and permeable material, although lateral movement can be substantial in rugged terrain. In environmental terms this process is termed metal leaching. In contrast, HYPOGENE (primary) mineralization refers to the primary sulphides and oxides formed from metals transported in generally ascending hydrothermal solutions. Supergene enrichment results in substantial reconcentration of metal values which essentially consists of the selective replacement of primary sulphides by secondary sulphides, and to a lesser extent the filling of interstices by oxide minerals, in the zone below the water table where oxygenated and acid leach solutions are reduced and neutralized. This process generally involves leaching of a large volume of rock and, providing the conditions at depth are favourable, metals can be redeposited as a higher grade concentration in a smaller volume of rock. There are varying degrees of metal leaching with respect to metal concentrations of metals vertically and laterally which implies multistage enrichment. Supergene enriched zones can range from a few meters to over 200 metres in thickness and generally have a hematitic leached cap up to tens of meters in thickness.

During the supergene processes, there is an increase in acidity and increased content of heavy metals and sulphates in surface waters and groundwaters. Essentially, it is supergene alteration of primary ore deposits which leads to a natural contamination of the local environment. The supergene processes generally produce pronounced mineral zonation. A typical zonation is shown in Figure 2 (below). The formation of oxide - supergene enriched mineralization may be derived from such primary deposits types as porphyry copper, nickel-sulphide bearing ultramafics (from which some nickel-iron laterites are formed), iron, zinc-lead-silver (Sangameshwar, 1983), gold (Butt, 1988) and manganese (Nimfopoulos et al., 1997). The zoning and paragenesis of the copper oxide, sulphide and iron minerals may be used to assess the maturity of supergene metals enhancement. Fossil enriched blankets (or beds) may occur high in the zone of oxidation, which subsequently can be destroyed through further leaching thereby resulting in the creation of a new supergene enriched zone. This natural leaching at the earth's surface may result in plant and aquatic life being non-existent due to metal poisoning or adapting to the present conditions. This scenario is also explicit in the historical ARGD and the impact upon paleodrainage. A study of supergene processes by Martycak et al. (1994) showed the importance of understanding the migration of heavy metals from ore into their adjacent surroundings, which is essentially known as geochemical dispersion. This study involved pore solutions from which they were able to characterize the environment where heavy metals were primarily released into their adjacent surrounding which impacted the quality of surface water, groundwater and soils. Geochemical dispersion will vary in metal composition and metal concentration with time and is dependant upon the paleoclimate.

Figure 2: Mature Gossan Profile

Descriptions of the various processes and substances involved in the creation of leached outcrops (leach cap - oxide zone) is discussed in detail by Blanchard (1968). The significance of leached material is paramount to understanding acid rock generation and drainage of natural and anthropogenic sulphide-bearing material and the various types of leach products. Various types of sulphides and sulphide-bearing material that oxidize under differing climatic conditions will generate different types of limonite. Limonite is the end product of iron oxidation and may occur in-situ or as transported limonite. Oxidation can occur at or near the surface as well as several tens of metres in depth due to structural controls such as faults. It is the impurities (trace elements) both in the sulphides and gangue that impact the geochemistry of the environment arising from the oxidation reactions.

The amount of copper actually lost from the hypogene to the leached zone and into the supergene enriched zone cannot readily be estimated, however if preserved material is available, a mass balance analysis can be attempted. A study of the La Escondida mine by Alpers and Brimhall (1989) examined the present day mineralogy in conjunction with acid rock drainage chemical processes, and they were able to reconstruct the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. Various people have attempted to calculate the amount of erosion and leaching involved in producing a supergene zone such as that occurring at La Escondida deposit, Chile (Mote and Brimhall, 1998) and at El Salvador deposit, Chile (Brimhall et al. 1985). A study by Alpers and Brimhall (1989) of limonite mapping within the leached capping at La Escondida permits reconstruction of the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. The aerial extent of a major porphyry deposit may cover several kilometres with its vertical depth in excess of several hundreds of metres. This large mass will impact surficial and ground waters for many kilometres.

A great variety of minerals are produced through the sequence of events from initial weathering to supergene development. By-products of ARGD include the formation of various iron oxides, some of which are known as yellow, brown or red ochre and which have been used by man as a colouring agent or pigment; malachite and turquoise are used for jewellery.

Acid Rock Generation and Drainage Through Time

Historical ARGD may be defined as geochemical processes which lead to the development of acid rock generation and drainage throughout geologic time. These processes may lead to the development of oxide, supergene and exotic mineral deposits which are influenced by parental rocks, climatic and structural history and landscape evolution on the formation and preservation of weathering crusts and paleodrainage. Weathering of exposed or near-surface sulphide mineralization results in the redistribution and zoned concentration of valuable metals, as well as the impact on the surrounding landscape with respect to metal leaching and deposition.

Landscape evolution changes with time and climatic conditions present at any one period during this evolution will impact the rate of ARGD. Butt (1997) discusses landscape models that describe the genesis and total geochemistry of the landscape which may have an impact upon ARGD and metal leaching (geochemical dispersion). The timing of surficial ore deposit weathering and development can also be used to evaluate paleoclimates and paleolandscapes. An example of a present day ARGD landscape that can be used to infer a paleolandscape is shown in Figure 3.

Figure 3: A recent picture of San Pedro De Atacama, Chile

Dating of oxide and supergene deposits may be done using conventional radiometric techniques of minerals within these deposits and from interpreted and inferred geology. The age of supergene oxidation and enrichment in the Chilean porphyry copper province has been demonstrated by the dating of alunite (Sillitoe and McKee, 1996). Their results indicated supergene processes were active from the early Oligocene (35 million years bp) to the middle Miocene (15 million years bp) covering a period of 20 million years. Supergene activity at individual deposits lasted for at least 0.4 to 6.2 million years. Darke et al. (1997) describe the supergene mineralization at the Kori Kollo gold mine in Boliva which began around 11 MA (and possibly up to 15 MA ago) and that supergene alteration lasted until around 4 Ma, covering a period of at least 7 million years. Sillitoe and Lorson (1994) describe the supergene oxidation of the epithermal gold-silver-mercury deposits at Paradise Peak, Nevada. These deposits range in depth from 60 to 250 metres with a sample of supergene alunite having an age K-Ar age of 10.0 +- 0.5 Ma.

There are numerous examples of present day in situ oxide and supergene development in environments ranging from arid to tropical to temperature climatic conditions. The Plesyumi copper prospect, Papua New Guinea, is an example of present day oxide - supergene development which essentially started some 250,000 years ago. The Windy Craggy (Claridge & Downing 1991, Downing & Giroux, 1993) massive sulphide deposits is an example of oxide - supergene development under temperate conditions where the development is essentially occurring under a glacier at this time. Present day mine waste dumps will essentially form oxide caps with minor supergene development and could become economic in future millennia. The time spectrum of these investigations indicate the perpetuity of ARGD development.

The conceptual sequences of acid rock generation and drainage leading to supergene mineralization and related deposits with time are shown diagrammatically in Figure 4 (below). This schematic represents the vertical and lateral weathering zonation and migration of the hypogene material to form the various ARD related deposits. The processes of oxidation and enrichment are long-lived and commonly repeated over time. They are controlled by climatic conditions, tectonics, levels of erosion, mineralogy of surrounding lithologies, and level of the water table.

Figure 4: Formation of mineral zones derived from hypogene material

Case History - Kemess District

The Kemess District lies within the early Mesozoic Quesnel Belt. The northwest-trending Quesnel Belt is some 1200 km long, and contains a number of important gold-copper porphyry mines and deposits. In the vicinity of the Kemess properties mafic to intermediate volcanic and sedimentary rocks of the Upper Triassic to Lower Jurassic Takla Group are intruded by large numbers of monzonitic dykes, sills and small stocks of middle Jurassic age. Large alteration zones hosting porphyry-type gold-copper mineralization are frequently associated with these intrusions. After an extended period of arid weathering, during which supergene zones were developed over exposed porphyry copper deposits, the Takla Group and the felsic intrusions were unconformably overlain by Upper Cretaceous to Tertiary Sustut Group continental clastic sediments and minor basalt flows. The overlying Toodoggone volcanics have been eroded, exposing several large monzonite intrusions with disseminated sulphide mineralization and associated hydrothermal alteration. The resulting disseminated sulphide system measures at least 9 km from north to south, and 6 km from east to west. It contains two known gold-copper deposits, Kemess South and Kemess North, and four other significant gold-copper occurrences.

The Kemess South and Kemess North deposits are described and discussed in papers by Rebagliati et al. (1995), Diakow and Metcalf (1997) and Rogers and Houle (1998). The Kemess South deposit was discovered by stream sediment and soil sampling surveys. There were no surface exposure of the either the leach - supergene or hypogene zones. The Kemess North deposit was discovered from the large surface gossan exposure.

Kemess South Deposit

The Kemess South is a porphyry deposit, which contains estimated reserves of 212 million tonnes, in two zones of mineralization, an upper Leach Cap-Supergene Zone containing approximately 20% of the mineral inventory and a lower Hypogene Zone containing the remaining 80%. Mineral grades in the deposit average 0.22% copper and 0.61 grams/tonne gold. The geometry and continuity of the reserve are such that it is conducive to efficient open pit mining techniques, at a nominal extraction rate of 53,000 tonnes per day, or 19.3 million tonnes per year.

The Kemess South deposit occurs in a flat-lying, near-surface quartz monzodiorite intrusion, called the Maple Leaf, and is one of the sill-like bodies intruded into the Takla Group rocks. The eastern portion of the Maple Leaf intrusive is nearly exposed at surface, covered only by a thin cover of glacial till, while the western portion underlies a westward thickening cover of Cretaceous - Tertiary pebble conglomerates and gravels.

Hydrothermal metasomatism has overprinted the effects of metamorphism in many areas, which together have resulted in five types of alteration, namely: 1) potassium silicate alteration, 2) sericitization, 3) silicification, 4) hematite-carbonate-clay silica alteration and 5) propylitization. The significance of this alteration on the overprinting of the leach and supergene zones is apparent in the continuing evolution of mineralogical changes in these ARGD zones.


Copper and gold mineralization have developed within the Maple Leaf intrusion, and to a much lesser extent within the underlying Takla Volcanics. Two types of ore are present. The Hypogene Zone contains 80% of the mineral inventory with sulphide minerals such as pyrite, chalcopyrite, bornite and minor molybdenite. The Supergene Zone which may be up to 70 metres in thickness, contains approximately 20% of the mineral inventory predominantly in the form of native copper and minor chalcocite.

A Leach Cap - Supergene zone has developed along the irregular paleosurface of the Maple Leaf Intrusive. Glaciation has removed the leach/supergene mineralization in the eastern half of the deposit, but in the western portion, the deposit has been protected from further erosion by the deposition of the Early Jurassic Hazelton Toodoggone Formation tuffs and epiclastics. The Leach Cap is very low in copper grade, while gold grades remain strong within this horizon. The main copper mineral in the Supergene horizon is native copper, with minor occurrences of chalcocite, cuprite and malachite.

Mineralization in the Hypogene portion of the Maple Leaf Intrusive, and the Takla Volcanics, consists of chalcopyrite, pyrite and magnetite, with minor amounts of bornite and molybdenite. Gold is associated with the chalcopyrite.

Immediately overlying the pluton, locally, is a unit referred to as LAG (ferruginous gravel). This one to five metre weakly mineralized horizon was formed by the breakup of the immediately underlying supergene intrusive material during arid weathering conditions, without significant transport of the fragments being involved.

Kemess North Deposit

The Kemess North deposit is conspicuous by its large well developed gossan. Outcrops are generally broken and limonite-stained. Ferricrete may be localized up to 10 meters in thickness occurring where groundwater emerges as springs. This deposit is underlain predominantly by Takla Group volcanic rocks intruded by Lower Jurassic porphyritic monzodiorite dikes. Gold-copper porphyry-style mineralization occurs mainly in the Takla Group rocks. Mineralization is similar to the hypogene in the Kemess South deposit, but with more pyrite. The deposit is underlain by an intensely broken relatively flat-lying 80 metres thick zone extending down to an irregular base. In this zone gypsum and anhydrite are common, a product of natural acid rock generation. Beneath this zone, minor amounts of supergene covellite and chalcocite coat chalcopyrite and pyrite grains. This flat-lying zone may be a result of a thrust fault, along which groundwater travels.

Natural ARGD has resulted in the formation and development of the leach cap and supergene zone. Known reserves are estimated at 157 million tonnes, grading 0.18% copper and 0.38 grams of gold per tonne, and it remains open along the strike and down the dip.

Historical Environment and ARGD Formation

The possible sequence of events through geologic time that resulted in the formation of the historical and present day acid rock drainage of the Kemess South and Kemess North deposits are as follows:


Historical Acid Rock Drainage
In the Kemess South deposit, a leach cap and supergene zone have been developed over the chalcopyrite-pyrite mineralized intrusive host rock. The leach zone contains approximately 21 million tons and the supergene zone is approximately 25 million tons with economic gold and copper grades. The mineralogy and geochemistry of this zone is indicative of an acid rock generation/drainage process. Age relationships of the lithologies indicate that this process occurred during the Cretaceous period, approximately 130 million years ago (age of the dinosaurs). Natural acid rock generation and weathering processes led to the development of the leach and supergene zones over the exposed Kemess South porphyry copper deposit. This event was followed by the overlying deposition of the Cretaceous Sustut Group sedimentary rocks and later modified by Quaternary glacial erosion and overlain by glacial material. This is in effect an ancient (historical) acid rock generation deposit developed over possibly million of years that was subsequently buried, has been partially exhumed and is now in the process of being mined.

Reconstruction of the paleoclimate from knowledge of the ARGD processes and environment indicates that the climate has changed from semiarid in Cretaceous time to temperate at present. Rocks were exposed to alternating periods of oxidation (during dry seasons) and leaching (during snow melt and rainy seasons).

The original leach-supergene zone may have been on order of several hundred metres thick, if one compares present day analogues from Chile and United States. The amount of erosion of the supergene zone before burial of the overlying lithologies is not known. This paleosurface will depend upon time of erosion and weathering environment(s).

Historical Metal Leaching
The drainage immediately surrounding the Kemess North deposit has high metal values in streams and stream sediments (Rebagliati et al, 1995). High metal values are reflected in the soils overlying the mineralized sections of Kemess South (Downing and Connell,1999). These metal values are a result of natural metal leaching of mineralized areas. Hence natural ARGD has a large impact on natural metal leaching in the environment, which has essentially been continuous since the Cretaceous time. Since the supergene zone is partially covered by sediments, one can assume that the aerial extent would have been be much greater than today and thus the influence of metal leaching on the environment would also impact a greater area.

The Kemess North is a modern day analogue of the Kemess South. This area has continually undergone natural acid rock drainage since the Cretaceous period. By mining the rock and dumping waste rock with some potential acid rock generation, the continuance of in-situ ARG is ongoing. Groundwater has probably been active in ARG since major ARG event but is now slower due to temperate climate with winter months of very little ARG.


Supergene processes has not been recognized as an acid rock drainage process by most ARGD practitioners, regulators, environmentalists and the public. It is in this context that this paper has been written. Few geologists interpret geology with respect to acid rock generation processes and environmental practitioners do not interpret ARGD from geological and geochemical processes.

The concept of acid rock drainage as a "perpetual process" has a long range effect on the environment, implied in the "Mother Nature" and anthropogenic perpetual care of ARGD sites. The impact of historical acid rock drainage is immense as observed in the reported tonnages, which would have had a major impact upon the environment, geochemistry and biotic life. The effects we observe today with respect to ARD would certainly have occurred in the geologic past. It might be appropriate to conclude that low grade waste dumps (anthropogenic acid rock generation) could in fact become mineral enriched (supergene) with geological time. The affects of ARGD have economic benefits or costs depending upon the time frame in which it is viewed. The environment and landscape geochemistry adapts to changing conditions throughout geological time.


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