Foras Teamhrach
News and analysis of crucial issues that affect Ireland
Latest video news
Death in Mexico: The Assassination of Bety Cariño

The Irish Media and the Corrib Gas Project

Home      Mineral Resources of Ireland Part 2
Print this pageAdd to Favorite
The Mineral Resources of Ireland – Part II: “The Land of Mineral Opportunities” [1]
Brian Guerin 

[A Geological Glossary is provided at the end of the article.]

“... the Government remains firmly committed to strongly encouraging exploration and mining in a
manner compatible with sustainable development.”

Mr. John Browne T. D. Minister of State at the Department of Communications, Marine and Natural Resources, September 2003
“... I can assure the industry that mineral exploration and development continue to have the full support of myself and my Government colleagues.”
Mr. Noel Dempsey T. D., Minister for Communications, Marine and Natural Resources, February 2005

Ireland has a complex and widely varied geological framework. The Lower Carboniferous carbonate rocks of the Irish Midlands are host to one of the great orefields of the world. Since 1960, 14 significant (resource >1Mt) zinc-lead deposits have been discovered, including the world class orebody at Navan (>70Mt). Ireland is ranked first in the world in terms of zinc discovered per square kilometre, and second in the world with respect to lead. The high grade, shallow occurrence and clean metallurgy of the orebodies, all result in a relatively low cost of mining for the Irish-type Zinc and Lead deposits. [2]
Large tracts of Ireland are underlain by the metasediments and metavolcanics of Proterozoic and Lower Palaeozoic age. These lithologies are known to contain significant VMS  mineralization (e.g. Avoca, 16Mt @ 0.6% of Copper) and auriferous mesothermal quartz veins. The latter style of mineralization has
been the focus of extensive exploration efforts across the island of Ireland, and in 1999 an opencast gold mine with quartz veins was established at Cavanacaw in Northern Ireland (at present estimated to be 2Mt @ 6.9g/t of Gold).

In addition to Ireland’s gold and base metal potential, its varied and complex geological framework renders it a strong prospect for other minerals. In the last few years, exploration has been undertaken for the following commodities:
Platinum - group mineralization associated with mafic intrusive complexes in northeast Ireland;
REE metals and other speciality metals: (Lithium, Tantalum, Tungsten or Wolfram and Tin) associated with pegmatites that cross-cut the Caledonian Leinster granite batholith in Southeast Ireland;
Nickel and chromite associated with these ultramafic intrusions in the west of Ireland;
Diamonds and other gemstones associated with Pre-Cambrian rocks are to be found in Donegal.

Ireland also has significant potential for industrial minerals. Gypsum, dolomite, brick shale, fireclay and dimension stone (marble, granite and limestone) are all currently exploited in Ireland and significant deposits of barite, coal, mineral sands and talc have also been delineated. [3]

Ireland’s mining industry dates back to copper mining during the Bronze Age (2000-400 B.C.), The earliest evidence of mining is provided by Bronze Age (1,300 BC) copper mines at Mount Gabriel in Co. Cork,
Southwest Ireland. The first written record is a reference by Ptolemy to copper mining at Avoca, Co. Wicklow, in 150 AD, while there are written accounts of ancient mining at Silvermines, Co. Tipperary (silver and lead), Abbeytown, Co.Sligo (silver and lead) and Allihies, Co. Cork (copper). (EMD, zinc and lead in Ireland, 2006). During the l9th century there was considerable mining activity throughout Ireland, with almost every coastal county yielding at least one mine. This however had ceased by the turn of the century, and up to the 1950s, the only mining of any note was for coal, leading to the incorrect assumption that Ireland was devoid of significant mineral resources. [4] Ireland is now internationally known as ‘major base metal territory with a string of discoveries over the past 40 years.’ [5]

For geological purposes, Ireland is divided into various mineral ‘provinces.’ In its 2004 press release, the Department of Communications, Marine and Natural Resources stated that these mineral provinces are “endowed with a diverse suite of base and precious metals as well as industrial mineral deposits.”

The Main Minerial Provinces of Ireland:

1. The North-Western Basement - (PROTEROZOIC era):
This province contains Pre-Dalradian orthogneisses overlain by variably metamorphosed Dalradian clastics, carbonates and volcanics, intruded by Palaeozoic granites.
The potential for base metals is indicated by the numerous 18th and 19th century mines in the region, which were aligned with the presence of strata-bound base metal and barite mineralisation. Quartz veins and shear zones are prime gold targets in Curraghinalt and Cavanacaw, in Co. Tyrone, and gold is also associated with such massive sulphide deposits. Skarn and porphyry style Molybdenum and Copper deposits are associated with granite deposits. The worked dimension and ornamental stone include the famed green Connemara marble. Diamond and other gemstone ‘targets’ have also been identified in the Inishowen Peninsula in County Donegal.

2. Longford-Down Massif – (Lower Palaeozoic era):

This is a major geological feature stretching from County Longford in the Republic of Ireland to County Down in Northern Ireland. [6] Three principal groups of metallic mineral deposits occur in the Longford-Down inlier. Epigenetic vein deposits are found here, mainly composed of lead and Zinc, but also including antimony, arsenic and gold (e.g. Clontibret). A number of these vein deposits have been exploited in the past. Stratiform manganiferous iron deposits are also to be found; these are of probable volcano-sedimentary origin. Several of these deposits were mined during the late19th century. Minor porphyry copper type copper-molybdenum mineralization is associated with small granodiorite intrusions. At Cashel Rock in Co. Tyrone, Ordovician silica-flooded brecciated rhyolites grade up to 30.5g/t over a 3.6m interval with associated copper and lead values. The area is under licence to Strongbow Resources and is included in their agreement with Tournigan Gold Corporation.

There is a major gold deposit at the Tullybuck – Lisglassen property which straddles both County Monaghan and Co. Armagh, located in the Armagh-Monaghan gold belt, in the Longford-Down Massif. The Armagh — Monaghan Gold Belt is an area of approximately sixteen kilometres by three kilometres. It lies along the major strike-slip feature known as the Orlock Bridge Fault (locally called the Clontibret Shear Zone). It is underlain by greywacke of andesitic composition, mainly of Ordovician age, and is divided into fault blocks by faults trending approximately north-south. This significant venture gold mine will doubtless be followed by the larger international groups once the mining operation has commenced. [7]

Leinster Massif
This area hosts the large low-grade VMS copper-pyrite deposit at Avoca, in Co. Wicklow, sub-economic tungsten or wolfram deposits and lithium mineralization associated with the Leinster granite batholith, numerous minor vein-type copper and lead deposits and the historically mined alluvial gold deposit at the “Gold Mines River” near Glendalough in Co. Wicklow. In the past, the Avoca area produced some 12Mt of copper ore from VMS deposits, and the northern extension of the mineralizing system has potential for gold and zinc mineralization. At Kilmacoo-Avoca, twenty drillholes have indicated the presence of 300,000-500,000t grading up to 2g/t of gold over a strike length of 125m. The area is under licence to Strongbow Resources. [8]

Mayo - Curlew Basin:
In this basin, significant vein/shear-hosted gold deposits are known to be located at Lecanvey and Cregganbaun, the latter occurring in a 30km long shear zone. Other deposits include talc - magnesite in Westport and copper in Charlestown, in Co. Mayo.

3. The Munster Basin Upper Palaeozoic era:

In the Munster Basin, Devonian terrestrial clastics with minor volcanics are overlain by Carboniferous marine sandstones and shales with subordinate carbonates. Major folding and strike faulting has resulted from the Armorican orogeny. Vein - hosted Copper and barite in Clonakilty, Co. Cork, are among a
variety of mineral deposits mined in the past. These Devonian sequences contain potential Gold deposits, while shale-hosted Zinc-lead mineralisation has been recently discovered in the Carboniferous strata.

5. The Central Ireland Basin:
Thick Lower Carboniferous carbonates in a number of sub-basins are located in this vitally important Zinc and Lead province. According to the Department, these known deposits contain some 11 Mt of zinc metal. The ‘target’ horizons are stratabound ‘Irish type’ (MVT) deposits in the basal Navan Group: (Navan, Tatestown, Oldcastle, (Co. Meath),  Keel, Moyvoughly (Co. Westmeath), the overlying Waulsortian limestones (Tynagh, (Co. Galway), Silvermines, (Co. Tipperary), Ballinalack, (Co. Westmeath), Galmoy, (Co. Kilkenny), Lisheen (Co. Tipperary), and stratigraphically higher cross-cutting deposits of the MVT type in Harberton Bridge, (Co. Kildare). Vein and massive replacement copper and silver deposits in Gortdrum and Aherlow in Co.Tipperary and in Mallow, Co. Cork are associated with the structurally controlled southern margin of the Basin. Other gold deposits include an epithermal style mineralization in the Silurian metasediments of Co. Galway, where values up to 190g/t of gold over 1m have been identified at Bohaun, Co. Mayo.  Nearby at Dooros, gold mineralization was drilled in massive pyrrhotite-amphibolite pods in a Dalradian volcaniclastic sequence.  Grades of up to 5.8g/t of gold occur over 4.6m. In Co. Kildare, spectacular visible gold deposits have been discovered in quartz float associated with Lower Palaeozoic greywackes. [9]

Other significant deposits include barite (Ballynoe, Co. Cork and Tynagh, Co. Galway), gypsum (Glangevlin, Co. Cavan), calcite (Kilbreckan, Co. Clare), and dolomite. There are possible ‘Carlin-type’gold deposits in major structural zones in Navan, Co. Meath, Silvermines, Co. Tipperary and Mallow, Co. Cork. [10]

The Geological importance of the Irish Midlands MVT region: The main focus for the exploration and development of Ireland’s lead-zinc deposits has been concentrated in the Carboniferous sedimentary rock of the Midlands region. [11] The mineralization process here is similar to that of the Mississippi valley type in the United States (known as MVT), and is hosted in carbonate units. Tournigan Ventures Corp. and European Gold Resources are exploring the Omagh gold project in County Tyrone, Northern Ireland [12] The Omagh project, located 5km southwest of the town of Omagh, covers an area of 189 km2. Previous exploration work by Rio Tinto Limited identified 15 mineralized structures in the eastern one-third of the concession.

It is apparent that Ireland’s geology is favorable for the occurrence of base metals and industrial minerals (Newman, 2002d). Ireland is now a major European Union producer of zinc. The main focus for exploration and development of Ireland’s lead-zinc deposits are the Carboniferous sedimentary units of the Midlands region. This exploration, which is now intensifying, the discovery of relatively new base and precious metal deposits, and the favorable geologic setting suggests that additional deposits may be discovered, particularly in the areas where the favorable units are overlain by thin Mesozoic and Cenozoic sedimentary cover.
[13] The mineralization in the Midlands Region is similar to that of the Mississippi Valley type (MVT), in the United States and is hosted in strata-bound carbonate units. The major mines worked to this date are located at Lisheen, Arcon, Galmoy, and Tara, now the largest zinc mine in Europe and the fifth largest in the world. An associated mine at Tatestown, Co. Meath, west of Navan, is now being developed by the New Boliden corporation, the owners of Tara Mines. Industrial mineral production is mainly limestone for cement and lime, and gypsum. [14]

The two stratigraphic intervals within this Lower Carboniferous carbonate sequence, which are the stratigraphically lowest units of non-argillaceous carbonate rocks, host all the known Irish zinc deposits: the Navan Group in the central and northern portion of the Irish Midlands and the Waulsortian Limestone in central and southern Ireland. The geological processes which resulted in the formation of the carbonate-hosted Irish zinc-lead deposits (including Galmoy) remain the subject of intense debate on the part of geologists. However, they share characteristics of both sedimentary-exhalative ("Sedex"),   largely syngenetic, and Mississippi Valley-type ("MVT"), largely e0pigenetic, deposits which in themselves are unique enough to merit their own class; "Irish-type." Irish-type carbonate-hosted deposits are stratabound, massive sphalerite, galena, iron sulphide and barite lenses with associated calcite, dolomite and quartz in dolomitized platformal limestones. The deposits are structurally controlled, and are commonly wedge shaped adjacent to normal faults. Deformed deposits are irregular in outline and commonly elongate parallel to the regional structural grain. [15]
These Irish-type deposits have host rocks and sulphide textures similar to those of MVT deposits, however, the Irish deposits differ in containing extensive zones of truly massive sulphides and have a metal suite containing more copper, silver and iron than most MVTs. [16]
Irish Pre-Carboniferous Geology:

The Irish Carboniferous carbonate sequence hosting the Irish zinc-lead deposits lies above the Upper Devonian to Lower Carboniferous red beds. These red beds rest on lower greenschist facies, and faulted greywackes, slates, volcanic rocks, and volcaniclastic sediments of the Lower Palaeozoic and Precambrian ages. The basement rocks are cut by late Silurian to early Devonian granitic  intrusions. The Old Red Sandstone is the basal component of the major transgressive sequence which covers the Irish Midlands. The Upper Old Red Sandstone forms a northward-thinning wedge which ranges in thickness from approximately 6 km in SW Ireland to several tens of metres in the north-central Midlands. Regional thickness variations in Upper Old Red Sandstone indicate that it was deposited in a 70 to 100 km wide, East-North East-trending basin: the Munster Basin. The northern margin of the Munster Basin has been interpreted as a synsedimentary fault zone from the presence of rapid thickness variations, extremely coarse conglomeratic sequences, and bimodal volcanic rocks. Another normal fault zone developed 50 to 70 km south of the northern edge of the Munster Basin at the end of the Upper Devonian. The area south of this fault zone is termed the South Munster Basin. [17]

Lower Carboniferous or Mississippian, period:

Tournasian Stage - (Early Carboniferous: 360-349 Ma)

This basal Carboniferous red bed sequence tends northward. It is overlain in the south and central Midlands by a 50 to 100 m thick, marine carbonate sequence which has been termed the Lower Limestone Shale. North of an East-South-East-trending line from Galway to south of Dublin, these basal marine sediments gradually thicken and become less argillaceous. In the northern Midlands the basal Carboniferous carbonate sediments are informally termed the Navan Group or Mixed Beds. They consist of a thin basal section of terrigenous sediments overlain by mudstones, sandy and bioclastic grainstones, and oolite grainstones.

The Navan Group contains minor disseminated sphalerite and thin sphalerite-galena veinlets throughout the north-central Midlands, one economic orebody (Navan), and a number of zinc-lead prospects in Tatestown, (Co. Meath), Moyvoughly (Co. Westmeath), Oldcastle, (Co. Meath), Keel). At Navan and Tatestown, high-grade mineralization commonly occurs in undolomitised micrites or grainstones beneath sandy or shaley dolomitic horizons, suggesting that these contacts facilitated the lateral flow of ascending hydrothermal fluids. At Moyvoughly, mineralization occurs within a mixed sediment mixture of grainstones, siltstones, sandstones, and micrites lying above a basal mudstone and below a calcareous sandstone unit; sulphide zones are most laterally persistent within grainstones. At Oldcastle, sulphides occur primarily within micrites. Sulphides at Keel (Co. Longford), are more fault-controlled than stratabound, occurring throughout the Navan Group and within the underlying Old Red Sandstone. [18]

The Ballysteen Limestone overlies the Lower Limestone Shale. A nearly identical type, termed the Argillaceous Bioclastic Limestone, or ABL, overlies the Navan Group in the north-central Midlands. ABL is generally poorly mineralized. It consists of a relatively homogeneous sequence of mildly argillaceous, bryozoan-rich, fossiliferous packstones and grainstones with thin argillite bands. Locally in southern and central Ireland, such as in the Silvermines and Galmoy/Lisheen areas, the Lower and Middle ABL contains intervals of oolitic limestone, which contain sulphide bodies immediately adjacent to mineralized faults.
Throughout southern and central Ireland, the ABL is overlain by the Waulsortian Limestone. Waulsortian deposition was initiated in the mid-Courceyan along the northern edge of the South Munster Basin and then moved northwards, reaching the northern edge of the Irish Midlands by the late Courceyan era.
Waulsortian Limestone consists of poorly bedded, dense, pale grey mudstone-wackestone and fine-grained packstone-grainstone.

This Waulsortian Limestone contains the majority of Irish zinc, lead and barite deposits and prospects although it has been stated that the combined tonnage of these deposits and prospects does not equal that of the Navan deposit. The major Waulsortian-hosted deposits are Galmoy, Lisheen, Silvermines and Tynagh; significant prospects include Ballinalack, Garrycam, Courtbrown, Carrickittle, Crinkill and Derrykearn. With the exception of Garrycam and Ballinalack, the Waulsortian-hosted deposits occur where the underlying basal Carboniferous section is comprised of Lower Limestone Shale. At Garrycam and Ballinalack, sulphides are also present in the underlying Navan Group. Unlike the Navan Group, the Waulsortian Limestone is generally barren of sulphides, except in the immediate vicinity of deposits and prospects. Mineralization occurs primarily at, or near, the base of the Waulsortian in the Galmoy, Silvermines and Lisheen deposits and the Garrycam, (Co. Longford), Courtbrown and Carrickittle, (Co. Limerick), and Derrykearn (Co. Laois), prospects.

Chadian-Arundian Sub-Stage: (Carboniferous 349-340 Ma)

The layer-cake Courceyan sequence gave way in the Chadian stage to a complex mosaic consisting of closely juxtaposed basinal and shallow marine sediments indicative of a strong structural control over facies development. Subsidence increased during the Chadian in both the Shannon Trough and the Dublin Basin. Basic volcanic ash layers, typically l–3 cm thick, spread widely over the Irish Midlands during the latest Chadian to early Chadian. They were derived from isolated volcanic centres scattered throughout the central and northern Midlands, which have been recognized in outcrop and from regional aeromagnetic data. Two small volcanic fields are known. One in the Limerick area consists of a number of basaltic vitric tuff rings and surface alkali basalt and trachyte flows while the other at Croghan Hill in County Westmeath, is comprised of several basaltic vents and flows.

The carbonate rocks in the Irish Midlands host one of the world's major zinc-lead orefields. This orefield includes the world-class Navan orebody (70 million tonnes; 10% of zinc, 2.6% of lead), four significant deposits (Lisheen, Silvermines, Galmoy, and Tynagh), and a number of smaller prospects that are currently regarded as being either marginally economic or sub-economic. The mineralized area covers approximately 8,000 square kilometers. The deposits occur in a transgressive sequence of Lower Carboniferous marine carbonate rocks which lie above a wedge of Upper Devonian continental beds. The orefield is regionally zoned; deposits in the southern section of the orefield have elevated copper and silver contents while those in the northern section are dominated by zinc. [20]

Part III of the Mineral Resources will appear shortly.

Geological Glossary

Lower Carboniferous: the fifth period of the Paleozoic era of geologic time from 350 to 290 million years ago.

Historical Geology of the Period

The Carboniferous period was marked by vast, coal-forming swamps and a succession of changes in the earth's surface that, continuing into the Permian period, ended the Paleozoic era. The Carboniferous is often split into two divisions, the Mississippian and the Pennsylvanian; in the United States the break in the geologic sequence is so sharp that each division is commonly considered an independent period.

The Lower Carboniferous Period:

In the Lower Carboniferous, or Mississippian, period, the submersion—on several occasions—of the interior of North America under shallow seas resulted in the formation of limestone, shale, and sandstone. In the Appalachian region, especially in Pennsylvania, great deposits of sandstone and shale were laid down by the erosion products from the eastern coastal highlands. In the far west the Rocky Mt. region was covered by shallow seas that deposited the Madison and Redwall limestones of the Grand Canyon.

The Lower Carboniferous in Europe was a period of submergence and great volcanic activity. East of the Rhine, shales, sandstones, and conglomerates were deposited; and in Russia, the Coal Measures formed. The close of the Lower Carboniferous was marked by mountain building in New Brunswick, Nova Scotia, the S Appalachian region, the SW United States, and Europe.

The Upper Carboniferous Period:

In the Upper Carboniferous, or Pennsylvanian, period, there was at least one great submergence. In the E United States great deltas of sediments, now represented by the Pottsville conglomerate, were formed during the early Pennsylvanian. In Kansas, Nebraska, Arkansas, and Texas, the Pennsylvanian beds are chiefly shale, sandstone, and coal; over the Rocky Mountains region, marine limestone, with little coal; on the Pacific coast from California to Alaska, limestone and shale. The sea level also oscillated during the period and caused the formation of great marshes with extensive vegetation that was later transformed into coal, with Pennsylvanian strata containing the largest U.S. coal deposits. The Pennsylvanian coal fields of North America include the anthracite field of E Pennsylvania; the Appalachian field, from Pennsylvania to Alabama; the Michigan field; the eastern interior field, in Indiana, Illinois, and Kentucky; the western interior and southwestern field, stretching from Iowa to Texas; the Rhode Island field; and the Acadian field of SE Canada.

In the Upper Carboniferous of Western Europe, the Millstone Grit (the equivalent to the Pottsville conglomerate) is followed by the Coal Measures, which include the Welsh, English, Belgian, Westphalian, and Saar Basin fields. In the Mediterranean region and parts of Asia, the Upper Carboniferous environment resembled that of W. North America.

The Upper Carboniferous was a period of marked disturbances caused by collisions of crustal plates. Gondwanaland, the supercontinent containing the continents of Africa and S America, had formed; Euramerica, part of Europe and N America, had fused into a continent to the north; and Angara, today's Asia, was also to the north of Gondwanaland. In Europe the Paleozoic Alps were thrust up; in Asia, the Altai and the Tian Shan; in North America, the Arbuckle and Wichita mts. and the ancestral S Rockies. The Indian peninsula became an active site of deposition; in the Himalayan geosyncline and much of China, mountain building was dominant. Crustal movements in the Andean geosyncline of South America affected the pattern of sedimentation over much of the continent.

Carbonate: A class of sedimentary rock whose chief mineral constituents (95% or more) are calcite and aragonite (both CaCo3) and dolomite [CaMg(CO3)2], a mineral that can replace calcite during the process of dolomitization. Limestone, dolostone or dolomite, and chalk are carbonate rocks. Although carbonate rocks can be clastic in origin, they are more commonly formed through processes of precipitation or the activity of organisms such as coral and algae. Carbonates form in shallow and deep marine settings, evaporitic basins, lakes and windy deserts. Carbonate rocks can serve as hydrocarbon reservoir rocks, particularly if their porosity has been enhanced through dissolution. They rely on fractures for permeability.

Metasediment: In geology, metasediment is sediment or sedimentary rock that shows evidence of having been subjected to metamorphism. Metasediment is a metamorphic rock formed from sedimentary rock.

Metavolcanic: In geology, metavolcanic rock is a type of metamorphic rock. Such a rock was first produced by a volcano, either as lava or tephra. Then, the rock was buried underneath subsequent rock and was subjected to high pressures and temperatures, causing the rock to recrystallize.

Metavolcanic rock can contain the minerals quartz, feldspar, amphibole, pyroxene. Less common minerals can include biotite, garnet, actinolite, epidote, chalcedony, prehnite, and wodginite.

Proterozoic era: or Precambrian era:  The name of a major division of geologic time from c.5 billion to 570 million years ago. It is often divided into the Archeozoic and Proterozoic; in other countries, the Precambrian is broken into other divisions, including the oldest Archaean period, Aphebian, Riphean, and Vendian. Precambrian time includes 80% of the earth's history.

Precambrian rocks are mostly covered by rock systems of more recent origin, but where visible they commonly display evidence of having been altered by intense metamorphism. Precambrian rocks often occur in shields, which are large areas of relatively low elevation that form parts of continental masses. One of the largest exposed areas of Early Precambrian rocks is the Canadian Shield. It covers most of Greenland, extends over more than half of Canada, and reaches into the United States as the Superior Highlands and the Adirondack Mts.

The rocks of this region, and of the Early Precambrian as a whole, are generally granite, schist, or gneiss. The most notable formations are the Keewatin and Coutchiching of Minnesota and the adjoining part of Canada; the Grenville of Ontario, which, however, may be Late Precambrian; and the widely distributed Laurentian. The Keewatin series of rocks is composed chiefly of metamorphosed lava, with some sediments; the Coutchiching series is chiefly of sedimentary gneisses and schists. The Grenville limestone, marble, gneiss, and quartzite are predominantly metamorphosed sediments; the Laurentian gneiss and granite are probably younger than the other series, having been forced up through the Grenville as igneous rock. After the appearance of the Laurentian, the Temiskaming, or Sudburian, sediments were deposited, and a second series of gneisses and granites, the Algoman, was formed. Elsewhere in North America, Early Precambrian rocks are exposed in the Grand Canyon of Arizona and in the Teton Range of Wyoming. Among the other shield areas composed of Early Precambrian rocks are the Angara Shield in Siberia, the Australian Shield, the Baltic Shield in Europe, the Antarctic Shield, and the African Shield comprising most of the African continent. In South America, the Amazon River basin separates the Guiana and the Brazilian shields. Fossils have been reported from this era, but few have been found in strata universally acknowledged to be Early Precambrian. Evidence such as bacteria and algallike spheroids, supports the belief that rudimentary life existed. During the early Precambrian, radioactive heat from the new planet may have been so great that little permanent crust could survive.

By the latter Precambrian, heat dissipated enough to allow the continental crust to form; crustal rifting, mountain building, and volcanic activity then dominated, as did sedimentation. The life of the Late Precambrian is poorly represented by fossils, but a few invertebrates including creatures resembling jellyfish and worms have been discovered. The best evidence that there probably were numerous forms of life is the variety and complexity which suddenly appears in Cambrian fauna. Mineral deposits associated with Precambrian rocks have yielded most of the world's gold and nickel in addition to large quantities of copper, silver, radium, and uranium.

Palaeozoic: The Palaeozoic Era spans 322 million years, beginning with the Cambrian period 570 million years ago, and finishing with the end of the Permian  period 248 million years ago.

Cambrian period: [Lat. Cambria=Wales], first period of the Paleozoic geologic era (see Geologic Timescale, table) extending from approximately 570 to 505 million years ago.

Dalradian: is a geological term that describes a series of metamorphic rocks, typically developed in the high ground which lies southeast of the Great Glen of Scotland. This was the old Celtic region of Dál Riata (Dalriada), and in 1891 Sir A. Geikie proposed the name Dalradian as a convenient provisional designation for the complicated set of rocks to which it is difficult to assign a definite position in the stratigraphical sequence. In Sir A. Geikie's words, "they consist in large proportion of altered sedimentary strata, now found in the form of mica-schist, graphite-schist, andalusite-schist, phyllite, schistose grit, greywacke and conglomerate, quartzite, limestone and other rocks, together with epidiorites, chlorite-schists, hornblende schists and other allied varieties, which probably mark sills, lava-sheets or beds of tuff, intercalated among the sediments. The total thickness of this assemblage of rocks must be many thousand feet." The Dalradian series includes the "Eastern or Younger schists" of eastern Sutherland, Ross-shire and Inverness-shire, the Moine gneiss, as well as the metamorphosed igneous and sedimentary rocks of the central, eastern and southwestern Scottish Highlands. The series has been traced into the northwestern counties of Ireland. The whole of the Dalradian complex has suffered intense crushing and thrusting.

Clastic:  Made up of fragments of preexisting rock; fragmental.   

Barite: Used as a heavy additive in oil-well-drilling mud, in the paper and rubber industries, as a filler or extender in cloth, ink, and plastics products, in radiography (“barium milkshake”), as getter (scavenger) alloys in vacuum tubes, deoxidizer for copper, lubricant for anode rotors in X-ray tubes, spark-plug alloys. Also used to make an expensive white pigment.

Shear zones:
In many respects, shear zones are the deep-level equivalents to faults. They should accommodate relative displacement of the surrounding rocks just as faults do but rather than be surfaces, they consistute bands of rock that have undergone deformation. Some shear zones can be narrow - rather like faults. Others can be tens of kilometres wide - the deep-lithosphere equivalents of fault-dominated plate boundaries seen at the Earth's surface today. This site provides a brief introduction to shear zones, illustrated by outcrop scale examples from the field.

Antimony: Antimony in its elemental form is a silvery white, brittle crystalline solid that exhibits poor electrical and heat conductivity properties.   Commercial forms of antimony are generally ingots, broken pieces, granules, and cast cake.  Other forms are powder, shot, and single crystals.

Estimates of the abundance of antimony in the Earth's crust range from 0.2 to 0.5 parts per million.  Antimony is chalcophile, occurring with sulfur and the heavy metals, lead, copper, and silver.  Over a hundred minerals of antimony are found in nature.  Stibnite (Sb2S3) is the predominant ore mineral of antimony.

Inlier:  An area or formation of older rocks completely surrounded by younger layers.

Epigenetic / Epigenesis: Change in the mineral content of a rock because of outside influences.

Granodiorite:  This is a strange, mixed rock. It has a high quartz content, like a granite, but also a high mafic (amphibole/biotite) content (10-25%) more like a diorite.
Granodiorite is typically intermediate colored with a subequal mixture of light colored sodium plagioclase/quartz, and dark colored amphibole and biotite. Appearance, like diorite, is often described as "salt and pepper" because of the mix. In this specimen some of the quartz grains can be seen as smooth light gray grains scattered among the white feldspars.
(Granodiorite, like diorite, is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas).
Talc: (derived from the Persian via Arabic talq) is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it is the widely used substance known as talcum powder. Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as pyroxene, amphibole, olivine and other similar minerals in the presence of carbon dioxide and water. This is known as talc carbonation or steatization and produces a suite of rocks known as talc carbonates.

Magnesite:  is magnesium carbonate, MgCO3. Iron (as Fe2+) substitutes for Mg with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts. Dolomite, (Mg,Ca)CO3, is almost indistinguishable from magnesite. Magnesite can be formed via talc carbonate metasomatism of peridotite and other ultrabasic rocks. Magnesite is formed via carbonation of olivine in the presence of water and carbon dioxide, and is favored at moderate temperatures and pressures typical of greenschist facies;

Magnesite can also be formed via the carbonation of magnesian serpentine (lizardite) via the following reaction:
Serpentine + carbon dioxide → Talc + magnesite + Water.

Lithologies: Of or pertaining to the character of a rock, as derived from the nature and mode of aggregation of its mineral contents.

Auriferous: Containing gold; gold-bearing.
[From Latin aurifer, gold-bearing : aurum, gold + -fer, -fer.]

VMS: Volcanogenic massive sulphide deposits, VMS deposits are an ore deposit typically comprising a lens of massive sulphide minerals (>60% sulphide) formed by volcanic processes normally on the sea-floor. VMS deposits are important sources of copper, lead and zinc.
The deposits represent major sources of copper, zinc, lead, gold and silver in a high grade
low tonnage ratio.

(Au-VMS): Gold-rich volcanogenic massive sulphide deposits (Au-VMS) are a sub-type of both volcanogenic massive sulphide (VMS) and lode gold deposits. Their diagnostic features are strata-bound to discordant lenticular volcanic-hosted massive sulphide lenses with associated discordant stockwork feeder zones in which gold grades (in g/t) exceed associated combined copper, lead, and Zinc grades (in weight per cent). Gold is thus the main commodity. The Au-VMS deposits are present in both recent seafloor and deformed and metamorphosed submarine volcanic settings. They occur in a variety of submarine volcanic terranes from mafic bimodal through felsic bimodal to bimodal siliciclastic in greenstone belts of all ages, typically metamorphosed to greenschist and lower amphibolite facies, and intruded by sub-volcanic intrusions and dyke-sill complexes. The deposits are commonly located in proximity of intermediate to felsic volcanic centers, at or close to the interface between intermediate to felsic volcanic domes and basalt-andesite or clastic sediments.

REE metals: The rare earth elements (REE) form the largest chemically coherent group in the periodic table. Though generally unfamiliar, the REE are essential for many hundreds of applications. The versatility and specificity of the REE has given them a level of technological, environmental, and economic importance considerably greater than might be expected from their relative obscurity. The United States once was largely self-sufficient in these critical materials, but over the past decade has become dependent upon imports. In 1999 and 2000, more than 90% of REE required by U.S. industry came from deposits in China.

Mafic: In geology, any igneous rock dominated by the silicates pyroxene, amphibole, olivine, and mica. These minerals are high in magnesium and ferrous iron, and their presence gives mafic rock its characteristic dark colour. It is usually contrasted with felsic rock. Common mafic rocks include basalt and gabbro.
Pegmatite Rock: Is an igneous rock with extremely course grain size. To elaborate, a pegmatite has the same base constituents as granite (quartz, feldspar, mica) except the crystals are larger in size. In basic granite, the rock forming minerals usually crystallize in sizes between 0.4 and 1 inch. Pegmatites begin as a concentrated residual rock, rich in water, chlorine, boron and other elements deep within the earth under tremendous pressure. As the surrounding rock begins to solidify, the minerals become more concentrated. Eventually the concentrated material cools off and if the cooling is slow enough, large crystals begin to form. After eons of uplift and erosion, the pegmatites are exposed to the surface. Pegmatites are basically dike shaped bodies but because of their form they are better thought of as veins. The shape of pegmatite is influenced by the type of rock that surrounds it. Pegmatites may be spherical, bowed or curved, pipe-like, tear shaped or irregularly branched. Most often they are lens shaped or table like.

Batholith: An enormous mass of intrusive igneous rock, that is, rock made of once-molten material that has solidified below the earth's surface. Batholiths usually are granitic in composition, have steeply inclined walls, have no visible floors, and commonly extend over areas of thousands of square miles. Batholiths are formed either as one large mass or many smaller masses at great depths in the earth's crust and are exposed at the surface only after considerable erosion of the overlying mountain mass. They are commonly associated with lithospheric plate boundaries, where the interactions between plates can produce sufficient heat to melt crustal rocks on a large scale and form batholiths. One of the largest single batholiths in North America is the Coast Range batholith of W Canada and Alaska, encompassing an area of about 73,000 sq mi (182,500 sq km). Important batholiths in the United States include the Idaho batholith, 18,000 sq mi (45,000 sq km), and the Sierra Nevada batholith, 16,000 sq mi (40,000 sq km).

Chromite: is the most important ore of chromium from which it derives its name. Chromium is an important metal and has a wide range of industrial uses.

Chromite forms in deep ultra-mafic magmas and is one of the first minerals to crystallize. It is because of this fact that chromite is found in some concentrated ore bodies. While the magma is slowly cooling inside the Earth's crust, chromite crsytals are forming and because of their density, fall to the bottom and are concentrated there.
Chromium (Cr) has a wide range of uses in metals, chemicals, and refractories. It is one of the United States’ most important strategic and critical materials. Chromium use in iron, steel, and nonferrous alloys enhances hardenability and resistance to corrosion and oxidation. The use of chromium to produce stainless steel and nonferrous alloys are two of its more important applications.   Other applications are in alloy steel, plating of metals, pigments, leather processing, catalysts, surface treatments, and refractories.

Ultramafic: Or Ultrabasic - extremely basic, specifically: very low in silica and rich in iron and magnesium minerals.

Skarn deposits: are significant sources of tungsten, copper, iron, gold, molybdenum, lead, zinc and tin, as well as being minor sources of silver and bismuth. Industrial minerals, including wollastonite, mica, talc and graphite, are also produced from skarn deposits.

Based on the elements that are present, skarns can be divided into seven categories: iron, tungsten, copper, zinc-lead, molybdenum, gold and tin.

Mineral deposits containing skarn typically form at or near the contact between predominantly carbonate-rich rocks (limestone or dolomite) and an igneous intrusive body, or in carbonate veins along faults or fractures.

They form when hot magmatic fluids from the intrusion react with the host carbonate-rich rock, producing calcium, iron, manganese and magnesium silicates (also known as calc-silicates). This process is called metasomatism, meaning that new minerals grow in the host rocks when chemically active pore fluids are introduced into it from an external source. This new growth causes only minor textural or structural disturbances in the original rock.

The new minerals are typically coarse-grained crystals that grow over or replace the fine-grained or massive host rock. The calc-silicate minerals include garnet (calcium-rich grossularite and andradite to magnesium-rich pyrope), pyroxene (diopside to hedenbergite), epidote, olivine (forsterite to fayalite), wollastonite, amphibole (actinolite-tremolite to hornblende) and scapolite.

Garnet and pyroxene are the predominant minerals in most skarns, but not all other minerals develop in every skarn. The mineralogy of the skarn depends on factors including the composition of both the intrusive and carbonate rocks; the structural or relative permeable nature of the host rocks; and the level of intrusion.

In order for skarns to form, host rocks must be permeable so that metasomatic fluids can flow into and through them. If the host rock is impermeable to fluids, the build-up of heat from the cooling intrusion will cause thermal metamorphism, which bakes the rocks and leads to the formation of hornfels (fine-grained rock in which new minerals are created by thermal metamorphism of existing mineralogy).

Although hornfels is typically fine-grained, it can be overgrown by such coarse-grained minerals as andalusite or cordierite. Because of the differences in the permeability of the host rock, carbonates develop skarns, while impermeable calcareous shales develop hornfels.

Skarns are classified as either calcic, if they formed in a limestone, or magnesian, if they formed in a dolomitic host rock. Silicate skarns form when intrusives come into contact with calcium-rich silicate rocks, such as amphibolite. Endoskarn is skarn that develops in the intrusive, whereas exoskarn develops in the surrounding carbonate-rich rocks. Endoskarn is igneous rock-hosted; exoskarn, sedimentary rock-hosted.

Typically, skarns are zoned, their mineralogy changing with distance from the intrusion. Closer to the intrusion, garnet is more abundant than pyroxene. Farther from the intrusion, pyroxene becomes more abundant before grading into unaltered carbonate host rocks.

There are also subtle changes in the chemical compositions of the minerals, particularly in the iron-to-manganese ratio in pyroxene. Closer to the intrusive, pyroxene is iron-rich; farther away, it becomes manganese-rich. Garnets in copper and other skarns change in colour from dark-brown nearest the intrusive to yellow at greater distances.

Skarns form in three stages: 1. country rock is heated by an intrusive magma, resulting in thermal metamorphism of the rock into hornfels. 2. Dissolved metals are deposited during a water saturation phase that follows crystallization in the magma. This process is similar to that of the boiling phases that form porphyry deposits. This vapour-fluid phase infiltrates permeable country rock, causing metasomatism, and leads to skarn formation. Metal deposition (typically as sulphides) takes place in the later, cooling stages of the metasomatic event. 3. retrograde alteration occurs in the cooling of the system. This alteration develops through circulation of ground waters from country rock.

Molybdenum: Molybdenum does not occur free in nature; it is usually found in molybdenite ore, Molybdenum Disulphide, and wulfenite ore, PbMoO4. Molybdenum is also recovered as a by-product of copper and tungsten mining. It is a silvery-white metal of the chromium group. It is very hard and tough, but it is softer and more ductile than tungsten. It has a high elastic modulus. Of the readily-available metals, only tungsten and tantalum have higher melting points. Molybdenum is an important alloying agent which contributes to the hardenability and toughness of quenched and tempered steels. It also improves the strength of steel at high temperatures. It is used in certain heat-resistant and corrosion-resistant nickel-based alloys. Ferro-molybdenum is used to add hardness and toughness to gun barrels, boilers plates, tools, and armor plate. Almost all ultra-high strength steels contain 0.25% to 8% molybdenum. Molybdenum is used in nuclear energy applications and for missile and aircraft parts. Molybdenum oxidizes at elevated temperatures. Some molybdenum compounds are used to color pottery and fabrics. Molybdenum is used to make filament supports in incandescent lamps and as filaments in other electrical devices. The metal has found application as electrodes for electrically-heated glass furnaces. Molybdenum is valuable as a catalyst in the refining of petroleum. The metal is an essential trace element in plant nutrition. Molybdenum sulfide is used as a lubricant, particularly at high temperatures where oils would decompose. Molybdenum forms salts with valencies of 3, 4, or 6, but the hexavalent salts are the most stable.

Carlin – type gold deposits: Carlin-type deposits are characterized by relatively high levels of gold and silver, enrichment in arsenic, antimony, mercury, and thallium and by the dominance of "invisible gold" as ions or submicron-sized particles in iron sulfide. The deposits are generally but not always hosted by Paleozoic carbonate rocks, and it has been proposed that they are largely controlled by deep-seated, ancient structures.

In Nevada in the US, , in the massive, ‘carlin’ deposits of abundant carbonaceous residue (thermally degraded petroleum) in the siliceous Paleozoic rocks, accompanied by anomalous gold, arsenic, copper, molybdenum, nickel, selenium, vanadium, and zinc, at present it has been speculated that fossil petroleum reservoirs -- a key geochemical trapping component of Carlin-type gold deposits - are present in the subsurface beneath the siliceous Paleozoic sedimentary rocks.
Sedimentary-exhalative ("Sedex"), largely syngenetic: The main economic constituents of SEDEX ores are Zinc, Lead, and Silver, and these are hosted primarily by sphalerite and galena in the bedded ore facies.
SEDEX deposits are stratiform, massive sulphide lenses formed in local basins on the sea floor. This is usually as a result of hydrothermal activity in areas of continental rifting. They represent major sources of lead and zinc with minor amounts of gold, barium and copper. Alteration is common especially in the form of silicification. Sedex deposits have many similarities with VMS deposits.

Epigenetic: (epigenesis). Change in the mineral content of a rock because of outside influences.

Sulphides: One of several minerals containing negative sulfur ions bonded to one or more positive metallic ions.

Greenschist facies: One of the major divisions of the mineral facies classification of metamorphic rocks, encompassing the rocks that formed under fairly low temperatures (480–660 °F, or 250–350 °C) and pressure conditions and usually produced by regional metamorphism.

The minerals commonly found in such rocks include quartz, orthoclase, muscovite, chlorite, serpentine, talc, and epidote; carbonate minerals and amphibole (actinolite) may also be present.

Red Beds: The term red beds usually refers to strata of reddish-colored sedimentary rocks such as sandstone, siltstone or shale that were deposited in hot climates under oxidizing conditions.[1] The red color comes from iron oxide in their mineral structure. Although they have been deposited throughout the Phanerozoic, they are most commonly associated with rocks deposited during the Permian and Triassic periods. Red beds have economic significance since many of them contain reservoirs of petroleum and natural gas.

Terrigenous sediments:  are produced by the physical and chemical "weathering" (alteration) of rocks exposed on continents. Sedimentary particles are then eroded and transported from land to the oceans. Most sediment is carried by rivers, but winds and glaciers are also important transport agents. River-transported sediment is deposited on continental margins and mostly stays there. Only a small fraction is transported to the open ocean (e.g., by turbidity currents).   

Bryozoans: or "moss animals," are aquatic organisms, living for the most part in colonies of interconnected individuals. A few to many millions of these individuals may form one colony. Some bryozoans encrust rocky surfaces, shells, or algae. Others, like the fossil bryozoans shown here, form lacy or fan-like colonies that in some regions may form an abundant component of limestones. Bryozoan colonies range from millimeters to meters in size, but the individuals that make up the colonies are rarely larger than a millimeter. Colonies may be mistaken for hydroids, corals, or even seaweeds.

Vitric: Tuff composed principally of volcanic glass fragments.

Trachyte: Trachyte: A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase.

Granodioritic: Granitic minerals are those having low melting temperatures, and are therefore the last minerals to crystallize in the Bowen reaction series. Granitic minerals have a high silicon content ( 70%) and are therefore quite viscous. Granitic minerals include amphibole, potassium feldspar, quartz, and sodium feldspar. Granitic minerals are light in color.

Transgressive: Transgression: A rise in sea level relative to the land which causes areas to be submerged and marine deposition to begin in that region.

Facies: The term facies refers to all of the characteristics of a particular rock unit. For example, you might refer to a "tan, cross-bedded oolitic limestone facies". The characteristics of the rock unit come from the depositional environment.

Every depositional environment puts its own distinctive imprint on the sediment, making a particular facies. Thus, a facies is a distinct kind of rock for that area or environment.

Greywacke: (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly-sorted, angular grains of quartz, feldspar, and small rock fragments set in a compact, clay-fine matrix. It is a texturally-immature sedimentary rock generally found in Palaeozoic strata. The larger grains can be sand-to-gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume.

Dolomitization: Conversion of limestone to dolomite rock by replacing a portion of the calcium carbonate with magnesium carbonate.

Replacive: Replacement deposit: A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity.

Sphalerite: also known as Blende or Zinc Blende, is the major ore of Zinc. When pure (with little or no Iron) it forms clear crystals, usually red (known as Ruby Blende), but as Iron content increases it forms dark, opaque metallic crystals (known as Marmatite).

Galena: is the natural mineral form of lead sulfide. It is the most important lead ore mineral.

Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms. It is often associated with the minerals sphalerite, calcite and fluorite.

Lens: In geology a lens is a body of ore or rock or a geological deposit that is thick in the middle and thin at the edges, resembling a convex lens (adjective: "lenticular"). A lens can also refer to an irregular shaped formation consisting of a porous, permeable sedimentary deposit surrounded by impermeable rock.

Calcite: Calcium carbonate, commonly with some impurities of either iron, magnesium, manganese, and occasionally with zinc and cobalt.

Oolitic: An oolite (egg stone) is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word oiòn for egg. Strictly, oolites consist of ooids of diameter 0.25-2 mm: rocks composed of ooids larger than 2 mm are called pisolites.

Argillaceous: Describing rocks or sediments containing particles that are silt- or clay-sized, less than 0.625 mm in size. Most have a high clay-mineral content, and many contain a sufficient percentage of organic material to be considered a source rock for hydrocarbon.
Turbidites: Sedimentary deposits formed by turbidity currents in deep water at the base of the continental slope and on the abyssal plain. Turbidites commonly show predictable changes in bedding from coarse layers at the bottom to finer laminations at the top, known as Bouma sequences, that result from different settling velocities of the particle sizes present. The high energy associated with turbidite deposition can result in destruction of earlier deposited layers by subsequent turbidity currents.

 Bioclastic: rock formed from organic remains

Micrites: Or lime mud; CaCO3, the mineral calcite. Micrite is the equivalent of clay (rock = shale) in clastics. Originally deposited as microscopic aragonite needles, but now converted to calcite and then calcite cemented to form the rock.

Calcareous: Calcareous refers to a sediment, sedimentary rock, or soil type which is formed from or contains a high proportion of calcium carbonate in the form of calcite or aragonite.

Galena:  is the primary ore mineral of lead. Worked for its lead content as early as 3000BC, it is found in ore veins with sphalerite, pyrite, chalcopyrite, fahlore etc., and in sedimentary rocks as beds or impregmentations. The crystals are bright when fresh but often receive a dull tarnish after exposure to air.
Galena forms in low- and medium-temperature ore veins, along with other sulfide minerals, carbonate minerals, and quartz. These can be found in igneous or sedimentary rocks. It often contains silver as an impurity, and silver is an important byproduct of the lead industry.

Footnotes and References:

[1] Department of Communications, Marine and Natural Resources’ Exploration and Mining Division, Press Release, 2004.



[4] (Land of Mineral Opportunities : Exploration and mining Division, 2004).

[5] (ibid),





[11] (p.66, United States Geological Service, ((USGS)), Geology and Nonfuel Mineral Deposits of Greenland, Europe, Russia, and Northern Central Asia, 2005).

[12] (Newman, 2002d).

[13] (p. 82., ibid).

[14] (p.67.).







© The Tara Foundation, 2007