• Tourmaline and Aquamarine Deposits from Brazil

• Jade: Occurrence and Metasomatic Origin

• Brazilian Gem Provinces

• Emerald Deposits - A Review

• Gem Tourmaline Chemistry and Paragenesis

• Origin of Gem Corundums from Basaltic Fields

• The Geology of Australian Opal Deposits

• Viewpoint - Much Ado About Nothing

(Follow this link for abstracts of past issues)

TOURMALINE AND AQUAMARINE DEPOSITS FROM BRAZIL

J. César-Mendes¹, H. Jort-Evangelista¹, R. Wegner^2
1 Federal University of Ouro Preto, Brazil
² Federal University of Paraiba, Brazil.

ABSTRACT
Brazil is one of the world’s largest producers of aquamarine and gem-quality tourmaline. These gemstones are found in granitic pegmatites from the Oriental Pegmatitic Province (OPP) in the states of Minas Gerais, Bahia, and Espírito Santo, and the Northeastern Pegmatitic Province (NPP) in the states of Ceará, Paraíba, and Rio Grande do Norte. Aquamarine occurs in only slightly differentiated pegmatite bodies associated with mica (muscovite and biotite), K-feldspar, and quartz. Tourmaline (verdelite, indicolite and rubellite) occurs mainly in pockets of heterogeneous, rare element-bearing pegmatites containing lepidolite, albite, tantalite and rose beryl. More than 500 gemstone-bearing pegmatites are found in the OPP. In Minas Gerais, the most important tourmaline mines are located near São José da Safira (Morro do Cruzeiro and Chiar mines), Coronel Murta (Barra do Salinas mine) and Conselheiro Pena (Itatiaia and Jonas mines). Aquamarine is mainly extracted from pegmatites near Coronel Murta and Santa Maria do Itabira (Minas Gerais), Pancas (Espirito Santo) and Medeiros Neto (Bahia). In the NPP there are more than 50 elbaite-bearing pegmatites, as well as those hosting the brilliant blue copper-bearing tourmalines from São José da Batalha (Paraíba). Aquamarines from Tenente Ananias (Rio Grande do Norte) are famous for its excellent colour. Geochemical data have shown that the Brazilian aquamarines are formed in iron-bearing systems, with iron its main chromophore. Of the thirteen (13) tourmaline-forming molecules, Brazilian gem tourmalines are always rich in the elbaite molecule.

JADE: OCCURRENCE AND METASOMATIC ORIGIN

G.E. Harlow¹, S.S. Sorensen^2
1 American Museum of Natural History, New York, NY, U.S.A
² Smithsonian Institution, Washington, DC, U.S.A.

ABSTRACT
True jade refers to two extremely tough, monomineralic rocks used for carvings and gems: nephrite or tremolite-actinolite [Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂] rock with a felted, microcrystalline habit; and jadeite [NaAlSi₂O₆] rock, or jadeitite, with micro- to macro- crystalline textures. Nephrite is more common, with important deposits in British Columbia (Canada), Kunlun Mountains (China), East Sayan Mountains (Russia), Cowell (South Australia), and South Island (New Zealand). Nephrite bodies result from contact and/or infiltration metasomatism of either dolomite by magmatic fluids, or silicic rocks by serpentinite fluids. Conditions of formation range from upper-greenschist-amphibolite facies (<550 °C) in the dolomite-derived type, or moderate to low temperatures (~400 to ~100 °C) in ophiolites at moderate to low pressure (<2kbar?). Jadeitite is rarer than nephrite, with important deposits in northern Myanmar (Burma), Motagua Valley (Guatemala), Polar Ural and Borus Mountains (Russia), and Itmurundy (Kazakhstan) Jadeitite is very uncommon and only occurs as bodies in subduction-related serpentinite along major fault zones. Rhythmically zoned jadeite crystals in jadeitites indicate crystallization from an aqueous fluid, undoubtedly as veins in host serpentinite. Jadeite indicates high pressure, but the absence of coexisting quartz requires pressure only above 5-6 kbar for its low temperature environments (200 to 400 °C). Jadeitite formation requires devolatilization, primarily sediment dewatering, within a subducting slab at depths down to the blueschist-to-eclogite transition; with fluids channelled in serpentinite diapirs rising through faults in a mantle wedge. Thus, most jade deposits record events at convergent margins that involve fluid interactions in and around serpentinizing peridotite.

BRAZILIAN GEM PROVINCES

ABSTRACT
Brazil is one of the world’s largest producers of gemstones, principally those gems containing Be, B, F, and the quartz group. The principal gemstones produced are aquamarine, alexandrite, emerald, tourmaline, topaz, amethyst, agate and opal. The main deposits are genetically related to pegmatite and pneumatolitic-hydrothermal fluids in mafic-ultramafic rocks; to hydrothermal fluids in psamitic metasediments; and to volcanogenic concentrations in continental basaltic flows. The main pegmatitic provinces are situated in the structural provinces of Mantiqueira, Borborema and Tocantins within Neoproterozoic mobile belts affected by intense granitic intrusion and gem mineralization. The gemstones rich in Be (aquamarine and other varieties of beryl, chrysoberyl, alexandrite) are mainly related to the Neoproterozoic granitic intrusion. Gem-quality tourmaline is found mainly in pegmatite intruded into metasediments. Emerald occurs in ultramafic schist in the vicinity of granitic bodies. Gemstones of the quartz group (rock crystal, citrine, amethyst, chalcedony, and agate) are related to hydrothermal veins, principally in Proterozoic quartzose rocks and in druses in Mesozoic basaltic flows of the Parana and Parnaiba basins. The deposits of Brazilian coloured gems are related mainly to two geotectonic events: 

• i. consolidation of the Gondwana supercontinent during the Brasiliano-Pan-African Cycle, when
  intense granitic intrusion in the Brasiliano mobile belts occurred; and,
• ii. opening of the South Atlantic with related extrusion of plateaux basalt in the Parana and Parnalba basins.

EMERALD DEPOSITS - A REVIEW

D. Schwarz¹, G. Giuliani ^2
1 Gübelin Gem Lab Ltd., Lucerne, Switzerland
² Centre de Recherches Pétrographiques et Gochimiques, Vandoeuvre lés Nancy, France

ABSTRACT
Emerald, the green variety of beryl, is scarce because its chromophoric elements (chromium and vanadium) are geochemically not related to beryllium. Sources of chromium and vanadium are mafic-ultramafic igneous rocks, as well as sedimentary formations, like black shales. Sources of Be are aluminous magmas, black shales and metamorphic rocks.

The juxtaposition of Cr/V and Be, in nature, requires exceptional geological and geochemical conditions. The principal mechanisms responsible for emerald crystallization are fluid-rock interactions, which allow the combination of the incompatible elements. 

Formation of most emerald deposits in the world are associated with granitic intrusions. Hydrothermal processes related to granitic-pegmatitic systems lead to the crystallization of emeralds in mafic-ultramafic or in (meta-)sedimentary rocks. In general, the mafic-ultramafic hosts of emeralds are schistose rocks of varying composition (e.g. in most African and Brazilian deposits, as well as in the Ural Mountains.) In Eidsvoll (Norway) and Emmaville-Torrington (Australia), emeralds are hosted by (meta-)sedimentary rocks. 

A second group of emerald deposits is not directly related to granitic intrusions. In these, tectonic phenomena (thrust faults and shear zones) are the controlling factors for the formation of emerald mineralisations. Circulation of fluids along these regional tectonic structures resulted in emerald formation in volcano-sedimentary series, e.g. Santa Terezinha (Brazil), Habachtal (Austria), or in oceanic suture zones such as the Swat Valley (Pakistan), and the Panjsher Valley (Afghanistan). The famous deposits in the Colombian Cordillera Oriental have a unique formation through the thermochemical reduction of evaporitic sulphate brines, with the participation of organic matter from the surrounding black shales in the reactions.

GEM TOURMALINE CHEMISTRY AND PARAGENESIS

W.B. Simmons, K.L. Webber, A.U. Falster, J.W. Nizamoff
Deptartment of Geology, University of New Orleans, USA.

ABSTRACT
Gem tourmaline is known from many highly fractionated pegmatites throughout the world, and considerable chemical variation exists in tourmaline from locality to locality. This study examines tourmalines from six worldwide pegmatite locations (Transbaikalia in Russia, two central Madagascar locations, two North American locations, and northern Brazil) to determine their chemical characteristics and pegmatite paragenesis. 

All the tourmalines are dominantly elbaite or liddicoatite. Darker coloured varieties contain a minor schorl component. Y-site chemistry strongly correlates with colour and appears to be strongly influenced by the chemical characteristics of the pegmatites in which the tourmaline occurs. X-site vacancies of all tourmalines are less than 0.3 apfu. Fe and to a lesser extent Mn, Ti and Cu are the principal chromophores. Pink and green tourmalines from Transbaikalia are essentially elbaites, however, yellow zones within these crystals show a substantial liddicoatite component, coupled with elevated Ti and Mn contents. Tourmalines from the Antandrokomby pegmatite, Madagascar, are members of the schorl-elbaite series with significant Mg and Ca contents. Even though the pegmatite occurs in a metadolomite, tourmaline Ca content is less than that of tourmaline from the classic Fianarantsoa and Anjanabonoina regions of Madagascar, where the tourmaline is liddicoatite and the pegmatites occur in pelitic country rocks. Tourmalines from both Madagascar locations have the lowest Y-site Al content. Tourmalines from San Diego County, California, and Newry, Maine, USA, are very similar chemically and belong to the schorl-elbaite series, with lighter coloured, gemmy varieties approaching end-member elbaite. Paraiba tourmaline from Brazil is elbaitic in composition but contains significant Cu, which imparts a vivid blue colour.

ORIGIN OF GEM CORUNDUMS FROM BASALTIC FIELDS

F.L. Sutherland¹, D. Schwatz^2
1Australian Museum, Sydney, Australia
²Gubelin Gem Laboratory, Lucerne, Switzerland.

ABSTRACT
Favoured basalt fields yield gem corundums among their xenocrystal offerings. They are recorded in six (6) continental regions, within 15 countries and involve over 40 main basalt fields. The corundums commonly include ‘magmatic’ blue, green to yellow, colour-zoned sapphires and less commonly ‘metamorphic’ various coloured sapphires and ruby. Magmatic suites (60 % of basalt fields) dominate over mixed magmatic/metamorphic suites (25 %) and metamorphic suites (15 %).

Magmatic sapphires contain diverse, but characteristic, mineral inclusions. Co-existing zircon yields uranium-lead isotope formation ages and presumably also the sapphire crystallisation ages. These ages are usually close to host basaltic eruption ages. Rare sapphire-bearing felspathic xenoliths suggest a coarse syenitic origin. Crystallisation of magmatic sapphires has been variously ascribed to mid-crustal carbonatitic-silicic hybrid melt interactions or to lower crust/mantle felsic melts. Low volume melting of hydrous mantle to produce such syenitic melts was proposed from zircon/basalt dating within Australian sapphire fields. Growth from buffered high pressure syenitic melts undergoing fugitive alkali carbonatitic volatile loss is supported by recent studies in Scotland.

Metamorphic sapphire and ruby suites incorporate mineral inclusions and trace element contents that indicate a range of metamorphic source assemblages. Rare corundum-bearing metamorphic xenoliths suggest contact metamorphic, alumino- silicate regional metamorphic and lower crust granulitic sources. 

In eastern Australia, trace element contents in corundums sampled from a 3000km long basaltic tract differentiate corundums into several separate magmatic and metamorphic fields. A few corundum suites show intermediate geochemical characteristics between those of ‘magmatic’ and ‘metamorphic’ origin and their precise origin requires further study.

THE GEOLOGY OF AUSTRALIAN OPAL DEPOSITS

I.J. Townsend
Department of Primary Industries and Resources, 
Office of Minerals and Energy, Adelaide, South Australia

ABSTRACT
Australian opal is hosted predominantly by sedimentary rocks of the Cretaceous, Great Artesian Basin (GAB). Silica, derived from weathering, forms spheres which are deposited in a regular array. Once a certain size is reached precious opal begins to form. 

Host rocks contained a variety of voids formed by the weathering process; leaching of carbonate from boulders, nodules, fossils, along with existing cracks, hollow centres of ironstone nodules and horizontal seams. Most opaline silica deposited is common opal (or potch). It does not show a play of colour. Opal also fills pore space in sand size sediments cementing the grains together forming deposits known as matrix or opalised sandstone. Opal is often associated with lineaments or faults which break up the rock providing waterways for the movement of ground water. These have been found useful in locating opal at Lightning Ridge in New South Wales (NSW) and pursued in other states. In addition, opal has been found associated with ancient palaeochannels in Queensland, and Lightning Ridge (NSW) often adjacent to these channels, which provide water channels. 

Variations in the types of opal depend on a number of factors. Firstly the climate provides alternating wet and dry periods, creating a rising or importantly falling water table which concentrates silica in solution. The silica itself is formed by deep weathering of Cretaceous clay sediments producing both silica and kaolin. Silica spheres are deposited in a regular array in voids from a receding water table forming precious opal in a variety of host materials.

VIEWPOINT
MUCH ADO ABOUT NOTHING

W.Wm.Hanneman
Poulsbo, Washington, USA

ABSTRACT
An opinion about high pressure - high temperature treatment of diamond to commercially produce colourless to near colourless diamond, and the difficulties and practicalities of accurately determining this treatment.