Continued Positive Sentiment Points To Further Gains In Gold Prices: Kitco Gold Survey

Friday February 12, 2016

(Kitco News) - Sentiment in the gold market remains strong  after prices hit a one-year high in a parabolic move and the majority of people surveyed  expecting more gains in the near-term, according to this week’s Kitco News Wall Street vs. Main Street Gold Survey.

Gold is preparing to end its third positive consecutive week, with gains of more than 5.3%, the biggest weekly percentage gain since late October 2011.

Analysts have noted that the yellow metal’s push back above $1,200 an ounce, generated a lot of attention and positive sentiment among fearful investors looking for a safe-haven. Strong positive sentiment is confirmed in Kitco’s weekly survey, which continues to attract record participation.

This week, the online survey brought in 1,953 votes, of which 1,678 participants, or 86%, said they are bullish on gold next week. This is the second week the survey has hit that level and is the fourth straight week it has been above 80%. At the same time, 177 people, or 9%, said they are bearish on gold next week and 98 people, or 5%, are neutral.

Although the spread is much narrower, a majority of market analysts also expect gold prices to move higher in the near-term. Out of 34 market experts contacted, 17 responded, of which nine, or 53%, said they expect to see higher prices next week. At the same time seven professionals, or 41%, said they see lower prices and one analyst was neutral on the market. Market participants include bullion dealers, investment banks, futures traders and technical-chart analysts.

Many analysts are bullish on gold, expecting sentiment to continue to grow as prices have broken through key technical barriers, culminating in a weekly high at $1,263.90 an ounce.

“Sentiment towards gold has changed dramatically, and gold has even moved up on some days in the face of a dollar or stock market rally. With a change in sentiment, those underweight or waiting on the sidelines will start buying and this will fuel gold’s rally for a little longer,” said Adrian Day, president of Adrian Day Asset Management.

While analysts are positive on the gold market, they are not ruling out some weakness at the start of the week.

Phillip Streible, senior market analyst for RJO Futures, said that he remains bullish on gold as prices remain above $1,230 an ounce, a level that he said is an important pivot point, representing a 50% retracement from Thursday’s considerable rally.

“We could see some profit taking early in the week but the gold engines will only rev up again. The problems we have seen that sparked this rally can’t be fixed overnight,” he said.

Bill Baruch, senior commodity broker at iiTrader, agreed that $1,230 will be an important support level to watch; however, he added that he is still a buyer of gold as prices remain above $1,200 an ounce.

“The market does seem a little over done but that doesn’t mean we can’t see another push,” he said.

On the negative side, analysts say they expect to see lower prices next week on a technical move, explaining that the market needs to consolidate after its recent parabolic drive.

“I just think [gold] shot up way too fast, the chart looks parabolic and is way overbought. [The market] is due for a correction. I also think [gold] will hold $1,200 and continue to trend higher over the longer term as the [US dollar] retreats,” said Colin Cieszynski, senior market strategist at CMC Markets.

Wall Street

Bullish53%

Bearish41%

Neutral6%

VS

Main Street

Bullish86%

Bearish9%

Neutral5%

By Neils Christensen

Ouro ultrapassa o melhor preço em 12 meses. Mineradoras estão bombando.

Publicado em: 2/12/2016 

Desde o início do ano o ouro está em uma alta alucinante. Em poucas semanas a onça subiu US$180. Uma aceleração para ninguém botar defeito

A volatilidade dos mercados, o baixo crescimento da China, a queda das commodities a política do Banco Central Americano são as principais razões que levam os investidores apostar no metal amarelo.

É sempre assim. Época de crise corra para o ouro...

A subida é tão pronunciada que alguns analistas, com medo, já começam a duvidar se o momento é de compra ou de realização de lucros.

Enquanto isso as mineradoras de ouro singram os mares conturbados da economia mundial sem problemas.

Somente neste ano a Yamana Gold subiu 81,86% a AngloGold 64,3% a Barrick Gold 64,68% e a a Kinross, com a melhor performance de todas, subiu impressionantes 108%.

Existe vida na mineração!!!

SURFICIAL PLACERS

SURFICIAL PLACERS
by Victor M. Levson
British Columbia Geological Survey
  

Ref: sedimento, aluvião, aluvionar, terraço, fluvial, eólico, glacial

Levson, Victor M. (1995): Surficial Placers, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Energy of Employment and Investment, Open File 1995-20, pages 21-23. IDENTIFICATION SYNONYMS: Holocene placer deposits; terrace placers; fluvial, alluvial, colluvial, eolian (rare) and glacial (rare) placers. COMMODITIES (BYPRODUCTS): Au, PGEs and Sn, {locally Cu, garnet, ilmenite, cassiterite, rutile, diamond and other gems - corundum (rubies, sapphires), tourmaline, topaz, beryl (emeralds), spinel - zircon, kyanite, staurolite, chromite, magnetite, wolframite, sphene, barite, cinnabar}. Most of the minerals listed in brackets are recovered in some deposits as the principal product. EXAMPLES (British Columbia - Canada/International): Fraser River (Au), Quesnel River (Au), Tulameen district (PGEs); North Saskatchewan River (Au, Alberta, Canada), Vermillion River (Au, Ontario,Canada), Rivière Gilbert (Au, Québec, Canada), Klondike (Au, Yukon, Canada), Rio Tapajos (Au, Brazil), Westland and Nelson (Au, New Zealand), Yana-Kolyma belt (Au, Russia), Sierra Nevada (Au, California, USA), Goodnews Bay( PGE, Alaska, USA), Emerald Creek (garnet, Idaho, USA), Rio Huanuni and Ocuri (Sn, Bolivia), Sundaland belt (Sn, Thailand). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Detrital gold, platinum group elements and other heavy minerals occurring at or near the surface, usually in Holocene fluvial or beach deposits. Other depositional environments, in general order of decreasing importance, include: alluvial fan, colluvial, glaciofluvial, glacial and deltaic placers. TECTONIC SETTINGS: Fine-grained, allochthonous placers occur mainly in stable tectonic settings (shield or platformal environments and intermontane plateaus) where reworking of clastic material has proceeded for long periods of time. Coarse, autochthonous placer deposits occur mainly in Cenozoic and Mesozoic accretionary orogenic belts and volcanic arcs, commonly along major faults. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Surficial fluvial placer concentrations occur mainly in large, high-order, stream channels (allochthonous deposits) and along bedrock in high-energy, steep-gradient, low-sinuosity, single-channel streams (autochthonous deposits). Concentrations occur along erosional surfaces at the base of channel sequences. Alluvial fan, fan-delta and delta deposits are distinct from fluvial placers as they occur in relatively unconfined depositional settings and typically are dominated by massive or graded sands and gravels, locally with interbedded diamicton. Colluvial placers generally develop from residual deposits associated with primary lode sources by sorting associated with downslope migration of heavy minerals. Glaciofluvial and glacial placers are mainly restricted to areas where ice or meltwater has eroded pre-existing placer deposits. Cassiterite, ilmenite, zircon and rutile are lighter heavy minerals which are distributed in a broader variety of depositional settings. AGE OF MINERALIZATION: Mainly Holocene (rarely Late Pleistocene) in glaciated areas; generally Tertiary or younger in unglaciated regions. HOST/ASSOCIATED ROCK TYPES: Well sorted, fine to coarse-grained sands; well rounded, imbricated and clast-supported gravels. DEPOSIT FORM: In fluvial environments highly variable and laterally discontinuous; paystreaks typically thin (< 2 m), lens shaped and tapering in the direction of paleoflow; usually interbedded with barren sequences. TEXTURE/STRUCTURE: Grain size decreases with distance from the source area. Gold typically fine grained (< 0.5 mm diameter) and well rounded; coarser grains and nuggets rare, except in steep fluvial channel settings where gold occurs as flattened flakes. Placer minerals associated with colluvial placer deposits are generally coarser grained and more angular. ORE MINERALOGY (principal and subordinate): Au, PGE and cassiterite (Cu, Ag and various industrial minerals and gemstones). GANGUE MINERALOGY: Quartz, pyrite and other sulphides and in many deposits subeconomic concentrations of various heavy minerals such as magnetite and ilmenite. ALTERATION MINERALOGY: Fe and Mn oxide precipitates common; Ag-depleted rims of Au grains increase in thickness with age. ORE CONTROLS: In fluvial settings, placer concentrations occur at channel irregularities, in bedrock depressions and below natural riffles created by fractures, joints, cleavage, faults, foliation or bedding planes that dip steeply and are oriented perpendicular or oblique to stream flow. Coarse- grained placer concentrations occur as lag concentrations where there is a high likelihood of sediment reworking or flow separation such as at the base of channel scours, around gravel bars, boulders or other bedrock irregularities, at channel confluences, in the lee of islands and downstream of sharp meanders. Basal gravels over bedrock typically contain the highest placer concentrations. Fine-grained placer concentrations occur where channel gradients abruptly decrease or stream velocities lessen, such as at sites of channel divergence and along point bar margins. Gold in alluvial fan placers is found in debris- flow sediments and in interstratified gravel, sand and silt. Colluvial placers are best developed on steeper slopes, generally over a weathered surface and near primary lode sources. Economic gold concentrations in glaciofluvial deposits occur mainly along erosional unconformities within otherwise aggradational sequences and typically derive their gold from older placer deposits. GENETIC MODEL: Fluvial placers accumulate mainly along erosional unconformities overlying bedrock or resistant sediments such as basal tills or glaciolacustrine clays. Basal gravels over bedrock typically contain the highest placer concentrations. Overlying bedded gravel sequences generally contain less placer minerals and reflect bar sedimentation during aggradational phases. Frequently the generation of more economically attractive placer deposits involves multiple cycles of erosion and deposition. ASSOCIATED DEPOSIT TYPES: Fluvial placers commonly derive from hydrothermal vein deposits and less commonly from porphyry and skarn deposits. PGE placers are associated with Alaskan-type ultramafics. Allochthonous fluvial placers are far traveled and typically remote from source deposits. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Anomalous concentrations of Au, Ag, Hg, As, Cu, Fe, Mn, Ti or Cr in stream sediments. Au fineness (relative Ag content) and trace element geochemistry (Hg, Cu) of Au particles can be used to relate placer and lode sources. GEOPHYSICAL SIGNATURE: Ground penetrating radar especially useful for delineating the geometry, structure and thickness of deposits with low clay contents, especially fluvial terrace placers. Shallow seismic, electromagnetic, induced polarization, resistivity and magnetometer surveys are locally useful. Geophysical logging of drill holes with apparent conductivity, naturally occurring gamma radiation and magnetic susceptibility tools can supplement stratigraphic data. OTHER EXPLORATION GUIDES: Panning and other methods of gravity sorting are used to identify concentrations of gold, magnetite, hematite, pyrite, ilmenite, chromite, garnet, zircon, rutile and other heavy minerals. Many placer gold paystreaks overlie clay beds or dense tills and in some camps these ‘false bottom’ paystreaks are important. ECONOMIC FACTORS TYPICAL GRADE AND TONNAGE: Deposits are typically high tonnage (0.1 to 100 Mt) but low grade (0.05-0.25 g/t Au, 50-200 g/t Sn). Placer concentrations are highly variable both within and between individual deposits. ECONOMIC LIMITATIONS: The main economic limitations to mining surficial placer deposits are typically low grades and most deposits occur below the water table. Environmental considerations are also an important limiting factor as these deposits often occur near, or within modern stream courses. IMPORTANCE: Placer gold deposits account for more than two-thirds of the world's gold reserves and about 25% of known total production in British Columbia. Recorded placer production has represented 3.5% of B.C.’s total gold production in the last twenty years. Prior to 1950, it was approximately 160 000 kg. Actual production was significantly larger. Placer mining continues to be an important industry in the province with annual average expenditures of more than $30 million over a survey period from 1981 to 1986. Shallow alluvial placers also account for a large part of world tin (mainly from SE Asia and Brazil) and diamond (Africa) production. REFERENCES:

Au SKARNS

Au SKARNS
by Gerald E. Ray
British Columbia Geological Survey

Ref: ouro, skarnito, metassomatismo, tactito

Revised 1997 Ray, G.E. (1998): Au Skarns, in Geological Fieldwork 1997, British Columbia Ministry of Employment and Investment, Paper 1998-1, pages 24H-1 to 24H-4. IDENTIFICATION SYNONYMS: Pyrometasomatic, tactite, or contact metasomatic Au deposits. COMMODITIES (BYPRODUCTS): Au (Cu, Ag). EXAMPLES (British Columbia - Canada/International): Nickel Plate (092HSE038), French (092HSE059), Canty (092HSE 064), Good Hope (092HSE060), QR - Quesnel River (093A121); Fortitude, McCoy and Tomboy-Minnie (Nevada,USA), Buckhorn Mountain (Washington,USA), Diamond Hill, New World district and Butte Highlands (Montana,USA), Nixon Fork (Alaska, USA), Thanksgiving (Philippines), Browns Creek and Junction Reefs-Sheahan-Grants (New South Wales, Australia), Mount Biggenden (Queensland, Australia), Savage Lode, Coogee (Western Australia, Australia), Nambija (Ecuador), Wabu (Irian Jaya, Indonesia). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Gold-dominant mineralization genetically associated with a skarn gangue consisting of Ca - Fe - Mg silicates, such as clinopyroxene, garnet and epidote. Gold is often intimately associated with Bi or Au-tellurides, and commonly occurs as minute blebs (<40 microns) that lie within or on sulphide grains. The vast majority of Au skarns are hosted by calcareous rocks (calcic subtype). The much rarer magnesian subtype is hosted by dolomites or Mg-rich volcanics. On the basis of gangue mineralogy, the calcic Au skarns can be separated into either pyroxene-rich, garnet-rich or epidote-rich types; these contrasting mineral assemblages reflect differences in the hostrock lithologies as well as the oxidation and sulphidation conditions in which the skarns developed. TECTONIC SETTINGS: Most Au skarns form in orogenic belts at convergent plate margins. They tend to be associated with syn to late island arc intrusions emplaced into calcareous sequences in arc or back-arc environments. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Most deposits are related to plutonism associated with the development of oceanic island arcs or back arcs, such as the Late Triassic to Early Jurassic Nicola Group in British Columbia. AGE OF MINERALIZATION: Phanerozoic (mostly Cenozoic and Mesozoic); in British Columbia Au skarns are mainly of Early to Middle-Jurassic age. The unusual magnesian Au skarns of Western Australia are Archean. HOST/ASSOCIATED ROCK TYPES: Gold skarns are hosted by sedimentary carbonates, calcareous clastics, volcaniclastics or (rarely) volcanic flows. They are commonly related to high to intermediate level stocks, sills and dikes of gabbro, diorite, quartz diorite or granodiorite composition. Economic mineralization is rarely developed in the endoskarn. The I-type intrusions are commonly porphyritic, undifferentiated, Fe-rich and calc-alkaline. However, the Nambija, Wabu and QR Au skarns are associated with alkalic intrusions. DEPOSIT FORM: Variable from irregular lenses and veins to tabular or stratiform orebodies with lengths ranging up to many hundreds of metres. Rarely, can occur as vertical pipe-like bodies along permeable structures. TEXTURE/STRUCTURE: Igneous textures in endoskarn. Coarse to fine-grained, massive granoblastic to layered textures in exoskarn. Some hornfelsic textures. Fractures, sill-dike margins and fold hinges can be an important loci for mineralization. ORE MINERALOGY (Principal and subordinate): The gold is commonly present as micron-sized inclusions in sulphides, or at sulphide grain boundaries. To the naked eye, ore is generally indistinguishable from waste rock. Due to the poor correlation between Au and Cu in some Au skarns, the economic potential of a prospect can be overlooked if Cu-sulphide-rich outcrops are preferentially sampled and other sulphide-bearing or sulphide-lean assemblages are ignored. The ore in pyroxene-rich and garnet-rich skarns tends to have low Cu:Au (<2000:1), Zn:Au (<100:1) and Ag/Au (<1:1) ratios, and the gold is commonly associated with Bi minerals (particularly Bi tellurides). Magnesian subtype: Native gold ± pyrrhotite ± chalcopyrite ± pyrite ± magnetite ± galena ± tetrahedrite. Calcic subtype: Pyroxene-rich Au skarns: Native gold ± pyrrhotite ± arsenopyrite ± chalcopyrite ± tellurides (e.g. hedleyite, tetradymite, altaite and hessite) ± bismuthinite ± cobaltite ± native bismuth ± pyrite ± sphalerite ± maldonite. They generally have a high sulphide content and high pyrrhotite:pyrite ratios. Mineral and metal zoning is common in the skarn envelope. At Nickel Plate for example, this comprises a narrow proximal zone of coarse-grained, garnet skarn containing high Cu:Au ratios, and a wider, distal zone of finer grained pyroxene skarn containing low Cu:Au ratios and the Au-sulphide orebodies. Garnet-rich Au skarns: Native gold ± chalcopyrite ± pyrite ± arsenopyrite ± sphalerite ± magnetite ± hematite ± pyrrhotite ± galena ± tellurides ± bismuthinite. They generally have a low to moderate sulphide content and low pyrrhotite:pyrite ratios. Epidote-rich Au skarn: Native gold ± chalcopyrite ± pyrite ± arsenopyrite ± hematite ± magnetite ± pyrrhotite ± galena ± sphalerite ± tellurides. They generally have a moderate to high sulphide content with low pyrrhotite:pyrite ratios. EXOSKARN MINERALOGY (GANGUE): Magnesian subtype: Olivine, clinopyroxene (Hd2-50), garnet (Ad7-30), chondrodite and monticellite. Retrograde minerals include serpentine, epidote, vesuvianite, tremolite-actinolite, phlogopite, talc, K-feldspar and chlorite. Calcic subtype: Pyroxene-rich Au skarns: Extensive exoskarn, generally with high pyroxene:garnet ratios. Prograde minerals include diopsidic to hedenbergitic clinopyroxene (Hd 20-100), K-feldspar, Fe-rich biotite, low Mn grandite garnet (Ad 10-100), wollastonite and vesuvianite. Other less common minerals include rutile, axinite and sphene. Late or retrograde minerals include epidote, chlorite, clinozoisite, vesuvianite, scapolite, tremolite-actinolite, sericite and prehnite. Garnet-rich Au skarns: Extensive exoskarn, generally with low pyroxene:garnet ratios. Prograde minerals include low Mn grandite garnet (Ad 10-100), K-feldspar, wollastonite, diopsidic clinopyroxene (Hd 0-60), epidote, vesuvianite, sphene and apatite. Late or retrograde minerals include epidote, chlorite, clinozoisite, vesuvianite, tremolite-actinolite, sericite, dolomite, siderite and prehnite. Epidote-rich Au skarns: Abundant epidote and lesser chlorite, tremolite-actinolite, quartz, K-feldspar, garnet, vesuvianite, biotite, clinopyroxene and late carbonate. At the QR deposit, epidote-pyrite and carbonate-pyrite veinlets and coarse aggregates are common, and the best ore occurs in the outer part of the alteration envelope, within 50 m of the epidote skarn front. ENDOSKARN MINERALOGY (GANGUE): Moderate endoskarn development with K-feldspar, biotite, Mg-pyroxene (Hd 5-30) and garnet. Endoskarn at the epidote-rich QR deposit is characterized by calcite, epidote, clinozoisite and tremolite whereas at the Butte Highlands Mg skarn it contains argillic and propyllitic alteration with garnet, clinopyroxene and epidote. WEATHERING: In temperate and wet tropical climates, skarns often form topographic features with positive relief. ORE CONTROLS: The ore exhibits strong stratigraphic and structural controls. Orebodies form along sill-dike intersections, sill-fault contacts, bedding-fault intersections, fold axes and permeable faults or tension zones. In the pyroxene-rich and epidote-rich types, ore commonly develops in the more distal portions of the alteration envelopes. In some districts, specific suites of reduced, Fe-rich intrusions are spatially related to Au skarn mineralization. Ore bodies in the garnet-rich Au skarns tend to lie more proximal to the intrusions. GENETIC MODEL: Many Au skarns are related to plutons formed during oceanic plate subduction. There is a worldwide spatial, temporal and genetic association between porphyry Cu provinces and calcic Au skarns. Pyroxene-rich Au skarns tend to be hosted by siltstone-dominant packages and form in hydrothermal systems that are sulphur-rich and relatively reduced. Garnet-rich Au skarns tend to be hosted by carbonate-dominant packages and develop in more oxidising and/or more sulphur-poor hydrothermal systems. ASSOCIATED DEPOSIT TYPES: Au placers (C01,C02), calcic Cu skarns (K01), porphyry Cu deposits (L04) and Au-bearing quartz and/or sulphide veins (I01, I02). Magnesian subtype can be associated with porphyry Mo deposits (L05) and possibly W skarns ( K05). In British Columbia there is a negative spatial association between Au and Fe skarns at regional scales, even though both classes are related to arc plutonism. Fe skarns are concentrated in the Wrangellia Terrane whereas most Au skarn occurrences and all the economic deposits lie in Quesnellia. COMMENTS: Most Au skarns throughout the world are calcic and are associated with island arc plutonism. However, the Savage Lode magnesian Au skarn occurs in the Archean greenstones of Western Australia and the Butte Highlands magnesian Au skarn in Montana is hosted by Cambrian platformal dolomites. Note: although the Nickel Plate deposit lies distal to the Toronto stock in the pyroxene-dominant part of the skarn envelope, the higher grade ore zones commonly lie adjacent to sills and dikes where the exoskarn contains appreciable amounts of garnet with the clinopyroxene. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Au, As, Bi, Te, Co, Cu, Zn or Ni soil, stream sediment and rock anomalies, as well as some geochemical zoning patterns throughout the skarn envelope (notably in Cu/Au, Ag/Au and Zn/Au ratios). Calcic Au skarns (whether garnet-rich or pyroxene-rich) tend to have lower Zn/Au, Cu/Au and Ag/Au ratios than any other skarn class. The intrusions related to Au skarns may be relatively enriched in the compatible elements Cr, Sc and V, and depleted in lithophile incompatible elements (Rb, Zr, Ce, Nb and La), compared to intrusions associated with most other skarn types. GEOPHYSICAL SIGNATURE: Airborne magnetic or gravity surveys to locate plutons. Induced polarization and ground magnetic follow-up surveys can outline some deposits. OTHER EXPLORATION GUIDES: Placer Au. Any carbonates, calcareous tuffs or calcareous volcanic flows intruded by arc-related plutons have a potential for hosting Au skarns. Favorable features in a skarn envelope include the presence of: (a) proximal Cu-bearing garnet skarn and extensive zones of distal pyroxene skarn which may carry micron Au, (b) hedenbergitic pyroxene (although diopsidic pyroxene may predominate overall), (c) sporadic As-Bi-Te geochemical anomalies, and, (d) undifferentiated, Fe-rich intrusions with low Fe2O3/FeO ratios. Any permeable calcareous volcanics intruded by high-level porphyry systems (particularly alkalic plutons) have a potential for hosting epidote-rich skarns with micron Au. During exploration, skarns of all types should be routinely sampled and assayed for Au, even if they are lean in sulphides. ECONOMIC IMPORTANCE TYPICAL GRADE AND TONNAGE: These deposits range from 0.4 to 13 Mt and from 2 to 15 g/t Au. Theodore et al. (1991) report median grades and tonnage of 8.6 g/t Au, 5.0 g/t Ag and 213 000 t. Nickel Plate produced over 71 tonnes of Au from 13.4 Mt of ore (grading 5.3 g/t Au). The 10.3 Mt Fortitude (Nevada) deposit graded 6.9 g/t Au whereas the 13.2 Mt McCoy skarn (Nevada) graded 1.5 g/t Au. The QR epidote-rich Au skarn has reserves exceeding 1.3 Mt grading 4.7 g/t Au. IMPORTANCE: Recently, there have been some significant Au skarn deposits discovered around the world (e.g. Buckhorn Mountain, Wabu, Fortitude). Nevertheless, total historic production of Au from skarn (more than 1 000 t of metal) is minute compared to production from other deposit types. The Nickel Plate deposit (Hedley, British Columbia) was probably one of the earliest major Au skarns in the world to be mined. Skarns have accounted for about 16 % of British Columbia's Au production, although nearly half of this was derived as a byproduct from Cu and Fe skarns

SEDIMENTARY ROCK-HOSTED OPAL

SEDIMENTARY ROCK-HOSTED OPAL

Ref: opala, sedimentos

Paradis, S., Townsend, J. and Simandl, G.J. (1999): Sedimentary Rock-hosted Opal; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines. IDENTIFICATION SYNONYMS: Australian opal deposits. COMMODITY: Gem quality opal (precious and common). EXAMPLES (British Columbia - Canadian/International): Lightning Ridge and White Cliffs (New South Wales, Australia) , Mintabie, Coober Pedy, Lambina and Andamooka (South Australia) Yowah, New Angledool (Queensland, Australia). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION:  Most of the Australian opal occurs in cracks, partings, along bedding planes, pore spaces and other cavities in strongly weathered sandstones generally underlain by a subhorizontal barrier of reduced permeability. The barriers consist mainly of claystones, siltstones and ironstone strata. TECTONIC SETTINGS:  The tectonic setting at the time of deposition and lithification of the opal-bearing lithologies is not indicative of favourable environment for opal. However, the presence of a terrestrial (non-marine) environment at the time of intense weathering is essential. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Clastic sediments were deposited in the shallow inland basins. Subsequently, these areas were affected by climatic/paleo-climatic changes (transformation into desert environment) that have resulted in rapid fluctuation in water table levels and entrapment of silica-rich waters. AGE OF MINERALIZATION:  In Queensland, Australia the host rocks are Cretaceous or Paleozoic and have been affected by deep weathering during the Early Eocene and Late Oligocene. The latter period is believed by some to be related to opal precipitation. Similar conditions favourable for opal deposition could have prevailed in different time periods in other parts of the world. HOST/ASSOCIATED ROCKS:  Sandstones, conglomerate, claystone and silty claystone. Associated lithologies are feldspathic rocks weathered to kaolinite, silcrete and siliceous duricrust, shales and shaley mudstones, limestones, dolostones and ironstones. Exceptionally, precious opal may be found in weathered crystalline basements stratigraphically underlying the lithologies described above. DEPOSIT FORM:  Opal occurrences are stratabound. Favorable subhorizontal, precious opal-bearing intervals can exceed 10 m in thickness, and are known to persist for distances of one to over 100 km. The distribution of individual precious opal occurrences within favorable areas is erratic. Veins are subhorizontal to subvertical and locally up to 10 cm thick. They pinch and swell, branch or terminate abruptly. A single vein can contain chalky to bony to blue, gray or milky common opal and precious opal. TEXTURE/STRUCTURE:  Opal occurs as veinlets, thin seams in vertical and horizontal joints, desiccation cracks in ironstone layers, lenses and concretions, and replacing fossils (shell and skeletal) and wood fragments. Opal also forms pseudomorphs after glauberite4. In places opal seems to follow cross bedding. In unusual cases opal pieces eroded from the original host are incorporated into younger sediments. In silicified sandstones precious opal may form the cement around detrital quartz grains, in other areas, the opal may be cut by gypsum or alunite-filled fractures. The lithologies above the opal may contain characteristic red-brown, gypsiferous silt-filled tubules. 4 Glauberite: 4[Na2 Ca(SO4 )2 ], widespread as a saline deposit formed as a precipitate in salt lake environments, also occurs under arid conditions as isolated crystals embedded in clastic sediments. ORE MINERALOGY: Precious opal. GANGUE MINERALOGY [Principal and subordinate]: Host rock, common opal, gypsum and gypsum-shot opal, alunite, hematite, limonite/goethite. ALTERATION MINERALOGY:  N/A. WEATHERING:  Feldspathic rocks strongly altered to kaolinite typically overly the Australian precious opal-bearing deposits. Opal exposed to arid weathering environments may desiccate, crack and lose its value; however, gem quality opal may be found at depth. ORE CONTROLS: 1) Regional configuration of impermeable layers permitting groundwater pooling. 2) Local traps within regional sedimentary structure, such as bedding irregularities, floored by impermeable layers, porous material (e.g. fossils) or voids where opal can precipitate. GENETIC MODELS:  Australian opal is hosted mainly by strongly weathered sandstones which are underlain by claystone, siltstone and ironstone that form relatively impermeable barriers. Periods of intense weathering are evidenced by indurated crust horizons. Silica-transporting solutions derived from intense weathering of feldspar within sandstones percolated downward to the contact between the porous sandstone and the underlying impermeable layers. During a subsequent dehydration (dry) period silica was progressively concentrated by evaporation. The last, most concentrated solutions or colloidal suspensions were retained within bedding irregularities at the permeable/impermeable rock interface, in joints and in other traps. Gem-quality opal was formed by ordered settling and hardening of silica microspheres of uniform dimensions. Disordered arrangement of silica microspheres or variability in microsphere size results in formation of common opal. ASSOCIATED DEPOSIT TYPES:  Possibly clay deposits (B05). COMMENTS: There is good reason to believe that a similar mode of opal formation could also take place in porous terrestrial and waterlain pyroclastic rocks, assuming favorable geological and paleo-climatic setting. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: N/A GEOPHYSICAL SIGNATURE:  Most opal fluoresces brightly if exposed to ultraviolet light. Limited success was achieved using magnetic field and resistivity to find ironstone and ironstone concretions that commonly contain precious opal in Queensland. OTHER EXPLORATION GUIDES:  Unmetamorphosed or weakly metamorphosed areas known for: 1) prolonged periods of deep chemical paleoweathering characterized by rock saturation and dehydration cycles; 2) broad sedimentary structures permitting shallow underground solution pooling; 3) local traps where opal could precipitate from nearly static, silica-bearing ground waters; and 4) presence of common opal. ECONOMIC FACTORS TYPICAL GRADE AND TONNAGE: No reliable estimates of grade or tonnage are available for individual deposits. Until 1970 the only records of production were annual returns submitted by opal buyers. Miners fear that reporting the true production would be used for taxation purposes. As with other gemstones, reporting the grades in terms of grams or carats per tonne may be strongly misleading. Large and exceptional quality stones command very high prices. Precious opal may be transparent, white, milky-blue, yellow or black. It is characterized by the internal play of colors, typically red, orange, green or blue. The best opal from Lightning Ridge was worth as much as $Aus. 10 000.00 per carat in cut form and Mintabie opal varied from $Aus. 50.00 to 10 000.00 per ounce of rough. Most of the white to milky colored opal from Coober Pedy was worth $Aus. 10.00 to 100.00 per ounce of rough, but the prices of top quality precious black and crystal opals exceeded $Aus. 5 000.00 per ounce. The value-added aspect of the gem industry is fundamental. An opal miner receives 1 to 50% of the value of cut and polished stone. ECONOMIC LIMITATIONS: In Australia mining is largely mechanized, either underground or on surface. Opal-bearing seams are generally found at shallow depths (< 30 metres). Opal is still recovered from old tailings by hand sorting over conveyer belts using ultraviolet light. Large and exceptional quality stones command very high prices and the unexpected recovery of such stones may change an operation from losing money to highly profitable. Stones from these deposits are believed to have better stability under atmospheric conditions than opal from most volcanic-hosted deposits. END USES: A highly priced gemstone that is commonly cut into solid hemispherical or en cabochon shapes. Doublets are produced where the precious opal is too thin, needs reinforcement or enhancement; plastic cement, a slice of common opal or other support is added to the back of the opal. IMPORTANCE:  Australian sedimentary-hosted opal deposits account for most of the opal produced today. The situation is likely to continue since these deposits recently attracted important Japanese investment. In 1990, the Coober Pedy, Andamooka and Mintabie produced opal worth over $Aus. 47 million. Total production estimates for Australia are in the order of $Aus. 100 million annually.