Opala em vulcânicas

Opala em vulcânicas

PRECIOUS OPAL IN VOLCANIC SEQUENCES by S. Paradis1, G.J. Simandl2 and A. Sabina3

Ref: opala, vulcânica, jásper, ágata

Paradis, S., Simandl, G.J. and Sabina, A. (1999): Opal Deposits in Volcanic Sequences; 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: Hydrothermal or "volcanic opal". COMMODITIES (BYPRODUCTS): Precious opal (common opal, chalcedony, jasper, agate). EXAMPLES (British Columbia - Canada/International):   Klinker (082LSW125), Northern Lights (093E 120), Whitesail Range (maps 93E10W and 93E11E) and a precious opal occurrence near Falkland, Eagle Creek (093K 095); pale green and apple green common opal occurs at Savona Mountain (092INE158); Queretaro Mines (Mexico), Virgin Valley (Nevada, USA), Tepe Blue Fire Opal Mine (Idaho, USA). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Opal occurs commonly in seams of volcanic ash or lahars sandwiched between successive lava flows. It occurs mainly as open space fillings and impregnations. Common opal, opalized wood and to some extent "fire opal" are widespread within Triassic or younger volcanic sequences, but precious opal is rare. Where opal occurs in massive volcanic rocks, it occurs also as open space fillings, however the opal-bearing areas are much smaller. Regardless of volcanic hostrock, the precious opal occurrences are discrete, whereas common opal occurs over large areas. TECTONIC SETTINGS:  Volcanic arcs, rifts, collapsed calderas, hot spot related volcanism and others. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Volcanic sequences formed in subaerial or shallow marine environments where porous, pyroclastic or lacustrine rocks are interbedded with lava flows. AGE OF MINERALIZATION: Tertiary or younger, commonly Miocene. HOST/ASSOCIATED ROCKS: Common host rocks are rhyolite, basalt, andesite and trachyte lavas, lahars and other volcaniclastic rocks. Associated rocks are perlite, bentonite, scoria, volcanic ash and diatomite; volcanic rocks may be intercalated with lacustrine sedimentary rocks. DEPOSIT FORM: Favourable opal-bearing horizons are commonly stratabound. Occurrences of precious opal within these horizons are commonly considered as erratic, controlled by permeability at the time of opal deposition. Individual precious opal-bearing fractures or lenses may grade into common opal and agate over distances of centimetres. TEXTURE/STRUCTURE:  Opal occurs as open space fillings in irregular cavities, narrow discontinuous seams, partially-filled pillow tubes, fractures, vesicles, matrix in volcaniclastic rocks and replacing wood fragments and logs. Common opal may form miniature stalagmites and stalactites within cavities, nodules in clay or diatomite beds and "thunder eggs". ORE MINERALOGY [Principal and subordinate]: Precious opal; "fire opal", chalcedony, agate, common opal. GANGUE MINERALOGY [Principal and subordinate]: Common opal, agate, fragments of host rock, clays, zeolites, quartz, jasper, celadonite, manganese and iron oxides. ALTERATION MINERALOGY:  Opal-bearing cavities may have zeolite and celadonite coatings, but so do the barren cavities. There is no known alteration which is specific to precious opal. WEATHERING: In arid environments, opal in surface outcrops may desiccate, become brittle and crack. Such material is not suitable as a gemstone. However, these opal bodies may be gem-quality at depth. ORE CONTROLS: Open spaces and other permeable zones open to the silica-bearing solutions. GENETIC MODELS: In many large opal districts, it is believed that during the longer periods of volcanic inactivity, shallow lakes developed. Forests grew along the lake-shores and driftwood accumulated in the lakes. Volcanic eruptions covered everything with pyroclastic materials capped by lava flows resulting in aquifers, perched water tables, and anomalies in the thermal gradient. This in conjunction with subsequent brittle tectonic deformation resulted in ideal conditions for the formation of hydrothermal systems. A variety of silica forms, including silica sinter, opaline silica, chalcedony and common opal are believed to have formed by deposition of silica-bearing fluids. The dissolved SiO2 content in water is well known to be temperature dependent with the maximum dissolution at around 325°C, however, the conditions needed for the precipitation of precious opal in volcanic environment are not well understood. At least a portion of the opal-CT in volcanic rocks is believed to precipitate directly from supersaturated solutions. The temperatures of formation for precious opal are expected to be relatively low by analogy to sedimentary-hosted precious opal deposits, but temperatures as high as 160°C are reported from fluid inclusion studies. No precious opal is reported from active hydrothermal fields, such as Geyser Valley, Yellowstone or Whakarewarewa (New Zealand). This suggests that the precious opal forms only under very specific physico-chemical conditions. Eh and definitely pH may be important. Chemical composition of hydrothermal fluids in terms of silica concentrations, as well as Na, K, Cl, Ca, SO4, HCO3, B, Li and other elements may be important. The composition of the silica-bearing fluid is probably modified during migration through the permeable host rock, specially if the latter contains zeolites and/or clays. Zeolites act as molecular sieves and are well known for their cation exchange properties. ASSOCIATED DEPOSIT TYPES:  Associated deposits can be beds of diatomaceous earth (F06), volcanic ash (E06), zeolite deposits (D01, D02), perlite and a variety of semi-precious or ornamental silica gemstones, such as jasper (Q05), moss agate (Q03), and chalcedony. Other deposit types occurring in the same setting are hot-spring Au-Ag (H03), hot-spring Hg (H02), agate (Q03) and hydrothermal Au-Ag-Cu: high sulphidation (H04). It is possible that these deposit types are the source of primary amorphous silica. COMMENTS: Precious opal is characterized by a play of color. The term common opal, as used here, covers any opal that does not show this play of colors. Some common opal specimens may be used as gemstones, but in general they have substantially lower value than precious opal. The term "Fire Opal" describes a common opal having a transparent orange to red-orange base color. Such opal is commonly faceted. Precious and common opal coexist within the same deposits. Common opal and opaline silica are also commonly associated with the spectacular hydrothermal systems characterized by hot springs pools and geysers, mud pots, geyser terraces and fumaroles where it may be deposited as common opal, opaline silica or silica sinter. The well known examples of such systems are: Yellowstone hot springs; Geyser Valley in Kamchatka and now inactive Waimangu Geyser (Taupo volcanic zone, New Zealand). It is possible that some of the precious opal is formed by the dissolution of the previously formed common opal, silica sinter in the same conditions as sedimentary rock-hosted precious opal deposits. EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Mn oxide fracture coating was observed in the proximity of the Klinker deposit. In some cases the indicator elements used in exploration for epithermal metalliferous deposits such as Hg, Sb and As may be indirectly applied to precious opal exploration. GEOPHYSICAL SIGNATURE: N/A, except for detecting perched water tables and faults (mainly VLF and resistivity). Thermometry may have use where precious opal is associated with recent hydrothermal activity. OTHER EXPLORATION GUIDES:  Boulder tracing is commonly used in opal exploration. Unmetamorphosed or weakly metamorphosed (zeolite facies) terrains (gem opal deteriorates and becomes brittle if subject to moderate temperatures); Tertiary or younger volcanic rocks. Areas containing known occurrences of precious or common opal, opalized wood and possibly chalcedony. Opal occurrences hosted by volcaniclastic rocks are commonly confined to the same lithologic unit over a large area. The presence of warm springs in an appropriate setting may also be considered as an indirect exploration indicator. At the Klinker deposit, mineralogical zoning within vesicule fillings may be used to delimit the most favourable areas. For example the common opal occurs only within broad areas of agate mineralization and precious opal only in small areas within the common opal mineralization. ECONOMIC FACTORS TYPICAL GRADE AND TONNAGE:  Grade and tonnage for volcanic-hosted opal deposits are not well documented, largely because the opal extraction is done by individuals or family type businesses. The precious opal distribution within most deposits is erratic, "Bonanza-type". The deposits at Querétaro were discovered in 1835 and are still in production. Furthermore, the term "grade" as commonly used for metalliferous deposits is much harder to apply to gemstone deposits and especially to opal deposits. For example "fire opal" ranges in value from $CDN 5 to 300 per gram. Average commercial precious opal will sell probably around $CDN40 per gram, the top quality stones may sell for $CDN 1400.00 per gram. ECONOMIC LIMITATIONS: Some of the common opal specimens may be used as semi-precious or ornamental stones, but in general they have substantially lower value than precious opal. Gem opal contains up to 10% water, which contributes to the translucency of the specimens. Precious opal from some localities, such as Virgin Valley in Nevada, are generally not suitable for gems because they crack too easily; however the opal from many other volcanic-hosted occurrences is as stable as that from the Australian sedimentary-hosted deposits. Deposits located in intensely weathered terrains are easier to mine than deposits in unaltered rocks. Prices of the best quality opal have risen steadily since 1991. There is a relatively good market for precious opal, nevertheless strong marketing and value-added processing are considered essential parts of successful opal mining operations. END USES:  Precious opal is highly priced gemstone; "fire opal" may be faceted, opalized wood is a speciality ornamental stone commonly used for book ends. IMPORTANCE: Volcanic rock-hosted opal deposits are numerous, but most of today's high quality opal production comes from Australian sedimentary-hosted deposits.