Mawsonite

Abstract: Trace mineral assemblages in the bornite- and chalcopyrite-rich Cu-Fe zone of the Ocna de Fier-Dognecea skarn deposits, Banat, SW Romania provide additional constraints on the genesis of this classic zoned skarn system. Observed assemblages substantiate a model, in which the Cu-Fe zone forms the proximal fluid-plume root of the system. Observed trace mineral assemblages in the magnesian forsterite-bearing skarns crystallized at ~650°C in a volatile-rich environment, evidenced by widespread phlogopite, ludwigite, valleriite and apatite. The entire assemblage thus belongs to the initial stage of skarn formation. Prolonged cooling led to sequential exsolution of trace mineral phases from bornite and chalcopyrite during the retrograde stage, although still at temperatures in excess of 500°C. Bornite is typified by the abundance of exsolved phases along cleavage planes and along crystal margins, notably chalcopyrite and pyrrhotite, but also cobalt pentlandite, carrollite, wittichenite, galena, mawsonite, silver and electrum. Chalcopyrite hosts cobalt pentlandite, carrollite, wittichenite, galena and a sequence of Se- and Te-bearing minerals (kawazulite, bohdanowiczite, hessite, volynskite), along, although not restricted to, grain margins. The assemblage bornite-chalcopyrite-magnetite, with the trace phases, cobalt pentlandite, carrollite, wittichenite and various Se- and Te-bearing minerals represents a characteristic assemblage common to a disparate range of deposits formed at temperatures in excess of 500°C in the presence of volatiles and typified by relatively low f s 2 fluids.

Abstract: The Basin Lake copper-gold prospect lies in western Tasmania’s Mount Read Volcanics and is hosted in a series of calc-alkaline andesites, quartz-feldspar porphyries, mudstones, carbonates, and sandstones between the Tyndall Group and the Central Volcanic Complex. Alteration at the Basin Lake prospect occurs over a strike length of 1.4 km and includes thin, strata-bound pyrophyllite-quartz-paragonite-kaolinite-pyrite-alunite alteration zones, up to 12 m wide and containing up to 50 wt percent pyrophyllite, with local fluorite veining. These zones grade out to paragonite-muscovite-kaolinite-quartz-pyrite and muscovite-carbonate-chlorite alteration zones. Extensive propylitic alteration (chlorite-carbonate-epidote) affects most other rocks outside these zones. Mineralization consists of thin strata-bound zones of massive and vein pyrite, tennantite, and chalcopyrite, with trace covellite and galena, hosted mainly within an intensely silicified core of the pyrophyllite-quartzsericite alteration zone. Pyrite has δ34S values of –1.4 to +6.9 per mil, although marginal vein pyrite in the propylitic zone has δ34S values around 12.4 per mil. Large silicified glacial erratic boulders at surface contain massive and vein pyrite, enargite, and tennantite, with minor barite, and trace covellite, stannoidite, and mawsonite. Pyrite and enargite have δ34S values of 1.7 to 6.8 per mil; barite has δ34S values around 35.2 per mil with 87Sr/86Sr around 0.7108. The alteration and mineralization at the Basin Lake prospect is similar to that associated with high-sulfidation copper-gold systems formed by acidic, relatively oxidized fluids. A new geochemical vector, here termed the “advanced argillic alteration index” [AAAI = 100 (SiO2)/(SiO2 + 10MgO + 10CaO + 10Na2O)], has been devised to help quantify the intensity of alteration. The values of the AAAI at Basin Lake are similar to those of several high-sulfidation epithermal deposits. The low sulfide δ34S values are similar to those at other sulfide occurrences in the Mount Read Volcanics that have previously been considered to be barren, are lower than those of nearby volcanic-hosted massive sulfide deposits, and may indicate a magmatic fluid component. However, the δ34S and 87Sr/86Sr values of Basin Lake barite at the assumed highest exposed level of the system and higher δ34S values in pyrite from marginal veins are similar to those of Cambrian volcanic-hosted massive sulfide systems, indicating the involvement of reduced seawater sulfate at these locations. Calcite carbon and oxygen isotope values, silicate oxygen isotope values, and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic, magmatic fluids were likely neutralized by seawater. This occurrence strengthens the case for prospecting the Mount Read Volcanics and other similar submarine volcanic belts for copper-gold and gold-only deposits that formed by the actions of hyperacid oxidized fluids.

Abstract: Properties, phase relations; stannite, kesterite, stannoidite, mawsonite, and unknown phase; New Brunswick

Abstract: Sn-W-Cu-Zn-Pb-Ag vein deposits in Japan, belonging to xenothermal class, occur in or in close proximity to regions of rhyolitic (and andesitic) rocks intruded by granitic rocks. The rhyolitic (and andesitic) rocks are products of the same igneous activities as the granitic intrusives. The geologic age of the igneous rocks is late Cretaceous or early Tertiary. The Akenobe, Ikuno, and Tada mines in western Japan and the Ashio mine in eastern Japan belong to the class.The fissures containing vein materials are strike-slip faults and tension cracks formed by the lateral pressure which caused the folded structures in each region. They are also normal faults due to the upheaval and withdrawal (including cauldron subsidence) of the granitic magma.The vein materials at the Akenobe mine are as follows: Ore minerals: cassiterite, wolframite, scheelite, magnetite (lamellar magnetite), arsenopyrite, molybdenite, pyrite, native bismuth, bismuthinite, chalcopyrite, stannite, sphalerite, galena, primary bornite, stannoidite (hexastannite), mawsonite, tennantite, primary chalcocite, roquesite, native silver, argentite, polybasite, stephanite, etc. Gangue minerals are quartz, topaz, apatite, fluorite, orthoclase, chlorite, calcite, rhodochrosite, etc. Indium and selenium are relatively concentrated in the ores from this type of deposit.Similar paragenesis is observed at the Ikuno, Tada, and Ashio mines. At the Ikuno mine, the distribution of the vein materials shows a remarkable zoning. There are zones, vertically from lower to higher and horizontally from center to margin, of (1) tin-tungsten, (2) tin-copper, (3) copper-zinc, (4) zinc-lead, (5) silver. The distribution of the zones generally coincides with the zoning described by Emmons (1924). As a whole it belongs to monoascendant zoning. At the Akenobe mine similar zoning is recognized. It is inferred that at the Akenobe mine the mineralizing fluid may have ascended along the normal faults and have flowed into the tension cracks. At the Tada mine native silver and stromeyerite are characteristically recognized in the silver zone of the deposit. These silver minerals belong to the latest stage of mineralization.At the Ashio mine a reverse zoning is recognized, i.e., the tin-tungsten zone occurs at the higher level and the copper and zinc zones exist below the tin-tungsten zone.From fluid inclusion studies, the decrepitation temperatures of the minerals in the Akenobe mine are determined to be 355 degrees to 155 degrees C. The fluid inclusions in the quartz from the Ashio mine are two-phased (liquid and vapor). The homogenization filling temperatures by the heating microscope, and NaCl equivalent concentration by the cooling microscope, of the fluid inclusions in the quartz from the Ashio mine are 350 degrees to 200 degrees C and 0 to 9 percent, respectively. In some cases, it is observed under the microscope that in the same quartz crystal from the veins of the Ashio mine the fluid inclusion predominating liquid phase coexists with the one predominating vapor phase. This indicates that the boiling phenomenon of the ore-forming fluid has occurred in the course of mineralization.In areas composed of Permo-Carboniferous sedimentary rocks intruded by the granitic rocks of the late Cretaceous or Early Tertiary period which are adjacent to regions of the above ore deposits, there occur hypothermal tungsten and tin veins, and hypothermal or mesothermal chalcopyrite and pyrrhotite deposits. In the hypothermal tungsten and tin veins, primary bornite, stannoidite, and mawsonite are lacking, which are found in the above xenothermal type. In the xenothermal type, cubanite (mackinawite) and pyrrhotite, often found in the hypothermal and mesothermal types, are not recognized. At the Tada mine the following succession of Cu-Fe-Sn-S mineralization is recognized: Pyrite, Chalcopyrite --> Chalcopyrite, Stannite --> Chalcopyrite, Stannoidite --> Chalcopyrite, Bornite, Mawsonite. From hydrothermal experiments mawsonite is apparently stable under conditions of higher sulfur fugacity or lower temperature than the assemblage of bornite + stannoidite + chalcopyrite. Similarly, stannoidite would be stable under conditions of higher sulfur fugacity or lower temperature than the assemblage of stannite + bornite + chalcopyrite. The mineralization sequence at the Tada mine thus proceeds toward lower sulfur fugacity and lower temperatures, depositing the native silver at the latest stage.