Skarn-type Mineralization Research Articles

The Xinqiao Cu–S–Fe–Au deposit in the Tongling mineral district, China: synorogenic remobilization of a stratiform sulfide deposit

The Cu–S–Fe–Au deposit at Xinqiao is located in the Tongling mineral district, Anhui Province, People's Republic of China. It is a large-scale polymetallic deposit with drilling indicated reserves of 0.5 million tonnes of Cu averaging 0.71%, 75.5 million tonnes of S averaging 29.3% and 24.9 million tonnes of Fe averaging 46%, as well as 11.2 tonnes of Au averaging at 4.7 g/tonne plus 248.4 tonnes of Ag averaging at 248.4 g/tonne. There are two major mineralization types in the Xinqiao deposit: one is the economically significant stratiform orebody hosted in the Lower Carboniferous Gaolishan siltstone (C 1g) and overlying Middle Carboniferous Huanglong limestone (C 2h), while the other is the less important skarn-type mineralization restricted to the contact between the Yanshanian diorite (110–168 Ma) and the Lower Permian Qixia marble (P 1q). The ore in the Xinqiao deposit is dominated by massive pyrite with minor amount of chalcopyrite, magnetite, pyrrhotite, galena, sphalerite, quartz and dolomite. Four stages of deformation have been recognized to affect the sedimentary rocks in the Xinqiao area. The first two (D 1 and D 2) produced mainly NE–SW and NNE–SSW trending open folds, whereas D 3 is characterized by normal faults striking NNE–SSW and D 4 is dominated by post-mineralization strike-slip faults segmenting the orebody. The structural information obtained by this study not only revealed the transgressive nature of the stratiform orebody, but also identified a strong structural control on the formation of the deposit. It is concluded that the Xinqiao polymetallic deposit was formed through remobilization of a Carboniferous sulfide source bed by hydrothermal solutions related to late Yanshanian plutonism (110 Ma), during which D 3 normal faulting played an important role in localization of the mineralization. Such a two-stage ore-forming model is consistent with the most recent genetic interpretations of the deposit.

40 Ar/ 39 Ar dating and mineral paragenesis for carlin-type gold deposits along the Getchell Trend, Nevada; evidence for Cretaceous and Tertiary gold mineralization

The orebodies at the Getchell and Twin Creeks mines were studied through mineral paragenesis, geologic relationships, and 40 Ar/ 39 Ar dating. Mineral paragenetic relationships are based on observations made during the logging of 18,000 m of drill cuttings and core and crosscutting relationships recognized in the field and Main pit at the Getchell mine. Ages for igneous and mineralizing events were determined through 40 Ar/ 39 Ar incremental heating analyses of 15 samples of biotite, K feldspar, sericite, and vein adularia. A thermal history for the area was also developed using K feldspar multiple diffusion domain results and age determinations on cogenetic minerals, which have different argon closure temperatures.Mineral paragenetic relationships and 40 Ar/ 39 Ar dating at the Getchell and Twin Creeks deposits document that gold mineralization occurred in five stages between 95 and 42 Ma.Stage 1 pyrrhotite-quartz-arsenopyrite-chalcopyrite-biotite gold mineralization in skarn is associated with 95 Ma dacite dikes and granodiorite plugs. Stage 2 chalcopyrite-sphalerite-galena-pyrite-arsenopyrite + or - gold and silver mineralization in skarn is related to the 92 Ma Osgood Mountains stock. Secondary K feldspar and sericite in stage 3 quartz-pyrite-kaolinite-gold mineralization formed at 83 Ma and are not related to 95 or 92 Ma igneous activity. Stage 4 low-grade quartz-pyrite-gold mineralization formed at about 75 Ma, in the matrix of a breccia pipe that cuts 83 Ma silicic and argillic ores. Adularia intergrown with stibnite-orpiment-pyrite-quartz-gold + or - realgar mineralization yields precise 40 Ar/ 39 Ar plateau ages of 42.11 + or - 0.43 and 41.90 + or - 0.25 Ma for stage 5.This study documents the temporal complexity of mineralization and igneous activity at the Getchell and Twin Creeks deposits. Minor gold in skarn-type mineralization (stages 1 and 2,) associated with 95 and 92 Ma igneous intrusions preceded and was not genetically related to Carlin-type gold mineralization (stages 3 and 5). The Carlin-type deposits at Getchell and Twin Creeks therefore represent a composite orebody formed by the overprinting of 83 Ma stage 3 quartz-pyrite-gold by 42, Ma stage 5 orpiment-stibnite-pyrite-gold mineralization.

Fluid inclusion and phase equilibrium studies at the Cannivan Gulch molybdenum deposit, Montana, USA: Effect of CO 2 on molybdenite-powellite stability

The molybdenum deposit at Cannivan Gulch is characterized by both stockwork- and skarn type mineralization. The vein mineral assemblages, however, differ considerably between the various host rocks. In the Cannivan stock, molybdenite occurs in typical quartz + K-feldspar or quartz + muscovite veins. In the country rocks, molybdenite occurs in chlorite + pyrite + magnetite veins containing variable amounts of quartz, calcite, and epidote. Primary fluid inclusions in quartz and fluorite contain aqueous liquid, CO 2 liquid, and CO 2 vapor (average X CO 2 = 0.05). Microthermometric and Laser Raman microspectrographic analyses indicate CO 2 is the principal volatile phase. First melting temperatures in the aqueous portion indicate the dissolved salt is principally NaCl. Final CO 2-clathrate melting temperatures indicate NaCl contents ranging from 2 to 7 wt% relative to H 2O. Homogenization temperatures range from 196 to 272°C. Four chlorite temperatures from two samples range from 296 to 319°C. In P- T space, the four chlorite univariants intersect four fluid inclusion isochores (from the same samples) at temperatures of 300 ± 20° C and 1500 ± 300 bars, which are the inferred P and T of molybdenite formation. Mineral equilibria in vein assemblages from country rocks at Cannivan Gulch indicate f O 2 and f S 2 conditions of molybdenite formation between the pyrite-pyrrhotite-magnetite and pyrite-magnetite-hematite buffers. Formation of a calcite-molybdenite assemblage over powellite suggests that even small amounts of CO 2 ( X CO 2 = 0.05) in an ore fluid greatly extend the stability field of molybdenite to higher oxygen fugacities.

Fluid inclusion studies of the skarn-type tin mineralization at Red-A-ven, Northwest Dartmoor, England

Two principal hydrothermal events have been recognized, which resulted in the tin-bearing skarns at Red-A-ven, Northwest Dartmoor. Tin was enriched during both the early (occurring as tin-bearing andradite) and late hydrothermal events (mainly as malayaite stannite and cassiterite, accompanied by boron silicates and minor tungsten). Thermometric measurements of fluid inclusions in diopside, garnet, malayaite, tourmaline-quartz and axinite were carried out, suggesting that the early skarns were formed at the temperature above 600 °C and the salinities ranging from 10 to 20 wt. %NaCl eq., while the late skarns at the temperatures of 350–460°C and the salinities ranging from 30 to 50 wt. % NaCl eq. The estimated minimum depths are 2856 m (850 bar) for the early event and 2526 m (185 bar) for the late event. The birefringence of the andradite might be related with the superimposition of late hydrothermal fluids.