Abstract

The JC tin deposit is one of several stanniferous skarns associated with the mid-Cretaceous Seagull batholith. At JC a ridgelike lobe of the Seagull batholith granite has intruded a thick sequence of Mississippian quartzite containing a 30- to 35-m carbonate horizon. Skarn has replaced this carbonate along its elongate contact with the granite ridge for a distance of more than 850 m.The paragenetic sequence in the JC skarn is contact metamorphic (skarnoid): Stage 0--grossularite + diopside + quartz + calcite; early skarn: stage I--andradite + hedenbergite, stage IIA--Fe amphibole + magnetite, stage IIB--Fe amphibole + pyrrhotite + or - chalcopyrite + or - sphalerite, stage III--epidote + quartz + calcite + or - axinite + or - Sn-Al sphene; late skarn: stage IV (early veins)--beryl + quartz + or - tourmaline + or - danalite, stage IV (main stage)--biotite + quartz + fluorite + or - arsenopyrite + or - chalcopyrite + or - cassiterite, stage V--Fe chlorite + muscovite + calcite, stage VI (veins-breccias)--calcite + pyrite + or - marcasite.The sequential stages of skarn development display a progression from massive skarn replacement of the protolith calcite marble (stages I and II) to more diffuse or veinlike replacement of earlier skarn assemblages (stages III to VI). Many of the early skarn silicates contain appreciable tin; especially andradite (up to 0.79 wt % SnO 2 ), Fe amphibole (0.07-0.61 wt % SnO 2 ), epidote (0.12-1.56 wt % SnO 2 ), and the Sn-Al sphene. The replacement of earlier (stage IIA) skarn at stage IV was an important ore-forming process, precipitating tin as cassiterite in a narrow zone above the skarn-granite contact.Microthermometric measurements of primary fluid inclusions in epidote and axinite yield a temperature range for stage III skarn formation of 410 degrees to 500 degrees C, at an estimated pressure of 750 bars. Stage III fluids ranged in salinity from 20 to 27 wt percent (4.5-6.5 molal total Cl) and had high CaCl 2 /NaCl ratios (>11). These fluids are interpreted to be the result of mixing of originally more saline magmatic fluids with meteoric water during the latest stage of early skarn formation. Low-salinity pseudosecondary inclusions in epidote may represent a relatively unmixed sample of this meteoric water component. Stage III fluids appear to have become CaCl 2 enriched as a consequence of skarn-forming reactions.Primary fluid inclusions in cassiterite imply a temperature range of 430 degrees to 560 degrees C and salinities of 25 to 43 wt percent (6-12 molal total Cl) for stage IV fluids. These fluids contained predominantly CaCl 2 + NaCl + H 2 O and a lesser component of KCl, with CaCl 2 /NaCl(+ KCl) ranging from 0.6 to 7.7. The apparent increase in both fluid temperature and salinity at stage IV is attributed to a renewed episode of fluid release from the underlying granite, with concomitant greisen alteration of the upper part of the JC granite ridge. A trend toward very high CaCl 2 /NaCl(+ KCl) ratios in the stage IV fluids may be attributable to the progress of CaCl 2 -producing (KCl-consuming) replacement reactions within the skarn. However, variations in the overall salinity, and other compositional characteristics of these fluids, may have been largely controlled by the process of aqueous phase separation from the JC granite.The high salinity of the stage IV fluids implies derivation from a granitic melt which exsolved a subcritical (two-phase) aqueous fluid. The observation of extremely saline two-phase fluids in greisens, and other types of granite-hosted deposits, provides more direct evidence of such a process. The relatively low salinity observed in some other examples of granite-related Sn and W mineralization may be a direct consequence of supercritical, rather than subcritical, aqueous fluid separation from the associated magmas.

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