The geochemistry of zoning in skarn minerals at the King Island (Dolphin) Mine

Abstract

Trends of chemical zoning in crystals of garnet, pyroxene, scheelite, and amphibole in 34 samples collected from the pluton9s contact updip to 500 m from the contact in the main ore horizon at the King Islnd scheelite (Dolphin) mine show systematic variations. Garnet cores up to 400 m from the contact are the most andradite rich, reaching values of 98 mole percent nearest the contact. Andradite values decrease systematically both for a particular part of the garnet crystals away from the contact and across individual grains to the outer edges. The mole percent andradite in core and outer core zones decreases to the 500-m mark while in the midsection and edge zones the values decrease to 400 m and then increase. A similar relationship occurs in pyroxenes, which nearest the contact contain hedenbergite values of 98 mole percent. Scheelite occurring as inclusions in garnet reach values of scheelite 65 powellites 34 for core zones nearest the pluton while scheelite interstitial to garnet have maximum values of scheelite 72 powellite 27 . Powellite contents decrease both toward the edges of individual grains and along the traverse to the 400-m mark where a reversal of trend occurs continuing to 500 m from the contact. Amphibole compositions show a linear decrease of the ratio Fe (super +2) /Fe (super +2) + Mg + Mn from 98 nearest the contact to 88 at 390 and then decreasing abruptly to 65 at 500 m.Fluid inclusion filling temperatures vary systematically in time and distance from the pluton, with a maximum near 800 degrees C for garnet midsections nearest the pluton to temperatures as low as 180 degrees C for interstitial calcite and quartz. The salinities decrease from near 65 weight percent total dissolved salts in the early formed minerals (i.e., garnet) nearest the pluton to less than 3 weight percent for later formed minerals farthest from the contact (i.e., calcite). These decreases occur out from the contact and progressively toward the end of the paragenetic sequence. Scheelite initially crystallized at higher temperatures, but interstitial scheelite precipitation occurred from a concentrated Na-K(-Ca)-Cl solution (approximately 10 molal, total dissolved salts) at temperatures of approximately 500 degrees C at the pluton9s contact to 300 degrees C at a distance of 500 m from the contact. Chemical variations of fluid compositions for all but garnet core zones beyond 400 m from the contact occurred due to mixing of solutions derived from the Northern Boundary fault. The interface between solutions which have permeated through and reacted with the developing skarn unit and those derived from the fault marks the maximum outward edge (from the pluton) of boiling of the solutions, the highest concentrations of W ore, reversals or inflection points in mineral composition profiles, and changes in the elemental ratios of constituents in the brine solutions.Hot saline solutions initially cooled to approximately 380 degrees C on entering the colder marble horizon. Flow along this horizon caused the loss of heat, dilution, and chemical change of the solution, this latter in response to both reactions with the marble and hornfelsic units as well as the precipitation of the skarn. Permeability was produced by the dissolution of CaCO 3 to produce CaCl (sub 2(aq)) and faulting. With time, thermal equilibrium was reached between the evolving fluid and the contact rocks until boiling of the solution occurred with the loss of a CO (sub 2-) -rich vapor. During this event the Northern Boundary fault was tectonically active so that primary fluid now entered the forming skarn both from the fault and the contact of the pluton. Mo-rich scheelite began to precipitate at the time of boiling. Subsequent to garnet + pyroxene crystallization, after the boiling event and contemporaneous with the crystallization of amphibole + some quartz, Mo-rich scheelite not armored in garnet was dissolved, redistributed, and redeposited as Mo-poor scheelite and molybdenite. This type of scheelite forms most of the ore being mined.