Skarns Related to Porphyry–Style Mineralization at Caicayén Hill, Neuquén, Argentina: Composition and Evolution of Hydrothermal Fluids

摘要: Located in the Main Cordillera of northwest Neuquen, Argentina, the Caicayen district shares following features with Cu skarn–porphyry copper deposits other parts South, Central, and North America, as well Asia and South Pacific: (1) alteration mineralization are related to I–type, magnetite series, calc–alkaline porphyritic sills intermediate composition with stockwork veining, brittle fracturing, brecciation; these features indicative a relatively shallow environment formation (~500 bars); (2) skarns have an oxidized mineralogy dominated by garnet (Ad 45–97 ), clinopyroxene (Hd 18–60 ), epidote, hematite–magnetite; (3) skarns contain up 15 percent sulfides (pyrite >> pyrrhotite–chalcopyrite) locally associated distal massive magnetite hematite lodes; (4) massive silica–pyrite bodies replace prograde skarn marble; (5) pyroxene–skarn silica– pyrite the potassic core Fe Zn enriched, respectively; (6) mineralized igneous rocks exhibit characteristic early potassic–propylitic late phyllic alteration that skarns can be correlated prograde garnet–pyroxene and retrograde silica–pyrite, (7) Cu–Au anomalies occur in exposures within potassic phyllic halos. Fluid inclusions from igneous rocks record multiple fluid events: a hypersaline inclusion population (up 67 wt % NaCl equiv for halite + sylvite–bearing inclusions, 2–36 KCl equiv, 31–45 equiv) homogenized by salt dissolution between 355° 400°C, interpreted been trapped at lithostatic pressure approximately 500 bars; (2) second hypersaline inclusion homogenized vapor bubble disappearance at 215° to 385°C may trapped similar temperatures but lower pressures relative first population; (3) coexisting NaCl–saturated inclusions (average salinity 35 vapor–rich in igneous close skarn (326°–360°C) that formed boiling low–salinity 8 NaCl) at hydrostatic pressures 120 170 bars. Skarn minerals also hypersaline fluids homogenization those measured nearby igneous slightly salinities (23.3–26 equiv). Inclusions limestone next temperatures and (33 rock inclusions. As evidenced fluids zone supercritical in silica–pyrite, hydrostatic regime during of skarn porphyry–style alteration. The phase nearby produced plume responsible for retrograde cavernous porosity carbonate the skarn front. Overall, Caicayen is worldwide copper systems emplaced at shallow levels, it lacks intrusive phases successive fluid pulses; thus, resulting grades than mined skarns.