The Empire Cu-Zn Mine, Idaho: Exploration Implications of Unusual Skarn Features Related to High Fluorine Activity

The Empire Cu-Zn skarn deposit is unusual because of the proximal position of Zn mineralization, abundance of endoskarn, and the extremely vermicular texture of quartz phenocrysts in the related intrusive rocks. Cu-Zn skarn occurs at the contact between Upper Mississippian White Knob limestone and the granite porphyry phase of the Mackay Stock which consists, from early to late, of quartz monzodiorite, granophyre, granite porphyry, porphyritic granite, and many dikes. The late phases have high F and also extremely vermicular quartz phenocrysts. Endoskarn is more abundant than exoskarn. The earliest alteration of the intrusive rocks consists of disseminated diopsidic pyroxene (Di64Hd36 to Di88Hd12), actinolite, and titanite. This assemblage was cut by early scapolite (Me18 to Me35, mostly Me18-26) and/or green pyroxene (Di14Hd80 to Di20Hd77) veinlets, with or without wollastonite halos. These early veins were then cut by main-stage endoskarn veins that typically have a garnet + minor pyroxene inner zone, a wollastonite and/or pyroxene ± Ca-rich plagioclase (An56 to An89) envelope, and a halo containing disseminated, fine-grained alteration minerals of the same assemblage as the envelope. Some veins contain only the envelope assemblage and are interpreted to represent the alteration front. The inner zone locally contains vesuvianite. Where many veins intersect, endoskarn is massive. Pyroxene is zoned around fluid conduits; the distal pyroxene is Fe rich (hedenbergitic) whereas the proximal pyroxene is Fe poor (diopsidic). The garnet changes in the opposite way, being Fe poor-Al rich (grossularitic) in locations distal to the fluid conduits, and Fe rich (andraditic) in proximal locations. In contrast, in the exoskarn, all pyroxene is diopsidic and garnet is andraditic. Weak, retrograde alteration composed of quartz + calcite + chlorite with minor fluorite, talc, and epidote overprinted both endoskarn and exoskarn. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskarn. Sphalerite precipitated together with chalcopyrite in proximal locations and is associated with retrograde alteration. Other ore minerals include minor molybdenite, bornite, pyrite, galena, arsenopyrite, native Au, as well as supergene minerals such as chrysocolla, malachite, azurite, native Cu, and limonite. Fluid xenoliths from pyroxene in early endoskarn veinlets homogenize at >600°C. Massive endoskarn and exoskarn replacing limestone inclusions in granite porphyry formed at 500° to >700°C, whereas the highest temperature inclusions, >700°C, occur in narrow garnet + minor pyroxene veins. Fluid inclusions in exoskarn replacing wall rock have homogenization temperatures of 500° to 650°C. Retrograde alteration and Cu-Zn mineralization occurred at 250° to 300°C. Fluid inclusions in prograde minerals contain daughter minerals, whereas fluid inclusions in retrograde minerals do not, indicating a decrease in salinity with time. Late-stage fluids have low eutectic temperatures, indicating the possible presence of KCl, NaCl, FeCl2, CaCl2, MgCl2, K2CO3, and/or Na2CO3. Formation of the unusually abundant endoskarn, the proximal position of Zn mineralization, and the extremely vermicular texture of quartz phenocrysts are all believed to have been promoted by the high F content of the magmatic fluid. These features may serve as exploration indicators of associated high F mineralization such as buried porphyry Mo deposits.

[1]  G. Dipple,et al.  World Skarn Deposits , 2005 .

[2]  L. Meinert,et al.  The magmatic–hydrothermal transition—evidence from quartz phenocryst textures and endoskarn abundance in Cu–Zn skarns at the Empire Mine, Idaho, USA , 2004 .

[3]  A. H. Clark,et al.  The Lithologic, Stratigraphic, and Structural Setting of the Giant Antamina Copper-Zinc Skarn Deposit, Ancash, Peru , 2004 .

[4]  Z. Chang Magmatic-hydrothermal transition, skarn formation, and mineralization at the Empire Mine, Idaho , 2003 .

[5]  S. Salvi,et al.  Experimental study of aluminum speciation in fluoride-rich supercritical fluids , 2002 .

[6]  J. Schott,et al.  Experimental study of aluminum-fluoride complexation in near-neutral and alkaline solutions to 300 C , 2002 .

[7]  A. M. Aksyuk Estimation of Fluorine Concentrations in Fluids of Mineralized Skarn Systems , 2000 .

[8]  D. M. Lawrence Gold in skarns related to epizonal intrusions , 2000 .

[9]  Dave B. Mayes,et al.  Geology, zonation, and fluid evolution of the Big Gossan Cu-Au skarn deposit, Ertsberg District, Irian Jaya , 1997 .

[10]  L. Meinert Application of Skarn Deposit Zonation Models to Mineral Exploration , 1997 .

[11]  J. Sáez,et al.  The Cu-(Au) skarn and Ag-Pb-Zn vein deposits of La Paz, northeastern Mexico; mineralogical, paragenetic, and fluid inclusion characteristics , 1994 .

[12]  Donald E. Canfield,et al.  Geochemica et cosmochimica acta : Erratum to D. E. Canfield and D. J. Des Marais (1993) , 1994 .

[13]  T. Mernagh,et al.  Fluid inclusion studies of zoning in the Dachang tin-polymetallic ore field, People's Republic of China , 1993 .

[14]  L. Meinert Skarns and Skarn Deposits , 1992 .

[15]  H. Krouse,et al.  An oxygen, hydrogen, sulfur, and carbon isotope study of carbonate-replacement (skarn) tin deposits of the Dachang tin field, China , 1991 .

[16]  T. Lowenstein,et al.  Melting behavior of fluid inclusions in laboratory-grown halite crystals in the systems NaClH2O, NaClKClH2O, NaClMgCl2H2O, and NaClCaCl2H2O☆ , 1990 .

[17]  A. B. Wilson,et al.  Mineral resource potential and geology of the Challis National Forest, Idaho , 1989 .

[18]  R. W. Le Maitre,et al.  A Classification of igneous rocks and glossary of terms : recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks , 1989 .

[19]  W. D'angelo,et al.  Chemistry of aqueous solutions coexisting with fluoride buffers in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O-F 2 O (sub -1) (1 kbar, 400 degrees -700 degrees C) , 1988 .

[20]  J. Webster,et al.  Phase equilibria of a Be, U and F-enriched vitrophyre from Spor Mountain, Utah , 1987 .

[21]  T. Shepherd,et al.  A Practical Guide to Fluid Inclusion Studies , 1985 .

[22]  F. Haynes Determination of fluid inclusion compositions by sequential freezing , 1985 .

[23]  D. Manning The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with excess water at 1 kb , 1981 .

[24]  C. R. Knowles,et al.  Phase relations in the systems PbS-Sb 2 S 3 -Bi 2 S 3 and PbS-FeS-Sb 2 S 3 -Bi 2 S 3 , 1980 .

[25]  E. McBride,et al.  The Mississippian and Pennsylvanian (Carboniferous) Systems in the United States: Texas , 1980 .

[26]  R. Iler The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica , 1979 .

[27]  P. Cloke,et al.  Compositionally distinct, saline hydrothermal solutions, Naica Mine, Chihuahua, Mexico , 1979 .

[28]  T. Nilsen Paleogeography of Mississippian Turbidites in South-Central Idaho , 1977 .

[29]  R. Potter Pressure corrections for fluid-inclusion homogenization temperatures based on the volumetric properties of the system NaCl--H/sub 2/O , 1977 .

[30]  C. P. Ross,et al.  Geology of part of the Alder Creek mining district, Custer County, Idaho , 1968 .

[31]  G. Anderson,et al.  Reactions of quartz and corundum with aqueous chloride and hydroxide solutions at high temperatures and pressures , 1967 .

[32]  C. P. Ross Upper Paleozoic Rocks in Central Idaho: GEOLOGICAL NOTES , 1962 .

[33]  J. G. Stone Ore genesis in the Naica district, Chihuahua, Mexico , 1958 .

[34]  F. W. Farwell,et al.  Geology of the Empire Copper Mine near Mackay, Idaho , 1944 .

[35]  A. Wandke,et al.  Pyrometasomatic vein deposits at Tepezala, Aguascalientes, Mexico , 1935 .

[36]  J. Umpleby The genesis of the Mackay copper deposits, Idaho , 1914 .

[37]  C. Fenner,et al.  Study of a contact-metamorphic ore deposit; the Dolores Mine, at Matehuala, S. L. P., Mexico , 1912 .