Description: Quantitative determination of the mineralogical composition of hydrothermally altered rocks was performed by means of the Rietveld method using X-ray powder-diffraction data. Initially, experiments were carried out to minimize systematic errors arising from preferred orientation of particles as well as micro-absorption. The precision of the proposed method was tested by independent replicate sample-preparation and analyses. The closeness of the replicate phase-determinations showed that random within-laboratory errors were comparatively small. Expressed as chemical compositions, the quantitative results are in good agreement with the major oxide concentrations determined by X-ray fluorescence. The results indicate that the relative abundances of phases and refined element substitutions were accurately determined. The method developed was applied to hydrothermally altered rocks from the Waterloo volcanic-rock-hosted massive sulfide (VHMS) deposit in Queensland, Australia. Hierarchical cluster analysis led to the discrimination of several mineralogically distinct alteration-induced assemblages. These mineral assemblages are characteristic of specific zones of alteration. The strong spatial zoning with respect to the mineralized body and the distinct mineralogical assemblages of the alteration halo are interpreted to result primarily from varying degrees of hydrolytic decomposition and potassium metasomatism of the wallrocks. Based on these results, we suggest that quantitative phase-analysis by the proposed method represents a new powerful tool in studies of alteration halos.

Description: The Glacier Creek volcanogenic massive sulfide (VMS) deposit, Alaska, is hosted within Late Triassic, oceanic back-arc or intraarc, rift-related bimodal volcanic rocks of the allochthonous Alexander terrane, known as the Alexander Triassic metallogenic belt. The Alexander Triassic metallogenic belt is host to the world-class Greens Creek Zn-Pb-Ag VMS deposit near Juneau in the south and the giant Windy Craggy Cu-Co VMS deposit in British Columbia, about 250 km to the north. The Glacier Creek deposit, located ~80 km southeast of Windy Craggy, consists of four tabular massive sulfide lenses within a bimodal mafic volcaniclastic and rhyolitic sequence. The mineralization-hosting stratigraphy is folded by a deposit-scale anticline and offset by a thrust fault near the axial surface of the fold. A resource of 8.13 Mt has been inferred from drilling, with grades of 1.41% Cu, 5.25% Zn, 0.15% Pb, 0.32 g/t Au, and 31.7 g/t Ag. Six main mineralization types are recognized, dominated by massive barite-sphalerite-pyrite, which is replaced at the base and center of the main lenses by massive and semimassive chalcopyrite-pyrite-quartz. The flanks and tops of the lenses are carbonate rich and consist of interbedded calcite-dolomite, barite and sulfide, resedimented massive barite-sulfide, and mineralized massive carbonate rocks. Tuffaceous hydrothermal sediment, with a distinct positive Eu anomaly, overlies the massive sulfide. Pyrrhotite and chalcopyrite in stringers constitute the main “feeder zone.” Stringer-style sphalerite-pyrite mineralization occurs above and below the lenses. Fe-poor sphalerite is dominant throughout the lenses, whereas Fe-rich sphalerite occurs at the stratigraphic top and bottom of the lenses in pyrrhotite-rich zones. Galena, tennantite-tetrahedrite, and arsenopyrite are the most important trace minerals within massive barite-sphalerite-pyrite mineralization, which is generally enriched in Sb, Hg, and Tl. Mineralization-related gangue minerals include barite, quartz, barian muscovite, calcite, dolomite, albite, chlorite, hyalophane, and celsian. Four types of alteration are recognized in the dominantly basaltic host rocks: pervasive muscovite-rich alteration, quartz-pyrite alteration associated with sulfide stringers, stratabound carbonate-bearing alteration, and background epidote-bearing alteration. Mass balance calculations indicate gains of S, Fe, Si, and K with coincident losses of Ca, Na, and Mg in all of the alteration types. Trace elements, Tl, Sb, Hg, Ba, Zn, Cu, and As were added to the rocks, whereas Sr was lost. Short wavelength infrared (SWIR) spectroscopy shows an increase in the wavelength of the AlOH absorption feature toward mineralization at a scale of 30 to 50 m, coincident with a general decrease in the Na, K, and Al and increase in the Fe, Mg, and Ba content of muscovite. The Glacier Creek deposit is transitional in character between Greens Creek, which is more Zn, Pb, and precious metal rich, and the Windy Craggy deposit, which is more Cu and Co rich, reflecting differences in the basement rocks and depositional settings within the Alexander Triassic metallogenic belt. Mineral-chemical studies and sulfur isotope data suggest that the Glacier Creek deposit formed under initially oxidized and sulfate-rich conditions that evolved to more reduced conditions in the latest stages of mineralization. The abundant argillite and presence of hyalophane rather than barite in the immediate hanging wall of the deposit may be an indication of a deepening basin and development of local anoxia, similar to Greens Creek.

Description: The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of ~25 km2, including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki and Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y., from ~3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks and volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, ~2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including the ~1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the ~0.02 Ma Lapangan tuff.Extensive advanced argillic alteration crops out for ~7 km along the unconformity between the basement rocks and the Imbanguila dacite formation and consists of quartz-alunite ± pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite ± kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is ~4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the ~1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches ~600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore.This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion: The alunite absorption peak at ~1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates that the Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Pb, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu