Potrerillos Stable Isotope Sample Descriptions

Sample Location Description
EH-15 Co. Silica Au-bearing quartz veins cutting sericitized porphyry. Granular quartz veins contain native gold, pyrite, jamesonite, and stibnite, and are surrounded by selvedges of sericite. Jarosite replaces pyritohedral pyrite, and forms veins cutting quartz veins.

EH-17 Cobre Porphyry Haulage adit. Annitic biotite phenocrysts from the least altered Cobre Porphyry available. Hornblende is fresh with glassy cleavage surfaces. Plagioclase is glassy and free of sericite or orthoclase rims. Groundmass is largely free of secondary orthoclase.

EH-20 Cobre Porphyry Haulage adit. Late stage pyrite-tennantite-chalcopyrite-ankerite vein with prominent sericite selvedge.

EH-21 Cobre Porphyry Haulage adit. Early bornite-chalcopyrite veinlets in pervasively K-silicate-altered porphyry. Bornite is present as fine grains disseminated along short, discontinuous microfractures, and within discontinuous, mosaic-textured quartz-orthoclase veins. Veinlets contain selvedges of turbid, pink, perthitic orthoclase, and scattered fine-grained flakes of phlogopitic biotite. Hornblende phenocrysts that are traversed by veinlets are completely converted to aggregates of fine-grained, "shreddy" phlogopitic biotite. Anhydrite is present as small grains within quartz grains. Large fluid inclusions in vein quartz contain multiple cubic and tabular birefringent daughter minerals and a vapor bubble in varying proportions.

EH-27 Cobre Porphyry Haulage adit. Coarse-grained, phlogopitic biotite filling vein with chalcopyrite. Selvedges of vein are turbid, pink, perthitic orthoclase.

EH-56 Co. Silica 15 mm wide vein of very fine-grained alunite cutting illite-altered porphyry. Alunite vein cuts all quartz veinlets.

EH93-20 Co. Silica Very fine-grained alunite replacing illite from 1-3mm diameter white "moteada" spots in silicified marine sedimentary rocks. Pyrite and leucoxene are disseminated largely in the gray siliceous matrix surrounding white spots. Alunite presumed to be supergene replacement of hydrothermal illite based on texture and inconsistent presence of alunite in white spots.

EH93-22 Co. Silica Au-bearing silicified marine sedimentary rock, consisting of a porous mosaic of 0.1 to 0.5 mm diameter granular quartz. Solid inclusions in quartz comprise rhodochrosite, pyrite, rutile, and apatite. Quartz is cut by 0.5-5.0 mm wide barite veins.

EH93-24 Co. Silica Nodular, very fine-grained alunite encased in massive kaolinite from the basal fault in the Cerro Silica open pit. This fault forms the lower limit of oxidation, and is marked by abundant sooty chalcocite, covellite, and native sulfur coating fracture surfaces.

EH93-29 Co. Silica Massive kaolinite associated with nodular, very fine-grained alunite at same location as EH93-24. Infrared reflectance spectrometry shows this kaolinite to be very poorly crystallized, typical of supergene kaolinite.

EH93-31 Sedimentary wallrocks of Cobre Porphyry Haulage adit - Callovian black calcareous, pyritic, argillaceous anhydrite bed located more than 1000 meters from the margin of the Cobre Porphyry stock. Pyrite is very fine-grained and framboidal in texture. Interpreted to be diagenetic pyrite.

EH93-32 Cobre Porphyry Haulage adit. Collected from the thrust fault that truncates the Cobre stock at depth. Shear zone consisting largely of mylonitized anhydrite containing broken mosaic-textured quartz veins and pyrite crystals. Might represent a fault whose location was controlled by a preexisting anhydrite evaporite layer in the wallrock sequence, or could be a sheared pyrite-anhydrite hydrothermal vein that was localized in the fault zone and consequently sheared.

EH93-34 Cobre Porphyry Haulage adit. Cross-fiber gypsum veinlet that cuts all other vein types in the porphyry, and that encrusts open fractures in the ribs of the adit. Similar veinlets in other places are associated with chalcanthite.

EH93-35 Cobre Porphyry Haulage adit. Late-stage quartz-tennantite-pyrite-chalcopyrite-galena-sphalerite-gold -anhydrite-gypsum-sericite vein, reopened and filled by coarse-grained, pink ankerite.

EH93-38 Sedimentary wallrocks of Cobre Porphyry Haulage adit - Callovian evaporite bed located within the outer fringe of the metasomatic aureole of the Cobre porphyry. Underlying sandstones have been altered to green epidote-vesuvianite-quartz rock. Gypsum bed is cut by meter-scale stylolites. Digestion of gypsum in HCl reveals insoluble epidote, vesuvianite, quartz, and possibly larnite. No veins of gypsum are observed in the underlying sandstone. Partial dissolution of this evaporite bed during emplacement of the Cobre Porphyry seems plausible.

EH93-39 Sedimentary wallrocks of Cobre Porphyry Haulage adit - Callovian evaporite bed located 500 meters further from porphyry than EH93-38. Gypsum bed contains lenses of, and is underlain and overlain by, red sandstone that lacks epidote, vesuvianite, or other minerals indicative of metasomatism. No stylolites present. Stringers of satin spar are abundant in the enveloping sandstone. Evidence of extensive dissolution of this gypsum bed is lacking.

EH93-54 Co. Silica North porphyry Coarse-grained alunite replacing plagioclase phenocrysts in porphyry, accompanied by pyrophyllite, zunyite, woodhouseite, pyrite, rutile, and granular quartz. Coarse-grained dickite in veinlets and irregular masses cut alunite and replace pyrophyllite. Jarosite replaces octahedral pyrite, and jarosite veinlets cut dickite veinlets.

EH93-64 Co. Silica Rhythmically banded illite//very fine-grained, gray quartz. Unusual variant of the "moteada" texture observed in altered marine sediments.

EH93-72 Co. Silica Intensely alunitized pyroclastic rock. Sample consists entirely of friable, coarse-grained alunite. Sample collected from drill core a few meters above EH93-79, which contains abundant relict pyrite enclosed in dense silica.

EH93-79 Co. Silica Strongly silicified pyroclastic rock which contains jarosite and abundant relict pyrite. Sample collected from drill core a few meters below EH93-72, which consists entirely of friable, coarse-grained alunite.

Sample	Location	Description
EH-15	Co. Silica	Au-bearing quartz veins cutting sericitized porphyry. Granular quartz veins contain native gold, pyrite, jamesonite, and stibnite, and are surrounded by selvedges of sericite. Jarosite replaces pyritohedral pyrite, and forms veins cutting quartz veins.
EH-17	Cobre Porphyry	Haulage adit. Annitic biotite phenocrysts from the least altered Cobre Porphyry available. Hornblende is fresh with glassy cleavage surfaces. Plagioclase is glassy and free of sericite or orthoclase rims. Groundmass is largely free of secondary orthoclase.
EH-20	Cobre Porphyry	Haulage adit. Late stage pyrite-tennantite-chalcopyrite-ankerite vein with prominent sericite selvedge.
EH-21	Cobre Porphyry	Haulage adit. Early bornite-chalcopyrite veinlets in pervasively K-silicate-altered porphyry. Bornite is present as fine grains disseminated along short, discontinuous microfractures, and within discontinuous, mosaic-textured quartz-orthoclase veins. Veinlets contain selvedges of turbid, pink, perthitic orthoclase, and scattered fine-grained flakes of phlogopitic biotite. Hornblende phenocrysts that are traversed by veinlets are completely converted to aggregates of fine-grained, "shreddy" phlogopitic biotite. Anhydrite is present as small grains within quartz grains. Large fluid inclusions in vein quartz contain multiple cubic and tabular birefringent daughter minerals and a vapor bubble in varying proportions.
EH-27	Cobre Porphyry	Haulage adit. Coarse-grained, phlogopitic biotite filling vein with chalcopyrite. Selvedges of vein are turbid, pink, perthitic orthoclase.
EH-56	Co. Silica	15 mm wide vein of very fine-grained alunite cutting illite-altered porphyry. Alunite vein cuts all quartz veinlets.
EH93-20	Co. Silica	Very fine-grained alunite replacing illite from 1-3mm diameter white "moteada" spots in silicified marine sedimentary rocks. Pyrite and leucoxene are disseminated largely in the gray siliceous matrix surrounding white spots. Alunite presumed to be supergene replacement of hydrothermal illite based on texture and inconsistent presence of alunite in white spots.
EH93-22	Co. Silica	Au-bearing silicified marine sedimentary rock, consisting of a porous mosaic of 0.1 to 0.5 mm diameter granular quartz. Solid inclusions in quartz comprise rhodochrosite, pyrite, rutile, and apatite. Quartz is cut by 0.5-5.0 mm wide barite veins.
EH93-24	Co. Silica	Nodular, very fine-grained alunite encased in massive kaolinite from the basal fault in the Cerro Silica open pit. This fault forms the lower limit of oxidation, and is marked by abundant sooty chalcocite, covellite, and native sulfur coating fracture surfaces.
EH93-29	Co. Silica	Massive kaolinite associated with nodular, very fine-grained alunite at same location as EH93-24. Infrared reflectance spectrometry shows this kaolinite to be very poorly crystallized, typical of supergene kaolinite.
EH93-31	Sedimentary wallrocks of Cobre Porphyry	Haulage adit - Callovian black calcareous, pyritic, argillaceous anhydrite bed located more than 1000 meters from the margin of the Cobre Porphyry stock. Pyrite is very fine-grained and framboidal in texture. Interpreted to be diagenetic pyrite.
EH93-32	Cobre Porphyry	Haulage adit. Collected from the thrust fault that truncates the Cobre stock at depth. Shear zone consisting largely of mylonitized anhydrite containing broken mosaic-textured quartz veins and pyrite crystals. Might represent a fault whose location was controlled by a preexisting anhydrite evaporite layer in the wallrock sequence, or could be a sheared pyrite-anhydrite hydrothermal vein that was localized in the fault zone and consequently sheared.
EH93-34	Cobre Porphyry	Haulage adit. Cross-fiber gypsum veinlet that cuts all other vein types in the porphyry, and that encrusts open fractures in the ribs of the adit. Similar veinlets in other places are associated with chalcanthite.
EH93-35	Cobre Porphyry	Haulage adit. Late-stage quartz-tennantite-pyrite-chalcopyrite-galena-sphalerite-gold -anhydrite-gypsum-sericite vein, reopened and filled by coarse-grained, pink ankerite.
EH93-38	Sedimentary wallrocks of Cobre Porphyry	Haulage adit - Callovian evaporite bed located within the outer fringe of the metasomatic aureole of the Cobre porphyry. Underlying sandstones have been altered to green epidote-vesuvianite-quartz rock. Gypsum bed is cut by meter-scale stylolites. Digestion of gypsum in HCl reveals insoluble epidote, vesuvianite, quartz, and possibly larnite. No veins of gypsum are observed in the underlying sandstone. Partial dissolution of this evaporite bed during emplacement of the Cobre Porphyry seems plausible.
EH93-39	Sedimentary wallrocks of Cobre Porphyry	Haulage adit - Callovian evaporite bed located 500 meters further from porphyry than EH93-38. Gypsum bed contains lenses of, and is underlain and overlain by, red sandstone that lacks epidote, vesuvianite, or other minerals indicative of metasomatism. No stylolites present. Stringers of satin spar are abundant in the enveloping sandstone. Evidence of extensive dissolution of this gypsum bed is lacking.
EH93-54	Co. Silica North porphyry	Coarse-grained alunite replacing plagioclase phenocrysts in porphyry, accompanied by pyrophyllite, zunyite, woodhouseite, pyrite, rutile, and granular quartz. Coarse-grained dickite in veinlets and irregular masses cut alunite and replace pyrophyllite. Jarosite replaces octahedral pyrite, and jarosite veinlets cut dickite veinlets.
EH93-64	Co. Silica	Rhythmically banded illite//very fine-grained, gray quartz. Unusual variant of the "moteada" texture observed in altered marine sediments.
EH93-72	Co. Silica	Intensely alunitized pyroclastic rock. Sample consists entirely of friable, coarse-grained alunite. Sample collected from drill core a few meters above EH93-79, which contains abundant relict pyrite enclosed in dense silica.
EH93-79	Co. Silica	Strongly silicified pyroclastic rock which contains jarosite and abundant relict pyrite. Sample collected from drill core a few meters below EH93-72, which consists entirely of friable, coarse-grained alunite.