Lawrence D. Meinert

Magmatic characteristics of the Paleocene Cerro Nevazón region and other Late Cretaceous to Early Tertiary calc-alkaline subvolcanic to plutonic units in the Neuquén Andes, Argentina

Abstract Voluminous Late Cretaceous–Early Tertiary calc-alkaline magmatism in the northwest part of the Neuquen province, Argentina, is characterized by a NS-trending belt of olivine basalt-augitic andesite volcanic and amphibole-rich diorite subvolcanic facies that were grouped in the Neuquen-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazon area, two were dated by K/Ar in amphibole and one by Ar/Ar in plagioclase; together, they yield ages between 60.1±1.6 and 56.0±1.7 Ma. The igneous rocks contain normally zoned plagioclase (64–74%), amphibole (magnesiohastingsite and edenite; 15–20%), biotite (5–10%), quartz (4–18%), minor enstatite–ferrosilite (Wo2.5–3En71–73Fs24–25), and augite (Wo42–43En42–43Fs14–15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase+olivine+pyroxene±magnetite±ilmenite is responsible for the evolution of the parental magmas, which originated in approximately 10% melting of a mantle source contaminated by fluids from the oceanic subducted lithosphere, with sparse or no residual garnet. The gabbro rare earth element patterns are similar to those of the Antuco volcano (37°25′S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37–41°5′S) emplaced in a relatively thin continental crust (≈30 km). Although the Cerro Nevazon granitoids share some geochemical signatures with the Paleogene Caicayen igneous units, also located in the northwestern region of Neuquen, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicayen rocks. With similar SiO2 contents, the Upper Cretaceous–Paleocene Campana Mahuida subvolcanic rocks from the same region are richer in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U) than either the Nevazon or Caicayen units, which suggests that their parental magmas were contaminated with crustal material. The low Fe2O3/FeO ratios in the Nevazon igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg+Fe+2) ratios in mafic minerals, suggest that the Nevazon magmas evolved under more reduced conditions than did the Caicayen or Campana Mahuida magmas.

Tectonic setting and petrogenesis of the Çelebi granitoid, (Kırıkkale-Turkey) and comparison with world skarn granitoids

Abstract Many studies have shown systematic correlations between the composition of plutons worldwide and the metal content of associated skarns. This is the first report of similar correlations between the composition of Celebi granitoid and skarns of the Celebi district in Central Anatolia, Turkey. The Celebi district is well known for its polymetallic Fe–W and Cu vein ores. These are hosted by calcic skarn zones. Both exoskarns (pyroxene–garnet) and endoskarns (epidote–pyroxene) occur in the district formed mainly along the granitoid contacts and along the fractures within the marble. Based on mineralogy, petrology and geochemistry, two different igneous rocks were recognized in the Celebi granitoid, referred to as leucocratic (felsic) and mesocratic (intermediate) Celebi granitoid. The leucocratic Celebi occurs as dominant rock type, and is classified as granite. The mesocratic Celebi is not widespread and is classified as adamellite, tonalite, quartz monzonite and quartz monzodiorite. The mesocratic Celebi has I-type characteristics, and have subalkaline, calc-alkaline and metaluminous characteristics like most worldwide skarn granitoids. A post-collisional tectonic setting is proposed on the basis of field evidence, the relative timing of intrusions with respect to metamorphic and obducted ophiolitic rocks and trace element geochemistry. The high abundance of La and Ce and the enrichment of V in mafic components suggest that Celebi granitoids are formed by partial melting of mantle rocks, but have been contaminated by interaction with continental crust involving possible magma mixing processes (i.e. mixing of coexisting felsic and mafic magmas). In the district, the mesocratic type and mafic microgranular enclaves (MME) mainly within leucocratic type represent a mafic underplating magma that was mixed with and/or injected into felsic magma of the leucocratic type. The present study shows that Fe mineralization is associated with mesocratic Celebi type, whereas W mineralization is associated with leucocratic type. Mesocratic Celebi granitoid is significantly different from the worldwide average of plutons associated with Fe skarns. In particular, MgO vs. SiO 2 , FeOt+CaO+Na 2 O/K 2 O vs. SiO 2 , Fe 2 O 3 /Fe 2 O 3 +FeO vs. SiO 2 and V vs. Ni vary from typical values (are lower than values typical for plutons associated with Fe skarns) for plutons associated with Fe skarns. Instead, it resembles the geochemical characteristics of plutons associated with worldwide Cu and possibly Au skarns. This suggests new exploration possibilities for copper and gold in the Celebi district.

Acceptance of the Society of Economic Geologists Silver Medal for 2009

Mr. President, ladies and gentlemen: Thank you for your attendance this evening and a special thank you to Brian Skinner for his very kind words of introduction. Also, I should point out that Brian received the very first Silver Medal award in 1981. Thus, I am doubly honored that he is my citationist tonight. It is customary at awards ceremonies to reflect upon how one arrived at this juncture in life and with President Hedenquist’s forbearance I will look not only at the past but also at the present and the future. I am a product of a public school education in the United States, from kindergarten through high school, at a time when public school was both the grand mixer of peoples of all backgrounds and home to many truly great teachers, some of whom were in public education because of wars, discrimination, or sexism that made it difficult for them to find other employment commensurate with their talents. Overall, I received an excellent education but there was at least one unintended consequence—like most American students I was never introduced to geology. I literally had not heard of the word geology when I left home for university. But a series of serendipitous events led to my standing at the podium today. First, I had the good fortune to attend Carleton College in Minnesota. Carleton is a very good school, but there are many excellent colleges and universities in the world. What made Carleton special is that it seems to have a magical ability …

Endoskarn and Cu-Zn mineralization at the Empire mine, Idaho, USA

The Empire mine is a Cu-Zn skam associated with the granite porphyry phase of the Mackay Stock, which consists of quartz monzodiorite, granophyre, granite porphyry, Mackay Granite, and numerous dikes. Both granite porphyry and Mackay Granite have high F and also have unusual, extremely vermicular quartz phenocrysts. Both endoskarn and exoskam are developed at the Empire mine, with more endoskam than exoskarn. The alteration of the intrusive rocks began with weak disseminated diopsidic pyroxene, actinolite, and titanite. Further endoskam formed by veins or as massive replacements of intrusive rocks. The earliest formed endoskarn veinlets contain scapolite, with or without wollastonite halo. This was followed by wollastonite-dominant (± Carich plagioclas and hedenbergitic pyroxene) veins as fronts or envelopes on garnet-dominant veins. Early pyroxene is diopsidic whereas pyroxene in distal/late veinlets is hedenbergitic. Similarly, garnet becomes more Fe-rich with time. In exoskam, all the pyroxene is diopsidic and garnet andraditic. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskam. Zn sulfide precipitated together with Cu in proximal locations, associated with retrograde quartz+calcite+chlorite. Massive endoskarn and exoskarn replacement formed at 500–550°C, whereas slightly higher temperatures were recorded by late minerals at the metasomatic front, >600°C. The highest temperatures, >700°C, occur in gamet-dominant veins that probably represent conduits insulated by earlier skarn. During retrograde alteration, quartz, calcite, chlorite, fluorite, and chalcopyrite precipitated in both endoskarn and exoskarn at 250–300°C. The extremely vermicular texture of quartz phenocrysts, abundant endoskarn, and proximal deposition of Zn, are all caused by the high F contents of the magma and magmatic fluid.