Mafic Wall Rocks Research Articles

BIF-hosted iron mineral system: A review

BIF-hosted iron mineral system: A review

Primary Nb-Ta minerals in the Szklary pegmatite, Poland: New insights into controls of crystal chemistry and crystallization sequences

An assemblage of primary, extremely As- and Sb-rich, Nb-Ta minerals from the Szklary pegmatite includes columbite-(Fe), columbite-(Mn), tantalite-(Mn), stibiocolumbite, stibiotantalite, an as yet unnamed (As,Sb,U)-rich (Ta,Ti)-oxide, Mn3UAs2Sb2Ta2Ti2O20, and holtite. Anomalous trends of Mn-Fe and Ta-Nb fractionation in the columbite group and crystallization sequences in the primary assemblage can be explained by the contamination of the pegmatite-forming melt by ultramafic and mafic wall-rocks, the competition among these minerals for Ta and Sb and with biotite and tourmaline for Mg, Fe, and Ti, and local variations in melt composition. A hot magmatic fluid, exsolved from the parental melt, reacted with the primary Nb-Ta oxides, inducing two different patterns of alteration. The columbite-group minerals were altered to fersmite, pyrochlore, and bismutopyrochlore locally grading to plumbopyrochlore, whereas stibiocolumbite, stibiotantalite, and the (As,Sb,U)-rich (Ta,Ti)-oxide altered to stibiomicrolite, uranmicrolite grading to betafite, and then to bismutomicrolite or Bi-dominant betafite. In all of the secondary pyrochlore-group minerals, Ta-Nb fractionation is comparable to, or only slightly greater, than that in the primary Nb-Ta oxides, indicating a modest differentiation of the residual melt coexisting with the fluid.

The El Teniente porphyry Cu–Mo deposit from a hydrothermal rutile perspective

Mineralogical, textural, and chemical analyses (EPMA and PIXE) of hydrothermal rutile in the El Teniente porphyry Cu–Mo deposit help to better constrain ore formation processes. Rutile formed from igneous Ti-rich phases (sphene, biotite, Ti-magnetite, and ilmenite) by re-equilibration and/or breakdown under hydrothermal conditions at temperatures ranging between 400°C and 700°C. Most rutile nucleate and grow at the original textural position of its Ti-rich igneous parent mineral phase. The distribution of Mo content in rutile indicates that low-temperature (∼400–550°C), Mo-poor rutile (5.4 ± 1.1 ppm) is dominantly in the Mo-rich mafic wallrocks (high-grade ore), while high-temperature (∼550-700°C), Mo-rich rutile (186 ± 20 ppm) is found in the Mo-poor felsic porphyries (low-grade ore). Rutile from late dacite ring dikes is a notable exception to this distribution pattern. The Sb content in rutile from the high-temperature potassic core of the deposit to its low-temperature propylitic fringe remains relatively constant (35 ± 3 ppm). Temperature and Mo content of the hydrothermal fluids in addition to Mo/Ti ratio, modal abundance and stability of Ti-rich parental phases are key factors constraining Mo content and provenance in high-temperature (≥550°C) rutile. The initial Mo content of parent mineral phases is controlled by melt composition and oxygen fugacity as well as timing and efficiency of fluid–melt separation. Enhanced reduction of SO2-rich fluids and sulfide deposition in the Fe-rich mafic wallrocks influences the low-temperature (≤550°C) rutile chemistry. The data are consistent with a model of fluid circulation of hot (>550°C), oxidized (ƒO2 ≥ NNO + 1.3), SO2-rich and Mo-bearing fluids, likely exsolved from deeper crystallizing parts of the porphyry system and fluxed through the upper dacite porphyries and related structures, with metal deposition dominantly in the Fe-rich mafic wallrocks.

GEOLOGY OF THE NORTH HALF OF THE MT. ABBOT QUADRANGLE, SIERRA NEVADA, CALIFORNIA

Research Article| October 01, 1958 GEOLOGY OF THE NORTH HALF OF THE MT. ABBOT QUADRANGLE, SIERRA NEVADA, CALIFORNIA DONALD G SHERLOCK; DONALD G SHERLOCK 1 Deceased. U. S. GEOLOGICAL SURVEY, FEDERAL CENTER DENVER, COLORADO Search for other works by this author on: GSW Google Scholar WARREN HAMILTON WARREN HAMILTON U. S. GEOLOGICAL SURVEY, FEDERAL CENTER DENVER, COLORADO Search for other works by this author on: GSW Google Scholar GSA Bulletin (1958) 69 (10): 1245–1268. https://doi.org/10.1130/0016-7606(1958)69[1245:GOTNHO]2.0.CO;2 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DONALD G SHERLOCK, WARREN HAMILTON; GEOLOGY OF THE NORTH HALF OF THE MT. ABBOT QUADRANGLE, SIERRA NEVADA, CALIFORNIA. GSA Bulletin 1958;; 69 (10): 1245–1268. doi: https://doi.org/10.1130/0016-7606(1958)69[1245:GOTNHO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The north half of the 15-minute Mt. Abbot quadrangle lies across the crest of the Sierra Nevada. The Cretaceous granitic rocks that underlie most of the area form eight large discordant plutons that range from quartz diorite to alaskite; the largest pluton is coarse prophyritic quartz monzonite. Pre-batholithic metasedimentary and metavolcanic rocks and metagabbro and metadiorite are minor.The porphyritic quartz monzonite sent gently dipping dikes as much as 200 feet thick into its walls. These dikes in places make up more than half the height of exposed contact zones. A broad border zone of protoclastic flaser gneiss was formed in part of the pluton and in the adjacent wall rocks and dikes.A contact between granodiorite and calcareous metasedimentary rocks swarms with dark inclusions. Their origin, puzzling in most places, is here clearly due to progressive hybridization of calc-silicate xenoliths; the changes take place within a few tens of yards of the contact. The xenoliths were amphibolitized concentrically and reconstituted to typical dark-inclusion texture and mineralogy and were drawn out from blocky xenoliths into spindle shapes.Elsewhere, large irregular xenoliths of metadiorite were assimilated by alaskite. Within the resulting complex is a thick series of alaskite flow layers crowded with xenocrysts showing graded “bedding”.All the granitic plutons were intrusive; assimilation of mafic wall rocks by felsic magmas may have caused much of the diversity. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.