Paper No. 9
Presentation Time: 10:40 AM
ANALYSIS OF THE ROLE AND TIMING OF FLUID FLOW DURING ORE SYNTHESIS AND VONSENITE CRYSTALLIZATION IN THE JAYVILLE IRON DEPOSIT, NW ADIRONDACK MOUNTAINS, NYS
The Jayville vonsenite/ magnetite ore body in the NW Adirondack mountains in New York State was developed via a complex series of fluid infiltration events through lenses and pods of calcsilicate gneiss engulfed with a late Proterozoic granite body. The Ore body is located in the footwall of the Carthage Colton Shear Zone (CCSZ) and ore formation attends exhumation of the footwall during the close of the Ottawan Orogeny. Initial ore development was the result of granite intrusion into and while the footwall was at upper amphibolite to granulite facies conditions. During this stage, the engulfed calcsilicate gneiss (DIOP + QTZ+PLAG+KSPAR) was transformed into Hornblendite (ferro-Hastingsite) and Annite skol. Subsequent to this event, the ore was modified by a series of low temperature (Greenschist facies) fluid infiltration events. Backscattered electron imagining (BSE) reveals the extent and timing of these infiltration events. Two discrete ore forming fluids have been identified. The first is an oxidizing and HF-bearing fluid that penetrates the ore resulting in widespread breakdown of the annite component in the skols and fluorine replacement of hydroxyl in the surviving biotite and hornblende. In biotite, magnetite lenses form along disrupted (001) cleavage planes forming delicate reaction textures. The second event involves a boron-rich fluid that infiltrates the ore through a network of fractures that give the host ore a shattered appearance in BSE. Where these veins intersect magnetite, the latter is partially converted to vonsenite. Vonsenite also occurs as veins that lace throughout the host ore. The source(s) for these fluids is not related to the intrusion of the granite which occurs at near peak regional P-T condition as these infiltration events occur long after (70-90My) the regional granulite/upper amphibolite facies conditions have waned. Preliminary d 11B data are consistent with a carbonate/evaporite source but do not preclude volcanic/exhalative source for these fluids. Since no igneous activity is known at the time when the footwall would have reached greenschist facies P-T conditions, we conclude that the fluids are most likely sourced from marble and meta-evaporite deposits in the adjacent hanging wall during late-stage collapse along and fluid flow through the CCSZ.