title: The Origin of the Rock Lake Stratabound Copper-Silver Deposit, Rock Lake, Montana
Creators: name: Hemstad, Christopher B.
Publisher: name: Western Washington University
Year: 1996

description: <p>The Rock Lake copper-silver deposit is a zoned, stratiform, red-bed type deposit, occurring within the Revett Formation of the Proterozoic Belt Supergroup. Structurally the deposit occurs in the west limb of an overturned syncline and is bounded to the west by the Rock Lake normal fault and to the east by the Libby Lakes thrust. At Rock Lake the copper-silver minerals are found in a zonally distributed copper-sulfide system consisting of seven gradational zones representing a migrating redox interface. The sulfide system is wedge shaped with a thick pyrite-galena core near the Rock Lake fault. Vertically and horizontally, from the pyrite-galena core (zone I) outward, the copper-silver minerals form concentric shells successively dominated by chalcopyrite <em>+!- </em>pyrite +/- magnetite (zone II), chalcopyrite <em>+ </em>bornite <em>+</em><em>1</em><em>- </em>magnetite <em>+!- </em>silver (zone III), bornite +/- silver <em>+!- </em>magnetite/hematite, (zone IV), bornite <em>+ </em>chalcocite <em>+!- </em>silver +/- magnetite/hematite (zone V), and an outer shell of chalcocite +/- magnetite/hematite (zone VI) grading into the hematite bearing Revett Formation (zone VII). From the pyrite-galena core outward this mineral zonation resembles a reverse roll front sequence, reflecting a gradual increase in oxygen fugacity and/or a decrease in sulfur activity. Along the outer/eastern fringes of this distinct wedge, minor oppositely zoned wedges extend into the overall geometry reflecting normal roll front processes, whereby the reverse mineral zonation in present. The deposit as a whole is stratabound within the lower Revett Formation, but individual zone boundaries cross bedding at various angles, suggesting an epigenetic origin for the Rock Lake deposit.</p>
<p>Log f0<sub>2</sub> vs. aΣS activity diagrams show that the stability fields of hematite, magnetite, pyrite, chalcopyrite, bornite, chalcocite, and native copper are consistent with the observed mineral boundaries. End member hydrothermal fluid estimates indicate that the solubility of copper as a bisulfide complex is too low to account for the amount of copper at Rock Lake. Therefore, it is likely that copper and silver were carried as metal-chloride complexes rather than as metal- bisulfide complexes.</p>
<p>Lead isotope compositions of sulfides from the pyrite-galena zone are extremely non-radiogenic and plot near the 1.0 Ga mixing line isochron between the upper crustal shale curve of Godwin and Sinclair (1982) and a calculated curve representing evolution of lead in the upper mantle. With distance from the Rock Lake fault the lead becomes increasingly more radiogenic suggesting that lead originated from two sources, the upper mantle and the upper crust.</p>
<p>Based on the distinct geometry of the deposit, thermodynamic modeling, and lead isotopes, three models are proposed to account for the origin of the Rock Lake deposit: (1) The one fluid model. In this case a reduced sulfur-rich fluid, enriched in both copper and silver, flowed up the Rock Lake fault and encountered the permeable sands at the top of the Lower Revett Formation. The reduced fluid permeated into the Revett and reacted with hematite and magnetite present in the Revett, becoming progressively more oxidized with distance from the fault, thereby successively precipitating pyrite, chalcopyrite, bornite, and chalcocite. (2) The two fluid mixing model. In this model a reduced fluid flowing up the Rock Lake fault permeated into the Revett sands and mixed with an oxidized metal-rich fluid flowing through the Revett aquifer. Mixing of the oxidized fluid with increasing proportions of the reduced fluid would have created a migrating redox interface outward from the Rock Lake fault. Successive precipitation of pyrite, chalcopyrite, bornite, and chalcocite took place at this interface. (3) The two successive fluids model. In this model a reduced pyrite-rich fluid flowed up the Rock Lake fault, leaked into the Revett Formation and precipitated a thick wedge of pyrite and galena. A later oxidized, metal-rich fluid, flowing laterally through the Revett encountered this thick wedge of pyrite and reacted around the margins, locally penetrating along permeable zones. Copper-sulfides were successively precipitated around the reduced wedge in a normal roll front sequence of hematite, chalcocite, bornite, and chalcopyrite.</p>
<p>Because copper and silver are more easily carried as chloride complexes, models two and three are more likely than model one. The lead isotope analysis shows that lead compositions are increasingly more radiogenic outward from the Rock Lake fault. This suggests leaching of upper crustal lead by a fluid flowing long distances through the Revett and later mixing with a reduced fluid carrying non- radiogenic lead. Therefore the two fluid mixing model is the best model for the origin of the Rock Lake stratabound copper-silver deposit.</p>