calendar Add meeting dates to your calendar.

 

Paper No. 6
Presentation Time: 9:20 AM

THE FORMATION OF AU-RICH PORPHRY DEPOSITS: WHERE IS THE CU?


SIMON, Adam1, MUNTEAN, John2 and AILLAUD, Brian1, (1)Geoscience, UNLV, 4505 Maryland Parkway, Las Vegas, NV 89154, (2)Nevada Bureau of Mines and Geology, Unversity of Nevada, Reno, Reno, NV 89557-0088, adam.simon@unlv.edu

A global analysis of porphyry-type ore deposits indicates significant variation in the metal tenor and Cu/Au ratios among deposits within a single tectonic regime. For example, an analysis of porphyry deposits along the Andean arc reveals the presence of Cu-rich, Au-poor deposits within tens of kilometers of Cu-poor, Au-rich deposits. The variation in Cu/Au does not correlate strictly to bulk magma composition or the timing of deposit formation. If one assumes that the observed variation in metal tenor and Cu/Au ratios is not caused by significant variation in the initial metal abundances or Cu/Au ratios of the parental pre-differentiated silicate melts along the arc, then processes occurring during magma evolution, i.e., melt/crystal and melt/fluid partitioning, must be responsible for the variation in the observed tenor and metal ratio(s) among deposits that form quasi-contemporaneously from compositionally similar magmas in a relatively restricted volume of crust. In this study, we focus on the evolution of magmatically-derived formative ore fluids in the Pancho and Verde East and West deposits of the Refugio District of the Maricunga Belt, Northern Chile. Pre-,syn- and post-ore fluid inclusions were characterized by using CL and microthermometry, and Cu and Au concentrations were quantified by using LA-ICP-MS. The data constrain fluid chemistries at discrete space-time steps throughout deposit formation. The fluid inclusion data indicate that the Cu/Au ratios of pre-, syn- and post-ore fluids remains relatively constant and that the formative ore fluids possess a low Cu/Au ratio. These data are consistent with a magmatic control on the Cu/Au ratio of the mineralizing fluid. This implies that the fluid chemistry was controlled by processes in the magma plumbing system prior to the exsolution of the ore-forming fluid from the Cu- and Au-carrying silicate melt. The likely magmatic control involves the selective fractionation of a Cu-Fe-S phase(s) that preferentially sequestered Cu from the melt, resulting in a low Cu/Au ratio of the melt prior to volatile saturation. The data elucidate potentially critical controls on the formation of Au-rich porphyry deposits and the Cu/Au ratio of porphyry deposits in general that may be used as exploration criteria.
Meeting Home page GSA Home Page