40 39 AR/ AR AGE AND ORIGIN OF JAROSITE MINERALIZATION AT THE HANSONBURG DISTRICT, NEW MEXICO Virgil W. Lueth1 and M. T. Heizler1 1New Mexico Bureau of Mines & Mineral Resources, Socorro, NM, 87801, [email protected] 3+ Jarosite [K2Fe6 (SO4)4(OH)12] is formed in oxidized portions of sulfide ore deposits, precipitated as a primary mineral in some thermal springs and also as result of the production of H2S from organic matter. Jarosite occurs late in the ore mineral paragenesis at the Snake Pit, Royal Flush, Mex-Tex, Portales, and Sunshine mines where it coexists with fluorite, barite and quartz. Coarsely crystalline and euhedral jarosite is occasionally included in the quartz and fluorite. Petrologic and paragenetic relationships indicate the jarosite mineralization is hypogene. Additional support for this hypothesis is seen in the paragenetic relationships displayed by the lead mineralogy. The fIrst formed lead mineral is galena (PbS) followed by anglesite (PbSO4) and fInally by cerussite (PbCO3). Mineral textures show sharp boundaries between galena and anglesite, characteristic of epitaxial overgrowth and not weathering. The cerussite is probably of secondary origin. The unusually diverse sulfate mineralogy at Hansonburg also suggests a hypogene oxidation event associated with the late stages of mineralization resulted in the formation of the complex sulfate mineral assemblage (anhydrite, antlerite, barite, beaverite, brochantite, caledonite, celestine, chalcanthite, creedite, corkite, cyanothrichite, goslarite, gypsum, jarosite, langite, linarite, plumbojarosite, serpierite, spangolite, tsumebite). Putnam (1983) modeled the mineralization at Hansonburg based on fluid inclusion compositions and determined that early sulfide mineralization was followed by a sulfate stage due to precipitation of sulfides or cooling of the mineralizing fluids. The coarse grained, euhedral jarosite also formed during this late sulfate stage of mineralization. 40Ar/39Ar geochronologic analyses were performed on hand-picked jarosite separates (confirmed by x-ray diffraction) by both CO2 laser and resistance furnace incremental heating techniques. The jarosite from the Snake Pit deposit gave well-behaved plateau age spectra and high radiogenic yields. The simple systematics provide precise ages ranging from 6.36±0.1 to 5.98±0.06 (2σ) Ma and are significant regardless of the mode of origin for the jarosite at Hansonburg. If the jarosite mineralization is hypogene, consistent with the geologic evidence, then the age represents the time of late stage sulfate mineralization at Hansonburg. Compared to other jarosite-bearing deposits such as the Copiapo jarosite deposit (40Ar/39Ar age = 5.0 ±0.3 to 4.6 ±0.06 Ma) at Webb Gap, New Mexico, the ages are similar and result from rift-related pulses of mineralization. Interpretation of the data suggests that basin dewatering and/or migration of H2S up boundary faults resulted in sulfide and sulfate (including jarosite) mineralization. Accordingly, galena-fluorite-barite mineralization probably correlates to major episodes of rift activity. If the jarosite mineralization is due to weathering, then the age represents a time of significant oxidation weathering of the Hansonburg deposits. Such a period of weathering would be due to uplift on the margins of the Oscura Mountains during the late Miocene. Keywords: jarosite, Hansonburg mining district, geochronology, argon, 40/39, pp. 18 1997 New Mexico Geological Society Annual Spring Meeting April 18, 1997, Macey Center.