Detection of copiapite in the northern Mawrth Vallis region of Mars: Evidence of acid sulfate alteration

The Mawrth Vallis region on Mars is associated with extensive layered deposits containing a stratigraphic sequence of Fe/Mg smectites overlain by Al phyllosilicates. Earlier studies have reported restricted exposures of the ferric sulfate mineral jarosite on top of the sequence. In this paper we have used CRISM data covering the northern portion of the Mawrth Vallis region to find a new jarosite exposure and multiple occurrences of the mixed valence Fe-sulfate mineral copiapite (Fe²⁺Fe³⁺₄(SO₄)₆(OH)₂·20(H₂O)). HiRISE imagery indicate that the copiapite exposures lie on top of the Al phyllosilicates and thus post-date that unit either as a coating or as extensive veins. The presumed copiapite exposures are associated with high values of a "SINDX" parameter derived from CRISM data. Application of several spectral matching metrics over a spectral subsection indicated several candidates for the high SINDX phase including copiapite, ferricopiapite and metavoltine (another mixed valence Fe-sulfate mineral). Visible and near infrared CRISM spectra of the high SINDX areas are most consistent with the phase being copiapite. On Earth copiapite generally occurs as efflorescent coatings in acid mine drainage environments or in association with acid sulfate soils. The presence of jarosite and copiapite indicates the presence of acidic waters. Such acid waters could have contributed to the formation of the underlying Al phyllosilicate minerals. A possible mode of origin for these minerals in this region would involve a fluctuating ground water table and the weathering of Fe sulfide minerals.