Zarita as diminutive of Sara is just a misspelling (not as diminutive of Zara, Zahara or Zaria, e.g., of course). The spelling rules in Spanish are clear and affect first names (see http://www.rae.es); then, there are very few possibilities of innovation in spellings (not as in English). The confusion Z/C/S (and B/V, G/J, etc.) among some Spanish speaker people are not unusual (in names or in words), but are examples of low knowledge of orthography.
Probably, it is an name from Arabic origin unknown in some places and assimilated to Sarita; just as Zara, known nowadays in Spain basically because of Zara Phillips, but unknown in a lot of Spanish speaking countries, where it is assimilated to Sara (and here the coincidence in their Spanish pronunciation is very important). Or just as with Ibán/Iván among a lot of Spaniards: two spellings, two origins and two meanings, but often they are mixed up by people because they sound the same. Or as with the Spanish Álvaro and the English Alvar (in a lot of Spanish names books, Alvar is classified as English version of Álvaro, but it is just an error, isn't it?).
I suspect an Arabic origin , among others, because I found some use of this name (Zarita/Zahrita) in people with Arabic surnames or/and in Muslim countries, as Malaysia: "Zainuddin, Zarita Universiti Sains Malaysia Department of Mathematics 11800 Minden"
"65. Zahrita Hamzah 63.75%. No. 28, 1st Floor, Jln Opera C U2/C, Taman Ttdi Jaya, 40000 Shah Alam, Selangor " (Malaysian Yellow Pages)