If you don't use any additional code, the footnotes in the minipage environment will continue with the number of the last footnote + 1. Is it possible to start footnotes with an arabic 1 again in a minibox when there occurred a number of footnotes before?
If you use 'alphanotes' of egreg Change footnote "numbering" temporarily, there is an alphabetic numbering in the text and an arabic numbering in the footnote section.
\documentclass[a4paper,11pt,oneside]{article} %Aber noch mal wegen Subsublines und so schauen
\usepackage[ngerman]{babel} %dt. Silbentrennung
\usepackage[applemac]{inputenc} %Richtiges Sprach-Funktionieren am Mac
\let\thempfootnote\thefootnote
% ---------
\newenvironment{alphafootnotes}
{\par\edef\savedfootnotenumber{\number\value{footnote}}
\renewcommand{\thefootnote}{\alph{footnote}}
\setcounter{footnote}{0}}
{\par\setcounter{footnote}{\savedfootnotenumber}}
%------------
\begin{document}
This is a document.\footnote{\dots}
\begin{minipage}[t]{.42\textwidth}%\centering
\begin{alphafootnotes}
Example.\footnotemark{}\footnotetext{\label{foot:1}Vgl. Quellenangabe, S. 234.}
Example.\footnotemark{}\footnotetext{\label{foot:2}Vgl. Quellenangabe, S. 234.}
\end{alphafootnotes}
\end{minipage}
\end{document}

\renewcommand{\thempfootnote}{\arabic{mpfootnote}}sufficient? Using\footnotein the minipage, of course. – egreg May 21 '14 at 13:33