Stackoverflowexception for more than 50,000 values

Question

The code sorts an array of up to 50,000 values from a txt and stored in a array for quicksort, but for 60,000 values or more, it’s Overflow. I’ve changed values to Int64 and persist... (Obs: I change the vector size a for the amount of values before executing)

using System;
using System.IO;
using System.Text;
using System.Diagnostics;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

class myclass
{
static void Main()
{

    int counter = 0, aux = 0, aux2 = 0, flag = 0;
    string line;
    Int64[] a = new Int64[10000];
    Stopwatch sw = new Stopwatch();
    sw.Start();


    System.IO.StreamReader file = new System.IO.StreamReader("C:\\Users\\Marco\\Desktop\\test1.txt");
    while ((line = file.ReadLine()) != null)
    {
        a[counter] = Convert.ToInt32(line);
        counter++;
    }


    file.Close();


    Console.WriteLine("Iniciando ordenação");
    Ordenar(a);
    Console.WriteLine("Ordenação finalizada!");
    sw.Stop();
    Console.WriteLine("Quantidade de itens: " + counter);
    Console.WriteLine("Tempo: " + sw.Elapsed);

    do
    {
        Console.WriteLine("Deseja exibir o vetor?");
        Console.WriteLine("1[sim] ou 2[não]");
        aux2 = Convert.ToInt32(Console.ReadLine());

        if (aux2 == 1)
        {
            do
            {
                Console.WriteLine(a[aux]);
                aux++;
            } while (aux < counter);
            flag = 1;
            Console.ReadLine();
        }
        else if (aux2 == 2)
        {
            flag = 1;
            Console.ReadLine();
        }
        else
        {
            Console.WriteLine("opção incorreta!");
        }

    } while (flag != 1);





}


    public static void Ordenar(Int64[] vetor)
    {
        Ordenar(vetor, 0, vetor.Length - 1);
    }

    private static void Ordenar(Int64[] vetor, Int64 inicio, Int64 fim)
    {
        if (inicio < fim)
        {
            Int64 posicaoPivo = Separar(vetor, inicio, fim);
            Ordenar(vetor, inicio, posicaoPivo - 1);
            Ordenar(vetor, posicaoPivo + 1, fim);
        }
    }

    private static Int64 Separar(Int64[] vetor, Int64 inicio, Int64 fim)
    {
        Int64 pivo = vetor[inicio];
        Int64 i = inicio + 1, f = fim;
        while (i <= f)
        {
            if (vetor[i] <= pivo)
                i++;
            else if (pivo < vetor[f])
                f--;
            else
            {
                Int64 troca = vetor[i];
                vetor[i] = vetor[f];
                vetor[f] = troca;
                i++;
                f--;
            }
        }
        vetor[inicio] = vetor[f];
        vetor[f] = pivo;
        return f;
    }
}